Meat was usually salted in barrels in late autumn when the livestock was slaughtered (usually November), when stable cold weather arrived but the ground had not yet frozen solid. The weather itself created the perfect conditions for the meat to cool down and cure smoothly without spoiling. Since tracking the temperature down to the exact degree was impossible, people compensated it by using massive amounts of salt, which served as a powerful antiseptic. Barrels were thoroughly scalded with boiling water and smoked with juniper or sulfur to kill off bacteria, every piece of meat (typically beef or pork) was literally dredged in salt on all sides and packed tightly into the barrel without any air gaps. Each layer was then heavily smothered with more salt. Then a wooden disc and a heavy stone were placed on top so under this pressure the meat released its juices, which mixed with the salt to turn into a highly concentrated brine.

The density of the brine was tested using a raw egg or a potato, which had to float to the surface. If they sank, more salt was added. In such an environment, putrefying bacteria physically could not multiply, even if the temperature crept a bit above 39°F. The resulting salt pork or corned beef was so incredibly salty and tough that it had to be soaked in several changes of water for 12 to 24 hours before eating, and then boiled for a very long time.

Also, even 300 years ago people mixed potassium or sodium saltpeter into the salt 'cause it preserved the meat's red color, and helped kill off deadly botulism bacteria.

Also, during the winter, people chopped thick blocks of ice from rivers and lakes, packed them tightly into the bottom of an icehouse or root cellar, and layered them with straw or sawdust for insulation. In a cellar like this, the temperature stayed right around 34°F to 37°F year-round. That was where the barrels of meat were put.

Victron Corporate video featuring a home brew system with self made batteries and a JK BMS. https://www.youtube.com/watch?v=t8Vdizkkzyw