Effects of chemical treatments combined with high-pressure steaming on the chemical composition and in vitro digestibility of crop by-products

Abstract

Samples of barley straw, pea straw and sugarcane bagasse were treated with sodium hydroxide, ammonium hydroxide, anhydrous ammonia, sodium chlorite, sulphur dioxide or sodium metabisulphite before, during or after high-pressure steaming at 170°C for 60 min. The steaming treatment included an additional 2-h period during which the steaming vessels cooled to room temperature. Control samples were untreated, treated with each chemical without steaming or steamed alone. The amounts of water or chemical solution added were insufficient to cause separation of a fluid fraction and chemical residues were not removed by subsequent washing. Changes in the by-products were assessed in terms of chemical composition and in vitro organic matter digestibility. Untreated barley straw had an in vitro digestibility of 38% and this was increased to 54% by high-pressure steaming alone. The highest digestibility of barley straw, 71%, was achieved by treatment with 6 g NaOH/100 g dry matter before steaming. Other treatments produced intermediate responses; treatment with anhydrous ammonia before or after steaming caused reductions in digestibility compared with the steaming treatment alone or the chemical alone and this also occurred when sulphur dioxide was applied before steaming. Untreated pea straw was 40% digestible. With the steaming treatment alone digestibility increased to 55% and with sodium hydroxide treatment alone, to 54%. No other treatment, either a chemical alone or a chemical treatment combined with high-pressure steaming, resulted in a digestibility higher than these and, in many cases, the combined treatments resulted in digestibility values similar to that of the untreated pea straw. Untreated bagasse was 27% digestible and with steaming alone it was 49% digestible. The highest digestibility of bagasse, 66%, was achieved when sodium hydroxide was provided during steaming. High-pressure steaming alone was more effective than any chemical alone, except sodium hydroxide. Combined treatments produced variable responses. The treatments caused some large reductions in the hemicellulose content of the by-products and smaller changes in contents of neutral detergent fibre and lignin.