At normal temperatures, the normal progression of changes in fruit juices is alcoholic fermentation by yeasts, followed by oxidation of the alcohol to acetic acid by acetic acid bacteria. When enough acetic acid is created, vinegar is the result. Vinegar is a condiment produced from sweet or starchy sources through alcoholic fermentation followed by acetic fermentation. Legal vinegar must contain at least 4g of acetic acid per 100 ml (or 40 grains).

Kinds of Vinegar 

• Vinegars can be categorised based on the substances from which they are produced: (1) those from the juices of fruits, e.g. apples, grapes, oranges, pears, berries, etc.; (2) those from starchy vegetables, e.g. potatoes or sweet potatoes, whose starch must first be hydrolyzed to sugars; (3) those from malted cereals, e.g. barley, rye, wheat, and corn; (4) those from sugars, e.g. syrups, molasses, honey, maple skimm
• Vinegar can be made from anything that contains sufficient sugar or alcohol and is not unpleasant as food.
• Typically, the vinegar’s name is derived from the substance from which it was produced: cider vinegar from apple juice, alegar from ale, malt vinegar from malted grains, spirit vinegar from alcohol, etc.
• In the United States, cider vinegar is the most common type of table vinegar, hence the term vinegar typically refers to cider vinegar.
• Vinegar made from grapes (wine) is the most popular in France, whereas vinegar made from malt liquors (alegar) is the most popular in the British Isles.

Fermentation

As previously stated, the production of vinegar from saccharine materials requires two processes: (1) the fermentation of sugar to ethyl alcohol and (2) the oxidation of alcohol to acetic acid. The first step is an anaerobic fermentation performed by yeasts, either those naturally present in the raw material or, preferable, additional cultures of high-alcohol-producing strains of Saccharomyces cerevisiae var. ellipsoidus. A simplification of the process equation is:

In reality, a sequence of intermediary reactions occur, and minor amounts of additional end products, such as glycerol and acetic acid, are created. In addition, there are trace amounts of chemicals derived from non-sugar molecules, including succinic acid and amyl alcohol. The second stage, alcohol oxidation to acetic acid, is an aerobic reaction carried out by acetic acid bacteria:

The intermediate chemical in this reaction is acetaldehyde. There are trace levels of aldehydes, esters, acetoin, etc., among the end products. Bergey’s Manual, ninth edition, proposed three families of Acetobacter with a total of nine subspecies. Gluconobacter comprised species capable of oxidising ethanol to carbon dioxide.

acetic acid. The second volume of Bergey’s Manual (1986) mentioned a family called Acetobacteriaceae with two genera named Acetobacter and Gluconobacter. The inability of subsequent species to convert acetic or lactic acid to carbon dioxide distinguishes them from Acetobacter. The current definition of Acetobacter includes bacteria that can oxidise acetic or lactic acid to carbon dioxide. Numerous “acetic acid bacteria” are currently identified as members of the genus Gluconobacter. Although the majority of the vinegar fermentation “work” may be performed by Gluconobacter sp., the commercially utilised cultures are a variety of strains that have been acclimated to the fermentation process.

Methods of Manufacture 

There are “slow” methods, such as the home, or “letalone,” method and the French, or Orleans, method, and “fast” procedures, such as the generator process or the fogging procedure, for producing vinegar. During acetification, the alcoholic liquid is not in motion during slow processes, but it is during rapid methods. For the synthesis of acetic acid, the slow procedures often employ fermented fruit juices or malt liquors, whereas the rapid methods are utilised mostly for the manufacturing of vinegar from spirits (alcohol). Fruit or malt liquors contain ample food for the vinegar bacteria, but to maintain active vinegar bacteria in generator methods employing alcohol, ethyl acetate, or vinegar, it must be supplemented with “vinegar food,” a combination of organic and inorganic compounds that varies depending on the compounder. Various combinations of compounds, including dibasic ammonium phosphate, urea, asparagine, peptones, yeast extract, glucose, malt, starch, dextrins, and salts, have been reported for application.

