Bio 101 question re homemade yogurt

7river7wed7

If I use a previous batch of homemade yogurt as my starter yogurt for a new batch why doesn't it go bad? Everything I've read says homemade yogurt is only good in the fridge for 7-10 days so I'm scratching my head over how it can stay good just because I mix it with new milk.

MaryWithoutSound

First off, I know absolutely nothing about yogurt.  Having said that, my guess is that the bacteria count in the yogurt grows too high to consume by days 7-10, but when you dilute a small portion in new milk, the levels drop back into the range that works for eating. 

uncannycanuck

I don't know, but it works.

I would guess it goes the other way - the bacteria consumes most of it's food in a week and starts to die back, allowing other fuzzy mold monsters to take hold. A new batch of milk provides another food source compatible with yogurt bacteria and lets them dominate again.

cme25

Pretty sure canuck is right. It's the same with sourdough starter, or that awful Amish Friendship Bread. The yeast (bacteria) eat through the flour, sugar, milk, whatever it eats, and will die when there's nothing else to eat. That leaves yeast/bacteria "poop" and the food they couldn't eat, which will start to grow mold. I've got to add flour and water to my sourdough starter probably twice a week to keep it alive... if I let it go 10 days without a feeding, it would die and start growing mold I'm sure.