What is cooling?
In modern times, cooling means storing something in the refrigerator, where it is kept at a constant temperature of between 0°C-5°C / 32°F-41°F. For our cave-dwelling ancestors, cooling meant storing food underground or underwater. And for our more immediate ancestors, cooling meant building houses with pantries, cellars or outdoor iceboxes. Unlike other preservation techniques such as salting or pickling, cooling does not change the taste or texture of the food. For this reason, it is the most popular form of preservation, because it keeps food as close to the natural, fresh state as possible.
How does refrigeration preserve food?
Storing food below 5°C / 41°F but above freezing point will not kill bacteria, but it will significantly slow down their growth, and so increases the amount of time it takes for food to spoil. This is because the optimum temperature for bacteria to reproduce is between 5°C-60°C / 40°F-140°F – the so-called ‘danger zone’. This is why you should not leave cooked food at room temperature for longer than 2 hours before refrigerating.
Does refrigeration alter the natural state of food?
Refrigerators are cold and wet places. Some foods do not keep well in this environment – in fact, storing them in the refrigerator actually hastens decay. Also, refrigerators do not maintain a uniform temperature and humidity throughout – some areas are cooler or wetter than others. So before you place foods in the refrigerator, you need to know whether it is a suitable environment.
Which foods can I keep in the refrigerator?
The water content of food varies – all the way from a cucumber which is around 96% water, to dried pasta which is around 10% water. Dry foods with a water content of 10% or less will naturally keep for a long time, without the need for refrigeration. This is because bacteria need water to grow, as they use it for dissolving their food. In fact, dehydration is another method of food preservation (think of sun-dried tomatoes, desiccated coconut, or strips of beef jerky).
Most fresh foods are made up of around 70% water, and so need to be refrigerated to stop bacteria from growing. Fruits, vegetables and soft herbs have particularly high water content, usually between 90-95%. Likewise, meat, poultry and fish have around 70-75% water content, and should always be kept in the refrigerator. Other foods that should be kept in the refrigerator include animal fats such as lard or dripping, cooked food, and opened tins of food that have been exposed to air.
Which foods should I avoid putting in the refrigerator?
Most fresh foods will reside happily enough in the refrigerator, retaining their freshness and texture. However, some foods are not suitable for refrigeration. For example, the cold environment stops fruit from ripening – which is unsuitable for green bananas or firm avocados. It can also alter the texture of foods; coagulating olive oil, drying out baked goods and turning tomatoes into grainy, flavourless mush.
Below is a list of common foods that do not need to be kept in the refrigerator. They are best stored in wire baskets / slatted boxes that will allow air to circulate around them. Most fruits are best stored out of the refrigerator, where they can continue ripening. If in doubt about whether to refrigerate a vegetable, a general rule of thumb is to refrigerate anything that is green (for examples, spring onions, courgettes, cabbage, broccoli etc.).
|Foods that can be stored on a worktop, out of direct sunlight:||Foods that should be stored in a cool, dark place, such as a cupboard or drawer:|
* eat on day of purchase
* store in a paper bag
Do I need to store fruits and vegetables separately?
As picked fruits and vegetables age, they produce a plant hormone called ethylene. This is released in the form of a gas that is completely harmless and flavourless, but that acts as a ripening agent. Ethylene speeds up decay, softening and browning fruit, and wilting vegetables.
Fruits and vegetables do not have a uniform response to ethylene; some release higher amounts (ethylene releasers), while some are more sensitive to the ripening effects (ethylene absorbers). This means that when ethylene absorbers are stored with ethylene releasers in a sealed environment that traps the gas, these foods will decay much quicker. This can be advantageous when you have fruit that needs ripening – for example, if you have green bananas, you can speed up the ripening process by sealing them in a paper bag with some apples. Similarly, if you have a bag of apples, you should remove any soft or damaged apples to stop them from spoiling the rest. Basically, you can view ripening as a process of contagion!
