phenolic autoxidation of carrot pureepdf

8/10/2019 Phenolic Autoxidation of Carrot Pureepdf

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P h e n o l ic A u t o x i d a t i o n I s R e s p o n s i b le f o r C o lo r D e g r a d a t i o n i n

P r o c e s s e d C a r r o t P u r e e

S. T. Talcott and L. R. Howard*,

Departm ent of Horticultur al Sciences, Texas A&M Un iversity, College Stat ion, Texas 77843-2133

Strained carrots were thermally processed with reduced oxygen pretreatments and exposed toelevated storage t emperat ures to accelerate physicochemical changes (40 C for 4 weeks). Strainedcarrot pretreatments prior to processing included a nitrogen sparge (N 2), blanch/frozen with nitrogensparge (BFN 2), oxygen sparge (O2), an d a control (C) tha t received no pretreat ment. Cha nges incolor, tota l soluble ph enolics, total carotenoids, phen olic acid m olecular weight , suga rs, a nd pH weremonitored during storage. Greater losses of color, total soluble phenolics, and total carotenoidsoccurred in control and O2-sparged samples a s compa red to N 2-sparged and BFN 2samples. Molecularweight of phenolic acids wa s lower in n itrogen-sparged sa mples th an contr ol and oxygen-spargedsam ples. Phen olic polymerization due to a utoxidation was responsible for color loss in processedstrained carrots. Processing treatments that reduce residual oxygen may result in better colorretention after processing and during storage. Determining the mechan ism(s) and magnitude of these reactions are important for devising strategies to prevent quality loss in strained carrots.

K e y w o r d s : S trained carrots; phenolic acids; autoxidation; color; carotenoids

INTRODUCTION

Brown color formation in processed st rained carrotsis perceived as a quality defect, an d meth ods to preservefresh carrot color a re importa nt for imp roving consumeracceptance. Maceration of carrot roots followed byheat ing in holding tank s causes oxidative chan ges thatresult in brown color and possible off-flavor formation.Processing in enclosed systems resulted in better flavorscores for pears that were attributed to fast inactivationof enzymes a nd essence captur ing (Leonard et a l., 1976).Numerous browning reactions in food systems have

been at tribut ed to cha nges in pH (Cilliers a nd Singleton,1990; Yeo and Shibamoto, 1991) and t ime an d tem per-ature of processing (Friedman and Molnar-Perl, 1990;Reyes et al., 1982). Alkali tr eatm ent of cooked, grat edca r r ot s r e su lt e d i n p r od u ct d a r k e n in g a s sh ow n b ydecreased lightness values (Archan a et al., 1994). Reac-tion rates for Maillard and phenolic nonenzymaticbrowning (NEB) are greatly influenced by tempera tur ean d pH . Ther ma l processing of low acid foods, involvingelevated temperatures for extended periods of t ime,impacts the overall quality of carrots (Simon, 1985).Maillard browning reactions in strained carrots havebeen proposed but not conclusively proven. Maillardbrowning was postulated to contribute to color, nutri-tional, an d flavor losses in r etorted str ained carr ots (Luh

et al ., 1969), and studies r eporting loss of reducingsu ga r s a n d a m i n o a c id s i n p r oce sse d ca r r ot s w er econsistent with this type of browning (Howard et al.,1996; Toribio and Lozano, 1986). However, reportedlosses of amino acids in many food systems can result

from complexation with o-quinones, which are interme-diates in phenolic acid oxidation (Cheftel et al., 1985).Since Maillard r eactions do not occur extens ively belowpH 6.0 (reta il carr ot puree pH 5.0-5.5) or at commercialstorage temperatures, other sources of browning ma ybe significant.

