Food preservation involves preventing the growth of bacteria, fungi (such as yeasts), or other micro-organisms (although some methods work by introducing benign bacteria or fungi to the food), as well as retarding the oxidation of fats that cause rancidity. Food preservation may also include processes that inhibit visual deterioration, such as the enzymatic browning reaction in apples after they are cut during food preparation.
Cooling preserves foods by slowing down the growth and reproduction of micro-organisms and the action of enzymes that cause food to rot. The introduction of commercial and domestic refrigerators drastically improved the diets of many in the Western world by allowing foods such as fresh fruit, salads and dairy products to be stored safely for longer periods, particularly during warm weather.
Freezing is also one of the most commonly used processes, both commercially and domestically, for preserving a very wide range of foods, including prepared foods that would not have required freezing in their unprepared state. For example, potato waffles are stored in the freezer, but potatoes themselves require only a cool dark place to ensure many months’ storage. Cold stores provide large-volume, long-term storage for strategic food stocks held in case of national emergency in many countries.
Heating to temperatures which are sufficient to kill microorganisms inside the food is a method used with perpetual stews. Milk is also boiled before storing to kill many microorganisms.
Salting or curing draws moisture from the meat through a process of osmosis. Meat is cured with salt or sugar, or a combination of the two. Nitrates and nitrites are also often used to cure meat and contribute the characteristic pink color, as well as inhibition of Clostridium botulinum. It was a main method of preservation in medieval times and around the 1700s.
The earliest cultures have used sugar as a preservative, and it was commonplace to store fruit in honey. Similar to pickled foods, sugar cane was brought to Europe through the trade routes. In northern climates without sufficient sun to dry foods, preserves are made by heating the fruit with sugar “Sugar tends to draw water from the microbes (plasmolysis). This process leaves the microbial cells dehydrated, thus killing them. In this way, the food will remain safe from microbial spoilage.” Sugar is used to preserve fruits, either in ananti-microbial syrup with fruit such as apples, pears, peaches, apricots and plums, or in crystallized form where the preserved material is cooked in sugar to the point of crystallization and the resultant product is then stored dry.
Smoking is used to lengthen the shelf life of perishable food items. This effect is achieved by exposing the food to smoke from burning plant materials such as wood. Smoke deposits a number of pyrolysis products onto the food, including the phenols syringol, guaiacol andcatechol. These compounds aid in the drying and preservation of meats and other foods. Most commonly subjected to this method of food preservation re meats and fish that have undergone curing. Fruits and vegetables like paprika, cheeses, spices, and ingredients for making drinks such as malt and tea leaves are also smoked, but mainly for cooking or flavoring them. It is one of the oldest food preservation methods, which probably arose after the development of cooking with fire.
Pickling is a method of preserving food in an edible anti-microbial liquid. Pickling can be broadly classified into two categories: chemical pickling and fermentation pickling.
In chemical pickling, the food is placed in an edible liquid that inhibits or kills bacteria and other micro-organisms. Typical pickling agents include brine (high in salt), vinegar,alcohol, and vegetable oil, especially olive oil but also many other oils. Many chemical pickling processes also involve heating or boiling so that the food being preserved becomes saturated with the pickling agent. Common chemically pickled foods include cucumbers, peppers, corned beef, herring, and eggs, as well as mixed vegetables such as piccalilli.
Sodium hydroxide (lye) makes food too alkaline for bacterial growth. Lye will saponify fats in the food, which will change its flavor and texture. Lutefisk uses lye in its preparation, as do some olive recipes. Modern recipes for century eggs also call for lye.
Canning involves cooking food, sealing it in sterile cans or jars, and boiling the containers to kill or weaken any remaining bacteria as a form of sterilization. It was invented by the French confectioner Nicolas Appert. By 1806, this process was used by the French Navy to preserve meat, fruit, vegetables, and even milk. Although Appert had discovered a new way of preservation, it wasn’t understood until 1864 when Louis Pasteur found the relationship between microorganisms, food spoilage, and illness.
Foods have varying degrees of natural protection against spoilage and may require that the final step occur in a pressure cooker. High-acid fruits like strawberries require no preservatives to can and only a short boiling cycle, whereas marginal vegetables such as carrotsrequire longer boiling and addition of other acidic elements. Low-acid foods, such as vegetables and meats, require pressure canning. Food preserved by canning or bottling is at immediate risk of spoilage once the can or bottle has been opened.
