What is freezing?
Freezing is the process of cooling liquids until they become solid. This happens when the temperature drops below 0°C/32°F, as this is the point at which water crystallises into ice. Before the invention of electrical freezers, this was achieved by using purpose-built icehouses. Icehouses were brick-lined chambers that were built below ground, and insulated with thick walls and straw. During winter months, ice would be transported to the icehouse from a nearby lake, where it would remain frozen until the following winter.
How does freezing preserve food?
Living things are largely made up of water – an adult human is 60% water, and most fresh foods are between 70-90% water. Because of their high water content, fresh foods freeze solid when stored below 0°C/32°F. Once the water has been converted into ice crystals, it is no longer available to bacteria. As bacteria need water for digesting food, this means that bacteria cannot grow on frozen food, and so the food can be kept for a long time without spoiling. Freezing is therefore more effective than cooling, which only slows the rate of decay, rather than stopping it altogether. However, once food has been thawed, bacterial growth will resume. Freezing does not kill bacteria, it only puts them into a state of hibernation.
Does freezing alter the natural state of food?
When water freezes, the molecules rearrange themselves to form ice crystals. In a crystal lattice, the molecules are spaced further apart, creating a more open structure that is less dense. This is why icebergs float, and why sheets of ice form on the top of ponds and lakes, rather than sinking to the bottom. Because the molecules are spaced further apart, liquids take up more space when they are frozen. In fact, frozen volume increases by 9-10%. This is not a problem in open expanses of water like ponds and lakes, but in confined spaces the pressure can cause a rupture, such as bursting water pipes in the winter. Similarly, in plants and animals, the expansion of sharp-edged crystals burst the rigid cell walls and membranes that maintain the structure of the organism. This means that frozen foods become softer when thawed, altering the texture and cooking behaviour.
The damage can be mitigated if food is frozen very quickly. Ice crystals start to grow as soon as food is placed in a freezer, and continue to grow until all the water content has frozen solid. The quicker the water is used up, the smaller the ice crystals. For this to happen, freezing needs to be complete within several hours. This is not possible in home freezers which run at temperatures of -18°C/0°F at best. The frozen food industry relies on a process called blast freezing to preserve the quality of frozen food. In a commercial freezer, temperatures are brought down quickly by blowing very cold air over the food. Once frozen, food can be transferred to a conventional freezer.
Which foods can I keep in the freezer?
You can freeze anything you like, but some foods will be altered beyond recognition after they have undergone the freeze-thaw process. Ultimately it is up to you to decide whether the compromise in quality outweighs the convenience. We all know that fish are best eaten on the day of purchase, but few of us are able to make a special trip to a fishmonger’s every time we fancy fish for dinner. Likewise, if you grow your own fruit and vegetables, then freezing is a great way to ration out seasonal gluts and avoid food waste.
Which foods should I avoid keeping in the freezer?
Foods with a high water content suffer the most damage from freezing. In particular, fruit and salad vegetables become very mushy. For this reason, it is best to freeze cooked food, as changes to the texture are less noticeable when the ingredients have already been softened by the cooking process.
However, even cooked foods are altered by time in the freezer. The consistency can change and become unstable as the bonds that hold the molecules together get broken down. Flavours can change, too, with some ingredients intensifying over time to become stronger or bitter – curries and heavily spiced dishes are particularly prone to flavour changes. Salt, meanwhile, loses flavour, and causes fat to go rancid more quickly. Rancidification is a chemical reaction that causes fat molecules to break down, resulting in an unpleasant smell and taste. This happens when enzymes in the fat react to oxygen in the air, and starts as soon as an animal is butchered and exposed to air. Although the process is slowed down by storing in an airtight container and at a lower temperature, enzyme activity continues even during frozen storage.
