The fermented beverage produced after the main fermentation of wort is called green beer, also known as new beer. At this stage, the carbon dioxide content in the beer is insufficient, and volatile flavor substances such as diacetyl, acetaldehyde, and hydrogen sulfide have not been reduced to an appropriate level. Additionally, the beer is not clear enough, so it generally requires several weeks or months of post-fermentation and storage. The maturation and clarification of the beer occur during this period.
During the post-fermentation process, there are still ongoing biochemical, chemical, and physical changes, and their main roles are as follows:
Carbon Dioxide Saturation
After the young beer is consumed, the initial post-fermentation temperature is maintained at 3-5°C, allowing residual fermentable sugars (mainly maltose and maltotriose) to continue fermenting. The carbon dioxide produced is dissolved continuously in the closed beer storage container, saturating the beer. Meanwhile, at a relatively high post-fermentation temperature, diacetyl can still be rapidly reduced.
The longer the beer is stored after carbon dioxide saturation, the more stable its absorption of carbon dioxide becomes, resulting in better beer quality. Carbon dioxide in saturated beer is generally considered to be in three states: gaseous carbon dioxide dissolved in the beer under different temperatures and pressures; carbon dioxide absorbed by colloidal substances in beer; and carbon dioxide combined with beer components.
Brewery Equipment Fermenters and Bright Beer Tanks
During the post-fermentation period, the promotion of beer maturation mainly involves the emission of carbon dioxide, which eliminates green-flavored substances such as diacetyl, hydrogen sulfide, and acetaldehyde contained in young beer. These substances are discharged in large quantities, reducing the immature taste of the beer and accelerating its maturation. The diacetyl content in mature beer should be below 0.1 mg/kg. Although a small amount of hydrogen sulfide is an essential component of beer flavor, excessive amounts are harmful and can give the beer an unpleasant green flavor, which must be avoided. The acetaldehyde content is particularly high during the early stages of fermentation and decreases sharply as carbon dioxide is emitted, remaining low until the beer storage period is completed.
Fermentation Tanks/Unitanks Customization
The clarification of beer refers to the sedimentation of suspended solids present in the beer during its storage. Yeast cells, cold coagulation proteins, hop resins, protein-polyphenol complexes, and other substances constitute the suspended solids in beer. After the post-fermentation is maintained for 7-10 days, the temperature of the beer is gradually lowered to 0-1°C, entering the maturity phase of beer storage. Over the long storage period, the suspended matter in the beer gradually settles at low temperature and low pH, resulting in gradual clarification of the beer and a more mature taste. At lower storage temperatures, certain protein-polyphenol complexes that tend to cause turbidity precipitate first or are removed through filtration. This improves the non-biological stability of the beer and increases the shelf life of the final product.
During the post-fermentation period, it is crucial to prevent any contact between the beer and air in order to maintain a reduced state. Otherwise, oxidation can significantly impact the flavor, foam, color, and non-biological stability of the beer, and make it susceptible to contamination by miscellaneous bacteria and spoilage.