annealing of starch a review

International Journal of Biological Macromolecules27 (2000) 1–12

Review

Annealing of starch — a review

Richard F. Tester *, Stephane J.J. DebonFood Research Laboratories, School of Biological and Biomedical Sciences, Glasgow Caledonian Uni6ersity, Glasgow G4 0BA, UK

Received 22 July 1999; accepted 1 December 1999

Abstract

Annealing processes, involving specific heating protocols, have been used by man for centuries to impart desirable propertiesto materials — especially metals and particularly tools and weapons. The terminology has also been applied to biopolymers suchas starches, where the effects of the processing have been known for decades although the molecular basis has not been at all wellunderstood. Because of the marked effect the annealing process has on starch functionality and consequently industrialapplications, it is critical that the underlying molecular events are understood. This review is an attempt to clarify the process ofstarch annealing with an emphasis on data generated in the authors’ laboratory. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: Annealing process; Heat–moisture treatment; Starch

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1. Background and definitions

The annealing process, when related to starches, hasbeen variously described. Both annealing and heat–moisture treatments are related processes, where thestarch to moisture ratio, temperature and heating timeare critical parameters to control. Jacobs and Delcour[1] have discussed the difference between annealing andheat–moisture treatment of starch. They state thattreatments in excess (\60% w/w) or at intermediate(40–55% w/w) water contents represent annealing whiletreatments below 35% (w/w) can be described as heat–moisture treatment. Also, they state that both processesoccur at above the glass transition temperature (Tg) butbelow the gelatinisation temperature. However, theterm heat–moisture is often used to describe high tem-perature treatments, like 100°C (up to 16 h at 27%moisture) [2]. Stute [3] has also discussed the differencebetween annealing and heat–moisture treatments, ac-knowledging that for work conducted in the early partof last Century, annealing and heat–moisture wereused as synonymous terms. More recently Collado and

Corke [4] have helped to clarify the situation. Theystate that annealing represents ‘physical modification ofstarch slurries in water at temperatures below gelatini-sation’ whereas heat–moisture treatment ‘refers to theexposure of starch to higher temperatures at very re-stricted moisture content (18–27%)’.

These authors propose that the terminology is stan-dardised — which has implications in terms of thedefinition of gelatinisation. Hence, the following defini-tions are proposed with respect to starch (and relatedpolymeric systems).

1.1. Glass transition temperatures

Glass transition temperatures are very importantparameters that affect polymeric physical properties.The transition is similar to a second-order thermody-namic transition and has been well described by Bili-aderis et al. [5]. The term describes the temperatureinduced transition of an amorphous glassy polymersystem to a progressively more rubbery state when it isheated (usually in the presence of a solvent/plasticiser,when applied to polysaccharides). In the case of com-pletely glassy polymers, Tg is relatively distinct, wherean inflection (increase) in the specific volume and en-thalpy as a function of temperature occurs and is

* Corresponding author. Tel.: +44-141-3318514; fax: +44-141-3313208.

E-mail address: [email protected] (R.F. Tester)

0141-8130/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.PII: S 0 1 4 1 -8130 (99 )00121 -X

R.F. Tester, S.J.J. Debon / International Journal of Biological Macromolecules 27 (2000) 1–122

reflected in discontinuity in the specific heat capacity(Cp). Because starch contains both amorphous andcrystalline material, the exact thermal event represent-ing Tg is difficult to detect. However, high sensitivitydifferential scanning calorimetry (DSC) has allowed forthe measurement of Tg in amorphous and nativestarches with various levels of crystallinity [6–8]. Wateris a very effective plasticiser of amorphous starch (andhence Tg), where the ratio of starch to water is criticalwith respect to the temperature at which Tg occurs (Fig.1).