If you are preparing food directly for the freezer, it is best to undercook it, and to wait until reheating before adding any crunchy toppings, thickeners or seasoning. Fat should be skimmed from liquid and semi-liquid dishes. This is easily done by leaving overnight in the refrigerator. After several hours chilling, the fat will rise to the top and congeal to form a solid layer on top. Gently scrape off with a metal spoon, then transfer to the freezer in a suitable container.
|Problem caused by freezing:||Affected Foods:|
|Crisp coatings become soggy||
|Consistency breaks down and weeps||
|Fat becomes rancid (accelerated by salt)||
What is the best way to organise food in the freezer?
Freezer organisation is primarily about stock control. You need to keep track of what you have in your freezer, and how long it has been there. It is difficult to do this by sight alone, and no one enjoys a forced game of defrost surprise halfway through preparing a meal. Even if you are able to accurately identify all the food parcels in your freezer, it is not energy efficient to keep the door open for minutes at a time while you rummage around trying to find what you need with increasingly numb fingers.
- Label food: use a permanent pen to label all items. Write down what it is, the date, the portion size and whether it is raw or cooked.
- Keep a stock list: keep a list of all the food that you have in the freezer. Add a new entry each time you put something in the freezer, including the freeze date and the best-before date (more of which later). Each time you take something out of the freezer, cross it off Review the list periodically to identify any food that needs using up.
- Rotate stock: place the oldest items on the top shelf, at the front of the compartment, so that they are next in line for use. Send all new items to the back of the queue by placing on the bottom shelf, at the back of the compartment. Gradually promote stock up the freezer in age order.
- Freeze flat: square or rectangular parcels can be lined up corner-to-corner and stacked on top of each other, reducing wasted space. Flexible freezer bags make the best containers, as they can be flattened into thin parcels to speed up freezing and thawing. Freezer bags are made from a heavy-duty plastic that is designed to be water-vapour-proof, so should not be substituted with flimsy sandwich bags. To flatten, place your filled bags on a tray and transfer the whole lot to the freezer. Once the contents have frozen solid, remove the tray.
- Use plastic containers: be wary of glass containers, unless labelled suitable for freezing, as the extreme cold may cause them to shatter. Rigid plastic containers are good for storing liquid dishes like soups. If you do not want your container to be held hostage in the freezer, line with a freezer bag before filling. Once the contents have frozen solid, lift out the bag and remove the container.
- Insert dividers: freezer shelves are tall and deep. To make it easier to get to items at the bottom or the back, invest in a set of removable and stackable plastic trays. Store similar items in the same container to make it easier to find things.
- Keep your freezer full: unlike refrigerators, freezers work more efficiently when they are full. Each time you open the freezer door you are reducing the temperature by allowing cold air to escape and warm air to enter. Once the door is shut, the freezer has to work extra hard to cool the warm air and bring the temperature back down. If there are more items in the freezer, then there is less air, and so less cold air is transferred.
- Defrost regularly: each time you open the freezer door, water droplets in the air freeze and form ice crystals. Over time, a thick coating of ice builds up on the sides of the freezer. To defrost, first remove the food and store in an ice box. Unplug the freezer and line the surrounding floor with old towels to absorb the melt water. Fill a couple of large bowls with just-boiled water, and place on the freezer shelves. Shut the door and leave for 10 minutes, to allow the steam from the hot water to loosen the ice. Repeat several times with fresh bowls of hot water until it is loose enough to be scraped off. Use a plastic spatula to avoid damage. When you have finished, plug the freezer back in and wait for it to reach temperature before putting the food back in.
How do I store meat in the freezer?
Freezer burn happens when food is exposed to air and loses moisture through evaporation. Although we think of evaporation as starting from a liquid state, water evaporation can occur at all temperatures, even below freezing point. This is because water molecules can go straight from a solid state to a gas state, in a process called sublimation. After the water has left the food, the vapour turns back into a solid and forms ice crystals. A build-up of ice crystals on the surface of food is a sign that moisture has been pulled out and lost. Over time, food in the freezer becomes dehydrated and shrinks in size. When the food is defrosted, there will be clear signs of damage. In meat, freezer burn looks shows itself as brown patches of shrivelled tissue, and in other foods as pale, discoloured patches. Although the food is safe to eat, it will taste off and have a tough texture.
