In the days before ripe Chilean tomatoes in the snowiest of winters, year-round beets, and the never-ending zucchini season, if you had a hankering for a summer vegetable in the middle of January, you had to wait six months. That is until 1810, when canning was invented and along with it, the possibility of anticipating your winter desires two seasons ahead.
Compared with the sourness of fermentation, the desiccated texture of drying, or the cloyingness of candying, canning altered the flavor and texture of preserved produce only slightly, a technological innovation that turn-of-the-19th-century society went mad for.
In basic terms, canning is the heating of food isolated in hermetically sealed containers. The heat deactivates plant enzymes and kills off harmful microbes. Along with a tight seal preventing re-contamination, food properly tucked into a canning jar can be stored at room temperature without spoiling.
1858 was a watershed year for home-canning. Prior to this date, home-canners had to use ‘wax sealers’, which featured glass tops and bottoms sealed together with wax. It sounds quaint, but in practice was time consuming and error prone. And discovering just before dinner that instead of a jar of fresh from the vine tomatoes, you had in fact been storing moldering mush, is a sad error indeed. When the young tinsmith John L. Mason invented the threaded lip and two-piece sealing lid, home-canning became far more foolproof.
Mason jars are made with thicker glass than single use commercial jars, allowing them to withstand the boiling temperatures of the canning process without cracking. The sealing compound on the lids creates a barrier impenetrable to microbes, and the screw band holds the lid in place. After boiling to create the seal, pressure form outside on the cooling lids makes a taut indentation. Press down on the lid, and if pops, the jar hasn’t sealed and you know to either give it another shot, or put the jar in the refrigerator and get to eating it quickly.
Hot on the heels of the success of mason jars, in 1882 Henry William Putnam filed a patent that combined the all-glass construction of ‘wax sealers’ with the gasket seal of mason jars. Called lightning jars, Putnam’s invention kept food from touching metal and had easy to open wire closures. Lightning jars were made for home canning up until the 1960s, and these more beautiful, decorative jars still use the wire clasp today and the aesthetic appeal of all-glass canning jars continue to maintain a strong following.
Canning is a satisfying activity. Making a pile of produce into a neat line of shiny, filled jars can make for a wholesome Sunday afternoon. All sorts of fruits and vegetables can be canned, from oranges and plums, to kale and beets. In the winter months, I prefer my canned tomatoes to the tasteless out-of-season varieties, and I don’t mind the constant task of preserving or jellying the beautiful yellow jewels of my over-active Meyer lemon tree.
Produce for canning is minimally processed. Lemon marmalade may need a few extra steps to develop pectin and candy the sugar, but tomatoes need only to be peeled and stuffed into jars. The majority of the work goes into sterilizing and processing the jars. In other words: boiling. You have to boil the glass and the lids before filling them, fish them out of hot boiling water (fix a magnet onto a stick to get the metal lids out, and get your tongs ready for the jars), then boil the filled jars again in a canner to create the all-important seal. Under-processing can result in spoiled food, and over-processing may overcook the vegetables. Pay attention to the recommended boiling times to get it right.
Of course the last thing you want to think about while you dream of canning is botulism, but it is important to be aware of its causes. Clostridium botulinum thrives in low-acid, airless conditions. Like most toxins, it is killed off by boiling. However, the spores are hardy and can survive prolonged boiling and proliferate into active bacteria as the cans cool down. Bulging caused by the pressure of gasses produced by the bacteria is a clear indicator, and those should be thrown out. Boiling the food again after opening will also kill the toxin. The high acid content of most tomatoes and citrus fruit inhibits the growth of the bacteria, but other vegetables with pH of 5 or 6 should be processed in a pressure canner for 30 to 90 minutes at 240°F to kill any possible spores.