The objective of starter culture propagation is to attain a preparation of active cells at high density so that fermentation is initiated as rapidly as possible. Providing adequate nutrients and controlling pH and incubation temperature are necessary to achieve this objective. Even though milk and whey are traditional growth media for lactic cultures, they provide neither optimal nutrition nor needed pH control. Consequently, various media formulations and culture growth systems have been devised to improve on traditional culture propagation.
Lactic acid bacteria cannot synthesize various vitamins and amino acids. Lactococci require niacin, pantothenic acid, pyridoxine, and biotin for growth. S. ther-mophilus requires these vitamins plus nitroflavin, whereas lactobacilli require pantothenic acid, niacin, and nitroflavin, with some species also requiring cobala-min (Mayra-Makinen and Bigret, 1993). In regard to amino acids, lactococci and S. thermophilus cannot synthesize the needed branched chain amino acids (isoleucine, leucine, valine) or histidine; some strains also require arginine and methionine (Monnet et al., 1996). Lactobacilli require these amino acids in addi tion to several others. Leuconostoc spp. require valine and glutamate, and some species may have additional requirements. The presence of amino acids other than those required often stimulates growth. Although milk contains many of the essential amino acids for starter culture microorganisms, these are not present in sufficient quantity to sustain maximal growth rates (Monnet et al., 1996). Lactic acid bacteria with greater proteolytic ability have less need for amino acid supplementation of milk-based growth media.
Ingredients commonly used to formulate starter culture media have been described by Whitehead et al. (1993) and are presented in Table 5. Lactose is always used as the major carbohydrate, although low concentrations of maltose, sucrose, or glucose are sometimes added to stimulate growth (Sandine, 1996). Yeast extract is a source of nitrogen as well as a supplier of vitamins, minerals, and other growth stimulants. Casein hydrolysates are added to provide readily available amino acids. Also, addition of whey protein concentrate to whey or UF (ultra filtered) whey permeate broth stimulates growth of lactic acid bacteria (Bury et al., 1998). Heat-stable a-nucleotide, nonprotein nitrogen, or some peptidases could be responsible for this stimulatory effect (Bury et al., 1998). Corn steep liquor, although a good source of vitamins, is not often used, because its supply is limited (Sandine, 1996). Sandine (1996) questioned the need for added antioxi-dants in media formulations, because acceptable growth can often be achieved in their absence. Neutralizers, such a ammonium or potassium hydroxide, help prevent excessive acidity. Phosphates are commonly used in culture media, because they act both as acid-neutralizing and phage-inhibitory agents.
3. Phage-Inhibitory Media
One of the first improvements in whey- and milk-based culture media was development of phage-inhibitory media (see Sec. V).
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