Why does flower food produce foam/froth/bubbles upon contact with water?

Rest assured that the foaming you observed is perfectly normal and will not adversely affect the life of your flowers. It is caused by the physical agitation of adding water to the flower food, and will dissipate in a relatively short period of time. The foaming action you observed can be more severe if less than the recommend amount of water is added to the flower food. To minimize the amount of foam produced, add the recommended amount of water to your vase first before adding the flower food sachet, then gently stir with a non-metallic spoon.