In vitro evaluation of starch degradation from feeds with or without various heat treatments

An in vitro method was used to evaluate starch degradation from various feeds with or without heat treatments in four studies. The method was based on incubation of feed samples with a buffered rumen fluid solution and subsequent enzymatic analysis of the remaining starch. In all studies, heat treatment of the feed samples increased rate or extent of starch degradation to glucose. In Study 1, measurements of remaining starch, after 5 h in vitro incubations, demonstrated substantial effects of cooking on starch degradation in potatoes, and a trend to faster degradation from autoclaving peas. Up to 0.60 of the starch remaining after a 5 h of incubation was not recovered by centrifugation at 3000 × g for 10 min. In Study 2, cooking increased in vitro starch degradation rate from isolated potato starch (from 0.038 to 0.197/h). Intact starch in barley and wheat grain had similar rates of degradation (0.117 and 0.109/h, respectively). In Study 3, both autoclaving time (15, 30, 60 min) and temperature (115, 130 and 145°C) affected in vitro starch degradation rates in peas, and, in no case did autoclaving for only 15 min increase degradation rates. For the 30 min autoclaving time, only the highest temperature (145°C) increased the degradation rate of the pea starch compared to the untreated peas (0.175 versus 0.110/h). When autoclaving for 60 min, both 130 and 145°C resulted in a considerable increase in starch degradation rate (0.211 and 0.193/h, compared to 0.110/h for the untreated peas). In Study 4, the proportion of starch degraded at 8 h of in vitro incubation was increased by heat treatment of pure potato starch (0.155 versus 0.870), peas (0.491 versus 0.815), barley (0.686 versus 0.913) and maize (0.351 versus 0.498). Measurements of volatile fatty acid production in the fermentation tubes showed a lower acetate:propionate ratio for the faster fermenting heat-treated feeds. Heat treatment generally increased starch degradation in vitro.