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REVIEWS

Chemistry, Biochemistry, and Dietary Role of Potato Polyphenols.A Review

Mendel F riedman

Western Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 800B u cha na n St reet , A lb a ny, Ca lifornia 94710

P otatoes and other plant foods accumulate a variety of secondar y metaboli tes, including phenoliccompounds, phytoalexins, protease inhibitors, a nd glycoalkaloids, a s a protection a gainst adverseeffects of mechanical bruising, light, and injury by predators including beetles, fungi, and insects.Since these phytochemicals a re consumed by insects, a nimals, a nd huma ns a s part of their norma ldiet , a need exists to develop a better understanding of the role of these compounds in both theplant a nd the diet. To contr ibute to this effort , this multidisciplinar y overview describes ana lyticaland compositional aspects of phenolic compounds in potatoes; their biosynthesis, molecular genetics,a nd r ole in host-plan t r esista nce relationships; bruising-, ferrous ion-, a nd h eat -induced discolorat ionssuch as after-cooking blackening and blackspot formation, which affect appearance and sensoryproperties of potat oes; polyphenol-oxidase-cata lyzed enzymatic browning reactions an d theirprevention by chemical and plant molecular biology techniques; and effects of baking, cooking,microwaving, light, and-ra diat ion on t he sta bility of the m ajor potat o polyphenol, chlorogenic a cid.Also covered a re beneficial effects of phenolic compounds in t he diet a s a ntioxidan ts, a ntimu ta gens,anticarcinogens, antiglycemic, and hypocholesterolemic agents; adverse effects on protein nutritionalqua l i ty; and recommenda tions for future research. Und erstanding t he biochemical basis of stress-induced formation of polyphenols in plants, the chemistry of their transformations in the plant andin foods, a nd t heir functions in pla nt physiology, food science, nutrit ion, a nd hea lth should stimula teinterest in maximizing beneficial sensory, nutritional, and health effects of polyphenols in the diet.Such efforts should lead to better foods a nd improved huma n hea l th.

Keywords: Ant ioxidan t; blackspot; browni ng; browni ng pr eventi on; chl orogeni c acid ; food quali ty;

food safety; glycoalk aloid s; health ; host-plan t resistance; nu tr it ion; pl ant geneti cs; pl ant physiology;polyph enol oxidase; polyph enols; potatoes;-radiati on; Solanu m tuberosum ; storage

INTRODUCTION

Polyphenolic compounds are secondary plant metabo-lites found in numerous plant species, including pota-toes (Deshpa nde et a l . , 1984). The oxidat ion productsof phenolic compounds appear to be involved in thedefense of pla nts a ga inst inva ding pat hogens, includingbacteria, fungi , a nd viruses. P olymeric polyphenoliccompunds seem to be more toxic to potential phyto-p a t h og e ns t h a n a re t h e p h en ol ic m o n om e rs, s u ch a schlorogenic acid, from which t hey a re derived. Thepolyphenol-oxidase-catalyzed polymerization helps seal

the injured plant surface and begins the healing process,an a logous t o the forma tion of fibrin blood clots in injuredh u m a n s .

Enzyme-catalyzed browning reactions (Hurrel l andFinot, 1984; Lee and Whitaker, 1995; Schwimmer, 1981)involve th e oxidation of phenolic compounds by theenzyme ty rosinase (polyphenol oxidase, P P O) to quino-nes, followed by tr a nsformat ion of the quinones to darkpigments. These result in deteriora tion of fla vor, color,a n d n u t r i t i on a l q u a l i t y a n d c on t i n u e a f t e r t h e f ood i sha rvested. Therefore, prevention of enzyma tic brown-ing in frui ts and vegetables has been a major concernof food scientist s.

P olyphenolic compounds ha ve a lso been shown to

possess antimutagenic, anticarcinogenic, antiglycemic,a nd a ntioxidat ive beneficial properties. These proper-ties can be uti l ized in the prevention of rancidi ty andin the development of health-promoting food ingredi-ents. They may a lso adversely affect protein nutr itiona lq u a l i t y .

To cross-fertilize informa tion a mong severa l d isci-plines (including plant science, entomology, food science,nutrition, and medicine) interested in plant phenolics,I have at tempted to integrate and correlate the widelyscattered l i terature on the role of potato polyphenolsduring plant development a nd growth a nd after ha rvest.Specifically covered are the following relevant aspects:ana lysis; composit ion; biosynthesis; host-plant resis-tance; chemistry of enzymatic browning, blackspots, andot h e r di s col ora t i on s ; h e a t -, l i gh t -, a n d -radiat ion-induced changes; pre- and postharvest browning pre-vention; and beneficial effects following consumption.Suggestions for future resear ch ar e a lso mentioned t ocata lyze progress in minimizing t he a dverse effects ofpota to polyphenols a nd enhancing t he desirable ones.

Since chlorogenic acid constitutes up to 90% of thet o t a l p h en ol ic con t e n t of p ot a t o t u b ers , m os t of t h ediscussion centers a round this compound. Figure 1s h ow s t h e s t ru ct u re s of p ot a t o p h en ol ics , i n cl u din gchlorogenic acid and its isomers.

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S0021-8561(96)00900-4 This article not subject to U.S. Copyright. Published 1997 by the American Chemical Society

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ANALYSIS AND COMPOSITION

Current analyt ical methods for potato polyphenolsinclude gas-l iquid chromat ography (GLC ), high-per-forma nce l iquid chromatography (HP LC), thin-layerchromatography (TLC), and UV spectrophotometry, butsurprisingly, not immunoassays. Before ana lysis, thecompounds ha ve to be extracted an d purified. Voigt a nd

Noske (1964) describe optimized extra ction of chloro-genic acid from potatoes with the aid of acetone, ethanol,an d metha nol. The order of effectiveness wa s metha nol>ethanol > a cetone. They found tha t th e chlorogenicacid content of stewed pota toes wa s the sa me as t ha t ofraw potatoes.

Phenolic compounds are distributed mostly betweenthe cortex and skin (peel) tissues of the potato (Reeveet al . , 1969; Figure 2). About 50% of the phenoliccompounds were loca ted in t he pota to peel an d a djoiningtissues, while the rema inder decreased in concentr at ionf ro m t h e o u t s i de t o w a rd t h e c e n t e r o f p o t a t o t u b e rs(Hasegawa et al . , 1966).

Several isomeric chlorogenic acids have been found

i n p ot a t oe s. Th e m a j or on e , n o w de si g n a t e d 5-ca f -feoylquinic acid by the I nterna tional U nion of Pur e andApplied Chemistry (IUPAC), is complemented by 3-and4-caffeoylquinic acids. The IU P AC numbering syst emdiffers from tha t used by B ran dl a nd H errman (1984).Whether a l l of these isomers occur nat ural ly, or someare a rt i facts formed during extraction a nd isolat ion, isan unresolved issue (Molgaard and Ravn, 1988).

Tisza et al. (1996) used a new GC-MS method for thequanti tat ion of chlorogenic, ci tric, malic, and caffeicacids and of fructose, glucose, and sucrose in freeze-dried potat o samples. The chlorogenic acid content offreeze-dried potato tubers, but not sprouts, correlatedwith values obtained by UV spectroscopy.

B ra ndl a nd H errma nn (1984) an d Na gels et a l . (1980)

describe HPLC separation of chlorogenic acid isomers

in potat oes. Different potat o cultivars grown under thesa me conditions conta ined the following isomers: 3-O-caffeoylquinic (n-chlorogenic a cid), 4-O-caffeoylquinic(crypto-chlorogenic acid), 5-O-caffeoylquinic (n eo-chlo-rogenic acid), 3,4-dicaffeoylquinic, and 3,5-dicaffeoylqui-nic.

B oussenadji et al. (1993) developed a liquid chroma to-graphic t echnique with electrochemical detection t omeasure electroactive antioxidative polyphenols addedto foods a nd drugs t o inhibi t their oxidation. Relatedstudies from this laboratory describe HPLC of glyco-

alkaloids with pulsed amperometric detection (Fried-ma n a nd Levin, 1995; Friedma n et a l . , 1994). Electro-chemical detection methods may therefore be useful inthe ana lysis of pota to glycoalka loids a nd polyphenols.

G riffiths et al . (1992) describe a colorimetric methodfor measuring chlorogenic acid in freeze-dried pota tot u b ers b a s e d o n t h e re a ct i on of t h e a ci d w i t h n i t rou sa cid. Application of this method to 13 pota to cultiva rsrevealed th at , in most cases, the chlorogenic acid contentof t h e s t ol on e n d o f t u b ers cou l d b e re la t e d t o t h esusceptibility t o a fter-cooking-blackening. However, intwo genetical ly r elated cult ivar s, the a mount of black-ening wa s great er tha n would be predicted solely on thebasis of chlorogenic acid content.

Spectrophotometric a na lysis of potat o chlorogenic acidg a v e h i gh e r v a l u es t h a n di d a n a l y si s b y H P L C or G L C(Ma lmberg a nd Thean der, 1985). Spectrophotometrymay give high values because chlorogenic acid isomerscontribute to the total a bsorbance. Since GL C requiresde riv a t i z a t i on , H P L C m a y b e p ref era b l e a s a g en e ra lm e t h od. H o w e ve r, H P L C a n a l y s is m a y n o t a l w a y s b esa tisfa ctory beca use chlorogenic a cid undergoes a time-and light-dependent change.

Because of this unexpected problem we encounteredwith the H P LC method (Da o and Friedma n, 1992, 1994;Friedman and Dao, 1990; Friedman et al . , 1989), weevalua ted th e effectiveness of a UV method for measur-ing chlorogenic acid in several va rieties of fresh pota toesas well as in part s of the pota to plan t , processed potato

products, and weed seeds (Tables 1-4). Chlorogenic

Figure 1. Structures of pota to polyphenols a nd chlorogenicacid isomers.

Figure 2. C ross section of a potat o tuber: (A) Reeve et a l .(1969); (B ) Olss on (1989).

