Can bacteria survive the freezer?
Bacteria will freeze along with the food they are on when they are stuck into a freezer. The pathogenic bacteria on foods that cause human disease are primarily mesophiles or organisms that grow best at around 37 degrees Celsius. In a freezer at -20 degrees Celsius, their bacterial enzymes cannot function because the water inside the cell has frozen, so they cannot grow. It has been reported that if you put a raw chicken into your freezer (considered a slow freeze), you may reduce the number of bacteria by approximately 90%. This is primarily due to the formation of ice crystals in the bacterial cytoplasm which then ruptures the bacterial cell wall and kills the cell, reducing the number of viable bacteria. In many food processing plants, food is flash frozen in a way that fewer ice crystals form in the food so more bacteria survive as well (up to 70 percent survive as opposed to 10 percent).
So the problem is the exponential growth capacity of bacteria. If the food is thawed improperly, (at room temperature) surviving bacteria on the surface can quickly reproduce to pre-freeze levels (USDA source). So when is this a problem?
Let’s use chicken as an example food. In a Consumer Reports article in 2013, chicken from retail stores around the country were tested and Salmonella was found on 10.8 percent and Campylobacter on 43 percent of all samples tested. Just this spring in the UK, they tested over 4000 samples and found that 73 percent of all chicken tested were positive for Campylobacter. Worse yet, of those, 19 percent were deemed heavily contaminated. So even ten percent of these organisms on the thawed bird would be a serious contamination issue.
The other way that we know that bacteria survive freezing is just to check the news and the various outbreaks of illnesses linked to frozen foods. There was an outbreak in March, 2013 of E. coli O121 that sickened 24 people in 18 states linked to frozen chicken quesadillas. The most recent outbreak is actually ongoing, where they have linked raw, frozen, Stuffed Chicken Entrees by Barber Foods to a Salmonella outbreak that has sickened 9 people in four states. Not only have these people become seriously ill, but the CDC identified that the strain of Salmonella responsible for the outbreak is a multi-drug resistant strain impervious to both tetracycline and ampicillin.
Best case practices in the case of contaminated foods? For pre-cooked food such as the quesadillas, reheat according to directions on box. Bacteria are killed by reaching temperatures of 165F, even muti-drug resistant bacteria.
For raw food, such as chicken, make sure to use aseptic technique when handling the food. Don’t touch surfaces of the kitchen with hands that have come into contact with the bird. Watch where the juice from the thawed chicken lands (it will be full of viable bacteria). Don’t wash your chicken in the sink, it aerosolizes the bacteria, and they may land on other surfaces that you are not aware of. NPR had a nice overview of what to do if you want to brine your chicken or marinade,-just remember that this fluid has bacteria in it and be careful when you are pulling the chicken out as to where the water droplets go. Thaw food in your refrigerator, in cold water, or in brine.
I always have a dish of hot soap water ready and wash my hands a lot when handling raw chicken. I turn on my faucet to wash my hands with my arm. Washing hands with soap is sufficient to prevent contamination. I also have my sink empty and all counter surfaces clear so that if anything DOES land there, it is a simple soap and water clean up.