Starvard.
Starvard University · Canning
Put up food safely by modern USDA rules: water bath for acid foods, pressure canning for low acid foods, and a clear eye for which old recipes to retire.
Hello, my name is Finn, and I'm going to help you understand this course.
Here it is, explained like you are 10:
Canning is a way to keep summer in a jar. You put good food in clean glass jars, heat them the right way, and the heat kills the tiny germs that make food spoil, while the lid seals shut so no new germs can get in. Sweet jams need only a bath of boiling water. Vegetables and meats need a special pressure cooker that gets much hotter, hot enough to kill the toughest germs of all. Follow the tested rules exactly, and your shelves fill with safe, delicious food you made yourself.
Food spoils because invisible living things, moulds, yeasts, bacteria, and the enzymes inside the food itself, keep working after harvest. Canning stops them with two ideas: enough heat, held long enough, to destroy the spoilage agents in that particular food, and a hermetic seal that keeps new ones out while the jar sits on the shelf. The United States Department of Agriculture tested these processes in laboratories and publishes the only complete, current guide home canners should trust.
One number runs the whole subject. Foods with a pH of 4.6 or lower are acid foods, and boiling water at 212 degrees F is hot enough to make them safe. Foods above pH 4.6 are low acid, and they can shelter Clostridium botulinum spores, which survive boiling and can grow in a sealed jar without air. Only a pressure canner, reaching 240 to 250 degrees F, destroys those spores.
Older books, including Sarah Tyson Rorer's charming 1887 guide, used methods science has since retired. This course teaches you to read those old recipes with respect and still follow modern rules, every single time.
The whole course in brief
Safety first, always: the USDA rules in this course are the line between a proud pantry and a dangerous one.
The seven lessons
Lesson 1
Fresh food carries moulds, yeasts, and bacteria on its skin and in the air around it, and its own enzymes keep ripening it past ripe into rot. Left alone, those agents sour, soften, mould, and sometimes poison food. Canning answers both problems at once: heat destroys the spoilage agents and stops enzyme action, and a sealed jar keeps new agents out for months or years of shelf storage.
The heat a food needs depends on its acidity, measured as pH. The USDA dividing line is pH 4.6. Acid foods, at pH 4.6 or below, include most fruits, jams, jellies, and properly made pickles. In that acid environment, the dangerous bacterium Clostridium botulinum cannot grow, and boiling water processing makes the food safe. Low acid foods, above pH 4.6, include vegetables, meats, poultry, seafood, and soups. Their jars have no acid protection, and botulism spores inside them survive boiling water, so only the higher heat of a pressure canner, 240 to 250 degrees F, destroys the spores. Botulism toxin gives no reliable smell, taste, or bulge warning, which is why the rules are not negotiable.
Lesson 2
Safe canning needs the right vessels. A boiling water canner is a deep pot with a fitted lid and a rack that keeps jars off the bottom and lets water circulate, with room for water to cover jars by 1 to 2 inches. A pressure canner is a heavy pot with a locking lid, a rack, and either a dial gauge or a weighted gauge that holds steam pressure at 10 to 15 PSI. Have a dial gauge tested for accuracy each year. A jar lifter, a wide mouth funnel, a bubble remover, a lid wand, and a clean timer complete the basic set.
Use only jars made for home canning, and inspect every jar before filling: run a finger round the rim and reject any jar with nicks, chips, or cracks, because flaws stop seals and invite breakage. Wash jars in hot soapy water, keep them hot until filling, and prepare modern two piece lids exactly as the maker directs, usually warming the flats in hot, not boiling, water. Never reuse a flat lid, and never use old zinc caps, glass bail lids with rubber rings for processing, or paraffin as a seal.
Lesson 3
Jams and jellies are acid fruit preserves, so they belong in the boiling water bath. Work from a tested recipe and do not change the fruit, sugar, pectin, or acid amounts, because the set and the safety both depend on them. Cook the fruit as directed, skim, and test for the gel stage. Fill hot, sterilised jars at once, leaving a quarter inch of headspace, wipe the rims spotless, set on warmed flats, and screw rings finger tight, snug but not forced.
Adjust process time for altitude, because water boils cooler as elevation rises. That adjustment is part of the recipe, not an extra.