1.2. Annealing

Annealing represents the physical reorganisation ofstarch granules (or appropriate polysaccharide matriceslike amylose–lipid complexes) when heated in water (orappropriate plasticiser) at a temperature between Tg

and the onset of gelatinisation (To) of the native starch(or polymeric system). It is recognised that annealingcan be associated with partial gelatinisation. However,these authors believe the definition should be appliedonly where gelatinisation does not occur and hence To

Fig. 1. State diagram of the starch–water system. The experimental data for the glass transition (Tg) are from amorphous starch [8] while thetheoretical Tg is derived from the Couchman–Karasz equation [9]. The experimental data for the melting transition (Tm) are from DSC(Tconclusion) of wheat starches at different moisture content [10–13]. The theoretical Tm is fitted from the Flory–Huggins equation [14].

R.F. Tester, S.J.J. Debon / International Journal of Biological Macromolecules 27 (2000) 1–12 3

Fig. 2. DSC thermograms of a commercial wheat starch (BDH 30265): (a) native; (b) after annealing in excess water (45°C, 100 days).

must not be exceeded. In addition, according to thisdefinition, the enthalpy of gelatinisation post-annealingcannot be less than for the native starch. Annealingleads to elevation of starch gelatinisation temperaturesand sharpening of the gelatinisation range (definedbelow) as shown in Fig. 2.

The annealing process has important industrial impli-cations. Starches may be deliberately annealed to im-part novel processing characteristics. However, thereare few commercial processes where annealing may bejustified in terms of energy and time to generatestarches with higher gelatinisation temperatures — es-pecially when many inexpensive chemical processes canbe employed, over a short time frame, to selectivelymodify starch characteristics. Often annealing isachieved unintentionally. One example is the wetmilling of maize when used to extract starch.

1.3. Gelatinisation

Gelatinisation is a term used to describe the molecu-lar events associated with heating starch in water.Starch is converted from a semi-crystalline, relativelyindigestible form to (eventually) an amorphous (readilydigestible) form. The gelatinisation process (in excesswater) is believed by these authors to involve primaryhydration of amorphous regions around and above Tg,with an associated glassy-rubbery transition. This inturn facilitates molecular mobility in the amorphousregions (with reversible swelling) which then provokes

an irreversible molecular transition. This irreversiblestep involves dissociation of double helices (most ofwhich are in crystalline regions) and expansion of gran-ules as the polymers (and granule interstices) hydrate.The onset temperature (Tonset or To, typically �45°C)by DSC reflects the initiation of this process, which isfollowed by a peak (Tpeak or Tp, typically 60°C) andconclusion (Tconclusion or Tc, typically 75°C) temperature(Fig. 2). After Tc, all amylopectin double helices havedissociated, although swollen granule structures will beretained until more extensive temperature and shearhave been applied. Beyond �95°C an amorphous gel isformed. The temperature range Tc–To represents thegelatinisation period.

After gelatinisation, a-glucan chains re-form doublehelices if the conditions are desirable. This process —retrogradation — occurs when, for example, breadstales. Sometimes, annealing type processes are con-fused with retrogradation. However, annealing ofstarch granules is a process that retains granular struc-ture and original order. Retrogradation occurs asamorphous a-glucan chains form double helices and,perhaps eventually, align themselves in crystallites.

Blanshard has discussed gelatinisation type processesin depth elsewhere [15–17], and how interactions be-tween starch and other groups (especially water andsolutes) modify the temperature driven transitions.Readers are referred to these publications for moredetail.


1.4. Heat–moisture

Heat–moisture treatments represent the control ofmolecular mobility at high temperatures by limiting theamount of water and hence gelatinisation. In commonwith annealing, physical reorganisation is manifested.The low levels of water in the system lead to an elevationof Tg — the trigger for polymeric reorganisation — asdiscussed below. Hence, high temperatures are requiredto cause physical reorganisation within granules.

Heat–moisture treatments of starches may be con-ducted deliberately by industry to impart novel charac-teristics. One example is the pre-treatment of starches forinfant foods. Other examples of industrial applicationsof the process include processing of potato starch toreplace maize starch in times of shortage, creation ofexcellent freeze–thaw stability and improvement of thebaking quality of potato starch [4].

2. Relationship between Tg on both annealing andgelatinisation

We view the gelatinisation process as a co-operativeevent between amorphous and crystalline regions instarches. In unlimited water, amorphous regions imbibewater as the starch granules are progressively heated.Perhaps the relative large amorphous growth ring typeregions are the primary amorphous regions to hydrate,followed by amorphous lamellae ‘sandwiched’ betweenthe crystalline lamellae. The plasticisation of the amor-phous lamellae and annealing of double helices is repre-sented in Fig. 3.

