Figure 4(13) shows typical DSC thermograms for representative samples (1 : 1 mixtures with water) of a cookie/cracker flour, a commercial, rotary-molded, high-sugar cookie, and a commercial saltine cracker. Wheat flours of the type used to make such baked goods typically contain about 13% moisture (wet basis) and approximately 85% starch (dry basis), the latter in the native form of partially crystalline, partially amorphous granules containing the two starch polymers amylopectin (Ap) and amylose (Am) (14). Thus, the flour sample represented in Figure 4A comprises a 40% starch-in-water slurry, and the appearance of the DSC thermogram is that widely recognized to be typical of native granular wheat starch in mixtures of approximately 1 : 1 with water (8–10, 14 and references therein). The characteristic biphasic endotherm, with onset temperature (T) of 53°C, completionT of 88°C, peak T of 64°C, and shoulder at about 75°C, is generally acknowledged to represent a combination of glass transition, of water-plasticized amorphous regions, followed by nonequilibrium melting, of microcrystallites of the partially crystalline glassy Ap in the ‘‘fringed-micelle’’
structure of the granules of native wheat starch, in a process known as starch gelatinization (1, 5, 8–11, 26–31 and references therein). The appearance of this gelatinization endotherm in the thermogram inFigure 4Asignifies, as expected, that the starch in the flour was native, until it was gelatinized as a consequence of heating to 90°C during the DSC measurement. The characteristic higher-T endotherm, with onsetTof 89°C, completionTof 117°C, and peakTof 104°C, is recognized to represent the melting of amylose-lipid (Am L) crystalline inclusion complex [1, 5, 8–11, 32 and references therein].
InFigure 4B,the DSC thermogram for the high-sugar cookie sample shows two principal endothermic events, aside from a pair of small fat-melting endo- therms below 40°C (representing the fat ingredient in the cookie formula, which
Figure 4 Typical DSC curves for representative samples (1:1 mixtures with water) of:
(A) a cookie/cracker flour; (B) a commercial rotary-molded high-sugar cookie; and (C) a commercial saltine cracker. See text for explanation of peak labels. (From Ref. 13.)
is not of interest in this discussion). The smaller of the two endotherms of interest, occurring above 100°C, had previously been observed in the thermogram of wheat starch isolated from a wire-cut cookie, as reported by Kulp et al. (seeFig.
6in Ref. 16). As described forFigure 4A,this endotherm represents the melting of crystalline Am L complex. The larger endotherm, with onsetTof 66°C, com- pletionTof 88°C, and peakTof 77°C, resembles the gelatinization endotherm inFigure 4A(same completionT), although it is obviously narrower and shifted to higher peakT. In fact, the appearance of this endotherm is characteristic of that for the gelatinization of native granular wheat starch in the presence of sucrose-water solution rather than water alone (8). Based on the known amounts of flour and sucrose in the cookie formula, and thus the corresponding known amounts of flour, sucrose, and water contained in the DSC sample pan, the ap- pearance of this endotherm can be explained, to begin with, by recognizing that the sample pan contained, in essence, a 1 : 4 mixture of starch and 20% sucrose solution.
It is well known that the presence of sucrose causes the gelatinization tem- perature of starch [taken as the peakTof the gelatinization endotherm inFigure 4A(8–11 and references therein)] to be elevated (29–31). This effect of sucrose has been explained by a concept of ‘‘antiplasticization’’ (by sugar-water relative to water alone) (8), which has received wide support in recent years (28, 33 and references therein). According to this ‘‘antiplasticization’’ mechanism (8), sugar, in the presence of native starch and excess water, behaves as a plasticizing cosol- vent with water, such that the sugar-water coplasticizer, of higher average molec- ular weight (MW) [and lower free volume, so higher glass transition temperature (Tg) (34)] than water alone, plasticizes [i.e., depresses the temperature of the glass transition of the amorphous regions, which immediately precedes the gelati- nization of native, partially crystalline starch (8, 9, 26)] starch less than does water alone. Thus, the gelatinization temperature (as well as theTgthat precedes it) in the presence of sugar is elevated (hence, ‘‘anti’’) relative to the gelatiniza- tion temperature of starch in water alone. Moreover, with increasing concentra- tion of sugar in the three-component mixture [thus, a sugar-water coplasticizer with increasing average molecular weight (MW), decreasing free volume, and increasingTg, relative to water alone], the magnitude of the antiplasticizing effect increases, and so doTgand the gelatinization temperature (8).
