Long Term Use of a Cheddar Starter and Development of Phages with Homology to its Bacteria

Abstract

A mixed, homofermentative, mesophilic starter was used for more than 11 years as the only starter for production of Cheddar cheese at a Danish cheese factory. The activity of the starter at the factory was carefully recorded, and whey samples were regularly tested for bacteriophages. During the years bacteriophages, homologous to an increasing number of the strains of the starter culture, gradually evolved. After 4 years bacteriophages, homologous to more than 50%—and after 6 years to more than 90%—of 62 bacterial isolates from the starter, had appeared. For most of the strains the virulence of their phages was low at first and thereafter increased over several years. It was, however, only after more than 11 years that the cheese factory first began to observe cases of reduced activity of the bulk starter; presumably because the virulence of the bacteriophages had developed to such a degree that very low levels of contamination of the milk for bulk starter could impair the subsequent activity of the bulk starter in the curd.