I made pizza dough for the first time (smitten kitchen recipe) and it didn't rise. Could it be because the yeast packet had been open a few weeks?
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Possibly. If could be that you just purchased a bad package of yeast. I work in a bakery and recently we had to dump a whole batch of dough because the yeast was dead. Other potential issues: if the salt and yeast came in direct contact, salt can kill yeast, or the water added was too warm-above 138-140F temps can also kill yeast. If you still have the dough, try mixing in some more yeast (obviously from a different packet) with a pinch of sugar and see if that'll do the trick.
Good advise from spiffypaws. Did you proof the yeast? That's done by putting the yeast in some of the water called for in the recipe, warmed to body temperature, with a pinch of sugar. It should get foamy in 5 to 10 minutes. If it doesn't, then you need different yeast.
If you proof the yeast in warm water with a teaspoon of sugar, you should see a good amount of foam on top of the water after about 10 minutes. If there is no foam, the yeast may be dead or the water is too cool. I have kept an open package of yeast for a few weeks with no problem.
Letting the dough rise in a warm location is really important. I've struggled to find someplace warm enough. I usually heat the oven to 100 degrees, then turn it off and let the dough rise in the warm oven.
My guess would be the yeast, or the rising temperature. How old was the yeast to begin with? I doubt that having the packet open for a week or two killed the beasties.
Note that while you *CAN* proof all kinds of yeast, there is a difference in the activity between "active yeast" and "instant or Rapid Rise yeast". Active yeast (and live, compressed yeast cakes) need to be proofed with tepid (90-115°) water and a pinch of sugar or flour. They will bubble and poof and act like they're alive. Instant yeast, at that temp, will not. It will sit there and fake you right out into thinking it's dead. If you want to proof "Instant or Rapid Rise" yeast, use a higher temperature of water (up to 125°) and no other food. And don't expect to see the same kind of reaction as you get with "active" yeast. They're both the same critter, just processed differently. Instant yeast is smaller granules so it disperses more readily. Usually you can just dump it into your mix without dissolving it as you do with active.
I have problems finding a warm enough spot to raise my doughs as well, since my kitchen tends to be cold unless I'm actually cooking in it. If you have a gas oven with a pilot light, that's a good option. Or a closed oven with the oven light turned on. Heating it to 200°, then turning it off works, as does putting a shallow baking pan of boiling water in the bottom, and putting your rising container on the top rack. I've used all these tricks, except for the pilot light. I've also heard of people putting a Pyrex measuring cup of water in their microwave, bringing it to a boil, then putting in the rising container.
Even with that, it may rise more slowly than you expect. Dough is a living thing, and each one is different. If you saw no action in your pizza dough at all, I'd first suspect the rising temp. If you saw a little, but not what you expected, I'd have kept it going longer. Which, I know, can be frustrating if you want dinner !
Thanks everyone! Excellent suggestions! Next time I'll try both increasing the rising temperature and proofing the yeast. (Neither of which I did!)