The absorption of water into amorphous regions iscertainly possible, as for example, potato starch canreversibly absorb up to 0.53 g water/g dry starch beforethe irreversible steps within the gelatinisation process areexceeded [17]. The water induces a transition of theamorphous regions from a rigid glassy state to a mobilerubbery state which in turn facilitates the hydration anddissociation of double helices in crystallites. The dissoci-ation of the crystallites begins after Tg of amorphousregions, and at this temperature (To), limited dissociationof amylopectin double helices (most of which are incrystallites) is associated with limited swelling of gran-ules. Gelatinisation proceeds as the temperature is in-creased, progressively uncoiling all the double helices andconverting crystalline material to amorphous material.

If water is sufficiently low so as to restrict gelatinisa-tion, the primary gelatinisation endotherm (G) developsa high temperature trailing shoulder (M) [19,20], asshown in Fig. 4. As the volume fraction of water (61) isreduced to B0.45, the shoulder becomes distinct andrepresents the only endotherm observed. In high mois-ture food systems, starch granules are completely gela-tinised and often no granule form is discernible (e.g.

custard). Drier food products have often been processedunder high moisture conditions and equally, little granuleform is apparent (e.g. wafer biscuits). Where water islimiting, however, like fat rich shortbread biscuits, essen-tially native granule form is apparent under the micro-scope-although this starch has presumably been heatmoisture treated.

The Tg must be reached or exceeded for annealing tooccur. Many authors accept that this is a prerequisite ofthe annealing process [17,22,23]. Indeed, the annealingprocess has been discussed in terms of the process itselfimproving Tg without facilitating the gelatinisation pro-cess [17]. If starches are heated at progressively highertemperature above Tg, they do eventually completelygelatinise, having gone through an early phase involvingenhanced mobility of amorphous regions. It is logicalthat this phase is comparable to the phase that initiatesand forms part of the annealing process [19,24–27].

Perhaps because of the difficulty associated with mea-suring Tg, some authors claim that starches can beannealed below Tg [28,29]. However, this would meanthat structural reorganisations of the crystalline compo-nent of starch granules occur independently of reorgan-isation of the amorphous phase. This is an almostimpossible situation to imagine in view of the relativelyimpenetrable nature of these regions by water molecules,with no associated passage through (and associatedreorganisation of) amorphous regions.

It is relatively straightforward to measure the gelatini-sation endotherm of starch using DSC, although this isnot true of Tg. Whilst Tg has reportedly been determinedprior to gelatinisation [24], it is in fact very hard to detectand quantify [5,22] — unless high sensitivity DSC is usedat low moisture contents [6–8]. Primarily this is becauseit is both small and submerged into the thermogrambaseline. Model polymeric systems, for example polyan-hydroglucose compounds [30] have, however, been usefulin developing an understanding of how a-glucan struc-ture itself moderates Tg. For example branched regions(which are also found in the amorphous zones of starch)seem to depress Tg similarly to plasticisation by smallmolecules [30].

3. Effects of starch-to-moisture ratio on Tg andannealing

The original work of Gough and Pybus [31], which wasthe first to describe how wheat starch could be annealedby heating in excess water at 50°C, showed that gelatini-sation temperatures could be increased while thegelatinisation range could be sharpened. Many studieshave been conducted on the effects of annealingon different starches, with different starch-to-moistureratios and different storage times [23,29,32–46]. Some-times, the annealing process is conducted as a


Fig. 3. Pictorial representation of the effect of hydration and subsequent annealing on the semi-crystalline lamellae (amylopectin double helicesare represented as rectangles): (a) dry starch with glassy amorphous regions; (b) hydrated annealed starch with rubbery amorphous regions(adapted from [18]).


