Inhibitory Compounds Produced By Starter Cultures

One of the valuable properties of starter cultures is their ability to inhibit growth of undesirable microorganisms. The main preservative action of lactic starter cultures is a result of acid production. Acids produced by lactic acid bacteria include not only lactic acid but also lesser amounts of acetic and formic acids. Production of acids other than lactic acid increases the preservative effect of the culture because, at equivalent pH, acetic and formic acids have greater inhibitory power than lactic acid.

Lactic starter cultures also produce nonacidic microbial inhibitors. These include hydrogen peroxide (which can act by itself or in concert with the lactoper-oxidase system as previously discussed), carbon dioxide, low molecular weight carbonyl compounds, and bacteriocins. Production of nonacidic inhibitors by lactic starter cultures is not necessarily advantageous. Undesirable effects include autoinhibition resulting from hydrogen peroxide (produced when oxygen is present in the milk) and an inability to be used in multiple-strain cultures as a result of bacteriocin production.

A. Low Molecular Weight Nonacidic Metabolites

Kulshrestha and Marth (1974a, 1974b) observed that many nonacidic low molecular weight metabolites of lactic acid bacteria have antimicrobial activity but at concentrations higher than produced in cultured milk. The metabolite with greatest inhibitory activity is the flavor compound, diacetyl (2,3-butanedione). Jay (1982) found that yeasts and gram-negative bacteria are inhibited by 200 ppm diacetyl and that gram-positive bacteria are inhibited by 300 ppm. Although such levels are not found in cultured dairy products, diacetyl may act in combination with other compounds to enhance the preservative effect of starter cultures.

  1. linens, when growing in a cheese-containing medium, produces an antimicrobial agent with a broad spectrum of activity, being active against yeasts and molds, Clostridium botulinum, Staphylococcus aureus, Salmonella spp., Bacillus cereus, and many yeasts and molds (Grecz, 1964). Volatile sulfur compounds are at least partially responsible for this activity (Beattie and Torrey, 1984).
  2. Bacteriocins

Bacteriocins are proteins or polypeptides with potent bactericidal activity. They typically have a narrow spectrum of activity against species closely related to the producing organism. Their production and immunity to their action is plasmid encoded (with some exceptions). Variation in the presence of immunity genes may be responsible for the large variation in bacteriocin sensitivity of lactic acid bacteria (Eijsink et al., 1998). Some bacteriocins are especially interesting, because their broad spectrum of activity may make them useful for inhibiting specific pathogenic or spoilage microorganisms. Activity of some bacteriocins against Listeria spp. is presented in Table 6. Ent. faecium suitable for use as a starter culture may produce enterocin B, which is active against nisin-resistant mutans of L. monocytogenes (Schillinger et al., 1998). Production of bacteriocins by lactic acid bacteria is common, as shown by data in Table 7. However, strains of lactic acid bacteria selected for use in multiple-strain cultures generally do not produce bacteriocins so they do not dominate the mixture. Bacteriocins produced by lactic starter cultures can be divided into three biochemical groups (Barefoot and Nettles, 1993): lanthionine-containing peptides such as nisin and lacticin 481; small non-lanthionine-containing proteins or peptides such as lacticin F, lacta-cin B, and lactococcin A; and large heat-labile proteins such as helveticin and caseicin 80.

Table 6 Activity of Some Bacteriocins Against Listeria Species

Producer organism

Bacteriocin

Inhibition

Lactobacillus

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