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acid underwent a t ime- an d l ight-dependent cha nge inthe methanolic and ethanolic extracts of potatoes (Fig-ure 3). The decrease of t he chlorogenic a cid peak onchromatograms was accompanied by a correspondingincrease of a new peak. U se of ultra violet spectropho-tometry to estimate chlorogenic acid was reproducible,appa rently beca use the new ly formed compound(s) hadan absorption ma ximum similar to tha t of chlorogenic

acid (Figure 4). Nearly al l of the chlorogenic acid in

spiked pota to powders w as recovered. Thus, the U Vm e t h od m a y h a v e a dv a n t a g e s ov er H P L C . H o w e ve r,UV spectroscopy measur es total content , while HP LCcan mea sure specific polyphenols in a mixtur e. Thus,when H P LC is used, extra cts of plant , food, an d a nimalt issues should be analyzed immediately to minimizeforma tion of new products th at ma y coelute with knownpolyphenols on HP LC colunms. Genera l ly, HP LC, UV,a n d G C -M S m e t h ods n e ed t o b e f u rt h er com p a re d,correlated, and val idated.

Using UV spectroscopy, w e found t hat pota toes a ndpota to plan ts conta ined a n a vera ge of 754 mg/100 g of

fresh weight for sprouts, 224 mg/100 g for lea ves, 26mg/100 g for r oots, a nd 17 mg/100 g for t ubers. Oven-baked potatoes contained no chlorogenic acid, boi ledpota toes 35%, an d microwaved pota toes 55% of theoriginal a mount. French fried pota toes, mashed pota toflakes, and potato skins contained no chlorogenic acid.Leaves contained high levels of chlorogenic acid andg l y coa l k a l oi ds (D a o a n d F ri edm a n , 1996; L y on a n dB a rker, 1984; Ta ble 5). Sosulski et a l . (1982) report atotal phenolic acid content of potato flour of 410 ppm,with free chlorogenic acid contributing 83.2% to thetotal. Figure 5 and Table 4 show changes in chlorogenicacid during baking.

Although potato processors are generally interestedin impar ting desira ble sensory (organoleptic) a tt ributes

to pota to products, the discovery of several beneficialeffects of dietary phenolic compounds, described below,implies t ha t food-processing conditions ma y n eed to beoptimized to minimize destruction of chlorogenic acidwhile mainta ining sensory q ual i ty. The fate of chloro-genic acid and other polyphenols during food processingis largely unknown. All such stud ies depend on reliableana lytical methods for phenolic compounds; these a restill being refined.

P L ANT P H YS I O L O G Y

Biosynthesis. Chlorogenic acid and related polyphe-nols are present in numerous plant species includingthose of the Solanaceae family (Deshpande et al., 1984).

Their ma in function in plants appears t o be to defend

Figure 3. HPLC chromatogram of chlorogenic acid in metha-nol: (A) freshly prepared; (B) a fter 1 day; a nd (C) after 7 days

(Da o a nd Friedman, 1992).Table 1. Chlorogenic Acid Content (Milligrams per 100 gof Fresh Weight) in Different Potato Varieties (Dao andFriedman, 1992, 1994)

va r iet y ch lor ogen ic a cid

Russet (sm a ll com m er cia l) 9.65 ( 0.49Russet (la rge com mer cia l) 14.22 ( 0.73sm a ll r ed (com m er cia l) 13.30 ( 0.69S im plot I (exper im en t al) 13.14 ( 0.94S im plot I I(exper im en ta l) 17.36 ( 1.19ND A 1725 (exper im en ta l) 17.36 ( 1.19pot a to 3194 (exper im en ta l) 18.71 ( 2.10

Table 2. Distribution of Chlorogenic Acid (Milligramsper 100 g of Fresh Weight) in Parts of a Potato Plant(Dao and Fri edman, 1992, 1994)

s a mpl e ch lor og en ic a ci d s a mple ch lor og en ic a ci d

t uber s 17.36 ( 1.19 lea ves 223.53 ( 0.95r oot s 26.34 ( 0.82 spr out s 754.06 ( 25.17

Table 3. Heat Stabili ty of Chlorogenic Acid (Milli gramsper Gram of Freeze-Dried Weight) in Cooked PotatoesDetermined by UV Spectrophotometry (Dao andFriedman, 1992)

pot a t o ch lor og en ic a ci d pot a t o ch lor og en ic a cid

fr esh 0.800 ( 0.05 boiled 0.319 ( 0.01ba ked 0.000 m icr ow a ved 0.434 ( 0.02

Table 4. Stability of Chlorogenic Acid during Baking ofMixed Heavenly Blue Morning Glory and Wheat Flour(Friedman and Dao, 1990)

sa m ple % loss

unba ked m ixed flour 0convection oven baked muffin

cr ust fr a ct ion 100cr um b fr a ct ion 65.1 ( 1.9

m icr ow a ve oven ba ked m uffin 77.0 ( 0.92

Figure 4. UV spec t r a of (A) c hlor og e nic a c id and (B ) anethan ol extra ct of potato roots (Dao a nd F riedman, 1992).

Table 5. Effect of Growing Time in a Greenhouse onChlorogenic Acid Content [Milli grams per 100 g of FreshWeight (n ) 3)] of Freeze-Dried Potato LeavesDetermined by UV Spectrophotometry (Dao andFriedman, Unpublished Results)

chlorogenic a cidt i m e(w e ek s) m e a n v a l ue 95 %C I (coe ff ic ie nt s of v a r i a t io n)

3 148.0 131.9, 164.14 152.1 135.9, 168.36 305.7 289.6, 321.97 423.1 406.9, 439.3

9 257.8 241.6, 274.0

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against phytopathogens, as described below for potatoes.B iosynt hetically, chlorogenic acid is derived from phen-y l a l a n i n e, w h i ch i n t u rn i s f o rme d v i a t h e c l a s s ica lshikimate pathway starting from phosphoenol pyruvateand erythrose 4-phosphate, a s described by H errmann(1995) and Schmidt and Amrhein (1995). Figure 6shows the biosynthetic transformations of phenylala-nine t o chlorogenic acid (Villegaa s a nd Kojima , 1986).Chlorogenic acid biosynthesis has an absolute require-ment for oxygen. Ph enylalanine is a lso a precursor forflavonol biosynt hesis in pota toes (va n E ldik et a l., 1997).

Detai led discussion of the enzymes involved in thei l lustra ted biosynthetic tra nsforma tions is beyond t hescope of this paper. Useful descriptions can be foundi n M ori g uch i e t a l . (1988) a n d Ta n a k a a n d K oji m a

(1991).Onta rio pota toes (which a re very susceptible to pre-

cooking blackening) grown at high levels of potassiumfertilization had higher cytochrome oxidase activity thant h os e g row n a t l ow e r l ev el s (M on dy e t a l . , 1967).Al t h ou g h p o t a s s iu m i n t h e s o il di d n o t a f f ect P P Oactivi ty, th e phenolic content a nd the discoloration ofthe ha rvested pota toes w ere affected by the potassiumfertilizer. Temperat ure and orga nic content of the soila lso seem to a ffect t he chlorogenic acid content of RussetBurbank potatoes (Kaldy and Lynch, 1983).

Plant Molecular Biology. B r u is in g a n d ot h e rstress condit ions induced PPO biosynthesis in potatoplan ts (Belkna p et a l., 1990; Dyer et al., 1989). Rela ted

studies by Thipyapong et al . (1995) show tha t onlypotato tissues which are developmentally competent toexpress PPO mRNA respond to wounding by increasedaccumulation of PP O mRNA. Studies by Thygesen eta l. (1995) revealed t ha t five dist inct cDNA clones encodea m u l t ig en e P P O f a m i ly i n p ot a t oe s. A pos s ib le a p -p ro a c h t o re du c i n g P P O a c t i v i t y a n d t h e c o n s e q u e n tenzymatic browning reactions in pota toes is t o cha rac-terize an d inactivate the genes coding for P P O. Suchinactivation was achieved by generating antisense RNAspecific for target enzymes (Bachem et al . , 1994; Huntet a l., 1993). The possible sign ifican ce of this a pproachf or b row n i n g p rev en t i on i s di sc us s ed b el ow u n derResearch Needs.

Host-Plant Resistance. Lyon (1989) ma kes t he

fol lowing points about the contribution of phenolic

compounds to phytopathogen resista nce: (a) P henoliccompounds a nd derivat ives ar e an importa nt pa rt of thegeneral defense mechanism, especially against soft rotb a ct e ri a . Th e y m a y f u n ct i on b y i n h i b it i n g b a ct e ria lgrowth , by inhibiting cell wall-degra ding enzymes, and/or as precursors in th e forma tion of physical bar riers.(b) Various phenolic compounds exhibit different speci-fici t ies against E r w i n i a spp. (c) P henolic compoundsi n h ib it e n zy m es i n v ol ve d i n p a t h o ge n es i s, s u ch a spolygalacturonase from R hi zobia solani a n d g lu ca nsynthase from B eta vul garis. (d) P olyphenols react withproteins to form insoluble ta nnins. The precipita tedt a n n i n s i n h ib it p ect i n a s e i n t h e ce ll w a l ls b y cros s -linking reactions. (e) B oth phenolics an d phytoalexinsappear to be involved in the resistance of potatoes toE r w i n i a spp.

Other st udies confirmed t ha t polyphenolic compoundsi mp a r t r es is t a n ce t o p ot a t o t u be rs a g a i n s t s of t r otcaused by Erw inia c arotov ora, a major cause of lossesof stored pota toes (Kuma r et a l . , 1991). This effect isp r e s u m a b l y d u e t o t h e a n t i b a c t e r i a l a c t i v i t y o f t h ephenolics or quinones formed by the action of PPO onphenolics. Although pota to cult ivar s w ere more sus-ceptible tha n somat ic hybrids toE . carotovora-induceds of t rot , n o re l a t i on w a s ob s erv ed b et w e e n s o f t rot

resista nce and a ctivity of polyphenol oxidase or peroxi-dase in intact, injured, or inoculated tissues (Loikowskaand Holubowska, 1992). I t is noteworthy tha t caffeic,chlorogenic, a nd ferulic acids were more effective aga instE. carotovorawh en used as mixtur es in the proportionsf ou n d i n t h e p eri derm of p ot a t oe s t h a n w h e n u s edindividua lly (G ha nekar et a l . , 1984). The chlorogenica c i d i n w o u n de d p e ride rm w a s g en e ra l ly h i g h er i ninfected t ubers tha n in t he uninfected ones.