Fig. 4. Influence of water content on the differential scanning calorimetry thermograms profile of potato starch: (a) the onset () and conclusion(�) gelatinisation temperatures (adapted from [21]); (b) the corresponding DSC thermograms (adapted from [19]).

single event (single-step) whilst at other times it isconducted as two starch–water/temperature/time events(double-step) or even many individual steps (multi-step)as discussed elsewhere [1]. This double or multi-stepapproach is often used to promote annealing withoutgelatinisation, and the double step process potentiallyproduces higher gelatinisation temperatures than thesingle step process [1]. The lack of standardisation ofannealing conditions makes it difficult to compare re-sults between the different studies.

The effect of the starch-to-water ratio, temperatureand time on annealing of wheat starch has been investi-gated in detail [23]. This study demonstrated how criti-cal the interrelationship of these parameters is. Theannealing process could be initiated when the moisturecontent exceeded 20% by weight, (because Tg is aroundroom temperature when this moisture content is ex-ceeded, Fig. 1) but was restricted (in terms of its effecton increasing gelatinisation temperatures) unless it ex-

ceeded 60%. Although annealing could be initiated at15°C below To by DSC, the effect was more marked thecloser the annealing temperature was set to (below) To.Similar studies have been conducted on starches ofdifferent botanical origins [32,40] and demonstrate theadditional complication of species specific variation.

During annealing of starches, there are in essence twothermally driven processes which are intimately relatedand reflect the moisture content of the system — theelevation of Tg and gelatinisation temperatures (espe-cially To). Low moisture causes elevation of (the rela-tively unplasticised) Tg of starches [6,7,22,47] and modelpolymeric systems [30,48,49] which, in the case ofstarch, intimately reflects the increase in gelatinisationtemperatures. Indeed, the elevation of Tg implies a moreglassy state and hence reorganisation of amorphousregions. This is associated with improved order ofcrystalline regions (below). The situation with respect toTg of starch in food systems is very complex because


Fig. 4. (Continued)

of the raft of potential interactions. More details con-cerning (general) glass transitions in model systems canbe found elsewhere [50]. Similarly, sub-Tg transitions ofstarches (which probably represent enthalpy relaxation[51]) have been described by other authors [51,52].

4. Environmental considerations

The effect of environmental temperature on starchsynthesis and properties has been the subject of muchrecent research. Apart from direct growth temperatureeffects on the activity of enzymes involved in starchbiosynthesis (which are discussed in some detail in arecent publication by these authors [53]), there is adistinct effect on starch physico-chemical properties. Ingeneral, for mature cereal and tuber starches [54–58]there tends to be a small effect on the fine structure(chain length distribution) of amylose or amylopectin.Granule size decreases as growth temperature increases,while amylose content remains approximately the same.In the case of lipid (lysophospholipids and free fattyacids) in cereal starches (only), there is a distinct tem-perature dependent increase. Growth temperature alsocauses a distinct increase in gelatinisation temperatures

of starches (which can also be modelled using potatomicrotubers [53]), and parallels have been drawn be-tween laboratory based annealing processes (in vitroannealing) and environmentally driven reorganisations(in vivo ‘annealing’) of starch granule architecture[23,53,57,58].

Hence, apart from species and cultivar specific varia-tion in starch physico-chemical properties, there is aprofound environmental effect on gelatinisation charac-teristics. This effect is, in the experience of these au-thors, far greater than cultivar specific (and hencegenetic) variation. The implications are, that productquality is not simply a cultivar specific trait — but islargely dependent on environmental conditions experi-enced during starch deposition.

5. Molecular basis for annealing

It has been difficult to define, at the molecular level,what happens to the internal structure of starch gran-ules when they are annealed. Some authors have dis-cussed the molecular event in terms of increasinggranule stability [32], reorganising granule structure[39,40] or lowering free energy [17]. These descriptive


terms do not, however, give readers a clear molecularpicture associated with the reorganisations involvedwithin granules when they are annealed. A number ofauthors have discussed annealing with more emphasison the crystalline and amorphous domains. Crys-tallinity and crystalline ‘perfection’ (optimisation ofcrystalline order) have been discussed in detail in thiscontext [28,36,38,42,43,59]. Similarly, granular reorgan-isations have been discussed in terms of rigidity [33]and realignments and partial melting [45,60]. Othersrecognise the importance of interactions between, andmobility of, amorphous and crystalline regions [3,61]and the constituent amylose and amylopectin molecules[37].

