Alcoholic beverages containing higher concentrations of alcohol than can be obtained by wine fermentation have to be prepared by distillation. After the distillation process all the ingredients of the original fermented mash or liquid, except volatiles, remain in the waste, called stillage, spent wash, slop, or vinasse. The composition of this distillery waste depends on the original raw materials used: sugarcane- or sugar beet molasses, corn, wheat, barley, cassava, potato, rice, fruits, etc. These materials have to be prepared for alcoholic fermentation by dilution or mashing, including eventual hydrolysis and/or addition of nutrients. Stillages remaining after distilling off the alcohol are troublesome wastewaters that are either pulpy materials or clear fluids containing settling particles. The stillages are about 10-14 times the volume of the alcohol produced and have a high content of organic substances (Shojaosadati et al. 1999). The pollution load is quantified by biochemical oxygen demand (BOD) and chemical oxygen demand (COD). Distillery stillages are characterized by a very high BOD, very high COD, and a low pH value. In addition, with certain types of distillery wastewater such as fruit slops, the separation of solids from the liquid is almost impossible (Friedrich et al. 1986). There are substantial pressures to develop a proper treatment for making wastes acceptable. A text on organic waste conversion (Bewick 1980), as well as several review articles, discusses the problem of distillery wastewater (Costa Ribeiro and Castello Branco 1980; Sheehan and Greenfield 1980; Weathers 1995; Wilkie et al. 2000). There are many potential solutions for these problems, including simple disposal, sewage treatment, and recovery of useful ingredients. In addition, many attempts have been made to exploit the organic compounds from stillages in biotechnological processes for the production of commercially valuable products. Industrial and experimental utilization and disposal of effluents and properties of stillage of different origins have been reviewed and compared by Wilkie et al. (2000).
The presence of a large amount of organic substances of natural origin opens up the possibility of bioconversion by microorganisms. While bacteria participate in anaerobic processes producing biogas, fungi are suitable for aerobic bioconversion. There have been several attempts to use fungi for treating distillery wastewaters. The aim of microbiological treatment is to purify the effluent by consumption of organic substances, thus, reducing its COD and BOD, and at the same time to obtain some valuable product, such as fungal biomass for protein-rich animal feed, or some specific fungal metabolite. The present review deals with the application of fungi for treating distillery stillages or slops. Suitable fungi, the bioconversion process, possible products and effects, together with economic considerations are described.
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