In its strictest sense, fermentation (formerly called zymnosis) is the energy-yielding anaerobic metabolic breakdown of a nutrient molecule, such as glucose, without net oxidation. Fermentation yields lactate, acetic acid, ethanol, or some other simple product.
Fermentation is also used much more broadly to refer to the bulk growth of microorganisms on some medium. No distinction is made between aerobic and anaerobic metabolism when the word is used in this sense.
This process is often used to produce or preserve food. Fermentation typically refers to the fermentation of sugar to alcohol using yeast, but other fermentation processes include the making of yogurt. The science of fermentation is known as zymology.
Fermentation usually implies that the action of the microoganisms is desirable.
Brewing is the production of alcoholic beverages through fermentation. This is the method used in beer production, although the term can be used for other drinks such as sake, mead and wine. The term is also sometimes used to refer to any chemical mixing process.
Brewing has a very long history, and archeological evidence tells us that this technique was used in ancient Egypt. Descriptions of various beer recipes can be found in Sumerian writings, some of the oldest known writing of any sort.
The brewing industry is part of most western economies.
All beers are brewed using a process based on a simple formula. Key to the process is malted barley which has been allowed to germinate and is then dried in a kiln and sometimes roasted. Depending on the amount of roasting, the "malted barley" will take on dark colour and strongly influence the colour and flavour of the beer.
The malt is ground into a coarse powder, known as grist, which is mixed with heated water in a vat called a "mash tun" for a process known as "mashing". During this process, natural enzymes within the malt break down much of the starch into sugars which play a vital part in the fermentation process. Mashing usually takes 1 to 2 hours, and during this time various temperature rests activate different enzymes depending upon the type of malt being used, its modification level, and the desires of the brewmaster. The Mash Tun generally contains a slotted "false bottom" or other form of manifold which acts as a strainer allowing for the separation of the liquid from the grain.
A mash rest at 104 F or 40 C activates beta-glucanase , which breaks down gummy beta-glucans in the mash, making the sugars flow out more freely later in the process. In the modern mashing process commercial fungal based beta-glucanase may be added as a supplement. A mash rest from 120F to 130 F (49C to 55C) activates various proteinases, which break down proteins that might otherwise cause the beer to be hazy. But care is of the essence since the head on beer is also composed primarily of proteins, so too aggressive a protein rest can result in a beer that cannot hold a head. This rest is generally used only with undermodified (i.e. undermalted) malts which are decreasingly popular in Germany and the Czech Republic, or non-malted grains such as corn and rice, which are widely used in North American beers. Finally, a mash rest temperature of 149 to 160 F (65 to 71 C) is used to convert the starches in the malt to sugar, which is then usable by the yeast later in the brewing process. Doing the latter rest at the lower end of the range produces more low-order sugars which are more fermentable by the yeast. This in turn creates a beer lower in body and higher in alcohol. While a rest closer to the higher end of the range creates more higher-order sugars, which are less fermentable by the yeast so a fuller-bodied beer is the result, with less alcohol.
After the mashing of the resulting liquid is strained from the grains in a process known as lautering. At this point it is known as wort. The wort is moved into a large tank known as a "copper" or kettle where it is boiled with hops and sometimes other ingredients such as herbs or sugars. The boiling process serves to terminate enzymatic processes, precipitate proteins, isomerize hop resins, concentrate and sterilize the wort. Hops add flavour, aroma and bitterness to the beer. At the end of the boil, the hopped wort settles to clarify it in a vessel called a "whirl-pool" and the clarified wort is then cooled.
The wort is then moved into a "fermentation vessel" where yeast is added or "pitched" with it. The yeast converts the sugars from the malt into alcohol, carbon dioxide and other components through a process called Glycolysis. After a week or so, the fresh (or "green") beer is run off into conditioning tanks . After conditioning for a week or longer, the beer is often filtered to remove yeast and particulates. The "bright beer" is then ready for serving or packaging.
According to Steinkraus (1995), traditionally food fermentation serves five main purposes:
Fermentation has some benefits exclusive to foods. Fermentation can produce important nutrients or eliminate antinutrients . Food can be preserved by fermentation, since fermentation uses up food energy and can make conditions unsuitable for undesirable microorganisms. For example, in pickling the acid produced by the dominant bacteria inhibit the growth of all other microorganisms.
Since fruits ferment naturally, fermentation precedes human history. However, humans began to take control of the fermentation process at some point. There is strong evidence that people were fermenting beverages in Babylon circa 5000 BCE, ancient Egypt circa 3000 BCE, pre-Hispanic Mexico circa 2000 BCE, and Sudan circa 1500 BCE. There is also evidence of leavened bread in ancient Egypt circa 1500 BCE and of milk fermentation in Babylon circa 3000 BCE. The Chinese were probably the first to develop vegetable fermentation.
Yeast produce ethanol and carbon dioxide gas. When the ferment has a high concentration of pectin minute quantities of methanol can be produced. Usually only one of the products is desired; in bread the alcohol is baked out, and in alcohol production the carbon dioxide is released into the atmosphere.
Products produced by fermentation, such as ethanol and lactic acid, are used as a substitute for oxygen. During cellular respiration, oxygen is the final electron acceptor. However, when no oxygen is available, ethanol and lactic acid is used in place of oxygen.
Bacteria generally produce acids. Vinegar (acetic acid) is the direct result of bacterial fermentation. In milk, the acid coagulates the casein, producing curds. In pickling, the acid preserves the food from pathogenic and putrefactive bacteria.