I ran the milkshake machine that Summer, (1969). We got Sealtest milkshake mix in half gallon cartons. The man also showed me how to disassemble the machine every night, sterilize it and then reassemble it for the next morning.
That same day he told me he would make the best milkshake I ever tasted. He put a half gallon of ice milk mix in the machine and later made a strawberry shake for me. I tried it and he was right. The best milkshake I had ever tasted.
I suggested that we use that for all our milkshakes. He told me it was illegal. The state of North Carolina would not allow it. He also said it tended to make people sick.
Sure enough, maybe a half hour later I began to feel a little nausea. Never did throw up but realized he was right.
Maybe 25 years later, I visited the same snack shop and noted they still had the same machine making milkshakes. I told the guy running it that I had once operated that same machine. I asked him if they still disassembled the machine and sterilized it every night.
He replied that it had not been cleaned the entire Summer.