The proteins of rye grain-classification, analysis, and functionality
P. Koehler
Deutsche Forschungsanstalt fuer, Garching, Germany
Cereal Foods World 54:A7
Although rye and wheat are comparable in their overall composition, there are specific differences in the functionality of the flours. This is especially valid for the rye proteins, which do not form a cohesive, viscoelastic matrix comparable to wheat gluten. With respect to the Osborne fractions rye contains a considerably higher proportion of albumins/globulins as well as a much lower content of glutelins than wheat. The storage proteins of rye have been termed secalins. Same as for wheat they can be classified into the alcohol-soluble prolamins and the alcohol-insoluble glutelins. Four types of proteins are present in rye, namely high-molecular-weight (HMW-), gamma-75k-, omega-, and gamma-40-k secalins. Except the gamma-75k-secalins these proteins are homologous to protein types present in wheat gluten. In contrast to wheat rye neither contains low-molecular-weight (LMW-) subunits, nor omega5- or alpha-gliadins. The amino acid sequences of gamma-75k-secalins are homologous to gamma-gliadins, however, the molecular mass is considerably higher and they are present as aggregates linked via intermolecular disulfide bonds. Rye protein types can be analyzed quantitatively by means of a combined extraction/HPLC method, in which the absorbance area of the peaks at 210 nm is used for quantitation. gamma-75k-Secalins are the most abundant (46%), followed by the gamma-40k- (25%), omega- (17%), and HMW- (7%) secalins. Specific structural features of rye proteins in comparison to wheat proteins are responsible for the poor breadmaking performance of rye flour. Bread baked from rye flour only gains 60% of the loaf volume a wheat flour bread achieves. HMW-subunits of rye have a different pattern and number of cysteine residues, and this obviously inhibits polymerization and in consequence high molecular mass aggregates. Furthermore, rye flour lacks LMW-subunits which also contribute to the formation of gel protein (GMP) in wheat. The importance of the structure of the HMW secalins can be demonstrated by genetic engineering. The incorporation of HMW subunits from wheat into rye considerably improves the GMP content and the breadmaking performance. The formation of high molecular mass aggregates by cross-linking rye proteins with transglutaminase leads to comparable results.
