Biodiesel's trash is a biorefineries' treasure: the use of "dirty" glycerol as an industrial fermentation substrate.

"Dirty" glycerol from biodiesel production is having a considerable environmental impact since its disposal is expensive and difficult. The increased biodiesel production in the last two decades has forced glycerol prices down, thereby making it now unprofitable for chemical companies to produce. The problem lies with the impurities of the biodiesel conversion process usually ending up within the crude glycerol fraction. These impurities are often too costly to purify with current processes, particularly for small scale producers. A wide variety of industries, including the paint, tobacco, food and pharmaceutical industries, utilize glycerol as part of their technology or products. However, the crude glycerol from biodiesel production is not of a high enough grade to be used in these industries. Biodiesel-produced crude glycerol is therefore cheap, readily available and presents itself as an attractive carbon source for industrial microbial production systems synthesizing value-added products. This mini-review will look at (a) microbial production processes which use crude glycerol to produce high-value products (product-driven research) and (b) genetic engineering of microbes which is aimed at improving microbial "dirty" glycerol utilization (substrate driven research).