A major unknown is the contribution to plant resis-ta nce of each of the n a tive phenolics, such a s chlorogenicacid an d tyr osine and their oxidation a nd condensationproducts including semiquinones, quinones, melanins,and suberins.

A histochemical t est ba sed on th e rea ction of chloro-genic with ni trous acid demonstra ted a n a ssociat ion ofthe polyphenol with physiological internal necrosis ofpota to tubers (Dinkle, 1964). P olyphenols, includingflavonols, cinnamic acid derivatives, and coumarins,a c cu m u la t e i n t h e s ou n d t i s s ue a dj a c en t t o i n j u redt i s s ue i n m a n y t y p es o f v eg et a b l e s a n d f ru it s . C om -pounds a ccumulating in I rish pota to t issues as a resultof wounding, exposure t o pat hogens, a nd virus infectioninclude chlorogenic acid, scopoletin, scopolin, aesculetin,a nd caffeic acid. This a ccumula tion of polyphenols insound t issue adjacent to injured t issue seems to givehost-plant r esista nce towa rd t he pathogen. This resis-tance could result from the increased rate of metabolisminduced by the injury a nd/or a l terat ion of th e normal

m e t a b o l i c p a t h w a y s b y t h e i n j u ry , l e a di n g t o t h e a c -cumulat ion of unused polyphenols norma lly meta bolized(oxidized) by plan t enzymes.

The formation of such wound-barrier layers occurs insweet potatoes, Irish potatoes, carrots, beets, parsnips,squash , and turnips (Cra ft a nd Audia, 1962). The UVspectra of extra cts of wounded sweet potat oes, Irishpota toes, a nd carr ots exhibited enhanced ma xima a t 325nm, associated with chlorogenic acid from suberizedt i s s ue s . Th e p ea k de cre a s ed i n i n t en s i t y f ol low i n goxidation.

These observations raise the possiblity of synergismbetween structural ly related phenolic compounds inimparting resistance to bacteria. Such synergist ic ac-

t ion could guide plant breeders to produce improved

Figure 5. E ffect of baking on th e sta bility of chlorogenic a cidmeasured by UV spectroscopy in a muffin prepared from amixture of chlorogenic acid containing morning glory seed flouran d wh eat flour: (A) chlorogenic acid; (B ) etha nolic extra ct ofmorning glory seeds; (C) ethanolic extract of crumb fra ctionof oven-baked m uffin; (D) extra ct of microwav e-bak ed muffin;and (E) extract of crust fraction of oven-baked muffin (Fried-man an d D ao, 1990).


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Figure

6.

Biosyntheticpathwaystowa

rdchlorogenicacid(Herrmann,1995;M

origuchietal.,1988;SchmidtandAmrhein,1995;TanakaandKojima,1991;

VillegasandKojima,1986).


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potato cult ivars with the right mixture of structural lydifferent phenolic compounds.

G l a n du la r t r i ch om e -b ea ri n g l ea v e s of w i l d p ot a t ocu l t iv a rs s u ch a s Solanum berthault i H a w k e s a r e o finterest to plant breeders because they confer resistanceto several major foliage-feeding insect pests (Ave et al.,1986). Type A glandula r t richomes confer insect r esis-ta nce to the wild pota toes by oxidatively polymerizingt h e ir con t e n t s f ol low i n g b rea k a g e , l ea di n g t o i n s ectentra pment (Kowa lski et al . , 1993). B ecau seSolanu m tuberosum trichomes contain low amounts of PPO, theydo not significantly contribute to host-plant resistance.Th e a u t h o rs s u gg es t t h a t a n E L I S A for P P O i n t h eglandular trichomes may assist in rapid introgressionof the trichome-resistant trait of the wild potatoes intocommercial S. tuberosum cul t ivars.

Selection of aphid-resista nt hybrids can be improvedby a n improved enzymic browning assa y (EB A) basedon release of the exudate from tetralobulate (type A)trichomes of three leaflets in a test tube (Ave et al . ,1986). The rea gents cha nge from pink to violet mea -sured a t 470 mu. The intensi ty of the color is relatedto the PPO and phenolic substrates of the trichomes,which interact to form a viscous substance as a resultof oxidative tra nsformat ion to polymeric materia ls.

Polyphenol oxidation is also important in the resis-ta nce of potatoes a gainst slugs, which cause consider-a b le d a m a g e t o m a i n cr op p ot a t o es (J oh n s t on a n dP e a rc e, 1994). Wh e n p ot a t o t i s su es a re da m a g ed,polyphenols ar e enzymatical ly oxidized to quinones,which then polymerize to dark pigments. The ra te ofdark pigment formation seems to impart resistance. Asi s t h e c a s e f o r o t h e r p a t h o g e n s , q u i n o n e s a n d p o l y -phenolic tannins may inhibi t slug digestion of potatotissue by binding to digestive enzymes in the gut and/or binding to proteins lowering protein digest ibility. (Seesection below on Protein Nutritional Quality.) However,i t is a lso possible tha t t he slugs have a n a version to theoxidized polyphenols ba sed on sensory perception.

Although differences in glycoalkaloid and sugar con-

t e n t cou l d b e re l a t e d t o s u s ce pt i b il it y (t o a t t a c k ) ofpota toes t o wireworm, Agrioles obscurus, t h i s w a s n o tthe case for chlorogenic a cid (J onasson an d Olsson,1994; Olsson, 1989). Total glycoa lkaloid content wa sthe key factor in predicting larva l feeding, a ccountingf or 65% of t h e t o t a l v a ri a t i on . D i f fe ren ce s i n s u g a rlevels of the diet (fructose plus glucose) accounted for13%. Differ ences in chlorogenic acid or sucrose levelsdid not significantly a ffect the survival of the la rvae.

No correlation was observed between chlorogenic acidcontent of di fferent potato clones a nd their r esista nceto infection with pota to leafroll virus (Lyon an d B ar ker,1984). Although levels of chlorogenic a cid increasedf ol low i n g i n fe ct i on of p ot a t o t u b ers w i t h t h e f u n gu s

Phytophthora infestans,no differences in t he levels wereobserved in resistant an d nonresista nt cult ivars (Phu-kan and Baruah, 1991).

The cited studies suggest that although polyphenolsma y be involved in host-plant resista nce, their involve-ment is selective; that is, they protect only a gainst someb u t n o t a l l p ot a t o p h y t op a t h o ge n s. A n e ed e xi s t s t obetter define th eir contr ibution to th e overa ll protectionin w hich other resistance compounds also pa rt icipate.This is a challenging pr oblem since the concentra tionsof t h e v a r i ou s com pou n d s a p pe a r s t o b e cu lt i va r -dependent, so that specific combinations found in dif-ferent variet ies need to be tested to assess optimumcombinations for possible synergism in phytopathogenresistance (Fewell and Roddick, 1993; Rayburn et al . ,

1995).

STORAGE AND PROCESSING

Effects of Storage. Ontario and Pontiac potatoes,cultivars su sceptible and resista nt to precooking black-ening, respectively, had higher cytochrome and poly-phenol oxidase act ivi t ies wh en stored a t 50 F tha n a t40 F (M on dy e t a l . , 1966). L e ss P P O a c t i vi t y w a sobserved a t t he lower temperature. This may explainthe observed greater polyphenol content at the lowertemperature, since enzyme activity would be expectedto be indirectly relat ed monomeric polyphenol content .

That is , the greater the act ivi ty, the greater the trans-formation of monomeric polyphenols to polymeric ones.The higher PP O activi ty a t t he higher storage temper-at ure may account for the grea ter discolora tion due totra nsformat ion of polyphenols to polymeric pigments att h a t t e m pe ra t u re . L e ja (1989) f ou n d t h a t t h e t o t a lphenolic and chlorogenic acid content was the same instored mat ure potato t ubers with undeveloped peridermas in ma ture tubers. Although high-tempera ture stor-age did not affect the total phenolic and chlorogenic acidcontent, both levels increased on prolonged (5 week)s t o ra g e a t 0 C .

Freshly ha rvested as well as stored pota toes producedphenolic compounds and phytoalexins after infectionwith soft rot bacteria E. carotovora a n d dry ro t f u n g iF u s a r i u m sp p. (Rober, 1989). U ninfected contr ol pota -toes synthesized only low levels of these compounds.Infection of pota to t ubers evidently induces a cascadeof meta bolic transformations during storage, leading t othe production of secondary metabolites involved inhost-plant resistance.

In addition to changes in polyphenol content, potatostora ge induces other compositiona l chan ges dependingon c on di t ion s u s e d. F or e xa m p l e, N a k a g a w a e t a l .(1995) report the following findings for potato tuberss t ore d a t 4, 2 0, a n d 33 C . S p rou t i n g o ccu rre d a f t e rabout 30 days at 20 C but not at 4 or 33 C after 160da y s . Tu b ers t ra n s f erre d t o 33 C a f t e r 7 0 da y s a t 20 C did not sprout during the next 160 day s. Respira tion

ra tes were ini t ial ly high but decreased during storage.Reducing sugar content of tubers stored at 33 C wasl ow e r t h a n i n t u be rs s t or ed a t 4 C . P P O a c t iv it y i ntubers stored at 20 C increased 2-fold during storage,but rapidly decreased at 33 C.

Storage-relat ed light- and -radiation-induced changesin polyphenol content of potatoes are described below.

Effects of Light. P otatoes conta in polyphenoliccompounds, glycoalka loids, an d proteins. Some of thep rot e in s h a v e t h e a b i l it y t o i n h i b it t ry p s in , ch y m o-trypsin, and carboxypeptidase (Brown et al . , 1986; Daoand Friedman, 1994; Lisinska and Leszczynski, 1989;Weder, 1989). Light , mecha nical injury, tempera tur ee xt re m es , a n d s prou t i n g i n du ce a n i n cre a s e i n t h e

glycoalkaloid content , which can range up to 5 t imesori g in a l l ev el s . E x p os u re t o l i g h t a f t er h a rv e s t a l s ol ea ds t o s u rfa c e g ree n in g , w h i ch i s a c com p a n i ed b yincreases in chlorogenic a cid and chlorophyll biosyn-thesis.

