Phenolic compounds affect several sensory components of wine, including red wine color, astringency, bitterness, and olfactory profile. Though present at low levels, their concentrations are a primary factor in the differences between wine types and styles. They are also important oxygen reservoirs and substrates for browning reactions.Their concentrations are largely due to processing considerations (for example, flavonoid content increases with increased skin contact time and temperature).
The basic phenol structure is carbolic acid (also known as hydroxybenzene; C6H5OH). Several hundred different phenolic compounds are naturally occurring in grapes, divided into two basic groups referred to as nonflavonoid and flavonoid phenols.
Nonflavonoid Phenols – The phenol content of grape juice is primarily nonflavonoid. For white varietals, nonflavonoids represent the overwhelming majority of finished wines’ phenol content as well. This is due to the fact that the majority of nonflavonoid phenols are sourced naturally from grape pulp: hydoxycinnamate derivatives present as free acids, ethyl esters, and tartrate-glucose esters. Nonflavonoid phenols levels are largely effected by fermentation; up to 20% of total nonflavonoids are absorbed by yeast, and many are hydrolized to free acid and ester forms including free cinnamic acids and ethyl phenols. Phenols arising from oak maturation are primarily hydrolizable nonflavonoids such as vanillin (oak influence on phenol content will be discussed in a following post).
Most nonflavonoids are present below their sensory threshold, though collectively they can have an impact on bitterness and astringency. Some nonflavonoids are also indicators of spoilage; for example, 4-ethyl phenol can be used as an indicator of Brettanomyces.
Flavonoid Phenols -Flavonoids have much more impact on a wine’s structure and color compared to nonflavonoids. They are found in skins, seeds, and stems of both white and red grapes; they represent 25% of total phenol content in white varietals made without skin contact, and represent 80-90% of total phenol content in red wines produced in a traditional manner. Flavonoids can exist in monomeric forms, or polymerized to other flavonoids, nonflavonoids, sugars, or a combination of these. Polymeric flavonoids make up the majority of total phenolics found in all stages of red winemaking; further polymerization yields flavenoid polyphenolic compounds (tannins and condensed tannins).
Catechins account for the majority of white wine flavonoid content (particularly those produced without skin contact), and up to 14% of total red wine phenol content. These are flavon-3-ols; catechins are negatively charged, while epicatechins are positively charged. Catechins and epicatechins are the precursors for browning and bitterness in both white and red wines. They polymerize to create procyanidins (condensed tannins).
Luecoanthocyanidins and luecoanthocyanins serve as precursors to larger polymeric forms (anthocyanins, which will be discussed in a following post). These compounds are very closely related to catechins; luecoanthocyanidins have an additional hydroxyl group, and luecoanythocyanins have an attached sugar molecule. These compounds have minimal effect on a wine’s bitterness, less than flavonols.
Flavonols are primarily found in grape skins, thus their concentrations in wines produced without skin contact are minimal or nonexistent. Quercitin commonly represents the majority of a wine’s flavonol content, though kaempforol and myricetin are also found in significant concentrations. These compounds have some effect on a wine’s bitterness.
As fermentation proceeds and yeast continue to propagate, they pass along a certain amount of cellular material to future generations. In juice/must without proper yeast rehydration, a gradual reduction in cell membrane thickness and decreasing amounts of nutrient reserve transfer from generation to generation is common as fermentation continues. This had led many producers to include the use of yeast rehydration nutrients to help make their yeast ‘happy’ during the rehydration process, and lead to a healthy yeast population, essential to successful fermentation.
There are several proprietary yeast rehydration nutrients available today, including Dynastart, GoFerm, and PreFerm. They are created from autolyzed yeast cells, and provide many essential micronutrients (including membrane lipids and sterols) and vitamins (including biotin, niacin, and thiamine) that are readily absorbed by active yeast cells. Providing these during the rehydration process reactivates the yeast’s internal metabolism quicker and leads to a substantial increase in cell volume; the original structures of the yeast’s plasmatic membrane are modified, leading to better viability, increased membrane fluidity, increased resistance to ethanol (essential towards the end of ferment), increased resistance to osmotic shock due to high sugar concentration (essential at inoculation), and increased aroma production (essential for good tasting wine!). Yeasts prepared with yeast rehydration nutrients also maintain a steadier metabolic rate throughout fermentation. Less stress also means far less volatile acidity formation and negative sulfur-containing compounds (hydrogen sulfide, disulfides).
The use of yeast rehydration nutrients is recommended at a 5:6 ratio with yeast (5 parts yeast to 6 parts rehydration nutrients, usually 250 ppm yeast with 300 ppm nutrient). I tend to use the recommended rate only when I know that strenuous fermentation conditions are inevitable (high brix levels, low fermentation temperature, low turbidity juice, historically deficient juice, etc.) or during yeast starter culture propagation. For normal fermentation conditions, I tend to use a 1:1 ratio (usually 200 ppm yeast and 200 ppm yeast rehydration nutrients).Winemakers also need to be wary of legal dosages of particular ingredients (i.e. thiamine) when using yeast rehydration nutrients in conjunction with other fermentation nutrients.
Please see here for yeast rehydration preparation.