A non-linear kinetic model to predict the consumption of different sugars (glucose, fructose and sucrose) as a substrate, during an apple wine yeast fermentation with Saccharomyces cerevisiae strain CCTCC M201022 is proposed. This model was used to predict sugar utilization by this yeast beginning at various initial sugar concentrations. After observation of the experimental data, a model based on the logistic equation of yeast growth, growthassociated production of ethanol with a lag time, and consumption of sugars for biomass formation and maintenance, was developed. After experimental model fitting, kinetic parameters in the model were estimated. The experimental verification of the model was performed using flask-scale fermentations, and the model obtained predicted the fermentation performance effectively, using different sugars as the substrate set at various initial sugar concentrations. Based on estimated kinetic parameters and the characteristics of sugar utilization, the yeast examined appeared to be glucophilic. The effects of different sugars with various initial concentrations on the fermentation performance by this yeast were investigated, and some applications of kinetic parameters are discussed.
Flavor volatiles are important components of alcoholic beverages. In this study, headspace solid-phase microextraction and gas chromatography-mass spectrometry (HS-SPME-GC-MS) were investigated for the determination of flavor-contributing volatiles for apple wine. HS-SPME parameters were defined, including fiber coatings, extraction time, and extraction temperature. Good linearity, recoveries, and repeatability of the SPME method using carboxen-polydimethylsiloxane (carboxen-PDMS) fiber were obtained with r 2 values, recoveries, and relative standard deviations ranging from 0.9112 to 0.9960, 80.23% to 110.21%, and 1.5% to 6.4% respectively using standard solution. No significant effects of ethanol concentration on headspace concentrations of analytes were observed when ethanol concentration changed from 0% to 12% (v/v), indicating that the HS-SPME-GC-MS method can be used for the determination of flavor volatiles in apple wine with an alcohol content below 12% (v/v) as well as in apple juice, and the method enables the monitoring of volatiles in mash during fermentation.
The effect of pure and mixed fermentation by Saccharomyces cerevisiae and Hanseniaspora valbyensis on the formation of major volatile components in cider was investigated. When the interaction between yeast strains of S. cerevisiae and H. valbyensis was studied, it was found that the two strains each affected the cell growth of the other upon inoculation of S. cerevisiae during growth of H. valbyensis. The effects of pure and mixed cultures of S. cerevisiae and H. valbyensis on alcohol fermentation and major volatile compound formation in cider were assessed. S. cerevisiae showed a conversion of sugar to alcohol of 11.5%, while H. valbyensis produced alcohol with a conversion not exceeding 6%. Higher concentrations of ethyl acetate and phenethyl acetate were obtained with H. valbyensis, and higher concentrations of isoamyl alcohol and isobutyl were formed by S. cerevisiae. Consequently, a combination of these two yeast species in sequential fermentation was used to increase the concentration of ethyl esters by 7.41-20.96%, and to decrease the alcohol concentration by 25.06-51.38%. Efficient control of the formation of volatile compounds was achieved by adjusting the inoculation time of the two yeasts.
Theanine (γ-glutamylethylamide) is the main amino acid component in green tea. The demand for theanine in the food and pharmaceutical industries continues to increase because of its special flavour and multiple physiological effects. In this research, an improved method for enzymatic theanine synthesis is reported. An economical substrate, glutamic acid γ-methyl ester, was used in the synthesis catalyzed by immobilized Escherichia coli cells with γ-glutamyltranspeptidase (GGT) activity. The results show that GGT activity with glutamic acid γ-methyl ester as substrate was about 1.2-folds higher than that with glutamine as substrate. Reaction conditions were optimized by using 300 mmol/l glutamic acid γ-methyl ester, 3,000 mmol/l ethylamine, and 0.1 g/ml of immobilized GGT cells at pH 10 and 50°C. Under these conditions, the immobilized cells were continuously used ten times, yielding an average glutamic acid γ-methyl ester to theanine conversion rate of 69.3%. Bead activity did not change significantly the first six times they were used, and the average conversion rate during the first six instances was 87.2%. The immobilized cells exhibited favourable operational stability.
An extracellular β-glucosidase from Aspergillus niger Au0847 was purified to homogeneity by precipitation with ammonium sulfate, anion exchange, and gel filtration. The purified protein was composed of two subunits with molecular masses of 110 and 120 kDa. Au0847 β-glucosidase exhibited relatively high thermostability and pH stability, and its highest activity was obtained at 65°C and pH 4.6, respectively. As a potential metalloprotein, its enzymatic activity was potently stimulated by manganese ion and DTT. The β-glucosidase displayed avid affinity and high catalytic efficiency for geniposide. Au0847 β-glucosidase has potential value as an industrial enzyme for the hydrolysis of geniposide to genipin.
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