To preserve quality, you need to freeze meat in airtight packaging, and expel all the air before sealing. Vacuum packaging offers the best protection from freezer burn, although few us have a vacuum sealer at home. If you are buying meat to store directly in the freezer, ask your butcher to vacuum package it.
- Remove packaging: if your meat is already vacuum packed, then it can be transferred, unopened, directly to the freezer. However, most meat is packaged for storing in the refrigerator, so uses oxygen-permeable materials to allow the meat to breathe. If meat is transferred directly to the freezer in its original packaging, air will circulate around the meat and cause freezer burn.
- Trim excess fat: over time, enzyme activity causes fat molecules to break down and go rancid. To prolong storage time, trim away excess fat.
- Reduce volume: as freezer space is limited, reduce the volume by removing any unnecessary bones or other non-edible parts.
- Break down into final form: prepare the meat as you intend to cook with it. If you intend to cook a stir fry, dice the meat before freezing. This will make packaging easier, and speed up freezing and defrosting.
- Separate into serving portions: to avoid defrosting more than you need, separate and freeze in serving portions.
- Freeze individually: to speed up defrosting, separate steaks, chops, sausages and patties with squares of wax-coated freezer paper to stop them from fusing together.
- Pad sharp edges: protruding bones may puncture airtight packages, so pad any sharp edges with butcher’s paper before wrapping.
- Double wrap: make an airtight package by wrapping meat tightly in a double layer of plastic food wrap. Although this stops air from circulating by clinging directly to the surface of food, it is too flimsy to ensure that the package stays airtight. Wrap the whole lot in a heavy-duty material, like a plastic freezer bag or wax-coated freezer paper secured with tape.
- Expel air: if you are storing meat in a flexible freezer bag, squeeze out as much air as you can before sealing. You can replicate a vacuum sealer by using a straw to suck out the air. Or you can force out the air by placing in a sink of cold water, making sure that the water level does not come above the top of the bag and allow water to enter.
- Fill containers: if you are using rigid containers, reduce air pockets by packing food tightly and filling the container as much as possible – not forgetting that liquid dishes need some headroom, as they will expand by 10% when frozen.
- Freeze quickly: freezing food quickly results in smaller ice crystals being formed, reducing damage. Bring the temperature down by placing in the refrigerator for two hours before transferring to the freezer. Split large quantities of food into several packages to speed up freezing. Do not stack unfrozen packages on top of each other. Instead, spread them out on different shelves, and wait until they have frozen solid before rearranging.
How do I store poultry in the freezer?
Poultry should be treated the same way as meat, and stored in airtight packaging to protect against freezer burn. If chicken is affected by freezer burn, you will see white patches of shrivelled tissue, which will be more visible on skinless portions.
- Remove giblets: if your bird came with edible organs, remove from the cavity and freeze separately. Offal thaws at a different rate, giving bacteria time to grow on the thawed areas while the rest continues to defrost.
- Freeze stuffing separately: wet stuffing creates a moist environment that encourages bacteria to grow. Because wet ingredients take longer to cook, by the time the stuffing reaches the recommended temperature of 74°C/165°F, the rest of the bird will be overcooked and dry. Freeze and cook stuffing in a separate dish.
- Process into joints: freezing food quickly results in smaller ice crystals being formed, reducing damage. Whole birds take a long time to freeze and take up lots of freezer space. Unless you intend to cook the bird whole, break down the carcass into the breasts, thighs, legs, and wings. Freeze like parts together, as they will defrost at the same rate. Use the non-edible parts for making stock.