The increase in glycoalkaloid content may be undesir-a b l e s i n ce i t cou l d i m p a rt a b it t e r t a s t e t o p ot a t oe s(K a a b er, 1993; J oh n s a n d K e en , 1986; M on dy a n dGosselin, 1988; Zitnak and Filadelfi-Keszi, 1988) andmake them less safe (Friedman and McDonald, 1997;Friedma n et al . , 1996). Since chlorogenic acid, glyco-alka loids, and protease inhibi tors may act as so-cal ledan tifeeding a gents in pota toes, protecting pota toes froma t t a c k b y p h y t op a t h og e ns a n d i n s ect s , t h e q u e st i onar ises wheth er suppression of biosynt hesis of alkaloids,

one of our current objectives (Moehs et al . , 1996a ,b,


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1997; S t a p le t on e t a l . , 1991, 1994), w i l l re su l t i ncompensa tory cha nges in t he biosynt hesis of chlorogenicacid a nd/or protea se inhibitors. Although t he biosyn-t h et i c p a t h w a y s l ea d i n g t o t h e f or m a t i on of t h es eresistance factors are probably unrelated, expectationsare tha t in response to stress a nd reduced formation ofone class of compounds, pota to plant s ma y increase th esynthesis of the others.

To develop a better understanding of the dynamicsof secondary meta bolite formation, potat oes w ere storedin the dark and under fluorescent light for various timeperiods and analyzed for chlorophyll, chlorogenic acid,R-chaconine,R-solanine, a nd inh ibitors of the digestiveenzymes trypsin, chymotrypsin, and carboxypeptidaseA (Da o and Friedma n, 1994). Specifica lly, exposure ofcommercial White Rose potatoes to fluorescent light for20 da ys induced a t ime-dependent greening of potat osurfa ces a nd a n increase in chlorophyll, chlorogenic a cid,and glycoalkaloid content, but no changes in the contentof inhibitors of the digestive enzymes trypsin, chymo-trypsin, and carboxypeptidase A. Storing pota toes int h e da rk di d n o t re su l t i n g re en i n g o r ch l orop h y llformat ion. Chlorogenic a cid a nd glycoa lkaloid levels ofdar k-stored pota toes did increase, but less th an in th el ight-stored pota toes. In the l ight , chlorogenic acid

concentra t ion increased from 7.1 mg/100 g of freshpotato weight to a maximum of 15.8 mg after greening.An approximate 300%light-induced increase for eachglycoa lkaloid wa s also observed. Experiments on delayo f g re e n i n g b y i m m e rs i o n i n w a t e r s u g g e s t t h a t t h econcentra tion of chlorophyll is 26 times great er, tha t ofchlorogenic acid and glycoalkaloids 7-8 t imes greater,and that of protease inhibi tors about 2-3 times lowerin the peel of the gr een potat oes th an in th e whole tuberafter storage.

Griffiths et al . (1995) also reported that exposure ofpotatoes to light induces significant increases in chlo-rog en i c a c i d con t e n t . Th i s i n cre a s e a p pe a rs t o b ecultivar-dependent since the magn itude of the increasecorrelated with the ini t ial value found in unexposed

pota toes. The light-induced increase in chlorogenic a cidmay result from light induction of phenylalanine dea mi-nase, which catalyzes the biosynthesis of chlorogenicacid (Zucker, 1965).

In related studies, Laanest et al . (1995) observed arapid increase in the content of chlorogenic acid andother phenolic compounds in light-exposed sliced pota-toes stored for 9 da ys. Forma tion of polyphenols in thet u be rs w a s a c com pa n i ed b y t h e a p pe a r a n ce of f la -v on oi ds . Th e a u t h or s s u gg es t t h a t a c t iv it y of t h es h i k i m a t e p a t h w a y s du ri n g h e a l i n g o f p o t a t o t u b e rsa f t er e xp os u re t o l ig h t i s n ot r a t e -l im it i n g t o t h ebiosynt hesis of phenols.

Effects of-Radiation. -Radiation effectively pre-

vents sprouting and protects potatoes during storagea g a i n s t da m a g e b y f u n gi a n d ot h e r p h y t o pa t h o g en s(Swa llow, 1989). This is a desirable objective sincewhen potatoes sprout, they decrease in weight, quality,a n d m a rk et v a l u e . S p rou t s m a y a l s o pre se n t a h e a l t hhaza rd, being high in glycoalka loids. Different inves-tiga tors report different results on t he effect of-radia-tion on chlorogenic acid an d rela ted phenolic content ofpota toes. Some of the relevan t studies are summar izedbelow.

P enner a nd Fromm (1972) used a qua nti ta t ive TLCmethod for th e direct determina tion of chlorogenic acidin irra diated potat oes. Chlorogenic acid content r osei mm ed ia t e ly a f t er i r ra d i a t i on a n d t h en r et u r n ed t onormal values within several weeks of storage.

Bergers (1981) found a time-dependent decrease in

chlorogenic acid and a glycoside of scopoletin afterpota to tubers were irra diated up to 3 kGy. Irra diat ionhad no effect on the glycoalkaloid content of the pota-toes. A dose a s low a s 0.1 kG y wa s sufficient to inhibitsprouting with minimal composit ional changes in thet r ea t e d p ot a t o es . I n f ect i on b y f un g i a l so l e d t o a na ccumula tion of scopolin (Clarke, 1976). Mondy a ndG osselin (1989) found th at irra diat ion increased discol-oration and phenolic content and decreased l ipid and

phospholipid content of pota toes. Irra diated pota toess t ore d a t 5 C h a d a h i g h er t o t a l p h en ol c on t e n t t h a nthose stored at 20 C. These aut hors also report t hat ara diat ion dose of 10 krad caused less darkening tha n ah i g h er dos e. Th e i n cre a s ed b la c k sp ot f orm a t i o n a th i g h e r do s e s m a y h a v e b e e n du e t o ru p t u re o f l i p i dm em br a n e s. Th i s r u p t ur e m a y l ib er a t e P P O f r ommitochondria, a llowing it to conta ct phenolic substra tesin the vacuole, causing enzymat ic dar kening. Da rken-ing could therefore be minimized by controlling the d ose.

I r r a d a t i on of t u be rs t o i n hi bi t s pr ou t in g ca u s edreduction in both total phenolic and chlorogenic acidcontents. Increased forma tion of pota to phenolics w asobserved during storage of tubers following irradiation

(R a m a r m u r t h y e t a l ., 1992; Ta b l e 7 ). Th e H P L Canalysis revealed that three chlorogenic acid isomersa ccounted for a bout 88%of the t ota l phenolics mea suredby HP LC. The forma tion and accumulat ion ofn eoa n dcrypto isomers of chlorogenic acid during storage afterexposure to ra diat ion a re a lso noteworthy.

-I r r a d ia t i on u p t o 10 k r a d h a d n o e ff ect on t h edefense mecha nism a t t he site of injury of potat oes, i .e.,the forma tion of q uinones from phenolic acids (Pend-har kar a nd Na ir, 1987). Irra dat ion of tubers to inhibitsprouting caused reduction in both total phenolic andchlorogenic acid contents. Specifical ly, i t reduced bya bout 50%the a bility of tubers to syn thesize chlorogenicand caffeic acids in the irradia ted t issue. The radia t iona lso impaired t he induction of cinna mic a cid-4-hydr ox-

y l a s e b u t n ot p h en y l a l a n i n e a m m o n ia l y a s e, b o t h of

Table 6. Millimolar Concentration of InhibitorsRequired To Reduce 400 Units of PPO/mL by 50% (I50) at25 C and PPO in Potato Suspensions (Friedman andBautista, 1995)a

inhibitor I50 (pure PP O) I50 (potato suspensions)

L-cyst ein y lgly cin e 0.43 1.85L-cyst ein e 0.35 1.09N-acetyl-L-cy st ein e 0.27 1.16h om ocy st ein e 0.23 1.17sodium bisulfit e 0.21 0.54L-cyst ein e et h y l est er 0.12 1.60r educed glut a t h ion e 0.12 2.18L-cyst ein e m et h yl est er 0.12 1.60

a The lower th e value, the more potent the inhibitor.

Table 7. Changes in Phenolic Compounds (Milligramsper 100 g Fresh Weight) in Potatoes during Storagefollowing Irradiation [Adapted from Ramarmurthy et al.(1992)]

storage period (days)

ph en en olic com poun d 0 4 6 8 10 15

3-O-caffeoylquinic acid(neo-chlorogenic a cid)

13. 6 24. 2 24. 2 31. 4 11. 7

4-O-caffeoylquinic acid(crypto-chlorogenic acid)

5.8 10.4 10.4 13.4 3.9

5-O-caffeoylquinic acid(chlorogenic acid)

4. 5 3 4. 1 60. 8 78. 8 3 8. 2 2 8. 5

ca ffeic a cid 1.8 5.7 6.4 7.8 6.8 4.9p-coum a r ic a cid 1.6 4.5 5.0 5.1 4 .4 3 .5fer ulic a cid 1.3 3.1 4.0 4.4 4.0 3.2

t ot a l 9.3 66.7 110.8 140.9 75.8 56.7


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which are involved in chlorogenic acid biosynthesis. Arelated study on the phenylpropanoid metabolism in-irradia ted a nd unirra diated potato tubers revealed adepletion of chlorogenic acid following irra dia tion dur ingthe 21-day postirradiat ion storage period (Penharkara n d N a i r , 1995). Th e l os s i n ch l orog en i c a ci d w a sa s s oci a t e d w i t h b o t h i t s i m pa i re d s y n t h e s is a n d i t saccelerat ed conversion t o ferulic and sin apic acids. Thelatt er were deposi ted in the l ignin par t of the tuber.

General ly, -radiation can suppress wound periderm

formation, thus favoring pathogen access to wounds. Adefense mechan ism a gainst this effect is the format ionof quinones via oxida tion of phenolic compounds by P P Oreleased by rupture of the potat o tissue. The quinonesthen polymerize to defensive polymers a t t he site of thetissue injury, as discussed earl ier.

