I think they didn’t know. Pasteurization didn’t come about until the mid 1800s. Maybe they suspected heating played a part - Wikipedia says it was a known metho

I think they didn’t know. Pasteurization didn’t come about until the mid 1800s. Maybe they suspected heating played a part - Wikipedia says it was a known method of preservation in China since the 1100s, but I think it took longer to get to Europe.