I have never heard of this. I'll have to dig out some silver dimes and give this a try. I try to drink a cup of milk at bedtime, so I do go through a half gallon of milk before it goes bad.I try to only buy a pint of milk at the time but sometimes all the store has is 1/2 gallons. So, I always drop a couple of silver dimes (pre-1965) in the jug. The silver kills the bacteria and the milk stays fresher longer. I also put dimes in my water pitcher and my berkfeld water filter. Again, kills any bacteria, water always tastes fresh.
I have lived in areas where the milk was almost gone when you pick it up at the store. This trick adds days to the life of milk and has saved me many gallons of milk over the years. I wish my parents knew this trick as I grew up on powdered milk and evaporated milk.I’ll have to try the salt in milk trick.