Seven mesophilic starter cultures were grown separately in sterilized skim milk for 8 h at 25°C. At the end of the incubation period, samples were analyzed for agglutination behavior by using microscopic examination. Samples were also assayed for proteolytic activity by the spectrophotometric o-phthaldialdehyde (OPA) and trinitrobenzenesulfonic acid (TNBS) methods. This experiment was replicated three times in a randomized block design. Microscopic examination showed that some cultures existed as single cells, diplococci, or short chains; other cultures tended to form long chains. Short-chain-forming cultures had high proteolytic activities, whereas long-chain-forming cultures had low proteolytic activities as measured by the TNBS and OPA methods. Proteolysis measured by both methods ranked starter cultures in the same order (R2 = 0.82). Low detectability values (obtained by dividing the standard error of the sample estimate for each method by its slope ratio) for OPA and TNBS methods indicated that the two assays do not differ significantly (P > 0.05).

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