Principles of Cereal Science and Technology, 3rd Edition.pdf

Fig. 6.4. Glass

1990)

transition temperature ( T g) of zein protein as a function of moisture. (Reprinted from Lawton

Fig. 6.5. Farinogram

of a zein-starch dough mixed at 35°C. The r esistance of the dough against the rotating blades of the farinograph instrument is measured as torque, expressed in farinograph units (FU), and recorded and plotted on-line as a function of time. (Reprinted from Lawton 1990) STARCH

Starch, as it occurs in nature, is a partially crystalline polymer system (see Chapter 2). As such, it should go through a glass transition. The granules take up only about 30% water at room temperature. Figure 6.6 shows its T g as a function of moisture content. When gelatinized, all the crystals are melted, and the resulting starch freely hydrates and swells in water. With excess water, the T g of gelatinized starch has been shown to be about −7°C. When the T g levels of native and pregelatinized starches are compared as a function of moisture content, under limited moisture contents, the T g of the native starch is always higher than that of the pregelatinized starch. One way to explain this difference is to consider the native, partially crystalline starch as a form of cross-linked (see above) starch, in which the free movement of the chains is restricted and which therefore has a higher T g than its gelatinized counterpart. However, Figure 6.6 is not universally accepted. Other workers have suggested that the glass transition of native starch occurs at higher temperatures. A thorough discussion of the arguments is beyond the scope of this book.

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