Several food preservation techniques involve heating, but the most widely recognized are pasteurization, sterilization (including canning), and blanching. Each uses a different temperature range and duration to achieve a specific goal, from killing dangerous bacteria to deactivating enzymes that cause spoilage. Heat-based preservation works by destroying microorganisms or their spores before they can multiply and break down food.
Pasteurization
Pasteurization is the most common heat-based preservation method and the one most people encounter daily through milk, juice, and other beverages. It uses moderate heat to kill harmful bacteria without drastically changing the food’s flavor or nutritional value. There are two standard approaches: heating to 63 °C (145 °F) for 30 minutes in a batch process, or heating to at least 72 °C (161 °F) for just 15 seconds in the high-temperature short-time (HTST) method used by most commercial dairies. HTST processing runs milk continuously through a heat exchanger, making it far more efficient for large-scale production.
A step beyond standard pasteurization is ultra-high temperature (UHT) processing, which heats milk above 135 °C (275 °F) for a few seconds. This destroys virtually all microorganisms, giving UHT milk a shelf life of 34 to 36 weeks when stored at room temperature or below. That’s why you’ll find UHT milk in unrefrigerated cartons at the grocery store. The tradeoff is a slightly cooked flavor that some people notice.
There’s also a gentler option called thermization, used mainly in traditional cheesemaking. It heats milk to just 57 to 68 °C for at least 15 seconds. This is mild enough to preserve much of the milk’s natural bacterial community, which cheesemakers rely on for developing flavor, while still reducing pathogen levels.
Sterilization and Canning
Canning takes heat preservation further than pasteurization by aiming to eliminate nearly all microorganisms, including heat-resistant bacterial spores. The primary concern is a spore-forming bacterium that produces a deadly toxin responsible for botulism. These spores cannot grow in acidic environments below a pH of 4.6, which is why the acidity of a food determines exactly how it needs to be canned.
High-acid foods like pickles, most fruits, and tomatoes (when their pH is confirmed to be low enough) can be safely processed in a boiling water bath at 212 °F (100 °C). A typical batch of pickles needs 10 to 20 minutes at that temperature. The heat kills active bacteria, and the food’s own acidity prevents any surviving spores from germinating.
Low-acid foods, including vegetables, meats, and soups, require a pressure canner that reaches 240 °F (116 °C). This is the temperature needed to destroy dangerous spores outright. Processing times are much longer: canned meat, for example, needs 75 to 90 minutes under pressure depending on jar size. Skipping the pressure canner for low-acid foods is one of the most common and dangerous mistakes in home food preservation.
Blanching
Blanching is a brief heat treatment used primarily before freezing or dehydrating fruits and vegetables. Its main purpose isn’t to kill bacteria but to deactivate enzymes that would otherwise cause off-flavors, color changes, and texture loss during storage. Plants contain several heat-resistant enzymes, and food scientists typically test for one called peroxidase to confirm that blanching was thorough enough, since it’s the most heat-resistant and widely present enzyme in plant tissue.
The process is simple: vegetables are submerged in boiling water or exposed to steam for a short time (usually one to several minutes depending on the food), then immediately cooled in ice water to stop the cooking. Without this step, frozen vegetables develop unpleasant flavors and mushy textures within weeks, even at proper freezer temperatures.
Heat-Assisted Dehydration
Drying food is one of the oldest preservation methods, and most modern dehydration relies on controlled heat. The general target temperature is around 140 °F (60 °C), which sits at the upper boundary of the temperature range where harmful bacteria can multiply. Maintaining this temperature removes moisture quickly enough to prevent bacterial growth while the food dries. Fruits like bananas are typically dried at around 135 °F, while meats and jerky often need slightly higher starting temperatures to ensure safety. The key principle is speed: the faster moisture drops to a level that can’t support microbial life, the safer the final product.
Tyndallization
A lesser-known technique called tyndallization uses repeated heating cycles to sterilize foods or liquids that can’t tolerate the extreme temperatures of pressure canning. Instead of one intense blast of heat, the substance is heated to below 100 °C three separate times, with rest periods in between. During each rest period, any surviving spores are allowed to germinate into their vulnerable, active form. The next round of heating then kills those newly active cells. By cycling through this process three times, tyndallization can achieve sterilization without ever exceeding boiling temperature. It’s rarely used in commercial food production today but remains relevant in certain laboratory and specialty food applications.
How Heat Affects Nutritional Value
Every heat-based preservation method involves some nutrient loss, and the extent depends on temperature, duration, and the specific nutrient. Vitamin C is the most vulnerable. It’s both water-soluble and highly sensitive to heat, and boiling can destroy anywhere from 26% to nearly 100% of the vitamin C in vegetables, with leafy greens like chard losing the most. B vitamins and folate are also susceptible, though typically to a lesser degree.
Fat-soluble vitamins (A, D, E, and K) and most minerals hold up better under heat. The practical takeaway is that shorter, higher-temperature methods like HTST pasteurization tend to preserve more nutrients than prolonged heating. Canned foods, which undergo the most intense heat treatment, lose the most vitamins during processing but remain nutritionally valuable overall, especially for minerals and fiber that heat doesn’t affect.