Fermentation Primer - Lesson X

In last month’s lesson we discussed Lachancea thermotolerans and its ability to ferment to a high ABV, along with its ability to produce a high rate of lactic acid (compared to S. cerevisiae).  In this month’s lesson we take a close look at another fascinating yeast.

Q: Are there any yeast species or varieties that show potential for the alcohol industry in future decades?

Alcohol producers (breweries, wineries, distilleries, fuel-alcohol plants) have different needs when selecting yeasts for their fermentations.  Some of these needs are:

  • Biomass yield, or the volume of yeast cells produced, which ties directly with the fermentation speed.
  • Growth under anaerobic conditions.
  • Ability to dominate other yeasts in a mixed medium, versus having to inoculate the medium (expensive) prior to introducing the desired yeast.
  • Ability to ferment to high ABV yields or with the right combination of congeners.
  • Yeast cell size, for biomass separation.  

The larger the cells, the easier it is to remove them.

Enter Torulaspora delbrueckii

The genus Torulaspora includes at least six species: T. delbrueckii (anamorph Candida colliculosa), T. franciscae, T. pretoriensis, T. microellipsoides, T. globosa, and T. maleeae.  The taxonomy of Torulaspora is changing rapidly, and we expect species reassignments and new species identification in the near future.

According to an article published by the American Journal of Enology and Viticulture in 1955 (Yeasts of wine fermentations from various regions of Italy), T. delbrueckii was the first non-Saccharomyces yeast proposed for industrial use in wine fermentation.  Unfortunately, however, T. delbrueckii shows less growth than S. cerevisiae under strict anaerobic conditions, and has less fermentation vigor and slower growth rate than S. cerevisiae under usual wine fermentation conditions, being quickly overcome by wild or inoculated S. cerevisiae strains.  These shortcomings are being addressed by researchers, as described below.

According to a paper published in 2018 by Manuel Ramírez and Rocío Velázquez, “a new wine T. delbrueckii killer strain has recently been characterised. It secretes a killer toxin (Kbarr-1) that is encoded in a double-stranded RNA virus (ScV-Mbarr-1) with broad antifungal activity against S. cerevisiae (killer and non-killer strains) and other non-Saccharomyces yeasts. This negative interaction has already been applied to promote the dominance of T. delbrueckii in white and red table wines. This killer strain had the advantage of dominating must fermentation in the presence of S. cerevisiae relative to the non-killer isogenic strains."

”With regard to the properties that are desired of yeasts for them to perform industrial alcoholic fermentations efficiently, the tolerance to high glucose concentrations (up to 700 g/L) of S. cerevisiae (a poor osmotolerant) was increased by fusing it with the heat-treated protoplasts of an osmotolerant T. delbrueckii strain. The resulting hybrids were able to grow in high glucose concentration media, producing increased amounts of ethanol.

The small cell size of T. delbrueckii cells (which leads to biomass separation problems and lower yields) seems to be because it propagates vegetatively as a haploid yeast, while the industrial strains of S. cerevisiae propagate as diploid or polyploid yeasts. This cell size can be enlarged by increasing the number of chromosomes (ploidy) inducing diploids through perturbed protoplast regeneration or the UV irradiation of intact cells.  Stable diploid strains have already been constructed from baker’s haploid T. delbrueckii strains. These diploid strains were about three times larger than the original parental haploid, while both types had similar biomass yield, stress resistance, gassing power and sweet dough-leavening ability.

While more research is still necessary, the above results are truly encouraging and may some day lead to additional non-Saccharomyces yeasts with desirable organoleptic and commercial traits!

Join us again next month as we continue to explore this fascinating topic.