Monday, August 26, 2013

Proofing Yeast



A while back we had a string of bread loaf duds.  As in, we were making the bread the same way as we always had, it just wasn't rising as much as it was supposed to.  After eating miniature sandwiches for weeks on end, we began to suspect that our yeast was dying off.  That was an unexpected conclusion for us, since it was only about six months old. (We normally get the 1-lb bricks, then keep it in a quart-size canning jar in the fridge.)  We tested our hypothesis by getting a new brick and proofing them side-by-side.  Sure enough, the old yeast was not nearly as vigorous as the new stuff.  At the time, we didn't have the sense to take pictures for a blog post, so we repeated the experiment this weekend.

All of the demonstrations we've seen online for how to test if the yeast is good or not say simply to mix yeast, sugar, and water, and wait to see if it bubbles.  The thought is that if bubbles form, it's alive, and you're good to go.  But, as we'll show below, it's sometimes more complicated than that.  It's not like the yeast all dies at once, after all--it kind of slowly fades away and eventually the 'active dry yeast' becomes just 'dry yeast.'  Now, a biologist might say something like, "but yeast cells multiply rapidly and the growth is exponential, so even if there's just a few active cells in the yeast, it shouldn't take long before it's cranking out CO2 just like the fresh stuff!"  True.  But in the lag period while it's catching up, there's other stuff going on with the bread dough, too, and even if we take pains to keep the temperature and humidity optimal, it seems like our less ambitious yeast just can't make as poofy a loaf as the fresh yeast.  Anyway, this is how our comparison of fresh and old yeast played out.

We started out with two glasses, each with about 0.25 cups water in them.
The water is 100-110 °F.
This is the yeast.  New brick on the left, old stuff on the right.  Get ready, because in the next photo they're going to flip flop.  The active dry yeast is so active, it moves to the right side of the photo.  'Dry' refers to its sense of humor.
To the water we added a heaping tablespoon of sugar and about one tablespoon of yeast.  We used powdered sugar here since it was the first thing we found in the pantry, but regular sugar works, too.  After about a minute and a half, they look pretty similar, except a lot of the old yeast is floating.  The newer stuff is better-dispersed through the liquid.  Katie says, "why is your phone so dirty?"  "Because I had dirt in my pocket, obviously."
After a little while, the yeast will start to build up a frothy malted layer on top of the liquid.  At 9.5 minutes, the fresh yeast is clearly ahead.  It's hard to see, but the frothy layer also goes almost all the way to the bottom of the cup for the fresh stuff, but the old stuff clearly has a line above the phone.
At 11 minutes, the fresh yeast breaks over the plane at the top of the glass.  It's the winner!
Hey! OK! The race is over, you can stop now!  Ahh! It's out of control!
Katie says, "What happened to the yeasty stuff in the cup?" 
"I dunno.  Want to have a burping contest?" 
Katie says, "No.  What did it taste like?" 
"Very malty and yeasty flavored, kind of sweet from the residual sugar, but there was definitely some ethanol in there, too.  Braaaapppp."
video


The video shows a time-lapse (x6) of the frothy layer growing in the cups for the old and fresh yeast.   The fresh yeast is clearly more ambitious and harder working.

So although the old stuff is still kind of active, it's key to have a fresh 'standard' to compare to.  If you don't want to open a new brick of yeast just for the test, you could make a video like the one above when you do open a new package, and six months later when you suspect the yeast is kind of phonin'-it-in, watch the movie next to a current proof test to see how the activity compares.  (Or show the movie to the yeast for inspiration.  "You used to be able to do that, little yeasties!")

So what do we do with the less-than-active-yeast (sedentary dry yeast?), now that it won't make good bread anymore?  We have been using it in things that don't need as much leavening, like pizza crusts and pancakes.  We could probably make some sourdough starter with it, too, but we haven't tried that yet.  Don't worry--it's not going to waste!



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