利用瘤胃細菌以木質纖維素物質為原料生產生質酒精

Abstract

由於石油能源枯竭，再生能源已經受到世界各地的矚目。其中生質酒精為最有潛力的替代能源。現今大部分生質能源多以甘蔗和玉米為原料，然而這些原料也提供給人類和動物當作食物，因此會造成競爭效應，導致物價上漲。最近木質纖維素原料被認為是具有潛力的替代原料，木質纖維素原料包含農業廢棄物、森林廢棄物及能源作物。然而以木質纖維素原料生產酒精最主要的障礙為其難被分解的特性。直接由瘤胃微生物水解木質纖維素原料後發酵成酒精可能是一個不錯的方式。 本試驗以大豆殼、五節芒和稻草為原料，將瘤胃纖維分解菌Ruminococcus albus 7和瘤胃中可生產酒精者Treponema saccharophilum PB 或Lachnospira multiparus D32共培養，於0、4、8、12、24、36、48小時收樣品並測試乾物質降解率、pH值、還原糖濃度及酒精濃度。 當R. albus 7和T. saccharophilum PB共同培養時乾物質降解於0-8小時劇烈上升，8小時後趨緩但繼續進行。同時，pH值於0-8小時隨著總還原糖濃度劇烈下降而下降。之後還原糖濃度變化不大，顯示由原料釋放出的糖立即被利用而生成酒精。酒精濃度到達最大值在8-12小時之間，而後變化不大。當R. albus 7和L. multiparus D32共同培養，乾物質降解和還原糖濃度變化與R. albus 7和T. saccharophilum PB共同培養時有類似趨勢。然而其酒精濃度在24小時前達到高峰而後隨時間下降。雖然大豆殼於兩個實驗均有較佳的乾物質降解率，然而搭配T. saccharophilum這組酒精產量在各原料間沒有差異，但L. multiparus這組則是以五節芒為原料下酒精產量較高。另外，當添加25 μM 3-phenylpropanoic acid (PPA) 於R. albus 7和T. saccharophilum PB共同培養液，使五節芒的降解率及其酒精產量均顯著提升(P < 0.05)。 綜上所述，混合瘤胃細菌可以利用木質纖維素物質生產生質酒精，然而不同物質適合不同的微生物利用，此外添加PPA可以有效提升較難分解的木質纖維素物質之降解率並提升酒精產量。

Because of the depletion of the fossil fuel reserve, it has attracted great attention in the renewable energy resources around the world. Bioethanol is one of the most promising alternative energy resources. Nowadays, most bioethanol is produced from sugarcane and corn. However, these materials are also provided as food for human or animals. Therefore, it will cause a competitive threat, resulting in high-rised price. Currently, lignocellulosic materials are considered as potential substitutes. They include agricultural residues, forestry residues and dedicated energy crops. Difficulty of hydrolysis is the major barrier of bioethanol production from lignocellulosic material. Directly hydrolyzing the biomass and fermenting to ethanol by ruminal bacteria may provide an alternative way for the bioethanol production from recalcitrant lignocellulosic materials. In this study, silvergrass, rice straw and soybean hull were used as substrates to produce ethanol by coculture of ruminal cellulolytic bacteria Ruminococcus albus 7 with one of major ethanol producers in rumen Treponema saccharophilum PB or Lachnospira multiparus D32. Samples from coculture incubation were harvested at 0, 4, 8, 12, 24, 36 and 48 h for measurements of dry matter digestibility, pH value, reducing sugar concentration and ethanol concentration. When R. albus 7 was cocultured with T. saccharophilum PB, substrates’ DM digestibility increased markedly from 0~8 h, then slowed down after 8 h. At the same time, the pH values decreased from 0~8 h along with the dramatical decrease of total reducing sugar concentrations. Afterwards, total reducing sugar concentration remained fairly constant, indicating sugar produced by cellulolytic bacteria was consumed by saccharolytic bacteria instantly. Concentrations of ethanol reached a maximum between 8 and 12 h and then remained fairly constant. When R. albus 7 was cocultured with L. multiparus D32, the substrates’ DM digestibility and reducing sugar concentration had a similar change pattern as R. albus 7 with T. saccharophilum PB. However, ethanol production increased to a maximum before 24 h and then decreased by time. Although soybean hull had the greatest digestibility in both coculture experiments, the ethanol yield was no significantly different from other two substrates by R. albus 7 and T. saccharophilum PB coculture, and the greatest ethanol yield was achieved with silvergrass by R. albus 7 and L. multiparus D32 coculture. Additionally, when adding 25 μM 3-phenylpropanoic acid (PPA), both of the DM digestibility of silvergrass and the yield of ethanol increased significantly in the coculture of R. albus 7 and T. saccharophilum PB (P < 0.05). In conclusion, mixed ruminal bacteria can produce bioethanol from lignocellulosic materials rapidly. However, different materials were suitable for different microorganisms to utilize. In addition, adding PPA can significantly improve substrate digestibility and ethanol yield for harsh lignocellulosic materials.