Ph enolic polymerization reactions du e t o au toxidat ioncan result in brown-colored pigments that are detri-mental to processed food quali ty. Browning rates of caffeic acid solutions are pH dependent with yellowpigments formed initially, followed by brown pigmentsas pH increases (Cilliers and Singleton, 1990). The

limiting factor for ph enolic aut oxidat ion is t he pr esenceand concentra tion of the ph enolate ion, wh ich declineswith decreasing pH (Cilliers and Singleton, 1989). Thephenolate ion is a highly reactive species that catalyzesoxidative reactions in phenolic acid systems. Acidifiedphenolic solutions will generally remain colorless unlesspolymerization occurs directly from molecular oxygenpresent in the system. Therefore, oxygen exclusioncoupled with product pH is important for controllingphenolic polymerization reactions (Bucheli and Robin-son, 1994). Major phenols in carrots include chlorogenic,caffeic, and p-hydroxybenzoic acids along with numer-ous cinnamic acid derivatives (Babic et al., 1993). Actingas a phytoalexin, phenolic acids may increase up to7-fold in carrot peel due to abiotic stress during post-harvest handling and storage (Sarker and Phan, 1979).Excluding oxygen in low and medium acid food systemsmay prevent condensation of phenolic acids, resultingin greater color retention. This study was undertakento explore the contribution of phenolic NEB reactionsto color degradat ion of processed stra ined carr ots.

MATERIALS AND METHODS

Ma t e r ia l s a n d P r o c e s s i n g . C a r r o t s (Da u cu s car ota L.)were purchased from a local market and stored at 4 C untilprocessed. Roots were wa shed, h and-peeled, cut int o 2-cm

* T o w hom cor r es pon d en ce s h ou ld b e a d dr es se d[phone, (501)575-2978; fax, (501)575-2165; e-mail, [email protected]].

Present address: University of Arkan sas, Institute of FoodScience and Engineering, 272 Young Ave, Fayetteville, AR72704.

2109J. Agric. Food Chem.1999,47,2109!2115

10.1021/jf981134n CCC: $18.00 1999 American Chemical SocietyPublished on Web 04/28/1999


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pieces, steam blanched for 30 min, an d cooled in a n ice waterbath . Roots were th en combined with 75% wat er (carrot weightafter blanching) and blended into a puree using a kitchen-scale blender. All blends were combined to create a homoge-n e ou s p ool ed s a mp le . T r ea t me n t s a p p li ed t o s u b sa mp le sincluded (1) a vacuum/nitrogen sparge (N 2) where the pureew a s d e a e r a t e d f o r 1 0 mi n i n a 4 - L s i d e a r m f l a s k u n d e r avacuum pressure of 635 mmHg a nd spar ged for 20 min withnitrogen gas, and the jar hea dspace was flushed with nitr ogenprior to capping; (2) an oxygen spar ge (O2) where the samplewas sparged for 20 min, and the jar headspace was flushed

with oxygen prior to capping; an d (3) a blanch/frozen withnitrogen sparge (BFN 2) treatm ent wher e peeled whole carrotswere steam blanched, cooled in ice water, packaged in poly-ethylene pouches, and frozen at -20 C. After being thawedt o r o om t e mp er a t u r e , c a r r ot s w er e b le n d ed t o a p u r e e a n dt r e a t e d i n a ma n n e r s i mi l a r t o t h e N 2-sparged samples. Acontrol (C) was processed directly from the pooled puree withno pretreatment. For all treatments, carrot puree was hand-fil led into 11-oz glass jars, heated to 60 C, and thermallyprocessed at 121 C for 30 min in a rotar y retort (Stock Pilot-Rotor 900, Stock America Inc., Milwaukee, WI) at 19 rpm (Fo> 6). An unprocessed puree sample was analyzed to determinephysicochemical changes during processing. Treated sampleswere stored 4 weeks at 40 C and a nalyzed 0, 1, 2, 3, and 4weeks after processing for physicochemical at tribut es.

I n a s e pa r a t e e xp e r ime n t , c a r r ot p u r e e w a s t h e r ma l ly

processed, as described above, to simulate a natur ally highoccurrence of phenolic acids or carrots that had experiencedstress-induced synthesis of phenolic acids. Two trea tmen ts, acontr ol and a phenolic spike, were a pplied to monitor th e ra teof phenolic oxidation an d subsequ ent physicochemical changesduring storage. Phenolics spiked into raw carrot puree in-cluded equal amounts of gentisic, p-hydroxybenzoic, m -cou-maric, o-coumaric, and caffeic acids (1250 mg/kg t otal).C a r r ot s w er e t h e r ma l ly p r oce s se d a n d mon i t or e d w ee k lyagainst a nonspiked control for 4 weeks at 40 C.

