Why not re-freeze thawed out frozen food?

The purpose of our freezer is to store our food, to stop it becoming spoiled. These two kitchen appliances, the freezer and the refrigerator, do a great job in maintaining our health. We can now enjoy fresh vegetables and berries all year round and benefit from all their vitamins. Previously, the way to preserve berries was to boil them, but that destroyed a lot of the vitamins. No longer do we need to eat heavily smoked or salted meat, increasing our blood pressure and causing a risk of cancer. Finally, our intake of microbial toxins is certainly much lower than before.

Bacteria producing toxins are resistant to freezing

Freezing will not kill the bacteria that spoil our food. They resist frost as happily as the perennial flowers in our garden. In laboratories, bacteria can be stored frozen, for decades or longer. This means that if there are any bacteria causing spoilage or producing toxins in the product we put into a freezer, then they will still be there when we take them out prior to consumption.

When meat or berries are put into a freezer, their cellular structure is usually undamaged, which means that the surface had prevented, at least to some extent, microbes from penetrating deeper into the material. When meat is hung in a cold store to cure it, the surface of the meat becomes dry and this forms a protective cover. That kind of steak will keep for a long time without any problems of spoilage. Ground or minced meat, however, will go off within a day or two, because surface bacteria get mixed with the whole mass. Berries are still living when they are picked, and they resist microbial attack. After the active resistance is broken, berries or fruit rot very quickly.

A frozen product is a risk product after thawing

What happens when you take meat or berries out of a freezer? Everyone has noticed that there is some liquid which drips out of the container. This is because ice crystals have damaged the cell structures. Consequently new routes have become available to microbes to penetrate into the tissue, routes that are not present in the unfrozen product. That is why frozen steak or berries will spoil much faster than their fresh equivalents.

Moreover both freezing and thawing take time. The microbes in a frozen product had therefore some extra time to multiply especially if the product is warmed up to room temperature. Refreezing means we are now freezing a product chock-full of bacteria, and after subsequent rethawing, bacterial production starts immediately at an incredibly fast pace, and the time that it is safe to eat is much shorter than the consumer might think. This can cause unexpected food poisonings.

It is often stated that bakery products may be refrozen. This is true with products like simple bread unsliced or in the original package. With moist and sweet products such as cakes the situation is not very different from meat: if toxin-producing staphylococci started to multiply before freezing, the growth of the bacteria is triggered immediately after thawing. Therefore freezing for a couple of days after a birthday may be a better option than overeating. But there is one thing you have to remember when you take the cake out from the freezer: how long it was on the table before freezing. You should not think of it as a fresh product. A longer storage may be risky, if you forget that maxim. You have to remember the previous period before freezing and thus there may be surprisingly few good hours left after you take it out of the freezer. It is a good idea to stick a piece of freezer tape on the container with accurate information on the product if it was not frozen fresh.

Is it wise to freeze something prepared in microwave oven?

Microwave ovens are intended for rapid warming of food. Rapid warming always includes the risk that though most will be warmed up to 60–70 ºC – there may be some parts which only reach 40 ºC, and this then creates ideal conditions for microbial growth. Therefore it is safer to use a microwave oven for the purpose which is intended: heating one portion of food that is eaten at that sitting. There is a clear risk of microbial growth when freezing warmed-up food. For the same reason, food made from scratch in microwave oven carries a greater microbial risk than food cooked in the traditional way. Especially if raw chicken is cooked in a microwave oven, one has to be very careful and make sure it is all thoroughly heated in order to destroy salmonella.

Even ice-cubes may contain bacteria

For similar reasons, fresh, good quality water has to be used to make ice cubes. If there is bacterial growth in water kept in room temperature before freezing, these bacteria will also be found in the ice cubes. This may be especially important when travelling abroad to tropical countries: ice cubes can be risky.

The freezer is an important appliance in promoting health, but it has to be combined with some understanding of the laws of microbial growth and death. Freezing will not kill bacteria; they wait like the Sleeping Beauty for a prince to wake them up.

Notes and references

One level up: Is man defiled by what goes to his mouth?

Previous chapter: May even our cooking habits hurt us?

Next chapter: The dirty dozen – not just an old movie?