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Resistant starch IIType 2 resistant starch (RS 2) includes granules of rawstarch of some plant species, e.g. potato or banana. Theresistance of raw potato starch to the activity of amylolyticenzymes was first observed in 1937 by a Polish scientist –Nowotny [Nowotny, 1938]. By subjecting the raw starch ofnumerous plant species to the activity of an amylolyticpreparation, Nowotny has observed the potato starch toundergo enzymatic hydrolysis to a small extent. Forty yearslater, similar results were reported by Japanese researchers[Fuwa et al., 1977; Sugimoto, 1980]. Further investigationshave also confirmed their results [Kelly et al., 1995].The phenomenon of raw starch resistance to the activityof amylolytic enzymes has not been fully explored yet. Theresistance of raw potato starch has been attributed to largesizes of its granules, hence the limited area of their availabilityto enzymes [Ring et al., 1988]. Still, the fine-grainhigh-amylose maize starch demonstrates the same resistanceto enzymatic activity as the coarse-grain potato starchdoes [Planchet et al., 1995]. Starch hydrolysis requiresenzyme adsorption on the surface of starch granules[Leloup et al., 1992]. Its extent is determined by the size andstructure of starch granule surface. The specific surface areaof potato starch granules is few times smaller than that ofcereal starches. It seems, therefore, that a considerablylower degree of enzyme adsorption on the surface of potatostarch granules than on the granule surface of some cerealstarches may be the reason for a difference in the susceptibilityof those starches to the activity of amylases. Norelationship has, however, been reported between theextent of enzyme adsorption on granules of different starchesand the degree of their hydrolysis [Kimura & Robyt,1995].Potato starch contains relatively high amounts of amylopectinand demonstrates a relatively high degree of crystallisation.The amylolytic enzymes first degrade the amorphousregions, hence crystallinity of starch granules couldhave been the reason for their resistance to the activity ofthose enzymes. Still, the degree of starch crystallinity is notalways linked with its resistance to the activity of amylases.Cereal starches, characterised by a high crystallinity degree(type A), are susceptible to the enzymatic activity, comparedto potato starch (type B), demonstrating a twofoldlower crystallinity degree, which is resistant to enzymatichydrolysis [Quingley et al., 1998]. An increasing amylosecontent of maize starch is accompanied by a decrease in itscrystallinity degree and increased resistance to enzymatichydrolysis. Waxy maize starch, containing nearly 100% ofamylopectin, demonstrates 40% crystallinity and is susceptibleto the activity of amylases. The crystallinity of highamyloseenzyme-resistant maize starch accounts for 15%[Brown, 1996]. That relationship does not, however, pertainto all types of high-amylose maize [Fujita et al., 1989].Potato starch, similarly to high-amylose maize starchresistant to the activity of amylolytic enzymes, crystallises tothe B form. Also starches of grain legumes, demonstratingthe crystalline pattern of C type being a mixture of forms Aand B, are to a high extent resistant to enzymatic hydrolysis[Garcia-Alonso et al., 1998]. Starch granules of differentplant species subjected to the activity of amylases have beenobserved with the use of an electron microscope [Fuwaet al., 1977; Sugimoto, 1980; Soral-Œmietana, 2000]. Thoseobservations indicated that the granules of cereal starch(type A) undergo deep enzymatic hydrolysis. They are centrallydegraded by a-amylase, which results in the formationof tunnels directed towards granule interior accompaniedby solubilisation of amorphous layers on the granule rim. Inthe case of potato starch granules (type B), the enzymescause damage only to the surface of the granules, and theiractivity results in the formation of small pits [Sugimoto,1980; Sarikaya et al., 2000]. It may indicate that the B typestarches, whose shape is formed by large “blocklets”, aremore resistant to the activity of amylases, compared to theA type starches.The resistance of starch granules to the activity of amylasesmay be enhanced by annealing of starch which consistsin keeping the starch for a longer period of time in waterwith a temperature lower than that of gelatinisation.Granules kept in water at a temperature of 20°C increasetheir volume by ca. 30%. At higher temperatures, water diffusioninto the granule interior is more intensive. Keepingstarch in water at an increased temperature results in waterpenetration into the granule interior, hence in a slightlyenlarged granule size. Hydrogen bonds are disrupted andwater particles bind to released hydroxyl groups. At temperatureslower than the starch gelatinisation temperature,it does not bring damage to the granules, but changes theirproperties. The resultant changes are determined by thebotanical origin of starch, temperature and time of annealing,as well as by the concentration of starch suspension inwater. The annealing of starches of different plant speciesresults in an increase in the crystallinity degree, andstrengthening of crystalline forms of the granules, ”stiffening”and ordering of starch chains both in the crystallineand amorphous layer. Those changes in the starch granulestructure evoke an increase in temperature of starch gelatinisationand increased enthalpy of that process [Leszczyñski,1992]. Annealing of potato starch induces additionalelongation of double helices and reduction of damagecaused to crystals [Genkina et al., 2003]. In maize starch, itis also responsible for the formation of new double heliceswith the share of amylose [Tester et al., 2000]. Starch componentsare observed to interact and granule stability isincreasing [Hoover & Vasanthan, 1994]. Those changesresult in decreased solubility and swelling capability ofstarch, and in increased granule resistance to the activity ofamylolytic enzymes [Hoover & Vasanthan, 1994, Thompson,2000].A similar effect can be obtained upon heating the granulesof high-amylose maize starch at a temperature of ca.100°C and moisture content below 35% [Ito et al., 1999].A decrease in a swelling degree and an increased resistanceto enzymatic activity obtained in this ”heat-moisture”process are likely to result from the formation of additionalcrystallites with the share of amylose, as well as from orderingand binding its chains in the amorphous regions[Hoover & Manuel, 1996]. In the case of potato starch,however, this process results in disorders of granule structure,change of the crystallinity pattern from B- to A- or C--type, and increased susceptibility to enzymatic degradation[Kuge & Kitamura, 1985]. Crystalline structures aredegraded, a degree of starch crystallinity decreases, anddouble helices are disrupted in the amorphous regions.This, in turn, results in a decreasing degree of granuleswelling, increasing temperature of their gelatinisation, andenhanced susceptibility to the activity of enzymes[Gunaratne & Hoover, 2002].
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