T o c h e mica l ly s imu l a t e s t r a i n ed ca r r o t p u r e e, a mod e lsolution containing 3 g/100 g sucrose, 0.5 g/100 g glucose, 0.5g/100 g fructose, 50 mg/L caffeic acid, 50 mg/L syringic acid,100 mg/L cysteine and citric/malic acids (1:1 m ixture for pHadjustment) was h eated for 2 h at 100 C to simulate residencetime of carrots in batch and hold tanks in a commercial setting.

Variat ions in m odel solution pH r an ged from 5 to 7.0. Phen olicbrowning was differentiated from Maillard browning by theexclusion of cysteine in identical models and calculating thedifference in absorbance at 420 nm.

P h y s i c o c h e m i c a l A n a l y s is .Color, total soluble ph enolics,total carotenoids, m olecular weight of phenolic acids, sugars,and pH were m easured in processed str ained carrots. CarrotpH was m easur ed using a Corning model 125 pH met er. Browncolor formation in model systems was measured at 420 nm(Gar cia et al., 1992). Tristimulus color wa s mea sured using aHunter Labscan model 5100 colorimeter. H unter a* a n d b*values were converted to hue (arctan b*/a*) and chroma (a*2

+ b*2)0.5 values. Total wat er-soluble phenolics were extr actedin deionized water (5 g/50 mL) and measured using the Folin -Ciocalteau assay as described by Swain and Hillis (1959).

Sugars were extracted in water (5 g/50 mL) using a Tekmar

model TP 18 t issuemizer (Cincinnati , OH). After fil teringthrough Miracloth (Calbiochem, San Diego, CA), the isolatewas pa ssed thr ough a pr ep-column conta ining 1 g of 200-40 0mesh Bio-Rex 5 anion-exchange resin (Bio-Rad, Richmond, CA)to remove organic acids. Samples were filtered through a 0.45-m filter an d injected int o a Spectra P hysics model P100 HPLCequipped with a 300 7.8 m m Aminex HP X-87C (Bio-Rad,Richmond, CA) carbohydrat e column heat ed a t 85 C. Mobilephase consisting of 100% Milli-Q water, run isocratically at0 .8 mL /mi n , w a s mon i t or e d w it h a Sh od e x mo d el R I -7 1refractive index detector. Sucrose, glucose, a nd fructose werequantified using external sta ndards.

Total carotenoids were extr acted (2 g/25 mL) with a solut ioncontaining acetone/ethanol (1:1) with 200 mg/L BHT added.Samples were extracted in th e dark, filtered thr ough Whatm anNo. 4 filter paper , and wa shed un til the r esidue was colorless.

Samples were adjusted to 100 mL, and the absorbance wasmeasured using a Hewlett-Packard 8452A diode a rray spec-trophotometer. Total carotenoids were calculated according toGross (1991) using th e equation (A V 10 6)/(A 1% 100 G),whereA is the absorbance at 470 nm, Vis th e total volume ofextract, A 1% is the extinction coefficient for a mixture ofsolvents arbitrarily set at 2500, and G is the sample weightin grams.

Molecular weight determ ination of soluble phenolic com-pounds was performed at 4 C using Sepharose CL-6B agaroseresin (Sigma Chemical Co., St. Louis, MO) packed in a glasscolumn (4.5 cm 100 cm). Soluble phenolic acid isolates wereobtained by centrifuging strained carrots until supernatantcould be passed thr ough a 0.45-m filter. Deionized wa tercarried 3 m L of strained carrot isolate th rough the column,and fractions were collected every 125 drops using a dropcount er. Dextra n sta ndar ds were used to determine molecularweight distributions from the column and fractions analyzedfor total carbohydrate (DuBois et al., 1956) by mixing 1 mLfr om e a ch fr a c t ion w it h 1 mL of 5 % p h e n ol a n d 5 m L of concentrated H 2SO 4 in a test tube. Tubes were heated for 10min at 80 C and cooled, and the absorbance was read at 490nm. Fr actions from the carrot isolate were measur ed at 280nm (Litridou et al., 1996), and th e data were reported as t otalsoluble phenolics.

