Selective Isolation as a Result of Sequential Biochemical Activity

The production of vinegar represents a unique sequence of environmental and biochemical events. The sugars in a fruit juice are first fermented anaerobically by yeasts to ethanol, which is then subjected to vigorous aeration, resulting in the oxidative conversion of ethyl alcohol to acetic acid (usually 4-5%) by resident acetic acid bacteria. Ethanol as an intermediate product will sustain the growth of fewer microorganisms than will glucose because it contains less energy than glucose and is therefore more restrictive with respect to microorganisms capable of attacking it. Acetic acid contains even less energy than ethanol, and, at a level of 4%, results in complete microbial preservation or stability in the absence of oxygen.

The methane fermentation, which is frequently used to digest solid waste materials, involves the initial anaerobic formation of intermediate metabolic products such as etha-nol, and lower fatty acids such as acetic acid and butyric acid, by members of the genus Clostridium and various facultative anaerobes. These metabolic intermediates are then converted anaerobically to gaseous methane by methane bacteria with the result that the initial solid matter is converted to a gaseous product.

The selective isolation of notably acid tolerant lactic acid bacteria in contrast to moderately acid tolerant lactics can be achieved using the dynamics of the sauerkraut fermentation. Here, 2.5% NaCl is uniformly applied to sliced cabbage in a suitable container. The first brine formed from osmotic expression of water from the tissue by the salt is nearly at a saturation level with respect to NaCl and will not allow microbial growth. As more and more water is drawn from the tissue, along with sugar and other nutrients, the NaCl concentration in the brine becomes progressively less inhibitory. At about 6.5% NaCl coliforms will begin to develop at the initial pH of about 6.5 and will reduce the pH to about 5.5. As these organisms undergo autolysis following their growth, the brine becomes nutritionally enhanced so as to allow a mixture of moderately and notably acid tolerant lactic acid bacteria, that are far more nutritionally fastidious, to initiate growth under anaerobic conditions. The organisms of highest acid tolerance, such as Lactobacillus plantarum and Pediococcus lactis, will be the last remaining viable organisms by the time a final pH of 3.7 has been reached at an NaCl concentration of about 3.0%.

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