Comparative quality assessment of wine produced from green and purple grapes using Saccharomyces cerevisiae isolated from fermented milk

Abstract

This research aimed at the comparative quality assessment of wine produced from green and purple grapes using saccharomyces cerevisiae isolated from fermented milk. The yeast was isolated using the pour plate technique and identified using cultural morphology, microscopic appearance, the carbohydrate utilization (sugar) test and ethanol tolerance test. Green and purple grapes were placed in different conical flask. The flask were designated as GGW and PGW for each sample respectively. Each sample (1000 ml) in the flask was inoculated with 2 ml 1.0 x 10₆ CFU of S. cerevisiae / mL , mixed with 0.2 ml 10% sodium metabisulfite, fermented for 5 days at room temperature, degassed and stirred daily, pasteurized at 60°C to stop the fermentation process and evaporated in a rotary evaporator at 60°C to remove the alcohol. Changes in temperature, pH, titratable acidity, volatile acidity, alcohol content, specific gravity (physicochemical parameters) were determined at intervals using standard methods. The must and the produced wines were subjected to several analysis. The produced wines were subjected to a proximate analysis, microbial analysis (total bacteria, coliform and total yeast counts) and sensory evaluated using nine-point hedonic scale by 10 experts (employees and students) and a shelf life of the produced wine were also determined. The yeast isolate was microscopically suspected to have the same colony morphology as Saccharomyces cerevisiae. As it was observed that glucose, maltose, fructose, galactose and sucrose were used up by the yeast isolates: The yeast isolate had an ethanol tolerance of up to 14%. The physicochemical parameters were relatively stable throughout the wine production process. The temperature ranged from 27°C - 28°C. The pH rose after fermentation. The specific weight decreased in all produced wines. Results of the proximate composition of green and purple grape wines showed varied percentages. The microbial analysis showed that bacterial isolates were dominated by yeast isolates. The sensory evaluation showed that both green and purple grape wine had an overall acceptance. The shelf life results showed that purple wine got spoilt faster than green wines. This study has shown that locally isolated yeast strains can be used to produce wines from locally available fruits which can yield good nutritional and microbiological quality.