Gelatinisation starch

The important issue is to produce a layer between the bubbles that is sufficiently stable that the bubbles do not burst either before the food is cooked or served. In bakery products the foam has to be stable until the surrounding proteins have been denatured and the starch gelatinised. 

Once the final expansion has occurred the proteins start to denature and the starch gelatinises. The proteins also undergo Maillard reactions, particularly at the outside of the product, which is the hottest part. 

Rye Starch. Rye starch gelatinises at or around the temperature at which a-amylase has its maximum activity (55-70°C). a-Amylase activity tends to be high in rye flour so steps have to be taken to minimise it. One step is to acidify the dough either by adding acid or... 

Cooke, D., Gidley, M. J. (1992). Loss of crystalline and molecular order during starch gelatinisation origin of the enthalpic transition. Carbohydrate Research, 227, 103-112. 

If amylose and amylopectin are mixed in water, an amylose-rich upper phase and an amylopectin-rich bottom phase are formed. We can regard the starch gelatinisation itself as such a phase separation, where amylopectin remains in the water-swollen granules and amylose forms the outside solution. It is interesting to see how the shape of the granules changes successively during this process, which is illustrated in the case of wheat starch in Fig. 8.6. Thus there is a clear tendency of deformation so as to obtain an outer surface with... 

In short, the kneading of dough introduces air cells, and during fermentation the carbon dioxide will increase the volume of these cells until a foam is obtained. In the oven the foam opens to give a pore system and the structure is "fixed" due to starch gelatinisation. Tlte process involves different steps where proteins and lipids compete at the interface, which we will consider in more detail. 

Donald model of starch gelatinisation on cooking, (a) in limiting water (b) in excess water. 

Due to the significant difference in the mid-infrared spectra of proteins and polysaccharides, it is possible to study simultaneously and independently their behaviours in mixtures.6 In this study ATR-FTIR was used to monitor starch gelatinisation and the evolution of the water content dependent hydrogen bonding in the gluten fraction of starch/gluten mixtures as a function of heating. 

Figure 1 shows the variation in the ratio 1047/1020 cm obtained for a wheat flour/water dough, heated at 2°C/min, plotted as a function of temperature, compared to a DSC experiment performed on the same sample with identical heating rate. The decrease in the ratio 1047/1020cm-1 was closely correlated with the DSC data for flour-water mixture. This finding validated the use of FTIR to follow starch gelatinisation in situ. 

Lipids (fats and oils) and monoacylglycerols used as emulsifiers form inclusion compounds with amylose, slow down the swelling of starch granules and the extent of starch gelatinisation. For example, around 96% of starch is fully gelatinised in white bread, which is low in fat. Bakery products rich in fat, especially in the surface layers with lower water activity, contain a considerable proportion of ungelatinised starch. Small concentrations of sodium chloride have only limited impact on gel formation. 

A typical symptom of retrogradation is aging (hardening) of bread and pastries. Re-heating of such bread or bakery products, for exampe in microwave ovens or during bread toasting, causes new starch gelatinisation, but such products harden very quickly. 

Starch gelatinisation process (a) raw starch granules made up of amylose (linear) and amylopectin (branched) (b) addition of water breaks up crystallinity and disrupts helices (c)) addition of heat and more water causes swelling, amylose diffuses out of the granule (d) granules, mostly containing amylopectin, are collapsed and held in a matrix of amylose (adapted from Lai and Kokini, 1991). 

Liu Q, Charlet G, Yelle S, Anil J, (2002) Phase Transition in Potato Starch-Water System I. Starch Gelatinisation at High Moisture Level . Food Research International, v35 n4 397-407. 

A particularly profitable line of enquiry involves the exact temperatures at which order, as measured by the various techniques, is lost. Gidley [9] has looked at this in detail in order to shed light on the origin of the endotherm in starch gelatinisation. Table 9.2 shows a comparison using several techniques of the overall level of order in starches. 

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