Chemistry SIA 2007 Research – Glutinous Rice Beer (Choujiu)

What Is Glutinous Rice Wine?

Glutinous Rice Beer, or Choujiu “稠酒” is a rich-flavoured liquid that is made primarily from glutinous rice and yeast. The yeast will react with the water-soluble starch of glutinous rice to form ethanol and carbon dioxide, resulting in a sweet red-colored brew.
Fermentation Process Of Yeast (Enzymology)

Enzymology is highly important and plays the vital role in the making of choujiu. Fermentation is a process of energy production in a cell in an anaerobic environment. If oxygen is present in the yeast’s environment, the sugar will choose an aerobic mode (use of oxygen) over anaerobic mode, and fermentation will not take place. Also, glucose will be broken down into carbon dioxide, water, and energy, for the yeast to grow and reproduce. However in alcoholic fermentation, or in anaerobic conditions, the yeast will break down the substrate into carbon dioxide, water, and ethanol, leading to fermentation to produce rice wine. Also, Ethanol will react with oxygen in the air to form ethanoic acid which is assisted by microbes and is the cause of ethanol going sour if oxygen is present.

ethanol + oxygen -----> ethanoic acid + water
C2H5OH(l) + O2(g) -----> CH3COOH(aq) + H20(l)

In fermentation, sugar is the common substrate, and the fermented products are ethanol, lactic acid, and hydrogen. The Sugars involved are glucose and fructose. Both can be used by yeasts for fermentation and they share the same formula (C6H12O6).

Fructose and glucose
In the case of the fermentation of yeast, the end result is the production of ethanol in beers, wines, and other alcoholic drinks, along with a large quantity of carbon dioxide.

Here is an overview of the fermentation of yeast:

Reaction in yeast fermentation
The reaction of yeast fermentation depends on the type of sugar being used and the product produced. Supposing the sugar is glucose (C6H12O6) (the simplest sugar), the product will then be ethanol (2C2H5OH). This is one of the fermentation reactions carried out by yeast, and is used in food production. Here is a chemical equation and word equation of the fermentation process of yeast:

Chemical Equation
C6H12O6 → 2C2H5OH + 2CO2 + 2 ATP (Energy Released:118 kJ/mol)

Word Equation
Sugar (glucose, fructose, etc) → Alcohol (ethanol) + Carbon Dioxide + Energy (ATP)

In the word equation, it can be seen that the substrate in the yeast fermentation, or sugar is broken down by the enzymes of the yeast cell, into alcohol, carbon dioxide and energy.
The final step of fermentation is the conversion of pyruvate to fermentation end-products. This process does not produce energy, however, it is a crucial step for an anaerobic cell since it regenerates (NAD+), which is required for glycolysis. This is important for normal cellular function, as glycolysis is the only source of ATP in anaerobic conditions.

What is the role of Pyruvate?

Pyruvate is the carboxylate anion of Pyruvic acid (CH3COCO2H), which plays an important role in biochemical processes. Pyruvic acid is a colorless liquid noted for a resemblance in smell to acetic acid. It is miscible with water, and soluble in ethanol and diethyl ether. Pyruvate is an important chemical compound as it is a key intersection in the network of metabolic pathways and unites several key metabolic processes.

Products - When yeast ferments, it breaks down the glucose (C6H12O6) in sugar into exactly two molecules of ethanol (C2H6O) and two molecules of carbon dioxide (CO2). Ethanol fermentation (performed by yeast and some types of bacteria) breaks the pyruvate down into ethanol and carbon dioxide. This is important in wine-making.

Factors affecting fermentation of yeast
- Yeast are greatly affected by temperature; the temperature has to be just right as too cold and they go dormant, too hot and they indulge in fermentation that cannot be cleaned up by conditioning. The best temperature for the yeast is about 37oC. At lower temperatures yeast does not grow well and at higher temperatures the enzymes are denatured.

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