Optimization of steam treatment as a method for upgrading rice straw as feeds

The objective of the present study was to study the effects of different conditions of steam treatment on nutritional value of rice straw (RS). The steam treatment was accomplished in a high pressure vessel at pressures 15, 17 and 19 kg/cm² for a range of treatment time (0, 5 and 10 min) at a water-to-straw ratio of 1:1. The effect was also studied of the amount of water added (0:100–50:50 (w/w)) on the nutritive value of treated RS, where treatment was conducted at 15 bar pressure for 5 min. The effects of steam treatment were evaluated in terms of dry matter loss, fibrous portions, sugar profiles and in vitro gas production (GP). Steam treatment increased water solubility of RS. There was no significant effect of steam pressure on dry matter loss, pH, fibrous fractions (NDF, ADF and hemicellulose), and sugar profiles (total and individual) in either water extracts or insoluble residues. Dry matter loss was however, significantly greater in longer treatment time (10 min) than in shorter time condition (0 and 5 min). In the water insoluble residues of the steam-treated straw, hexoses were dominantly higher than pentoses, indicative of the hemicellulose hydrolysis during treatment. Steam treatment of RS also resulted in an increased soluble carbohydrate content, which decreased with the increasing pressure and prolonged time. Compared with the untreated RS, the treated straw had a higher 24 h GP, and a faster rate of GP but the potential GP was similar. All these GP parameters tended to be higher at lower steam pressure and shorter time of treatment, though the effect was not significant (P>0.05). Significant differences (P<0.01) were found in all parameters between lower and higher amount of water added. Compared to the lower water-to-straw ratio (below 2:8), the treated straw at the high moisture had a lower dry matter loss (P<0.01), higher water soluble carbohydrates (P<0.01) and higher GP (P<0.01). From these results and considerations of energy economy, it is inferred that the optimal treatment condition is likely at a steam pressure of 15 bar for 5 min, and that water-to-straw of above 3:7 seem to be needed to obtain optimal treatment results.