Starmerella bacillaris, previously known as Candida zemplinina, has received wide spread attention by wine yeast researchers as a result of its fructophilic nature and lower ethanol producing abilities. The yeast occurs naturally in spontaneous fermentations, but to date no commercially produced ADWY starter cultures are available. The aim of this study was to determine if S. bacillaris can be desiccated (dried) and rehydrated similarly to Saccharomyces cerevisiae and still have the desired effects in fermentations. For the purpose of this summary only some results of interest will be presented. Refer to the article for full experimental layout and all results.
Experimental layout
- Cell suspensions of five strains of S. bacillaris were desiccated in the presence of different trehalose concentrations (10, 20, 30, 40, 50%), by exposure to dry air for 20 hours at 28°C.
- Cell rehydration was performed at 30, 40 and 50°C for 10, 20 and 30 minutes.
- Viability was determined by plating out after rehydration and incubating for 48 hours at 28°C. Colony forming units (CFUs) were counted.
- Vinifications were performed with one strain (Sb3) in 450 ml Tempranillo must containing 100 g/L glucose and 140 g/L fructose (thus imbalanced).
- Three fermentation strategies were followed: EC1118 control, EC1118 + Sb3 in co-inoculation, and Sb3 inoculated when EC1118 reached 2% alcohol.
- Fermentations were performed at 25°C
Main results
- It was determined that the best viability results were obtained when S. bacillaris was desiccated in the early stationary growth phase. This is in contrast with drying S. cerevisiae where best viability results are obtained when drying the yeasts in the late stationary growth phase.
- The presence of 10% trehalose in the cell suspension before desiccation showed an increase of 18% in yeast viability. Trehalose naturally occurs in yeast cells and helps to protect yeasts against stress conditions.
- Best results were obtained with 30 minute rehydration time and S. bacillaris could also be rehydrated successfully at both 30 and 40°C. S. cerevisiae needs to be rehydrated at, at least 38°C.
- Viability was also improved if 0.5% galactose was included in the rehydration water.
- EC1118 fermentations took six days to dryness. Co- and sequential inoculations took five days.
- EC1118 fermentations on average had 12.3% alcohol. Co- and sequential inoculations had on average 11.9 and 11.4% alcohol.
- At the end of fermentation EC1118 had 95% viability. Sb3 was 2 and 10% respectively in the co- and sequential inoculations.
- The volatile compounds were two-fold higher for the co- and sequential inoculated wines. This result is in contrast with some other studies showing decreased volatile compounds in the S. bacillaris wines. Volatile compound outcomes are thus strain dependent.
Significance of the study
The dried and subsequently rehydrated yeast (Sb3) retained its positive traits of preferential utilisation of fructose and slightly lower alcohol production. The study therefore demonstrates the possibility that S. bacillaris can be produced and dried for commercial purposes.
Reference
Roca Domènech, G., López Martínez, G., Barrera, E. et al. Ann Microbiol (2018) 68: 667. https://link.springer.com/article/10.1007/s13213-018-1373-8