Tester et al. [23] (working on wheat starches), havediscussed annealing in the context of hydration andswelling of amorphous regions (temperature range be-tween Tg and To), which facilitates ordering of doublehelices in crystalline regions. This ordering of doublehelices could be associated with minor optimisation ofdouble helix length, although no additional doublehelical material is formed [23]. The amorphous materialpost-annealing probably becomes more ‘glassy’ (morerigid and less mobile) whereupon Tg is elevated. Theconstancy of double helix content pre- and post-anneal-ing has also been shown by Jacobs [1] for a range ofstarches (pea, potato and wheat).

With respect to the effects of annealing on the doublehelix content of starches, the situation in amylomaize isfar more complex than for waxy or normal starches. Inamylomaize starch there is evidence from NMR thatamylose also forms some double helices and that uponannealing there is partitioning of amylopectin and amy-lose helical structures [62,63]. This different structuralmodel within amylomaize starch granules helps to ex-plain the characteristic gelatinisation characteristics ofthese starches (below).

Whilst NMR can be used to quantify the number ofdouble helices within starch granules, it does not mea-sure crystallinity per se. For this purpose, wide angleX-ray scattering (WAXS) may be employed. Earlywork on annealing of wheat starch indicated that therewas little detectable effect on the X-ray diffractionpattern [31]. This has been confirmed for potato starch[3]. However, other workers have reported a smallincrease in intensity of the diffraction pattern (but withlittle or no effect on d-line spacings) for wheat, oat,lentil [32,64] and barley starches [46] but with a de-crease in intensity for potato starch [32]. It is veryuseful to link together both NMR and WAXS data forstarches, as they measure different levels of order. Onecould, for example, have non-crystalline double helices(outside crystalline domains) within starch granulesthat give a strong NMR signal but not a WAXSdiffraction pattern. The relative significance of thesetechniques for determining starch structure and order

during annealing and gelatinisation may be supportedby the use of scanning electron microscopy [65], wheredimensions of amorphous and crystalline lamellae maybe estimated.

Overall, the NMR and WAXS data support the samegeneral picture that annealing causes no significantincrease in crystalline material formed within starchgranules by either of two possible mechanisms: (i)formation of double helices (which need not necessarilybe associated with existing crystalline domains) or; (ii)major increase in amount of crystallinity as a conse-quence of ordering of previously amorphous regions.Rather, the enhanced ordering of double helices, due toimproved registration (alignment), with associated in-creased rigidity of amorphous regions, probably under-lies the annealing process.

Unlike WAXS which quantifies crystalline orderthroughout starch granules, small angle X-ray scatter-ing (SAXS) quantifies differences (periodicity) at thelevel of amorphous-crystalline lamellae radiating fromthe hilum to the periphery of starch granules. Usingthis technique, Jacobs et al. [36] showed that (for wheatand potato starches) the repeat distances of the crys-talline and amorphous lamellae remain unchanged(10.5 nm in wheat and 9.9 nm in potato), althoughthere was an increase in peak intensity. A pictorialrepresentation of the length scales within starch gran-ules together with techniques used for their quantifica-tion are presented in Fig. 5. More detailed discussionregarding the application of this technique to under-stand structural, gelatinisation and annealing mecha-nisms of starches can be found elsewhere [65]. Thoseauthors, however, reported that the lamellar repeatdistance for wheat starch is smaller (8.85 nm) than thefigures quoted by Jacobs et al.

