Fermentation variables
Important physical and chemical
variables for alcoholic fermentation
Sirromet Wines Pty Ltd
850-938 Mount Cotton Rd
Mount Cotton Queensland, Australia 4165
www.sirromet.com
Courtesy of Jessica Ferguson
Assistant Winemaker & Site Chemist
Downloaded from seniorchem.com/eei.html
A cautionary note…
• Remember that a hydrometer does not
measure ‘sugar’ or for that matter, ‘alcohol’,
directly
• A hydrometer measures density
• Any component in solution that affects
solution density will affect S.G.
• Sugar increases density, alcohol decreases
density
• You cannot calculate either sugar content or
alcohol content from an S.G. reading where
both sugar and alcohol are present!
‘Investigations’ in Fermentation
• A common assignment seems to be to
investigate the effects of changing one
variable in the must/juice
• Important to remember when performing
such experiments that fermentation is a
biological process
• Any ‘effect’ observed will be fundamentally
due to a change in yeast metabolism/viability
Typical variables
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pH
Acidity or varying acid profile
Initial sugar concentration
Type of sugar (glucose, fructose, sucrose)
Temperature
Yeast strain
Yeast preparation
Usually investigated against a ‘control’
What sort of results?
Can we draw conclusions?
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Failure of fermentation onset
Increased/decrease lag phase period
Increase total fermentation period
Failure of fermentation completion - ‘stuck’
Increased/decreased alcohol production
Final residual sugar levels
Variability in other fermentation products
Changes in pH or acidity pre- to postfermentation
• Other changes – colour, smell, clarity
pH
• Yeast will ferment sugar to alcohol over a very large pH
range
• Winemaking pH range is typically 3.0-4.0
• Changing initial pH generally has little effect on
fermentation kinetics or products, or final alcohol levels
• Very low pH (<3) will impede yeast
• Higher pH >4 will favour bacteria and other competing
organisms (Acetobacter)
• Very high pH >4.5 will favour other pathways of sugar
catabolism (reduced alcohol production)
• pH will affect role of any SO2 present as action of SO2 is
pH dependent
• pH does not usually change much during normal ferment
Acidity
• Acids in fruits are weak organic acids
• Acid profile varies with fruit (handout)
• Most acids do not take significant part in
fermentation metabolism
• Tartaric acid may precipitate as tartrate salt (loss
of acidity)
• Malic acid may be metabolised to lactic acid (loss
of acidity) by yeast or MLF bacteria
• Faulty ferment may produce excess acetic acid
(increased acidity)
• Acidity and pH may change slightly due to
production of alcohol (changes buffer capacity)
Changes in Acidity and Acid
Profile during Fermentation
• Acidity (TA) may increase or decrease overall
• Succinic acid, acetic acid produced via normal
alternative pathways (increase)
• Some yeast strains may produce malic acid,
more may convert some of malic acid to lactic
acid (increase or decrease)
• Tartaric acid is stable to microbial action but
can precipitate with liberated potassium ions
(as potassium tartrate or potassium hydrogen
tartrate)
Sugar
• Sugars in fruit are usually a combination of
glucose, fructose and sucrose
• Grapes approx 1:1 glucose:fructose, trace
sucrose (other fruits, see handout)
• Yeast may ferment glucose faster than
fructose.
• Sucrose is inverted by yeast enzymes to
glucose + fructose
Sugar Concentration
• Typically 20-25% in winemaking
• This is high enough to delay onset of
fermentation (longer lag phase)
• High sugar >250g/L – cell viability reduced
- cell division retarded
- possible increased sensitivity
to alcohol toxicity
- increased production of acetic acid
- greater likelihood of stuck ferment
Temperature
• Along with sugar concentration, temperature is
one of the most important fermentation variables
• Growth rate of yeast strongly temperature
dependent
• Cell division: every 12 hours at 10˚, every 5 hours
at 20˚, every 3 hours at 30˚
• At temperatures over 20, yeast viability declines
rapidly at the end of ferment
• For many reasons, the preferred temperature for
winemaking is below that known to be optimal for
ethanol production or yeast growth
Low temperature ferments
• 15-20˚ typical for white wine styles
• Yeast growth retarded, but yeast viability
enhanced (reduces toxicity effects of
alcohol)
• Slower ferment rate – longer to complete
fermentation (note: too cold will arrest
fermentation)
• Higher production of alcohol
• Increased synthesis and retention of fruit
esters and fatty acid ethyl esters
• Better flavour concentration for whites
Higher temperature ferments
• 24-27˚ for reds
• Higher temperatures favours extraction of
anthocyanins (colour) and tannins
• Shorter lag phase = earlier alcohol production,
which also favours colour and tannin extraction
• Higher temps can favour undesirable
consequences such as increased production of
acetic acid, aldehyde and acetoin, lower ester
production
• will be less noticeable in reds due to their more
complex composition
Final Thoughts - Temperature
• Consider a juice at 23˚ Brix
• Theoretically can increase its own
temperature by 30˚ during fermentation
• However this heating occurs over daysweeks, not all at once (luckily for yeast)
• Rise in temperature due to fermentation can
easily reach levels critical to yeast survival
if not controlled
Yeast Strain
• Yeast strains vary considerably in many
factors, such as:
• Alcohol production and toxicity tolerance
• Temperature range
• Acetic acid production
• SO2 production
• Sugar metabolism (glucophilic, fructophilic)
• Flavour production and metabolism
• Selection of yeast strain is a critical decision
in commercial winemaking