The fermented liquid of wort after the main fermentation is called green beer, also called new beer. At this time, the carbon dioxide content of the wine is insufficient, the volatile flavor substances such as diacetyl, acetaldehyde, hydrogen sulfide have not yet been reduced to a reasonable level, the taste of the wine is immature and not suitable for drinking; a large amount of suspended yeast and condensed substances have not yet completely settled , The liquor is not clarified enough, and generally requires several weeks or months of post-fermentation and storage period. The maturation and clarification of beer are completed during the post-fermentation and storage period. In the post-fermentation process, some biochemical, chemical and physical changes are still slowly progressing, and their main effects are as follows:
1. Carbon dioxide saturation
After the tender beer is brewed, the initial post-fermentation temperature is still maintained at 3~5℃, so that the remaining fermentable sugars (mainly maltose and maltotriose) continue to ferment, and the carbon dioxide produced is continuously dissolved in the closed wine storage container The wine is saturated. At the same time, at a relatively high post-fermentation temperature, diacetyl can still be reduced relatively quickly.
After the carbon dioxide is saturated, the longer the wine storage time, the more stable the adsorption effect of the wine on carbon dioxide, which is more beneficial to the beer quality. Carbon dioxide in saturated beer is generally considered to be in three states: the gaseous carbon dioxide dissolved in the wine under different temperature and pressure; the carbon dioxide absorbed by the colloidal substances in the beer; the carbon dioxide combined with the beer components.
2. The maturity of beer
In the post-fermentation period, the role of promoting beer maturity is mainly to use carbon dioxide emissions to eliminate the green flavor substances contained in tender beer, such as diacetyl, hydrogen sulfide and acetaldehyde, which are simultaneously discharged in large quantities, reducing the immature taste of beer. Speed up beer maturation. The diacetyl content of mature beer should be below 0.1mg/kg; a very small amount of hydrogen sulfide is a necessary component of the beer flavor, and excessive amounts are harmful. It can give beer an unpleasant green taste, which must be eliminated; the acetaldehyde content is vigorously fermented The period reaches a peak, and then gradually decreases. In the early stage of wine storage, with the emission of carbon dioxide, the content of acetaldehyde begins to decrease sharply, and remains low until the wine storage is completed.
3. Clarification of beer
The clarification of beer is the precipitation of suspended solids contained in the wine during storage. The suspended solids in beer are mainly yeast cells, freeze-solid proteins, hop resins, protein polyphenol complexes and so on. After the post-fermentation is maintained for 7 to 10 days, the wine temperature is gradually reduced to 0 to 1 ℃, and the wine storage mature period. During the longer storage period, the suspended solids in the beer will slowly settle down at low temperature and low pH value, so that the liquor will gradually clarify and the taste of the liquor will become more mature. At a lower storage temperature, some protein-polyphenol complexes that are easy to form turbidity gradually precipitate and precipitate or are removed by filtration, which improves the non-biological stability of the beer, thereby increasing the shelf life of the finished beer.
During the post-fermentation period, beer should absolutely avoid contact with air and keep the wine in its reduced state. Otherwise, not only the flavor, foam, color and non-biological stability of the beer will be seriously affected by oxidation, but also easy to be infected with bacteria and even spoiled.