Sometimes, we have to go back to basics to get things right. This is true with food safety as well. Although, it is not the most exciting subject, we must always strive to keep food safe from contamination, whether it be biological, chemical or physical. In regards to biological contamination, the best thing we can do is control time and temperature which means basically, keeping food out of room temperature as much as possible to avoid happy pathogens and growth of bacteria.
As per the Food and Drug Administration, the Temperature Danger Zone (TDZ) is the temperature range between 41°F and 135°. This translates to our having to hold cold food to an internal temperature of 41° or below and hot food to an internal temperature of 135° or above. In what I call the “magic four-hour rule,” the FDA states that food must be discarded at an accumulation of four hours. This means at a minimum limit, when holding food for service, a temperature check must be held every four hours and should the food temperature measure above 41°F or below 135°F at four hours, your only corrective action is to discard the food. Four hours is really not a long time, when you consider all the possible steps in dealing with food throughout its flow from receiving, storage, preparation, cooking, holding, cooling, storage, reheating and serving.
The purpose of keeping food out of the TDZ overall is to avoid replication/duplication of bacteria. Generally, one bacteria is capable of replication every 20 minutes. To put this in perspective, one bacteria is capable of becoming a billion at 10 hours when held at room temperature (TDZ)! Thus – the four hour cut off. Plain and simple: There is nothing we can do to fix the food to make it safe at four hours. The only corrective action available is to discard the food at the accumulation of four hours in the TDZ. In contrast (this still always amazes me), food prepared in-house and kept with temperature control under refrigeration at 41°F or lower can be held with a seven day expiration date. Please note that if food is made on the first of the month, this means that the food must be discarded on the seventh of the month. The date of preparation counts as the first day.
According to the FDA, the temperature range between 70°F-120°F is considered to be the most dangerous part of the TDZ. When cooling or reheating food, we are causing the food to go up or down through the TDZ more than once. Passing up or down again through the most dangerous part of the TDZ within two hours is very important when reheating food for the purpose of hot holding, or when cooling food.
When cooling food, it is important to get through this range as quickly as possible because the exponential velocity of replication of bacteria is so much faster, i.e. hundreds of times in an hour versus every 20 minutes. This explains why when cooling, we must get from 135°F-70°F within two hours. The FDA cooling rule is a two-stage process. Cool food from 135°F to 70° or lower within two hours and then continue cooling to 41°F or lower in the next four hours. This means after cooking food, we can’t put hot food directly in the refrigerator. We can refrigerate it when it is at 70°F or lower. Again, because we have to pass the food through the most dangerous part of the TDZ, we are limited to only two hours. That is not a lot of time. That is why when cooling food, we first divide the food into smaller sections, use ice water baths or stir the food to dissipate the heat – these are all actions to facilitate faster cooling. In all reality, if you can reach 70°F or lower in the first stage of cooling in a “hot kitchen,” in two hours or less, the second stage will be faster in your cold refrigerator. If you don’t reach 70°F or lower within two hours in the first stage, you have to correct it using one of two applicable corrective actions. One, is correct it by correct reheating to 165°F within two hours and then try the cooling process all over again or your other corrective action available is to discard the product.
In addition, when reheating food for hot holding, we must heat all the way from 41°F to 165° within two hours, so that the time spent in the TDZ is minimized. This means when you take food out of the fridge (41°F or lower) and put it on the stove, you have a time limit of two hours to reach 165°F. This is so important that when reheating food for hot holding, if you don’t reach a minimum temperature of 165°F within the allowed two hours, your only corrective action is to discard the product.
I hope this information helps to better understand the Basics and why to keep food out of the TDZ and how we have to follow time and temperature rules. Please don’t forget to use your thermometer.