Fermentation in beer production
The aerobic respiration is carried out in the early stage of beer fermentation, which is mainly the proliferation of yeast cells, and the anaerobic fermentation is carried out in the later stage. The yeast cells use the nutrients in the wort to produce alcohol, fusel oil and organic acids.
Traditional fermentation is generally divided into two stages: main fermentation and post-fermentation.
In order to shorten the fermentation cycle and increase the utilization rate of equipment, the one-tank fermentation method is now generally adopted, that is, the two stages of main fermentation and post-fermentation are no longer strictly divided, and they are carried out in the same cone-bottomed fermentation tank to increase the yield.
1. The main fermentation process:
①. Material changes during the main fermentation process:
Under aerobic conditions, yeast proliferates in large quantities, oxidizes glucose to carbon dioxide and water, and releases a large amount of heat energy. As dissolved oxygen is exhausted, anoxic conditions are created. The yeast anaerobic fermentation produces ethanol, carbon dioxide, lactic acid, acetic acid, citric acid and malic acid. And other organic acids.
About 96% of fermentable sugars are converted into ethanol and carbon dioxide, 1.5~2.0 are used for new cell generation, and 2.0~2.5% are converted into fermentation by-products.
The pH of the fermentation broth decreased slowly before and after, and the final pH of the fermentation was 4.2-4.5.
Several amino acids are called and converted.
There are two types of fermentation by-product flavors.
One type is directional substances (higher alcohols and esters, etc.), which mainly determine the aroma of beer, and controlling it within a certain concentration range is beneficial to beer quality.
Common higher alcohols (also known as fusel oils) mainly include isoamyl alcohol and isobutanol.
Esters are the main carrier for beer, mainly produced in the vigorous stage of the main fermentation, second only to higher alcohols in quantity.
Both the higher alcohol formation and the ester content of the upper fermentation exceed those of the lower fermentation.
Rapid high-temperature fermentation, too high wort concentration, too much dissolved oxygen or insufficient amino acid content will result in a high content of higher alcohols in the product.
Pressurized fermentation and increasing the amount of yeast added can reduce the production of higher alcohols.
High wort concentration, high amino acid content, low ventilation, low-temperature main fermentation, extended post-fermentation time or pressurized fermentation will increase the production of esters; measures to promote yeast proliferation will reduce the production of esters; use ventilation and stirring to produce esters during fermentation The amount is less.
Another type of fermentation by-products are green-flavored substances (such as diacetyl, aldehydes, sulfides, etc.), and the quality of beer is not good.
The threshold value of diacetyl is only 0.10~0.15mg/l, and beer above this value has a rancid taste, which seriously affects the taste.
The maturity of beer can be determined according to the content of diacetyl.
Yeast cells reduce diacetyl to acetoin, which can also cause a prominent bitter taste and a musty taste, which is further reduced to 2,3-butanediol.
Increase the temperature and the concentration of yeast cells, the wort has enough α-amino acids (especially valine) and adding an appropriate amount of α-acetolactase is beneficial for diacetyl reduction.
More than 50 aldehydes have been detected in beer, mainly acetaldehyde (normal concentration is 8-10mg/l). Full ventilation of the wort and increasing the temperature and yeast concentration in the post-maturation stage are conducive to the decomposition of aldehydes.
Sulfide seriously affects beer flavor, in which non-volatile S-S compounds and volatile dimethyl sulfide are brought in by hops and wort;
The yeast produces volatile hydrogen sulfide, dimethyl sulfide, and mercaptans during fermentation.
② Process operation of main fermentation:
The main fermentation is also called pre-fermentation. It is actually carried out in two steps. Part of the wort (concentration 10-12%) cooled to 5~7℃ is pumped into the multiplication tank, and 0.8~1.0L/100L is generally added for recovery and storage. The yeast paste is mixed with the wort at a ratio of 1:1, and sterile air is introduced to suspend the yeast cells and sent to the yeast multiplication tank, or directly use the 0.5-0.6 expanded yeast liquid to fully mix with the wort. Mix well, pass in sterile air (wort dissolved oxygen control is about 8mg/L), leave it for 12-20 hours, the yeast cell concentration will reach 2.0x107 cells/mL, the liquid surface will form a layer of white foam, change the tank to remove the condensed solids And dead yeast.
Yeast culture broth and new wort (to ensure sufficient valine content and place to generate excess diacetyl) at 5~7℃ flow into the main fermentation tank at the same time, and fill up within 12~18 hours, and the dissolved oxygen in the wort has been basically consumed. At the end, anaerobic fermentation is carried out for 5~7 days, the highest product temperature is controlled to 8~10℃, the final temperature is reduced to 3.5~5℃, the appearance sugar content is reduced to 3.5~5.5%, and the appearance fermentation degree is 50~60%.
It is mainly taught that the temperature drops sharply on the last day, which is conducive to the sedimentation and recovery of yeast.
The main fermentation can be divided into four periods: low foaming period, high artillery period, falling foam period and foam cap forming period according to the appearance and disappearance of foam.
2. Post-fermentation process:
①. Material changes in the post-fermentation process:
After the main fermentation is completed, the fermentation liquid is called new wine or tender beer, and the post-fermentation (also known as the post-ripening or storage stage of beer) is suitable for drinking for a long time.
The fermentable sugar remaining in the tender beer is continuously metabolized by the yeast to produce alcohol, carbon dioxide, higher alcohols, organic acids, esters and aldehydes. The initial fermentation is obvious, sugar consumption is slightly faster, and gradually slower until almost completely stopped.
