Alcoholic fermentation by yeast cellsIn brewing, alcoholic fermentation is the conversion of sugar into carbon dioxide gas (CO2) and ethyl alcohol. This process is carried out by yeast cells using a range of enzymes. This is in fact a complex series of conversions that brings about the conversion of sugar to CO2 and alcohol. Yeast is a member of the fungi family which I like to think of as plants but strictly they are neither plant nor animal. To be specific yeast is a eukaryotic micro-organism. Not all yeasts are suitable for brewing. In brewing we use the sugar fungi form of yeast. These yeast cells gain energy from the conversion of the sugar into carbon dioxide and alcohol. The carbon dioxide by-product bubbles through the liquid and dissipates into the air. In confined spaces the carbon dioxide dissolve in the liquid making it fizzy. The pressure build up caused by C02 production in a confined space can be immense. Certainly enough to cause the explosion of a sealed glass bottle. Alcohol is the other by-product of fermentation. Alcohol remains in the liquid which is great for making an alcoholic beverage but not for the yeast cells, as the yeast dies when the alcohol exceeds its tolerance level.
Yeast fermentation of alcohol process
simple chemistry one mole of glucose is converted into two moles of ethanol and two moles of carbon dioxide but in reality it is far from this clear. There are many by products. In addition to CO2 and alcohol, the sugar is incorporated into other by products such as yeast biomass, acids (pyruvic, acetaldehyde, ketoglutaric, lactic), glycerol. Hence if you read many home brewing books there is a table estimating the conversion of sugar into alcohol. These tables tend to be derived from measurements rather than a set formula. The efficiency of the yeast and fermentation conditions alters the proportions of various by-products meaning a simple single formula is not available. Wine makers will see different efficiencies to beer makers. Fermentation conditions such as temperature vary the production of by products. This knowledge is used by wine makers to get fuller bodied wines by brewing in conditions that causes fermentation to produce more of the by-product glycerol.
Fermentation by-product Glycerol gives wine its body. From time to time you read in the press a very shocking tale of people adding anti-freeze to wine but this statement on its own does not tell you the full extent of the danger. Bear in mind Glycerol can be used as an anti-freeze and is a natural by product of fermentation but not all anti-freeze use glycerol, most use very toxic alternatives. So the statement "Anti-freeze added to wine" does not tell you if highly toxic chemicals were added or just Glycerol to supplementing the natural Glycerol content.
simple chemistry one mole of glucose is converted into two moles of ethanol and two moles of carbon dioxide but in reality it is far from this clear. There are many by products. In addition to CO2 and alcohol, the sugar is incorporated into other by products such as yeast biomass, acids (pyruvic, acetaldehyde, ketoglutaric, lactic), glycerol. Hence if you read many home brewing books there is a table estimating the conversion of sugar into alcohol. These tables tend to be derived from measurements rather than a set formula. The efficiency of the yeast and fermentation conditions alters the proportions of various by-products meaning a simple single formula is not available. Wine makers will see different efficiencies to beer makers. Fermentation conditions such as temperature vary the production of by products. This knowledge is used by wine makers to get fuller bodied wines by brewing in conditions that causes fermentation to produce more of the by-product glycerol.
Fermentation by-product Glycerol gives wine its body. From time to time you read in the press a very shocking tale of people adding anti-freeze to wine but this statement on its own does not tell you the full extent of the danger. Bear in mind Glycerol can be used as an anti-freeze and is a natural by product of fermentation but not all anti-freeze use glycerol, most use very toxic alternatives. So the statement "Anti-freeze added to wine" does not tell you if highly toxic chemicals were added or just Glycerol to supplementing the natural Glycerol content.
In fact Glycerol is used in health foods and is essential in fine wines. Wine produced in conditions where there was low production of the glycerol by-product can tempt the producer to add something to boost the wine's body. Adding food grade Glycerol to boost a wine's body is not ideal but no need for panic as glycerol is natural and is often used in food products. Adding toxic anti-freeze to boost a wine's body can and does kill people.