Fruits release more ethylene than vegetables, which is why fruits are traditionally stored in a separate fruit bowl. You will notice that there is a lot of cross-over between the list of ethylene releasers, and the list of foods that should not be stored in the refrigerator. This is because a refrigerator is a sealed environment that traps ethylene and hastens decay. By storing ethylene-releasing foods in open baskets, you allow the ethylene to escape and increase the shelf life of the fruit. You will also notice that it is possible for a food to appear on both lists, being an ethylene absorber when unripe, and an ethylene releaser once ripe.
|Ethylene Releasers:||Ethylene Absorbers:|
What is the best way to organise food in the refrigerator?
As we have seen, fresh foods do not decay uniformly, accelerating or stalling in different environments. Happily, refrigerators do not present a uniform environment, containing pockets of higher or lower temperatures and humidity. If we can map these microclimates, then we can make sure that food is placed in the optimum environment.
You can also ensure a longer shelf life by keeping your refrigerator clean, as spills and leaks fester, creating a breeding ground for bacteria. As with most things, prevention is the best cure. Line the door shelves and crisper drawers with sheets of kitchen towel, which can be regularly replaced. Similarly, invest in a set of removable and stackable plastic trays for the main compartments.
|Door Shelves = warmest temperature
Store foods that are not sensitive to a fluctuating temperature each time the door is opened, such as condiments and preserves.
Top Shelf = warmest temperature
Store ready-to-eat food that does not need to be heated, such as deli counter items like cured meat and dips.
|Middle Shelf = most consistent temperature
Store food that will go off quickly and needs using up, such as leftovers and cut fruit and vegetables.
|Bottom Shelf = coldest temperature
Store food that needs to be cooked at a high temperature, such as raw meat, poultry and fish.
|REFRIGERATOR DRAWERS: Crisper Drawers = highest humidity
Store food that needs to be kept moisture and is in danger of drying out, such as vegetables.
How do I store meat in the refrigerator?
Instinctively, we rely on the colour of meat to tell us how old it is and whether it is good to eat. Who can resist a juicy red steak? Freshly slaughtered beef actually has a darker colour, closer to purple, when it is butchered. This colour comes from a pigmented protein found in muscle tissue, called myoglobin, which is responsible for transporting oxygen. When the interior is exposed, oxygen in the air reacts with myoglobin, and they bind together to form the compound oxymyoglobin. This chemical reaction is called oxidation (the gaining of oxygen by a substance), and it is what gives meat its characteristic cherry-red colour. However, after a few more days’ exposure, oxymyoglobin begins to break down, and the iron molecule in the middle of the molecular structure becomes oxidised. A new compound called metmyoglobin is formed, which turns the meat a grey-brown colour. Meat that is very old will turn a grey-green colour, and should be avoided. It will usually smell sour and have a slimy texture.
As colour can be used as an indicator of age, some retailers ‘cheat’ the reaction by using Modified Atmosphere Packaging (MAP). This is a type of airtight packaging where the composition of the air around the meat is controlled to prevent chemical reactions from causing discolouration. The air in the packaging is evacuated and replaced with a different gas mixture, such as higher levels of carbon dioxide, carbon monoxide or nitrogen. This means that the true age of meat can be masked until it is taken home and opened. So despite the convenience offered by packaged supermarket meat, the best cuts are to be found at a butcher’s counter, where they are displayed on open trays.
Wherever you buy your meat from, it will make up a large portion of your food expenditure. Meat is expensive, so taking a few minutes to prepare it for refrigeration will preserve quality and optimise taste and texture.
- Wrap up small cuts: small cuts with a large surface area such as diced meat and ground meat, and wetter cuts such as sausages and offal, should be kept wrapped to prevent them from drying out. The packaging needs to be water-vapour-proof to stop moisture loss through evaporation, but it also needs to be oxygen-permeable to allow air in so that the meat can breathe. For best results, wrap the meat in peach paper (also known as butcher’s paper), then seal in a plastic bag. Wet cuts spoil more quickly, as moisture encourages bacterial growth.
- Remove packaging from large cuts: sealing meat in airtight packaging may well stop oxygen and contaminants from getting in, but it also stops moisture from getting out. This causes meat to ‘sweat’, which affects the flavour and texture. Allowing the meat to breathe means that water is lost through evaporation, which intensifies the flavour by removing a dilutant. It also extends the shelf-life by a day or two.
