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Phosphate Heat Treatment of Milk to Prevent Bacteriophage Proliferation in Lactic Cultures

Dairy Products Laboratory, Eastern Utilization Research and Development Division, USDA Washington, D. C.

ABSTRACT

The effects of various prosphate salts on bacteriophage proliferation in milk cultures and the binding of calcium in milk were studied, with the aim of developing a practical and reliable method for preventing bacteriophage development in lactic starter cultures. The kind and concentration of phosphate, pH, and heat treatment had a marked effect on phage inhibition, and the free calcium content of milk. Of the group of phosphate salts tested individually in milk, orthophosphates showed the greatest degree of bacteriophage inhibition. Most of the phage types tested were suppressed by 2% orthophosphate (salt), but the most resistant types required 3%. Heating the milk after adding the phosphate was essential to sufficiently bind most of the free calcium. Usually, the free calcium content of the treated milk ranged from 10 to 30 p.p.m. The phosphated milk became more effective as the pH of the phosphate buffer and milk was increased from pH 6.4 to 7.0. The best combination for phage inhibition, minimum milk precipitation, and economy was obtained when milk was heated with 1.7% orthophosphate salt, pH 6.6, followed by the addition of 0.3% pyrophosphate. Thirteen different lactic strains grossly contaminated with their respective phages were freed of phage within three to four subcultures in phosphated milk. In most instances, the activity of the cultures in phosphated milk was as great as or greater than in the controls.