Abstract
The capacity of wine yeast to utilize the nitrogen available in grape must directly correlates with the fermentation and growth rates of all wine yeast fermentation stages and is, thus, of critical importance for wine production. Here we precisely quantified the ability of low complexity nitrogen compounds to support fast, efficient and rapidly initiated growth of four commercially important wine strains. Nitrogen substrate abundance in grape must failed to correlate with the rate or the efficiency of nitrogen source utilization, but well predicted lag phase length. Thus, human domestication of yeast for grape must growth has had, at the most, a marginal impact on wine yeast growth rates and efficiencies, but may have left a surprising imprint on the time required to adjust metabolism from non growth to growth. Wine yeast nitrogen source utilization deviated from that of the lab strain experimentation, but also varied between wine strains. Each wine yeast lineage harbored nitrogen source utilization defects that were private to that strain. By a massive hemizygote analysis, we traced the genetic basis of the most glaring of these defects, near inability of the PDM wine strain to utilize methionine, as consequence of mutations in its ARO8, ADE5,7 and VBA3 alleles. We also identified candidate causative mutations in these genes. The methionine defect of PDM is potentially very interesting as the strain can, in some circumstances, overproduce foul tasting H2S, a trait which likely stems from insufficient methionine catabolization. The poor adaptation of wine yeast to the grape must nitrogen environment, and the presence of defects in each lineage, open up wine strain optimization through biotechnological endeavors.
Highlights
Inoculation of selected yeast into wine must, rather than relying on spontaneous fermentation, is an established oenological practice that allows better control of organoleptic wine characteristics and guarantees the homogeneity of successive fermentations
To quantify variations in the nitrogen source utilization among wine strains, strains PDM, ARM, RVA and TTA were microcultivated at low and intermediate concentrations (30 and 140 mg N/L) of 23 individual nitrogen substrates. These substrates covered the entire width of the low complexity nitrogen compounds utilizable as sole nitrogen sources by the yeast lab strain S288c
We developed a strategy to assess the ability of four commercial wine strains to utilize all the low complexity nitrogen sources supporting yeast growth and we observed a substantial quantitative variation between both strains and sources
Summary
Inoculation of selected yeast into wine must, rather than relying on spontaneous fermentation, is an established oenological practice that allows better control of organoleptic wine characteristics and guarantees the homogeneity of successive fermentations. Most commercial wine production is based on such commercial starter wine yeasts, which were originally selected mainly from natural varieties of the Wine/European genetic clade [1], given their superior fermentation properties. The overall suitability of wine yeasts to grape wine production, which imposes demands for a large number of genetically complex traits, has not been stringently evaluated. The vast variability among natural yeasts [2], in combination with widespread antagonistic pleiotropy, suggests that any one strain selected from a natural stock is unlikely to possess an ideal combination of oenological characteristics. It is easy to envision a substantial potential for optimization of existing wine yeasts
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