Lesson 4
Pickles are vegetables made safe by added acid. Cucumbers, peppers, and green beans on their own are low acid foods, well above pH 4.6, but a tested pickling recipe brings the whole jar to pH 4.6 or below with vinegar of known strength, and that is what allows water bath processing. Use vinegar labelled 5 percent acidity, use the exact vinegar, water, salt, and vegetable proportions in a tested recipe, and use canning or pickling salt, because table salt additives can cloud brine.
Never dilute the vinegar in a recipe, never reduce the acid to suit your taste, and never invent a salsa or pickle mixture and can it by guesswork, since the thickest part of the jar is what must reach safe acidity. If a pickle tastes too sharp, adjust the next batch by choosing a different tested recipe, not by weakening this one. For crisp pickles, start with fresh produce, trim a thin slice from the blossom end of cucumbers, which carries softening enzymes, process promptly, and do not overprocess. Old recipes that pickle in weak vinegar, brine alone, or crocks of uncertain strength are for reading, not for canning shelves.
Lesson 5
Vegetables, meats, poultry, seafood, and soups are low acid foods, and boiling water cannot make them safe, because botulism spores survive 212 degrees F. The pressure canner solves this with steam under pressure, reaching 240 to 250 degrees F at 10 to 15 PSI depending on altitude and gauge. Follow a tested recipe for food, jar size, pack, pressure, and time, and never shorten any of them.
Keep the gauge honest, follow the altitude adjustment for your home, and write down what you canned and when.
Lesson 6
Sarah Tyson Rorer's Canning and Preserving, from 1887, is delightful and often shrewd: choose only sound, fresh fruit, she insists, work cleanly, and examine every jar after a week. Keep those habits. But her processes predate botulism science, and several standard methods are now plainly unsafe.
Rorer canned lima beans, peas, asparagus, and corn by boiling jars for three hours in a wash boiler, which is water bath canning of low acid vegetables. Boiling water never reaches the 240 to 250 degrees F those foods require, so that method is retired: pressure can those vegetables or do not can them at all. Her tomatoes were cooked and sealed boiling hot with no timed process, an open kettle style method the USDA no longer accepts for any food. Her jelly directions seal glasses with paper, bladder skin, or the jelly's own surface, and period practice widely used paraffin on top: none of those is a hermetic seal, so use two piece lids and a timed water bath instead. Oven canning appears in other period books and is also banned by modern rules, because oven heat is uneven and jars can explode.
Lesson 7
Canning is one tool in a bigger pantry. Freezing is the simplest safe method for many foods: blanch vegetables briefly in boiling water to stop enzyme action, cool them fast in ice water, drain, pack in freezer containers with a half inch of headspace for expansion, and freeze at 0 degrees F or below. Fruits freeze well packed in sugar, syrup, or dry, on trays first for loose packs. Drying removes the moisture microbes need: dry herbs, fruit leathers, and vegetables in a dehydrator or low oven until brittle or leathery as the directions specify, then condition dried fruit in jars for a week, shaking daily, before final storage.
Store sealed canning jars with the rings removed, so a failing seal cannot be held shut by its ring, in a clean, cool, dry, dark place at 50 to 70 degrees F. Label every jar with contents and date, use the oldest first, and for best quality use home canned food within one year. Before serving, check each jar again: the lid must be firmly sealed and concave, with no leaks, rising bubbles, spurting liquid, mould, or off odour. When in doubt, throw it out, without tasting.
Sources
This course is taught from classic texts that are free for anyone to read. Every method in the lessons traces back to one of these books.
Graduation
You now know the one line that runs all of canning, pH 4.6, and the two processes that honour it: boiling water for acid foods, pressure at 240 to 250 degrees F for low acid foods. Keep tested recipes close, label and date every jar, and store them cool and dark. A shelf of food you canned safely is one of the most satisfying sights a home can offer.
Keep learning
This course is part of Starvard University, striving for greatness.
A note on accuracy: Much of what you read at Starvard was created with help from AI agents, working alongside people. We check what we can, but we cannot guarantee that everything here is true, current, or complete. If something really matters, your health, your money, a grade, a job, please verify it with a trusted source before you rely on it.