Simultaneous Saccharification and Co-Fermentation using co-culture for conversion of mango leaves into bioethanol

The food versus fuel competition has spurred nation’s interest on secondary bioethanol, which is produced from lignocellulosic (LC) biomass. Mango leaves (Mangifera indica) have been recognized as abundantly available non-edible agricultural waste that could be a LC based bioethanol substrate for their high polysaccharide content. Besides, simultaneous saccharification and co-fermentation (SSCF) is identified as an effective method to maximize this secondary bioethanol production. However, a feasible co-culture to simultaneously convert both hexose and pentose sugar in the mango leaves into bioethanol is yet to be discovered. Therefore, the present work emphasizes on conversion of mango leaves into bioethanol through co-culture of Saccharomyces cerevisiae and Trametes versicolor via SSCF. Initially, a comparison of biomass composition among mango leaves varieties indicated that Harum Manis mango leaves are the most desirable bioethanol crop for its highest holocellulose content of 78.73 %, maximum concentration of fermentable sugar (163.83 ± 5.48 mg/g of biomass) and bioethanol (0.21 ± 0.02 mg/mL) compared to Chokanan and Sunshine. Subsequently, this study aimed to elucidate the structural changes of Harum Manis mango leaves after single-stage (acid and alkaline) and two-stage (acid-alkaline and alkaline-acid) chemical pretreatments. Acid-alkaline pretreatment with the maximum delignification effect (86.97 ± 1.26) facilitated the biomass to yield the highest holocellulose composition, fermentable sugar content and bioethanol concentration of 95.26%, 415.02 ± 7.01 mg/g and 1.57 ± 0.06 mg/ mL, respectively. Additionally, the structural characterizations also validated the acid-alkaline pretreated biomass to be the most feasible substrate for bioethanol production in this study. Next, cellulase and xylanase with enzymatic activity of 0.413 ± 0.014 FPU/mL and 0.514 ± 0.009 IU/mL, respectively, were synthesized from Aspergillus niger in this study. These enzymes were proven to be viable for enzymatic saccharification during the bioethanol production. Thereafter, this study also demonstrated that S. cerevisiae and T. versicolor were compatible as a co-culture to significantly enhanced the co-fermentation of glucose and xylose in the mango leaves into bioethanol. With this, the designated SSCF based bioethanol production was screened and optimized. Consequently, the optimum condition at temperature: 30.68oC, pH: 5 and incubation time: 5.08 days yielded a maximum bioethanol production of 79.59 ± 0.32 %. Lastly, kinetic study of this bioethanol production was evaluated. The Logistic model and Modified Gompertz model were discovered to proficiently understand the co-culture cell synthesis, substrate consumption, and product formation that occurred during the bioethanol production. Overall, the scale up potential of this SSCF process could be explored using the obtained kinetic insights for an efficient conversion of mango leaves into bioethanol.