Acetic bacteria

The conversion of ethyl alcohol by way of acetaldehyde into acetic acid is the chemical expression equivalent to acetic fermentation. In this process the acetic bacteria utilise atmospheric oxygen in order to bind the hydrogen. That the hydrogen which has to be removed is activated, and not the oxygen (as was formerly thought), is shown by experiments in which oxygen is eaxluded and replaced by quinone the bacteria produce acetic acid from alcohol as before and the quinone is reduced to hydroquinone. 

The organism also fermented acrylate, as do other Clostridia, to a 2 1 molar mixture of propionate and acetate. Bacteria that ferment aciylate, derived from DMSP, probably abound in coastal marine sediments (Kiene and Taylor, this volume). Kiene and Taylor (this volume) observed that acrylate was immediately metabolized in anoxic slurries of coastal marine sediments with the appearance of a 1 2 molar mixture of acetate and propionate. The acetate and propionate were consumed by sulfate-reducing bacteria. Acetate is a quantitatively important substrate in anoxic marine sediments (491 and DMSP may be a significant precursor of acetate in such environments. 

Landete, J.M., Pardo, I. Ferrer, S. (2007b). Biogenic amine production by lactic acid bacteria, acetic bacteria and yeast isolated from wine. Food Control, 18, 1569-1574. 

Sugar acids come from enzymatic oxidation of sugars, especially important in wines affected by Botrytis cynerea and by rot in general acetic bacteria also produce these compounds. Some data about their level in wines are presented in Table 7.6. 

Organic acids (citric, lactic, acetic) Bacteria (both gram negative and gram positive) 2-3 70-1500 ppm 1-5 Often need mineral acid to lower pH active agent is nonionized form... 

Lactic and acetic bacteria develop in dough from the start of fermentation and play some part in giving- bread its flavor. If bread dough is fermented too long these bacteria produce so much acid that the bread is sour. 

Concerning the subject LeEevre says, Sometimes failure occurs during the acetic fermentation. Acetification may be slow in starting or may stop entirely, owing to the fact that the acetic bacteria are present only in small numbers or that those present are of a weak... 

This trouble can be checked by the addition of a large amount of pure culture of acetic bacteria. Steam and hot water sterilization of equipment is also practiced as a preventive measure. 

Abnormally high volatile acidity levels, however, are due to the breakdown of residual sugars, tartaric acid and glycerol by anaerobic lactic bacteria. Aerobic acetic bacteria also produce acetic acid by oxidizing ethanol. 

Finally, acescence in wine is linked to the presence of ethyl acetate, the ethyl ester of acetic acid, formed by the metabolism of aerobic acetic bacteria (Section 2.5.1). 

This chapter deals exclusively with organoleptic defects that develop during aging. Some are of chemical origin (oxidation, reduction and contact with certain materials), but microbiological processes are often involved, even in the development of cork taint and spoilage due to sulfur derivatives. The various problems caused by anaerobic lactic bacteria are described, as well as the role of acetic bacteria. The mycodermic yeasts responsible for/tor are included in Section 8.3. 

Excessive amounts of acetic acid in wine are due to the action of anaerobic lactic bacteria or aerobic acetic bacteria. Together with other molecules, this acid plays a major part in organoleptic defects of bacterial origin. On the other hand, myco-dermic (Section 8.3.4) and Brettanomyces yeasts (Section 8.4.6) cause defects that do not involve accumulations of this acid. 

These exceptions may be linked to cases of difficnlt alcoholic fermentation, leading to abnormally high concentrations of acetic acid produced by yeast. It is, however, possible to detect by analysis whether the acetic acid in a wine was produced exclusively during fermentation, without the involvement of bacteria (Volume 1, Section 14.2.3). Lactic bacterial activity leads to concentrations higher than 200 mg/1 of isomers of lactic acid and acetic bacteria produce ethyl acetate levels above 160 mg/1. 

There are several types of acetic bacteria (Volume 1, Chapter 7) with different metabolic properties. These are responsible for serious problems due to acescence, sometimes called acetic spoilage . Wine is only affected by Acetobacter, or vinegar ferment. The main reaction consists of the oxidation of ethanol to produce acetic acid. In the presence of ethanol, this same bacterium may also esterify acetic acid to produce ethyl acetate. Acetic bacteria develop in the form of a white bloom that may take on various appearances. Prolonged development produces a viscous mass, known as vinegar mother . 

