Fermentation of Ultrafiltered Skim Milk Retentates with Mesophilic Lactic Cheese Starters

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Abstract
Acid production and its relation to pH changes by commercial, direct-set frozen concentrated lactic starters in skim milk and 2:1 skim milk retentates were studied. Retentates resisted pH change < pH 5.2 despite the production of large amounts of lactic acid by starter bacteria. Control skim milk required 6 h at 32.degree. C to attain pH 4.6, but skim milk retentates incubated similarly could not be fermented to this pH even after 8.5 h. Doubling the starter inoculum in the retentate led to pH 4.6 in 7.5 h. Direct-set starter DS1, with more bacteria numbers than direct-set starter DS2, fermented skim milk and 2:1 skim milk retentate more rapidly.