Background

Abboud and Hoseney (14, 15) were among the first to apply DSC to cookie doughs—in effect, using the instrument as a ''microbaking'' oven to record the occurrence of thermal transitions. Typical sugar-snap cookie doughs (60% sucrose, flour weight basis [f.b.]) produced three endothermic peaks, identified as fat-melting (30°C), sugar-dissolving (70°C), and starch gelatinization (120°C). Although evidence was later presented showing that the third, high-temperature peak actually resulted from water vaporization (Ref. 2 and infra), it was noteworthy that no starch gelatinization endotherm was present below the maximum internal temperature reached by doughs during baking (about 100°C); this finding explained the virtual absence of gelatinized starch in such cookie products.

Kulp et al. (16) reported similar results in a DSC study of wire-cut cookies (45% sucrose, f.b.), namely, that starch in the cookie dough remained unchanged during the baking process. The DSC curves ''showed no apparent difference'' in peak areas for both gelatinization and amylose-lipid melting, between starch samples isolated from cookie flour and from the corresponding baked products. In confirmation of the DSC results, no significant differences were found between the two isolated starch samples when examined by scanning electron microscopy (SEM) and when tested for such physicochemical properties as viscosity (by Visco-Amylograph), swelling power and solubility, and iodine affinity.

Wada et al. (17) evaluated cracker-type products prepared from model doughs containing gluten/corn starch mixtures and no added sugar. The

DSC curves of the doughs differed according to the type of corn starch used: In doughs with high-amylose corn starch (HACS), no thermal events were visible up to about 120°C, other than a fat-melting peak at 36°C; with waxy corn starch (WCS), however, a broad, shallow gelatinization endotherm starting at about 88°C and peaking at about 114°C also appeared. This difference was reflected both in the degree of gelatinization (by enzymatic assay) of the baked products obtained from HACS and WCS doughs (i.e., 0.8% and 2.0%, respectively) and in the gelatinization temperatures of the two native starches (lower for WCS).

Doescher et al. (18) used DSC to evaluate the effects of various cookie dough ingredients on the Tg of commercial hard wheat gluten. Formula water alone (20%, gluten basis) produced a significant depression in the gluten Tg, from 82°C originally down to 33°C; sodium bicarbonate (around 1%) shifted the Tg nearly back up to its original temperature; glucose (60%) in water also shifted the Tg upward, but only to 68°C; the combination of water (20%), glucose (60%), soda (1%), salt (1%), nonfat dry milk (3%), and fat (30%) produced nearly the same shift in Tg (to 66°C), as did the water-glucose pair. Results of their study led these workers to propose a mechanism for the ''setting'' of cookie dough during baking: i.e., gluten goes through a glass (to rubber) transition and expands into a continuous web, causing an increase in viscosity sufficient to halt further flow of the dough piece (19). This hypothesis has met with some disagreement, based in part on data that indicate the actual occurrence of a reduction in cookie dough viscosity during baking (5, 13).

Piazza and Schiraldi (20) investigated the effect of dough resting time on the Tg of semisweet biscuit doughs (19% sucrose, f.b.). They determined by DSC analysis that the Tg of doughs allowed to rest for 120 minutes after mixing increased by about 2°C over the Tg of freshly mixed doughs. The increase in Tg conformed to rheological data showing comparable changes with resting time in elongational viscosity, tensile elastic modulus, and tensile stress at break, changes that implied that structural ripening and the growth of protein networks (gluten polymerization) had occurred in the doughs during resting. Perhaps unexpectedly, in three-point bending tests on the baked biscuit products, values for elastic modulus decreased with dough resting times—i.e., the biscuits made from rested doughs were more leathery and lower in breaking strength. Since the moisture content of the baked products also increased with dough resting times (from about 0.2% for 1-minute doughs to 2.3% for 120-minute doughs), plasticization of the biscuit matrix by residual water, which reduces the Tg (1), had evidently occurred to a greater extent in products obtained from the longer-resting doughs.

In a study of moisture effects on the textural attributes of model flour-water crackers, Given (21) used DSC to determine the unfreezable water content (UWC) of samples prepared with variations in dough moisture, oven temperature, and mixing time. The maximum UWC was essentially constant (ca. 26%) and well above the baked sample moistures, which ranged from 3 to 20%; hence, all of the water present in the test crackers was unfreezable. Moisture content, i.e., UWC, correlated with sensory texture scores for perception of moistness (R2 = 0.96), fracturability (-0.90), and hardness (-0.73). It was suggested that unfreezable water, acting as a plasticizer, effected the incremental reductions in Tg of the cracker system, which were responsible for the macroscopic changes in texture. Aubuchon (22) demonstrated by modulated DSC (MDSC) that the Tg of a commercial cookie sample decreased from 60°C when fresh from the package to below room temperature (about 10°C) after overnight exposure to ambient conditions, which resulted in moisture uptake and plasticization. And MDSC was reported to be more effective than standard DSC in resolving the glass transition of this product.

Thermal analysis methods other than DSC have also been used for the product types discussed in this chapter. Nikolaidis and Labuza (23), for example, have studied the glass transitions of commercial crackers and their doughs by means of dynamic mechanical thermal analysis (DMTA). The Tg of crackers was reported to decrease with increasing moisture content, from ca. 155° to 12°C, a drop of 15°C per 1% moisture. Tg-vs.-moisture curves were very similar for the baked cracker and its dough, an indication that oven temperatures did not significantly modify ingredient functional behavior and that gluten or its glutenin fraction (rather than gelatinized starch) was primarily responsible for the mechanical behavior of the dough.

Miller et al. (19) determined the apparent Tg of model cookie dough systems by use of a thermomechanical analyzer (TMA), which also was reported to be more sensitive than DSC for lower-intensity thermal transitions, such as the glass transition. Doughs (25% moisture) made with hard wheat flour had a lower apparent Tg, compared to doughs made with soft wheat flour (78° vs. 71 °C). The difference between doughs could not be attributed to inherent differences in flour properties, however, since Tg values for the two flours were identical (ca. 30°C, at 13% moisture). Decreasing the level of sucrose in the doughs from 60% (control) down to 50%, 40%, or 30% (f.b.) brought about corresponding decreases in Tg as well as in cookie ''set time'' and diameter.

In the following sections, we review an application of DSC as a diagnostic assay, or analytical ''fingerprinting'' method, that has been used to characterize the thermal properties of wheat starch in low-moisture, wheat-flour-based baked products, including cookies, crackers, and hard pretzels (13). This use of DSC has enabled us to relate starch thermal properties, on the one hand, to starch structure, and on the other hand, to starch functionality, in terms of baking performance and finished-product quality. Such DSC ''fingerprinting'' has been used as an aid to successful product development efforts (e.g., Ref. 25), by identifying matches between appropriate ingredient functionality/baking performance and superior finished-product quality.

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