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Fermentation

Key Concepts

  • Fermentation of fermentable sugars by a suitable yeast produces ethanol (ethyl alcohol).

  • Fermentable sugars are:

            - glucose (also known as dextrose, blood sugar, or grape sugar), C6H12O6
            - mannose, C6H12O6
            - galactose, C6H12O6
            - fructose (also known as levulose), C6H12O6

  • Yeast is a single celled fungus containing a number of enzymes useful during fermentation.
    Brewers yeast and wine yeast, for example, contain zymase, an enzyme that can convert glucose to ethanol.

  • The overall chemical equation for the fermentation of a fermentable sugar such as glucose by an enzyme such as zymase is:
    glucose zymase
    -------->
    ethanol + carbon dioxide gas
    C6H12O6(aq) zymase
    -------->
    2C2H5OH(aq) + 2CO2(g)

  • The reaction conditions for fermentation are:

          - aqueous environment
          - warm temperature (different enzymes will have different optimum temperatures)
          - anaerobic environment (no oxygen present)
          - slightly acidic pH

  • Fermentation will cease if

          - the ethanol concentration becomes too great
          (~5% by volume ethanol for brewers yeast, ~14% by volume ethanol for wine yeast)
          - the temperature is high enough to kill the yeast
          (> ~27oC for brewers yeast)
          - the pH falls below ~6

  • Distillation of the aqueous mixture produced from fermentation can produce a solution containing ~95% by volume ethanol.

Preparation of Ethanol

Fermentation of Glucose to Produce Ethanol

  1. Glucose is dissolved in warm water in a conical flask.

  2. Some yeast is added to the glucose solution and cotton wool is used to plug the top of the conical flask.

  3. The flask is left in a warm place to allow fermentation to take place.

  4. When bubbles of carbon dioxide are no longer forming, the fermentation reaction has ceased.

The first step in the fermentation reaction is the conversion of glucose (C6H12O6) to pyruvic acid (CH3-CO-COOH) using the hydrogen carrier NADH present in the yeast:

C6H12O6 + 2NAD+ → 2CH3-CO-COOH + 2NADH + 2H+

Zymase, an enzyme in the yeast, converts the pyruvic acid to ethanol and carbon dioxide using the hydrogen carrier NADH:

CH3-CO-COOH + NADH + H+ → C2H5OH + CO2 + NAD+

So, the overall equation for the fermentation of glucose is:

C6H12O6 + 2NAD+ 2CH3-CO-COOH + 2NADH + 2H+
2CH3-CO-COOH + 2NADH + 2H+ 2C2H5OH + 2CO2 + 2NAD+

C6H12O6 2C2H5OH + 2CO2

In reality, the chemistry of fermentation is actually much more complex than this, with the glucose being incorporated into other by-products such as pyruvic acid, acetaldehyde and glycerol.

Distillation of Fermentation Products to Produce Ethanol

Distillation is a process that separates different compounds with different boiling points.
At 1atm (101.3kPa) the boiling point of pure water is 100oC while the boiling point of ethanol is 78.3oC.

  1. The mixture containing ethanol is placed in a round-bottomed flask or Claisen flask.

  2. Since ethanol is flammable and has a boiling point below that of water, the flask is placed in a water bath to gently heat the mixture.

  3. Using a fractionating column reduces the number of distillations required to complete the separation of the ethanol as it provides a continuous series of partial condensations of the vapour and partial vaporisations of the condensate.

  4. A thermometer is used to ensure that only the fraction that boils at 78.3oC is collected as this is the ethanol fraction.

  5. The ethanol vapour entering the Liebig condenser is cooled and condenses to liquid ethanol.
    One end of a flexible tube is connected to the bottom of the Liebig condenser and the other end is connected to a water tap.
    One end of a flexible tube is connected the top of the Liebig condenser and the other end of this tube is placed in the sink.
    When the tap is turned on, water flows into the bottom of the Liebig condenser and out of the top of the Liebig condenser.
    This constant stream of cool water allows heat to be transferred from the hot vapour in the Liebig condenser to the cooler water which is flowing away, thereby condensing the vapour to the liquid.

  6. The product of the distillation is called the distillate and is collected in a suitable flask (often a conical flask).
    The distillate will contain ethanol as long as the temperature of the vapour as measured by the thermometer is 78.3oC.
    When the temperature of the vapour begins to rise, the flask containing the distillate should be removed and replaced with a clean flask to prevent contamination of the ethanol distillate, and the bunsen burner turned off.

In practice, it is not possible to obtain pure 100% ethanol using this distillation method because the constant boiling point mixture collected as the distillate contains ~95% ethanol and 5% water.
Further concentration of the ethanol is possible using other separation techniques such as vacuum distillation, or, by adding another substance such as benzene to the mixture.


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