The winemaker's bug: From ancient wisdom to opening new vistas with frontier yeast science

Abstract

The past three decades have seen a global wine glut. So far, well-intended but wasteful and expensive market-intervention has failed to drag the wine industry out of a chronic annual oversupply of roughly 15%. Can yeast research succeed where these approaches have failed by providing a means of improving wine quality, thereby making wine more appealing to consumers? To molecular biologists Saccharomyces cerevisiae is as intriguing as it is tractable. A simple unicellular eukaryote, it is an ideal model organism, enabling scientists to shed new light on some of the biggest scientific challenges such as the biology of cancer and aging. It is amenable to almost any modification that modern biology can throw at a cell, making it an ideal host for genetic manipulation, whether by the application of traditional or modern genetic techniques. To the winemaker, this yeast is integral to crafting wonderful, complex wines from simple, sugar-rich grape juice. Thus any improvements that we can make to wine, yeast fermentation performance or the sensory properties it imparts to wine will benefit winemakers and consumers. With this in mind, the application of frontier technologies, particularly the burgeoning fields of systems and synthetic biology, have much to offer in their pursuit of "novel" yeast strains to produce high quality wine. This paper discusses the nexus between yeast research and winemaking. It also addresses how winemakers and scientists face up to the challenges of consumer perceptions and opinions regarding the intervention of science and technology; the greater this intervention, the stronger the criticism that wine is no longer "natural." How can wine researchers respond to the growing number of wine commentators and consumers who feel that scientific endeavors favor wine quantity over quality and "technical sophistication, fermentation reliability and product consistency" over "artisanal variation"? This paper seeks to present yeast research in a new light and a new context, and it raises important questions about the direction of yeast research, its contribution to science and the future of winemaking.

Figure 1. To the winemaker, yeast is integral to crafting wonderful, complex wines from simple, sugar-rich grape juice. Grape juice is converted into wine by the action of wine yeast. Some wine components are wholly generated by yeast as part of metabolism while others are essentially created by the grapevine. The large number of compounds synthesized or modified by wine yeast have a major impact on wine quality and style.

Figure 2. Reducing alcohol levels in wine: several GM-based strategies have been explored to generate wine yeasts that partially divert sugar metabolism away from ethanol production. (A) Two glycerol-3-phosphate dehydrogenase isozymes, GPD1 and GPD2, can be harnessed to divert carbon from glycolysis to glycerol production. However, increased glycerol production was accompanied by undesirable increased concentrations of acetic acid. This problem was alleviated by knocking out ALD6. (B) Wild-type yeast convert most of the sugar they consume into ethanol and CO₂.

Figure 3. Commercial yeast strains possess different abilities to form and modulate compounds that impact on wine sensory properties. These compounds are produced as a result of yeast metabolic processes.

Figure 4. There are powerful synergies between Sauvignon Blanc grapes and yeast strains in formation of the compounds responsible for tropical fruit flavors: 4-mercapto-4-methylpentan-2-one (4MMP), 3-mercaptohexan-1-ol (3MH) and 3-mercaptohexyl acetate (3MHA). Odorless cysteine and glutathione conjugates are converted to aromatic thiols by carbon-sulfur-lyase enzymes. Alcohol acetyl transferase further modifies 3MH, converting it to the more potent 3MHA.

Figure 5. Building on knowledge from work utilizing GM strategies, a classical, non-GM mutagenesis approach was used to develop three “low-H₂S” strains. These strains have impaired sulfite reductase activity due to mutations in their MET10 and MET5 genes.

Figure 6. There are two options to genetically engineer extraneous malate utilization in order to deacidify wine. One approach utilizes the Schizosaccharomyces pombe malate transporter gene (mae1) and the O. oeni malolactic enzyme gene (mleA), enabling yeast to perform malolactic fermentation in parallel with alcoholic fermentation. Alternatively, Saccharomyces cerevisiae can be modified by the introduction of mae1 and the S. pombe malic enzyme gene (mae2), thereby enabling the conversion of malate into ethanol.

Figure 7. A wine yeast has been genetically engineered to reduce ethyl carbamate production during fermentation. Through increased expression of DUR1/DUR2, this yeast breaks down urea to ammonia and CO₂ before it is able to react with ethanol.