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Sunday, July 22, 2012

Production of Wine from Grapes & Estimation of Percentage of Alcohol, Total Acidity, Volatile Acidity in wine


The making of wine involves the fermentation of soluble sugar of juices of grapes into carbon dioxide and ethanol. After grapes are harvested they are crushed to produce a raw juice or "must", a highly acidic liquid containing 10 - 25% sugar by weight. The mixed yeast flora on the grapes can serve as the inoculum for the fermentation or the true wine yeast, Saccharomyces cervisae. Must from both the red & white wine grape is white & results in white wine. Since the colour of red grapes is in the skin, and red wine are made by fermentation. Following fermentation, new wine must be clarified, aged and stabilized to produce a satisfactory final product. These process require months & for high quality red wine even years.During the first year, many wines undergoes a second spontaneous fermentation, malo-lactic acid fermentation which can be caused by a variety of lactic acid bacteria. This fermentation converts malic acid to lactic acid & CO2. thus converting a dicarboxylic acid to monocarboxylic acid, hereby reducing the acidity of wine.

The different types of the wine are:
  1. Fortified Wine
  2. Sparking Wine
  3. Champagne
  4. Dry Wine

Wine Production : Procedure
  1. Clean the grapes by the removing stems & wash under tap water.
  2. Grapes are then soaked in 5% NaCl solution and then washed with distilled water.
  3. 250 grams of grapes are then weighed and crushed into pulp in a blender along with 25g of sugar and 200ml of water and transferred into a brown bottle.
  4. 4-5 granules of yeast granules are re-suspended in water and added to the grape extract as inoculum.
  5. The bottle is then tightly covered and incubated overnight at room temperature.
  6. The remaining grapes are then crushed along with 50g of sugar & 300ml of water. This is taken in a 200ml bottle fitted with a one-holed cork.
  7. A pinch of potassium meta bi-sulphite is added to this & the inoculum is transferred.
  8. The contents are mixed well & a bent glass tube is inserted into one holed cork.Little water is added to the glass tube to serve as air trap.
  9. The contents are mixed are mixed well & incubated for 7 days.
  10. Presence of air bubble in the glass tube indicative of fermentation.

White Wine Making - Image Source

Estimation of percentage of Alcohol, Total Acidity & Volatile Acidity 

Specific gravity is the density (weight per unit volume) of a substance divided by the density of water. A specific gravity of 1.05 indicates that the substance is 5% heavier than equal volume of water.

Variation with temperature: The densities of water & wort vary with temperature. when using specific gravity, reference temperatures for the wort sample and water must be specified along with the value of specific gravity.


Specific Gravity bottle  - Image Source

Procedure:

  1. The specific gravity bottle is cleaned and dried, its weight (W1) is noted.
  2. The bottle is filled with alcohol and the weight of bottle + alcohol is noted (W2).
  3. Wash and dry the bottle, the fill it with distilled water, weigh of bottle + distilled water is noted (W3)
Specific Gravity of Alcohol =  ( (W2 - W1) / (W3 - W1 ) ) * Density of Water

W2 - W1 -  GIves the weight of alcohol
W3 - W1 - Gives the weight of water

Estimation of Totall Acidity & Volatile Acidity

Acids are very important structural components of wine. if a wine is too low in acid., it tastes flat & dull. if a wine is too high in acid, it tastes too tart and sour. so maintaining proper acid content is important as it has impact on the taste of wine.

Total Acidity

The total acidity (TA) of wine is measured assuming all the acid present in it is tartaric acid. This allows one to determine a value for the total acidity, ie, content. A high TA of 10% most people would find this level of acidity too tart  & Sour for consumption.A low TA result in flat tasting of wine.
The principle acids found in grapes are tartaric acid, potassium hydrogen tartarate, malic acid and potassium hydrogen malate.

Volatile Acidity
Both tartaric and malic acids are non-volatile which means that they don't evaporate or boil off when the wine is heated. This is to be distinguished from volatile acidity (VA) in wine that represents acetic acid. Acetic acid does boil off and high VA is undesirable in wine.A VA of 0.3 - 0.6 % is produced during fermentation and is considered as normal level.

Procedure
  1. 10ml of alcohol is taken in a conical flask.
  2. 10 ml of distilled water is added to this along with few drops of Phenolphthalein as indicator.
  3. This solution is titrated against 0.1N NaOH taken in burette.
  4. The end point is determined by change in colour from dark green to permanent pink.
  5. The titration is repeated to get concordant values.

Total Acidity Estimation / Calculation 

Total Acidity can be calculated from the following formula:

Total Acidity  = (Volume of Alkali added * Normality of Alkali * 7.5) / Weight of the sample

Volatile Acidity Estimation / Calculation

Volatile Acidity(Volume of Alkali added * Normality of Alkali * 6) / Weight of the sample