1. Fermentation variables
Important physical and chemical variables for alcoholic fermentation
Sirromet Wines Pty Ltd
Mount Cotton Rd
Mount Cotton Queensland, Australia 4165
Courtesy of Jessica Ferguson
Assistant Winemaker & Site Chemist
Downloaded from seniorchem.com/eei.html

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
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

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 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

10. 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

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