It was previously found that yeasts can negatively affect wine colour in two ways: via their β-glycosidase enzymatic activity (removing the sugars from monomeric anthocyanins and thus rendering them colourless), and through direct adsorption of anthocyanins on the yeasts’ cell walls. Brazilian researchers evaluated the effect of five different commercial yeasts on red wine colour. The yeasts were previously grouped into three categories namely low, medium and high yeast pigment adsorption phenotypes. This grouping was done by the same researchers during their development of a method to test the adsorption capacity of yeasts.
The current research
Small scale fermentations were conducted using a 50/50 Merlot/Tannat must combination that underwent a laboratory scale thermovinification process. The researchers measured the anthocyanin adsorption overall, as well as that of individual cells.
What they found
- The researchers found that yeasts begin to adsorb anthocyanins towards the end of fermentation.
- In general, yeasts with a high adsorption phenotype adsorbed more colour than the lower adsorption phenotypes.
- However, within the same high adsorption yeast population low and high anthocyanin adsorption was observed. This was not the case for the low adsorption yeasts where the population was more uniform.
- The researchers found a correlation between cell viability (actually, cell morbidity) and anthocyanin adsorption, meaning: dead cells adsorb colour and not live ones. This finding is supported by previous research that found that more colour is adsorbed by commercial yeast hulls (cell walls) than the same concentration of intact viable yeast cells.
- The researchers concluded that the main driver for colour adsorption by yeast cells is actually their ability to stay alive during fermentation and not so much their capacity to bind colour pigments.
Lessons to learn from this research
- Dead yeast cells bind colour and remove it from your wine. If you have issues with obtaining enough colour in your red wines, say with Pinot noir, high yielding blocks or virus infected vines, it is best to use very stress resistant yeast strains with high viability towards the end of fermentation. Examples of high alcohol tolerant strains are the “PDM type” strains (Saccharomyces cerevisiae subsp. bayanus) or certain hybrid strains.
- Spontaneous fermentations can contain a big percentage of dead cells since they comprise big populations of non-Saccharomyces yeasts that die during fermentation. This can be very advantageous for the surviving S. cerevisiae cells in terms of providing nutrition for the yeasts, as well as the subsequent MLF bacteria growth if MLF is desired. Not so good for colour if the colour is already a less ideal situation.
- Ageing on gross lees where most of the yeasts are dead is also not a good idea, unless of course if you have colour to lose and don’t mind they funky smells they also release after death (built up H2S and other sulphur compounds due to a nitrogen shortage during fermentation).
There are various ways to ensure high yeast viability throughout fermentation of which using the correct yeast at the correct dosage for the prevailing conditions, maintaining moderate fermentation temperatures, and feeding the yeast appropriately, are the most important ones.
Reference
Sergio Echeverrigaray, Fernando Joel Scariot, Morgana Menegotto, Ana Paula Longaray Delamare (2020). Anthocyanin adsorption by Saccharomyces cerevisiae during wine fermentation is associated to the loss of yeast cell wall/membrane integrity. International Journal of Food Microbiology, Vol. 314, 108383. https://doi.org/10.1016/j.ijfoodmicro.2019.108383.
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