Food may be preserved by cooking in a material that solidifies to form a gel. Such materials include gelatin, agar, maize flour, and arrowroot flour. Some foods naturally form aprotein gel when cooked, such as eels and elvers, and sipunculid worms, which are a delicacy in Xiamen, in the Fujian province of the People’s Republic of China.
Meat can be preserved by jugging. Jugging is the process of stewing the meat in a covered earthenware jug or casserole. The animal to be judged is usually cut into pieces, placed into a tightly-sealed jug with brine or gravy, and stewed. Red wine and/or the animal’s own blood is sometimes added to the cooking liquid. Jugging was a popular method of preserving meat up until the middle of the 20th century.
Burial of food can preserve it due to a variety of factors: lack of light, lack of oxygen, cool temperatures, pH level, or desiccants in the soil. Burial may be combined with other methods such as salting or fermentation. Most foods can be preserved in soil that is very dry and salty (thus a desiccant) such as sand, or soil that is frozen.
Fermented foods are created by allowing one type of microbe to act on a food substance in order to convert some of its components into alcohols or acids. Alcohols are fermented by yeasts, while most foods are fermented by lactic acid bacteria. Some mold-ripened cheeses are created by the work of fungi, and other cheeses are fermented by the work of bacterial cultures. This family of preserved foods includes some of the world’s greatest culinary treasures, including bread, cheese, pickles, sauerkraut, chocolate, beer, coffee, wine and a whole host of cured meats, to name but a few.
The bacteria, yeasts and fungi necessary to ferment different foods can be naturally occurring or wild, as it was for each of these marvelous foods to have been discovered in the first place. Or it can be purposefully cultured with ingredients obtained via cheesemaking or home brew suppliers. To try fermentation at home, start with the simple recipe for sauerkraut below.
Many foods last longer if they are simply dunked in a bath of vinegar. Just as vinegar rids dirty clothes and kitchen countertops of infectious germs, it can be put to the same—though tastier—use with fruits, vegetables and herbs. The most famous vinegar-preserved foods are cucumbers (though some cucumber pickles are actually fermented), but many other foods are delicious in vinegar, too—turnips, beets, radishes, carrots, leeks, kale, garlic scapes, Swiss chard, green beans, cauliflower, zucchini, peppers, asparagus, cantaloupe and green tomatoes are just some of the options that are delicious when pickled. From balsamic and apple cider to rice and champagne, a wide world of vinegar flavors awaits. The recipe above provides a supersimple way to preserve fresh cucumbers using acidification.
Dehydrating food makes it less attractive to moisture-loving bacteria. Removing the water also concentrates flavors in a mighty tasty way, and it’s a fitting trick for fruits and veggies of all kinds. Dried foods take up the least pantry space of all the preserved treats you might make, and the benefactors of this technique are limited only by your imagination. Food dehydrators make easy work of drying food, but it’s easy to do in an oven set on low heat, too. Once they’re dried to a crisp, store foods in an airtight container. Some foods, such as plum tomatoes, are also great halfway dried and then stored in oil.
The practice of “putting food by” refers to storing produce for long periods of time, and it is often quite simple. Many foods will last weeks or months if kept in a cool, dark spot. Some foods benefit from being coated first in oil or being stored in a bucket of sand; others will keep when simply set on a shelf. You can build a fancy ventilated root cellar if you’ve got the space, time and inclination, but a cool, dark corner of your garage or basement will probably do nicely.
From simple canned tomatoes to homemade soups, canning is a great way to preserve the peak-harvest flavors of many fresh foods. However, it is extremely important to do it right, so be sure to learn the basics. While canning is simple and safe with the proper instruction, improperly canned foods can make you and your family sick. The easiest method of canning is called water-bath canning, and it’s a great way to preserve acidic foods such as pickles and tomatoes. For nonacidic foods, you will need to rely on the more complex system of pressure canning. We recommend starting with the easier water-bath canning to learn the technique before moving on to pressure canning.
Freezing helps prevent food from spoiling before we’re ready to eat it. Several tricks and tips can improve your freezer strategies. Many foods freeze well, but a few—such as lettuce, cream sauces and whole eggs in shells—really just don’t. Obviously, it’s helpful to know the difference. Some foods, such as blueberries, can be frozen as is; others, such as greens, must be blanched first. To blanch, bring a pot of water to boil, dunk food for a brief time, then pat the food dry and freeze in freezer proof containers. To make efficient use of freezer space, try freezing liquids such as soups in baggies laid flat on a baking sheet. Once frozen, they can be stacked neatly elsewhere in the freezer. You can also fill ice cube trays with sauces such as pesto and then pour the cubes, ready for single-serving uses, into a freezer container.