Fermentation: Yeast, Temperature and Vessels

Question 1

What is alcoholic fermentation?

Answer 1

The conversion of sugar into ethanol (also known as ethyl alcohol) and carbon dioxide carried out by yeast in the absence of oxygen (‘anaerobically’).

This conversion also produces heat

Question 2

Define Yeast:

Answer 2

Yeast is the collective term given to the group of microscopic fungi that convert sugar into alcohol and affect the aroma/flavour characteristics of wines.

Question 3

Describe how yeast works:

Answer 3

Initially, yeast need oxygen to multiply quickly, but once any oxygen is used up by the yeast (in aerobic respiration), they switch to fermentation

They need:
- a viable temperature range
- access to yeast nutrients (especially nitrogen)
- the absence of oxygen

Low levels of nitrogen can stress the yeast causing them to produce undesirable sulfur compounds (giving a rotten egg smell) or even to stop fermenting (resulting in a stuck fermentation)

Yeast nutrients: Diammonium phosphate (known as DAP) or thiamine (vitamin B1) can be added as yeast nutrients.

Question 4

What does the fermentation produce?

Answer 4

• Alcohol
• Heat
• C02
• volatile acidity
• very small amounts of naturally-produced SO2
• wine aromatics (precursors: Thiols, Terpenes / created by yeast: esters)
• glycerol (which increases the body of the wine)

Question 5

What is the most common species of yeast used in winemaking?

Answer 5

Saccharomyces cerevisiae

It can withstand well the high acidity and increasing alcohol level of the must as it ferments and, hence, it reliably ferments musts to dryness.

It is also fairly resistant to SO2 in comparison to other yeast species.

Question 6

Define/Describe ambient yeast:

Answer 6

• Ambient yeast (also called wild yeast) is present in the vineyard and the winery.
• include a range of yeast species (e.g. Kloeckera and Candida), most of which will die out as the alcohol rises past 5 per cent.

Question 7

Advantages/Disadvantages of ambient yeast

Answer 7

Advantages:
* Can add complexity resulting from the presence of a number of yeast species producing different aroma compounds.
* It costs nothing to use.
* Some studies have shown that the yeast population in a must can be unique to a place or region, especially where widely available cultured yeast has not been used (TERROIR)
* Using ambient yeast may also be used as part of the marketing of the wine.

Disadvantages:
* Fermentation may start slowly (Can lead to unwanted volatile acidity and the growth of spoilage organisms (such as Brettanomyces) and bacteria, potentially leading to off-flavours)
* Fermentation to dryness may take longer, which may not be desirable in a high volume winery. There is also increased risk of a stuck fermentation (fermentation ceases or slows) leaving the wine in a vulnerable state to spoilage organisms.
* A consistent product cannot be guaranteed, which can be a drawback, especially for producers looking for consistency over many large vessels or across vintages.

Question 8

Define/describe Cultured yeast

Answer 8

Yeast strains that are selected in a laboratory and then grown in volumes suitable for sale. (often single strains of Saccharomyces cerevisiae)

The must may be cooled down to prevent fermentation by ambient yeast and then the cultured yeast added, which quickly overwhelm the natural yeast population.

Another option is to add SO2 to the must to suppress ambient yeasts. A starter batch, made up of fermenting grape must which has been activated with the cultured yeast, is then added to the tank of must to be fermented.

Question 9

Advantages/Disadvantages of Cultured yeast

Answer 9

Advantages:
* Reliable, fast fermentation to dryness.
* Low levels of volatile acidity
* Less danger from spoilage organisms and bacteria (given its speed and reliability)
* Helps to produce a consistent product from one vintage to another.
* With a large selection of cultured yeast strains available commercially, the winemaker’s
choice can also affect the style of wine created. For example, the winemaker may choose to use a neutral yeast for a sparkling wine base or a strain of yeast to boost the aromatic character in Sauvignon Blanc (e.g. for mid-priced Marlborough Sauvignon Blanc).
Other species of yeast are used for particular wines, for example, Saccharomyces bayanus is sometimes used for must with high potential alcohol or for re-fermenting sparkling wine

Disadvantages:
* Some believe that using cultured yeast leads to a certain similarity of fruit expression
* Adds the cost of using a commercial product.

