Effect of particle size and extrusion processing parameters on in vitro starch fractions, in vivo starch digestibility and glycemic index of field pea in dogs

Abstract

A meta-analysis was performed to determine the effect of hydrothermal processing of peas on starch digestibility in monogastric species. From a total of 415 studies on hydrothermal processing of peas, nine studies were identified for inclusion in the meta-analysis. Effect sizes were standardized by converting them to Cohen’s d (CD). The combined nine studies showed a significant increase in pea starch digestibility by CD = 6.94 (95% CI; 4.50-9.37; P < 0.001) after hydrothermal treatment. A regression of processing temperature on the effect size showed a nearly significant quadratic response (CD = -0.009(temp)2 + 2.345(temp) – 146.103, r² = 0.303; P = 0.096). This suggests that the rate of pea starch digestion can be manipulated by controlling processing temperature. The hypothesis of this research was that processing parameters, namely particle size and extrusion, would alter pea starch in vitro degradability, and in vivo digestibility and glycemic response in laboratory beagles. A preliminary experiment found that, although not significant (P = 0.07), pea starch had a lower total tract apparent digestibility coefficient (TTADC) than rice starch (81% vs. 100% respectively) (n = 6). A second experiment found no significant effect of pea particle sizes 195, 309, and 427μm on glycemic index (GI) in laboratory beagles (n = 6). A third experiment was performed to determine the effect of extrusion processing on pea starch. The experiment used a completely randomized 2 x 2 x 2 x 2 factorial design with 2 levels of temperature (110 vs. 150°C), moisture (20 vs. 28%), particle size (288 vs. 407 μm) and cooling method (freezing vs. drying). Extrudates were analyzed for their rapidly digestible starch (RDS), slowly digestible starch (SDS) and resistant starch (RS) contents. Particle size was the only significant effect; large particle size increased RS and decreased RDS (P < 0.05). There was also significant negative correlation between particle size and RDS and SDS fractions (P < 0.05) and a trend toward particle size being positively correlated with RS content (P = 0.059). Subsequently, four of the 16 extruded treatments were selected for the measurement of GI in beagles (n = 6): 3) 150°C, 288 μm, 20% H20, dried; 7) 110°C, 288 μm, 20% H20, dried; 10) 150°C, 407 μm, 28% H20, frozen; 14) 110°C, 407 μm, 28% H20, frozen. There was no relationship between GI and particle size, but GI was negatively and RDS was positively correlated with temperature (P < 0.05). These results suggest that in vitro starch fractions are not good predictors of GI in dogs. However, the rate of pea starch digestion may be manipulated by controlling processing temperature. Further studies are needed to determine the effect of multiple temperatures on the GI of starch.