Lactic Culture Activity Tests Using pH and Impedance Instrumentation

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Abstract
A culture activity test was based on pH changes after .25 and 5% inoculum volumes were added to sterile reconstituted nonfat dry milk and incubated 5 h at 37.degree. C. The test predicted inoculum concentrations and time required to make Cheddar cheese. It could be used with culture concentrates such as those produced with a pH-controlled culture medium. The test successfully evaluated both proteinase-positive and proteinase-negative lactic cultures. Changes in conductance and impedance detection times could both be used in a lactic culture activity test. Two milliliters of media, inoculated at .25 to 5%, were transferred into Bactometer wells and incubated at 30.degree. C in the processor incubator. The best response for proteinase-positive and proteinase-negative cultures, and strains with slow or fast rates of acid production, resulted when yeast extract was added to the nonfat dry milk substrate. Low correlations between change in pH and impedance detection times were due to nonlinearity associated with changes in pH at high inoculum volumes. Adequate activity estimates were available in 1 h using conductance. Both methods can be used over a wide range of cell concentration, however, data with the imdedance method were more linear at higher cell mass.