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.Several models have been pro-posed to predict the influence of water on melting transitions in starch.In earlierstudies, the Flory Huggins equation (52), which is used to describe the melting-temperature-depressing effect of diluents in synthetic polymers, was applied topredict the melting temperature of starch as a function of water content (5355).Several investigators reported that melting temperatures predicted using theFlory Huggins equation were in agreement with the experimentally determinedvalues for volume fractions of water between 0.1 and 0.7 (42).However, a sig-nificant deviation between theoretical and experimental melting temperatures wasreported for rice starch above a volume fraction of water of 0.7.Also, the meltingtemperature for dry starch was underestimated (54).Furthermore, the presenceof more than one endotherm in DSC thermograms cannot be predicted or ex-plained using the Flory Huggins equation.After the pioneering work of Sladeand Levine (50), it was recognized that starch melting is a nonequilibrium phe-nomenon and, therefore, it cannot be analyzed by Flory Huggins theory basedon equilibrium thermodynamics.Slade and Levine s study (50) on wheat andCopyright 2003 by Marcel Dekker, Inc.All Rights Reserved.waxy corn starch with 55% (wt) water demonstrated that the melting process isirreversible, kinetically controlled, and mediated indirectly by water plasticiza-tion, as it affects the stability of glassy regions.The proposed   fringe micelle network model for starch structure supports the observation of a glass transitionpreceding the crystalline melting, as the melting of interconnected microcrystal-lites depends on the mobility of the continuous glassy regions on which waterexerts a plasticizing effect.Observation of more than one melting endotherm atlimited moisture content ( 25 60%) can be attributed to nonuniform moisturedistribution or the presence of microcrystalline domains with different thermalstabilities.Gelatinization enthalpies must be interpreted with caution when used asindices of starch crystallinity, because they represent net thermodynamic quanti-ties of different events: granule swelling and crystallite melting (endothermic)and hydration and recrystallization (exothermic).Furthermore, significant contri-butions to the "H value from amorphous regions have been also suggested (42).Gelatinization enthalpies are used to quantify the starch modification as a resultof extrusion processing.The degree of starch conversion (DC) is described bythe reduction in the area of the gelatinization endotherm before and after extrusion(56, 57):"H0 "HiDC(%) 100 (8)"H0where "H0 is the enthalpy of gelatinization of the native starch and "Hi is theenthalpy of gelatinization of the extruded starch.Starch conversion is sometimes used synonymously with degree of gelatini-zation (58).It should be remembered that the application of mechanical energyduring extrusion results in structural changes to starch, including granule disrup-tion and fragmentation of starch molecules into smaller-size polymers.Althoughsome gelatinization is expected, the reduction in the gelatinization peak area can-not be attributed only to the gelatinization of native starch during extrusion.C.Amylose Lipid ComplexLipids are present naturally in cereal flours in varying amounts (1.2% in wheatflour, 1.4% in degermed corn flour, 5 7% in oat flour).Furthermore, lipids areadded during extrusion processing, mostly in the form of emulsifiers.The addi-tion of emulsifiers is reported to modify the product characteristics, includingexpansion, cell size and distribution, and texture (59 61).Emulsifiers are re-ported to form complexes with amylose during extrusion processing (62).Extru-sion operating conditions influence the creation of a product, with its structureand physical properties both depending on the severity of treatment and on theCopyright 2003 by Marcel Dekker, Inc.All Rights Reserved.provision of suitable conditions for the ingredients to react with each other.Co-lonna et al.(63) claim that molecular modification is reduced because lipids mayact as lubricants.Each type of lipid has a distinct effect on the material propertiesduring extrusion processing that requires modification of the extrusion operatingparameters.Most emulsifiers are compounds that have both hydrophobic and hydro-philic ends on the same molecule.Glycerol monostearate (GMS) and sodiumsteroyl lactylate (SSL) are two small-molecular-weight emulsifiers commonlyused in food applications.The hydrophobic ends of emulsifiers are believed toform a complex with the amylose fraction of starch during cooking, retardingstarch gelatinization and decreasing swelling.Starch granule swelling and solu-bility decline with an increase in complex formation (64).Galloway et al.(62)reported that the formation of amylose GMS complex resulted in a decreaseddegree of gelatinization, water solubility, and expansion of wheat flour extrudates.Scanning electron microscopy studies of wheat flour extrudate microstructureshowed that addition of GMS and SSL to wheat flour extrudate increases thesize and uniformity of the cells (60).Amylose forms complexes with iodine (65), alcohols (66), and fatty acids [ Pobierz całość w formacie PDF ]