Dispelling the myths about freezing and defrosting food

Can food that has already been defrosted be refrozen? Do vegetables and fish lose vitamins when frozen? Is it possible to freeze legumes? What is the best type of container to use for storing frozen food? How long can frozen food be kept? Biologist and UOC Faculty of Health Sciences professor Alicia Aguilar provides the answers to these and other questions and dispels some of the myths about the process of freezing and defrosting our food.
 
According to the expert, "Almost no nutrients are lost as the result of freezing food as long as you implement good practices in terms of transport and handling once you have made your purchase".  The process of freezing acts to keep the microorganisms in food from proliferating and to slow some of the chemical processes that lead to food degradation or alteration. "In the case of fish, it is therefore better to buy it already frozen than buy it fresh and freeze it yourself. Fish is a foodstuff that begins to degrade quickly and, therefore, it is easier to freeze when fresh, providing it is done immediately after it has been caught", she adds.
 
Almost all types if food can be frozen but to preserve all its organoleptic properties (taste, texture, appearance, etc), experts recommend following the appropriate practices when it comes to shopping, handling and cooking. "Otherwise you could find that, as in the case of potatoes and pasta, the texture of the product changes after freezing, either becoming harder or softer", explains Aguilar, who also forms part of the UOC FoodLab research group.

As for bread, the expert recommends freezing it when fresh and just bought. "Much of the bread we buy today is in the form of part-baked baguettes which quickly go hard if they are not put in the freezer straight away", she says. In the case of fruit, vegetables and legumes (beans, peas, etc), it is advisable to blanch them  (immersing them in water at 100°C for 1 to 2 minutes) and cool them quickly prior to freezing them.


How long can food be stored in the freezer?

It is important to freeze food in suitable covered containers that have been labelled with the date of freezing. “Glass containers are a good choice because they allow you to see what is inside and are resistant to temperature changes, plus the food can then be heated in a water bath or in the microwave and they are reusable and easy to clean”, says the biologist. Freezer bags are also good, especially for certain fresh (uncooked) foods, and they allow you to store more food in the freezer because they occupy less space.
 
Aguilar points out that the expiration date of frozen foods is determined by a number of factors, such as the type of food, the storage temperature, the freshness prior to freezing (it may already have been stored in the fridge for several days...), the type of packaging (vacuum or not), etc. "To give an idea, food can be stored in the freezer for about six months in the case of meat and for a maximum of three months in the case of other foods (fish, cooked dishes, etc)", she says.


Food should never be refrozen

It is advisable to defrost food by refrigerating it, ie by moving it from the freezer to the refrigerator and leaving it for 24 hours. "If a microwave is being used, glass containers are the best option as they are resistant to sudden changes in temperature and the food should be defrosted on the lowest possible temperature setting or using the ‘defrost’ mode", the UOC professor advises. Vegetables and legumes can be defrosted by placing them directly into boiling water and, in the case of cooked foods with sauces or juices, the use of a water bath is recommended, making sure the dish is enclosed in a tightly sealed container.

“Once the food or dish has been defrosted, it should not be refrozen and should be consumed as soon as possible”, warns Aguilar, going on to reiterate that the freezing process does not kill microorganisms but merely prevents them from proliferating. Therefore, when the food is returned to room temperature, the microorganisms are once again able to multiply, resulting in the freshness potentially being compromised when refrozen. “If you want to refreeze food that has been defrosted, you first need to cook it at a high temperature and for the amount of time needed to eliminate the microorganisms”, she adds.

When talking about the purchase of frozen products rather than home freezing, it is important to be aware that the cold chain needs to be maintained as far as possible. Recommended practices include the use of isothermal bags, separating frozen foods from those that are not so that they remain colder for longer and getting home as soon as possible and placing them in the freezer straight away.