Freezing food has been in practice for a while and have been used by farmers, fishermans and trappers to preserved food in unheated building during the winter season
The freezing turns water present in food into ice. After reaching -9.5°C bacterial growth is stopped and this is why freezing can preserve the food .
In the food industry there are used two processes: mechanical and flash (kriogenic) freezing. Generally, food which is frozen quickly, generates smaller ice cystals and doesn't damage the cellular structure.
So the faster the process is performed the better food quality and composition you will preserve.
The fastest method – flash freezing allows to freeze instantly due to liquid nitrogen temperature (-196°C) .
The most popular domestic freezing has been available since 20th centrury.
It was a big change in how food was preserved and it was able to decrease the amount of food wastes.
Food stored in household freezers usually hold temperatures from -23 to -18°C, and as mentioned before stop microbiological processes in food.
Domestic feezers can be included as a separete compartement in a refigenerator or can be standalone kitchen appliances.
A vapor compressions cycle is used in most of household frezers and refrigerator-freezers. Refrigerant such as R-134a (1,1,1,2-tetrafluoroethane) circulates in the cycle and due to its thermogenic properties can change its own state which can help to transfer the heat from food.
Freezing and nutritional value
The most popular frozen foods are: fruits, vegetables, meat, poultry and fish. In case of fruits and vegetables, most people do not have home gardens which can supply them recommended doses.
Frozen food may be a convenient solution available for the whole year. The most important concern is how freezing changes the nutritional value of food with a particular focus on fresh produce.
Most of the fresh produce that will be frozen are generally picked at peak ripeness, when they are at their most nutritious.
After harvesting the vegetables, they undergo washing, blanching, cutting and finally freezing and packing processes.
Fruits, are not blanched as it can affect their delicate nature and inherent acidity, however vitamin C (ascorbic acid - antioxidant) or sugar (sacharoze) can be added, which can prevent spillage .
In general, the results from scientific studies show that freezing preserves the nutritional value of the food and the difference between fresh and frozen products is not significant [3, 5, 6].
Ohter studies has shown that fat soluble vitamins (A, E, carotenoids) and dietary fibre content is similar and frozen food .
In terms of antioxidant levels, freezing does not cause major changes in antioxidant and acitivity .
What can you freeze?
There are plenty of food suitable for freezing:
All the fruits can be frozen, however remeber to wash, dry and divide into small portions prior freezing. Then, when you need it, you will just need to take it out from freezer, without worring about opening the entire bag.
When you store them properly, they can last up to a year in your freezer.
Similiary to fruits, most of the vegetables can be frozen. There are a few exceptions such as: cabbage, celery, cress, cucumbers, endive, letuce, parsley, radishes, because after thawing, they become limp, water-logged, quickly develop an oxidized color and aroma nad flavor .
It is important however to blanch them (dropping the veggies into boiling water, then instantly cooling and drying) prior to freezing. You can store them up to a year in your freezer.
Dairy products can safely be freezed – milk in the jug (pour out a little for the expansion), cheese (slice it, so it won't crumble) and even yogurt.
Eggs without the shell (raw or cooked) can be freezed as well, so you can easily separete yolks and whites for many recipes and keep the ready i.e. in ice-cube containers.
You can utilize meat with reduced price by freezing it immedietely after coming back home.
All meats are suitable for freezing either cooked or raw. It is best to remove them from its stored packaging and place in freezer bags.
The best method to thaw them is in your fridge. Freezing cooked meat can save you a lot of time when you have to eat something and don't have time to prepare it.
Products such as bread, rolls, dough, pizza, pancakes, waffles, muffins, tortillas are suitable for freezing so they may come handy if you do not have something to eat.
However irish potatoes, baked or boiled one are not the best one to freeze as may become soft, crumbly, water-logged and mealy .
Pasta, rice, macaroni
They are not the best fit for freezing as they can become mushy and tastes warmed over, however in any of above if you slightly undercook them (al dente in case of pasta) they will do better when defrosting and reheating.