S t a t i s t i c a l A n a l y s i s .Three jars from th e four t reatment swere analyzed in triplicate at each storage time. Chemical data

were a nalyzed by a nalysis of variance (SAS Institute, Inc.,1985), and mean separa tion was conducted u sing Duncansmu l t i p l e r a n g e t e s t (P < 0.05). Stepwise linear regressionana lysis was performed to predict carr ot color from chemicald a t a .

RESULTS AND DISCUSSION

Chemical Analysis.Browning reactions in processedstra ined carrots were pr imarily related to phen olic acidconden sat ion with a m inor cont ribution from car otenoidoxidation. Cha nges in color, pH, t otal soluble ph enolics,carotenoids, and sucrose were significantly affected byst o r a g e t i m e a n d t r e a t m e n t (P < 0.05). A decline insoluble phenolic content was consistent with an in-

creased level of oxygen incorporated into the samples(F i gu r e 1 A). S in ce n o e n zy m e a ct i vi t y r e m a in s i nther mally pr ocessed products, oxidation of phenolicacids occurred via autoxidation reactions. Levels of water-soluble phenolics declined th roughout t he studyw it h t h e g r ea t e st l osses occu r r i n g d u r in g t h e r m a lprocessing. Thermal processing accounted for 58.5% oftotal phenolic acids lost in the study for the control ,27.2% in N 2-sparged, 27.8% in BFN 2, and 51.4% in O2-sparged samples. Losses indicate that unit operationsthat exclude oxygen prior to t hermal processing arecritical for m ainta ining st rained carrot color. P henolicacids, as monitored by the Folin -Ciocalteu assay, werepolymerized enough to become water-insoluble since theassay will detect any soluble reducing compoundsincluding some conjugated phenols and reducing sugars.The decline in pH was greatest after thermal processing(0.9 unit) , which can be at tr ibuted to a reduction inpart icle size and subsequent release of organic acids oraddit ional unknown reactions (Figure 1B). The pHcontinued to decline dur ing storage at elevated t emper-at ur e an d rea ched its lowest value of 5.27 in O 2-spargedsamples. Decreased pH is det rimenta l to overall carr otflavor, and additional work is needed to underst and themechanism responsible for the decline in thermallyprocessed products.

Greater carotenoid retention was observed in N 2-s pa r ge d a n d B F N2 t r ea t m en t s a s com p a r ed t o O 2-sparged and control samples (Figure 1C). Carotenoid

2110 J. Agric. Food Chem.,Vol. 47, No. 5, 1999 Talcott and Howard


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concentrations declined expectedly in control (6.4%) andO2-sparged (40.6%) a s compared to N 2 t reatments.Greater retention in N 2-treated samples demonstratedt h a t O2 exclusion from stra ined carr ots was beneficialfor carotenoid and color retention. Hue angle values(Figure 2A) significant ly increased after therm al proc-essing in O2-sparged samples an d th en declined thr ough-out the study, which may be due to the formation of colored pigments from NEB phenolic polymerizationreactions. Nitrogen sparging treatments resulted inlower hu e an gles after t herma l processing and declinedat a slightly slower ra te over time a s compar ed to contr ola n d O

2-sparged samples. An elevated hue angle in O

2-

sparged and control samples was indicative of initialoxidation products (yellow pigments) of phenolic acidsthat were prevented from polymerizing in N 2-treatedsamples. Therefore, a high hue angle that declined overtime was indicative of product darkening as monomerunits polymerized int o brown pigments.