Question 10

Describe the role of temperature in fermentation:

Answer 10

The speed of fermentation is related to the temperature of the must, which in turn affects the style of wine being produced

Cool: 12–16°C / 54– 61°F
(Cool temperatures promote the production and retention of esters that give fruity aromas and flavours: fresher, fruitier white wines and rosé)

Mid-range: 17–25°C / 63–77°F
(For less fruity white wines, top of this range for barrel-fermented white wines: Easy-drinking fruity red wines to retain fruit aromas and for low tannin extraction)

Warm: 26–32°C / 79–90°F
(Maximum extraction of colour and tannins, but can result in some loss of fruity flavours: Red wines with pronounced flavour concentration and high tannins)

Above 35°C (95°F) the fermentation may slow down and stop as yeasts struggle to survive, with risk of a stuck fermentation

Question 11

Options of Temperature control:

Answer 11

• Temperature of the room in the winery
• Temperature control systems in the vessel (usually Stainless steel)
• water or glycol in jackets that surround the vessels
• Temperatures can also be reduced by pumping over/délestage, which releases heat

Question 12

Stainless steel vessels

Answer 12

• Easy to clean
• Large range of sizes
• Enables a high degree of temperature control
• Neutral vessels (very good at protecting the wine from oxygen / do not add any flavours)
• Most common type of vessel used in modern, high-volume wineries due to:
• price
• hygiene
• high level of mechanisation possible (automatic pump-over, temperature control, automatic emptying, etc.)
• Substantial initial financial investment
  (in the tanks themselves and in computerised temperature-control systems)

Question 13

Concrete vats:

Answer 13

• inexpensive option in the last century
• high thermal inertia: they maintain an even temperature much more efficiently than stainless steel
• Smaller, egg-shaped vessels in concrete
  (very expensive ) are said to set up convection currents that mix the fermenting must and mix the lees during maturation

Question 14

Wood vessels:

Answer 14

• Retains heat well
• Provides small amount of oxygen
• Great care has to be taken with hygiene as the pores in wood can harbour bacteria and spoilage organisms.
• Can be reused many times and so are inexpensive over the long term.
• Require capital investment when new large oak casks are bought
• White wines may also be fermented in small wooden barrels (Relatively rare for red wines due to the need to manage the cap of skins)

Question 15

Other type of vessels:

Answer 15

Terracotta:
- Historically used
- Amphora, Qvevri (Georgia) and Tinaja (Spain)

Plastic:
- light
- Versatile and useful for small-batch fermentations
- Permeable to oxygen
- Difficult to control the temperature

Question 16

Define/Describe Malolactic fermentation

Answer 16

• Result of lactic acid bacteria converting malic acid into lactic acid and carbon dioxide, and it produces heat
• Typically happens after alcoholic fermentation (occasionally during it)
• Certain conditions encourage it to happen: 18–22°C (64–72°F), a moderate pH (3.3–3.5) and low total SO2.
• Can be started by inoculating with cultured lactic acid bacteria
• Inhibit conditions: Temperature below 15°C (59°F), a low pH and moderate levels of SO2.
• Can be avoided by adding the enzyme lysozyme, which kills lactic acid bacteria or by filtering out lactic acid bacteria

Question 17

What are the outcomes of MLF:

Answer 17

• Reduction in acidity and rise in pH – This is because lactic acid is a weaker acid than malic acid. It results in a softer, smoother style of wine.
• Some colour loss in red wines – This is not a problem except in very pale red wines.
• Greater microbial stability – If the wine goes through malolactic conversion during or after alcoholic fermentation, this then prevents malolactic conversion from spontaneously happening later (e.g. when the wine is in the bottle) when it would be undesirable.
  However, in cases where the pH of the wine is high, raising the pH slightly makes the wine
  more vulnerable to spoilage organisms.
• Modification of the flavour – A slight loss of fruit character may occur with the addition of buttery notes (notably in white wine). The process will also increase volatile acidity.

Question 18

Post-fermentation adjustments:

Answer 18

• The acidity, pH and tannins can be adjusted
• Removal of Alcohol:
  Reverse osmosis – A form of cross-flow filtration that removes a flavourless permeate of alcohol and water, which can be distilled to remove the alcohol. The watery permeate is then blended back to recreate the wine.
  (most common high-tech option / generally costly)

Spinning cone – A device that first extracts volatile aroma compounds from wine and then removes the alcohol. The aroma compounds are then blended back into the wine of the desired alcohol level.
(Only financially viable for large volumes of wine)

• Colour : reduce unwanted colour tints by fining the wine / enhance colour intensity by adding very small amounts of the grape-derived colouring agent, MegaPurple.