Milk samples from two cheese plants with overlapping milk supplies were collected monthly for one year in an attempt to measure seasonal variation in the ability of milk and whey to support lactic culture growth . Treatments to control and minimize variability of milk or whey were evaluated to optimize stability in starter culture performance. Raw milk samples were tested for somatic cell counts, activity tests (modified Horrall- Elliker), acid degree values, and total plate counts. Activity (modified Horraii-EIIiker) and inhibitory tests were also performed on pasteurized, pasteurized-vacuumized and high heat milk treatments. Rennet whey (heated and unheated) was collected from raw and pasteurized-vacuumized milk and tested for lactic culture performance by monitoring growth under pH control for 16 h and measuring milli-equivalents of neutralizer (NaOH) added. Lactic culture performance and stability in raw milk was poor in all seasons. Culture performance in high heat milk was poor, but demonstrated good repeatability. Pasteurized milk supported good lactic culture performance and stability. Pasteurized-vacuumized milk provided excellent lactic culture performance and stability throughout the year. Culture performance during December through March demonstrated the greatest variation. The cultures performed more uniformly during April through August. September was a transition month. Cultures demonstrated uniformity and optimum culture activity during October and November.

A buffered growth medium was developed that sustained a significantly greater concentration of various strains of Streptococcus lactis and Streptococcus cremoris than did milk. The active buffering ingredients of this medium were magnesium hydroxide, ammonium and sodium salts of phosphate and citrate. This medium was entitled Lactic Culture Buffered Growth medium (LABGRO) and possessed about 8 times the buffering strength between pH 6.6 and 5.1. Significant differences in growth and acid production rates were noted among Prt+ and Prt- lactic strains evaluated. Also noted were significant differences in cellular acid production rates. The two Prt- strains having the slowest growth rate in milk were selected as test strains to evaluate stimulants. Three strains of Kluyveromyces fragilis were aerobically propagated in whey medium and afterwards subjected to autolysis conditions. Whey protein and casein were added to some yeast samples prior to initiation of autolysis. To some of these latter samples, pepsin was added. After autolysis the yeast and yeast-protein samples were dried and their stimulatory properties for lactic culture propagation were evaluated with a test strain and LABGRO medium. No product was found to be as stimulatory as a control commercial yeast autolysate. Yeast-protein samples with added pepsin were markedly more stimulatory than the other samples. The whey-derived yeast extract significantly improved the lactic culture growth stimulating properties of protein hydrolysates.

A whey product has been formulated for use as a lactic starter medium. Phosphate-treated whey medium (PWM) has been shown to support growth of lactic cultures and prevent phage proliferation. Comparisons were made of a commercial phage inhibitory medium (PIM), reconstituted non-fat dry milk (NDM) and PWM. PWM inhibited all phages tested and stimulated starter growth. PWM was not as stimulating as PIM but was better than NDM. Good Cheddar cheese has been made using PWM. PWM is more economical than PIM but NDM is the most economical one if cheese yield is considered.

A Spiral Plater and a Microtiter system were used to isolate and evaluate cultures for a paired strain culture program. Bacteriophage and temperature sensitivity data of 43 Streptococcus cremoris strains were introduced into a computer cluster program to pair the least similar strains. Selected pairs were challenged with phage. Resistant mutants were developed. Characteristics of proteinase positive and proteinase negative variants were examined. Proteinase positive isolates produced more changes in pH, cell mass and more generations in milk than their counter-parts. Paired proteinase negative cultures produced more change in pH and cell mass and more generations in milk than single strains. Whey based medium under pH control was superior to commercial internal pH control medium for proteinase negative culture propagation. Proteinase negative isolates achieved 90% of the cell mass obtained by their counterparts in nonfat dry milk-yeast medium. Proteinase negative starter culture endured significantly higher phage titers than proteinase positive cells. Proteinase negative variants sustained activity comparable with phage-free controls when challenged for seven cycles with high phage titers. Proteinase positive cells had impaired activity after the second cycle. Pairing of proteinase positive strains was advantageous for phage protection. Erythromycin, streptomycin and penicillin adversely affected the activity of both cell types, yet proteinase positive cells were significantly more inhibited. Pairing neither variant enhanced activity. Cheddar cheese was exclusively manufactured with 2% inoculum of proteinase negative cultures compared to 1.5% usage of the proteinase positive paired strains. Cheese quality and cheese making times were normal. Over 4200 consecutive vats of Cheddar cheese were made in 1982 employing one pair of proteinase positive culture. Acid control and cheese quality were improved. The cheese making times were more uniform. Smaller inocula volumes could successfully be used for bulk starter in cheese plants utilizing pH controlled starter propagation. A needle/syringe system for inoculating starter tanks provided better protection against contamination during inoculation.

Seven strains of Streptococcus cremoris and Streptococcus lactis were tested for temperature sensitivity on a temperature gradient bar. A temperature of 37 C was the optimum for differentiating between temperature-sensitive and insensitive strains. Temperature-insensitive strains produced cell masses with absorbance values of 0.1 or higher and activity levels of 1.0 (expressed as a decrease in pH) or above at 37 C. Temperature-sensitive strains failed to produce these cell masses and activity levels, Strain selection and resistant- mutant isolation made it possible to identify ten strains of Streptococci sufficiently free of phage sensitivity for use in a cheese plant rotation program. These resistant mutants failed after being successfully used in mixed cultures for a short period of time, They became either slow acid producers or were again attacked by new bacteriophages. More work is needed to successfully isolate phage-resistant mutants suitable for cheese starters.