Broths, soups, sauces
They are very good for freezing, as you can cook more and leave the leftovers for other days. Remember to put them into smaller, separate servings for latter convenience.
Why you should blanch your veggies before freezing?
Blanching (scalding veggies in boiling water or steam for a very short time) is a must for most of vegetables prior freezing.
The purpose is to inactivate naturally occuring enzymes that may be still active in the frozen product and may destroy the nutrients after defrosting.
Additionally it can clense the surface from dists and microorganisms, brighten the color and helps retard loss of vitamins.
Blanching time is critical and varies depending of type and size of vegetable.
Underblanching can increases the enzymes activity instead of inhibiting them.
Overblanching on the other hand, can contributes to flavor and color loss as well as drop in vitamin and mineral levels.
The important part after blanching is cooling which must be proceeded quicky and throughly to stop the cooking process.
To do it, plunge the vegetables instantly in large amount of cold water and change them frequently (similarily how you do with boiled eggs).
Cooling should take at least the same time as blanching. After that drain them as extra moisture can cause a loss of quality later on .
Blanching times for vegetables can be found here .
Few tips before you start
- never re-freeze your food (health risk)
- cool your food prior freezing (food can affect the temperature of your freezer andcontribute to freezer defrosting)
- keep your freezer packed (it is more efficient and economical to not cool the empty freezer)
- keep servings under control (control the size to not the frost the whole batch to get one item)
- don't freeze old food (try to always freeze food as soon as possible i.e. instantly after shopping)
- label your food (can help you to control your stock and do not keep food too long – write down the date when was frozen – remember First In – First Out)
- defrost your freezer (the same like you take care of your car you should maintain the proper freezer efficiency to not build up to much ice)
- stay away from ice-burns – (when you choose the food in your supermarket which have an icy surface i.e. ice creams – stay away from it as may indicate that the freezer was not working constantly but been turn off)
Freezing is an effective form of food preservation because it can stop microbiological process and spoilage.
If you freeze the food, the faster it is done, the better quality of the food you will receive after the frosting.
Most of the nutrients and antioxidants in frozen food are preserved and does not differ significantly from a fresh food.
Generally speaking most of the food are suitable for freezing, but some of them require extra preparation i.e. blanching when freezing vegetables.
The good practice is to use appropriate packaging, freeze food immediately after purchasing and use labels to maintain the state of frozen food.
- https://en.wikipedia.org/wiki/Frozen_food as seen on 26/07/2016
- Daniele De Pietri, DensityDesign Research Lab, Basic functioning of a refrigerator
- Rickman J.C., Barret D.M., Bruhn C.M., Review Nutritional comparison of fresh, frozen and canned fruits and vegetables. Part 1. Vitamins C and B and phenolic compounds. J Sci Food Agric 2007, 87: 930-944
- Rickman J.C., Bruhn C.M., Barret D.M., Nutritional comparison of comparison of fresh, frozen, and canned fruits and vegetables II. Vitamin A and carotenoids, vitamin E, minerals and fiber. Journal of the Science of Food and Agriculture 2007, 87: 1185-1196
- Bouzari A., Holstege D., Barret D.M., Mineral, fiber, and total phenolic retention in eight fruits and vegetables: a comparison of refrigerated and frozen storage. J Agric Food Chem. 2015, Jan 28, 63(3): 951-6
- Shofian N.M., Hamid A.A., Osman A., Saari N., Anwar F., Dek S.P., Hairuddin M.R., Effect of Freeze-Drying on the antioxidant Compounds and Antioxidant Activity of Selected Tropical Fruits. Int J. Mol Sci. 2011; 12(7): 4678-4692
- http://bfff.co.uk/wp-content/uploads/2013/09/Leatherhead-Chester-Antioxidant-Reports-2013.pdf as seen on 30/07/2016
- Andress E.L., Harrison J.A., So easy to preserve, 5 th ed 2006. Bulletin 989, Cooperative Extension Service, The University of Georgia, Athens
- http://nchfp.uga.edu/how/freeze/blanching.html as seen on 30/07/2016