Declines in pH, soluble phenolics, and total caro-tenoids all correlated with decreased chroma values (r 2

) 0.49, 0.90, and 0.70, respectively) in strained carrots(Figure 2B). Nonpolar and polymerized phenolic acidsma y ha ve affinity for t he lipid fraction in carrots, whichcont ain carotenoids, directly lowering chroma values orupon their subsequent oxidation. This a ction wouldserve t o dull th e vivid ora nge color foun d in n on-oxidized

purees. Linear regression analysis demonstrat ed th athu e an gle and chroma were significan tly affected by thelevels of soluble phenolics a nd pur ee pH (R 2 ) 0.91 an d0.83, respectively), which demonstrates the importanceof strict oxygen exclusion prior to thermal processing.

The in ability to completely r everse color form ed a fteroxygen sparging of phenolic acid model systems hasbeen demonstra ted in our lab; however, visible colorformed by alkali addition can be reversed if the pureeis acidified within a short period of time. Similar coloredpigment s formed in a caffeic acid model system abovep H 5 .6 0 a n d w er e a t t r i bu t e d t o q u in on e for m a t i onduring oxidation (Fulcrand at al., 1994). Strained car-rots that were held in long-term storage ( >2 yr) exhib-ited improved color (L * ) +1.59) when acidified, indi-cating that irreversible pigments from Maillard browningwere n ot the sole source of pigmentation formed. Asimilar increase in lightness due to acidification wasd e m o n st r a t e d b y A r c h a n a e t a l . ( 1 9 9 4 ) t h a t w a s a t -t r i bu t e d t o ch a n g e s i n ca r ot e n oi d i som e r s p r e se n t .Chan ge in pigmentat ion wa s a lso noticed in pH-alteredsweet potato puree where normal color returned as thesample pH was adjusted back to its original value (Iceet al., 1980). Lightness (L *) values in strained carrotsdeclined cont inua lly throughout t he sh elf life stu dy withthe largest decline occurring during processing (Figure2C). High L * values indicate a l ighter color while

F i g u r e 1 . Effects of pretr eatm ent on (A) total soluble phenolics, (B) pH, and (C) total carotenoids in stra ined carrots. Treat ment sinclude n o pretrea tmen t (contr ol), nitrogen spa rge (N2 spar ge), blanched/frozen with nitrogen spa rge (BFN 2), and oxygen spa rge

(O2 sparge). Unprocessed pur ee demonstrates changes during th ermal processing. Bars represent sta ndard error of the m ean.

Effect of Phenolic Acids on Carrot Puree Color J. Agric. Food Chem.,Vol. 47, No. 5, 1999 2111


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decreased chroma a nd h ue values correspond to dark ercolors (Yang and Yang, 1987). Based on lightness valuesof unprocessed strained carrots, control samples de-clined 9.8% after pr ocessing a s compar ed t o 5.8% and5.3% losses for N 2 a n d B F N 2 sa mples, respectively.During storage, O2-sparged samples slowly bleachedand were almost completely white from the headspacet o t h e ce n t e r of t h e p r od u ct , r e su lt i n g i n e le va t e dlight ness values. The presen ce of oxygen likely initiatedextensive phenolic an d lipid oxidation, r esulting in theformation of reactive peroxides responsible for caro-tenoid losses (R 2 ) 0.57). However, the regression modelfor l ightness was not greatly improved when the O2treatment was removed from the model (R 2 ) 0.64).Compared to control and O2-sparged samples, N 2 a n dBF N 2 t reatments resulted in higher l ightness values,demonstra ting a relationship with th e degree of oxida-tion.

Molecular weight (MW) determination of water-soluble phenolics was performed on samples isolatedfrom day 0 (immediately after t herm al processing) andafter 4 weeks storage (Figures 3 and 4, respectively).Molecular weight fractions near the maximum absorb-

ance at 280 nm were qua ntified by the Folin -Ciocalteuassay and corresponded to the retention of low molec-ular weight phenolic compounds (Table 1). Nitrogen andBF N 2trea tmen ts h ad h igher amount s of low molecularweight phenolics (252 and 253 mg/kg, respectively) thancontrol and O2-sparged samples (223 and 207 mg/kg,respectively) after processing, indicating tha t phenolicpolymerization was reduced with oxygen exclusion.Molecular weight determination of phenolic acids servedto further identify their role in product darkening andis apparently related to phenolic polymerization reac-tions resulting in the loss of these compounds.

