Agro-Industrial Wastes to Sustainable Bio-Oil Fuels, Enzymes and Biobased Chemicals in Yeast-Biorefineries

The International Energy Agency Bioenergy Task 42 defined biorefinery as “the sustainable processing of biomass into a spectrum of bio-based products (food, feed, chemicals, materials) and energy (fuels, power, heat)”. This includes biochemical, thermochemical, chemical, and mechanical processes. Yeast and fungi are ideal agents for biochemical biorefineries because of their metabolic versatility. This chapter will focus on yeast-biorefineries which can use inexpensive agro-industrial waste to obtain two main products: bio-oils (feed and food), and the enzyme lipase, which is a high-value product that can modify the bio-oil to get more products. The so-called “unconventional yeasts” stand out for having the ability to metabolize a variety of carbon sources. Yarrowia lipolytica, Cutaneotrichosporon oleaginosus and Rhodotorula toruloides have been described as the most efficient in terms of yield of bio-oil. For instance, Y. lipolytica can consume glycerol to produce bio-oil, lipases, organic acids, polyols and single-cell protein. Its genome is known and can be modified to overproduce the target compound or to engineer metabolic routes for other products of interest such as polyols. Therefore, this yeast has been proposed as a model for yeast-biorefineries. Besides, enzymes are one of the products with more added value that can be produced in a biorefinery. Yeasts are also very good enzyme producers or even hosts for heterologous expression of enzymes. Moreover, solid fermentation is a configuration particularly suitable to use agro-wastes in a biorefinery. Indeed, fermented solids of agro-wastes may be directly employed as biocatalysts allowing the production of more cost-competitive bio-oil fuels and biobased chemicals. This strategy may use inexpensive agro-industrial wastes as solid support/substrates for microorganism growth and lipase production, avoiding expensive steps of enzyme purification and immobilization. Recombinant lipase from Thermomyces lanuginosus produced by solid-state fermentation using agro-industrial wastes and its application to obtain biodiesel is described as example.