All You Need to Know about Fermentation


HanNing Tung (Brewing Researcher)

Last time, I introduced the mysterious creature in homebrewing- “yeast.” Now, let’s discover how it would behave under different circumstances. In low oxygen level, yeast consumes sugar to get energy that supposes its life. This sugar-consuming process also produces alcohol and carbon dioxide, but they are the side products that yeast do not need. This process is called “fermentation.”


If there is sufficient oxygen in the environment, yeast can apply oxygen molecules and sugar to get more energy and carbon dioxide, but there is no alcohol produced. This process that yeast uptakes oxygen, produces energy and carbon dioxide is called “aerobic respiration.” Yeast love the environment with a high oxygen concentration. In the aerobic condition, it will have vigorous growth until the oxygen level drops too low to uptakes enough oxygen that supports yeast to carry out aerobic respiration.

    Only in low oxygen concentration can fermentation happen and yeast make alcohol. In general, yeast always produces alcohol in brewing process because the oxygen concentration in brewers’ fermenters is not high. In the cytosol of yeast, one glucose molecule will degrade to 2 alcohol molecules and 2 carbon dioxide molecules, and yeast gains energy from this degradation metabolism process. Yeast can also ferment fructose, sucrose and maltose. However, yeast is unable to apply the polymers such as starch and protein. Therefore, there are two environmental requirements of fermentation: low oxygen concentration and sufficient amount of glucose, fructose or sucrose.

    Actually, yeast doesn’t like alcohol. When the alcohol concentration is higher than 10%, the growth rate of yeast will decline. Most bacteria don’t even have alcohol resistance, which means they would die when the alcohol concentration is higher than 2%. Therefore, alcohol makes yeast dominant the environment and evolved in the history. Then, it’s applied by human to make alcohol beverages and different yeast strains are selected to brew diverse products. Let’s try to use different yeast from ALCHEMA to make creative cider flavor!

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