Statistical optimization of lipase production in solid-state fermentation by NDA03a and application of the fermented solid as a biocatalyst for biodiesel production

Background and objective
Biodiesel, an attractive alternative fuel, is defined by the American Society for Testing and Materials (ASTM) as fatty acid methyl esters (FAME). Biodiesel is an ecofriendly fuel compared with many other transportation fuels. The aim of this study was to implement the statistical approaches for optimization of NDA03a mutant G lipase produced in solid-state fermentation (SSF), and then application of the dried fermented solid as a biocatalyst for biodiesel production from waste frying oil (WFO).
Materials and methods

NDA03a mutant (3G) was previously selected as a good lipase producer. Five oil residue meals were evaluated in the presence of wheat bran (WB) for their potential as enzyme inducers and substrates for the production of 3G lipase by SSF. The best oil residue meal was selected and used in subsequent experiments. The fermented solid thus obtained was collected, lyophilized, and used as a biocatalyst for waste frying oil transesterification to FAME. To optimize SSF conditions for lipase production using 3G, a Plackett–Burman design was used at first to screen the critical factors from several process variables, and finally, a central composite design was applied to further estimate the relationship between the variables and response as well as optimize the levels. Response was measured in terms of FAME yield. To verify the adequacy and accuracy of the model, validation experiments were also carried out.
Results and conclusion
The most favorable oil residue meal that enhances 3G lipase production by SSF was black cumin meal. Results of the Plackett–Burman design revealed that the factors contributing to the main effect were incubation temperature, incubation period, and moisture content. The optimal SSF conditions for lipase production were WB 10 g, black cumin meal 6% (w/w of WB), pH 8, temperature 28°C, moisture content 40%, molasses 1% (w/w of WB), and incubation period 3 days. Under these optimized conditions, produced FAME yield (65.55%) increased by 58% compared with the basal medium (41.46%). A good agreement between the experimental (65.55%) and predicted (65.03%) values was detected. The significance of this model was confirmed by its probability value and lack of fit (<0.05) and clearly showed that the model was sufficient to describe the correlation between the FAME yield and the tested variables. The obtained results ascertained the success of response surface methodology as an efficient technique to optimize the lipase production in SSF and consequently the ability of application of the dried fermented solid as a biocatalyst for biodiesel production.