Comparison of growth and survival of single strains of Lactococcus lactis and Lactococcus cremoris during Cheddar cheese manufacture.

Traditionally, starter cultures for Cheddar cheese are combinations of Lactococcus lactis and Lactococcus cremoris. Our goal was to compare growth and survival of individual strains during cheesemaking, and after salting and pressing. Cultures used were 2 strains of L. lactis (SSM 7605, SSM 7436) and 2 strains of L. cremoris (SSM 7136, SSM 7661). A standardized Cheddar cheese make procedure was used that included a 38°C cook temperature and salting levels of 2.0, 2.4, 2.8, 3.2, and 3.6% from which were selected cheeses with salt-in-moisture levels of 3.5, 4.5, and 5.5%. Vats of cheese were made using each strain on its own as biological duplicates on different days. Starter culture numbers were enumerated by plate counting during cheesemaking and after 6 d storage at 6°C. Flow cytometry with fluorescent staining by SYBR Green and propidium iodide was used to determine the number of live and dead cells in cheese at the different salt levels. Differences in cheese make times were strain dependent rather than species dependent. Even with correction for average culture chain length, cheeses made using L. lactis strains contained ∼4 times (∼0.6 log) more bacterial cells than those made using L. cremoris strains. Growth of the strains used in this study was not influenced by the amount of salt added to the curd. The higher pH of cheeses with higher salting levels was attributed to those cheeses having a lower moisture content. Based on flow cytometry, ∼5% of the total starter culture cells in the cheese were dead after 6 d of storage. Another 3 to 19% of the cells were designated as being live, but semipermeable, with L. cremoris strains having the higher number of semipermeable cells.

Galactose-positive adjunct cultures prevent gas formation by Paucilactobacillus wasatchensis WDC04 in a model gas production test.

Gas production by obligatory heterofermentative lactic acid bacteria such as Paucilactobacillus wasatchensis is a sporadic problem in Cheddar cheese and results in undesired slits and cracks in the cheese. Growth of Pa. wasatchensis is not rapid, which makes investigations of gas production difficult to consistently execute. A primary objective of this study was to develop a model gas production test that could be used to investigate the effect of galactose and ribose utilization on gas production by Pa. wasatchensis and determine whether galactose-fermenting adjunct cultures could prevent gas formation. Paucilactobacillus wasatchensis WDC04 was inoculated at 101 to 106 cfu/mL into carbohydrate-restricted MRS broth containing different ribose and galactose levels and incubated for up to 21 d at 23°C. Gas production in the broth was detected using a Durham tube inverted on a 6-cm-long capillary tube; cells were enumerated at 4, 8, and 12 d; and residual galactose was also measured. Gas production was sporadic except for when 105 cfu/mL of Pa. wasatchensis WDC04 was inoculated into broth containing 0.3% ribose and 0.7% galactose. In those tubes, gas production was consistently observed after 8-d incubation, by which time galactose levels had decreased to 0.15%. Co-inoculation of Pa. wasatchensis WDC04 with as few as 103 cfu/mL of a lactose-negative galactose-positive adjunct culture (Pediococcus acidilactici 23F, Lacticaseibacillus paracasei UW4, or Lactobacillus helveticus 7995) resulted in galactose depletion by d 4 and no observable gas production by d 12. With less galactose available to the slower-growing Pa. wasatchensis WDC04, its growth was limited to 108 cfu/mL when any of the adjunct cultures was co-inoculated, compared with 109 cfu/mL when grown on its own. We concluded that galactose-fermenting adjunct cultures have potential for preventing unwanted gas production in cheese by competition for resources and especially by removing the 6-carbon galactose before it can be utilized for energy by an obligatory heterofermentative lactobacilli such as Pa. wasatchensis and produce carbon dioxide.