As noted above, the high salt concentrations used in pickle manufacturing cause the fermentation to proceed quite differently from
Box 7—3. Starter Cultures and Fermented Vegetables
The modern manufacture of most fermented vegetables, in contrast to cheese, sausage, and other fermented food products, still relies on a natural fermentation. In large part, this is because vegetable fermentations occur as a succession, and duplicating this process with "controlled fermentations" using starter cultures has not been a viable option. Also, vegetable fermentations are often conducted in less than aseptic conditions, so adding a culture to a raw material comprised of a complex, well-populated background flora is unlikely to be very effec-tive.Finally, paying for cultures to perform a step that ordinarily costs the manufacturer nothing makes little economic sense.
Yet, for all of the same reasons that eventually drove other industries to adopt pure starter culture technology (i.e., consistency, control, safety, and convenience), the fermented vegetable industries have indeed developed manufacturing procedures that depend on starter cultures, rather than the natural flora, to perform the fermentation. Research on pure culture technology for fermented vegetables actually began nearly fifty years ago, and cultures were developed in the 1960s (Daeschel and Fleming, 1987). Despite the availability of these cultures, however, they have not been widely used.
More recently, other factors have provided additional motivation for the use of starter cultures. One particularly relevant issue relates to the large volumes of high salt brines that are generated by pickle and olive fermentations.These salt solutions create significant environmental problems. The most obvious way to reduce the discharge of this material into the environment is to use less salt. However, since salt provides the major means for controlling the microflora, conducting a natural fermentation at low-salt concentrations is unlikely to be satisfactory. In contrast, less salt could be used if the background flora was controlled by other means (see below), and a starter culture was used instead to dominate the environment and to carry out the fermentation.
The first requirement for performing a controlled fermentation is to remove and/or inactivate the endogenous microflora. This can be done with chemical agents that kill organisms at the surface. Specifically, the raw product is washed first with dilute chlorine solutions, then with acetic acid. In reality, chemical pasteurization is possible only for cucumbers and olives and not for shredded cabbage due to surface area considerations.Although olives can tolerate a modest heat treatment, cabbage and cucumbers suffer severe texture defects if heated. Finally, a nitrogen purge drives out air and creates anaerobic conditions, and an acetate buffered brine is then added, followed by the starter culture.
The organisms that are currently available or are being considered for use as starter cultures include many of the same species ordinarily isolated from vegetable fermentations, e.g., Lactobacillus plantarum, Leuconostoc mesenteroides, Pediococcus acidilactici, and Lactobacillus brevis. Strain selection, however, is necessary and must be based on the specific application and desired characteristics of the particular fermented food (Table 1). For example, strains to be used as starter cultures for fermented olives must resist the antimicrobial phenolic compounds ordinarily present in olives.
As noted above, another inherent problem in controlled fermentation technology is the difficulty in establishing a microbial succession, such that a heterofermentative phase always precedes the homofermentative phase.That is, how can L. mesenteroides and L.plantarum both be added at the outset of a sauerkraut fermentation, yet have conditions controlled such that growth of L. plantarum is delayed until the later stages of the fermentation? Researchers at USDA and North Carolina State University in Raleigh, North Carolina, addressed this problem by developing a clever bio-controlled fermentation process (Breidt et al., 1995;Harris et al., 1992). In this model system, the starter culture consisted of an L. mesenteroides strain that resisted the bacteriocin nisin. As hypothesized, in the presence of nisin (either added directly or produced by a companion nisin-producing strain of Lactococcus lactis), the L. mesenteroides strain grew
Box 7—3. Starter Cultures and Fermented Vegetables (Continued)
fine, but growth of nisin-sensitive L. plantarum was inhibited.Thus, it was possible to prolong the heterofermentative (and flavor-generating) phase of the fermentation, while delaying the homofermentative phase.
Ultimately, the use of starter cultures for the manufacture of fermented vegetables is likely to increase as the size of the production facilities and the demand for speed, efficiency, and throughput both increase.In addition,the starter culture industry is now able to develop strains that have specific physiological properties, satisfy specific performance characteristics, are stable during storage, and are easy to use.
Table 1. Properties of starter cultures for fermented vegetables.
Minimum nutritional requirements
Able to grow at low temperatures
Able to ferment diverse carbohydrate substrates
Able to compete against wide array of organisms
Able to produce desirable flavor
Rapid growth and acid production
Tolerant to acids and low pH
Tolerant to salt
Tolerant to antimicrobial phenolics Resistant to bacteriophage Non-pectinolytic
Unable to produce dextrans or other polysaccharides Unable to produce biogenic amines Minimum loss of viability during storage
Breidt, F., K.A. Crowley, and H.P. Fleming. 1995. Controlling cabbage fermentations with nisin and nisin-resistant Leuconostoc mesenteroides. Food Microbiol. 12:109-116.
Daeschel, M.A., and H.P. Fleming. 1987.Achieving pure culture cucumber fermentations: a review, p. 141 — 148. In G. Pierce (ed.), Developments in industrial microbiology. Society for Industrial Microbiology,Ar-lington,Va.
Harris, L.J., H.P. Fleming, and T.R. Klaenhammer. 1992. Novel paired starter culture system for sauerkraut, consisting of a nisin-resistant Leuconostoc mesenteroides strain and a nisin-producing Lactococcus lac-tis strain.Appl. Environ. Microbiol. 58:1484-1489.
that in sauerkraut. Only those pickles made using brines at less than 5% salt will allow for growth of L. mesenteroides. Although hetero-fermentative fermentations may promote more diverse flavor development, the formation of CO2 is undesirable, because it may lead to bloater or floater defects (see below). Moreover, low salt brines may also permit growth of unwanted members of the natural flora, including coliforms, Bacillus, Pseudomonas, and Flavobacterium. At salt concentrations between 5% and 8%, growth of Leuconostoc is inhibited and instead the fermentation is initiated by Pediococcus sp. and L.plantarum.
Pickle fermentation brines typically contain high concentrations of salt and organic acids and have a pH less than 4.5.These conditions are especially inhibitory to coliforms, pseudomon-ads, bacilli, clostridia, and other non-lactic acid bacteria that would otherwise cause flavor and texture problems. This environment, in fact, is hard even on lactic acid bacteria. However, the latter have evolved sophisticated physiological systems that enable them to survive under very uncomfortable circumstances (Box 7-4).
After fermentation, salt stock pickles can be held indefinitely in the brine. However, these pickles cannot be eaten directly, but rather
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