STOCKPOTS AND SLOW COOKERS
How long before food starts to go bad while
cooking in a slow cooker or stock pot?
"Go bad" can just mean a change that you
were not expecting, not necessarily
decomposition. Let's take a microbiological
perspective. We can forget ALL food-borne
viral and parasitic agents, as none of them
are heat resistant. The non-heat resistant
bacteria will have been killed long before
72C (161F) is reached, especially for these
extended periods of time, meaning we can
expect no trouble from any of the Staph or
Strep groups, and Listeria, Yersinia,
Campylobacter, and the celebrated E. coli
suspects will all have disappeared from the world of the simmering stock pot. (For anyone who recalls S. aureus surviving heat, it is only the toxin, not the bacterium, that resists heat, and provided there is no bacterium, there is no toxin.)
This leaves the thermoduric and thermophilic spore-formers, including the aerobic Bacillus group (various types of B. cereus, B. subtilis, B. mesentericus), the anaerobic Clostridium perfringens, and its infamous distant relative, C. botulinum.
Heat-resistant spores are indeed able to survive varying degrees of heat, and will return to a vegetative (actively ‘living’) state and start multiplying once the temperatures return to a growth range. Density of cells would need to be close to 10^5 / ml, but if the pot has been retained at temperatures hostile to bacterial growth, that massive growth never happens, even if the pot is taken off the heat for a few hours overnight. Remember the heat profile is not a ‘stepwise’-function, changing suddenly from “too high” to “maximum growth” the moment the temperature reaches a critical level for that species. Instead, it’s a slow, steady gradient, over many hours, gradually reaching rapid growth around 37 C, and in many cases only after a delaying "lag-phase" has passed (no growth). The growth again slows as the pot cools further. In reality, if the lid is still on when the long-simmering pot is removed from the heat at the end of the day, and nothing new is added, there will be zero or negligible growth detectable by the time the heat is returned in the morning.
Having said that, longer periods without heat OR refrigeration could indeed be dangerous, but let’s take the most extreme case. Imagine that someone has forgotten to return the pot to boiling point for a couple of days. Some spores have survived, perhaps C. perfringens, or even of a proteolytic C. botulinum, type A, and the ambient kitchen temperature - say 20 C (68 F) - has allowed germination of those surviving spores, and production of neurotoxin BoNT/A. While toxin production can be measured at cell counts of 100/ml, multiplication of those cells would need to proceed to that level. But any toxin produced would be quickly denatured/destroyed when the pot was again brought to at least 90 C (194 F) for 10 minutes. Cooks from the older traditions always made sure the stockpot was brought to a boil every morning and then kept at a simmer.
So, would this be a valid and condoned practice in a modern large commercial operation? Most food safety authorities would not allow, interpreting prevailing legislation to mean that the pot would have to be rapidly cooled by a combination of stirring, and/or divided into smaller containers, and refrigerated. The reason is the need for more than one safety barrier in place when the risk is extended to very large numbers of patrons. Holidays, days off, substitute personnel, shift-work, and the complexity of large operations can add uncertainty and lack of continuity to what has to be careful nurturing of the stockpot. Hence it doubtless survives in small, proudly-defended, often private, domains, but is rarely seen in today’s mass-catering operations, where gravy/jus/soups/broths, etc. begin as a convenient ‘readymixed’ package (but sadly without that essence of the stock pot!).