- Pat dry: when it comes to cooking meat, the surface needs to be dry enough to allow the meat sugars to caramelise and form a crust. Wet meat will stew in its own juices, and will not develop the depth of flavour you get from browning. So before putting meat in the refrigerator, pat it dry with kitchen towel to remove excess moisture.
- Allow air to circulate: meat keeps best when cold air is free to circulate, as this ensures even cooling. Mimic the conditions of a butcher’s counter by placing meat on a plate or tray rather than in a sealed container. Cover meat loosely with tin foil to create an odour barrier and prevent it from drying out.
- Arrange in a single layer: the process of oxidation gives meat its characteristic cherry-red colour, so if you starve meat of oxygen then it will lose this rosy hue and turn brown. Piling cuts of meat on top of each other stops the overlapped areas from receiving oxygen and causes them to turn brown. This explains why mince can appear bright red on the outside but brown on the inside. Once exposed to air, the meat will usually bloom to the same red colour within 20 minutes.
- Prevent cross-contamination: keep meat in a dedicated meat tray to contain raw juices, and store on the bottom shelf of your refrigerator where it is coldest and at least risk of cross-contamination.
How do I store poultry in the refrigerator?
Poultry is classed as domesticated birds like chickens, turkeys, ducks and geese that are reared for eggs or eating. When it comes to storing poultry, food safety is the key consideration. Salmonella is a bacteria that lives in the intestinal tract of birds and animals. It is transmitted in the faeces of infected animals, causing food poisoning in humans when it gets into the food chain. Although we do not eat the intestines, intestinal bacteria can migrate to other parts of the carcass when they are removed during the slaughter process, and so contaminate the edible portions.
Salmonella contamination is particularly prevalent in poultry, due to the way it is processed after slaughter. Before a bird can be cooked it needs to be plucked to remove all the feathers. Most poultry sold in this country is processed through large-scale factories that run a mechanical production line. After the intestines have been removed by machine, the carcass is plunged into hot water to loosen the feathers for plucking. If there is faecal matter on the carcass, or the intestines have only been partially removed during mechanical evisceration, then the water in the scald tank becomes contaminated. As more carcasses follow down the production line, they are plunged into successively dirty water and contaminated with Salmonella. In the industry, this is charmingly referred to as ‘faecal soup’. It certainly provides a strong argument for buying poultry from a reputable source – and by this I don’t mean paying a premium at a higher-end supermarket.
Speaking of paying a premium, wild game birds carry less risk than poultry, because they are not subject to the same intensive processing. In fact, they will often be dry-plucked because the additional cost can be absorbed by asking a higher price. Dry-plucking is more labour intensive, but it results in a better product that will keep for longer because it does not leave behind moist skin that encourages bacterial growth. Like meat, game birds can be hung and aged for several days, during which time enzymes tenderise the meat and develop more complex flavours. If you are lucky enough to get hold of some game birds, treat them like meat by allowing air to circulate around them in the refrigerator.
Fortunately, Salmonella is killed by heat during the cooking process. So if you have unwittingly brought a contaminated bird, then cooking it to the recommended internal temperature of 74°C/165°F will destroy the bacteria. In the meantime, while the contaminated bird is sitting malevolently in your refrigerator, you need to ensure that it does not contaminate cooked or raw foods that are not going to be heated.
- Keep in packaging: leave poultry in its original packaging. This should be some form of plastic wrapping that completely surrounds the bird. If there are any leaks, then place inside another plastic bag. All packaging should be thrown away after use.
- Prevent cross-contamination: keep poultry in a dedicated poultry tray to contain raw juices, and store on the bottom shelf of your refrigerator where it is coldest and at least risk of cross-contamination.
- Do not share chopping boards: designate a chopping board that is only ever to be used for raw poultry, and share this rule with the rest of your household. Clean thoroughly after use, including dousing with just-boiled water from the kettle.
- Do no wash poultry: despite what you may have been told, only high temperatures kill Salmonella. So dousing over a bit of water will do nothing but spray contamination around your sink.
- Wash your hands: wash your hands thoroughly with soap each time you touch raw poultry.
How do I store fish in the refrigerator?
Fish is even more expensive than meat and poultry, and also more perishable. Because fish swim about suspended in water, they do not have weight-bearing muscles like terrestrial animals. This means that they have very little of the connective tissue that is needed to hold muscles to bone. Enzymes digest tender flesh much more easily than tough connective tissue, causing fish to spoil quickly.
