Bacteria, lactic acid multiplication

An improvement of the productivity of the NF-based process to 7.1gl h is obtained by running the process semi-continuously [64]. Final lactic acid concentrations in the permeate can reach values between 10 and 60 g 1 . The higher values are at the lower limit of concentrations found in UF- and MF-based processes [60]. Based on these data, a three-step repetitive process has been proposed [64]. The first step is the cell multiplication step during which pH can be controlled, the second step is an acid production step, followed by NF. In this approach a constant pH is assumed during each separate process step. However, other strategies in the acidification stage, which make use of natural acidification by the lactic acid bacteria are also possible. 

Of all the metabolic activities that lactic acid bacteria can carry out in wine, the most important, or desirable, in winemaking is the breakdown of malic acid, but only when it is intended for this to be removed completely from the wine by malolactic fermentation. Although the breakdown of malic and citric acids has considerable consequences from a winemaking perspective, it is also evident that lactic acid bacteria metabolise other wine substrates to ensure their multiplication, including sugars, tartaric acid, glycerine and also some amino acids. We will now describe some of the metabolic transformations that have received most attention in the literature, or which have important repercussions in winemaking. 

Recently, multiple PCR systems have been published that use different pairs of primers to simultaneously detect lactic acid bacteria producers of histamine, tyra-mine and putrescine, the majority wine biogenic amines (Marcobal et al. 2005b Constantini et al. 2006). 

In addition, the residual sugar, aldehydes and pyruvate are degraded so that less SO2 is required in a subsequent sulfur treatment step. The multiplication of the lactic acid bacteria is promoted by increasing the temperature to 20 °C and stirring up the yeast settlings. 

Unless the appropriate inhibitory treatments are applied, lactic acid bacteria are part of the normal microflora of aU white and red wines. From the start to the end of fermentation, and even during aging and storage, they alternate between successive growth and regression periods depending on the species and the strains. AU multiplication or survival involves a metabolism that is perhaps very active or, on the contrary, hardly perceptible and even impossible to detect with current analytical methods. Substrates are transformed and consequently organoleptic characters are modified. Some metabolic activities are favorable and others are without consequence, whUe some are totally detrimental to wine quaUty (Volume 2, Section 8.3). 

Yeasts are well adapted to growth in grape must. From the first days of fermentation, their multiplication is very rapid. Lactic acid bacteria also multiply very easily when inoculated alone in this same environment. Yet in practical conditions, yeasts and bacteria are mixed yeasts are always observed to dominate bacteria. The experimental inoculation of grape must with S. cerevisiae and diverse lactobacilli or S. cerevisiae and a mixture of 0. oeni clearly shows a behavioral difference between the bacteria. 

In addition to the influence of yeasts and other lactic acid bacteria, fungi and acetic bacteria present on infected grapes also affect wine lactic acid bacteria. The media precultivated by the above have varying effects on lactic acid bacteria multiplication with respect to the control media (San Romao, 1985 Lonvaud-Funel et al., 1987). Organic acids and polysaccharides accumulate in the medium and either impede or activate bacterial growth, but in practice they have little effect. Even if the grapes are tainted, these metabolites remain in insufficient concentrations to affect lactic acid bacteria. 

The principal factors affecting acetic acid bacteria development (as with lactic bacteria) are the alcoholic content, the pH, the SO2 concentration, the temperature, and the oxidation-reduction potential. The more the pH and temperature are increased, the more easily the bacteria survive. Their multiplication is quicker in the case of aeration. 

Having witnessed the difficulty of obtaining lactic acid bacteria growth in wine, Lafon-Lafourcade (1970) studied the possibility of obtaining malic acid degradation by using a biomass sufficiently abundant and rich in malolactic enzyme so that the reaction can occur without cellular multiplication. 

The growth of malo-lactic bacteria in wines is favored by moderate temperatures, low acidity, very low levels of S02, and the presence of small amounts of sugar undergoing fermentation by yeast. It is frequently possible to inoculate a wine with a pure culture of a desirable strain of bacteria and obtain the malo-lactic fermentation under controlled conditions. The pure-culture multiplication of the selected strain of bacteria is difficult, however. It is also difficult to control the time of the malo-lactic fermentation—sometimes it occurs when not wanted, and at other times will not go when very much desired. For the home winemaker it is probably most satisfactory to accept the malo-lactic fermentation if it occurs immediately following the alcoholic fermentation. The wines should then be siphoned away from deposits, stored in completely filled containers at cool temperatures, and have added to them about 50 ppm S02. If the malo-lactic fermentation does not take place spontaneously and the wine is reasonably tart, the above described regime of preservation will likely prevent its occurrence. When the malo-lactic transformation takes place in wines in bottles, the results are nearly always bad. The wine becomes slightly carbonated, and the spoiled sauerkraut flavors are emphasized. 

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