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
How would a winemaker start his fermentation with cultured yeast
1) cool the must to suppress the ambient yeast fermenting
add in the cultured yeast
must is warmed again, and cultured yeast quickly overwhelm the population of ambient yeast.
OR
2) the must is treated with so2 to stop ambient yeast fermenting
starter batch of must activated with cultured yeast
starter batch added to tank.
What is a start batch?
- Fermenting grape must activated with the cultured yeast
- Added to the tank of must to be fermented
What are the advantages / disadvantages of cultured yeast?
Advantages
- reliable, fast fermentation to dryness
- security - low levels of volatile acidity, less danger from spoilage yeasts and bacteria
- consistent product from one vintage to another
- large selection of cultured yeast strains available commercially, can affect style.
Disadvantages
- Some believe that using cultured yeast leads to a certain similarity of fruit expression (‘industrial wine’)
- Cost
Why is oxygen needed at the early stages of fermentation?
to enable yeast to multiply rapidly
What may also be added to kick start the fermentation? In what cases may this be necessary?
- yeast nutrients
1. diammonium phosphate (known as DAP)
2. thiamine (vitamin B1) - when Nitrogen levels are low
Why are low level of Nitrogens bad for fermentation?
- can lead to a stuck ferment
- reductive (rotten egg) smells from undesirable sulfur compounds created by stressed yeast
What two yeast nutrients are commonly added?
- diammonium phosphate (known as DAP)
2. thiamine (vitamin B1)
Comment on temperature ranges required for fermentation per style of wine?
Fresh/fruity white or Rose
- Cool: 12–16°C / 54–61°F
- Cool temperatures promote the production and retention of fruity aromas and flavours
Low Tannin Reds / non aromatic whites
- Mid-range: 17–25°C / 63–77°F
Red Wines
- Warm: 26–32°C / 79–90°F
- Maximum extraction of colour and tannins
- Can result in some loss of fruity flavours
What will cause fermentation to stop
- Above 35°C (95°F) Yeasts struggle to survive
- Adding so2 or grape spirit which will kill the yeast
- Chilling the wine below 5°C
- if concentration of sugar is so high the yeast cannot function.