All fish should ideally be eaten on the day of purchase, but particularly oily fish (anchovies, herring, mackerel, salmon, sardines, trout and tuna), which store fat throughout their flesh, rather than concentrated in their liver. The natural fats in oily fish lend a darker colour and stronger flavour to the flesh, but also cause it to deteriorate more quickly as they absorb oxygen and become rancid. Lean fish will usually keep in the refrigerator for 1-2 days.
- Keep on ice: fish are immediately put on ice after being caught. This is also how they are displayed at a fish counter, and the storage conditions that you want to mimic at home. This is because fish swim in very cold water, and so need to be kept at 0°C/32°F to remain in peak condition. The ambient air temperature of a refrigerator is closer to 5°C/41°F, so is not cold enough to slow down the enzyme activity that causes decay.
- Keep Drained: just like a fish counter, you need a drainage system for syphoning off the melt water from the ice. Fish that are left to sit in water will leach out flavour and texture. Take some ice cubes out of the freezer and crush them to create a larger surface area (you can do this in a food processor or blender, or by placing in a plastic bag and bashing with a rolling pin). Find a container that has holes in the bottom, such as a steamer basket or colander. Fill the container with the crushed ice, then sit within a larger container. As the ice melts, it will drip through the holes into the larger container. Make sure that the smaller container will sit above the water mark once the ice has melted– if your container needs to be raised, place on top of inverted ramekins or scrunched up tin foil. Store in the coldest part of your refrigerator (normally at the back of the bottom shelf), draining and replenishing the ice as it melts.
- Seal fish fillets: if dealing with filleted fish, remove any packaging, seal in an airtight plastic bag, and place on top of the crushed ice.
- Gut whole fish: if dealing with whole fish, ensure they are gutted. This should be done as soon as possible, as bacteria in the entrails accelerate spoilage, rotting from the inside out. Stand the fish upright on the ice to replicate a natural swimming position, and cover loosely with tin foil to create an odour barrier and allow the juices to freely drain.
- Marinade oily fish: sometimes it is not convenient to eat fish on the day of purchase. You can preserve fish by soaking in an acidic liquid, as this creates a hostile environment for bacteria. However, marinating fish for more than 30 minutes irreversibly alters the texture of fish, causing the protein molecules to denature and unravel. The denatured proteins then bond together, giving the flesh a firmer texture and opaque appearance. This is the basis for Ceviche, a Peruvian dish where fish is marinated in lime juice, oil and seasoning for several hours before being served raw. Start with good quality fish, remove the skin, and leave to marinade in the refrigerator in a non-reactive container. Oily fish will keep for 2-3 days, but lean fish will turn to mush.
- Salt lean fish: similarly, you can prolong the shelf-life of lean fish fillets for 2-3 days by lightly salting. Salt draws out water, which discourages bacterial growth. It is also absorbed into the flesh, which inhibits enzyme activity. This means that salted fish has a firmer texture and stronger flavour. How much salt you use depends on the volume of fish, and how long you leave it on for depends on the size of the fillet. Weigh the fish, then weigh out 20% of this weight in coarse sea salt (so 50g of salt for every 250g of fish). Scatter a thin layer of the salt on the bottom of a non-reactive container, then layer on the fish fillets, skin-side down. Scatter over the rest of the salt, sowing more heavily on thicker bits. Cover, and leave in a cool place. Do not refrigerate, as this will slow down the process. For fillets that weigh 250g, leave for 30 minutes, and then 30 minutes for each additional 250g. Once salted for the allotted time, gently rinse off the salt under the cold tap, pat dry with kitchen towel, and store in the refrigerator. You can also salt oily fish, although they will need longer to salt, as fat reduces salt absorption.
How do I store fruits and vegetables in the refrigerator?
Fruit and vegetables are the most forgiving food group when it comes to storage, as they are generally less expensive, less perishable and less dangerous than meat and fish – an aged green will not give you food poisoning. The goal with plant-based produce is not about eating it as quickly as possible before it has a chance to spoil, but about preparing it to withstand a longer storage time.