Yeast fermentation of alcohol process
Note: The sugars used can be a range of fermentable sugars. These sugars are converted by enzymes to glucose which is then converted to alcohol and CO2. Some sugars are not able to be fermented and will remain in the liquid.
Fermentation and yeast in brewing
Brewer's yeast tolerate up to about 5% alcohol. Beyond this alcohol level the yeast cannot continue fermentation. Wine yeast on the other hand tolerates up to about 12% alcohol. The level of alcohol tolerance by yeast varies from 5% to about 21% depending on yeast strain and environmental conditions.
The fermentation process has limits such as temperature. Greater than 27C kills the yeast less and than 15C results in yeast activity which is too slow.
Not all sugars are fermentable. Non fermentable sugars in solution will remain after fermentation and will result in a sweeter end product. Malt has non fermentable sugars which can be used to balance the bitterness of the hops. The amount of sugar in the solution can be too much and this can prevent fermentation. Some wine recipes suggest adding the sugar in parts throughout fermentation rather that all at the beginning. This is especially true if the brew is aimed at producing a high level of alcohol. Some yeast strains have evolved to handle higher sugar levels. Yeast such as Tokay and Sauterne handle high levels of sugar. The normal, home brewing, fermentation is in two parts.
Part 1
Aerobic (Oxygen is present)
This is the initial rapid process where the yeast is doubling its colony size every 4 hours.
(Usually 24-48 hours)
Part 2
Anaerobic. (No oxygen present)
Slower activity and the yeast focuses on converting sugar to alcohol rather that increasing the number of yeast cells.
(This process can take from days to weeks depending on the yeast and the recipe)
Fermentation and yeast in brewing
Brewer's yeast tolerate up to about 5% alcohol. Beyond this alcohol level the yeast cannot continue fermentation. Wine yeast on the other hand tolerates up to about 12% alcohol. The level of alcohol tolerance by yeast varies from 5% to about 21% depending on yeast strain and environmental conditions.
The fermentation process has limits such as temperature. Greater than 27C kills the yeast less and than 15C results in yeast activity which is too slow.
Not all sugars are fermentable. Non fermentable sugars in solution will remain after fermentation and will result in a sweeter end product. Malt has non fermentable sugars which can be used to balance the bitterness of the hops. The amount of sugar in the solution can be too much and this can prevent fermentation. Some wine recipes suggest adding the sugar in parts throughout fermentation rather that all at the beginning. This is especially true if the brew is aimed at producing a high level of alcohol. Some yeast strains have evolved to handle higher sugar levels. Yeast such as Tokay and Sauterne handle high levels of sugar. The normal, home brewing, fermentation is in two parts.
Part 1
Aerobic (Oxygen is present)
This is the initial rapid process where the yeast is doubling its colony size every 4 hours.
(Usually 24-48 hours)
Part 2
Anaerobic. (No oxygen present)
Slower activity and the yeast focuses on converting sugar to alcohol rather that increasing the number of yeast cells.
(This process can take from days to weeks depending on the yeast and the recipe)
Yeast fermentation of alcohol process
Alcohol fermentation is a well established technology which has long been practiced throughout the world. Continuous alcohol production with the use of immobilized yeasts, which give rise to more efficient fermentation, is described in this Section. A fermentation process incorporating the flash method which is seen as a means of improving alcohol productivity using yeasts with poor alcohol resistance.
Of the various methods used for continuous fermentation, that employing the use of immobilized yeast cells was selected. Methods of yeast immobilization evaluated included collagen casting, acetic acid-lactic acid cellulose microencapsulation, chitosan/glutar-aldehyde molding, carrageenan-entrapping method, and calcium alginate gel-entrapping. Of these, the calcium alginate gel-entrapping method was preferred because of its high enzymatic activity, simple manner of preparation, and stability.
Preparation of a uniform calcium alginate gel, necessitated maintaining the viscosity of the mixture of calcium alginate and yeast cells between 1000 and 2000 cps. The addition of a nonionic surfactant and an unsaturated fatty acid at the time of gelling was also found to improve cell retention and enzyme activity. Preparation of immobilized yeast cells.