Thomas and Delincee (1979) found that -radiationat sprout-inhibiting doses did not prevent suberizationof pota to tissues. P ostirrad iat ion healing of tissues wasaccompanied by the deposi t ion of suberin, a polymerderived from polyphenols a nd fa t ty acids. Such suber-izat ion prevents weight loss and perma nent da mage topotatoes.

An increase in polyphenol oxida se activity wa s noteda f t e r p ot a t oe s w e re i rra di a t e d a t 2 k ra d (C h e u n g a n d

H e n ders on , 1 972). I n con t ra s t , t h e a c t i vi t y of t h ee n zy m e de cre a s ed w h e n t h e ra di a t i on dos e w a s i n -creased. P rolonged postirra diat ion storage of the po-ta toes a lso resulted in lower enzyme activi ty.

C o ns i dera b l e b row n i n g occu rs i n s t ore d p ot a t oe swh ich are th en exposed to -ra diat ion, especially in thecortex and along the xylem (Ogawa and Uritani, 1979).O bs erv ed b row n i n g w a s a c com p a n i ed b y a m a rk e dincrease in polyphenol content and peroxidase activitya n d a t ra n s i e n t de cre a s e i n P P O a c t i v it y . Th e e xt e n tof browning strongly depended on the storage periodfrom ha rvest to irra dia tion. These a uth ors recommendtha t , to minimize browning, pota toes should be storeda t a m b ie nt t e mp er a t u r e f or a b ou t 1 m on t h b ef or e

irradiat ion.P e e li n g of p ot a t oe s p rior t o b oi li n g re duc es t h eintensity of-ra dia tion-induced a fter-cooking da rkeningof the tuber flesh (Thomas, 1981). P resuma bly, leachingof polyphenols from the flesh into the cooking waterminimizes the availability of polyphenols to form darkiron complexes. The a mount of leaching into th e cook-i n g w a t e r w a s 3-5 t imes greater in prepeeled tuberstha n in whole tubers. I t is a lso possible that leachingof f errou s com p ou n ds i n t o t h e cook i n g w a t e r a l s ocontributes to reduced darkening.

Leszczynski et al . (1992) found that (a) -radiationinhibited sprouting of potat oes, (b) the irr ad iat ed tubersh a d l ow e r s t a r ch a n d h i gh er s u cr os e con t en t s , (c)radiat ion produced darker potatoes, and (d) radiat ion

ha d no or minimal effects on th e qua lity of pota to chips

made from the treated pota toes. Since radia t ion inhib-i ted sprouting during storage, the production of goodq u a l i t y c h i p s w a s e a s i e r f r o m i r r a d i a t e d t h a n f r o mcontr ol potat oes.

The described studies report apparently contradictoryre su l t s a b ou t e le va t i o n or de cre a s es i n p ol y ph e n olcontent of pota toes fol lowing irra diat ion. P ossible ex-planations for these results include (a) di fferences inradiat ion doses and exposure t imes used by di fferentinvestigators, (b) storage history of the potatoes beforeexposure to rad iat ion, a nd (c) differences in genotypesusceptibi li t ies to effects of rad iat ion. These varieta l-related differences could be governed by differences inenzyme profiles involved in biosynt hesis of polyphenols,differen ces in moisture content a nd/or porosities (Lula iand Orr, 1995) of the potato tuber surfaces that mayaffect peneteration of radiat ion, or some as yet unde-f in e d p a ra m e t e rs. F or e a c h c u lt i v a r , a n e ed e x is t s t odefine minimum ra diat ion conditions to prevent sprout-ing and to destroy phytopathogens while minimizingundesirable compositional changes.

DIETARY SIGNIFICANCE

Bruising-Induced Potato Discolorations. P o t a -

toes may undergo browning and other discolorations(depicted in Del Zan and Baruzzini (1991) with colorphotographs of whole and sliced potatoes) both duringg row t h a n d a f t e r h a rv e s t . I n t e rn a l b l a ck s pot i n p o t a -t o e s i s c a u s e d b y i n t e rn a l e n z y m a t i c b ro w n i n g t y p ereactions initiated by PPO catalysis with tyrosine as theprimary substrat e. According t o Shetty et a l . (1991),bruising of pota toes induces t hese undesira ble pigmen-t a t i o n s a s w e l l a s s h ri n k a g e , ro t t i n g , a n d m a j o r e c o -nomic losses. This subsur face physiological discolora -tion resulting from mecha nical injury is a lead ing causeof re je ct i on of s h i pm e n t s i n p ot a t o p rodu ct i on . I naddit ion to blackspot formation, bruising induces thepostha rvest biosynth esis of glycoalka loids, w hich couldadversely impact sa fety (Friedman, 1992; Friedma n a nd

McDona ld, 1997).Mapson et al . (1963) found a correlat ion between

tyrosine, the a ct ivi ty of PP O, and the ra te of browningin di fferent potato va riet ies. No such correlat ion wa sf ou n d f o r c h lorog e n ic a c i d a n d b row n i n g . P a v ek a n dcolleagues extensively studied the relat ionship of ty-rosine to interna l blackspot resistance and the inheri t-ance of blackspot bruise resistance in pota to (Corsiniet a l., 1992; P a vek et a l., 1985, 1993; St a rk et a l., 1985).Enzyma tic discoloration wa s highly correlated w ith tota lphenolic (r) 0.89) and free tyrosine levels (r ) 0.85) inp ot a t oe s. D e a n e t a l . (1992, 1993) de scri be t h re etechniques for measuring blackspot pigment develop-ment in progeny of potato crosses with various degrees

of resista nce to blackspot forma tion. The tra nsfer ofresistance of bruise-resistant genotypes to progeny incrosses with susceptible genotypes correlated with ty-rosine content . The relat ive ra te of tyrosine synthesisin a blackspot-resistant pota to cult ivar wa s a bout 55%less tha n in t he susceptible cult ivar Lemhi Russet . Onthe other han d, Mondy a nd Munshi (1993) report tha tal though free tyrosine was posi t ively correlated withdiscoloration within a cult ivar, i t did not appear to bethe predominant factor determining blackspot suscep-tibility of potat oes, since t he bla ckspot-susceptible cul-t i va r , O n t a r io, h a d h i gh er a s cor b ic a c id a n d l ow e rtyrosine contents than the resistant cult ivar, Pontiac.Moreover, a scorbic a cid an d enzymatic discolorationwere not consistently correlat ed with each other in spite

of t h e f a c t t h a t t h e v it a m i n a p pe a rs t o f u n ct i on a s a n

Table 8. Oxidation Potentials (in Volts) of SelectedPhenolic Acids at pH 4.7 [Adapted from Felice et al.(1976)]

ph en olic a cid pot en t ia l

3,4-dih ydoxy cin n a mic (ca ffeic) 0.35ch lor ogenic 0.393 ,4 -d i h y d r ox y ph e n y lp r op io n ic (d i h y d r oc a f fe ic ) 0 .4 33,4,5-t rih ydr oxy ben zoic (ga llic) 0.463,4-d ih yd rox yben zoi c (pr ot oca t ech uic) 0. 524 -h y d r ox y -3 ,5 -d i me t hox y be n zoi c (s y r in g ic) 0. 554-h y dr ox y-3-m et h ox yci nn a m ic (f er u li c) 0. 574-h yd rox y-3-m et h ox yb en zoi c (v a nil li c) 0. 724-hydroxycinnamic (p-cour m a r ic) 0.734-h y dr oxyph en y la cet ic 0.774-h y dr oxyben zoic 0.99


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ant ioxidant in t he process of discoloration. The l ipidcontent of Chippewa and Katahdin potatoes, al thoughlow, is a lso importa nt in cellular integrity a nd resistan ceto bruising (Mondy an d Mueller, 1977). Evidently,tyrosine-initiated blackspot formation varies in differentpotato cultivars and may be modulated by ascorbic acida n d l ip ids . L e mh i R u s s et p ot a t oe s a re a v a i l a b le w i t henha nced resista nce to bruising-induced blackspot for-mation (Love et al., 1993).

The described results show that a variety of environ-menta l fa ctors influence storag e-induced compositionalchanges in potat oes. The observed cha nges should helpdefine consequences of potato breeding, blackening, andgreening for plant physiology, food qual i ty, and foodsafety. Specifical ly, this information should be usefulfor breeding potato varieties with an initial low chloro-genic acid content to minimize both content and rate ofincrease, e.g., to minimize a fter-cooking da rkening a nd/or enzymatic browning dur ing food processing, i f tha tis t he objective. However, since chlorogenic a cid isreported t o exert beneficia l effects on h ealt h a s describedbelow, a nother d esirable objective might be to breed forhigh-chlorogenic acid va rieties a nd/or enha nce the chlo-rogenic acid content by post-harvest exposure to lightand other stress condit ions. Possible implicat ions of

these changes for handling and marketing of potatoesare also discussed below under Research Needs.After-Cooking Darkening. C hlorogenic a cid seems

to be responsible for bluish-gray discoloration of boiledor steam ed pota toes following exposure to air. This so-called after-cooking blackening or darkening, whichcan occur within a few minutes a fter steam peeling, isperceived by some consumers a s undesirable. Chemi-cally, the blackening appears t o be due to the format ionof a colorless r educed ferrous ion-chlorogenic a cidcomplex in the pota to. The ferrous complex is thenoxidized to a da rk ferr ic complex following exposure t ooxygen in the air . Such dar kening of potat oes has beenextensively studied (Heisler et al . , 1969; Hughes andSwain, 1962; Muneta and Kaisaki, 1985; Putz, 1995).

Th e p o t en t i a l f or da rk e n in g ca n b e m e a s u red i n t w owa ys: (a) by a test involving a w ai t ing period of 1-12h for full color development after steaming; and (b) bya chemical test based on reaction of chlorogenic acidwith a mixture of urea, ta rta ric acid, and sodium nitri te(M a n n a n d L a m b er t , 1989). Th e l a t t e r i s a r a p id ,histological staining method based on the formation ofa cherry-red color of nitrosylat ed chlorogenic a cid. Thehistological method for visualizing chlorogenic acid inpota to t issues a greed with after-cooking da rkening ina blanching fry test .