- Remove any packaging: always remove any packaging or ties, and do not pack items too tightly together. It is important to give room for air to circulate, as this will have a cooling effect and keep humidity levels down. The exception to the rule is cut vegetables and fruits, which you should cover tightly with plastic wrap to create an airtight seal.
- Avoid washing: by all means gently shake off excess dirt, but do not wash fresh produce before storing, as moisture encourages bacterial growth. For produce that has lots of soil clinging to it, lay on top of a paper towel to contain the mess.
- Handle gently: if you handle fresh produce roughly, this will damage the exterior, causing cracking and bruising. This hastens decay by exposing the insides to bacteria and oxygen.
- Keep intact: resist the temptation to remove stems or stalks – this effectively damages the plant, and so accelerates the decay process. The exception to the rule are root vegetables that still have their green tops attached, such as carrots, beetroot, radishes and turnips. Here, the tops should be removed and stored separately, as they draw out moisture from the root.
How do I store fruits and vegetables that have already been cut in the refrigerator?
Ideally, you should chop fresh produce just before you intend to use it, as this will retain maximum freshness and nutrients. However, sometimes you may cook something that only calls for a half or a quarter of something, or you may need to prepare ahead for convenience. Just like bruised and damaged food, cut food will spoil quicker than food that has been kept intact. This is because when the insides are exposed, enzymes in the food react with oxygen in the air. A chemical reaction called oxidation occurs, which causes food to break down. This results in discoloration, dehydration, and degradation of flavour, texture and nutrients.
You can prevent oxidation by creating an airtight seal – either by covering tightly with plastic wrap, putting in an airtight container, or by placing cut-side down on a plate. Foil should not be used, as it will react with acidic foods. Generally, the smaller you chop the food, the faster the rate of spoil will be, as there is a larger surface area for oxidation to occur. Some fruits and vegetables turn brown once they are exposed to air. You can prevent browning by brushing the cut surface with lemon juice before sealing. Citric acid lowers the pH level, which deactivates the enzymes. For large pieces, cut a lemon in half and rub over the whole surface area. For smaller pieces, juice a lemon and dip the pieces into the juice, removing and shaking off any excess.
|Fruits that turn brown when cut:||Vegetables that turn brown when cut:|
How do I store herbs in the refrigerator?
Cut herbs are delicate items. Stored in an inhospitable environment, even the most verdant and fragrant of bunches are leached of colour and flavour. Herbs are particularly sensitive to moisture. If they are exposed to too much moisture, they become slimy and rotten. If they do not have enough moisture, they dry out and lose their flavour. So storing herbs is all about striking a delicate balance between the two.
In this respect, herbs are best stored in the refrigerator, where humidity levels can be carefully controlled. I find the best solution is to store herbs all together in one large, upright container with an airtight lid –preferably one that has a removable freshness tray at the bottom. Line the bottom of the container with a double layer of kitchen towel, place the herbs on top, then cover with a double layer of kitchen towel before snapping on the lid. This allows some air to circulate, but traps enough moisture to keep the herbs fresh and hydrated. If humidity levels start to build up, and moisture droplets form inside, simply leave out of the refrigerator with the lid off for an hour or so to air.
To prepare herbs for storage, first remove any packaging or ties, as closely-packed bunches and plastic bags trap moisture. Next, sort the herbs according to whether they are soft herbs with tender stems, or hard herbs with woody steams. Soft herbs are prone to yellowing and drying out, whereas hard herbs are prone to browning and rotting. Soft herbs can be placed loose in the airtight container, whereas hard herbs should be wrapped loosely in a piece of kitchen towel to absorb excess moisture.
|Soft Herbs:||Hard Herbs:|
* do not refrigerate
The exceptions to the rule are Basil and Mint, which are too sensitive to the cold, and will turn brown if kept in the refrigerator. They are best treated like cut flowers and stored at room temperature in a jam jar, with a couple of inches of water at the bottom. Trim the stems first, as this removes any dried and sealed ends that will prevent water from being taken up. Store out of direct sunlight and away from heat sources. Change the water as soon as it starts to discolour and smell, but realistically you need to eat basil and mint on the day of purchase. Basil is too delicate to grow in a British garden, but mint is incredibly prolific and loves a wet climate, so consider growing a pot in the garden.
