Glutamic acid reactivity in heated protein and protein-alginate mixtures

When heated at 185°C for 35 min, only soya and gluten (out of seven proteins tested) produced measurable quantities of additional water. In the presence of 2% manucol DM (an alginate rich in mannuronic acid residues), the formation of water was markedly increased in the soya and gluten samples but not in the other proteins. Amino acid analysis showed that on heating soya in the presence of manucol DM, the glutamic acid content decreased whilst the aspartic acid content increased. When manugel GMB (an alginate rich in guluronic acid residues) was heated with soya, no loss of glutamic acid was observed and no additional water was formed. Experiments with single acids confirmed that the reaction giving rise to the formation of water primarily involved glutamic acid. The chemical reactivity of glutamic acid with alginates rich in mannuronic acid residues explains the well-established effect of this type of polysaccharide in extrusion processing. In addition, the apparent reactivity of glutamic acid residues in these types of reactions may be a possible reason for the ease with which soya and gluten (proteins rich in glutamic acid residues) can be extruded, while proteins with less of these residues are more difficult to extrude.