In early 2025, as I tried to finish up my book Smart Yogurt, avian flu had infected a large number of dairy herds here in the United States.
I’m sure you heard that the milk from these herds was still safe, because it was pasteurized. But what you might not have heard was that milk from infected cows was different in content—thicker and yellowish, described as similar to colostrum. That’s the special milk that cows and other mammals briefly produce to feed newborns, providing extra protection from disease while their immune systems are still weak.
The difference in milk might not be easily seen when a cow had only recently been infected, or once its milk was mixed with other milk. But it still meant a change in enzymes, and some of these might not be disabled by pasteurizing.
What does this have to do with yogurt? In the middle of my yogurt making tests, I discovered that the store brand of milk I occasionally used had stopped firming up properly. In repeated tries, it wound up runny with clumps. I had never seen this before.
Of course, I ran more tests to make sure nothing was wrong with my starters, but they still worked fine with other milk. The trouble was limited to that particular brand. And I began to wonder if it was linked to the flu. Could flu-induced enzymes in the milk be curdling it before it could firm up properly?
I’m still wondering. A quick test with a colostrum powder supplement showed no problem in making yogurt with the colostrum added. But that didn’t guarantee the same results from milk of cows with the flu.
The upshot is this: If avian flu or another disease has spread among dairy cows, and your milk is having trouble firming, try a different milk! With reasonably fresh pasteurized milk, an active starter, and proper procedure, there is no good reason for your milk not to firm up.
Something else you might do is trace the source of your milk. In researching my problem, I discovered that dairy products carry a numeric code identifying the dairy they came from. For dairy codes in the United States, visit Where Is My Milk From?
