Microbial amino acid yield from in vitro incubation of cellulose or starch with rumen fluid

Two model substrates, cellulose (particle size 20 mu m) and powdered maize starch, were incubated in vitro in glass syringes with rumen fluid and buffer. The fermentation was monitored after 1 h and every 3 h, up to 24 h of incubation. At the end of each incubation period gas yield was read and the contents of three syringes were removed, separated and analysed. Bacterial dry matter was calculated from total amino acid yield, assuming that this represented 42% of bacterial cells, and substrate degradability was calculated as the difference between residual dry matter and estimated bacterial dry matter at the end of each incubation period. The results obtained for pH, ammonia nitrogen and gas production indicated that the fermentation of cellulose was active from 9 to 21 h of incubation For starch, the active period of fermentation was considered to occur between 3 and 15 h. The amino acid production was significantly different between substrate and times of incubation (P < 0.001); moreover the interaction of the main effects was also very high (P < 0.001). At the end of the selected incubation periods times (21 h for cellulose and 15 h for starch) the quantity of amino acid yielded from the bacteria growing in cellulose was much lower than that obtained from starch (59.4 vs. 89.8 mg AA g(-1) substrate incubated). The amount of lysine and methionine produced during the fermentation of cellulose and starch varied between substrate and time of incubation. The in vitro estimated degradability differed significantly for substrate, time of incubation and their interaction; at the end of the restricted period of incubation considered, the degradability reached a value of 52.7 and 91.8% for cellulose and starch, respectively. The efficiency of bacterial growth (mg AA nitrogen g(-1) substrate degraded) was numerically, but not significantly higher for cellulose than starch (19.3 vs. 15.8 mg AA nitrogen g(-1) respectively) and at the beginning of incubation for both the substrates. (C) 1997 Elsevier Science B.V.