Effect of saponins from Enterolobium cyclocarpum on in vitro microbial fermentation of the tropical grass Pennisetum purpureum

The effect of Enterolobium cyclocarpum saponins on the fermentation of Pennisetum purpureum was studied in vitro, with P. purpureum incubated alone (PP), in combination with E. cyclocarpum (EC), E. cyclocarpum without saponins (ER) or with saponins extracted from the same amount of E. cyclocarpum (SP). Gas production was monitored and contents sampled after 24 h for short-chain fatty acid (SCFA) and total microbial mass analyses. SCFA and microbial attachment to P. purpureum using (15)N as marker were determined from different bottles at 6, 12 and 24 h. Up to 8 h, gas production was highest for SP and lowest for ER (p < 0.05), probably because highly fermentable solubles were also extracted. Gas from PP was highest from 16 h (p < 0.05). Treatments including saponins (SP and EC) increased propionate (p < 0.001) and decreased acetate and butyrate (p < 0.01) proportions. Stoichiometrically estimated methane concentration was lowest for SP and EC (p < 0.01). Microbial attachment after 24 h was lower for EC and SP than for ER (p < 0.05). Saponins from E. cyclocarpum are responsible for the reduction in gas production when supplemented with P. purpureum and modify the fermentation pattern by increasing propionate proportion and decreasing methane production. Besides, saponins increase total microbial mass, although microbial attachment to the grass substrate was reduced.