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

Abstract

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 and barley 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.

Review: Effects of fibre, grain starch digestion rate and the ileal brake on voluntary feed intake in pigs

“…Until recently, this had only been determined in a few grain samples (Al-Rabadi et al, 2009;Mahasukhonthachat et al, 2010). However, a recent study (Ratanpaul et al, 2018) showed that plotting the inverse of the first-order digestion rate coefficient against the square of particle size ( Figure 1) gave linear plots for all of 16 diverse grain samples, consistent with a surface area-dependent rate-limiting step. The slope of this plot provides the diffusion coefficient for pure diffusion control, but as other surface area-dependent factors cannot be ruled out, we propose to designate this value as an apparent enzyme diffusion coefficient.…”

“…Smaller particles are digested faster in the SI than larger particles, providing more energy for normal body function and growth. Milled grain particle size has a major impact on starch digestion rate as found from in vitro studies (Al-Rabadi et al, 2009;Mahasukhonthachat et al, 2010;Ratanpaul et al, 2018). Smaller particles lead to a greater extent of growth and feed efficiency in pigs (De Jong et al, 2016;Al-Rabadi et al, 2017).…”

“…This coefficient is a measure of the intrinsic digestibility of milled grains (i.e. independent of particle size), and can be used to investigate grain effects on amylase digestibility between species and as a function of growth conditions (Ratanpaul et al, 2018).…”

“…Grains that are highly digestible lead to rapid increases in postprandial blood glucose concentrations and subsequently lead to increased insulin secretion compared to slowerdigested foods (Scazzina et al, 2013). Slow or incomplete Figure 1 (color online) Starch digestion in vitro for a sorghum sample (Waxy Isoline) with SEM: (a) Digestograms as a function of particle size, (b) first-order kinetics fitrate coefficient k (h -1 ) for each fraction is given by the slope, (c) relationship between square of particle size (mm 2 ) and inverse of rate coefficient, 1/k (h)the slope gives the ADC (Ratanpaul et al, 2018). The fit in (c) shows that the rate constant for digestion (Ki) at a particle size of siavg2 is determined by the diffusion coefficient D. This is consistent with an 'outside-in' surface erosion method of starch digestion from grain particles; the light (yellow) region represents digested and the dark (blue) represents undigested regions of a schematic particle after partial digestion.…”

Measures Matter—Determining the True Nutri-Physiological Value of Feed Ingredients for Swine

“…Consequently, understanding the kinetics of starch digestion may also be beneficial for enhancing growth performance of pigs. The kinetics of starch digestion has been measured by the amylase diffusion rate, and it appears that formulating diets with sources of starch that have high intrinsic digestibility increases growth performance of nursery pigs [ 328 ]. This enhanced growth performance may be due to the greater digestibility of starch in feed ingredient sources that have greater intrinsic amylase diffusion rates.…”