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WsetDip D1 > 13.6 General Winemaking Options: Alcoholic Fermentation > Flashcards

13.6 General Winemaking Options: Alcoholic Fermentation Flashcards

1
Q

What is alcoholic fermentation?

A
  • 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
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2
Q

What is yeast? What is the effect of yeast on wine?

A
  • the collective term given to the group of microscopic fungi
  • convert sugar into alcohol
  • affect the aroma/flavour characteristics of wines
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3
Q

How do yeast kick off fermentation?

A
  • yeast need oxygen to multiply quickly

- once any oxygen is used up by the yeast (in aerobic respiration), they switch to fermentation

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4
Q

What are the 3 “right conditions” needed for yeast to ferment sugar into alcohol?

A
  1. a viable temperature range
  2. access to yeast nutrients (especially nitrogen)
  3. the absence of oxygen
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5
Q

What are the 7 by-products of fermentation?

A
  1. alcohol
  2. CO2
  3. heat
  4. volatile acidity
  5. SO2 (small amounts)
  6. wine aromatics
  7. glycerol
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6
Q

What does glycerol add to wine?

A

Increases the body

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7
Q

What wine aromatics are added to wine during fermentation?

A
  1. from aroma precursors

2. created by yeast

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8
Q

What are aroma precursors? Give 2 examples.

A

Compounds that have no flavour in the must, but are released by the action of yeast and create aromas in wine

Examples:

  1. thiols (4MMP, which gives aromas of boxwood/gooseberry in Sauvignon Blanc)
  2. terpenes (e.g. linalool and geraniol, which give Muscat its floral, grapey aroma)
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9
Q

Give an example of a wine aroma that is created by yeast.

A

Esters, which give many fruity flavours

  • e.g. banana flavour in wines made with carbonic maceration
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10
Q

What undesirable aroma characteristics might result from some species or strains of yeasts?

A
  • reductive sulfur compounds (rotten eggs, rotten cabbage)

- acetaldehyde (bruised apple, paint thinner).

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11
Q

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

A

Saccharomyces cerevisiae

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12
Q

Why is Saccharomyces cerevisiae so commonly used?

A
  • withstands high acidity
  • withstands increasingly high alcohol levels
  • fairly resistant to SO2
  • reliably ferments musts to dryness
  • many strains within the species, which gives rise to the option to choose a strain (known as a selection) for particular outcomes
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13
Q

Give an example of yeast selection and how it effects the final wines.

A

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)
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14
Q

Why might a producer choose to use Saccharomyces bayanus?

A
  • Used for must with high potential alcohol

- Used for re-fermenting sparkling wine

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15
Q

What are ambient yeast?

A
  • AKA Wild yeasts
  • present in the vineyard and the winery
  • most of which will die out as the alcohol rises past 5%
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16
Q

What are the 4 advantages of ambient yeast?

A
  1. Add complexity resulting from the presence of a number of yeast species producing different aroma compounds.
  2. Costs nothing
  3. Dominant yeast population in a must is unique to a place or region, contributes to terroir
  4. Marketing of the wine
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17
Q

What are the 4 disadvantages of ambient yeast?

A

1 . Fermentation may start slowly.

  1. Fermentation to dryness may take longer
  2. Increased risk of a stuck fermentation (fermentation ceases or slows) leaving the wine in a vulnerable state to spoilage organisms
  3. Consistency not guaranteed
18
Q

Why might a slow fermentation start be dangerous?

A
  • Build-up of unwanted volatile acidity
  • Growth of spoilage yeasts (such as Brettanomyces) and bacteria
  • Potentially leads to off-flavours
19
Q

What are cultured yeasts?

A
  • 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
20
Q

What must a winemaker do when using cultured yeasts to suppress the start of fermentation by the ambient yeasts?

A
  • cool the must down to prevent fermentation by ambient yeast
    OR
    -add SO2 to the must to suppress ambient yeasts
21
Q

What is a starter batch?

