Hi, I have decided to test probiotics.
I bought four different kinds of probiotics.
I took two caplets/glass vials of each and dissolved them in a small plastic dish with milk.
I left them sit overnight.
Milk was supposed to go sour, but it did not. In all four cases.
What am I doing wrong if anything?
Can all four be dead?
Should I leave them out longer - for 24 hours maybe?
UPDATE on my experiment: two of them changed their consistency after 30 hours. One looks like really thick buttermilk and another one looks like thin buttermilk, but other two are just milk.
Back in microbiology lab we used something called a litmus milk test to help identify bacteria, pretty much the same thing but with a pH indicator. If the probiotics you were using had mostly lactobaccilus (aka "milk bacterium") they should turn the milk sour (other probiotics may not thought). But we were cautioned that this test is prone to false negatives.
It's common to keep the test running longer, several days. Of course this opens up the possibility of contamination or residual milk bacteria skewing the results. You can set up control milk with no probiotics to help with this problem.
I personally wouldn't feel good about the results you got, but it's far from a damning test in my opinion. I'd try letting them go for 24 hours, perhaps more. Do you have any pH strips? Those can help you measure the acidity without the subjective measurement of tasting it.