Food preservation involves treating food to maintain safety, but also to prolong the enjoyment of foods long after harvest. Food spoilage can render food unfit for consumption, caused by microorganism contamination, insect infestation, or enzymes naturally present within the food causing rancidity. Other factors contributing to food spoilage include physical and chemical changes that ultimately lead to food spoilage, such as oxidation. Among the many ways food can be preserved are canning, dehydration, freezing, refrigeration, all of which are detailed below with further resources to assist you in your preservation journey.

Refrigeration

The most common method of food preservation is low-temperature preservation which extends the shelf-life of foods. Refrigeration is a common method of low-temperature preservation, where temperatures are kept below 40 degrees Fahrenheit (4 degrees Celsius), creating an ideal environment to store foods such as fruits, vegetables, eggs, dairy, and meats. The low temperature delays the growth of microorganisms such as bacteria and fungi (mold). A refrigerator thermometer should be used to ensure the appliance is maintaining a temperature below 40 degrees Fahrenheit (4 degrees Celsius) and can be found cheaply at most large stores and online. More information about maintaining the safety of foods using refrigeration can be found through the USDA here.

Freezing

An excellent way to preserve the nutritional quality of food, temperatures should always be below 32 degrees Fahrenheit (0 degrees Celsius). Freezing inactivates microbes but does not kill microbes such as bacteria, yeasts, and molds; once thawed, the microbes can become active once more, necessitating proper handling to prevent foodborne illness. Foods should be frozen at the peak of freshness, with individual foods requiring special treatment like hot-water blanching prior to freezing. Additionally, if the freezer temperature is allowed to fluctuate, the ice crystals in food tend to enlarge, affecting the texture and palatability of food.  Foods should be tightly wrapped and packaged to prevent moisture loss (freezer burn) which results in surface dehydration. In the event of a power failure: a full freezer may maintain safe food for about 2 days, according to the USDA, but only if the door is kept shut.  A half-full freezer may maintain safe food for about 1 day. Please read more here for information about freezer preservation and safety from the USDA.

Canning

Canning dates back to the early 1800’s, but it wasn’t until 1896 when Clostridium botulinum was identified as the microorganism that produces a lethal toxin that causes botulism.  This discovery led to careful guidance to safely preserve foods by canning, beginning with pre-sterilization procedures to ensure the vessels in which the food will be canned are free from any contamination and the cleaned food to be heat-preserved is free from air pockets. The sterilization process considers time and temperature based on the typical microorganisms found on a particular food, the size of the container, the acidity of the food, and the method of heating.  Following the directions of a reputable source for recipes and directions is critical to avoid the lethal contamination that results in improper canning. Differing directions, such as water-bath canning vs. pressure canning, are provided for high-acid foods (pH less than 4.6) compared to low acid foods (pH greater than 4.6). Canned foods are best when consumed within a year and should be stored away from direct sunlight in a cool, dry place. Some nutrients may be lost during the prolonged heat of canning, such as vitamin B1 and vitamin C. Any bulging cans should be immediately discarded as it is a sign of the presence of microorganisms such as deadly Clostridium botulinum within the can, which produce gas as they metabolize the food within the can. Remember to always follow a recipe from a reputable source. More information about safely canning can be found here.

Pasteurization

Raising the heat of food and holding it at a specific temperature to kill microorganisms and inactivate enzymes is called pasteurization. Depending on the pH of the food, pasteurization times and temperatures vary, such as the case of fruit juices compared to dairy products. Following a heat treatment, a rapid cooling follows to bring the liquid back into a temperature-safe zone where bacterial growth is limited. Refrigeration is still necessary following pasteurization as the heat treatment is not adequate to render the liquid sterile. More information on pasteurization can be found here.

Blanching

Mostly used for vegetables, blanching is accomplished with either steam or a water bath for two to five minutes, depending on the food. Blanching cleans the product, deactivates enzymes that lead to spoilage of frozen foods, encourages trapped gases to exit the food-matter, and reduces the number of microbes on the food. Information on blanching can be found here.

Dehydration

Reducing the water-content of foods can significantly reduce microorganism activity, leading to the practice being applied to foods such as fruits, vegetables, skim milk, legumes, and even meats. Since foods can still pose a risk when rehydrated and consumed, the control of contamination in the dehydration process is critical. High-quality raw material may require pasteurization prior to dehydration, and safe storage practices afterward are necessary to prevent contamination from rodents, insects, or other environmental contamination. Air dehydration is the most common in-home preservation, but dehydration can also be done by freeze-drying at very low temperatures in carefully controlled processing conditions.

Fermentation and Pickling

Harnessing the power of beneficial microorganisms can yield a delightful array of foods such as sauerkraut, kombucha, and endless varieties of pickles. Under the right conditions, following directions accounting for salt and acidity, the growth of microorganisms can be controlled to preserve certain foods. For directions from state extension agencies, please see the attached information here, including a kombucha recipe.

Chemical Preservation

Although it may sound nefarious, included in this segment are preservation methods that include the addition of sugar, alcohol, salt, and acids, such as vinegar. Among other chemical preservatives are antioxidants, bleaching agents, acidulants, neutralizers, stabilizers, firming agents, and humectants. These chemical preservatives can include benzoates, although their use is limited in most countries due to negative health impacts. Other organic compound preservatives commonly used are vanillic acid esters, monochloroacetic acid, propionates, sorbic acid, dehydroacetic acid, and glycols.  Inorganic chemical preservatives include sulfur dioxide and sulfites, which commonly act in preventing molds and yeasts in fruits and vegetables. Oxidizing agents used in the curing of meats include nitrates and nitrites.

For more information, go to Cornell Cooperative Extension Herkimer County.