The earliest form of food preservation in history is perhaps the curing of various foods, particularly meat. The process involves the dehydration of liquids via drying, smoking and salting the meat in specialized rooms. There the food was hanged and after it was salted, it was left to dry out. With that, people managed to get rid of potential bacteria and to preserve the food longer. It was mostly consumed during the winter season, as foods became scarce or non-existent.
Before the invention of the refrigerator as a place to freeze foods in order to preserve them, people harvested ice, which was then stored in special ice cellars, or any room where the temperature would be a constant of 0 Degrees Celsius or even below. It was typically used for beverages, but later people used it to freeze any sort of food that was not already cured, which allowed them to cook it later for consumption.
Boiling was used typically for water and milk. In order to get rid of bacteria, people would heat water or milk to their boiling point, or the temperature at which the vapor pressure of the liquid was equal to the pressure exerted on the liquid by its surrounding atmosphere. As water was the staple for various cooking recipes in cultures, people often boiled their foods as they cooked.
The most common used method of food preservation is perhaps heating itself. At high amounts of heat the various foods placed in the pots were rid of bacteria, therefore, this method was mostly used when preparing stews and soups.
As sugar cane was brought into Europe via trade, it allowed for yet another process of food preservation. Fruits were at first dehydrated, and later placed in jars filled with boiled water and sugared either with crystalized or raw sugar, or any natural liquid with high sugar density such as honey, molasses or syrup. These jars would later be stored in storerooms and before they were going to be used, they were fermented.
Pickling was the counterpart of sugaring; only it was used for vegetables instead. This process was used to store foods in an environment where bacteria could not reach and infest them. People would put the vegetables in jars, which contained boiled water, brine, vinegar, alcohol or vegetable oil. Yet at times, the water was not boiled, allowing for the bacteria inside the jars to produce organic acids and become preservation agents, most typical of which was lactic acid that was formed during the fermentation process.
Requiring a strong stomach to muster the foods after they were exposed to lye, it allowed for the preservation of foods by making them too alkaline for bacterial growth. The lye also converted the fats in the food, changing their color, flavor and texture as well.
Used specifically for artisanal foods: alcohol, vinegar, olives, bread and cheese among others, this process allowed specific acids produced by some bacteria to enrich the flavor, texture and color of the foods and act as preservation agents, disallowing other forms of bacteria to infect and cause damage.