The first 2 to 3 days of open natural fermentation will release carbon dioxide and take away the volatile substances such as acetaldehyde, diacetyl and hydrogen sulfide in the wine, which greatly reduces the green taste of the beer and promotes the maturation of the beer.
The carbon dioxide gradually reaches saturation in the liquor (the mass fraction of the finished liquor is about 0.5%), and it has the functions of anti-oxidation, acid-increasing, and anti-corrosion.
The post-fermentation temperature and pH are low, so the suspended yeast, protein condensate, hop resin and protein-polyphenols in the wine gradually precipitate to promote the clarification of the wine.
② Process operation of post-fermentation:
Generally, the post-fermentation time is 7 to 10 days, and the temperature is maintained at about 4°C. The average daily sugar reduction is less than 0.3%.
The temperature of tender beer is 3.5~5℃, the content of fermentable sugar is 3.5~5.5%, and the number of yeast cells is controlled at (5~10)×106/ml.
The room temperature of the post-fermentation chamber is 3.5~5℃. The tender beer is pumped into the tank from the bottom entrance of the tank (called the wine), the yeast cells are suspended, the wine liquid absorbs oxygen, and the upper part leaves a gap of 5~15cm.
The open fermentation is relatively vigorous. After about 24 hours, there will be white foam rising on the liquid surface and a pungent smell of strong carbon dioxide. After 2 to 3 days, the brown hop resin and protein coagulum will overflow out of the tank, and the foam will fall in 2 to 3 days. , At this time, seal the can, and the sealing cover is sterilized and kept clean.
5~10 days after sealing, the tank pressure reaches 0.05~0.08MPa, which keeps stable, and promotes the temperature reduction and saturation of the wine.
After 15~20 days of post-fermentation, the wine temperature will drop to 1~2℃ within 6~7 days, maintain for about 15 days, cool to about 0℃, and mature after 15 days (the whole cycle is generally about 50 days) ).
At present, many manufacturers use cone-bottom fermentation tanks, and the entire production process of main, post-fermentation and wine storage is completed in one tank.
The wort of this method enters the tank in two batches. The first two batches are normally ventilated, and the subsequent batches are reduced or not ventilated. Generally, the tank is full within 12 to 20 hours, and the temperature is lower than the main fermentation temperature by 20°C. 0.6~0.8%), the number of generations used by yeast is less (only 4 times).
Fermentation temperature control generally adopts low-temperature fermentation and high-temperature post-cooking processes (fermentation temperature rises 9~10°C, keeps 3~4 hours, and raises the temperature to 12~13°C), keep the temperature until the diacetyl reduction is reduced to 0.1mg/l, and start with 0.2~ Cooling down from 0.3°C/hour to about 4°C mainly controls the temperature at the bottom of the conical tank, which is lower than the top temperature to form top-down convection and promote solid precipitation, which is beneficial to the recovery of yeast, wine clarification and carbon dioxide saturation. Keep it for 48 hours Recover yeast (the two-tank method uses fermentation tanks and wine storage tanks).
With the development of new fermentation technologies, fermentation methods such as direct application of active dry yeast and immobilized yeast cells for continuous fermentation and high-concentration fermentation to dilute beer have emerged successively in beer brewing.
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The Role of Yeast in Brewing
By: Owen Ogletree
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The Role of Yeast in Brewing
The fascinating microscopic organism that is yeast provides a tremendous impact on beer and brewing. Yeast plays a key role in creating the distinctive aroma and flavor components that make up an impressive variety of beer styles. Without the metabolism of yeast cells, the traditional production of alcoholic beverages would not be possible.
Yeasts are single-celled organisms with a nucleus that belong to the fungus kingdom. Unlike plant cells, yeast requires no sunlight, and the organism has been the catalyst used in baking and the creation of alcoholic beverages for thousands of years. During fermentation, the primary beer yeast species, known as Saccharomyces cerevisiae, produces energy for its cellular metabolism by converting certain sugars into carbon dioxide, alcohols and fermentation by-products.
Brewers in the middle ages had no idea about the presence of yeast and the role it plays in beer production. These rustic brewers would often stir a new vat of wort with a "magic" wooden paddle inoculated with yeast cells from previous batches. Fermentation would kick in within a few hours. Thankfully, modern brewers possess an intimate scientific knowledge of yeast types, metabolism, reproduction and flavor-production characteristics.
Basically, two types of brewing yeast exist – classified originally on whether fermentation takes place at the top of the fermenter or near the bottom. Beer yeast that prefers warmer temperatures and working near the top of the fermenter is known as "ale" yeast. Bavarian wheat beer yeast is also classified as a type of ale yeast. "Lager" yeast likes to hang out at the bottom of the fermenter and thrives in colder temperatures.
Ale yeast strains ferment best at temperatures in the range of 60 to 72 degrees Fahrenheit. They are quite diverse in metabolism and in the production of fruity, spicy aroma and flavor compounds imparted to the finished beer. Think of a traditional English brewery using ale yeast in open fermentation vats at a rather cool ambient temperature. The beer ferments in just a few days, and the resulting English bitter style offers elegant notes of dried fruit, stone fruit and perhaps just a hint of butterscotch - all of which are fermentation by-products from the ale yeast.
Ale yeast has a knack for excreting ester chemicals that can be perceived as various fruit notes. Being greatly influenced by temperature, ale yeast can experience a four-fold increase in ester production as a result of increasing fermentation temperature from 60 to 68 degrees Fahrenheit. Ales fermented at extremely warm temperatures will often exhibit intense fruitiness.
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