Where does the chlorine taste in yogurt come from?

My parents used to make yogurt some years ago. When they tried to make some last month, it turned out very firm and smooth in appearence, but tasted like chlorine.

The taste is so strong that even strawberries couldn’t get rid of it. Since then we have not tried making yogurt anymore.

Is the chlorine taste related to some fermentation level/age? Quality of milk?

Obs.: Though the chlorine taste was very strong, it wasn’t sour.