Fermentation variablesImportant physical and chemical

variables for alcoholic fermentation

Sirromet Wines Pty Ltd850-938 Mount Cotton Rd

Mount Cotton Queensland, Australia 4165www.sirromet.com

Courtesy of Jessica FergusonAssistant 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• 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?

• 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

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 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

Yeast Strain• Yeast strains vary considerably in many factors, such as:

• Alcohol production and toxicity tolerance

• Temperature range

• Acetic acid production

• SO22 production

• Sugar metabolism (glucophilic, fructophilic)

• Flavour production and metabolism

• Selection of yeast strain is a critical decision in commercial winemaking