I recently made my first batch of yogurt with a dannon starter culture and some "vitamin D" milk, which i think is a fairly dumb term for high milkfat. People are too afraid of fat nowadays. Thats another topic though. So I let it set for around six hours, pulled it out of the oven I was letting it ferment in, and it was perfect. Nice and solid, not broken up and chunky or anything, and not too sour. As soon as it had cooled in the fridge to stop fermentation i mixed in some raspberries and a little honey and tried it that way. It was some of the best yogurt i've ever had. The stuff on the shelf just doesnt compare at all.
Naturally I wanted to share some with my friends and show them how easy it is to make fantastic tasting yogurt. I sealed the jar, put it back in the fridge, and the next day it was more runny than when i put it in the night before. Then that night it was more or less a liquid. Is there a way I can keep this from happening, but still be able to add something to the yogurt? I don't have a problem adding something in at home, but if I want to take it somewhere i'd need somewhere to keep the fruit too, and these were frozen raspberries so just throwing them in a bag wasn't an option for me. Maybe just put in the add-ins but dont stir it till I want to eat it? How do the commercial makers do it? gelatine additives or something just as diabolical? The process that made the yogurt liquid again made me curious. Does anyone have an idea of what was happening at a molecular level to make the milk proteins change or abandon their structure?
Pardon my curiosity, and thanks in advance.

~Nick