- Avoid freezing young chickens: bones have a liquid core of bone marrow, which expands into sharp-edged crystals during freezing. This can damage the bones of young chickens, allowing a pigmented protein called myoglobin to escape through the soft, porous bones and into the surrounding tissue. When the chicken is cooked, the pigment will turn the bones and flesh a dark maroon colour. Although safe to eat, this has an unappetising appearance. Older chickens have had time to develop hard bones, so avoid freezing chickens that weigh below 1.6 kg / 3.5 lbs.
How do I store fish in the freezer?
Fats are classified as either ‘saturated’ or ‘unsaturated’, depending on their chemical structure. Fats are made up of long chains of carbon atoms, linked together by either single bonds (c-c-c) or double bonds (c=c-c). Double bonds can split apart and react with hydrogen, so that one bond continues to link across to the next carbon atom in the chain, while the other bond links up to an isolated hydrogen atom. In saturated fats, every double bond in the chain has split apart and bonded to a hydrogen atom, meaning that the chain holds as many hydrogen atoms as is possible, and is therefore ‘saturated’.
You might think that breaking up all the double bonds would make saturated fats less stable than unsaturated fats. However, the reverse is true. Saturated fats like lard and coconut oil tend to have a higher melting point and stay solid at room temperature. This is because fat chains linked by single bonds produce straight chains which pack closely together, whereas the double bonds introduce a kink to the chain.
Meat and poultry are higher in saturated fat, while fish and plants are higher in unsaturated fat. As unsaturated fats are less stable, this means that fish does not freeze as well and is prone to becoming rancid. This happens when oxygen reacts with enzymes in the fat, causing it to break down and spoil. Damage can be minimised by wrapping or glazing fish to keep out the air. However, the enzyme activity continues even during frozen storage, so it is best not to keep fish in the freezer for too long.
- Process into fillets: freezing food quickly results in smaller ice crystals being formed, reducing damage. Thin fillets will freeze more quickly than whole fish, and take up less freezer space. Leave the skin on, as this will form a protective layer against freezer burn. Use the non-edible parts for making stock, or discard.
- Remove guts: if freezing fish whole, ensure they are gutted to remove the bacteria that live in the entrails, and the digestive enzymes that break down the flesh. During freezing, the formation of sharp-edged ice crystals may rupture the gut wall, contaminating the flesh. During defrosting, the guts will thaw at a quicker rate, giving the bacteria time to grow and spoil the fish as it continues to defrost. Unless you intend to cook the fish whole, remove the head, fins and tail to reduce the volume.
- Treat in a solution: treating fish before freezing prevents moisture loss and prolongs storage. Fish are categorised according to whether they are lean and store fat concentrated in the liver (white fish and most river fish), or oily and store fat throughout their flesh (anchovies, herring, mackerel, salmon, sardines, trout and tuna). Over time, fat molecules get broken down by oxygen and become rancid, resulting in an unpleasant smell and taste. Oily fish should be dipped in an ascorbic solution for 30 seconds before freezing to protect the flavour (mix 1½ pints / 850 ml water with 2 tablespoons crystalline ascorbic acid / juice of 1 lemon). Lean fish should be dipped in a brine solution for 30 seconds before freezing to protect the texture (mix 1½ pints / 850 ml water with 4 tablespoons salt).
- Build a glaze: glazing involves building a thin crust of ice around individual fish or fillets. This creates an airtight seal that protects against freezer burn. Line a baking tray with parchment paper and arrange fish in a single layer. Cover with plastic wrap and place in the freezer. Once frozen, remove and dip for several seconds in a bowl of ice-cold water. The water on the surface of the fish will freeze, forming a thin and fragile glaze. Return to the freezer for 5-10 minutes to harden the glaze. Repeat several times until a uniform glaze is built, then wrap and seal for freezing. Over time, the glaze will evaporate and need to be renewed.
- Avoid freezing very fatty fish: oily fish do not have a uniform fat content, and some varieties are fattier than others. Avoid freezing herring, mackerel and sardines, as they have the highest fat content and are prone to going rancid.
How do I store fruits and vegetables in the freezer?
