Apparent amylase diffusion rates in milled cereal grains determined in vitro: potential relevance to digestion in the small intestine of pigs

Starch digestion in grain particles depends on the diffusion of amylase into the particles. Enzyme diffusion rate (EDR) is influenced by grain structure, and may be key in determining the extent of grain digestion in the small intestine. EDR values of sixteen grains (wheat, barley and sorghum) were compared with their ileal digestibility from pigs. Five fractions of each hammer-milled grain were digested in vitro under conditions mimicking monogastric digestion to obtain apparent amylase diffusion coefficients (ADC) from the inverse square dependence of rate-coefficients on particle sizes. The ADC values in sorghum were lower than in wheat (P < 0.05) and barley (P < 0.001) for samples grown under standard conditions. However, grain differences such as sprouting or immaturity, resulted in higher ADC values that were relatable to in vivo digestibility. Grain fibre had large effects on ileal digestibility, with an apparent optimum neutral detergent fibre level of 10-20% that resulted in maximum starch ileal digestibility. It is inferred that the true potential for efficient feed utilisation depends both on fast grain digestion, and on an appropriate rate of passage, so that small intestine residence time is long enough for starch in grains to be digested fully.