Simplified Pairing of Lactic Streptococcal Strains for Cheese Manufacture

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Abstract
A selected group of 43 strains from a culture bank of over 600 lactic culture isolates was tested for temperature sensitivity and phage typed with 28 phage isolates. Data were analyzed to maximize differences between paired strains. Only the best pairs, as determined by computer, were propagated in a buffered growth medium after which they were inoculated at 2% into sterilized nonfat dry milk containing 0.015% brom cresol purple pH indicator. Inoculated milk was dispersed into a microtiter plate. Filtered homologous phage stock was inoculated into these milk samples to produce serial dilutions from 100 to 10-11. The inoculated plate was sealed and incubated through a temperature cycle simulating cheese making. The pairs least affected by phage attack were evident through color differences. Measurement of pH in each well with a pH meter further refined selection. Several pairs selected in this manner were used successfully in rotation programs at 2 cooperating cheese plants.

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