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Notes on Oenology: Lactic and acetic bacteria

Notes on Oenology: Lactic and acetic bacteria



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Lactic acid bacteria

Lactic acid bacteria are defined as those microorganisms that from the fermentation of sugars mainly produce lactic acid. These bacteria are essentially ubiquitous and normally present in food products, they intervene in numerous natural fermentative processes and are also widely used on an industrial level. In the oenological field they are the agents of malolactic fermentation and are also the cause of some wine defects known with the term "filmed" or "stringy". These microorganisms can have a coconut or rod shape, are Gram positive, catalase negative (with the exception of some species of the genus Pediococcus), have non-mobile cells and do not form spores, are anaerobes or microaerophiles, that is, they multiply well in total absence or partial oxygen, even if many of them tolerate it without using it in energy production processes. According to their metabolism, they are classified into homofermentative and heterofermentative lactic bacteria. Homofermentatives produce almost exclusively lactic acid from glucose metabolism, heterofermentatives produce, in addition to lactic acid, also acetic acid, carbon dioxide and ethanol. Bacteria are more sensitive than yeasts to the various conditions of the medium, in particular they are very affected by the presence of sulfur dioxide, pH and temperature.
Malo-lactic fermentation is very important in the formation of red wines, the most important bacterial agent of which is the species Oenococcus oeni (Leuconostoc oenos). Fermentation can be carried out by the indigenous bacterial population naturally present in wine, however the presence of bacterial species other than Oenococcus oeni is not desirable, as their uncontrolled development can give rise to organoleptic defects in wine: these malolactic fermentations, in some cases , especially if they last for a long time, they can lead to high production of secondary compounds, such as acetic acid or acetaldehyde, compounds that give the product a bad smell. Oenococcus oeni, on the other hand, is capable of conducting a good malo-lactic fermentation, transforming malic acid into lactic acid and reducing total acidity. This type of fermentation has a particular effect on the organoleptic characteristics of the wine, making it softer and less aggressive.
Lactic acid bacteria begin to proliferate at the end of alcoholic fermentation when cell lysis which partially involves the population of yeasts releases nitrogen compounds necessary for growth. During this phase, when the population has reached a sufficient concentration, malolactic fermentation takes place. At the end of the process, the bacteria can survive for many months if the wine is not added with SO2 as they can also obtain the necessary energy from other organic compounds such as citric acid, glycerol and to a lesser extent tartaric acid. During the various stages of winemaking, there is generally a succession of the bacterial species present; in fact at the beginning (in the musts just arrived in the tanks) the microflora is more varied and above all Leuc are found. mesenteroides, P. damnosus, L. hilgardii, L. brevis, L. plantarum and L. casei. Subsequently, the micro-organisms belonging to the genus Lactobacillus, Pediococcus and Leuconostoc tend to disappear, while the species that is affirmed, especially in the case of particularly acidic wines, is O. oeni, more adapted to the low pH and alcohol content of the medium. bacteria of the genus Lactobacillus and Pediococcus can survive and even dominate at the expense of O.oeni.

Malo-lactic fermentation
Malolactic fermentation takes place according to the reaction described in the figure below.

Lactic and alcoholic fermentation (source: it-it.abctribe.com)

As for yeasts, it is advisable, in red wines, to inoculate the quantities of Oenococcus oeni immediately after the end of alcoholic fermentation, in order to prevent a negative evolution of malo-lactic fermentation by relying only on the bacteria present in the must. Products that contain bacteria of the species Oenococcus oeni are available on the market in the form of freeze-dried preparations. Given their low vigor and adaptability, it is necessary to gradually adapt the crops to be inoculated to wine, to increase their survival.

Acetic bacteria

Acetic bacteria are those that have intense oxidative metabolism, which metabolize sugars and ethanol oxidatively and produce acetic acid. Acetobacteria are Gram negative bacteria, approximately 0.5-0.8μm x 1-4μm in size. They have a rod or ellipsoidal shape, arranged singly, in pairs or in chains. They are obligatory aerobes and catalasipositive. These microorganisms are widespread in environments containing sugars, alcohol and acids, such as beer, wine, vinegar, cider and are also present in the production of fermented foods. Acetic bacteria belong to the Acetobacteraceae family which includes the genera Acetobacter, Gluconobacter, Gluconacetobacter, Acidomonas.
The genera associated with grapes and wine are Acetobacter, Gluconobacter and Gluconacetobacter. The most frequent oenological species belong to the genera Acetobacter and Gluconobacter and in particular are: G.oxydans, A. aceti and A.pasteurianus. Both genders have the ability to oxidize ethanol to acetic acid, but in Acetobacter, unlike in Gluconobacter, the oxidation of acetic and lactic organic acids continues until carbon dioxide and water are obtained. In musts and grapes, Gluconobacter is mainly isolated, because it prefers substrates with sugars and is sensitive to ethanol, while in wines or fermenting musts, Acetobacter is mainly isolated. In wines these microorganisms are considered contaminants because the high concentration of acetic acid they produce involves serious alterations making the wine unpleasant, a defect known as volatile acidity. Other alterations of wines caused by these microorganisms are the production of acetaldehyde and vinegar. In general, wines at the end of alcoholic fermentation have volatile acidity values ​​between 0.3 and 0.5 g / l of acetic acid naturally formed by the fermentation process. Higher values ​​indicate bad production processes.
Acetic bacteria, such as lactic acid bacteria and yeasts, come from indigenous populations in grapes and can be favored by particular conditions such as acid rot or attacks by parasites that tear the skin and promote their development; sometimes, however, important contamination can also occur from the cellar equipment, such as destemmers, pipes and tanks not washed diligently. Respect the right ripening periods of the grapes, without exposing yourself to unnecessary risks in the collection of overripe grapes with torn and therefore already contaminated grapes, then carry out the collection of grapes that have a good sanitary state, speed up the operations of harvesting and transporting the grapes in the cellar without exposing them to the hottest hours of the day, always wash the trolleys or in any case all the equipment that comes into contact with the product, use products with antioxidant and antiseptic action such as sulfur dioxide and finally respect the normal hygiene standards of the cellar: all these operations can certainly lead, if not to the elimination, to an adequate control regarding possible bacterial alterations of our product.

Bibliography
- Plant Research and Production Center (CRPV), Viticulture and Biological Oenology, Edagricole, 2004
- Dandrea Maura, Characterization and evaluation of the potential of use of lactic acid bacteria in the oenological field, Doctoral Thesis, 2010

Sitography
- www.ersa.fvg.it
- www.vinicoltura.it
- www.isii.it


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