We now take preserving food for granted. Pop something in the refrigerator or freezer and it will keep for as long as a week for fresh meat and vegetables, longer for some things. Commercially canned food will last for years. Food you "can" yourself (i.e. put up in sealed jars) will last a lot longer than it would in the refrigerator.
Medieval people had none of these options. Instead the primary ways of keeping food were salting and smoking. Most of us like smoked, salted food, which is why bacon is a perennial favorite, but modern ham and bacon are smoked and salted a lot less than the medieval versions would have been, which is why they still have to be refrigerated.
If you've ever had a true Smithfield ham you've had something closer to medieval ham, that is the ham where step one is cutting off the mold and step two is soaking overnight to get out the extra salt, before you even get to the baking step.
In northern Europe, the pig harvest came just as winter was setting in,
so some meat might be frozen (or at least semi-frozen) up in the attic
or in an out-building. Here your danger was January thaw.
They knew after all that food spoiled slower if it was cooler, even without understanding bacterial action (not understood until the nineteenth century). So milk would be stored in a springhouse, where a cool stream or spring ran through the bottom of the structure, where one could keep things at least moderately cold. Most milk, however, became cheese, which we still like for itself alone (as we like bacon), even though its original purpose was preservation.
The image above is a nineteenth-century springhouse, but the idea is unchanged since the Middle Ages: build a small structure right over a stream or spring, so that you can set jars right into the cool water.
There was nothing like canning food until the nineteenth century. One needs those glass jars with metal lids and rubber rings used for home canning, and those were far in the future. The difficulty is making sure bacteria don't multiply in the carefully sealed environment and kill you. Today home canners are advised to stick with acidic foods like fruits or tomatoes, which are put into jars at boiling temperatures. For other vegetables, you need to further cook your jars in a pressure canner, far above the boiling point, to make sure the bacteria are really dead—this is what commercial cans experience.
No wonder the medieval diet would seem extremely bland to us. It would consist primarily of bread, whatever fresh foods were available, and a selection of those things that could be preserved. All natural! Farm to table! Boring!
© C. Dale Brittain 2019
For more on medieval food and other aspects of medieval history, see my book Positively Medieval, available from Amazon and other online booksellers.