The reserves of fossil-based fuels, which currently seem sufficient to meet the global demands, is inevitably on the verge of exhaustion. Contemporary raw material for alternate fuel like biodiesel is usually edible plant commodity oils, whose increasing public consumption rate raises the need of finding a non-edible and fungible alternate oil source. In this quest, single cell oils (SCO) from oleaginous yeasts and fungi can provide a sustainable alternate of not only functional but also valuable (polyunsaturated fatty acids (PUFA)-rich) lipids. Researches are been increasingly driven towards increasing the SCO yield in order to realize its commercial importance. However, bulk requirement of expensive synthetic carbon substrate, which inflates the overall SCO production cost, is the major limitation towards complete acceptance of this technology. Even though substrate cost minimization could make the SCO production profitable is uncertain, it is still essential to identify suitable cheap and abundant substrates in an attempt to potentially reduce the overall process economy. One of the most sought-after in-expensive carbon reservoirs, agro-industrial wastes, can be an attractive replacement to expensive synthetic carbon substrates in this regard. The present review assess these possibilities referring to the current experimental investigations on oleaginous yeasts, and fungi reported for conversion of agro-industrial feedstocks into triacylglycerols (TAGs) and PUFA-rich lipids. Multiple associated factors regulating lipid accumulation utilizing such substrates and impeding challenges has been analyzed. The review infers that production of bulk oil in combination to high-value fatty acids, co-production strategies for SCO and different microbial metabolites, and reutilization and value addition to spent wastes could possibly leverage the high operating costs and help in commencing a successful biorefinery. Rigorous research is nevertheless required whether it is PUFA-rich oil production (for competing with algal omega oils) or neutral bulk oil production (for overcoming yield limitations and managing process economy) to establish this potential source as future resource.

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In the present work, a mild acid saccharification approach for rice (Oryza sativa) straw was developed by response surface optimization. A 2 3 central composite experimental design with variable factors, such as duration (minutes), mild acid concentration percentage (v/v) and minimum solid loading percentage (w/v) was chosen to optimize the hydrolysis process, fixing the operating temperature to a moderate minimum of 121 °C. The study determined a solid loading of 5%, an acid concentration of 0.75%, and a residence duration of 150 min to be optimum, for enhanced reducing sugar release. To compensate for the reduced saccharification efficiency under mild operating conditions, a novel approach of multitier saccharification was implemented under optimal conditions of saccharification. High-performance liquid chromatography (HPLC) analysis revealed the final reducing sugar yield of 0.76 g reducing sugar per gram of straw (47.9 g/L and 35 g/L concentration in two consecutive saccharification cycles) and glucose, galactose, and xylose as major reducing sugars in hydrolysate. On assessing the hydrolysate as a fermentable substrate with and without detoxification for oleaginous strain Mortierella alpina, nondetoxified hydrolysate surpassed detoxified hydrolysate for growth support, sugar consumption, and lipid accumulation. HPLC analysis indicated the complete absence of furfurals and hydroxymethylfurfurals in the nondetoxified hydrolysate. Therefore, the rice straw hydrolysate produced by multitier mild acid saccharification approach was suitable for microbial propagation without requiring detoxification, suggesting it to be a potential fermentable substrate for microbial lipid production, as well as for other prospective bioprocesses.