Beer fermentation and Degree of fermentation
Beer fermentation is a complex biochemical and material transformation process.
Yeast's main metabolites and fermentation by-products:-ethanol and carbon dioxide-alcohols, aldehydes, acids, esters, ketones and sulfides and other substances. These fermentation products determine the physical and chemical properties of beer, such as flavor, foam, color and stability, and give beer with typical characteristics.
Type of fermentation
Beer fermentation can be divided into two types: top fermentation and bottom fermentation due to the different yeasts used.
The top fermentation type beer uses top yeast and a higher fermentation temperature of 16-22°C;
The bottom fermented beer uses the bottom yeast and a lower fermentation temperature of 7-12°C.
The whole fermentation process can be roughly divided into 3 stages:
(1) Yeast adaptation stage;
(2) Aerobic respiration stage;
(3) Anaerobic fermentation stage.
Since the fermentation process is continuous, the fermentation changes in the main fermentation and post-fermentation should be regarded as an interrelated process. During fermentation, the by-products formed by yeast metabolism play a special role, and some of these by-products are decomposed again. The formation and partial decomposition of these by-products are closely related to the metabolism of yeast.
Fermentation of glucose to ethanol
The biochemical mechanism of glucose alcohol fermentation is the most basic theory of alcohol production and wine brewing.
For beer brewing, in addition to fermentation metabolites alcohol and CO2 are the most important components of beer, the EMP pathway in the metabolic process is also the basis for the production of many metabolites.
Wort is rich in nutrients and provides a good living environment for yeast cells. Yeast absorbs nutrients in wort and excretes metabolites. Carbohydrates account for about 90% of the wort extract. Among them, glucose, fructose, sucrose, maltose, maltotriose and raffinose are called fermentable sugars, which are the main carbon nutrients of brewer’s yeast and can be used in fermentation. Material used.
The DP9-DP12 dextrin, maltotetraose, maltopentaose to maltononaose in the wort are all non-fermentable sugars, also known as non-sugars. In actual production, the ratio of sugar to non-sugar is generally controlled to be 7:3.
Produce light-colored beer with slightly higher fermentable sugar content, high degree of fermentation, and refreshing taste;
For the production of thick beer, its non-sugar ratio is slightly higher to increase its mellowness.
Degree of fermentation
The percentage of the decrease in the extract in beer is called the degree of fermentation. Different types of beer have different levels of residual sugar and degree of fermentation. In the process of beer fermentation, sugar is continuously consumed, and the concentration of beer decreases accordingly. In order to accurately express the degree of fermentation of extracts in beer, the concept of degree of fermentation was born. It expresses the ratio of extracts in the wort that has been fermented, and the degree of fermentation is represented by "V". The difference between the extract content of the inoculated wort and the extract content of the beer at the time of measurement is the fermented extract content.
Appearance fermentation degree (Vs)
During the main fermentation, the extract concentration was measured with the aid of a brix meter. The degree of fermentation measured and converted by the brix meter is called the appearance degree of fermentation, which has a certain deviation from the true degree of fermentation of beer. Since the deviation value is directly proportional to the degree of fermentation, and the appearance of the extract concentration is easy to determine, the fermentation workshop of the brewery uses the appearance of the degree of fermentation to control production.
The appearance degree of fermentation of light beer is generally 68 to 75%.
The appearance fermentation degree of dark beer and strong beer is generally 64-73%.
Real fermentation degree (Vω)
First, steam out all the alcohol in the tested fermentation broth (or wine), then make up to the original volume with water, and then measure its concentration. This concentration is called the true concentration, and the degree of fermentation calculated with the true concentration is called the true degree of fermentation. The true degree of fermentation refers to the percentage of extract that has been consumed during the fermentation process. Because alcohol and CO2 are present in the tested fermentation broth, its relative density is reduced, resulting in the measured appearance of sugar content lower than the actual content of the extract. Therefore, the true fermentation degree is always lower than the appearance fermentation degree. The appearance fermentation degree is generally about 20% higher than the real fermentation degree. Using the following calculation formula, you can easily convert the true degree of fermentation. Vω≈0.819 VS coefficient 0.819 is an empirical value calculated by Bahrain in 1870.
Final fermentation degree (EV)
In order to have a standard for the degree of fermentation, it is necessary to know what proportion of the fermentable extract in the extract, that is, to determine the final degree of fermentation.
From wort inoculation to before filling, the decline in extract content is not uniform, and the decline in extract in the main fermentation stage is much higher than that in the post-fermentation stage.
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