Slow Methods 

• In the home, or let-alone, method, a fruit juice, such as apple juice, is allowed to undergo a spontaneous alcoholic fermentation, preferably to approximately 11 to 13 percent alcohol, by yeasts that are already present; then, a barrel is partially filled with the fermented juice and placed on its side with the bunghole facing upward and open.
• The alcoholic solution is then subjected to an acetic acid fermentation, known as acetification, conducted by naturally occurring vinegar bacteria until vinegar is created.
• On the surface of the liquid, a film of bacteria known as “mother of vinegar” should form and convert the alcohol to acetic acid.
• Unfortunately, the yield may be low due to a poor yield of alcohol during yeast fermentation, the absence of productive strains of vinegar bacteria, the oxidation of acetic acid by the vinegar bacteria if alcohol is in short supply, or the competitive growth of film yeasts and moulds on the surface, which destroy alcohol and acids, and of undesirable bacteria in the liquid, which produce unpleasant flavours.
• The procedure is extremely sluggish, and the end result is frequently of poor quality. In contrast to the batch procedure just described, the Orleans, or French, method widely used in Europe is a continuous process. However, both processes are often conducted in barrels.
• In the Orleans method, one-fourth to one-third of the barrel is filled with raw vinegar from a previous run, which serves to provide an inoculum of active vinegar bacteria and acidify the additional wine, hard cider, or malt liquor in order to suppress competing microorganisms.
• The vinegar is combined with enough alcoholic liquor to fill roughly half the barrel, leaving an air space above that is exposed to the outside air through the bunghole at the top and a hole in either end of the barrel above the level of the liquid.
• This shielding protects these gaps. The acetic acid bacteria living on the surface of the liquid convert alcohol to acetic acid over a period of weeks to months at temperatures between 21 and 29 degrees Celsius, after which a portion of the vinegar is withdrawn for bottling and an equivalent amount of alcohol is added to the barrel.
• This action is performed multiple times, so making the process more or less continuous.
• This very sluggish procedure can generate high-quality vinegar. The dropping of the gelatinous coating of vinegar bacteria and the resultant delay in acetification is a challenge of this approach.
• To circumvent this problem, a raft or floating framework is often used to support the film. It is asserted that an excessively thick bacterial coating will inhibit acetification.

Quick Methods 

• As previously stated, rapid techniques of vinegar production involve the movement of the alcoholic liquid during the acetification process.
• Typically, this liquid is dripped over surfaces on which vinegar bacteria have developed and which are supplied with an ample amount of air.
• The generator method is the prevalent one currently. The basic generator is a cylindrical tank that is available in various sizes and is typically constructed of wood.
• The interior is divided into three sections: the upper section, where the alcoholic liquid is introduced; the large middle section, where the liquid is allowed to trickle down over beechwood shavings, corncobs, rattan shavings, charcoal, coke, or pomace; and the lower section, where the vinegar is collected.
• The alcoholic liquid is brought in at the top through an automatic feed trough or sprinkling mechanism (sparger) and dripped down over shavings or other material on which a slimy growth of acetic acid bacteria has grown. These bacteria oxidise the alcohol to acetic acid.
• When air enters through the fake bottom of the middle part and warms, it rises and is vented above. As the oxidation process generates considerable heat, it is typically necessary to limit the temperature to between 29 and 30 degrees Celsius.
• This can be accomplished by employing cooling coils, regulating the rate of air and alcoholic liquid feeding, and either cooling the alcoholic liquid before it enters the generator or cooling the partially acetified liquid that is returned to the top of the tank for further action.
• Before vinegar can be produced, the slime of vinegar bacteria must be created in a new generator.
• First, the tank’s central section is filled with raw vinegar containing active vinegar bacteria to inoculate the shavings with the required bacteria, or this material is circulated through the generator.
• Then, an alcoholic liquid acidified with vinegar is gently recirculated through the generator to promote bacterial growth on the shavings. Before entering the alcoholic liquid into the generator, some producers acidify it with vinegar or leave some vinegar to acidify the new batch of liquid.
• A single pass of the alcoholic liquid through the generator can generate vinegar, or the vinegar collected at the bottom can be recirculated through the generator if insufficient acid was initially produced or too much alcohol remains.
• The liquid from the first tank is sometimes sent via a second or even a third generator. The Frings generator is a huge, cylindrical, airtight tank equipped with a sprinkler (sprayer) at the top, cooling coils around the lower portion of the middle section housing the shavings, and facilities for the cycling of the vinegar from the bottom collection chamber through the system.
• Modern versions of these generators are outfitted with automatic controls for feeding the alcoholic liquid, injecting filtered air, regulating the temperature, and recirculating the collected liquid from the bottom.
• These generators produce significant yields of acetic acid with minimal alcohol residual. Through jet nozzles, a fog or fine mist of a mixture of vinegar bacteria and nutritional alcohol solution is sprayed into a chamber during the Mackin process.
• The mist is kept in circulation by filtered air for a time before falling to the bottom to be collected, cooled, and reatomized before being reintroduced to the chamber. This procedure is repeated until practically all of the alcohol has been oxidised.
• The dipping generator consists of a tank with a basket containing beech-wood shavings that can be elevated or lowered into a solution of diluted alcohol in the lower portion of the tank.
• Aeration permits quick acetification by vinegar bacteria on the shavings while the basket is out of the liquid, and lowering the basket into the liquid provides more culture medium and eliminates the acetic acid produced.
• Submerged Procedure In submerged fermentation, a medium containing 8 to 12 percent alcohol (hard cider, wine, fermented malt mash, or spirits) is inoculated with Acetobacter acetigenum* and maintained at 24 to 29 degrees Celsius with regulated aeration by means of fine air.
• Hromatka detailed the use of a pure culture of Gluconobacter oxydans in a submerged vinegar fermentation in a future publication. The Frings Acetator, depicted in Figure, is an example of a piece of equipment used with this procedure. In a suspension of fine air bubbles and fermenting liquid, bacteria proliferate.
• An aerator with a unique design achieves the suspension. It is composed of a rotor (a) operated by an electric motor (c) positioned beneath the tank (d).
• The rotor draws in air, accelerates it after fully combining it with water, and evenly distributes the suspension across the tank’s cross-section.
• The self-priming rotor is encircled by a stator to ensure uniform distribution of the air bubbles (b).
• A pipe (e) attached to the rotor exits the tank’s interior towards the top and splits into two branches outside the tank.
• One of the branches links to a rotameter (f), which measures the volume of air entering the tank each minute.
• The opposite branch is connected to a condensate cylinder (g) through an exhaust air pipe (h).
• Four stabiliser boards I are attached to the tank’s inside circle. The stainless steel cooling coil (k) is attached to these.
• The coil receives cooling water through a pipe (l) connected to a pump (m) and a flowmeter (f) (n).
• A regulating thermometer (o) controls the pump to maintain a steady fermentation temperature.
• The nearby charging pump (p) pumps the mash through the charging pipe (q).
• The mash enters the tank in the centre of the top and runs directly into the rotor. Near the acetator, the discharging pump (r) empties the tank halfway after each fermentation cycle.
• Attached to the tank’s top-center is a defoamer that mechanically eliminates foam (Morton generators do not have a defoamer).
• The defoamer pumps the foam’s liquid component back into the tank. Air is expelled via the exhaust pipe.
• The generator is charged and discharged by the controller (t).