Blackening tendency appears to be directly relatedto the size of the potato tubers (Siciliano et al . , 1969).This trend presumably results from the fact that the

stems of large tubers have low citric acid content, highpotassium-ci t r ic a c id r a t i os , a n d l ow ci t r ic a c id-polyphenolic rat ios compared to sma ller ones. Ci tricacid a nd other chelat ing a gents inhibi t pota to blacken-ing by competing with chlorogenic acid for ferrous ionchelat ing sites. The aft er-cooking da rkening of Spar ta nP e a rl p ot a t oe s w a s corre la t e d w i t h p h en ol ic b u t n otcitric acid content (Silva et a l . , 1992).

Ma nn (1995) described a process for minimizing a fter-cooking dar kening in French fried pota toes: tr eat mentof w a t e r-b la n c h ed, f r ie d p ot a t o s t r i ps , w h i ch of t endi scol or on e xp os u re t o a i r , w i t h a com b in a t i o n ofcalcium acetat e an d a n oxidat ion inhibitor. The preven-tive effect is presumably due to both t he inhibi tor a ndthe complexation of the calcium ions with chlorogenic

a c id on t h e s u rfa c e o f t h e p ot a t o s t r i p s , p rev en t i n g

formation and oxidation of the chlorogenic acid-ferrousion complex t hat causes da rkening.

Ascorbic acid plus ferr ous iron forms a purple pigmentsimilar to the chlorogenic acid-iron complex (Munetaa n d K a i s a k i , 1985). Th e a s c orb a t e c om p le x i s l es ss t a b l e, h ow e v er. S t ro n g c h el a t i n g a g e n t s , i n c lu di ngcitric acid, EDTA, and sodium hydrogen pyrophosphate,suppressed color formation result ing from both com-plexes.

Muneta a nd Ka lbfleish (1987) describe a h eat -inducedconta ct discoloration in boi led pota toes, a n irregularbrown ring near the edge of the conta ct zone where theunpeeled pota to touches the bott om of the pan. P eelingpotatoes or keeping unpeeled potatoes away from thebottom of the pan prevents da rk ring formation. Theheat-induced conta ct discoloration may result fromenzymat ic-type browning rea ctions similar to those tha tma y be r esponsible for bla ckspot a nd prepeeling black-ening of pota toes. Specifica lly, rings ca n occur at somepoint along a decreasing temperature gradient wheret h e do m i na n t e ff ect i s t y ros i n a s e a ct i v a t i on b y h e a t(leading to browning), rather t han heat inactivation oft h e e n z y m e. At s om e s t i l l l ow e r t e m pe ra t u re , h e a tinactivat ion of the enzyme or leaka ge of substra tes fromheat-damaged cell membranes no longer occurs, stop-

ping heat ring formation.Although blackspot forma tion in potat oes ma y a risefrom nucleophilic a ddition rea ctions of a mino groups tooxidized polyphenols described below (Stevens an dDa velaa r, 1996), nonenzymat ic browning reactions offree amino acids and proteins with r educing sugar s sucha s g l ucos e cou l d a l s o e xp la i n t h e h e a t -i n du ce d a n drelat ed discolora tions. Such reactions are described indetail elsewhere (Felton et al . , 1992; Friedman, 1994,1996a,b ).

The green color of potato cooking waters appears tob e d u e t o a p ig m en t d er i ve d f r om t h e r ea c t ion ofchlorogenic acid and glut a mine (Adams , 1994). Wheth erthe heat-induced discolorat ions adversely a ffect t henutri t ional value of the boi led and darkened potatoes

is not known.Browning Prevention. The sulfhydr yl (SH or thiol)

compounds such as cysteine, N-acetyl-L-cysteine, andreduced gluta thione, as w ell as a scorbic and citric acids,ar e good inhibi tors of the enzyme P P O which cata lyzesenzymatic browning in frui ts a nd vegetables includingpota toes (Dudley a nd H otchkiss, 1989; Friedman et a l . ,1991; G olan-G oldhirsh et al . , 1994; Lan gdon, 1987;Ma theis, 1987a ,b; Math eis and Whita ker, 1984; Muneta,1981; Muneta and Walradt, 1968; Sanchez-Ferrer et al.,1993; Sapers, 1993; Sapers and Miller, 1992, 1995). Weshowed that SH-containing amino acids and peptidesar e good inhibitors of both enzymat ic and nonenzymat icb row n i n g i n f res h l y p rep a re d a n d com m erci a l f ru it

juices, apples, fresh and dehydrated potatoes, heateda m i n o a c i d-carbohydrate mixtures, and protein-richf oods (F ri edm a n a n d B a u t i s t a , 1995; F ri edm a n a n dMolnar-P erl, 1990; F riedma n et a l . , 1992; Molnar -P erland Friedman, 1990a,b; Figure 7).

In some applicat ions, the effectiveness of some ofthese compounds on a molar basis approached that ofsodium sulfite, the use of which is being discontinuedbecause ma ny a sthma tic individuals a re sensi t ive to i t(FD A, 1990). However, in sea rching for th e most potenta n d s a f e re pl a c em e nt f or s odi u m s u lf it e , i t b eca m ea p p a re n t t h a t t h e p o t e n c y o f di f f e re n t t h i o l s a s P P Oinhibitors va ries widely, depending on both the st ruc-t u re o f t h e t h i o l a n d t h e e n v i ro n m e n t i n w h i c h t h einhibition is carried out. To compa re rela tive effective-

ness of structura l ly di fferent thiols in a food mat rix as


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w e ll a s i n p ur e e nz y me s ol ut i on , w e e va l u a t e d t h eeffectiveness of a series of sulfhydryl compounds in PPOsolutions and in dehydrated potato suspensions at 25 C. These inhibi tors w ere less effective in the pota tosuspensions, and the relative effectiveness of structur-ally d ifferent t hiols differed from those observed in P P Osolutions. Ta ble 6 shows rela tive inhibitory potencies

of several thiols against pure PPO and in the suspen-sions.Although the reasons for these di fferences are not

apparent , possibil i t ies include slow di ffusion of th einhibitors to PP O a nd substra tes in the h eterogeneousde h y dra t e d p o t a t o s u s p e n s i o n , t h e a c t i o n o f P P O o nsubstrates other than tyrosine such as chlorogenic acid(Friedman and Smith, 1994), and the reaction of thethiols with carbohydrates. The lat ter reactions may a lsob e b e n ef i ci a l s i n ce t h e y m a y p rev en t n on e n zy m a t i cbrowning.

These considera tions suggest t ha t n o general conclu-sions can be made about the PPO inhibi t ion by thiolsin specific foods. Ea ch food cat egory has t o be evalua teds ep a r a t e ly w i t h e a ch s u lf h yd r y l com pou n d t o f in d

op t i mu m con di t ion s t o p rev en t b ot h e n zy m a t i c a n d

nonenzymat ic browning under specific condit ions ofstorage, tra nsport , an d processing.

Our st udies sh ow tha t SH-conta ining compounds,especially cysteine ethy l ester an d reduced glutat hione,a r e p ot e nt i n hi bi t or s of P P O . Th ey a r e t h er ef or epotential sul fi te substi tutes for browning prevention.Since th e effectiveness of a specific sulfhydr yl compound

i s i n f l u e n c e d b y i t s s t ru c t u re , a ddi t i o n a l s t u di e s a reneeded to optimize the antibrowning action of a varietyof structurally different SH-containing amino acids andpeptides with different physicochemical properties.

Since safety considera tions will determine regulat oryapproval and commercial use of SH -conta ining a minoacids and peptides as browning inhibitors, i t should benoted that L-cysteine is on t he G RAS (genera lly a ccepteda s s a f e ) l i s t a n d i s w i de l y u s e d a s a n a n t i o x i da n t i nbaking formula tions (Friedma n, 1996b). Reduced glu-ta thione is a na tura l ly occurring t ripeptide present inma ny foods (J ones, 1992). Alth ough N-acetyl-L-cysteined oes n ot occu r n a t ur a ll y, it is u sed a s a d ru g(Mucomyst) to reduce lung congestion (Friedman,1973). The compound is a n excellent nu tr itiona l source

o f c y s t e i n e f o r h u m a n s a n d a n i m a l s ( D e B e rn a rdi di

Figure 7. Tran sformation of tyrosine to melanin via DOP A and dopaquinone (enzymat ic browning) and postulated inhibition ofenzymatic browning by cysteine and ascorbic acid by trapping the dopaquinone intermediate.


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Valserra et a l . , 1989; Friedman a nd G umbma nn, 1984),a n d a c t s a s a n a n t i m u t a g e n a n d a n t i c a r c i n o g e n ( D eFlora et al . , 1989; Friedman et al . , 1982a; Stevens etal., 1995).

Efforts a re continuing to develop sulfi te substi tutesfor browning prevention in potatoes and other foods. Forexample, S apers and Mil ler (1995) inhibi ted potat odi scol ora t i o n f or 14 da y s a t 4 C b y u s i n g a dou b letrea tment , a scorbic/citric acid s olutions plus hea t, fol-lowed by dipping in a solution containing ascorbic andcitric acids plus sodium a cid pyrophospha te. Almeidaa n d M og u ei ra (1995) e va l u a t e d t h e e ff ect i v n es s ofcombina tions of several inhibitors a nd hea t in reducingP P O a ct i v it y i n f ru i t s a n d v eg et a b l e s. As corb ic p l uscitric acid and heat proved the best combination. Theapparent formation of inclusion complexes of chlorogenicacid with -cyclodextrins could a lso provide a basis forcontrolling enzymat ic browning in potatoes (Irw in et a l . ,1996).

Flavor and Taste. S inden et al . (1976) found acorrelation between glycoalkaloid content and undesir-able potato flavor, but no significant correlation betweenphenolic content and either bitterness or burning sensa-tion. In contra st, Mondy et a l . (1971) report a positivecorrelation between phenolic content and bitterness and

astringency of potatoes.Since potatoes contain both glycoalkaloids and poly-phenols in various a mounts depending on variety, t hen e t e f f e c t o n t a s t e a n d f l a v o r c o u l d b e t h e re s u l t o fcombined, possibly a dditive, synergistic, or an ta gonisticeffects of both components (J ohns an d Keen, 1986;Kaaber, 1993; Zitnak and Filadelfi , 1985).