Even though the freeze-thaw process causes irreparable damage to the texture of fruits and vegetables, if you have a glut of fresh produce it is better to freeze than throw away and waste. This also ensures a supply of seasonal produce throughout the year, allowing a burst of summer berries in the darkest days of winter. One way to mitigate against mushiness is to serve fruits partially thawed. Alternatively, frozen fruit can be saved for cooking, where textural changes will be less apparent.
- Blanch vegetables: briefly scalding vegetables in boiling water or steam deactivates the enzymes that continue to ripen and degrade food, even during frozen storage. Bring a pan of boiling water to the boil, then plunge for 2 – 5 minutes, depending on the type and size of vegetable. Start timing as soon as the water returns to the boil. Once blanched, drain immediately and immerse in a bowl of iced water to stop from cooking further. Drain and dry well before freezing. Bear in mind that the blanching process will reduce the cooking time.
- Treat fruit in a solution: fruits do not need to be blanched, as natural acids neutralise the enzymes. If you are freezing cut fruits that brown when exposed to air (apples, avocado, bananas, peaches, pears), the enzymes can be deactivated by dipping in an ascorbic solution for 30 seconds before freezing (mix ¾ pint / 425 ml water with 1 tablespoon crystalline ascorbic acid / juice of ½ lemon).
- Prepare for use: fruit and vegetables become mushy and delicate to handle once thawed. Wash before freezing and remove non-edible parts such as peel, cores and stones.
- Freeze flat: line a baking tray with parchment paper and arrange the pieces in a single layer. Cover with plastic wrap and place in the freezer. Once frozen, remove the plastic wrap, transfer to a freezer bag and seal. This stops the contents from fusing together.
How do I store herbs in the freezer?
Frozen herbs are a poor substitute for fresh herbs. The freeze-thaw process damages the cell structure, resulting in limp leaves that are useless for salads and garnishes. Frozen herbs can be used in liquid dishes like stews and sauces, although flavour and colour will be lost. As herbs have such a high surface area, they quickly lose moisture and suffer from freezer burn. If you must freeze herbs, then use as quickly as you can, and definitely within 2 months of freezing.
- Blanch: briefly scalding herbs in boiling water or steam deactivates the enzymes that continue to ripen and degrade food even under frozen storage. Bring a pan of boiling water to the boil, then briefly plunge – about 15 seconds for soft herbs, and 30 seconds for hard herbs. Once blanched, immediately drain and immerse in a bowl of iced water to stop from cooking further.
- Dry thoroughly: before freezing, remove any excess moisture by using a salad spinner, then blotting between sheets of paper towel.
- Chop soft herbs: soft herbs with tender stems like basil, chervil, coriander, dill, mint, parsley and tarragon should be removed from the stem and finely chopped.
- Keep hard herbs on stems: hard herbs with woody stems like bay leaves, curry leaves, oregano, rosemary, sage and thyme can be frozen on the stem.
- Freeze flat: place herbs in a plastic freezer bag, squeeze out the excess air, and flatten into a thin parcel to speed up freezing and thawing.
How do I store cooked food in the freezer?
Cooking large quantities of food in one go is an efficient way of ensuring a ready supply of healthy meals. Why spend three hours cooking a casserole for one meal, when you could double, triple or even quadruple the recipe to get more meals for the same effort? You could store the leftovers in the refrigerator, but they would need to be eaten within five days. Cooked food can be kept under frozen storage for months, allowing you to mix and match meals during busier times.
Liquid and semi-liquid braised dishes such as stews, gravies and soups are particularly well suited to freezing. Liquid dishes can withstand the freeze-thaw process because, unlike dry dishes like pies, they do not rely on crispness or layers of contrasting texture for appeal. Their free structure means that they do not need to be handled delicately, and there is no danger of the dish drying out during reheating. Lastly, the braising liquid creates an airtight seal around the ingredients, protecting them from freezer burn.
