Rheological Properties of Cereal Proteins

Abstract

The protein content of intact cereals ranges from about 5% in millet to over 15% in oat (Wieser et al., 1980). The protein content of wheat flour can vary from 7% to 17% (Redman, 1971). Besides contributing to the nutritional value of the cereal, proteins also give functional properties to foods. Among the cereal proteins, effects on functional properties are most evident in the case of wheat proteins, due to the unique viscoelasticity of gluten. Similar properties are shown to only a very small extent by proteins in rye and barley, and not at all by proteins in the other cereals. It is not surprising, therefore, that so much work has been done to find out what makes the wheat proteins umque. Several chemical and physicochemical methods, from determinations of amino acid sequences to measurements of rheological properties, have beenused to obtain a better understanding of the wheat proteins. For most meth- ods, however, a protein fraction is required that is essentially pure before measurements can begin. A wide range of fractionation procedures for wheat proteins has therefore been developed during the years. This chapter begins with a description of some chemical and physicochem- ical properties of cereal proteins. However, the main subject is the rheological properties of cereal proteins. These properties have not been investigated for all cereals. In fact, they have been investigated almost exclusively for wheat proteins. As a consequence, several important cereals are omitted (rice, mil- let), whereas others are included because they have been studied as additives to gluten (e.g., oat). The rheological properties of cereal proteins in solution are first discussed, followed by a treatment of A-gliadin, the cereal protein most thoroughly investigated by rheological methods (both in solution and in more concentrated systems). Thereafter the rheological properties of protein dispersions (again gluten) are discussed.