From: http://www.thriftyfun.com/tf47824676.tip.html.

"The pioneers who traveled west would put silver coins in their water barrels to keep the water from spoiling. Back before there was refrigeration, people would put a silver coin in their milk jugs to keep the milk fresh longer."

The anti-bacterial and anti-microbial qualities of silver have been know for ages.

Internet search:
colloidal silver
silver dollar milk
ionic silver