Process cheese sauce is typically prepared by combining one or more natural cheeses
of the same type or different varieties, mixing with emulsifying salts and other dairy
ingredients, and thermal processing to temperatures above 90°C. The processing renders
the product free of all viable organisms and is considered commercially sterile. Despite
the safety of process cheese sauce during processing, its low acid and high moisture
environment are conducive to support the growth of bacteria contaminated by the
consumer. Spore-forming bacteria such as Bacillus cereus and Clostridium perfringens
are particularly problematic because their spores state can survive temperatures above the
required reheating temperature of 60°C and the sub-lethal heating could induce
germination in the cheese sauce medium if the product is mishandled.
The goal of this project was to assess the survival and germination potential of sporeforming
bacteria in process cheese sauce through typical temperature-abuse schemes that
could happen in a food service setting. Commercial cheese sauce was inoculated with
cocktails of B. cereus and C. perfringens at 104 CFU/g and held at temperatures of 4, 12,
23, 37, and 45°C for short-term and long-term storage. Cyclic temperature scenarios
typical of commercial buffet restaurants were also evaluated, as well as the use of a
common antimicrobial nisin for B. cereus. Three strains of each bacterium were
combined and tested in each sample and bacterial counts were determined by direct
plating on PEMB agar for B. cereus and anaerobic blood agar for C. perfringens.
Bacillus cereus germinated well in the cheese sauce medium. At 37°C the vegetative
cell population reached 8 log CFU/g after 24 hours, but took 48 hours to reach the same level when the antimicrobial nisin was added. At temperatures of 23°C and 45°C,
vegetative cell populations reached 8 log CFU/g within 48 hours and at 12°C reached 7
log CFU/g after 1 month. When incubated at 4°C no growth was observed even after 2
months. Almost no increase in B. cereus spore count was observed at any temperature,
except at 23°C.
The B. cereus inoculated cheese sauce was subjected to cyclic temperature abuse
schemes that simulated a buffet restaurant schedule. One scenario depicted samples that
were not properly heated (below 60°C) on the buffet line. Short incubation times at
abusive temperatures did not influence germination, but after 24 hours of fluctuating
temperatures vegetative cells began to grow. However, when the contaminated sample
was reheated to temperatures above 60°C vegetative cells actually began to decrease.
The cheese sauce matrix did not support germination and growth of C. perfringens at
any temperature. Even during a worst case scenario where the cheese sauce was allowed
to cool from 75°C to 4°C in a 9 hour period, vegetative cell and spore counts declined
below a detectable level.
In almost all experiments the vegetative cell population level of bacteria was either
maintained or increased in the cheese sauce, while the spore population was maintained.
If the process cheese sauce were contaminated by the consumer the potential for an
outbreak may exist if left at temperatures favorable for bacterial growth.