At the same time, acetic bacteria are capable of esterifying the acetic acid that they form, producing ethyl acetate. The latter is responsible for the organoleptic characteristics of acescence, characterized by a very unpleasant, suffocating odor and an equally nasty impression of harshness and burning on the finish. The perception threshold of ethyl acetate (150 mg/1) is much lower than that of acetic acid (750 mg/1). 

Acetic bacteria are present everywhere on grapes, in wineries, on walls and floors, as well as inside empty wooden containers. Even if steps are taken to minimize contamination, wine always contains small quantities of bacteria, especially if it is not sulfured. If a young wine is allowed to remain in contact with air, it starts to produce a bloom (Section 8.3.4) and then acetic spoilage occurs. In older wines, spoilage occurs immediately. Consequently, it is essential to keep wine under conditions where bacterial development is as limited as possible. 

By isolating acetic bacteria, lactic bacteria and yeasts from red wines with phenol off-odors, it was demonstrated that Brettanomyces/Dekkera yeasts were the only microorganisms capable of producing several milligrams of ethyl-phenols per liter of wine. The species most prevalent in wine is Brettanomyces bruxellensis (Chatonnet et al, 1992b). 

Oil acts by extracting liposoluble substances. It is stirred vigorously to create an emulsion in the wine. This operation is repeated several times, until the oil is perfectly dispersed. When the mixture is allowed to rest, a layer of oil forms on the surface and is eliminated by careful racking. Liquid paraffin is mentioned in many books on this subject. It was not only recommended for eliminating unpleasant smells but also for isolating wine from the air in partly empty vats in order to protect it from acetic bacteria. In certain countries, solid paraffin discs, impregnated with allyl isothiocyanate, are used to create a sterile atmosphere. 

For selective counts of acetic bacteria, the lactic bacteria are inhibited by adding 0.001% penicillin. Incubation time 5-7 days. 

For selective counts of lactic bacteria, the acetic bacteria are inhibited by incubating the sample in Petri dishes under anaerobic conditions (with CO2 under pressure). Incubation time varies from 7 to 12 days, depending on the species under investigation. Incubation temperature 25°C. 

Another recommendation concerns the elimination of Drosophila fruit flies, likely to contaminate must and new wine with acetic bacteria. Systems that constantly release volatile insecticides are easily available. The same type of system may be used in bottle aging cellars to destroy moths and cork worms. 

Inert gases (N2 - - CO2) may be used to prevent chemical oxidation on the surface of wine and the development of acetic bacteria when it is stored in vats that are not completely full (Volume 1, Section 9.6.1.), although it is generally preferable to keep containers full. Regular topping up, to ensure that containers are always full, is another essential cellar operation. 

In addition to acetic bacteria Acetobacter sp.), harmful oxidative and acidifying yeasts Candida valida and Pichia vim) have also been identified (Chatonnet et al., 1993b). 

Fermentation conditions are ideal for bacterial growth and contanunation can retard or extend fermentation and cause off-flavors and odors. Typically specific gravity, pH, and flavor are checked while brewing and microbiological analysis is only carried out if issues arise during fermentation. Lactic and acetic bacteria and wild yeast are the main threats with a tolerance of less than lOcfu/ml and zero count, respectively. Media for their detection is given in Table 13.9. 

Lactic and acetic bacteria present in bright beer cause vinegary, sour astringent off-flavor and odor, excessive gassing, and strong head retention. Every batch should be... 

Fig. 12.1 Reichstein process for vitamin C production. The D-glucose was hydrogenated to form D-sorbitol. The D-sorbitol was converted into L-sorbose by acetic bacteria. The L-sorbose was further oxidized with protection to form 2-KLG. The 2-KLG was then esterified and lactonized to form vitamin C...

In the literatnre, increased viscosity in ciders and beers is attribnted to different lactic acid bacteria species, notably P. damnosus mAL. brevis—which are also fonnd in wine (Williamson, 1959 Beech and Carr, 1977). Luthi (1957) established that the symbiosis between lactic acid bacteria producing polysaccharides and acetic bacteria accelerates the increase in viscosity of the medium. 

This phenomenon is not exclusive to lactic bacteria, but certainly applies to many other microorganisms. It is easily demonstrated for acetic bacteria in winemaking. As soon as they are deprived of oxygen, the difference between the viable and VNC populations increases rapidly, then disappears completely as soon as the wine is aerated (Millet and Lonvaud-Funel, 2000). The same experiments showed that yeast and bacteria in VNC state decrease in size and some of them may pass through filters intended to eliminate them. 

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