Food Preservation Guide: Canning, Drying, and Fermenting
Food preservation is the practice of preventing the growth of microorganisms and slowing the chemical reactions that cause spoilage. Before refrigeration and global supply chains, preservation was essential for survival through winter months and long journeys. Today, it offers home cooks and food producers ways to reduce waste, enjoy seasonal foods year-round, and develop flavors that fresh ingredients cannot provide. The science behind each preservation method targets specific factors that microbes need to survive: water availability, temperature, pH, oxygen, and nutrients.
Each preservation method exploits vulnerabilities in the biology of spoilage organisms. Canning uses heat to destroy microorganisms and seals the container to prevent recontamination. Freezing slows microbial growth to a near halt by converting water into ice crystals, making it unavailable for biological processes. Drying removes water that bacteria and fungi require for metabolism. Fermentation creates acidic conditions that inhibit pathogenic bacteria while allowing beneficial microbes to thrive. Understanding the principles behind each method allows you to preserve food safely and effectively. The USDA Complete Guide to Home Canning provides authoritative guidance on safe practices that every home preserver should consult.
Canning
Canning is the most reliable method for long-term shelf-stable food preservation. It involves heating food in a sealed container to destroy spoilage organisms and create a vacuum seal that prevents new organisms from entering. Two distinct canning methods exist, and choosing the correct one is critical for food safety.
Water Bath Canning
Water bath canning is suitable only for high-acid foods with a pH of 4.6 or below. The high acidity naturally inhibits the growth of Clostridium botulinum, the bacterium that causes botulism. Foods suitable for water bath canning include most fruits, jams, jellies, pickles, tomatoes (with added acid), and fruit juices. The process involves submerging filled jars in boiling water for a specified time. The heat destroys molds, yeasts, and enzymes that cause spoilage while the acidity prevents bacterial spore germination.
The processing time depends on the food, jar size, and altitude. Above 1,000 feet, water boils at a lower temperature, so processing times must be increased to compensate. The valid processing times and altitude adjustments for each food are determined by USDA research. Shortcuts like reducing processing time or using improper jar sizes risk under-processing and potential foodborne illness. The jars must be sealed tightly, and the water must cover the jar lids by at least one inch throughout processing.
Pressure Canning
Pressure canning is required for low-acid foods with a pH above 4.6, including vegetables, meats, poultry, seafood, and soups. These foods do not have enough natural acidity to prevent Clostridium botulinum spore germination. Pressure canning achieves temperatures of 240 to 250 degrees Fahrenheit, well above the 212 degrees Fahrenheit boiling point of water. This higher temperature is sufficient to destroy botulism spores.
A pressure canner is not the same as a pressure cooker, though some appliances serve both functions. Pressure canners are designed to maintain precise pressure and temperature for the extended periods required for safe canning. The canner must have a functioning pressure gauge and a weighted vent to release excess pressure. Processing times for low-acid foods range from twenty to ninety minutes depending on the food, jar size, and altitude. It is essential to follow tested recipes from the USDA or the National Center for Home Food Preservation. Modifying recipes by changing ingredient ratios, adding thickeners, or substituting low-acid ingredients can create unsafe conditions.
Dehydration
Drying is the oldest method of food preservation, dating back at least 12,000 years. Removing water from food prevents the growth of bacteria, yeasts, and molds, all of which require moisture to reproduce. Dehydrated foods also concentrate flavors and reduce weight, making them ideal for snacks, camping, and long-term storage.
The Science of Drying
Microorganisms require water for their metabolic processes. The measure of available water in food is called water activity. Fresh produce has a water activity above 0.97, which supports rapid microbial growth. Dried foods have a water activity below 0.60, at which most bacteria cannot grow. Yeasts and molds can grow at water activities as low as 0.70 to 0.80, so foods must be dried sufficiently to achieve water activity below 0.60 for safe long-term storage.
The drying process involves three phases. In the first phase, surface moisture evaporates rapidly. In the second phase, moisture from the interior migrates to the surface and evaporates, but the drying rate slows. In the third phase, called the falling rate phase, moisture movement becomes very slow, and the food approaches equilibrium with the surrounding air. Increasing air temperature, reducing humidity, and increasing air flow all accelerate drying. However, temperatures above 140 degrees Fahrenheit can case-harden the food, where the surface dries and hardens while the interior remains moist, trapping moisture and leading to spoilage.
Drying Methods
Sun drying works in hot, dry climates with low humidity and requires several consecutive days of temperatures above 85 degrees Fahrenheit. Most home preservationists use electric dehydrators, which provide controlled temperatures between 95 and 155 degrees Fahrenheit. Oven drying at the lowest setting with the door slightly ajar is possible but less energy efficient. Freeze drying, a more advanced method, freezes the food and then sublimates the ice directly into vapor under vacuum, producing the highest quality dried food with minimal nutrient loss.
