Process optimization and kinetic evaluation for biosynthesis of d-isoascorbyl stearate

The synthesis of d -isoascorbyl stearate from d -isoascorbic acid and stearic acid with immobilized lipase (Novozym ® 435) as catalyst was studied. Response surface methodology and Box-Behnken design with six variables and three levels were employed to evaluate the effects of processing conditions on the conversion of d -isoascorbic acid. The results confirmed that the response surface method and statistical analysis were proved to be useful in developing optimal conditions for d -isoascorbyl stearate synthesis. The optimum conditions were predicted as follows: reaction temperature 48 °C, reaction time 17.7 h, immobilized lipase amount 50.0 % (w/w, of d -isoascorbic acid), substrate molar ratio 9:1 (stearic acid to d -isoascorbic acid), d -isoascorbic acid concentration 0.14 mol/L (based on solvent), 4A molecular sieve addition 200 g/L (based on solvent), and the optimal conversion was 90.6 %. Through the kinetics model fitting of the esterification, it was considered that the esterification conformed to a Ping–Pong bi–bi kinetic model with d -isoascorbic acid inhibition, and the obtained kinetic constants showed that the inhibition of d -isoascorbic acid and the enzyme affinity to substrate were abate with the increase of the reaction temperature.