Eggs – How long does it take to pasteurize something at 60C

Buttermilk is already thoroughly packed with live bacteria. During its manufacture, that bacteria already consumed some portion of the available lactose and turned it into lactic acid.

Because of the lack of food, acidity, and the extreme competition it is pretty hard for buttermilk to go bad. The good bacteria will stay active and the buttermilk will get thicker and more sour until it runs out of lactose. In fact- when your buttermilk container has about 1/2 cup left you can make more just by refilling the container with milk and leaving it to ferment on the counter for a day. If you use it up more quickly than the bacteria eat the lactose then you can keep this up indefinitely.

Don't worry if it is thicker- if it still smells good then it probably is. As Noctrine said- mold around the lip is the worst risk. I am not a food chemist and despite my personal experience- if you ever suspect that food is bad just throw it out. $2 of buttermilk isn't worth an unpleasant afternoon.

There are two basic mechanisms:

• the salmonella is on the shell and when the egg gets broken the interior gets contaminated; or
• the chicken's reproductive system is contaminated and lays an egg with contaminated interior.

To reduce the very common (a few percent) shell contamination, eggs sold commercially have their shells treated (typically washed). After washing only about 50 in every million eggs will still have some shell contamination. The shell contamination may have been there when the chicken laid the egg, from fecal matter, or as a result of plant processing.  

In some rare cases the surface salmonella can make it to the inside without breaking the egg: if it was there while the cuticle dried, if the egg is stored at room temperature, or if the environment is moist.