Antioxidative Activities. Polyphenolic compoundsin potatoes show a ntioxidative a ct ivi ty in several foods y s t em s . F or e xa m p l e, O n y e n eh o a n d H e t t i a a c h ch y(1993) evaluated the abilities of freeze-dried extractsfrom the peels of six pota to var ieties to prevent soybea noi l o xi da t i on . U s i n g a n a c t i ve o xy g en m e t h od, t h e yfound tha t 20 g of soybean oil trea ted w ith 50 mg of theextra cts ha d a lower peroxide value (PV, 22-28) than

di d a con t rol oi l s a m p l e (P V , 1 09). H P L C a n d TL Cstudies suggested that chlorogenic and protocatechuicacids were the main ant ioxidants in the extracts. Peelsfrom red potatoes contained greater amounts of poly-phenols than those from brown-skinned varieties.

In r elat ed stud ies, Rodriguez de Sotillo et a l. (1994a ,b)confirm the strong antioxidant activity of freeze-driedextra cts of pota to peel was te in sunflower oil . The tota lpolyphenolic content of the peel determined by HPLCconsisted of 50.3%chlorogenic a cid, 41.7% ga llic a cid,7.8% protoca techuic a cid, a nd 0.21% caffeic a cid. Al-Sa ikhan et al . (1995) found tha t the phenolic contentand antioxidative act ivi ty of four potato cult ivars wasg en ot y p e-de pe n de n t a n d n ot re la t e d t o f le s h col or .

These results suggest the possible value of potat o peelin the prevention of oxidative rancidity of food oils.As part of an effort to develop an HPLC method for

plant phenolics with a mperometric detection, Felice etal. (1976) measured the anodic oxidation potentials ofstructural ly di fferent compounds. The low va lues forcaffeic and chlorogenic acids (Ta ble 7) suggest tha t thepresence of the acrylic acid group in these compoundsconjugat ed with the a romatic ring fa cil i tat es oxidationto t he corresponding quinones. The free electron ofperoxy radicals present in oxidized fatty acids or lipo-proteins can be abstr acted. G enerally, phenols are morereactive with t he peroxy ra dicals tha n t he correspondingq u in on es . O n t h e o t h er h a n d , q u i n on es a r e b et t e rm e t a l -ch e la t i n g a n t i ox ida t i v e a g e nt s . B o t h a s p ect s

operat e in the a ntioxidative effect. The free electron on

t h e r es u lt i n g q u in on e r a d i ca l i s t h en s t a b il iz ed b ydissipa tion of the cha rge through t he conjugated syst em(Figure 8). The lat ter r adical is therefore much lessreactive than the peroxy radical (Halliwell et al . , 1995;La rson, 1988; Newma rk, 1984, 1987). Thus, t he oxida -tion potentials of polyphenols in various food environ-m en t s m a y b e u s ef ul f or p re di ct i n g t h e ir r e la t i v ep ot e n ci es a s a n t i o xi da n t s . I n re la t e d s t u die s, F o t i e ta l. (1996) describe st ruct ure-antioxidative activities ofphenolic compounds in micelles.

Elsewhere, we exa mined in deta il the kinetics, mech-anisms, and synthetic aspects of related nucleophil icaddition reactions of protein functional groups to con-ju g a t e d dou b le b on ds (C a v i n s a n d F ri edm a n , 1967,

1968; Friedman, 1966, 1973; Friedman and Romers-berger, 1966; Fr iedma n et a l., 1982b). The oxidat ion ofa polyphenol to a quinone followed by participation ofthe quinone in such nucleophilic a ddit ion reactions isalso mechanist ical ly a na logous to the correspondingoxidation of a porphyrin t o a dehydroporphyrin inter-mediate followed by the addition of nucleophiles to thedehydroporphyrin. These studies should facilita te thedesign of experiments for the synthesis of chlorogenicacid and related quinone adducts for biological evalu-ation.

Antimutagenic and Anticarcinogenic Effects.G e n e ra l ly , i n h ib it i on of m u t a g e n ici t y a n d of ca n c erdevelopment by polyphenolic compouds could be due to

t h e i r a b i l i t y t o s c a v e n g e a n d t ra p p o t e n t i a l l y D N A -damaging electrophiles, free radicals, and toxic metals,t o i n h i b it e n zy m e s t h a t a c t i va t e p reca rc i n og en s t ocarcinogens, and to induce carcinogen-detoxifying en-z y m es (F ri edm a n a n d S m i t h , 1984; Ta n a k a , 1994;Tan aka et a l . , 1993). The mechanisms of t hese pos-sibilities need to be further elucidated.

Reported antigenotoxic and anticarcinogenic effectsof both free and potato-bound chlorogenic acid includet h e f ol low i n g: (a ) N it r i t es i n f ood ca n r ea c t w i t hsecondary amines t o form muta genic and carcinogenicnitr osamines. Chlorogenic a cid a nd other polyphenolsare reported to block ni trosamine formation by com-p e t i t i v e l y re a c t i n g w i t h t h e n i t r i t e ( K i k u g a w a e t a l . ,1983). (b) Chlorogenic acid an d severa l other simple

phenolic acids also inactivat e the muta genici ty of a fla-

Figure 8. R e sonanc e st a b il iz at i on of a phenoxy l r ad i calt hr ough d e local i z at i on of t h e fr ee e le ct r on t hr oughout t heconjuga ted sys tem of chlorogenic acid: (top) resona nce formsin which the free electron is localized on oxygen or carbon;(bottom) delocalized electron. The actual resonance structureis a hybr id of t he depicted forms (Whela nd, 1955). The depictedq ui none r a d i cal i s m or e st a b le (has a l ow e r g r ound -st at ee ner g y ) t ha n, for e xam pl e, a l inole ic a c id per oxy r ad i cal .Mixing chlorogenic acid with such a peroxide would thereforer e sul t i n t r ansfe r of t he e l e c t r on fr om t he pe r oxi d e t o t heelectron sink of chlorogenic acid (antioxidative effect).


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toxin B 1 (Figure 9). (c) A model system conta iningcellulose plus chlorogenic a cid bound 100% of thecarcinogen benzo[a]pyrene compared to 66%by cellulosealone a nd 32%by a cellulose-quercetin mixture (Cam-ire et al . , 1995). (d) Pota to peel bound more of thebenzo[a]pyrene than did wh eat bran, cel lulose, or a ra-

binogala ctan. Extrusion of the peel at 110 C reducedthe a ffini ty of the carcinogen for t he pota to peel . Suchextrusion may result in the destruction of chlorogenicacid. Ca mire et al . (1995) speculat e tha t chlorogenicacid in the peel may interact with benzopyrene to forman insoluble complex, possibly as does chlorophyll withcar cinogenic heterocyclic amin es (Friedma n, 1996b). (e)The protective effect of caffeic acid esters a ga inst DNAdamage induced by hydrogen peroxide seems to be duet o t h e o-dihydroxy structure of the esters (Nakayamaet a l., 1996).

It should also be pointed out that peel polyphenolsmay have l i t t le dietary significance i f t he polyphenolsar e destroyed dur ing processing (baking, cooking, frying)and unless they are also present in the peeled potatoes(flesh). As ment ioned earlier, destr uction of chlorogenicacid appear s to be least during microwaving. Note tha tthe peel of baked or fried pota toes is the prin cipa l sourceof p ee l i n t h e h u m a n d ie t (B u s h w a y e t a l ., 1984;Friedman and Da o, 1992). It ma y therefore be desirableto develop new, health-promoting potato peel-rich foodformulations.

Glucose-Lowering Properties. Thompson et al .(1983) report that the polyphenol content of potatoes,

Figure 9. Inactivation of of mutagenic activity of aflatoxinB 1 by phenolic compounds [ada pted from Stich a nd Rosin(1984)].

Figure 10. Nucleophilic addition reactions of protein functional groups to the conjugated systems of chlorogenic acid quinone.Addition can ta ke place at the a crylate ester side cha in to form product A a nd a t t wo positions of the quinone ring to produce thederivatives B and C. In addition, each of the trigonal carbon atoms with which P-X combines becomes tetrahedral and asymmetric,creat ing th e possibili ty of t wo dia stereoisomers in each case. Losses of chlorogenic acid during ba king shown in Figure 5 m ay be

due to these and related transformations during food processing.


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l e g u m e s , a n d c e re a l s c o rre l a t e d n e g a t i v e l y w i t h t h eblood glucose response (glycemic index) of n ormal a nddiabetic huma ns consuming th em in a contr olled study.The glucose-lowering effect of th e polyphenols ma y a risefrom their abi l i ty to inhibi t amylases (which catalyzethe hydrolysis of sta rch to glucose), phosphorylases(which are involved in starch metabolism), and pro-teolytic enzymes (that catalyze the hydrolysis of proteinsto free a mino acids in the digestive tra ct) and/or fromtheir a bility to direct complexa tion between t he polyphe-nols a nd st ar ch, preventing digestion. Another pos-sibility is prevention of i n v i v o nonenzymatic browningbetween plasm a glucose an d a mino groups of hemoglo-bin, which occurs under physiological condit ions andwhich contributes t o the cause of diabetes (Friedman,199 6b ). P r e v en t i o n o f n o n en z y m a t i c b row n i n g b ypolyphenols could occur in tw o wa ys. First , a ntioxida-tive polyphenols could block oxidat ive steps in themultist ep nonezyma tic browning rea ctions. A less likelypossibility is tha t polyphenol-derived q uinones couldcompeti t ively int eract w ith a mino groups, as shown inFigure 10, thus minimizing amino-carbohydra te reac-tions.

Cholesterol-Lowering Effects. Chlorogenic acida n d ot h e r p ol y ph e n ol s a l s o e xh i bi t s t ro ng i n v it r o

antioxidant act ivi ty for heart disease-related l ipopro-teins (Vinson et a l . , 1995). Since i n v i vo oxidation oflipoproteins (LDL ) appear s to be a ma jor cause of hear tdisease, i t is possible that chlorogenic acid and otherpolyphenols may also lessen heart disease.

