physical or chemical stresses. Thus, the acid tolerance response may not only protect the cell against low pH, but also heat and oxidative stress.
In vegetable fermentations, the other important stresses encountered by lactic acid bacteria are high salt concentrations and high osmotic pressures. Salt concentrations in sauerkraut brines are around 0.4 M, giving an osmolality of about 0.8 Osm. Pickle and olive brines may contain more than 1.0 M salt, or osmolalities above 2 Osm. Salt is an extremely effective antimicrobial agent due to its ability to draw water from the cytoplasm, thereby causing the cell to become dehydrated, lose turgor pressure, and eventually plasmolyze (and die!).The ability of lactic acid bacteria to tolerate high salt conditions varies (just as it did for acid tolerance), depending on the organism. Given that L. plantarum usually dominates high salt fermentations, it should
Box 7-4. In a Pickle: How Lactic Acid Bacteria Deal with Acids and Salts (Continued)
be no surprise that this organism has also evolved physiological and genetic mechanisms that make it salt- and osmotolerant.
Like acid tolerance, the salt tolerance system depends on the activity of membrane pumps. However, in the latter case, the pumps are actually transport systems that take up a special class of molecules called compatible solutes. By accumulating these non-toxic solutes inside the cytoplasm to high concentrations, cell water is retained and osmotic homeostasis is maintained.
Compatible solutes are also referred to as osmoprotectants because they not only maintain osmotic balance, they also protect enzymes, proteins, and other macromolecules from dehydration and misfolding.Among the osmoprotectants accumulated by L. plantarum, the quaternary amine, glycine betaine (or simply betaine) is the most effective. Potassium ion, glutamate, and proline are also accumulated, but to lower concentrations, at least in lactobacilli. In L. plan-tarum, betaine is preferentially transported by the quaternary ammonium compound or QacT transport system (Glaasker et al., 1998).This transporter is a high affinity,ATP-dependent system whose activity is stimulated by high osmotic pressure (Figure 1B). In contrast, at low osmotic pressure, the efflux reaction is activated, and pre-accumulated betaine is released back into the medium.Analysis of the L.plantarum genome sequence indicates QacT may be encoded by the opuABCD operon, which is widely distributed in other Gram positive bacteria (Kleerebezem et al., 2003).The natural source of betaine, it is worth noting, is plant material, so perhaps it is no coincidence that L. plantarum is unable to synthesize betaine and instead relies on a transport system to acquire it from the environment (Glaasker et al., 1996).
Glaasker, E.,W.N. Konings, and B. Poolman. 1996. Osmotic regulation of intracellular solute pools in Lactobacillus plantarum.J. Bacteriol. 178:575-582. Glaasker, E., E.H. Heuberger,W.N. Konings, and B. Poolman. 1998. Mechanism of osmotic adaptation of the quaternary ammonium compound transporter (QacT) of Lactobacillus plantarum. J. Bacteriol. 180:5540-5546.
Kleerebezem, M., J. Boekhorst, R. van Kranenburg, D. Molenaar, O.I? Kuipers, R. Leer, R.Tarchini, S.A. Peters, H.M. Sandbrink,M.W. Fiers,W, Stiekema, R.M. Lankhorst, P.A. Bron, S.M. Hoffer, M.N. Groot, R. Kerkhoven, M. de Vries, B. Ursing,W.M. de Vos, and R.J. Siezen. 2003. Complete genome sequence of Lactobacillus plantarum WCFS1. Proc. Natl.Acad. Sci. U.S.A. 100:1990-1995. Lu, Z., H.P. Fleming, R.F. McFeeters, and S.S.Yoon. 2002. Effects of anions and cations on sugar utilization in cucumber juice fermentation. J. Food Sci. 67:1155-1161. McDonald, L.C., H.P Fleming, and H.M. Hassan. 1990. Acid tolerance of Leuconostoc mesenteroides and
Lactobacillusplantarum.Appl. Environ. Microbiol. 56:2120-2124. Van de Guchte, M., P. Serror, C. Chervaux,T. Smokvina, S.D. Ehrlich, and E. Maguin. 2002. Stress response in lactic acid bacteria. Antonie van Leeuwenhoek 82:187-216.
Destruction and softening of the pickle surface tissue is another serious defect. When this happens, the pickle loses its crispness and crunch and becomes slippery and soft. These pickles are neither edible nor can anything be done to salvage them. The defect is caused by pectinolytic enzymes produced by microorganisms that are part of the natural cucumber microflora. The organisms responsible include mostly filamentous fungi, especially species of Penicillium, Fusarium, Alternaria, Aschyta, and Cladosporium. Pectins are complex het-eropolysaccharides that serve as the main structural component of plant cell walls. Their hydrolysis requires the concerted action of three different enzymes—pectin methylesterase,poly-galacturonase, and polygalacturonate lyase. Although Penicillium and other fungi are capable of secreting these enzymes, they may also be produced by various yeasts, as well as by the cucumber flowers. Bacteria, however, do not appear to be a major source of pectin-degrading enzymes.
The fungi responsible for the softening defect gain entry into the fermentation tank via their association with cucumber flowers.Thus, excluding these constituents from the fermentation may reduce or minimize this defect. Other preventative measures include maintaining sufficient salt concentrations, acidity, anaerobiosis, and temperature. Another method used to control adventitious microorganisms and to ensure a prompt fermentation (especially when the salt concentration is at the low end) is to partially acidify the cucumbers with acetic acid to about pH 4.5 to 5.0.This practice of chemical pasteurization is also used when starter cultures and controlled fermentation methods are used to produce pickles (Box 7-3).
Olives: Products and Markets
Olives refer not only to the usually salty, acidic product known as table olives, but to the fruit from which they are made. The main use of raw olives, in fact, is as olive oil—more than 90% of the total worldwide olive production is used for oil and only 7% to 10% are consumed as table olives.
Olive trees are native to the Middle East and, due to their hardy nature (some trees live as long as 1,000 years), olives have long been a major agricultural crop throughout the region. Olives and olive oil are among the most frequently mentioned foods in the Bible. Olive production subsequently spread from the Middle East across the Mediterranean, to Greece, Italy, France, Spain, and Northern Africa. Olives were not introduced to the Americas until the 18th century. Currently, four countries—Italy, Spain, Greece, and Turkey—are responsible for 75% of the total worldwide olive production (between 9 billion Kg and 15 billion Kg. In contrast, the United States accounts for less than 1% of total world olive production, with essentially all 90 million Kg (200 million pounds) being produced in California (2003 statistics).
Currently, about 90% to 95% of the olives grown in California are used in the manufacture of table olives (making California among the leaders in table olive production).Although fermented olives are common in Europe and other olive-producing regions, most of the table olives produced and consumed in the United States are not fermented. In fact, more than 70% of the U.S. olive market consists of olives that are simply brined and canned (hence, this type is referred to as California-style olives).
There is also a small (but dedicated) market for fresh or raw tree-ripened olives.The famous Provence-style olives of France are of this type. More than thirty cultivars are grown worldwide; however, fermented olives are usually produced using one of eight main varieties: Manzanillo, Gordal, Picholine, Rubra, Mission, Sevillano, Ascolano, and Barouni. These culti-vars differ widely with respect to their composition, size, texture, color, and flavor. Olive properties also change during ripening; most olives are picked when they are still green, straw-yellow, or cherry-red, even if later they acquire a much darker appearance. Ultimately, their selection for table olive production is based on the style of the olive produced (discussed below).
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