Wine

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Carbon dioxide is toxic to yeast and can impact cell performance. The release of carbon dioxide helps to minimize toxicity and decreases the lag phase of yeast growth. This is the time in which juice is most sensitive to both enzymatic and chemical oxidation. Mixing during fermentation keeps the yeast in suspension, and helps to drive carbon dioxide out of solution, resulting in a lowering of carbon dioxide saturation. Mixing during fermentation may be important, regardless of the size and shape of the vessel. Some addition products contain inert compounds like micro-crystalline cellulose, the purpose of which is to help release carbon dioxide from solution.

Under the acidic conditions in wine (pH 3.4) a random bondbreaking and -making process among procyanidins was observed. This led to the production of molecules of increased size which eventually precipitate; thus leading to a decrease in astringency.

Oxidation, particularly of white wines, was a common fault in Australian white wines 40 years ago when our table wine technology was reasonably primitive, compared to that of today. Oxidation is much less common today with the application of refrigeration, inert gas blanketing during the production and packaging operations and effective sulfur dioxide management. The oxidation flavor is due to multiple compounds including a range of aldehydes.

Some wines are more sensitive to oxidation than others. White wines made from the "floral" varieties such as Riesling are very prone to oxidation, whereas red wines can withstand significant oxidation during handling due to the higher content of phenolic compounds, which are natural antioxidants. The sensory characteristics of oxidation range from a dulling of the aroma, to "cardboard", "straw" and "hay-like" aromas, to "sherry-like" and "madeirised". In extreme cases a "wet wool", "wet dog", or "varnish-like" aroma can be evident. Of course for some wine styles, such as sherry, oxidation is deliberately encouraged.

In wine, excessive oxidation can cause a loss of desirable sulfur compounds, such as the fruity "grapefruit, passionfruit" compounds critical to Sauvignon Blanc and many other fruity white wines.

It is common practice for winemakers to make N additions at the following times:

• Yeast rehydration to rebuild cell walls (rehydration nutrients consist of inactivated yeast and autolysates. They contain no inorganic N and only 3 mg/L N for every 100 mg/L added).
• Six to twelve hours after inoculation (2-3 Brix drop)
• End of growth/exponential phase (1/3 sugar depletion)

Note that at ½ sugar depletion the yeast cannot utilize N since alcohol accumulation prevents uptake. This residual N can then be utilized by other organisms such as Brettanomyces spp.

Of course, intermediate must YAN favors the best balance between desirable and undesirable chemical and sensory wine attributes. The key is to have timely and accurate YAN must concentration data immediately before primary inoculation.

Nitrogen is a key nutrient for yeast growth, and is necessary for the successful fermentation of grape juice and must into wine. Nitrogen compounds in juice, must, and wine affect not only the fermentation, but the clarification, aroma, and final chemical composition of the wine. For these reasons, the analysis of nitrogen in the winemaking process is recommended for ensuring a quality wine.

Supplements added after about half the fermentation is completed may not be used by the yeast because alcohol prevents their uptake. Therefore, adding nutrients to a stuck fermentation is seldom effective. With increasing ethanol concentrations the permeability of the plasma membrane to hydrogen ions increases. This requires intracellular enzymes and ATPases to pump protons back out of the cell in order to balance the internal pH of the yeast cell against the external pH of the juice/must. Due to the competing nature of these coupled transport systems, nitrogen is picked up by the cell only in the early stages of fermentation. It is stored in vacuoles and used upon demand. Nitrogen added late in the fermentation late cannot be transported into the cell (17). Once stopped due to nutrient stress, the fermentation may require significant effort to complete.

Slight aeration of yeast starters may play an important role in subsequent fermentative performance. Wahlstrom and Fugelsang (21) reported increased cell density and more rapid fermentations when aerated starters were used compared with non-aerated starters. In the absence of sulfur dioxide, grape-derived oxidative enzymes (tyrosinases) catalyze conversion of nonflavonoid phenols to their corresponding quinones. The reaction brings about rapid (but reversible) browning of the juice while consuming oxygen required by yeasts during the early stages of growth (Figure 3). Grape tyrosinase is easily inactivated by addition of SO2 to the juice/must. However, sulfur dioxide addition also inactivates thiamine. If additions of more than 50 mg/L SO2 occur, thiamine (in the form of nutritional supplements) should be added to the fermenter.

Sulfate exists naturally in grape juice in amounts as high as 700 mg/L or more.

Potential Resources

• https://www.apps.fst.vt.edu/extension/enology/online.html
• https://www.researchgate.net/publication/49842680_Micro-Oxygenation_of_Red_Wine_Techniques_Applications_and_Outcomes
• http://www.math.fsu.edu/~gmizell/mead/Lallemand%20Yeast%20Quick%20Reference.pdf
• https://www.grapecreek.com/blog/wine-education/jeff-s-corner-2-22-27/
• Proteomics of Grapevines and Wines
• Oxidation of Wine Phenolics: A Critical Evaluation and Hypotheses
• Phenolic Compounds of Grapes and Wines: Key Compounds and Implications in Sensory Perception
• Relationship between Sensory Perception, Volatile and Phenolic Components in Commercial Spanish Red Wines from Different Regions
• Comparative contents of some phenolics in beer, red and white wines.
• "Impact of oxygen on quality of white wine"
• "Oxidation mechanisms occurring in wines"
• Chapter 4 - Microoxidation in Wine Production
• https://www.ajevonline.org/content/69/2/141 [Fe(III)]:[Fe(II)] Ratio and Redox Status of Red Wines: Relation to So-Called "Reduction Potential"
• Characterization and Control of Hidden Micro-Oxygenation in the Winery: Wine Racking
• FAQs about YAN
• Se[ptember Northern Grapes Project News You Can Use – Yeast Assimilable Nitrogen
• YEAST ASSIMILABLE NITROGEN (YAN)
• Starting your fermentation right: nutrient supplementation
• Using Yeast Nutrients
• Yeast Assimilable Nitrogen (YAN) Optimization for Fermentation of Cold Climate Cultivars
• Advanced Nutrients in Meadmaking
• 2020 WINEMAKING HANDBOOK
• YAN Calculations / nutrient discussion
• Magnesium, Calcium and Fermentative Metabolism in Wine Yeasts