Thus, based on previously reported DSC results for native granular wheat starch in mixtures with sucrose solutions of varying concentration (8), the major endotherm inFigure 4Bfor the cookie sample is interpreted as representing the gelatinization of starch in 20% sucrose solution. This interpretation was con- firmed by DSC analysis of a sample of flour prepared so as to represent a 1 : 4 mixture of starch in 20% sucrose (thermogram not shown). Once normalized for sample weight, that thermogram for flour (containing fully native starch) in 20%
sucrose was found to be essentially identical in appearance to the one inFigure 4B (except for the fat-melting peaks below 40°C) for the 1:1 mixture of high- sugar cookie in water, in terms of both areas and temperature ranges for both of the endothermic peaks (labeled Ap and Am L) inFigure 4B.It should be noted that we prefer to express peak area in terms of deltaQ(for the change in total heat uptake), rather than the conventional deltaH(change in enthalpy) terminol- ogy used by the PE DSC 7 software (and unavoidably listed as such in the print- outs from the instrument), when a peak is known to comprise multiple thermal events, such as the glass and crystalline melting transitions represented within the gelatinization endotherm of starch (8, 9, 26). Further comparison of the peak areas for the Ap and Am L endotherms [first normalized for total sample weight, and then further normalized for flour (and therefore starch) weight] in the thermo- gram for flour : 20% sucrose [or in the equivalent (after normalization) thermo- gram in Figure 4B] with the corresponding peak areas in the thermogram for flour : water in Figure 4Ademonstrated that both the Ap and Am L peak areas inFigure 4Brepresent 100% of those for the native wheat starch represented in Figure 4A.This result indicates that the starch in the baked cookie was completely native prior to DSC analysis and was first gelatinized during DSC heating, thus demonstrating that the starch was not gelatinized at all during baking of this high- sugar cookie. This finding is in agreement with the conclusion reached previously in other studies (2, 3, 16, 29–31). To anticipate a question about why, if the native starch represented inFigure 4Bcould be completely gelatinized by heating to 88°C in the DSC, was it not gelatinized during baking of the cookie, wherein the internal temperature reached approximately 100°C, we point out that the gelatinization temperature of 77°C inFigure 4Bwas measured for starch in a 1 : 4 mixture with 20% sucrose solution. Thus, for the 20% starch slurry, plasticiza- tion by the sucrose solution (present in fourfold excess) depressed the gelatiniza- tion temperature down to 77°C. In contrast, during baking of the cookie dough (with a dry-basis composition of 87 : 54 : 35 flour : sucrose : water), the native starch in the flour was in an aqueous environment composed of a small excess of total solvent [⫽sum of sucrose⫹water (4)] comprising a 61% sucrose solu- tion. Under these conditions (i.e., less effective plasticization of the starch, by a lesser amount of a more concentrated sucrose solution), the gelatinization temper- ature would be expected, based on previously reported DSC results for starch : sucrose : water model systems, to be elevated to well above 100°C (8). Therefore, the native starch of the flour in the cookie dough would be unaffected (i.e., not gelatinized at all) by the maximum temperature reached by the cookie during baking.
InFigure 4C, the DSC thermogram for the saltine cracker sample shows two principal endothermic events, aside from a broad fat-melting endotherm, centered about 35°C (representing the fat ingredient in the cracker formula, which
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again is not of interest in this discussion). The smaller of the two endotherms of interest, with onsetTof 96°C, completionTof 118°C, and peakTof 109°C, is again identified (as inFigs. 4A and 4B)as that representing the melting of crystal- line Am L complex. The larger endotherm, with onsetTof 65°C, completionT of 95°C, and peakT of 75°C, again resembles the gelatinization endotherm in Figure 4A,although it is obviously smaller in peak area and shifted to a higher temperature range. We can begin to explain these differences by first noting that a saltine cracker is typically formulated with no added sugars (the presence of which would elevate the starch gelatinization temperature) and with enough water [i.e., at least about 27 parts water to 73 parts dry wheat starch (8)] in the dough (at least, early in baking) to allow starch in the flour to gelatinize during baking of the cracker, wherein the internal temperature reaches at least about 100°C. If we assume that the peak labeled Ap inFigure 4C represents what remained of the full gelatinization endotherm inFigure 4A,after some but not all of the starch in the cracker was gelatinized during baking, we can calculate the percentage remaining native Ap structure, by comparing the Ap peak areas inFigures 4C (1.49 J/g) and 4A (3.92 J/g) and then normalizing first for total sample weight and second for flour (and thus starch) weight. We obtain a value of 40%, indicat- ing that the extent of starch gelatinization during baking of the cracker was 60%.
Apparently, it could not reach 100%, because, as the content of plasticizing water in the dough decreased as baking progressed, the gelatinization temperature would have increased, evidently to well above 100°C by the end of baking. Since not all of the starch was gelatinized during baking, the portion remaining native was evidently subject to annealing (at temperatures within the range fromTgto Tmat the end of Ap crystallite melting), due to the heat-moisture treatment consti- tuted by baking (1, 8, 9, 26). The expected effect of this annealing treatment (35) is manifested by the Ap peak inFigure 4C,which is narrower by about 5°C and up-shifted by about 10°C, relative to the corresponding Ap peak inFigure 4A.
As with the Ap peak areas, we can compare the Am L peak areas inFigures 4A (0.63 J/g) and 4C (0.71 J/g), and, after the same normalizations as before, we obtain a value of 121% native Am L structure in the cracker sample represented inFigure 4C.Thus, evidently as a consequence of gelatinization of some of the granular starch during cracker baking, some previously uncomplexed amylose was made available for forming additional Am L complex (32) in the cracker.
The Am L peak inFigure 4Cis narrower by about 6°C and up-shifted by about 5°C, relative to the corresponding Am L peak in Figure 4A, apparently as a consequence of the same annealing treatment during baking, which similarly in- fluenced the Ap peak. As a final remark about the thermogram inFigure 4C,we point out what we view as the salient DSC features of this cracker sample (which is taken to represent an excellent eating-quality, commercial product with opti- mum properties): 40% remaining native Ap structure and 121% native Am L structure. As will be developed further in the discussion ofFigure 5that follows,
these features illustrate the basis of our application of DSC analysis as a diagnos- tic ‘‘fingerprinting’’ method that has allowed us to relate starch structure and thermal properties to starch function in, and associated finished-product quality of, low-moisture baked goods (13).