How do I store cooked leftovers in the refrigerator?
There is never a good reason to throw away perfectly good leftover food, but this is particularly the case when you follow the Paleo diet, where the demarcation between breakfast, lunch and dinner food is somewhat blurred. Even a couple of spoonsful of leftover curry can provide a welcome sop to a breakfast omelette. Indeed, aromatic foods such as curries and stews actually improve in flavour, as the spices release into the oil and meld together to marinate the meat. Rather than one or two flavour notes dominating, you get a much more rounded and complex taste. So any meal that contains aromatic ingredients such as garlic, ginger, onion, spices and herbs will continue to develop flavour in the refrigerator.
If you are lucky enough to have leftovers, they should be transferred to an airtight container that will keep out bacteria and prevent flavour transference. Store for a maximum of five days on the middle shelf of the refrigerator, where they will be in eye-line and so are less likely to be forgotten. When you are ready to eat, reheat the food until it is piping hot throughout, as this will kill the bacteria that cause food poisoning. For best results, use the original cooking method to reheat the food, as this should bring the food most closely back to its fresh state. For example, reheat fried foods in a frying pan and casseroles in the oven.
Liquid and semi-liquid braised dishes such as stews, gravies and soups should be brought to a rolling boil. Braised meat dishes often set when they have been kept in the refrigerator, as the protein fibres from the meat get broken down and released into the sauce. You will not normally need to add any water during reheating, as the cooking sauce will liquefy under gentle heat. However, be aware that meat sauces get consistently thicker each time they are cooled and reheated.
Dry dishes such as steamed vegetables or slices of meat should be heated until steaming hot. To stop the food from drying out, keep covered during reheating to lock in the moisture. You may even need to add moisture by wetting your hand under the tap and sprinkling over the food. Roasted or grilled meats can be wrapped in foil before being placed in the oven. Better yet, add a few tablespoons of stock to leftover meat before you put it in the refrigerator – as the meat cools, it will absorb some of the liquid. If the meat is completely fry and unpalatable, then chop up and fry in oil to rehydrate.
Can I put hot food straight into the refrigerator?
Many of us were brought up to believe that it was anathema to put hot food directly into the refrigerator. Food needed to be brought down to room temperature before it could be safely transferred. However, refrigerator technology has come on a lot since then, and modern refrigerators are perfectly capable of absorbing a sudden rise in temperature without overheating.
It is much more important to ensure that food is refrigerated within 2 hours. This is because the optimum temperature for bacteria to reproduce is between 5°C-60°C / 40°F-140°F – the so-called ‘danger zone’. The longer that food sits uncovered on a kitchen worktop, the greater the rate of bacterial growth. However, from an energy efficiency perspective, it is still best practice to bring the temperature of cooked food down as much as possible, and preferably to the point where it is just warm to the touch. You can speed up the cooling process by taking a couple of shortcuts. Firstly, increase the surface area by decanting liquid and semi-liquid braised dishes into shallow containers, and breaking down roasts. Secondly, decrease the surrounding temperature by placing in a sink filled with ice cold water.
How long can I keep food in the refrigerator before it goes bad?
Packaged foods that have a short shelf-life are stamped with a ‘use by’ date, which advises that the food may not be safe to eat after this date. Packaged foods that have a longer shelf-life are stamped with a ‘best before’ date, which advises that the food will deteriorate in quality after this date. However, it is possible for food to spoil before an official expiration date, or indeed to remain perfectly edible for some time after. To further complicate matters, loose produce is not stamped with expiration dates at all.
This is where olfactory and common sense come into play. Helpfully, spoilage odours are often produced as a chemical by-product when bacteria multiply and break down food. So if food smells bad or tastes bad, then throw it away. If it seems fine, then save for another day. Obviously it is useful to know how long you can expect different foods to keep for, as this will help you to plan meals and limit food waste. But I repeat that common sense should prevail over rigid rule-following, and that eating food that is beyond its best but still within safe bounds is preferable to throwing away perfectly good food. For example, fish should be eaten on the day of purchase when it is best, but that does not mean you should throw out a beautiful tiger-striped mackerel the next day because you ended up eating something else for dinner.
|Expected Shelf Life:
(best before, but may keep longer)