In an era of grocery stores and home refrigerators, it is easy to lose sight of the fact that, for most of history, people have been bound by the seasonality of food. This reality has long presented humanity with a conundrum: how to keep eating after the harvest is over?
In food-insecure regions of the world, the "hunger gap," or period of time between when the previous season's food resources run low and when the next harvest arrives, remains a real and persistent issue. A good harvest-and the ability to make it last as long as possible-has been a song of life and death for humanity for millennia.
Methods of food preservation to address lean seasons throughout history have taken fascinating twists according to region and culture. There are the much beloved stinky cheeses of France, the tart sauerkraut of Germany (which actually originated in China), miso paste in Japan, salted fish in Iceland and the cured meats of multiple cultures stretching into ancient history, to name just a few. Some methods of preservation have sought to minimize microbe involvement by making foods hostile to microbial settlement, while others have embraced microbial life to make many of the specialties we know and love today.
Warding Off Microbes: Curing, Drying and Canning
It may be no coincidence that some of the key components of flavor are also things that help protect food from microbial misanthropes! Let's look at some time-tested ways to make food hostile to microbes while still keeping it edible to humans.
Salt and Sun: Curing and Drying
Salting fish and meat goes back millennia in human history and is featured in many cultures around the world. Hunting or slaughtering a large animal provides a lot of nutrient-rich meat at once-much more than an average family can consume before it spoils. Before freezers became widespread around the 1940s, humans needed other ways to make the nutrients in meat last, and salt became a viable option.
Salt dehydrates meat and can act as an antiseptic. Large amounts of sodium reduce what is referred to as the 'water activity' of a food, or the amount of free water available for bacteria to use. While not all microbes are killed by high salinity, many potential pathogens die by osmosis as the salt draws all the water out of them. Others simply find it too energetically costly to survive.
The food industry still uses salt for food preservation today, though not in the ways one might think. For example, highly processed foods are notorious for being astronomically high in sodium (more than 70% of the average person's daily intake). This is, in part, so that packaged foods can sit on shelves for years without spoiling.
Salt is not the only way to dehydrate, however. The sun has also come in handy for many a gardener looking to sock away baskets upon baskets of tomatoes, fruit or other produce. Similar to salted foods, foods dehydrated by the sun (or modern dehydrators) lack much of the water that microbes need to survive, and this deters them from setting up shop.
Acid and Heat: Canning
Once a somewhat forgotten old-timey art (akin to churning butter), home canning is back in full force. While curing is traditionally done for meat and fish, canning shines for preserving fruits and vegetables, which have higher natural water content. The principle of canning has 2 primary elements: heat and acid. The temperature needed to sterilize the food will depend on the acid-content of the food being canned. This is because Clostridium botulinum, a spore-forming bacterium (of botulism notoriety), can persist and thrive when the pH is above 4.6, even after boiling. For this reason, many canned preserves incorporate naturally acidic foods or else supplement with lemon juice or other acids to reduce the pH in the canning environment.
Pressurization is another way to ensure proper temperatures are reached to eliminate spore-forming microbes. C. botulinum spores, the primary concern for canning safety, are very difficult to destroy at normal boiling temperature (212°F). However, by pressurizing the canning environment, temperatures can reach the ~250℉ needed to fully eradicate the spores of C. botulinum and other spore-forming microbial contaminants. Low-acid foods must therefore be canned in a pressurized canning system to ensure proper, safe temperatures are reached.
Fermentation: If You Can't Beat 'Em, Join 'Em!
Many people don't need to be encouraged to love some of fermentation's darlings, such as cheese, beer, wine, miso or kombucha. Moreover, as scientists have begun to unravel the mysteries of the gut microbiome, fermented foods have earned their proper reputation as gut health superstars. All the same, many fermented foods have humble beginnings in humanity's quest to keep food around.
Many fermented foods utilize lactic acid bacteria (LAB) to metabolize sugars into lactic acid and carbon dioxide. Similar to the benefits of acid in canning, the lactic acid produced by LAB lowers the pH of the preserved food and helps keep pathogenic microbes at bay. This acidity also gives many fermented foods their characteristic tangy or sour flavor profile. The gas production of lactic acid metabolism can also contribute to the fizzy or bubbly characteristics of beverages like kombucha or beer, or the rising of sourdough bread.
