Fermentation%20variables

1
Fermentation variables

• Important physical and chemical variables for
  alcoholic fermentation

2
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!

3
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

4
Typical variables

• 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

5
What sort of results? Can we draw conclusions?

• 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 post-
  fermentation
• Other changes colour, smell, clarity

6
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 (lt3) will impede yeast
• Higher pH gt4 will favour bacteria and other
  competing organisms (Acetobacter)
• Very high pH gt4.5 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

7
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)

8
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)

9
Sugar

• Sugars in fruit are usually a combination of
  glucose, fructose and sucrose
• Grapes approx 11 glucosefructose, trace sucrose
  (other fruits, see handout)
• Yeast may ferment glucose faster than fructose.
• Sucrose is inverted by yeast enzymes to glucose
  fructose

10
Sugar Concentration

• Typically 20-25 in winemaking
• This is high enough to delay onset of
  fermentation (longer lag phase)
• High sugar gt250g/L cell viability reduced
• - cell
  division retarded
• - possible increased
  sensitivity
• to alcohol toxicity
• - increased production of acetic
  acid
• - greater likelihood of stuck
  ferment

11
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

12
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

13
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

14
Final Thoughts - Temperature

• Consider a juice at 23 Brix
• Theoretically can increase its own temperature by
  30 during fermentation
• However this heating occurs over days-weeks, not
  all at once (luckily for yeast)
• Rise in temperature due to fermentation can
  easily reach levels critical to yeast survival if
  not controlled

15
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