Sucrose concentra tions were slightly h igher in BFN 2sa m p le s t h a n con t r o l, O2-s p a rg ed , a n d N 2-spargedsamples (data not shown). This result may be attributedt o t h e fr e ez e-t h a w cy cl e t h a t r e su lt e d i n ce ll u la rdisruption an d great er liberation of sugar s. Yan (1989)also found that processing frozen carrots into pureeresulted in higher Brix levels than puree obtained fromnonfrozen carrots. Glucose and fructose concentrationswere unaffected by processing treatments, indicatingthat reducing sugars were not consumed in Maillardreactions. In contr ast , Howard et al. (1996) reported a

F i g u r e 2 . Effects of pretreatment on (A) hue angle, (B) chroma, and (C) Hunter L * of strained carrots. Treatments include nopretr eatm ent (control), nitrogen sparge (N2spa rge), blanched/frozen with n itrogen spar ge (BFN2), and oxygen sparge (O2spar ge).Unprocessed puree demonstrates changes during therma l processing. Bars r epresent standard error of the mean.



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loss of reducing sugars in carrot puree, which werehypothesized to be consumed in Maillard browningreactions. Organic acids rema ined unchan ged through-out the study and were unaffected by processing treat-ments (data not shown).

Strained carrot samples spiked with phenolic acids

followed similar tren ds for color degradat ion as de-scribed in the accelerated shelf life study (data notshown). Soluble phenolic changes in control and spikedsamples declined at a constan t rate with significantlosses observed after processing (9.4% and 4.2%, respec-tively; Figure 5A). Strained carrot pH demonstrated aconsistent rate of decline, but spiked samples exhibiteda l ow er p H (5 .2 8) d u e t o a ci di t y i m p a r t ed b y t h ephenolics as compared to control sa mples (pH 5.82;Figur e 5B). This acidificat ion effect s erved t o lower t heinit ial decline in pH after pr ocessing for the spikedsamples (0.08 unit) as compared to the control (0.74unit). This small decline in pH after processing forsp ik e d sa m p le s d e m on st r a t e d t h a t e it h e r ox id a t iv ereactions or a change in the buffering abil i ty of thepuree r esulted in a pH decline du ring processing. Sinceoxidation ra tes ar e generally pH dependent , the rat e ofpH decline was slower for spiked samples as comparedto the control during t he first week of storage. I t washypothesized that increasing phenolic acid concentra-t i on w ou l d r e su lt i n m or e r a p i d l osses of sol u bl ephen olics, resulting in a faster decline in pH, an d wouldresult in rapid discoloration during storage. However,spiked samples resulted in a 16.6% loss of phenolic acidsas compared to a 31.8% loss in nonspiked sa mples.Results indicate tha t t he r eaction r ate of physicochem-ical chan ges would l ikely be constan t under similarstora ge conditions r egardless of phen olic concent ra tion,but a lower puree pH would serve to slow oxidative

reactions affecting strained carrot color. On the basisof percenta ge loss, th e effect of phen olic acid concentr a-t ion and pH decline was similar between spiked andcontrol samples. Individual phenolic acids have beenshown to oxidize at different rates (Garcia et al., 1992);therefore, concentrat ion and identi ty of t hese com-pounds after processing a nd st orage ma y impact stra inedcarr ot color.

M o d e l S y s t e m . Carrot model solutions were pre-pared to verify t he major r ole of phenolic oxidation incolor production and to determine if Maillard browningwas a potential source of pigmentation during com-mercial processing applications. After heating eachmodel system, solution pH was adjusted to originall ev el s a n d col or w a s m on i t or e d a t 4 20 n m . C ol orformation due t o Maillard browning was determined bysubtracting the absorbance due to phenolic acid con-densation (no cysteine added) from the total colorformed (with cysteine) and expressed on a percentagebasis (Table 2). Phenolic acid condensation was respon-sible for the yellow and brown pigments formed, with

F i g u r e 3 . Effect of pretreat ment on molecular weight distr i-bution (kDa) of phenolic acids in strained carrots immediatelyafter ther mal processing. Treatment s include no pretr eatmen t(control), nitrogen sparge (N 2 spa rge), blanched/frozen withnitrogen sparge (BFN 2), and oxygen sparge (O2 spar ge).

