TIMES AND TEMPERATURES
Some foodborne illnesses (e.g., Shigella, some serotypes of Salmonella, E. coli) require only a few bacterial cells, so simple 'presence' of cells is a danger. Large numbers are not important
For others, very large numbers are needed to cause illness (e.g., Bacillus cereus, Staphylococcus, Clostridium perfringens). To reach these huge numbers (e.g., 100,000 per gram of food), the bacteria have to multiply rapidly in suitable foods, at the correct temperatures. The growth is exponential:
100-> 200-> 400-> 800.. etc. These 'generation times' are critical, and depend upon temperature.
When does growth begin? It certainly does not suddenly begin at 5C (40F) and suddenly stop at 60C (140F)! This is the false impression given by repeated warnings about the danger zone. For most species, there is no growth at 0C (-18F), and then very slow growth can start as the temperature increases. For most (not all) human pathogens, fastest optimal growth rate is reached at human body temperature (37C 98F). And in between we have a gradual bell-shaped curve. As you can see from the chart (at right) showing growth of Salmonella in Chicken meat, 40F (4.4C) is where the bacteria haven’t begun to grow at all. At 5 C (50 F) the bacteria have just begun to wake up but are barely multiplying.
To get a better idea of what this means in terms of numbers of bacteria, the time needed for one ‘doubling’ is the mean generation time (MGT). From the chart at right, the doubling time (at neutral ph 6.5) at 10C (50F) is about 10 hours! You’d need to wait 100 hours (4 days) to reach million bacterial cells/g.
(The vertical axis is the mean (log) of the colony-forming units per hr. )
At 30 C (86F) the MGT is only 36 minutes, and you could reach a million/g in about 5 hrs - ONCE THE BACTERIA START TO MULTIPLY. (They actually first enter a “lag phase” (meaning no growth) for the first 4 hrs or so before they start to multiply.
.................................So PLEASE don’t waste good food!
Q: We left out cooked broccoli, carrots, potatoes - unrefrigerated - overnight. Are they safe?
Q: A local supermarket offers meat pies from a warming cabinet. Is this dangerous?
Q: I left the door slightly open all night and the temperature in the refrigerator is 48 degrees. Should I worry about the food?
A: If it were 48 degree Celsius, you would have VERY warm food, so let's assume it's 48 F (9C). At that temperature, most bacteria have not begun to multiply and those that have (e.g., Yersinia enterocolitica), are multiplying extremely slowly. See the chart above.
Q: We left out cooked broccoli, carrots, potatoes - unrefrigerated - overnight. Are they safe?
A: None of these are displayed for sale refrigerated! So why imagine they suddenly need refrigeration in your kitchen? These are foods not normally associated with foodborne illness unless you were to put them in jars with only boiling water-bath treatment, and leave them on the shelf for a few weeks or months; then, we could have a problem with botulism, but in the short term, in a kitchen setting, they will simply dry up or become moldy after about a week.
Q: A local supermarket offers meat pies from a warming cabinet. Is this dangerous?
A: Yes this is a potential hazard. A meat pie is particularly vulnerable because it has abundant protein, neutral acidity, high water activity, and no natural bacterial inhibitors. Several pathogenic species could flourish here, including C. perfringens, B. cereus (diarrhoeic), S. aureus, Salmonella, Listeria and many others. The temperature is critical. If the internal temperature of the pies is kept at 60C (140F) or more, then they're fine. If obviously lower than this, contact the local health unit and report.
Q: The power went off last night and the meat in the freezer started to thaw. Should we throw it out? A website says that if ice crystals remain in the meat it is safe. True?
A: This is another example where a correct decision for a commercial food company, (retail or wholesale) may not be appropriate for the domestic kitchen. Food being transported across the country may have had a failure of the freezer unit for an unknown number of days when it arrives. Who can tell? We would need to know how long it has been thawed; a day or a week?
But you know the chain of events for your unplugged freezer. The meat has begun to thaw overnight, but the meat is still cool. If it looks normal (no off-colours), smells good (not sour), and there's no slimy surface feel, then it will be fine to re-freeze and use later or to cook and use today. Even if it has completely thawed, there's nothing on raw meat that can cause hidden health problems. You can even re-freeze it several times if you wish. There will be a slight quality loss (loss of juices, appearance, texture changes), but absolutely nothing of a health or safety concern. Refreezing an expensive steak might be unwise because of appearance, but poultry, burgers, ground meats, chops, sausages, etc., go ahead. No need to waste perfectly good food. Finally, one of the real hazards is "drippage" of meat juices onto ready to eat foods or desserts. That can lead to serious contamination, and is another reason (aside from 'freezer burn'), to wrap your frozen foods very thoroughly.