Application of Streptococcus thermophilus DPC1842 as an adjunct to counteract bacteriophage disruption in a predominantly lactococcal Cheddar cheese starter: use in bulk starter culture systems

摘要

A significant amount of Cheddar cheese manufactured world-wide relies on bulk starter cultures instead of direct vat set (DVS) cultures. While the inclusion of S. thermophilus is sometimes used to counteract failure due to lactococcal phage in the latter system, it is considered difficult to implement in bulk starter systems and is normally avoided. This stems from the problem in controlling the ratio of S. thermophilus to lactococci during the bulk starter preparation such that suitable acidification rates can be achieved. The current study demonstrates how S. thermophilus numbers can be controlled during growth in the bulk starter medium prior to inoculation of a culture, based on three lactococcal strains and S. thermophilus DPC1842, into the cheese vat. The concentration of inorganic phosphate necessary to inhibit the growth of strain DPC1842 in a whey-based bulk starter medium was found to be 0.18 mol.L-1. Since higher levels of phosphate exist in different commercial bulk starter media used for mesophilic cultures, a number of these media can be used for propagation of this starter blend without domination of strain DPC1842 over its lactococcal counterparts. Strain DPC1842 is highly phage resistant and is particularly acid-fast in the cheese milk and reduces the pH efficiently at very low inocula. Cheddar cheese was manufactured in a commercial plant with this system and the resulting cheese had good flavour characteristics. This study demonstrates the effectiveness of the S. thermophilus DPC1842 component in rescuing the fermentation in the event of severe lactococcal phage attack.