Native cereal starch granules contain amylose–lipidcomplexes, as shown by NMR [67,68]. The lipid(lysophospholipid or free fatty acid) is immobilisedwithin the a-glucan helices and the corresponding lackof mobility of methylene groups can be determinedusing this technique. The biochemical significance ofthese complexes is uncertain, although they have asignificant effect on starch functionality. Whilst inmodel amylose–lipid complex systems annealing can beinduced [69] because of the relative ease of mobility,annealing between Tg of the starch and To of amy-lopectin in wheat starch has little effect on this amy-lose–lipid endotherm [34,35]. The effect of annealing atthese temperatures (e.g. 35–50°C) on amylose–lipidcomplexes is predictably very unlikely, because thetemperature is far too low. The peak transition temper-ature of these complexes is of the order of 95–115°C.At temperatures where complexes have been annealed(for example 80°C, [69]) the starch would be fullygelatinised in unlimiting water. Probably Tg for thecomplexes under these conditions is quite close to this


Fig. 5. Pictorial representation of the length scales within the starch granule together with techniques used to characterise the structural features(adapted from [66]).

temperature, although it is perhaps as mobile as Tg foramylopectin as a function of moisture content [23].

There is a relationship between the amount of starchphosphorylation and elevation of gelatinisation temper-atures as a consequence of annealing [70]. The shift inTp (or DTp) is greater for starches with low levels ofphosphorylation. The authors [70] proposed that this isbecause the number of potential dislocations is smaller.In other words, phosphate moieties restrict double helix(and consequently crystallite) formation. The higher thephosphate content, the greater the interference. Becauseof the detrimental effect of phosphate groups on crys-tallite formation, however, the increase in enthalpy islargest for the high phosphate starches indicating that

steric hindrance is diminished as a consequence of theincreased mobility during annealing. Chemically intro-duced phosphate groups have similar effects to natu-rally (during biosynthesis) inserted groups [71]. Thisshould be viewed in the context of phosphate estersaffecting the crystallinity of native starches [72], wherethe gelatinisation enthalpy is inversely related to thephosphate content.

6. Physical consequences of annealing

According to some authors [32], annealing causes noeffect on granule dimensions or shapes, although early


microscopic work indicated that wheat starch granuledimensions increase after annealing [31]. Clearly, how-ever, small differences in size cannot be accuratelyquantified using microscopy and care should be placedon reliance upon this data. The A-type diffractionpattern of wheat starch is retained after annealing [31],although the line intensity may increase as discussedabove. Although heat moisture treatments causes a B-to A-type transition for potato starches, this does nothappen during annealing [3]. Hence, the molecular re-orientation is more subtle.

The effect of annealing on gelatinisation characteris-tics is well established, particularly using DSC, wherethere tends to be an increase in To and Tp, decrease inthe gelatinisation range (Tc–To) and either constancy oran increase in gelatinisation enthalpy[3,23,28,31,32,34,37–41,43,46,64,70,71,73]. The increasein gelatinisation temperatures is associated with a de-crease in swelling power [23,32,41,46,73], provided thatsome granular structure is retained. This is reflected ina higher temperature onset of swelling and reducedswollen volume (below circa 90°C, provided that wateris not limiting). The effects of annealing on pastingcharacteristics are complex. In some studies the consis-tency (viscosity) of annealed starches (wheat andpotato) increases (with associated decrease of peakviscosity for potato starch) while for lentil and oat ittends to decrease [32]. Similar results have been re-ported for wheat and potato starch, with annealed peaand rice starches exhibiting increased viscosity [3,33,41].

Using the model proposed by these authors andco-workers [23], the physical properties discussed abovecan be explained on the basis of more glassy amor-phous regions within annealed starch granules and amore ordered registration of amylopectin double he-lices. These molecular events restrict ease of hydrationof the starch granules during gelatinisation and elevategelatinisation temperatures. In parallel, these eventsrestrict swelling. It is difficult to unravel the effects onpasting characteristics, because this system will bestrongly influenced by granule size and polysaccharidesolubilisation — more so than gelatinisation (by DSC)and swelling power determinations. This is probablywhy there is a lack of consistency in response toannealing for starches from different botanical origins.

7. Solubility

The annealing process itself leads to little solubilisa-tion of a-glucan [23]. This is important as it shows thatimproved order is a genuine molecular event ratherthan a consequence of leaching amorphous a-glucanand hence ‘concentrating’ crystalline material. Anneal-ing reduces solubilisation of a-glucan during swellingbelow 100°C [32,41,46,73]. As leachate is primarily

(amorphous) lipid free amylose (FAM) according to thedefinition of Morrison et al. [68], the amylose must bemore restricted from leaching out of the granules. Al-though at a given temperature post-annealing, the gran-ules will swell less than un-annealed starches and thiswill be the primary restraint to leaching. This does,however, strengthen the view that there is molecularreorientation in the starch granule which makes theamorphous material more glassy with an elevated Tg.

