Effect of temperature on carbohydrate hydrolase activity during dry anaerobic fermentation of mixed wheat straw

The production of biogas by anaerobic dry fermentation of straw could decrease environmental pollution from burning straw and at the same time produce abundant biogas residue that can be used as eco-organic fertilizer. However, the application of anaerobic dry fermentation of straw has been limited by the long processing cycle and slow rate of gas production. Hydrolysis has been identified as the factor slowing down straw fermentation, and carbohydrate hydrolase is a key element of hydrolysis. Biogas yield from the dry anaerobic fermentation of wheat straw was directly affected by hydrolase activity. Thus using wheat straw as the main component, carbohydrate hydrolase activities was studied during anaerobic dry fermentation under different temperature conditions in this study. The purpose of this study was to lay theoretical basis for increasing biogas production rate and shortening the residence time of straw fermentation through enzyme engineering. In the experiment, the fermentation materials were the mixture of wheat straw, inoculums and water with the proportion of 1∶7∶7. Total substance (TS) of the mixed materials was 35%. The experiment was carried out in wild-mouth bottle (as the fermentation container) and under different temperatures (20 ℃, 25 ℃, 30 ℃, 35 ℃ and 40 ℃) and with fermentation period of 35 days. The activities of cellulase, xylanase, amylase and sucrase were measured during the dry fermentation process of wheat straw mixture. The results showed that biogas production peaked at about 35 ℃. For the 20 40 ℃ temperature range, TS, volatilizable substance (VS) and carbon/nitrogen (C/N) ratio for the fermenting mixture decreased apparently with increasing temperature. With the end of fermentation, the pH of the fermenting materials in all the treatments declined under 7.0. The lowest pH was under the 40 ℃ condition. Cellulase activity was low during the initial period of fermentation while biogas production peaked later in the fermentation process. The rise in temperature was significantly enhanced cellulase activity. Initial xylanase activity was high, with a variation tendency of initial rise, a latter decline and a final smooth trend during the fermenting process. There were no obvious peaks for xylanase activity under the 20 25 ℃ during the fermentation process. Amylase activity was low at the initial stage and then rose rapidly afterwards, hitting peak values after about 10 days under temperatures of 30 ℃, 35 ℃ and 40 ℃. Sucrase activity tracked the same variation tend as amylase, except that sucrase activity rose more rapidly at temperatures above 30 ℃, declined more rapidly at 35 ℃ and then climaxed at 40 ℃ (which was on the 15th day of fermentation). This could been due to restrained microbial activity with substrate decline. In conclusion, the peak values of the four carbohydrate hydrolase enzymes activities were directly related with temperatures during the fermentation process. The peak value at 40 ℃ was higher than those at 30 ℃ and 35 ℃, apparently different from biogas production relationship with temperature. Biogas productions at 30 ℃ and 35 ℃ were higher than at 40 ℃. Hence temperature had different effects on different carbohydrate hydrolase activities during dry anaerobic fermentation.