The structure of barley endosperm--an important determinant of malt modification

Structural differences in barley grains have been classified as either mealy or steely and their relative proportions have been determined using a light transflectance method in three barley samples varying in the degree of steeliness, Target being the most steely and Chariot most mealy with Blenheim being intermediate. These structural differences were found to be associated with differences in the concentration of endosperm components, particularly proteins and beta-glucan. Analysis of nitrogen within the endosperm showed that protein was mainly concentrated in the embryo and distal regions with the inner, mid-endosperm containing lowest levels. As the total nitrogen (TN) of the grain increased, the mealier samples accumulated nitrogen mainly in the embryo whereas the steely sample had higher levels in the central endosperm. SDS-PAGE showed no differences in the protein banding pattern at different TN levels. Electron microscopy using immuno-gold labelling demonstrated that gamma-hordeins were present in sub-aleurone and outer endosperm whereas the C-hordeins were found throughout the central endosperm. However, steely areas of central endosperm contained gamma-hordeins. During malting, protein modification in Chariot was more extensive than in Target with 34kD and 97kD hordeins being completely degraded. In Chariot and Blenheim, level of beta-glucan was low and it was evenly distributed throughout the endosperm. In the steelier Target, however, the amount of beta-glucan was higher and was concentrated in the proximal and distal areas of the endosperm. Steely grains (containing high concentrations of protein and beta-glucan) displayed slower water distribution during steeping and later development and distribution of beta-glucanase during germination. As a consequence, the steely sample achieved a lower degree of modification during malting. The structure of the endosperm, therefore, has a prime influence on the evenness of distribution of moisture and enzymes which is crucial for homogeneous modification during malting.