Production of xylitol from xylose in batch fermentations of Candida mogii ATCC 18364 is discussed in the presence of glucose as the cosubstrate. Various initial ratios of glucose and xylose concentrations are assessed for their impact on yield and rate of production of xylitol. Supplementation with glucose at the beginning of the fermentation increased the specific growth rate, biomass yield and volumetric productivity of xylitol compared with fermentation that used xylose as the sole carbon source. A mathematical model is developed for eventual use in predicting the product formation rate and yield. The model parameters were estimated from experimental observations, using a genetic algorithm. Batch fermentations, which were carried out with xylose alone and a mixture of xylose and glucose, were used to validate the model. The model fitted well with the experimental data of cell growth, substrate consumption and xylitol production.
Mandarin and lime peels are rich sources of phenolic compounds including phenolic acids and flavonoids. The ultrasound-assisted extraction (UAE) proved to be an effective method in the extraction of phenolic compounds from plants due to its ability to save extraction time and increase extraction yields. In this research, factorial randomized complete experimental design has been used to investigate the effect of various factors on UAE using low power (50.93 W). Influence of citrus peel types and solvent types and concentrations on the total phenolic compounds, total flavonoids, and flavanone glycosides (hesperidin and naringin) were studied. Antioxidant activity of the extracts was measured by ferric reducing antioxidant potential (FRAP), azinobis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS), and 1,1-diphenyl-2-picrylhydrazyl (DPPH) assays. The optimum extraction conditions occurred when mandarin peel and 80 % acetone were used. In those conditions, the extract contained 3,083.61 mg gallic acid equivalent (eq) 100 g −1 dry weight (DW) of total phenolic compounds, 2,539.82 mg quercetin eq 100 g −1 DW of total flavonoid, 1,374.20 mg 100 g −1 DW of hesperidin, and the measured antioxidant activity was 0.61 mg mL −1 of DPPH IC 50 , 1.12 mg mL −1 of ABTS IC 50 , and 2,521.47 mg trolox eq 100 g −1 for FRAP. A strong correlation between total phenolic content and antioxidant activity was also observed.
A model-based optimization involving a genetic algorithm was used to establish the optimal substrate feeding profiles in fed-batch fermentations for maximizing the average production rate of xylitol from xylose. The best-case feeding profiles were verified experimentally in fermentations fed with glucose and xylose. The model-predicted profiles agreed remarkably well with the measured data even though the model was based on parameter values derived from a batch fermentation.
Summary
A 2‐step fermentation process was developed to enhance antioxidant activity of monascal waxy corn, fermented by Monascus purpureus TISTR 3090, compared with the conventional method. The results showed that, after 8 days of fermentation, monacolin K content and pigment intensity of monascal waxy corn from 2‐step fermentation were 62.89 mg kg−1 dry weight and 3072.70 unit g−1 dry weight, respectively, higher than those of the conventional method, while the residual reducing sugars were exhausted, giving the observed highest growth. The IC50 values of DPPH, ferric reducing/antioxidant powder (FRAP) and chelating ability on Fe2+ assays of the 2‐step fermentation were about 50% less than those of the conventional method. A significantly positive correlation was found between pigment intensity and monacolin K content, while significantly negative correlations were found among monacolin K content, IC50 of FRAP and chelating ability on Fe2+. An ideal negative correlation between pigment intensity and chelating ability on Fe2+ was found.
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