13.6 General Winemaking Options: Alcoholic Fermentation Flashcards
What is alcoholic fermentation?
- the conversion of sugar into ethanol (also known as ethyl alcohol) and carbon dioxide carried out by yeast in the absence of oxygen (‘anaerobically’)
- also produces heat, which has to be managed
What is yeast?
- the collective term given to the group of microscopic fungi
- convert sugar into alcohol
- affect the aroma/flavour characteristics of wines
When do yeast kick of fermentation?
- yeast need oxygen to multiply quickly
- once any oxygen is used up by the yeast (in aerobic respiration), they switch to fermentation
What are the 3 “right conditions” needed for yeast to ferment sugar into alcohol?
- a viable temperature range
- access to yeast nutrients (especially nitrogen)
- the absence of oxygen
What are the 7 by-products of fermentation?
- alcohol
- CO2
- heat
- volatile acidity
- SO2 (small amounts)
- wine aromatics
- glycerol
What does glycerol add to wine?
Increases the body
What wine aromatics are added to wine during fermentation?
Aromas from pre-cursors
- Thiols 4mmp - box tree - SB
- Terpenes - linalool, geraniol - fruity/floral in muscat e.g.
Aromas from the fermentation/byproducts
- Esters - isoamyl acetate (banana) and many other fruity aromas; ethyl acetate (vinegar smell)
- Acetaldehyde
- Diacetyl (from malic-lactic acid conversion) butter
- in some cases sulfur compounds (small amounts are “struck match” large amounts - reductive and nasty
What is the most common species of yeast used for winemaking?
Saccharomyces cerevisiae
Why is Saccharomyces cerevisiae so commonly used?
- withstands high acidity
- withstands increasingly high alcohol levels
- fairly resistant to SO2
- reliably ferments musts to dryness
- many strains within the species, menas option to choose for particular outcomes
Explain how yeast selection may affects the final wines.
Options:
- Yeast that boosts the aromatic character in Sauvignon Blanc (e.g. mid-priced Marlborough Sauvignon Blanc)
- Ambient or cultured yeasts that produce a more restrained fruit profile (e.g. in Sancerre)
Why might a producer choose to use Saccharomyces bayanus?
- Used for must with high potential alcohol
- Used for re-fermenting sparkling wine
What are ambient yeast?
- AKA Wild yeasts
- present in the vineyard and the winery
- most of which will die out as the alcohol rises past 5%
What are the advantages and disadvantages of ambient yeast?
Advantage
- Add complexity from the variety of possible species producing different aroma compounds.
- Costs nothing
- Thought to indicate “place” or “region” “terroir”
- Marketing /selling point
Disadvantage
1 . Fermentation may start slowly.
2. Fermentation to dryness may take longer
3. Increased risk of a stuck fermentation (fermentation ceases or slows) leaving the wine in a vulnerable state to spoilage organisms
4. Consistency not guaranteed
Why might a slow fermentation start be dangerous?
- Build-up of unwanted volatile acidity
- Growth of spoilage yeasts (such as Brettanomyces) and bacteria
- Potentially leads to off-flavours
What are cultured yeasts?
- AKA selected yeast or commercial yeast
- yeast strains that are selected in a laboratory and then grown in volumes suitable for sale
- Often single strains of Saccharomyces cerevisiae