Improvement for the ensiling pretreatment effectiveness of sweet sorghum bagasse by fortified with Lactobacillus plantarum and cellulolytic enzymes

Abstract: Sweet sorghum bagasse (SSB) is the resultant waste after extraction of sugar-rich juice from the stalks during bioethanol production. As a typical biomass, the SSB consists of moisture, unspent soluble sugar, and abundant lignocellulosic component (cellulose, hemicelluloses and lignin). The undisposed SSB can inevitably cause the environmental pollution and resource waste. Hence, the effective storage of SSB is necessary for the year-round stable operation of biofuel plants. Among them, ensiling (wet storage) can be an efficient technology available for the SSB preservation and utilization, which enable the SSB to be processed all year around. Moreover, ensiling can also act as a pretreatment strategy, due to the benefits for the cell wall degradation and the improvement of biomass bioconversion for subsequent processing. In this study, the potential of ensiling pretreatment was investigated to fortify with Lactobacillus plantarum inoculant and cellulolytic enzymes on the improvement of ensiling quality to modulate the performance of enzymatic saccharification of SSB. The SSB were ensiled with no additives (CK), Lactobacillus plantarum (L), cellulolytic enzyme (E), and a combination of L and E (LE) for 21 days. The dynamic changes of nutrient composition, lignocellulosic components, and ensiling fermentation characteristics were investigated, and then dynamic evolution of the bacterial community structure was analyzed by high-throughput sequencing at HiSeq2500 platform. Ensiling quality was comprehensively evaluated by membership function method, in order to screen the suitable silages additives in consideration of enzymatic hydrolysis performance. The results showed that the addition of Lactobacillus plantarum inoculant and cellulolytic enzymes either alone or in combination was facilitated the lactic acid fermentation to reduce the fermentation losses, as evidenced by the higher content of crude protein, starch, neutral detergent fiber, acid detergent fiber, hemicellulose and cellulose components than that of un-ensiled SSB. The content of dry matter, crude protein and water-soluble carbohydrates in silages with additives were higher than those in the CK, but the content of holocellulose lower. Furthermore, the content of dry matter in the E silages, the content of crude protein in the L silages, and the content of water-soluble carbohydrates in the LE silages were the highest in all silages, respectively. The pH value of all silages significantly decreased to below 4.2, accompanied by the ratio of lactic Acid (LA) and total organic acids (TOA) higher than 0.65 and the ratio of lactic acid (LA) and acetic acid (AA) higher than 2 during the whole ensiling for 21 days. The content of LA and AA in the L silages were significantly higher than those of silages at the CK (P<0.05), the ratio of LA/TOA and LA/AA in the E silages were significantly higher than those of silages at the CK(P<0.05). The membership function analysis indicated that the silages at the E group shared the highest comprehensive scores at 21 days. The bacterial community structure showed that the Proteobacteria was the main bacteria in all silages during ensiling fermentation. At 21 days of ensilage, Enterobacteriaceae, pantoea and leuconostoc were the main species in the silages at genus level. ACE, Chao1 and Shannon index in the E, L and LE silages were significantly lower than those of CK group (P<0.05). It infers that the ensiling pretreatment with the additives was effectively reduced the microbial richness and diversity. Principal Coordinate Analysis (PCA) was constructed using the relative abundance of bacteria at the genus level, indicating the outstanding separation in the SSB before and after ensiling pretreatment. After ensiling pretreatment, the yield of reducing sugar in all SSB silages significantly increased, compared with the un-ensiled SSB. Consequently, the reducing sugar yield of in the E silages increased by 117%. The highest net income of E silages fortified with cellulolytic enzyme was almost three-fold that of un-ensiled SSB using cost-benefit analysis. In conclusion, the ensiling pretreatment fortified by cellulolytic enzymes can be expected to serve as a cost-effective, eco-friendly and tech-feasible strategy for the preservation and pretreatment of SSB biomass. Especially, cellulolytic enzyme can be used to modulate the ensiling pretreatment performance. Therefore, the ensiling quality and the biodegradation performance of SSB silage can be effectively improved for the bioenergy utilization of sweet sorghum bagasse.