The effect of morphological structure on the digestibility of barley and wheat endosperms

Abstract

The main aim of this investigation was to identify the relationship between the structure and composition of barley and wheat endosperms and ease of conversion during malting and distilling. The research at BRF International focused on barley endosperm structure and its malting quality whereas the effect of endosperm structure of wheat on distilling was carried out at the International Centre for Brewing and Distilling (ICBD) at Heriot-Watt University.

Results

For both cereals, endosperm structure strongly influenced their ability to be convened during the malting and distilling processes. Structurally different areas of the grain, known as mealy and steely areas, were identified in single sections and in whole grains. These structural differences can be traced to the variety of the grain and also to the growing and environmental conditions. One of the factors which affected endosperm structure was the application of fungicide to the growing crop. This increased the mealiness of both barley and wheat grains, probably due to enhanced photosynthetic activity throughout the growth period of the plants.

In barley, malting varieties contained fewer steely grains than did the feed variety. Malt quality was inversely related to the proportion of steely grains. Endosperm structure was related to the concentration of structural components such as b -glucan and protein. For example, steeliness was associated with higher concentrations of b -glucan and specific proteins, particularly g -hordein. The rate of moisture uptake and re-distribution across the endosperm was greatly dependent on its structure. In turn, moisture distribution affected the distribution activity of key hydrolytic enzymes such as b -glucanase. The extent of endosperm modification during malting was therefore, affected by the actual location of steely areas within the endosperm. Thus steely patches in the proximal endosperm restricted the distribution of moisture into the central endosperm and reduced the effectiveness of hydrolytic enzymes. A method was developed to measure the proportion of steely and mealy areas in a given sample. This method depends upon the extent of light transflectance through the endosperm.

Wheat can be further sub-divided into soft and hard types according to endosperm structure, which is varietally determined. The small starch granules and matrix proteins of hard wheats produce a much more rigid structure than seen in soft wheat varieties. For both wheat and barley, steeliness was influenced by the growing conditions. During distilling, spirit yield is the paramount factor and soft wheat varieties normally produce the highest alcohol yield. Some hard wheat varieties were potentially useful for distilling, provided that the grain nitrogen content was low. The pentosan content of wheat was significantly higher than in barley and mealy grains of hard wheat had significantly higher levels than soft wheat. The effect of pentosans on foam during whisky production was studied and b -glucanase and protease were more influential in reducing foam whereas xylanase reduced wort viscosity.