F i g u r e 4 . Effect of pretreat ment on molecular weight distr i-bution (kDa) of phenolic acids in str ained car rots a fter 4 weeksstorage at 40 C. Treatments include no pretreatment (control),nitrogen sparge (N 2 sparge), blanched/frozen with nitrogensparge (BFN 2), and oxygen sparge (O2 spar ge).

Tabl e 1. Conc e n tr ati on of Wate r -Sol ubl e P he n ol i c s(mg/ kg) i n Str ai ne d Car r ots afte r The r mal P r oc e ssi ng(Day 0) and afte r 4 We e ks of Stor age (We e k 4) i n anAc c e l e r ate d She l f Li fe H e l d at 40 C for 4 We e ks a

t rea tm en t da y 0 (m g/k g) week 4 (m g/k g

con t r ol 223 188N 2 2 52 200BF N2 253 202O2 2 07 145

a A molecular weight column was u sed to separ ate phenolic acidfractions (ma x ) 280 nm) and quantif ication was conducted bythe Folin -Ciocalteu assay.



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l it t le contribution by Maillard browning. More pig-mentation was formed at higher pH values as reportedby numerous investigators (Cerrutti et al., 1985; Yeoan d Shibam oto, 1991; Friedma n a nd Molnar-Perl, 1990)which were at tribut able to both ph enolic acid condens a-

tion and Maillard browning. Results ind icat ed th at colorformation due to Maillard reactions would develop at aslow rate at the pH range of strained carrots with themajority of pigmentation (>88%) resu lting from ph enoliccondensation.

CONCLUSIONS

Pr ocessors should a void un it operations t ha t a llow forexcessive heat an d oxygen incorporat ion int o macerat edfruit and vegetable products due to phenolic aut oxida-tion potential. Utilization of nitrogen spar ge techniquesimmediately after macerat ion followed by headspa ceflushing with nitrogen prior to capping should improveoverall color retention in strained carrots. Heat in

combination with dissolved oxygen is detr imenta l tostrained carrot color, an d a ttempts should be made toreduce oxygen concentration in the system to preventphenolic au toxidation a nd carotene bleaching.

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F i g u r e 5 . Effects of pretreatment on (A) total soluble phe-nolics and (B) pH in stra ined carrots stored 4 weeks a t 40 C.Treatments included a nonspiked control and a phenolic acidspike (1250 mg/kg). Unpr ocessed puree demonstrat es changesduring therma l processing. Bars represent standar d error oft h e me a n .

Tabl e 2. Contr i buti on (%o f Total Col or F or mati on) of P h e n o l i c C o n d e n s a t i o n a n d M a i l la r d B r o w n i n g a t 4 2 0n m t o T o t a l Co l o r F o r m a t io n i n a S t r a i n e d C a r r o t M o d e lS o l u t i o na

total browning

i n it i a l p H p h e n ol ic b r ow n in g (%) M a i ll a r d b r ow n in g (%)

7.00 54.9 45.16.50 68.3 31.76.00 97.5 2.25.50 88.0 12.05.00 100 0

a Phenolic browning calculated as color formed without cysteineminus color formed with cysteine. Model solution contained 3%sucrose, 0.5% glucose, 0.5% fru ctose, 50 m g/L caffeic acid, 50 m g/Lsyringic acid, 100 mg/L cysteine, an d citric/malic acid (1:1) adjust edt o p H 5-7. Solutions were heated for 2 h at 212 F.



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Received for review October 14, 1998. Revised ma nuscriptreceived Mar ch 2, 1999. Accepted March 9, 1999.

JF981134N


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