I didn't have any whey, and I was unable to extract any from the yogurt I bought.
Being impatient I dumped a pro-biotic capsule into my beets and carrots.
Although the yogurt would have to contain the legally required specific lactobacillus and streptococcus required for yogurt, I know often bifidobacterium is in yogurt. All that's in the capsule are a couple of kinds of lacto, and bifido, no strep though.
I'll try and let you know if it kills me.
I'm expecting it to ferment REALLY fast.