Measurement Of Starter Activity

The term activity refers to the ability of starter cultures to produce desirable changes in fermented dairy products. Activity is a consequence of many factors, some of which are difficult to quantify, such as physiological state of cultures, growth conditions, harvesting, and packaging and storage conditions (Spinnler and Corrieu, 1989). Usually activity measurements are confined to the ability of starter cultures to acidify milk.

Most activity tests are based on rapid quantification of acid production for the purpose of strain selection, comparison of different combinations of defined starters, determining the best harvesting time, or determining culture stability during storage. Ideally, before activity measurement, cultures should be subcul-tured twice, cultured overnight in the appropriate broth medium at optimum growth temperature, and centrifuged at 20000 X g at 4°C for 5 min. The pellet is then washed at 4°C with 50 mM potassium phosphate buffer, pH 6.7, resus-pended in the same buffer at 5 mM to minimize buffer capacity, and adjusted to A650 for use as an inoculum (Demirci and Hemme, 1995). The classic way to determine starter activity is by measuring the pH of the culture at different time intervals. Maximum acidification rate (Vm) (Spinnler and Corrieu, 1989), the capacity of a culture to respond to a new environment (Barreto et al. 1991), and the biomass measurement (Olivares et al., 1993) may all be useful determinations. Activity can be measured by means other than pH, including conductance and impedance (Lanzanova et al., 1993; Tsai and Luedecke, 1989). These measurements estimate the activity of stored cultures within less than an hour. Activities other than acid production such as proteolysis (Dermirci and Hemme, 1995), lipolysis (Kenneally et al., 1998), and P-galactosidase production (Ord'Zez and Jeon, 1995) can also be determined by other means.

IX. STARTER CULTURES INTERACTIONS

Mixed starter cultures may be composed of various genera, species, and strains of lactic acid bacteria which together make up a dynamic, complex culture. Their strain components will differ in growth rate, acid production, aroma production, proteolytic activity, bacteriocin production and sensitivity, and phagic resistance. Milk composition can affect strain dominance; for example, the very low concentration of Mn2+ in winter milk leads to poor growth and low numbers of Leuco-nostoc in a mixed starter (DeMan and Galesloot, 1962).

Different types of interactions can occur in strain mixtures and affect culture performance (Hugenholtz, 1986) (Table 8). This could lead to slow acidification and modification of texture and organoleptic properties of fermented milk products. Sometimes more than one interaction can occur among particular strains. For example, the interaction between propionibacteria and Lb. plantarum in Swiss cheese changes from commensalism to mutalism when lactic acid is accumulated and Lactobacillus starts to benefit from its removal by propionibac-teria.

The nature of the interaction among mixed cultures is strain dependent. Day-to-day transfer of mixed cultures should be employed with caution unless they have previously demonstrated maintenance of the original proportions through several transfers.

X. ADJUNCT CULTURES

Adjunct or secondary cultures are those added to cheese for purposes other than acid production. Such cultures are used to intensify and modify cheese flavor and to accelerate flavor development. Because adjunct cultures grow during cheese ripening rather than during curd manufacture, they are unlikely to support phage production. Examples of adjunct cultures are Pr. freudenreichii subsp. shermanii in Swiss cheese, B. linens in surface smear-ripened cheese, and P. camemberti in surface mold-ripened cheeses. In Cheddar cheese making, adjunct cultures include lactose-negative Lc. lactis subsp. and attenuated (heat-shocked, freeze-shocked, or solvent-treated) cultures (Fox et al. 1998). Some adjunct cultures such as thermophilic Lactobacillus spp. that do not grow to a significant extent in Cheddar cheese can serve as a source of ripening enzymes. In addition, replacing part of a bitter-producing starter with thermophilic Lactobacillus cultures avoids bitterness development (Fox et al., 1998). However, using a high level

Table 8 Possible Interactions Among Dairy Starter Culture Strains

Type of interaction Definition Examples

Table 8 Possible Interactions Among Dairy Starter Culture Strains

Type of interaction Definition Examples

Competition

Populations of two species are

1. In Cheddar cheese (high

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