A broad range of ingredients is added to a wheat flour dough, and they can all be expected to exert an influence on the gelatinization behavior of the starch and thus have an influence on the DSC endotherm. Moreover, some added ingredients can certainly give rise to a transition of its own in the DSC thermogram, thus complicating the picture even more.
1. Sugar
When sucrose or any other mono- or disaccharide is added to starch, an increase in bothToandTmoccurs (48, 123, 124). How large the shift will be depends on the type of sugar and on the concentration. When the DSC scan is performed at water conditions giving rise to the double endotherm for starch gelatinization (for example, at a starch : water ratio of 1 : 1), the double endotherm is shifted into a single endotherm at a starch : water : sugar ratio of 1 : 1 : 1 (124, 125). At sugar levels high enough (1.7 g sucrose/g starch in the case of potato starch), almost the whole gelatinization temperature range is shifted to temperatures above 100°C. For products with a high sugar content, part of the starch might thus be ungelatinized even when the temperature has reached 100°C during baking.
The amount of unfreezeable water was determined for starch suspensions in the presence of sugars, and it was found that the onset temperature of gelatini- zation (determined using microscopy) increased when the amount of unfreezeable water increased (125).
The addition of glucose or glucose syrup will shiftTgof gluten to higher temperatures, compared to the hydrated gluten alone (106).
2. Salt
The effect of added salt on the thermal transitions is rather complicated. For certain salts and concentrations, a decrease inToandTmis observed, whereas for other salts or other concentrations an increase is observed (48, 126). However, in the case of NaCl, effects are observed at concentrations that are not possible for use in bread.
3. Lipids
The presence of endotherms due to the melting of fats or monoglycerides can be a problem when the effect of these components on gelatinization or retrograda- tion of starch is studied. One way to overcome this problem is of course to use lipids containing unsaturated fatty acids. For applications involving triglycerides, the presence of fat crystals must then not be a prerequisite for the effect (127).
In case of monoglycerides, the complexing ability with amylose could be impor- tant, and an unsaturated monoglyceride could be expected to perform more poorly than the saturated monoglyceride. However, the poorer complexing ability is a result of the phase behavior of the monoglyceride and not necessarily of its con- figuration (128). Therefore, unsaturated monoglycerides can be used as models for the saturated ones as long as the conditions are such that the complexing is made possible. Monoglycerides can be transferred into liposomes by dispersing them in sodium cholate solutions in order to facilitate the complex formation (128).
The change in DSC gelatinization characteristics observed for some lipid additives are given inTable 7.The addition of an emulsifier like sodium stearoyl lactylate (SSL) results in an increase inToandTm, whereas lysolecithin decreases these temperatures. The effect of saturated monoglycerides (SMGs) is somewhat difficult to interpret because of the overlapping between the endotherm due to the melting of SMG and the gelatinization endotherm. The decrease in∆Hgelhas been interpreted as a result of an exothermic complex formation occurring in the
Table 7 Effect of Added Lipids/Emulsifiers on the Gelatinization Characteristics of Wheat Starch Measured by DSC at Different Water Contents
Change in
Additivea To(°C) Tp(°C) ∆Hgel(J/g)
Starch : water⫽1 : 3
SSLb 1.4 1.0 ⫺2.4
SMGc ⫺0.3 ⫺0.8 0.1
Lysolecithin ⫺1.3 ⫺0.7 ⫺5.2
Lecithin ⫺0.3 ⫺0.5 ⫺0.5
Starch : water⫽1 : 1
SSLb ⫺0.5 3.3 ⫺4.0
Lecithin ⫺3.5 ⫺0.3 ⫺1.2
aAddition at the 5% level, calculated on starch.
bSodium stearoyl lactylate.
cSaturated monoglycerides.
Source: Data compiled from Refs. 16 and 50.
same temperature range as the gelatinization (27). That lecithin also causes a decrease in∆Hgelindicates that even a diacyl lipid might take part in complex formation, although not to the same extent as the monoacyl lipids (50).
When wheat starch is heated in corn oil, no transitions at all are detected in the DSC thermogram (129). The addition of low levels of shortening (6%
calculated on flour) does not influence the gelatinization of starch (112). Even an increase in the amount of triglyceride to 50% will not influence the gelatinization behavior of starch. It was found that at a wheat flour–water ratio of 1 : 1 and at a level of addition of oil of 50% calculated on wheat flour, no effects onTo,Tm, or∆Hgelwere obtained (127). However, if the fat was added dispersed as an L2 phase, the gelatinization endotherm decreased in size and shifted into a higher temperature range (130). At high levels of added triglycerides (coconut oil), an endotherm due to the melting of the fat is observed (131). Depending on the mixing temperature and hydration, the appearance of the thermogram will differ.
4. Enzymes
The effect of pretreatment of barley starch withα-amylase was investigated by DSC. Starch was kept for 3 h at 50, 55, or 60°C, either with enzyme added or without (132). There was a slight increase inTmfor the enzyme-treated starches, whereas the∆Hgelvalues did not differ significantly.
When wheat starch was annealed, i.e., kept at a temperature belowTo, an increase inTo andTmwas observed, whereas∆Tdecreased and∆Hgelremained unchanged (133). Pancreatin hydrolysis of the starches caused a slight increase inTo and Tmof native starch, whereas the DSC thermograms of annealed and hydrolyzed starch were essentially unaffected. If the starch was hydrolyzed before annealing, the annealing effect onToandTmincreased.
5. Hydrocolloids
The influence of hydrocolloids on the gelatinization behavior of starch has been investigated for some cereal starches, including corn (68). It was found thatTo, Tm, andTcincreased with increasing levels of hydrocolloid, resulting in a broad- ening of∆T.∆Hgelwas found to increase, but because the water content changed at the same time as the addition of the hydrocolloid, the interpretation was diffi- cult. However, when the samples were compared at the same volume fraction of water it was found that∆Hgelwas independent of the addition of a hydrocolloid.
The change in gelatinization temperature was interpreted as showing that water was less available for the starch in the presence of the hydrocolloid. The additions of certain hydrocolloids (guar gum and xanthan gum) at the 1% level did not affect the gelatinization peak temperature, whereas the gelatinization temperature range and ∆Hgel increased (122). The transition of the amylose–lipid complex was not affected by the presence of hydrocolloids.
IV. RELATIONS BETWEEN THERMAL BEHAVIOR OF STARCH AND PRODUCT QUALITY
The thermal behavior of cereal components, or more or less complicated doughs, is of course studied in order to learn more about the baking process and the baked product. In this section a few examples will be given on how thermal behavior has been used to understand, and even predict, the properties and quality of the end product. The staling of bread, which is routinely assayed using DSC, will be described in the next section.