Why do you need to use alumni foil in covering the yogurt? Why do you need to heat the milk mixture at 90 de?
Um, not sure I understand your first question, but yogurt is usually covered with aluminum foil or something else while it's incubating to keep any other bacteria besides the ones you're intentionally using to make your yogurt from getting into the milk and changing the taste/etcof the finished yogurt There are all kinds of bacteria (good and bad) floating around in the air at all times, but you'll want only the ones present in the culture you're using to ferment your milk Yogurt is also usually covered while it's in the refrigerator after incubation to keep flavors elsewhere in the frig from being absorbed by the finished yogurt(Uncovered foods can lose some of their water in the frig too because it's a drying environment.) As for having to heat the milk to 90 degrees, you don't say if you mean 90 centigrade or fahrenheit, and I don't know where you are so can't guess which it's more likely to be 90 degrees C is equal to 194 degrees F thoughGenerally for homemade yogurt, the best temperature for fermenting the milk is 115-120 F, which isn't really either of the possibilities for 90(Too much hotter than that temprange and the bacteria will be killed, but too much lower than that and the bacteria will just get slowed way down so fermentation takes much longer.) Also, before fermentation many people like to make a thicker yogurt than can usually be made at home (manufactured yogurts usually add starches, gelatins, etc.) so those people will often heat their milk to 185 F (which is close to 194 F, I guess) in order to make the finished yogurt thicker than it would have been if not heated that high (Usually then it's best to keep it at about that temp for a few minutes at least, and then cool it slowly and naturally, for the thickest results.) When the yogurt cools to 115-120, the culture is then added and fermentation beginsHTH, Diane B.
