Solubility of cereal proteins

Some cereal proteins are quite soluble in aqueous solution (albumins, globulins) but the functional proteins—prolamins and glutelins—are difficult to solubilize. This is important because, for example, we do not want wheat gluten proteins to be soluble in a dough system. However, in order to characterize proteins, it is usually necessary to have them in solution. Because the property of solubility is such an important one with respect to cereal proteins, we will dedicate this chapter to the topic. In keeping with the objectives of the book, we will attempt to approach the subject from the most general and fundamental basis. 

Why do some compounds dissolve in a solvent but others do not This is part of a more general question Why do some physical processes (such as solubilization) or chemical reactions proceed while others do not, and why do they stop at a certain stage Let us first try to answer this question and then consider how it applies to an understanding of solubility. 

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It is possible to classify proteins on several different bases. One basis is on the cereal from which they come and where in the seed they are found, another is the Osborne classification system, which is based on the solubility of the protein. Proteins can be further classified in chemical terms such as molecular weight and the presence or absence of sulfur. 

As mentioned before, proteins are the key to explaining dough s physical properties (Chapter 6) and they play a major role in cereal technology. For that reason, particular attention is given to proteins in Chapters 9 and 10. Cereal proteins (especially the gluten type) are known for the difficulty encountered in their solubilization. Chapter 9 deals with the topic of protein solubility. Solubilization is usually a necessary precursor for characterization. Some of the main techniques currently used for characterization of cereal proteins are discussed in Chapter 10. 

Figure 9.6 Effects of sodium fluoride (left) and sodium thiocyanate (right) on the solubility of gluten protein. (Data from Preston, K. R. 1981. Cereal Chemistry 58 317-324.)...

The discussion of protein solubility in Chapter 9 leads logically into the next topic because it is usually necessary to have proteins in solution in order to characterize them. Of course, some important characterization can be made by hydrolyzing the proteins (e.g., determination of the amino acid composition and sequence). What we are mainly concerned with in this chapter is the measurement of molecular composition and properties of proteins or their subimits in or close to their native state. This is required if our aim is to relate composition to fimctional properties. The classification of cereal proteins into four groups based on solubility was described in the previous chapter. This classification still proves to be valuable for researchers. However, in recent times, techniques have been developed that enable more accurate quantification of cereal protein composition. 

Deamidation of proteins induces the conversion of glutamine and asparagine residues to acid groups, with the concomitant release of ammonia. Deamidation is known to be an important method for improving the functional properties of proteins, for instance, the increase in solubility of cereal prolamins,... 

Rye Proteins. While rye is the only European cereal able to completely replace wheat in bread, rye protein is not as effective as wheat protein. One reason for this is that as much as 80% of the protein in a rye sour dough is soluble compared with 10% of soluble protein in a wheat dough. One factor that inhibits the formation of a gluten-like complex is the 4-7% of pentosans present, which bind water and raise the viscosity of the dough. The crumb structure is then formed from the pentosans in combination with the starch. 

The beneficial effects of dielary fiber, including both soluble and insoluble fiber, are generally recognized. Current recommendations arc for daily intakes of 20-35 g in a balanced diet of cereal products, fruits, vegetables, and legumes. However, the specific preventive role of dietary fiber in certain diseases has been difficult to establish, in pan because dielary risk factors such as high saturated fat and high protein levels are reduced as fiber levels increase. 

This association is greatest at the isoelectric point of the protein and being readily dissociated at pH s above or below the isoelectric point ( iJ.) Phytate association in cereal grains is less well defined but is contained in significant concentrations both in the bran and germ (12). There appears to be at least one ferric ion associated in the otherwise soluble phytate complex in wheat bran (13). Phytate in sesame seed appears to be the most unique and least soluble of all seeds. In this case magnesium appears to be the predominant cation (9 ) ... 

The seed contains a relatively high amonnt of essential amino acids (55) (Table VII). In contrast to other plant seed protein profiles, baobab seed protein contains a high amonnt of lysine. Becanse lysine is limited in most cereal plants, it may be possible to nse baobab seed protein to improve cereal protein quality, especially in weaning food mixtmes (55). The high protein solubility at acidic and alkaline pH snggests that the baobab seed protein could be an adequate food ingredient (55). 

Although Osborne s fractionation was a major milestone in the development of cereal chemistry, it must be understood that each of these fractions is a complex mixture of different polypeptides, and also that these polypeptides overlap in their solubilities. This is particularly true for the gliadins and glutenin proteins. Beckwith et al. (1966) detected by gel filtration studies a small fraction of the ethanol-soluble proteins as an excluded peak, which he called gliadin of high molecular weight (HMW) [5]. Nielsen et al. (1968) found that... 

Prolamins, a group of storage proteins occurring in cereals. They are soluble in 50-90% ethanol and can be so separated from the alcohol-insoluble glutelins. Prolamins are globular proteins (M, ... 

Cereals are an important protein source and are processed into bread, pasta and noodles, breakfast cereals, and fermented drink. For all these applications the quality is determined, to a greater or lesser extent, by the gluten proteins which account for about half of the total grain nitrogen. There are also opportunities to develop novel uses for cereal proteins in both the food and nonfood indns-tries. It is important to study the wheat gluten proteins, particularly the deamidated SWP (soluble wheat protein) in relation to their structure and function. 

Properties of proteins such as solubility and chromatographic behavior depend on the residues that are accessible to the solvent. The nonpolar residues buried in the interior of the molecules should not directly affect these properties. Therefore, the surface hydrophobicity may be the more relevant parameter. Of course, the ideal model depicted by the polarity ratio is never attained. Because of the steric constraints and the inevitable adjacency of polar and nonpolar residues in the amino acid sequence, a proportion of nonpolar residues will be found at or close to the periphery of molecules. For proteins whose structures have been determined (e.g., by x-ray crystallography), it is possible to calculate the surface hydrophobicity. Some values are recorded in Table 9.1. Because of their heterogeneity, cereal proteins are not easily amenable to structure determination, so their surface hydrophobicities have not been calculated. 

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