Microbial conversion of glycerol: present status and future prospects

Biodiesel has emerged as a potential alternate renewable liquid fuel in the past two decades. Total annual production of biodiesel stands at 6.96 million tons and 11.2 million tons in USA and Europe, respectively. In other countries, Asia and Latin America, biodiesel production has increased at unprecedented rate. Despite this, the economy of biodiesel is not attractive. An obvious solution for boosting the economy of the biodiesel industry is to look for markets for side products of the transesterification process of biodiesel synthesis. The main by-product is glycerol. However, this glycerol is contaminated with alkali/acid catalyst and alcohol, and thus, is not useful for conventional applications such as in toothpaste, drugs, paints and cosmetics. Conversion of this glycerol to value-added product is a viable solution for effective and economic utilization, which would also generate additional revenue for the biodiesel industry. Intensive research has taken place in area of conversion of glycerol to numerous products. The conventional catalytic route of glycerol transformation employs prohibitively harsh conditions of temperature and pressure, and thus, has slim potential for large-scale implementation. In addition, the selectivity of the process is rather small with formation of many undesired side products. The bioconversion processes, on the other hand, are highly selective although with slower kinetics. In this review, we have given an assessment and overview of the literature on bioconversion of glycerol. We have assessed as many as 23 products from glycerol bioconversion, and have reviewed the literature in terms of microorganism used, mode of fermentation, type of fermentor, yield and productivity of the process and recovery/purification of the products. The metabolic pathway of conversion of glycerol to various products has been discussed. We have also pondered over economic and engineering issues of large-scale implementation of process and have outlined the constraints and limitations of the process. We hope that this review will be a useful source of information for biochemists, biotechnologists, microbiologists and chemical engineers working in the area of glycerol bioconversion.