In a relevant study, Lazarov and Werber (1996) foundthat consumption of potato peel induced a lowering ofcholesterol in ra ts. They as cribe this result t o the fibercontent of the peel. However, it is l ikely tha t polyphe-n olic a n d ot h er a n t ioxid a nt s a s w el l a s g ly co-a l k a l o i ds i n t h e p e e l c o n t r i b u t e t o t h e o b s e rv e d h y -pochelesterolemia. Both tomato a nd potato glycoalka -loids have a strong affini ty for cholesterol (Friedmanet al., 1997; Roddick, 1979).

Protein Nutritional Quality. While behaving as

a n t i o x i da n t s i n f o o ds a n d i n v i v o , semiquinones andquinones formed on oxidation of polyphenols can alsosimultaneously react with other molecules includinga m i n o a ci ds a n d s t ru ct u ra l a n d f u n ct i on a l p rot e in s(Friedma n, 1994; Hur rell an d F inot, 1984; Ma theis a ndWh i t a k e r, 1 984; S t e v en s a n d D a v e l a a r , 1994). F o rexample, Figure 10 shows some possible protein deriva-tives that can form from reaction of chlorogenic acidquinone wit h a ctive-hydr ogen-bearin g protein functionalgroups such as -NH 2 groups of lysine, SH groups ofcysteine, and OH groups of serine and tyrosine. Notethat chlorogenic acid quinone can form protein adductsboth at the acryl ic acid side chain (adduct A) and ont h e b e n z e n e r i n g ( a ddu c t s B , C ) a n d t h a t t h e c a rb o n

atom to which the protein becomes at tached becomesa s y m m e t ric. I n e a ch ca s e , t h e a ddi t ion re a ct i on ca ntherefore create two diastereoisomers, analogous to theobserved forma tion of LD - a n d LL -lysinoala nine isomersderived from addit ion of -NH 2 groups to the doublebond of dehydroala nine (Friedman , 1977; Friedma n a ndP ear ce, 1989; Liar don et al., 1991). The nutr itiona l andtoxicological significance of consuming such proteinderivat ives is la rgely unknown.

P olyphenolic compounds and derivatives (tannins)bind to proteins in the gut , a dversely a ffecting absorp-tion of food in both insects and animals (Duffey andSt out, 1996; Fr iedman , 1989; Oste, 1989). Ra t feedings t u die s (G ri f f it h s , 1986) s u g ge st t h a t re duct i on i nprotein nutri t ional qua l i ty fol lowing consumption of

p ol y ph e n ol s m a y b e du e t o f orm a t i o n of p rot e in-

polyphenol complexes (Spencer et al . , 1988) a nd toi n h ib it i on of t h e di g es t i ve e n zy m es R-a m y l a s e a n dtry psin. P olyphenols a lso induced a n increa se in lipaseactivi ty in the digestive system of the rats. However,u n l ik e l eg u m e p rot e a s e i n h ib it o rs (G u m b m a n n a n dFriedman, 1987), polyphenolic compounds did not in-duce pan creatic hypertrophy. P ossible effects of poly-phenol-rich diets on protein nut rition need t o be betterdefined.

R E S E AR C H NE E DSThis integrated overview covers contributions from

several overlapping disciplines with a common concernfor theoretical and practical consequences of the func-tion of pota to polyphenols in the plant and in th e diet .It will hopefully cata lyze progress an d permit the w idestpossible intera tions of viewpoints an d expertise neededto find solutions to some of th e a bove-mentioned prob-lems. Such synergistic, mutua lly beneficial intera ctionw i l l b en e fi t b ot h t h e f a r m e r g r ow i n g p ol y ph e nol -conta ining plant foods an d the consumer w ho expectsimproved food qua lity, safety , a nd healt h. The followin gare some addit ional implicat ions and research needssuggested by the described roles of potato polyphenols

i n t h e p la n t a n d i n t h e di et .The complex dyna mics an d t he ra te of compositiona lchanges during storage of potatoes is affected by di f-ferent storage conditions including light, temperature,a n d h u mi d it y a n d m a y b e c ul t iv a r -r el a t ed . A n e edexists to minimize adverse compositional changes dur-ing ha ndling, sampling, storing, shipping, and mar ket-i n g of p ot a t o es . S p eci fi ca l l y, s t or a g e s t u di es w i t hcommercial and experimental cul t ivars should deter-mine effects of l ight and other environmental factorson glycoalka loid levels, polyphenol content (predictor ofbrowning a nd other d iscolorat ions), chlorophyll content(greening), protease inh ibitors a nd phy toalexins (resis-tance factors), and glucose and free amino acid content(indicat ors of nonenzyma tic browning). Such informa -

tion could serve as a guide for th e industr y to minimizepostharvest changes in potatoes for various end usesan d w ill help in the selection of specific potat o cultiva rsshowing the greatest resistance to stress-induced com-positiona l changes. Since resista nce to stress conditionsincluding phytopathogens causing late bl ight may bedue to glycoalka loid, phyt oalexin, polyphenol, and pro-tease inhibitor content , the profi les of composit iona lchanges of di fferent cult ivars wil l make i t possible toevalua te full-spectru m resista nce of a given line. Suchprofiles could serve as a guide to producing and market-ing cultivar s with increased resista nce without the needfor lengthy, large-scale field trials.

To minimize a dverse effects of stress-induced cha nges

during storage, a need also exists to develop specificindicat ors of stress. We recently discovered th at theenzyme solanidine glucosyltransferase (SGT) is inducedin wounded potato tubers and leaves by stress condi-t ions such as sl icing (Moehs et al . , 1996a,b; 1997,St a pleton et al . , 1991). St udies are needed to ascerta inwhether the level of SG T or of other enzymes such a sepoxide hydrolase, which is induced by wounding (Sta-pleton et al., 1994), can be measured by a simple ELISAand whether such enzymes can serve as general indica-tors for postha rvest bruising a nd blackspot forma tion,late bl ight , and other stress condit ions in susceptiblepota to t ubers.

A major concern of polyphenol research is whetherphytochemicals such as chlorogenic acid behave differ-

ently, depending on whether they a re consumed a lone


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or as part of a complex diet , which could affect theirsta bility, interaction with other dieta ry components, an dconsequent ly their effica cy. Another concern, as notedelsewhere (Friedma n a nd McDonald , 1997), is tha t pla ntb ree din g a n d m ol ecu l a r g e n et i c s t u die s de si g n ed t ocreate improved potato cultivars need continuing guid-a n c e f ro m c om p os i t ion a l a n d s a f e t y e va l u a t i o n s t oassure t he safety of newly developed potatoes. Thus,the practical significance for host-plant resistance andfor potato qual i ty and nutri t ion of enzymic browningprevention through suppression of genes involved in thebiosynthesis of polyphenol oxidase through ant isenseR N A (B a ch e m e t a l . , 1 994) i s a l s o w ort h y of s t u dy .Since preventing the biosynthesis of PPO may resultin accumulation of monomeric polyphenols at the ex-pense of polymeric ones, adverse effects on host-plantresistance in which polymeric compounds part icipateare a possibi l ity. On the other hand, increased mono-meric polyphenol content may ha ve beneficial dietaryconsequences, as discussed ear lier (Sim et a l . , 1997).

The preceding ana lysis of t he current knowledge ofpotato polyphenols in the plant and in the diet showsthat these studies consti tute an evolving, potential lybeneficial area of food science, food safety, and nutritionwith ma ny unsolved problems. In a ddit ion to recom-

mendations made earlier, we are challenged to respondto a ddit iona l research needs outl ined below.

1. Assess whether an tioxidat ive potencies of poly-p h e n o l s , i n t e rm s o f t h e i r a b i l i t i e s t o t ra p da m a g i n ghydroxyl (OH ), alkoxyl (RO), peroxyl (RCOO ) , a n dsuperoxide anion (O2-) radicals, ca n be predicted fromthe net electrochemical redox potentials of mixtures ofstructural ly di fferent polyphenols actual ly present inpota toes a nd other plan t foods.

2. Define relat ive hea l th-promoting properties ofstru ctura lly different chlorogenic a cid isomers. Deter-mine whether two or more polyphenols can act syner-gist ical ly as antioxidants.

3. Enha nce the content of the most potent a ntioxi-

dat ive polyphenolic compounds in pota toes by plantbreeding and plant molecular biology techniques.

4. Define effects of commercial a nd home food pro-cessing on polyphenols in pota toes.

5. Develop new, inexpensive, nutritious, and health-promoting high-chlorogenic acid pota to t uber, peel, an dleaf food formulat ions. The possible value of leaves a sa food source deserves additional comment. P reviously,m a j o r e f f o rt s w e re m a de t o re m o v e p i g m e n t s i n t h eprepara tion of edible lea f protein isolates (Bickoff et al . ,1973; Pir ie, 1973; Fr iedma n, 1996a ). Sin ce leav es ha vea high polyphenol content (Tables 2 and 5) and sincechlorophyll binds strongly to ca rcinogenic heterocyclicamines (Friedman, 1996b), i t may be worthw hile toreta in both of these leaf constituents in t he prepa ra tionof the isolates.

6. Ca rry out anima l and human feeding studies withhigh chlorogenic acid potato diets to assess whetherbeneficial effects of chlorogenic acid i n v i tr o are con-firmed i n v i v o .

The suggested research needs apply to chlorogenicacid-conta ining frui ts a nd vegetables in genera l . Alls u c h s t u di e s a re b a s e d o n t h e a s s u m p t i o n t h a t t h e reare significan t concentra t ions of polyphenols in theedible portions of pota toes an d other foods, a s consumed.In addition, high chlorogenic acid potatoes would haveto be acceptable to consumers with respect to blackspot,after-cooking da rkening, and sensory qua l i t ies. Theconsumer ma y h a ve to choose betw een perceived und e-

sirable appeara nce and real beneficial heal th effects.

ACKNOWLEDGMENT

I a m m o s t g ra t e f u l t o a j o u rn a l re f e re e f o r h e l p f u lcomments a nd to G . M. McDonald for valua ble contri-butions and for drawing the structural formulas.

LITERATURE CITED

Adams, J . B. Green colour development in pota to cookingw a t e r . Food Chem. 1994, 49, 295-298.

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