Finishing 

• Obviously, the composition of vinegar relies on the ingredients used to create it.
• Vinegars derived from fruits and malt liquors has flavours like those substances. The technique of production also affects the product’s characteristics.
• Due to the maturing that occurs during the lengthy preparation process, vinegars produced by slow procedures are less acidic than those produced by quick methods.
• When quickly manufactured vinegars are matured in tanks or barrels, their body, flavour, and fragrance increase.
• To clarify the vinegar, which should be very clear, filtration and “fining” (clarification by settling out of additional suspended components) are used.
• The majority of vinegar on the market is now pasteurised in bulk or in bottles. Temperatures and times vary, but brief heating at 60 to 66 degrees Celsius is one example.
• The vinegar’s strength is measured in grains, which is ten times the amount of acetic acid per 100 ml of vinegar.
• At 20 degrees Celsius, 40-grain vinegar has 4 grammes of acetic acid per 100 millilitres.

Vinegar Defects and Diseases 

As with wine, metals and their salts have the potential to cloud and discolour vinegar. Ferrous iron can oxidise into ferric iron and interact with tannins, phosphates, or proteins to form a haze. Cloudiness may also be produced by tin or copper salts. Iron interacting with tannin or oxidase activity may be responsible for vinegar’s darkening.

Microbial Defects 

• Defects caused by microorganisms may result in substandard materials utilised to produce vinegar or in the condiment’s inferior quality.
• Wine and hard cider, for instance, are susceptible to the issues outlined in the wine illnesses section.
• Lactobacillus and Leuconostoc species in fruit juice may not only be responsible for off-flavors, such as the “mousy” taste, but they may also create sufficient amounts of acetic acid to inhibit yeast fermentation.
• Under anaerobic conditions, bacteria that make butyric acid may produce their harmful acid. This difficulty can be mitigated by adding sulphur dioxide to the juices, but this chemical is toxic to the bacteria that produce vinegar.
• The primary flaws of vinegar are the creation of excessive sliminess in the bulk of vinegar microorganisms and the decomposition of acetic acid inside the product.
• It has been stated that a particularly heavy, thick, slimy coating of bacteria slows the pace of acetification during the sluggish process of vinegar production.
• However, excessive sliminess is far more detrimental to the generator process since it inhibits aeration.
• Sliminess is favoured by an alcoholic liquid that is a good culture medium, such as cider, wine, or a medium to which too much rich vinegar food has been added, but it is usually not a problem in the acetification of a poor culture medium, such as spirits used to make vinegar (alcohol).
• Several kinds of vinegar bacteria are capable of causing sliminess (cellulose), but Acetobacter aceti subsp. xylinum* is likely the most significant.
• During the vinegar-making process, acetic acid bacteria can convert vinegar’s acetic acid to carbon dioxide and water if there is an insufficient amount of alcohol or an excessive amount of oxygen.
• Film yeasts (“wine flowers”), moulds, and algae are additional species capable of oxidising acetic acid in aerobic conditions.