Issue link: http://winesandvines.uberflip.com/i/137110
GRAPEGROWING Using ecological diversity of yeasts to modify wine fermentations BY Dr. Kate Howell T he average concentration of alcohol (ethanol) in wine has been steadily increasing, particularly in red wines. This problem has many causes, as a consequence of grape sugars increasing at harvest. In Australia, grape sugar at harvest has been linked to crop yield reductions, soil drying, changing vineyard management practices and climate warming over a 60-year period (Webb et al. 2012 Nature Climate Change 2; 259-264). The problem is only likely to increase in Australia, with similar trends being noted in Europe (Mira de Orduna 2010 Food Research International 43; 1844 – 1855). Simple microbiology tells us that the ethanol in wine is produced from grape sugars by yeasts during fermentation. So, increased sugar in harvested grapes leads to more available sugar for yeasts to convert into ethanol. The problem is exacerbated by using Saccharomyces cerevisiae to conduct the fermentation. This yeast is particularly good at converting sugar to ethanol and relies on this to modify the fermentation environment and ensure numerical dominance. The metabolic pathways of S. cerevisiae are optimised to produce ethanol, and it is difficult to engineer these yeasts to produce less (see for example Kutyna et al. 2010 Trends in Food Science and Technology 21(6); 293 – 302). S. cerevisiae is just one part of the microbiological story in winemaking. There are considerable populations of non-Saccharomyces yeasts present on grapes and in wine, with differing abilities to produce ethanol. There are some unpleasant side effects to fermenting with non-Saccharomyces yeasts, which include sluggish or stuck fermentations and disagreeable aromas that lead to increased risk of fermentation failure. The non-Saccharomyces yeasts exist in grape must inoculated with S. cerevisiae, but appear not to make a big contribution to normal fermentation conditions. 56 p r acti c al w i ne ry & v i ne yard J U LY 20 13 Experiments looking at non-Saccharomyces yeast metabolism show that there is much biochemical and metabolic diversity in these yeasts. There is potential to harness this ecological diversity to alter aroma of wine, or, in this case, change the yield of ethanol in a normal fermentation. Controlled use of non-Saccharomyces yeasts is an emerging technique in winemaking, and using these yeasts in altered inoculation regimes could give the winemaker control over the fermentation outcomes. What has been given little attention is the ethanol yield of these yeasts, particularly when grown with S. cerevisiae. A review of the literature shows that the potential yield of ethanol, and the amount of sugar used varies widely between non-Saccharomyces yeasts (Figure 1). These studies have been performed in single-culture fermentations, where the inoculated non-Saccharomyces yeast is the sole fermentative microorganism. Clearly, this is an oversimplification of a wine fermentation, where these yeasts will co-exist with other yeasts, in particular S. cerevisiae. Yeasts such as Torulaspora delbrueckii, Hanseniaspora uvarum and Candida spp. are common fermentation partners, and investigation of the fermentation outcomes shows that common wine components are altered (Figure 1). Compounds such as acetic acid and glycerol could have a sensory impact on the resultant wine. And of course importantly, wine ethanol can be reduced by 1%–2%. Sequential inoculation is a technique where non-Saccharomyces yeasts are inoculated and allowed to ferment for several days before the addition of a selected S. cerevisiae to complete the fermentation. Our industry partners practice a form of this modified protocol at a well-known Victorian winery. The freshly crushed red grapes are allowed to sit at ambient winery temperatures for several days before inoculation with an industrial strain of yeast. The winemakers feel that these wines show more complex aromas and flavors than other wines, and they often see a disparity between the Baume to ethanol conversion (in most cases, 1°Bé converts to 1% ethanol (w/v). This indicates that non-inoculated yeasts could be using some of the sugar otherwise available for fermentation. Case Study Could it be that sugars could be diverted from S. cerevisiae's efficient sugar to ethanol conversion pathways, and into growth and metabolism of a non-Saccharomyces yeast? However, growth of these 'wild' yeasts can be a source of concern for the winemaker as other (un)desirable compounds could be produced to affect the aroma of the resultant wine. Careful strain selection and yeast combinations will allow intelligent use of yeasts for sequential fermentations. Different Grape juice/must containing 200 – 300 g/L glucose + fructose S. cerevisiae Candida spp. Hanseniaspora Pichia spp. Torulaspora spp. Sugar 100% Ethanol +++ C02 +++ Glycerol ++ Sugar 30 – 60% Ethanol ++ C02 + Glycerol +++ Acetic acid + Sugar 10 – 100% Ethanol + C02 + Glycerol + Acetaldehyde + Ethyl acetate +++ Sugar ? Ethanol ? C02 ? Thiols +++ Sugar ? Ethanol +++ C02 ? Succinic acid + Figure 1. A review of the literature cited shows that the usage of sugars and fermentation products differs widely in non-Saccharomyces yeasts. Sugar is given as a % of that converted into other compounds, and the ethanol is given from + < 4%; ++ < 10%: +++ < 17% measured as alcohol by volume.