Among the microbial defects that occur in pickles, the most common are bloaters and floaters (Table 7-4).The defect is caused by excessive gas pressure that subsequently results in internal cavity formation within the pickles. The CO2 gas is mainly produced by heterofermentative lactic acid bacteria (some of which may produce CO2 via the malolactic fermentation), although coliforms and yeasts may also be responsible. Floaters and bloaters can still be used for some processed products (i.e., relish), however, they cannot be used for the whole or sliced pickle market. The most common way to control or minimize this defect is to remove dissolved CO2 by flushing or purging the brine with nitrogen gas.
Box 7—4. In a Pickle: How Lactic Acid Bacteria Deal with Acids and Salts
The key requirement to ensure the success of vegetable fermentations is to create a restrictive, if not inhospitable, environment, such that most indigenous microorganisms are inhibited or otherwise unable to grow.Typically, this is initially accomplished by adding salt, excluding oxygen, and maintaining a somewhat cool temperature. As lactic acid bacteria grow and produce organic acids and CO2, the ensuing decreases in pH and Eh provide additional hurdles, especially for salt-sensitive, neutrophilic, aerobic organisms.These conditions, however, may not only affect resident enteric bacteria, pseudomonads, clostridia, fungi, and other undesirable microorganisms, but they can also impose significant problems for the lactic acid bacteria whose growth is to be encouraged.
In addition, there may be other ionic compounds present in the vegetable juice brine that interfere with growth of lactic acid bacteria. For example, acetate, lactate, and other buffer salts are often added to brines, especially when pure cultures are used to initiate the fermentation. These acids and salts may have significant effects on cell metabolism and growth (Lu et al., 2002). Thus, how plant-associated lactic acid bacteria cope with these challenges may be of practical importance.
Lactic acid bacteria are prolific producers of lactic acid (no surprise there), and can tolerate high lactic acid concentrations (>0.1 M) and low pH (<3.5), much more so than most of their competitors.At least several physiological strategies have been identified that enable these bacteria to tolerate high acid, low pH conditions.
First, lactobacilli and other lactic acid bacteria can generate large pH gradients across the cell membrane, such that even when the medium pH is low (e.g., 4.0), the cytoplasmic pH (the relevant pH for the cell's metabolic machinery) is always higher (e.g, 5.0). For example, in one study (McDonald et al., 1990),Lactobacillusplantarum and Leuconostoc mesenteroides maintained pH gradients of nearly 1.0 or higher, over a medium pH range of 3.0 to 6.0 (acidified with HCl). In the presence of lactate or acetate, however, somewhat lower pH gradients were maintained. This is because organic acids diffuse across the cytoplasmic membrane at low pH (or when their pKa nears the pH), resulting in acidification of the intracellular medium. For some bacteria, e.g., L. mesenteroides, the pH gradient collapses at low pH (the so-called critical pH). At this point, the cell is in real trouble, as enzymes, nucleic acid replication,ATP generation, and other essential functions are inhibited. In general, these results reflect, and are consistent with the observed lower acid tolerance of L. mesenteroides, as compared to L.plantarum.
If maintenance of a pH gradient is important for acid tolerance, then the next question to ask is how such a gradient can be made.That is, how can the protons that accumulate inside the cell and cause a decrease in intracellular pH be extruded from the cytoplasm? Although there are actually several mechanisms for the cell to maintain pH homeostasis, one specific system, the proton-translocating F0F1-ATPase (H+-ATPase) is most important.This multi-subunit, integral membrane-associated enzyme pumps protons from the inside to the outside using ATP hydrolysis as the energy source (Figure 1A).This enzyme is widely conserved in bacteria (in fact, throughout nature), but the specific properties of enzymes from different species show considerable variation.Thus, the H+-ATPases from lactobacilli have a low pH optima, accounting, in large part, for the ability of these bacteria to tolerate low pH relative to less tolerant lactic acid bacteria.
Although the H+-ATPase system is the primary means by which lactic acid bacteria maintain pH homeostasis, other systems also exist (Figure 1A). For example, deamination of the amino acid arginine releases ammonia, which raises the pH. Decarboxylation of malic acid, which is commonly present in fermented vegetables, also increases the pH by conversion of a dicar-boxylic acid to a monocarboxylic acid. In fact, when lactobacilli and other lactic acid bacteria are exposed to low pH, a wide array of genes are induced (Van de Guchte et al., 2002). Collectively, this adaptation to low pH is referred to as the acid tolerance response. Some of the induced genes code for proteins involved in the machinery used by the cell to deal with other
Box 7-4. In a Pickle: How Lactic Acid Bacteria Deal with Acids and Salts (Continued)
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