Adding bacteria to pasteurized milk? Yep! Technically, we call this post-pasteurization contamination.
"No," you say. "Why would anyone do that?"
A quick case study. A couple of years ago I was on a large dairy in China. All the young calves had diarrhea. The dairy had its own lab and could do bacteria counts on colostrum and milk.
We sampled "as-fed" colostrum, raw milk, milk directly from the pasteurizer and "as-fed" pasteurized milk.
- Colostrum - fair results, most samples had coliform counts under 5,000cfu/ml and total plate counts in the 25,000 - 35,000cfu/ml range.
- Raw milk - all under 200,000cfu/ml - that's acceptable for milk going into a pasteurizer.
- Milk from pasteurizer outlet - all samples under 1,000cfu/ml.
- "As-fed" pasteurized milk - all samples over 5,000cfu/ml coliforms with total plate counts in the 100,000's.
Ooooops! Clean coming out of the pasteurizer but badly contaminated in the pail for the calf to drink? Not good.
Culprits? All the usual suspects.
- Hose between pasteurizer and transport tank - it was not part of the wash cycle at any time. It turned out to be full of slime. Solution #1 - replace hose. Solution #2 - install a couple of fittings in the pasteurizer plumbing so that when the pasteurizer was washed this hose was also cleaned.
- Bottom and sides of the transport tank were spotless - shiny clean. By leaning into the tank I could run my fingers along the underside of the tank top. Ugh! The energetic worker cleaned what she could see - unfortunately neglecting about 1/3 of the tank surface - the upper invisible part.
- Feeding pails. The dairy had between 900 and 1,000 calves on milk. They had 200 metal feeding pails that were washed between each feeding. So, the first 200 calves got a clean pail, the next 200 calves used a pail that had been use once, and so on.
A sterile-water rinse sample from a "clean" pail grew a huge crop of bacteria - mostly Staph and Strep species. Problem? Wash protocol needed to be tuned up. They ran one huge sink of lukewarm water, added lots of detergent and all 200 pails were washed in this water (no pre-wash rinsing so lots of milk ended up in this water. The water started out about 100F).