Utilization of lactic acid-producing yeast to create novel, yogurt-like dairy products

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Bacteriophages are environmental viruses omnipresent in the air, on plants, animals, and liquids. They need a host culture to grow. The bacterial starter cultures that are commonly used in the fermented dairy industry become their potential host. Bacteriophage infects the starter cultures and changes their functionality disabling them to produce lactic acid from lactose as they should. Thus, the texture of the fermented product gets affected. Later, phages kill the host culture, reproduce more phages and lead to a fermentation failure. Yeasts (Saccharomyces cerevisiae) do not naturally produce lactic acid but can be used as an organism that can be engineered to exchange ethanol with lactic acid as a fermentation by-product. Sourvisiae® is a genetically modified strain of S.cerevisiae engineered to develop sour beer using one strain of yeast at a faster rate with fewer steps involved. Sourvisiae® has a single gene modification, a lactate dehydrogenase gene, that enables the yeast to produce high levels of lactic acid. For this project, we studied the fermentation kinetics of this engineered S.cerevisiae strain in milk. We aim to standardize the fermentation parameters using this yeast strain so that it can mimic the fermentation properties of traditional bacterial starter culture. This would allow us to find a potential solution to address the bacteriophage issue in the fermented dairy industry and also would help create a novel line of fermented dairy products using yeast as a single strain starter culture. One of the major parameters to consider for proper fermentation is the time taken to reach the food-safe pH of 4.6. To achieve that we considered the fermentation parameters to be standardized as incubation temperature, using different lactase enzymes, no lactase enzyme, and the addition of excess glucose to understand the conversion rate of glucose to lactic acid by the yeast, and the inoculation rate of the yeast. The parameters were standardized using UHT Skim milk and the fermentation was set up for 24 hours. The fermentation markers like pH, ethanol concentration, lactic acid production, and cell counts (before and after fermentation) were recorded for each trial conducted. All the experiments were conducted in triplicates. The final standardized parameters were found to be 30°C as the incubation temperature of the inoculated milk sample along with Maxilact A4 as the lactase enzyme and the inoculation rate of the yeast to be 10^8 CFU/ml. These parameters would help to reach a pH of 4.6 within a set timeframe. Following these parameters, the comparison between the viscoelastic properties of the fermentate using a traditional starter culture and Sourvisiae® as the starter culture was observed using the ElastoSensTM Bio. The results showed that both the fermentates had a similar viscoelastic property. Therefore, Sourvisiae® has the potential to be considered as a starter culture and replace the traditional culture based on the set parameters. A sensory study was consulted with the prototype to understand the consumer's thoughts about this new product. An unflavored fermented sample was tasted against a commercial store-bought kefir drink which had a similar ingredient panel with no additives. The results showed a huge difference in the acceptability. The store-bought was more acceptable in visual appearance and taste compared to the Sourvisiae® fermented prototype. Further shelf-life study to be conducted to understand the rate at which the pH and other parameters change with storage. Also, further product development to be done based on the sensory data to understand consumer preferences and to optimize the sensory profiles accordingly.

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