Sugar reduction process of purple sweet potato concentrated juice by microbial fermentation for improved performance of natural pigments

High sugar content of anthocyanin extract from purple sweet potato makes it susceptible to microbial contamination and pigment deterioration, leading to a lot of inconvenience to subsequent processing and application. This work was herein designed to develop a microbial fermentation method for effectively reducing sugar content in purple sweet potato concentrated juices with low anthocyanin loss. The effects of various factors on the sugar reduction and pigment retention were evaluated in terms of fermentation strain, initial sugar content, fermentation time, temperature, pH and strain addition. The combined effects of three variables including fermentation temperature, pH and strain addition on sugar reduction were analyzed via response surface methodology (RSM). The optimum conditions were determined as strain RA, initial sugar content 150 g/L, fermentation time 4 days, temperature 29.8 ℃, pH 4.10, strain addition 0.16%, for maximum sugar reduction of 99.02%. Moreover, the dynamic changes of yeast amount, sugar and the resulting alcohol during fermentation could be well predicted by SGompertz, DoseResp and Logistic models, respectively. In addition, the purple sweet potato concentrated juices after fermentation exhibited increased DPPH radical scavenging capacity and stabilities. The findings indicate that the microbial fermentation is promising for improving performance of pigment extracts, which provides a new idea for the storage of natural pigment crude extracts.