Glucose metabolism

The composition of wort for sugar metabolism in beer production includes rich nutrients such as sugars, nitrogenous substances, hops, acidic substances, lipids, tannins, dissolved oxygen, inorganic salts, vitamins and other solids. , provides a good living environment for yeast cells, active yeast absorb nutrients in wort and release metabolites.

In the extract of beer wort, sugar is one of the most important components, which accounts for about 90% of the extract of wort, of which: glucose and fructose account for about 10% of sugar, and sucrose accounts for about 5%. Maltose accounts for 45%-50%, maltotriose accounts for 10%-15%, oligosaccharide accounts for 20%-25%, and there are also a small amount of pentose and pentosan, β-glucose, isomaltose and so on. Among them, the sugars that can be fermented by yeast are called "fermentable sugars", such as glucose, fructose, sucrose, maltose, maltotriose, etc. are called fermentable sugars, which are the main carbon source nutrients of brewer's yeast and are also used in fermentation. The sugars that can be used in the process, their fermentation order is glucose>fructose>sucrose>maltose>maltotriose. 

Maltotetraose, maltopentaose to maltononose, and DP9-DP12 dextrin in wort cannot be utilized by brewer's yeast. These sugars are called non-fermentable sugars, also known as non-sugars. Generally, the ratio of sugar to non-sugar in actual production is generally controlled to be 7:3, which is more appropriate. In the production of light beer, the fermentable sugar content is slightly higher, the degree of fermentation is high, and the taste is refreshing; if the production of strong beer, the non-sugar ratio is Slightly higher to add body to the beer. The composition of fermentable sugars in wort varies due to different raw materials and process methods used, and also due to different auxiliary materials and different addition amounts of auxiliary materials, resulting in slight differences in the content of various sugars.

The yeast consumes fermentable sugars (aerobic respiration) in the oxygenated cold wort, the sugars are decomposed into water and CO2, and the obtained bioenergy makes the yeast cells rapidly proliferate and release excess heat. The fermentation process of glucose is relatively complicated. About 96% of the fermentable sugar is fermented by yeast into ethanol and CO2. Under the action of various enzymes of yeast, it is converted into Pyruvate is then processed by acetaldehyde under the action of pyruvate decarboxylase and alcohol dehydrogenase in yeast, and finally ethanol and CO2 are generated. The main fermentable sugar in wort is maltose. In addition, the sugar in the wort is not fermented at the same time, the yeast first uses the monosaccharide, and then the disaccharide and the trisaccharide. Fermented sugars are thus divided into primary fermented sugars (monosaccharides), primary fermented sugars (disaccharides), and post-fermented sugars (maltotriose).

Glucose and fructose first enter the yeast cells and directly participate in their metabolism; sucrose needs to be converted into glucose and fructose by the sucrase secreted by the yeast on the cell surface before it can enter the yeast cells for fermentation. The utilization of maltose and maltotriose by brewer's yeast varies with different types of yeast. For the following yeast, when the wort contains more glucose, it will inhibit the yeast cell to secrete maltose permease. 

Without the action of this osmotic enzyme, maltose cannot enter the cell, and the inhibitory effect can be eliminated only after the concentration of glucose and fructose is fermented to a certain extent. After being decomposed into monosaccharides by α-glucosidase, it can be hydrolyzed. This inhibitory effect of glucose is called "catabolic inhibition" or "glucose repression effect". Similarly, only after the concentration of maltose is reduced to a certain level, the yeast cells can secrete maltotriose permease, so that the maltotriose can enter the yeast cells for fermentation. In normal wort, this inhibitory effect is not prominent, but if a large amount of glucose or sucrose is added to the wort, the fermentation of maltose and maltotriose will be severely inhibited. Some top yeasts, in the presence of glucose, can still maintain their ability to ferment maltose and maltotriose, so the top yeasts ferment relatively quickly.