- Undercook: if you are preparing a meal directly for the freezer, take off the heat before it has finished cooking, otherwise it risks becoming overcooked during reheating. Leave out strong flavours that will intensify during reheating, and add during reheating instead.
- Ring fence leftovers: if you have cooked extra intending to eat some fresh and freeze the rest, divide into an eating portion and a freezing portion before serving. This removes the temptation to dredge through the leftovers and pick out the best bits.
- Leave headroom: remember that water expands by 10% when frozen, so allow enough headroom for liquid dishes, and do not fill containers right to the top.
- Submerge beneath liquid: if you have a liquid dish, ensure that the meat and vegetables are fully submerged to protect against freezer burn.
Can I put hot food straight into the freezer?
The critical factor when freezing food is to freeze it as quickly as possible. Food that takes a long time to freeze will grow larger ice crystals. These sharp-edged crystals damage the cell structure, resulting in a mushy texture when thawed. If you place hot food directly in the freezer it will take longer to freeze. It will also damage surrounding items by raising the temperature and causing a partial thaw.
Hot food should always be cooled completely before being placed in the freezer. Decant into shallow containers, then place in a sink filled with ice cold water. Transfer to the refrigerator within 2 hours, and wait until chilled before transferring to the freezer.
How long can I keep food in the freezer before it goes bad?
Although bacterial activity is stopped under frozen storage, enzyme activity is only slowed. Enzymes are responsible for ripening food, so will continue to gradually degrade the colour, taste and texture. This does not make food harmful to eat, but it does mean that food cannot be stored indefinitely without compromising the quality. Fatty foods deteriorate more quickly, as enzymes in the fat absorb oxygen and become rancid.
|Expected Shelf Life:
(best before, but will keep longer)
How do I thaw food from the freezer?
As freezing does not kill bacteria, bacterial growth will resume once thawing begins. The optimum temperature for bacteria to reproduce is between 5°C-60°C / 40°F-140°F, so it is not safe to thaw food at room temperature on a kitchen counter. The safest way to thaw food is in a refrigerator, where air temperature is kept at a consistent 5°C / 32°F to slow bacterial growth. Place on the bottom shelf where the temperature is coldest, in a tray that will contain leaking juices. Raw meat portions will take 1 day to thaw using this method, and whole roasts 2-3 days depending on the size.
If you need to thaw food quickly, then submerging in water will speed up the process. This is because water conducts heat more effectively than air, transferring the cold out of the food. Seal in a waterproof bag, to stop the food from absorbing water, then submerge in a large bowl of cold water. Do not use hot water, as this will encourage uneven thawing and allow bacteria to grow on the outer layers. Replace the water every 30 minutes. Raw meat portions will take 1 hour to thaw using this method, and whole roasts 2-3 hours depending on the size. This is preferable to microwave defrosting, which tends to partially cook and toughen meat.
Food thawed using an acceleration method carries the risk of uneven thawing. This means that it should be cooked immediately, because the defrosted outer layers may reach an unsafe temperature that encourages bacterial growth while the frozen core continues to thaw. Food thawed using the freezer-to-refrigerator method does not need to be cooked immediately – the ‘best before’ countdown clock has restarted, so it should just be eaten within the recommended number of days for refrigerated food (although remember to subtract any days spent in the refrigerator before you froze it).
Can I cook food from frozen?
If you do not have time to thaw food first, it is perfectly safe to cook it straight from the freezer, just expect cooking times to be increased by around 50% or more. However, there is a risk that larger items such as whole chickens will be overcooked and dry on the outside by the time the core reaches the recommended temperature of 74°C/165°F. If food starts to burn on the outside, then cover with a double layer of tinfoil. Cooking from frozen is best suited to smaller items such as vegetable or chicken pieces for a stir fry.
Can I refreeze food?
Food thawed in the refrigerator can be refrozen, although bear in mind that the formation of sharp-edged ice crystals will damage the texture for a second time. It is not safe to refreeze food that was thawed in a water bath or microwave.