Guide to home freezing including information on bacteria and enzyme activity.
Freezing food is an excellent method of food preservation. It enables many foods to be stored for weeks or even months longer than in the refrigerator and to be defrosted when needed, without loss or with very little loss of quality and nutrients.
This is particularly useful for those with large families; less time is spent shopping for fresh food, as food can be bought in bulk and less time is spent preparing meals, as several meals can be made at once and frozen; thus saving them time and money.
Not all foods are suitable for home freezing. However for those that are, as long as the guidelines for the correct way to freeze and defrost food are adhered to, there is no reason for there to be any loss of quality of texture, taste, colour, freshness or nutrients upon defrosting.
Other pages in our home freezing section
Top 8 tips for freezing food
Below are our top 8 guidelines for safe and correct home freezing for your reference, but for more in-depth and extra information, particularly on certain food types, continue reading the rest of the article.
- Ensure that food is in perfect condition before freezing.
- Divide food into small portions. This ensures rapid freezing and better quality on defrosting.
- Use the correct type of packaging or container.
- Make sure that food is tightly wrapped or sealed and that no air or water can get in or out.
- Check that the temperature of the freezer is at 0°F or below.
- Do not freeze too many unfrozen items at once.
- Leave space around newly introduced packages.
- Defrost items in the refrigerator or in cold water.
About the freezing process
Water is the primary element of fresh food and usually makes up between 50-90% of the weight of the product. Freezing is a way of maintaining these water levels and temporarily preventing the growth of micro-organisms and bacteria, as the lack of moisture renders them unable to multiply when the water freezes to ice.
When fresh food is quick frozen, the cold penetrates rapidly, freezing the water and forming a large amount of small ice crystals. The smaller the ice crystals, the better the food quality after defrosting.
If food is frozen slowly, a smaller amount of large ice crystals tend to form. This means that as the water freezes and expands, the cell walls of the food expand and rupture causing structural damage to the food. This is why sometimes certain foods will be limp or mushy when thawed or just taste and feel different on eating.
The faster a food is frozen the better, so make sure that you use the "fast freeze" option if you have one or that you place the unfrozen product at the bottom of the freezer, which is the coldest location, so that it freezes faster than it would in a warmer area.
If you have several items to freeze, spread them out away from each other until they have completely frozen and they will also freeze quicker.
The ideal temperature at which to store food items in the freezer for long periods of time is 0°F (-18°C) or below. Foods stored at temperatures above 0°F will be prone to minimal bacteria activity, food spoilage and deterioration. At 0°F bacteria are inactivated, nutrients are retained and deterioration is at its lowest.
Bacteria and freezing
Bacteria, whether harmful or not, are not destroyed through the freezing process, as they are through the cooking process. They are merely put out of activity for the duration of the time that the frozen product is actually frozen solid. During this time bacteria, yeasts and moulds are unable to multiply, as they need water or moisture to do so.
Once the product is being thawed though, it's another matter. As soon as food begins to defrost, temperatures rise and moisture is present, bacteria will start to grow. If the item is thawed at a temperature above 40°F (5°C) bacteria will multiply rapidly in a very short time and could lead to food poisoning and severe illness.
For this reason, it is very important to defrost food slowly and safely, preferably overnight in the refrigerator.
Food quality after freezing
It is true to say that the fresher the food that goes into the freezer, the better quality it will be when it comes out.
Fresh food should be frozen at peak quality, when it is in its best condition. This means that if you know that you are not going to use something immediately or within a few days, you should store it in the freezer as soon as it is brought home from the supermarket or picked from the garden.
The only foods that may not emerge from the freezer in such good shape are those that are not necessarily suited to freezing (see above) or those that have not been stored under the best conditions.
Nutritional value after freezing
Freezing food does not lessen the nutritional value of that item. In fact, nutrients are retained better in frozen fruits and vegetables than in those that are transported long distances to their destination.
Fruits and vegetables actually begin to lose valuable vitamins and minerals soon after picking, especially if they are stored at room temperature or above. However, if they are frozen within a few hours after harvesting, this will not happen.
Meat, fish and poultry barely lose any nutrients at all, as proteins and Vitamins A and D are unaffected by the freezing process.
Enzyme activity and freezing
Enzymes are protein substances that are naturally present in all animal and plant foods. They help promote different types of chemical reactions that take place before and after harvesting or slaughtering.
Before fruits and vegetables are harvested, the enzymes within them speed up chemical reactions that enable the plant to grow, mature and ripen, whereas after harvesting, enzyme activity promotes chemical reactions such as ripening and undesirable changes in texture and aroma, thus leading to the deterioration and break down of the plant.
Enzymes that cause food spoilage in fruit and vegetables are active particularly at high temperatures, which is why most fruits and vegetables should be stored in the refrigerator. However, as you know deterioration will still take place.
Freezing slows down enzyme activity but it doesn't stop it altogether. Even at 0°F (-18°C) fruits and vegetables will experience some deterioration over time. The only way to prevent enzyme activity during freezing is to halt it before freezing. This can be done by using two different methods.
Vegetables suitable for freezing are blanched (placed into fast-boiling water), steamed or microwaved for a specific amount of time and then rapidly cooled. This action will inactivate certain enzymes that would otherwise continue their activity even when frozen. The only downside to this is that a small amount of vitamins and nutrients are lost in the blanching process.
With some fruits, on the other hand, there is less enzyme action, as many of the enzymes are neutralised by the natural acids contained in the fruit. For others, a chemical compound such as ascorbic acid (Vitamin C) is usually added to stop the deterioration of commercially frozen fruit during freezing.
If freezing fruit at home there are several methods, which can be used, depending on how you are going to use the fruit afterwards. (For more information see our section on freezing fruit).
Rancidity in foods
Foods that contain high amounts of fat such as meat, fish or poultry may turn rancid when stored in the freezer. This could be due to a number of reasons. Firstly, this could happen because of contact with air, particularly if the food item has not been packaged properly and air from outside is allowed to come into contact with the food. For this reason, it is vital that animal products are carefully wrapped in airtight packaging and as much air is excluded as possible before wrapping.
Secondly, the fat itself will turn rancid when exposed to oxygen. For this reason, to extend the shelf life of the product in the freezer, trim all excess fat from meat in particular, before wrapping and freezing.
Finally, rancidity will occur naturally if a product is left in the freezer for an excessive amount of time. This will result in off flavoured products, which will spoil the taste of the food and ruin a meal. Make sure that you consume frozen animal products well within their recommended storage time, so that you do not end up having to throw away expensive meat, fish or poultry items and be left with a table full of hungry and disappointed people.
If a food item has been stored in the freezer for a long time and you are unsure of the quality, check the colour and smell of the product once it has been defrosted. If there is a rancid odour or too much discolouration, it is probably better to throw the item away.