How come some forms of food are both perishable and shelf-stable?
For example, putting a pound of hamburger on a shelf at room temperature is a big no no, but beef jerky is totally cool.
Gallon of milk needs to be refrigerated, but butter is usually left out at room temperature.
What creates the change?
Thanks
The short answer is Water Activity. It is basic to food science.
Water activity is the measure of how available the water within a food is for bacterial growth. Water activity increases or lessens with the temperature and the amount of moisture within a foodstuff.
Water activity slows down or stops spoilage around 8-10% moisture. Products can be stabilized at higher moisture with the addition of additives that will reduce the activity while retaining a higher level of moisture. The water is still there but it is unavailable for bacterial growth due to the lower water activity. The most common additives are salt and sugar, both which lower the water activity.
People commonly ask why food companies add so much salt and sugar to our food. Part of the answer is taste but the larger answer is to stabilize and extend the shelf life of the food. This is why processed foods contain more salt than freshly prepared foods, they need to last longer.
If you try to reduce the amount of salt you are getting from your processed food by buying low sodium products, read the label carefully, the salt has probably been mostly replaced by sugar. Both work as well but salt is more commonly used because it is much cheaper than sugar.
Salted butter is OK to leave out at room temperature, unsalted will spoil sooner if left unrefrigerated. Your hamburger is both higher in moisture than the jerky and the jerky contains both salt and sugar usually. You can either have hard, dried jerky or you can have tender, higher moisture jerky. What is the difference? Salt and/or sugar.
Why is there salt in butter? Cheese? Bread? Every damn thing in the store that isn’t dried out? Why doesn’t honey spoil?
Water activity.
In the case of jerky, the product is fatty and dessicated, so there is no moisture to support mocroorganisms. Butter is reduced to pure fat, again no moisture to support life. Also, both usually contain salt, which retards development of most life forms, and makes sauerkraut last a lot longer than cabbage.
Sauerkraut keeps mainly because of the lactic acid in it, not salt. It is fermented (which produces the lactic acid), so, in a sense, it has already “gone bad”, but in a controlled way. Similarly, pickled foods are preserved by the vinegar (acetic acid) in which they are steeped. It makes the environment too acidic for most things to grow.