In vitro starch disappearance procedure modifications

Four in vitro experiments evaluated the effects of ruminal fluid inoculum:artificial saliva ratios, grinder type, grind size, and diet of ruminal fluid donor on in vitro starch disappearance. Experiment 1 examined rates of starch disappearance and coefficients of determination obtained by linear regression of starch disappearance using five grain sorghum (Sorghum bicolor (L.) Moench) lines, a corn (Zea mays L.) control, and a wheat (Triticum aestivum L.) control. Grains were incubated for 4, 8, 12, 16, and 20 h with inoculum varying in proportion of ruminal fluid and artificial saliva (1:1, 1:2, 1:3, and 1:4). In vitro rates of starch disappearance and coefficients of determination were similar for the 1:2, 1:3, and 1:4 inoculum dilutions and were higher (quadratic, P < 0.01) than the 1:1 dilution. As the proportion of artificial saliva in the inoculum increased, in vitro pH increased (linear, P < 0.01). In Experiment 2, six grain sorghum lines and a corn control were ground through a 1-mm screen in a Udy (cyclone type) or Wiley mill. Starch disappearance for samples ground using a Udy mill were higher (P < 0.01) than for samples ground in a Wiley mill and approached 90% digestion in 8 h. Experiment 3 compared six grain sorghum lines and a corn control ground through a Wiley mill equipped with either a 1-mm or 2-mm screen. Starch disappearance of samples ground through a 1-mm screen were lower (P < 0.10) when compared with samples ground through a 2-mm screen. In Experiment 4, the effect of grain type (corn, sorghum, or 50:50 corn:sorghum) in 92.5% concentrate diets of ruminal fluid donors was compared in a replicated 3 ⇔mes; 3 Latin square design. Six grain sorghum lines and five corn hybrids were evaluated with in vitro digestion. Ranking by in vitro rate of starch digestion was not affected (P > 0.10) by grain type fed; however, absolute rates of digestion varied among inoculate sources. Grinder, grind size, and ruminal fluid inoculum:artificial saliva ratios affected rate of starch disappearance in samples digested in vitro. Diet of ruminal fluid donor affected the rate of starch digestion, but not the relative ranking of the grains. If rates are to be compared across in vitro runs for different grains, these processing and dietary factors must be kept constant.