Resistance of Certain Bacteria to Cottage Cheese Cooking Procedures

tIeat resistance in Cottag'e cheese whey at pH 4.6 was determined for selected strains of Pseudomonas fragi, Escherichia coli, Streptococcus cremoris, and Streptococcus lactis.Strains were selected for study on the basis of high heat resistance.Survivor curves in whey at various temperatures and thermal resistance curves were calculated.The time-temperature treatments used for cooking Cottage cheese curd in seven representative California plants and one hypothetical procedure were evaluated as to lethality for each of the selected cultures.Cooking procedures that did not have a maximum temperature of at least 130 ° F., and a holding time of at least 18 rain.at 130 ° F., proved unsatisfactory.Three of the most important defects of Cottage cheese are spoilage fronl psychrophilic bacteria, coliform bacteria, and sourness (6-8).Numerous reports deal with preventing contamination with psychrophiles and coliforms, and preventing microorganisms from producing defects.The development of sourness is related to lactic streptococci used in manufacturing the curd.Sourness may follow storage of cheese at 50 ° F. and above, inadequate cooking, or inadequate washing of the curd.Cottage cheese is cooked to expedite expulsion of whey from the cubes of curd.Heating also improves keeping quality.The various cooking procedures used, developed from practical experience over many years, differ considerably in total heat.Data on the resistance of certain important bacteria to the heat applied are inadequate for use as a guide in developing proper procedures.Adequate data are particularly needed now, in view of current interest in mechanization of curd production, recent attention to decreasing the time required in making curd by adding starter stimulants that expedite the expulsion of whey (II), and the constant demand for Cottage cheese with better keeping quality.An investigation was conducted to determine the heat resistance of selected psychrophiles, eoliforms, and lactic streptococci, and to use the data in evaluating representative cooking procedures in present use. ~[ETHODS Culture selection.Cultures were selected on the basis of heat-resistance determinations in pasteurized whey at pH 4.6.The heat-treatment procedure used is described below.Coliforms and lactic streptococci were tested at ]25 ° F. ; psychrophiles were tested at 115 ° F. The selection tests involved 21 single-strain cultures of Streptococcus cremoris, three of Streptococcus lactis, six of Pseudomonas fragi, two of Pseudomonas viscosa, three of Escherichia coli, three of