The DSC thermogram of a wheat flour–water mixture will show endotherms related to the transitions of starch. There is usually a difference in the gelatiniza- tion temperature range between the starch and flour, in that To,Tm, andTcare shifted toward higher temperatures in the wheat flour suspension compared to the starch–water suspension (16). The difference inTmcan be around 5°C. This means that components present in the flour will affect starch gelatinization. At low water content (water-to-starch ratio 0.70–0.72) there is also a difference in the appearance of the endotherm, in that the second peak (M1, see Fig. 2)be- comes much more pronounced in the wheat flour suspension (112). If the whole wheat kernel is heated, the DSC thermogram will show the endotherm expected for starch gelatinization at the actual water content (113). For a gelatinization endotherm to emerge it can thus be necessary to boil or steam the grain in order to increase the water content inside the kernel.
When a complete dough is studied it can be expected that other ingredients added to the dough will also exert an influence. This means that, for example, an increasing protein content can shift Tc of the starch to temperatures above 100°C; i.e., the starch will not be completely gelatinized during the baking pro-
cess. If this is the case, this will have an effect not only on the rheological proper- ties of the baked product, but also on the retrogradation behavior of the starch (52, 114). Ungelatinized starch can also be of interest from a nutritional point of view, for nongelatinized starch has a lower bioavailability than gelatinized starch (115). Certain ingredients are added with the aim of decreasing staling, and they should thus be expected to influence the retrogradation-related endo- therm. The addition of emulsifiers could be expected to show up in a transition related to the amylose–lipid complex, and the addition of enzymes could also be expected to result in changes in the thermogram. So far, no systematic study has been presented on the influence of mixing and other processing conditions on the DSC thermogram, although it has been reported that the amount of freezeable water was higher in a fully developed dough than in unmixed flours at the same moisture level (108).
1. Gluten
The main component, besides starch, in flour is of course protein(Table 1).As discussed earlier, the gluten proteins do not give rise to any detectable endotherms during the DSC scan, but they will shift To andTm for the thermal transitions related to starch gelatinization (116). It was found that for the two peaks obtained at water conditions where the gelatinization endotherm has the shoulder (G and M1 in Fig. 2), gluten causes a linear increase in the peak temperatures when added to wheat starch, at least at a level of addition up to 0.4 g gluten/g starch.
For the starch and gluten used in that investigation, the following relation was obtained for the G endotherm (116):
Tm(°C)⫽60.48⫹7.54∗GS (1)
where GS is the amount of gluten expressed as g gluten/g starch. Gluten from a flour of poor baking performance affected the temperatures in the same way as the gluten from the flour of good baking performance used for Eq. (1). It was also found that the enthalpy (∆H) for the G endotherm decreased with increasing gluten level:
∆H⫽ ⫺1.76∗GS (2)
When starch and gluten were mixed in a 1 : 1 ratio and analyzed at excess water conditions in the DSC, a slight increase inToandTmwas observed (117). How- ever, when the water-soluble fraction (obtained when the wheat flour was hand- washed into prime starch, tailings starch, gluten, and water solubles) was mixed with starch, the effects were much greater:Toincreased from 57.8 to 65.9°C, and Tmfrom 64.1 to 71.8°C. In the case of the starch : gluten mixture,Tcxwas found to decrease, whereas it increased for the mixture of starch and the water-soluble
fraction. The effect of the water-soluble fraction could be understood from its content of solutes, including sugars.
When the retrogradation was studied for wheat starch–gluten mixtures it was found that ∆Hc decreased somewhat in the presence of gluten, especially after long times (7 days) and high levels of gluten (0.4 g gluten/g starch) (118).
The source of gluten did not seem to influence the retrogradation significantly.
Determination ofTgfor 1 : 1 mixtures of gluten and amylopectin was used to study their miscibility (80). At most conditions, two well-separated transitions occurred, with theTgat the highest temperature attributed to amylopectin or an amylopectin-rich phase, andTgat the lowest temperature attributed to the gluten- rich phase. When the water content of the mixtures increased, the twoTgvalues approached each other and eventually overlapped. It was concluded that gluten and amylopectin could then be miscible under certain conditions.
2. Polar Lipids
When a wheat flour–water suspension is heated in the DSC, an endotherm due to the transition of the amylose–lipid complex is detected. This endotherm usually increases in size after a second heating (27), and the presence of uncomplexed lipids in the wheat flour explains an increase in ∆Hcx for flour, compared with starch (16, 89). Part of the complexation is thus believed to occur during the heating in the DSC. When a flour is heated, more lipids are present that can form the complex than when starch is heated. In wheat starch the main polar lipid is lysolecithin, whereas in the wheat flour other polar lipids are also present (109).
The∆Hcxof the complex is affected in cereal processing. Drum drying has been found to increase∆Hcx(89), whereas extrusion cooking seems to decrease
∆Hcx (119). In this connection the presence of different polymorphic forms of the complex is of interest (120). Depending on the processing conditions, com- plexes of different crystallinity could be expected, and it could be speculated that such complexes will differ in digestibility (120).
3. Nonstarch Polysaccharides
When 1 or 2% of a soluble arabinoxylan fraction was added to wheat starch at water contents corresponding to the situation in dough (50.8–53.1% water),Tm
increased whereas∆Hgelwas unaffected (121). The increase inTmwas 2°C when 2% of the soluble arabinoxylan fraction was added. When the water content was much higher (around 75%) there was no observable effect of the added arabinoxy- lan. For 40% (w/w) aqueous wheat starch slurries, the addition of 1% arabinoxy- lan or 1%β-glucan did not significantly change theTm, whereas the gelatinization temperature range and∆Hgelincreased (122). The transition of the amylose–lipid complex was not affected by the addition of nonstarch polysaccharides.
When the retrogradation behavior of wheat starch–arabinoxylan mixtures was studied, it was found that an increase in retrogradation could be obtained at certain conditions (121). The interpretation of the results is complicated because the addition of an arabinoxylan fraction to starch will also change the dry matter content, and this will affect starch retrogradation, as discussed earlier.