A
  • Fermenting grape must that has been activated with the cultured yeast
  • Added to the tank of must to be fermented
22
Q

What are the 4 advantages of cultured yeast?

A
  1. reliable, fast fermentation to dryness
  2. security - low levels of volatile acidity, less danger from spoilage yeasts and bacteria
  3. consistent product from one vintage to another
  4. large selection of cultured yeast strains available commercially, can affect style
23
Q

What are the 2 disadvantages of cultured yeast?

A
  1. Some believe that using cultured yeast leads to a certain similarity of fruit expression (‘industrial wine’)
  2. Cost
24
Q

Why is oxygen needed at the early stages of fermentation?

A

to enable yeast to multiply rapidly

25
Q

What may also be added to kick start the fermentation? In what cases may this be necessary?

A
  • yeast nutrients

- when Nitrogen levels are low

26
Q

Why are low levels of Nitrogen bad for fermentation?

A
  • can lead to a stuck ferment

- rotten egg smells from undesirable sulfur compounds created by stressed yeast

27
Q

What two yeast nutrients are commonly added?

A
  1. diammonium phosphate (known as DAP)

2. thiamine (vitamin B1)

28
Q

What temperature range is required for fresh, fruity whites and roses? Why?

A
  • Cool: 12–16°C / 54–61°F

- Cool temperatures promote the production and retention of fruity aromas and flavours

29
Q

What temperature range is required for easy-drinking fruity red wines to retain fruit aromas and for low tannin extraction, and less fruity whites?

A
  • Mid-range: 17–25°C / 63–77°F
30
Q

What temperature range is required for powerful red wines? Why? What is the downside?

A
  • Warm: 26–32°C / 79–90°F
  • Maximum extraction of colour and tannins
  • Can result in some loss of fruity flavours
31
Q

At what temperature does fermentation start to slow down or stop? Why?

A
  • Above 35°C (95°F)

- Yeasts struggle to survive

32
Q

How is temperature controled?

A
  • Temperature of the cellar (movement between rooms)
  • Temperature controlled vessels
  • Pumping over/délestage - releases heat
33
Q

How do temperature-controlled vessels work?

A
  • Use either water or glycol in jackets that surround vessels (typically stainless steels tanks) or in inserts that can be put into vessels.
  • Most monitored and controlled by computer
34
Q

What is the most standard fermentation vessel? Why?

A

Stainless steel

  • easy to clean
  • comes in a large range of sizes
  • enables a high degree of control over the temperature of the must or wine
  • protect wine from oxygen
  • do not add any flavours
35
Q

What is the most common fermentation vessel used in high volume wineries? Why?

A

Stainless steel

  • price
  • hygiene
  • high level of mechanisation possible
36
Q

What types of mechanisation is possible with stainless steel?

A
  • automatic pump-over
  • temperature control
  • automatic emptying
37
Q

What are the benefits of fermentation in concrete vats?

A
  • inexpensive option in the last century, with the vats being built in situ on a large scale
  • high thermal inertia: maintain even temperature much more efficiently than stainless steel
38
Q

Why might smaller, egg-shaped concrete vessels be used? What is the downside?

A
  • very expensive

- permit natural lees stiring (set up convection currents)

39
Q

What are the benefits of fermentation in wood?

A
  • retains heat well
  • Some winemakers value the small amount of oxygen that fermenting red wine in oak provides
  • can be reused many times; so are inexpensive over the long term.
40
Q

What are the downsides of fermentation in wood?

A
  • great care has to be taken with hygiene as the pores in wood can harbour bacteria and spoilage organisms
  • require capital investment when new large oak casks are bought.
41
Q

Why is red wine fermentation in small wooden barrels very rare?

A

due to the need to manage the cap of skins

42
Q

What are other fermentation vessels and what considerations are there for each?

A

Plastic
Pro: light, versatile, and useful for small-batch fermentations
Con: permeable to oxygen and it can be difficult to control the temperature in plastic vessels

Terracotta

  • historic (amphorae in Georgia)
  • small scale production today

WsetDip D1 (87 decks)

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