Non-Saccharomyces yeasts as antidotes to climate change: Fundamentals

In my post on the role of alcohol in wine balance I indicated that, in addition to unbalancing the wine, an excess of alcohol: makes the wine appear hot; will lead to a reduced perception of wine aroma; and can impart a sense of intoxication to the taster.  The winemaker may address the issue of excess alcohol by (i) reducing the concentration of sugar present in grapes or (ii) by removing "excess" alcohol from the wine.

Reducing grape sugar requires harvesting the grapes earlier than normal but this risks affecting wine composition and quality due to fewer aroma flavors, less color intensity, non-attainment of phenolic ripeness, and increased acidity. I have dealt with removing alcohol from the wine in a previous post and technologies/approaches employed include: (i) reverse osmosis, (ii) the spinning cone, and (iii) adding water to the wine. Reverse osmosis and the spinning cone are authorized in the U.S by wine regulation 27 CFR 24.248 Processes Authorized for the Treatment of Wine, Juice, and Distilling Materials.  Under this regulation the processes must be conducted at a Distilled Spirits Plant (DSP) or at a bonded winery that is authorized to alternate between a DSP and a bonded winery.

An additional alcohol-reducing approach has come to my attention by way of a recent Elin McCoy article which discusses how some Oregon producers were turning to non-Saccharomyces yeasts to aid in countering potential climate effects on the region's Pinot Noir wines. According to the article, wines made with the Pinot Noir grape, a famously finnicky variety, could become increasingly unbalanced in the future as the effects of climate change bring riper grapes (with higher potential alcohol) into the cellar.

Source: wineeconomist.com

According to Elin, Chapter 24, an Oregon-based producer was partnering with a professor at MIT to study the potential of non-Saccharomyces yeasts to reduce the alcohol content during fermentation without negatively affecting the aroma and flavor profile of the wines. I explore the science behind this effort in this series but begin with some background material on fermentation, Saccharomyces cerevisiae(SC), and non-Saccharomyces (non-SC) yeasts.

Wine is the result of applying yeasts to grape berries/must/juice in an anerobic environment in order to convert the resident sugars into alcohol.  The yeast that receives most of the credit -- and does most of the work -- is a species called Saccharomyces cerevisiae (SC) which is "specialized in metabolizing media with high sugar content and small quantities of nitrogenous compounds" (Suárez-Lepe and A. Marota, New trends in yeast selection for winemaking, Trends in Food Science and Technology 23 (2012), 39-50.).  According to Fugelsang (Overview of yeast selection and malolactic fermentation on aroma, flavor and phenols), the yeasts (i) extract compounds from the solids in the must/juice in order to form the "characteristic metabolites of fermentation (alcohols, esters, fatty acids, carbonyls, etc.) and (ii) cleave cysteine-containing precursors such that volatile thiols (aroma component of several varieties) can be released.  SC is the yeast species which completes the alcoholic fermentation process in both inoculated and spontaneous ferments.

Grapes in a vineyard are hosts to what Gourrand (Using non-Saccharomyces yeasts during alcoholic fermentations: taking advantage of yeast biodiversity) calls native microflora -- molds, lactic bacteria, acetic bacteria, Saccharomyces spp, and non-Saccharomyces yeasts (Pichia, Metchnikowia, Kloeckera, Kluyveromyces, Candida, Zygosaccharomyces, Torulaspora, Cryptoccus, Brettanomyces, and Hanseniaspora) -- and it is the yeast element of this microflora that the feral-yeast winemaking adherents seek to exploit.  Wild yeasts accumulate on the grapes from flowering through harvest with the presence of SC being pegged at 1 in 1000 grapes (Robert Mortimer, Vineyard Theory of Wild Yeast, UC Berkeley).  At harvest, SC is the least prevalent of the grape-resident yeast strains.

In the case of indigenous yeast fermentation, the process is kick-started and dominated initially by the "weakly fermentative" -- but numerically dominant -- non-Saccharomyces Kloeckera.  This initiation can take up to a week to begin due to the relatively small amount of wild yeasts present at startup (relative to the amount of yeast used to begin the process in the case of inoculated ferments).  For the first few days of fermentation, the weakly fermentative non-SC population dominates but is then replaced by more adaptive non-SC strains.  As the alcohol level continues to rise, the more alcohol-tolerant SC increases in number at a rapid rate such that at the end of the fermentation it is the dominant population.

Natural wine adherents assert that the progression from non-SC to SC fermentation in the vessel is an integral part of non-interventionist winemaking and adds complexity to the finished wine (Mortimer; Pretorius).  Critics of the approach see it as akin to Russian roulette because of the inherent risks (Ross; Pretorius): (i) the irregularity of natural fermentation and the associated risk of a stuck fermentation; (ii) in the event of rains around harvest time, the wild yeasts could be washed off the grapes; (iii) spoilage yeasts are often present in grape-derived yeasts; (iv) spontaneous ferments take longer to begin and longer to complete; and (v) while the positive characteristics of natural yeasts are not detectable after 6 or so months of aging, the negative characteristics tend to persist much longer.

For inoculated ferments, a large dose of SC is added to the juice/must in order to initiate fermentation.  The yeast strains utilized have traditionally been selected on the basis of the ability to start the fermentation quickly, the toleration of increasing alcohol levels, low acetic acid production, and resistance to sulfur dioxide (Ross; Suárez-Lepe and A. Marota).

The advantages that are perceived by "inoculants" are clear: (i) quick, effective, efficient fermentations: (ii) flexibility; (iii) lower risk production process; (iv) the ability to tailor the fermentation; and (v) the ability to take advantage of future advancements in commercially produced strains.  The disadvantage of the use of inoculation is, as perceived by the "naturalists," even more power placed into the hands of the winemaker to manipulate the dickens out of the wine; and the customer loses as a result.

As both Ross and Pretorius point out, the needs of large- and small-production wineries may lead to different emphasis in yeast-strain selection.  For the large producer, effective, efficient production and maintenance of quality is key and a strain that meets that need will be selected.  The smaller producer, on the other hand, is more likely to take advantage of varying yeast strains and temperature regimes as a means of enhancing the wine's aromatic and flavor characteristics.

To gain the benefits associated with both spontaneous and inoculated ferments, some winemakers are employing cocktails of strains hoping to get the "complexity of flavors ... without running the risk of contamination of spoilage yeasts" that comes along with the spontaneity.

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