Much of it was difficult for me to decipher but I found the following useful tidbits.

“yogurt prepared with unheated or inadequately heat-treated milk, is characterized by poor texture, weak gel and firmness, and increased susceptibility against wheying off.” They tested two heat treatments: 80°C (176F) for 30 min or 95°C (203F) for 10 min. Much longer than I have yet done.

“Generally, the addition of 2% of protein concentrates is considered adequate for amending the textural quality of yogurt.”

“The size of the inoculum was set at 2.5% (wt/wt).” So they added about 0.8 oz of starter to 1 qt of milk.

“Sensory evaluation revealed that the use of skim milk significantly improved (P < 0.001) all of the examined quality characteristics. Non-fat yogurts were characterized by a firm, consistent, and viscous texture. Moreover, non-fat yogurts exhibited enhanced flavor and they were also rated as more palatable. Brauss et al. (1999) observed that low fat yogurts (0.2% fat) release volatiles more quickly and at higher intensity but with less persistence than whole fat yogurts." So they claim skim milk produces the best tasting yogurt.

"Thus, yogurts fortified with SMP had enhanced firmness and consistency, good mouthfeel performance, and were less prone to syneresis." So adding 2% skim milk powder (dry non-fat milk) improved things all round.

"In general, xanthan gum had the best stabilizing effect, enhancing firmness and consistency without developing brittleness or excessive gumminess. Moreover, xanthan addition prevented the wheying off defect." They added it at 0.01% and clearly specified that was by weight.

Finally, "Skim yogurts containing skim milk powder or xanthan gum were found to be the most acceptable samples in terms of sensory quality."

For those who decide to click through to the link Gary provided above, “syneresis” means “the extraction or expulsion of a liquid from a gel.” Full definition with some more science-y words at wikipedia: http://en.wikipedia.org/wiki/Syneresis_%28chemistry%29

If you’re going to be adding probiotics before starting the fermentation process, they would have to be the kind that turn milk into yogurt; otherwise they might out-compete the yogurt cultures and you’d just end up with a soupy mess (if it even got to the point of being soupy). If the probiotics are not for culturing yogurt, then you’d have to wait to add them until after the fermentation is complete.

Once you have tried thickening with the agar-agar and/or the tapioca, I’d be very interested in hearing about your results – not just the thickening results, but taste and texture as well. I think there are a lot of people who would prefer not to use gelatin, for various reasons. Personally, I have had uneven results with it – sometimes I’ve ended up with little chunks of clear, tasteless jello in my yogurt, which is not the texture I was going for!