Comparative evaluation of direct in situ transesterification of wet and dry biomass for biodiesel production from a high-lipid-yielding Aspergillus terreus variant TB21.

This study evaluates direct transesterification of wet versus dry biomass for biodiesel production using an MNNG (N-methyl-N'-nitro-N-nitrosoguanidine)-induced Aspergillus terreus TB21 variant cultivated on a glucose-based lipid accumulation medium (LAM). Biomass was subjected to in situ acid transesterification for FAME (fatty acid methyl esters) recovery. Under glucose-LAM conditions, the TB21 variant showed enhanced lipid productivity compared to the wild type, with FAME yield increasing from 4.90 to 7.46 g L⁻¹ and FAME content from 34.50% to 48.18% in dried biomass. Wet biomass in situ transesterification significantly improved process efficiency, achieving rapid FAME recovery within 5 min under the parameter conditions for transesterification optimised earlier as compared to conventional methods requiring 2-24 h. Under optimized conditions, TB21 produced a maximum FAME yield of 95.16 g L⁻¹ with 82.17% FAME content, exceeding the wild type (66.98 g L⁻¹; 72.73%) and representing substantially higher FAME recovery than that obtained using conventional dry-biomass transesterification. Fatty acid analysis indicated a shift toward biodiesel-favourable composition, with total saturated fatty acids reaching 82.6% and polyunsaturated fatty acids decreasing to 5.05%, contributing to improved oxidative stability. The resulting biodiesel exhibited predicted fuel properties, including cetane number (58), iodine value (88 g I₂/100 g), and kinematic viscosity (4.1 mm² s⁻¹), which were within the acceptable ranges specified by ASTM D6751, EN 14,214, and IS 15,607 standards. The combined strategy of strain improvement and wet biomass direct transesterification enhanced lipid recovery, reduced processing time and provides an efficient approach for sustainable microbial biodiesel production.