14

I am pretty sure I know the answer to this, but I need to know for sure to settle an argument with someone... Is yeast still active after the bread has been baked and cooled?

  • 3
    Define active... active as in "there might be a few yeast cells alive" or "still happily bubbling away". – Stephie Feb 27 '15 at 21:31
15

Yeast dies at about 130-140F.

Bread is done baking at 200F or so.

Almost all the yeast is dead when the bread is done.

Edit
I wrote "almost" because in the context of cooking nothing is ever 100%. Not all alcohol boils it off a sauce. Not all microbes are killed. Etc.

The longer and hotter you cook the more are killed until there are too few to be an issue. We're content if 99.99% of bacteria or yeast are dead - but there are always a few that survive.

As an example- Russian kvass is made by putting well toasted black bread in water with a little sugar until it is carbonated. There is enough yeast alive in the bread even after baking and well toasting.

  • 1
    we posted at roughly the same time, I think we used the same source. You need to edit your temp. – moscafj Feb 27 '15 at 19:28
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    There are always exceptions, the way heat circulates, overly wet areas, , stronger yeast etc!. Pretty much 100% guarantee that some trace of yeast will survive. To get a 100% kill you will need a autoclave – TFD Feb 27 '15 at 20:45
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    Exactly @TFD. It is also easy to observe. Russian Kvas is made by fermenting black bread in sweetened water. It gets nice and yeasty in short order. – Sobachatina Feb 27 '15 at 21:14
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    @Sobachatina - couldn't that be yeast the same way you get yeast for sourdough starters though, regarding russian kvas? – rfusca Feb 27 '15 at 21:55
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    @rfusca- it's possible of course but it only takes two days and smells and tastes exactly like the bread it is made with. – Sobachatina Feb 27 '15 at 21:59
11

The thermal death point for yeast cells is 130° F–140° F (55° C–60° C).

Most bread is cooked when the internal temperature reaches 200 F or 100 C.

The yeast is dead.

  • But if 200F is the oven temperature (seems rather low to me) this isn't the temperature of the centre of the bread (which has a rather good insulating structure cooled by phase change of the water evaporating). Sticking a meat thermometer into a loaf fresh out of the oven would be a good way to test, but I don't have a meat thermometer (and I'm not sticking a thermocouple in food), and I'm not planning to bake bread this weekend. – Chris H Feb 28 '15 at 12:12
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    @ChrisH 200F is the internal temp of the bread when it is cooked. Oven temps are typically between 350 and 500F, depending on bread and recipe. – moscafj Feb 28 '15 at 13:31
  • That makes much more sense - it's a while since I baked bread (other than pizza) in the oven but after converting to °C that sounds about what I remember. – Chris H Feb 28 '15 at 13:55
  • I can confirm 200F because I used to make bread, it didn't turn out as I like it to, and instead started to take it out when the internal temp was 200F. It ended up being much better after it reached 200F inside – Joe Plante Jan 19 '16 at 19:14
5

The most realistic answer, including many correct comments above, is 99.9999..% dead.

Yeast and bacteria can sporulate, and spores can survive very harsh conditions. A spore is basically a solid: a cell which has been dried out, packed with sugars and wrapped in an extra thick cell wall. They are not metabolically active, so they can stay that way for thousands of years. And they can survive boiling temperatures for a little while too, that's why temperatures above boiling are needed for sterilization.

So if any of the yeast in your dough (or bacteria that are in there too) decided to sporulate before the bake, your could find them alive later. But they would not be active just after the bread is baked, the temperature at which they can grow is, as others have mentioned, quite a bit lower than boiling.

I don't know whether baker's yeast actually sporulates much, but it is said that brewers yeast are very unlikely to make spores. Maybe they've had it too easy at the brewery. But I'm pretty confident that after you've baked your bread, there will be more active yeast and bacteria falling onto the outside than there are live yeast on the inside.


BTW - if you used dried yeast from a package, they are not spores, they are actually made be freeze drying live yeast, which is why its so important to rehydrate them with water, as written on the package.

  • 3
    "if any of the yeast in your dough decided to sporulate" - this almost certainly didn't happen. Yeast sporulates when depleted of nutrients, and the whole dough making process is geared towards giving the yeast enough nutrition. It would have to be a yeast cell in very unusual circumstances. And then, yeast spores can also be destroyed with hot air, just not with temperatures as low as 50 Celsius. See ncbi.nlm.nih.gov/pubmed/15643934 - I don't have access right now, so can't read up which temperatures functioned. – rumtscho Feb 28 '15 at 7:33
  • To make it clear, I like it that you posted this answer, +1 for providing more information. I'm just pointing out that, while the theory is interesting, it is very unlikely to have happened in practice. – rumtscho Feb 28 '15 at 7:34
-2

There must be some live yeast in it because that's how they make wine in prison. Store bought bread seems to be a lot doughier though, so maybe that's why. I've never been locked up, but my friends and I were bored and made wine with a couple slices of bread, sugar, and crystal lite; it worked very well.

  • 1
    You're assuming that you didn't get any wild yeast in the process, or even yeast in the bakery finding its way onto the bread after it had been baked. – Joe Mar 5 '17 at 18:09

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