The spherical gel method was employed for the preparation of calcium alginate gels since this method does not require the use of specialized equipment. Spherical gels are readily obtained by adding sodium alginate solution to calcium chloride solution using a nozzle. No special granulation apparatus was used when the equipment was assembled, but a gel-dropping nozzle was provided at the top of the fermentor. The fermentor was filled with a calcium chloride solution prior to fermentation, and sodium alginate solution was added dropwise to form granules. The culture medium was then supplied to the fermentor to initiate the fermentation. This procedure was found to simplify the gel preparation process.
Of the various methods used for continuous fermentation, that employing the use of immobilized yeast cells was selected. Methods of yeast immobilization evaluated included collagen casting, acetic acid-lactic acid cellulose microencapsulation, chitosan/glutar-aldehyde molding, carrageenan-entrapping method, and calcium alginate gel-entrapping. Of these, the calcium alginate gel-entrapping method was preferred because of its high enzymatic activity, simple manner of preparation, and stability.
Preparation of a uniform calcium alginate gel, necessitated maintaining the viscosity of the mixture of calcium alginate and yeast cells between 1000 and 2000 cps. The addition of a nonionic surfactant and an unsaturated fatty acid at the time of gelling was also found to improve cell retention and enzyme activity. Preparation of immobilized yeast cells.
The spherical gel method was employed for the preparation of calcium alginate gels since this method does not require the use of specialized equipment. Spherical gels are readily obtained by adding sodium alginate solution to calcium chloride solution using a nozzle. No special granulation apparatus was used when the equipment was assembled, but a gel-dropping nozzle was provided at the top of the fermentor. The fermentor was filled with a calcium chloride solution prior to fermentation, and sodium alginate solution was added dropwise to form granules. The culture medium was then supplied to the fermentor to initiate the fermentation. This procedure was found to simplify the gel preparation process.
Yeast fermentation of alcohol process
Continuous fermentation tests were conducted using two 1-kL fermentors arranged in series. Continuous production of alcohol was carried out stably for 4000 hours while maintaining high enzymatic activity .
Alcohol productivity has been greatly improved through the development of bioreactors. Alcohol production by the batch method, immobilized continuous, and immobilized flash methods .
Alcohol inhibition of yeasts, does not pose any problems in alcohol fermentation by the batch method, since yeasts are used only once in batch processes. In such processes, yeast damage due to alcoholic inhibition is of relatively little significance. However, where immobilized continuous methods of alcohol production are used, stable yeast activity needs to be maintained, and the alcohol concentration in the fermentor must be relatively low. The relationship between alcohol concentration and fermentation rate, using an immobilized yeast fermentor . Several methods of continuously removing alcohol from fermentors as a means of avoiding alcoholic inhibition of yeasts have been proposed.
Alcohol productivity has been greatly improved through the development of bioreactors. Alcohol production by the batch method, immobilized continuous, and immobilized flash methods .
Alcohol inhibition of yeasts, does not pose any problems in alcohol fermentation by the batch method, since yeasts are used only once in batch processes. In such processes, yeast damage due to alcoholic inhibition is of relatively little significance. However, where immobilized continuous methods of alcohol production are used, stable yeast activity needs to be maintained, and the alcohol concentration in the fermentor must be relatively low. The relationship between alcohol concentration and fermentation rate, using an immobilized yeast fermentor . Several methods of continuously removing alcohol from fermentors as a means of avoiding alcoholic inhibition of yeasts have been proposed.
The fermentor used in our study was equipped with a distillation column (operable either under atmospheric or reduced pressure). Circulation of the fermentation broth through this distillation column, enabled the removal of alcohol, and sugar thereby facilitating the maintainance of low alcohol concentrations within the fermentor. Results obtained using a 200-L scale pilot fermentation.
Alcoholic inhibition of yeast was reduced while the fermentation rate increased. In addition, this method enabled raw material to be used at a high concentration, reducing the volume of waste liquor. However, the use of raw material at a high concentration necessitates a relatively longer retention time within the fermentor, thus increasing the accumulation of by-products, which inevitably reduces the fermentation rate due to by-product inhibition.
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