For a detailed look at the freeze drying process, see the freeze drying guide. For a comprehensive overview of dehydration techniques, visit the food dehydration article.
Fermentation
Fermentation uses beneficial microorganisms to preserve food by creating conditions that inhibit spoilage organisms. Unlike canning and freezing, fermentation does not sterilize the food. Instead, it creates an environment where carefully managed microbial activity prevents pathogens from establishing a foothold.
Lactic Acid Fermentation
In lactic acid fermentation, beneficial bacteria of the genus Lactobacillus consume sugars in the food and produce lactic acid. The accumulated acid lowers the pH of the food below 4.6, creating conditions where Clostridium botulinum and other pathogens cannot grow. Lactic acid fermentation preserves vegetables like cabbage (sauerkraut), cucumbers (pickles), and peppers (hot sauce) while also producing complex, tangy flavors.
The salt concentration in lactic acid fermentation is critical. At salt concentrations of two to three percent by weight, Lactobacillus species dominate while putrefactive bacteria are suppressed. At higher salt concentrations above five percent, fermentation slows dramatically, and the food is preserved primarily by the salt rather than by acid production. The fermentation temperature also affects which microorganisms thrive. Cooler temperatures between 60 and 65 degrees Fahrenheit produce cleaner, more lactic-acid-driven fermentations. Warmer temperatures above 70 degrees Fahrenheit allow a wider range of microorganisms to participate, producing more complex, funky flavors.
Acetic Acid Fermentation
Acetic acid fermentation, carried out by Acetobacter bacteria, converts ethanol into acetic acid — the main component of vinegar. This two-stage process begins with yeast fermentation that produces alcohol, followed by bacterial fermentation that converts the alcohol to vinegar. Wine, cider, beer, and other alcoholic beverages serve as the starting point for vinegar production. The acetic acid content of finished vinegar ranges from four to eight percent, which is sufficient to inhibit most spoilage organisms.
The relationship between fermentation and food safety is explored in depth in the food safety science article.
Freezing
Freezing is the most convenient preservation method for most home cooks. It preserves the texture, color, and nutritional value of foods better than any other method, provided the food is properly prepared and packaged. Freezing works by converting water into ice, making it unavailable for microbial growth and enzymatic reactions.
Freezer Burn and Quality
Freezer burn occurs when moisture on the surface of frozen food sublimates — turns directly from ice into vapor — exposing the food to oxygen. The affected areas become dry, tough, and discolored. Proper packaging that excludes air prevents freezer burn. Vacuum sealing is the most effective method. Alternatively, wrapping foods tightly in freezer paper, plastic wrap, and then foil provides a sufficient barrier for most home freezers.
Freezing speed affects the quality of frozen foods. Slow freezing produces large ice crystals that puncture cell walls, resulting in mushy texture upon thawing. Rapid freezing produces small ice crystals that cause less cellular damage. Commercial flash freezers freeze foods within minutes, but home freezers freeze slowly. To minimize ice crystal damage in a home freezer, set the temperature to zero degrees Fahrenheit or below, arrange food in a single layer so cold air circulates freely, and avoid overloading the freezer with unfrozen food at once.
FAQ
How long does home-canned food last?
Home-canned food retains peak quality for one to two years but remains safe indefinitely as long as the seal remains intact and the food shows no signs of spoilage. Store canned food in a cool, dark place between 50 and 70 degrees Fahrenheit for maximum shelf life. Discard any jars with bulging lids, leaking seals, or food that smells off when opened.
Can I reuse canning lids?
No, standard two-piece canning lids are designed for single use. The sealing compound on the lid forms a tight seal during processing and cannot be relied upon to seal again. The screw bands can be reused indefinitely as long as they are not rusted or bent. Some European-style canning systems use glass lids with rubber gaskets that are reusable when the gaskets are replaced.
Is it safe to can butter or flour?
No, butter and flour are low-acid foods that cannot be safely canned at home. Butter is a dairy product with insufficient acidity for water bath canning and excessive fat content, which insulates bacterial spores during pressure canning. Flour and other dry goods have very low water activity but should be stored in sealed containers in a cool, dry place, not canned.
What is the best way to store dehydrated food?
Store dehydrated food in airtight containers with oxygen absorbers in a cool, dark, dry place. Temperatures below 60 degrees Fahrenheit and humidity below 15 percent are ideal. Properly stored dehydrated foods retain quality for five to ten years or longer. Vacuum sealing provides the best protection against moisture and oxygen.
Why does my frozen fruit turn mushy when thawed?
The ice crystals formed during freezing puncture the cell walls of the fruit. When the fruit thaws, the cellular structure collapses and releases liquid, resulting in a soft, mushy texture. Rapid freezing produces smaller ice crystals and less damage. Fruits with high water content like strawberries and melons are most susceptible to texture changes from freezing.