8. Chemical hydrolysis

Annealing tends to reduce the amount of acid hy-drolysis of starch granules, although small granulessometimes exhibit little difference or even enhancedhydrolysis [32,34]. This discrepancy has to some extentbeen resolved by Tester et al. [23] who investigated acidhydrolysis for native and annealed wheat starch as afunction of time. They found that for the first phase ofacid hydrolysis (0–7 days, representing amorphous ma-terial hydrolysis) annealed starch was more extensivelyhydrolysed than native starch. After 7 days, wherecrystalline material is progressively hydrolysed, the ex-tent of hydrolysis for the native and annealed starcheswas essentially the same. The explanation for the en-hanced hydrolysis of amorphous regions after anneal-ing is that the amorphous regions become moreconcentrated due to the enhanced glassy structure. Onthe other hand, the similarity in hydrolysis patternduring the crystalline hydrolysis phase (\7 days) confi-rms that it is double helices (which remain constant)which are the primary contributor to the hydrolysisprofile during this phase. Amylose–lipid complexesmay affect the pattern of acid hydrolysis in cerealstarches, which could also influence enzymic hydrolysis(below).

9. Enzymatic hydrolysis

Certain studies have indicated that annealed wheat,barley and sago starches are more easily hydrolysed bya-amylases than native starches [31,46,74]. This has,however, been contradicted by other research on wheat,lentil and potato starches [32,41], although small starchgranules (oat) have been reported to be much moreeasily hydrolysed post-annealing [32]. However, the rateof a-amylase hydrolysis for different starches followstwo distinct phases: an initial rapid then subsequentslow phase. Annealing alters the extent of hydrolysis ofthese different phases as a function of botanical origin[35]. During the second phase of hydrolysis, annealedwheat and pea starches are more resistant to a-amylasehydrolysis whilst the inverse is true for potato starch[35]. Annealed potato starch is less easily hydrolysed by


amyloglucosidase than native starch [41]. In commonwith the statements above regarding acid hydrolysispatterns of starches, reported differences in a-amylasehydrolysis patterns may represent unjustified compari-sons between different phases of hydrolysis (amorphousand crystalline material) rather than necessarily starchspecific responses to enzyme hydrolysis post annealing.Although, the botanical origin of starch is importantwith respect to hydrolytic pattern, the surface area tovolume ratio is probably of more significance than theactual plant source. It is also possible that the anneal-ing process creates pores or fissures which alter thepattern of amylase hydrolysis from surface to internalerosion [74]. Hence, although amorphous and crys-talline lamellae become more ordered, accessibility tothe amorphous regions by enzymes is facilitated.

Annealing can be conducted in the presence of amy-lases to selectively hydrolyse amorphous regions andthe possibility of novel products with unique gelatinisa-tion and swelling characteristics [75,76]. Both potentialstarch and glucose syrup products are possible usingthis general approach.

10. Overview

Much data has been published on annealing ofstarches. Whilst the molecular basis is not absolutelycertain, these authors believe that the physico-chemicaldata are consistent with enhanced registration of amy-lopectin double helices within crystalline zones (withperhaps some slight enhancement of helical length) andgreater rigidity (more glassy) of amorphous regions.The consequence is that Tg and gelatinisation tempera-tures are increased, swelling is decreased and leaching isrestricted. The situation in high amylose starches isdifferent from waxy and normal starches where anneal-ing facilitates partitioning of amylose and amylopectindouble helices.

Annealed starch provides a very useful model toinvestigate amylopectin crystallisation. Whilst a verysimple technique, it provides a great deal of informa-tion about this process. There are important industrialimplications of the annealing process with relevance tostarch extraction and food production. As more sophis-ticated physical techniques are developed, the molecularbasis will be understood in far more detail.

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