Cellulase treatment of untreated and steam pre-treated rice straw — effect on in vitro fermentation characteristics

This study was conducted to evaluate the cellulase treatment of the untreated (URS) and steam pre-treated rice straw (STRS) in terms of dry matter loss, sugar composition and in vitro gas production (GP). The STRS was prepared at 15 bar for 5 min with a water-to-straw ratio of 30:70 (w/w). Enzymic hydrolysis of straws was accomplished in a 37°C incubator. Wet samples of the URS and STRS were saccharified with a commercial cellulase (Penicillium funicalosum) ranging from 0, 4, 8 to 16 unit per gram straw for 1, 2 and 3 weeks. At lower enzyme loading, dry matter loss from URS was low at 1-week treatment but significantly increased with the prolonged treatment time (P<0.01). With higher enzyme loading and longer incubation time, dry matter loss from URS was high (>150 g kg−1). However, dry matter loss from the STRS was low in all treatments. Water extracts of both the URS and STRS were increased by enzymic hydrolysis, with significantly higher content of water extracts in higher enzyme loading (P<0.01). Enzymic treatment had little effect on the composition of individual and total sugars in either water extracts or insoluble residues of URS, but did increase total soluble sugars of the STRS. Soluble carbohydrates increased significantly with the increase in enzyme dosage (P<0.01), but these were much higher in the STRS than in the URS. While the enzyme treatment had little effects on the GP parameters in the URS, the 24 h GP (P<0.05) and potential GP for the STRS increased with the increment in enzyme loading. Little effect of treatment time was observed in the GP parameters for both URS and STRS. Estimated organic matter digestibility was 46.8–48.7 and 52.0–54.5% for the enzyme treated URS and STRS, respectively. It is inferred that cellulase treatment may further improve the nutritional value of steam pre-treated rice straw, but little could be expected for untreated rice straw only using a pure cellulase.