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In my post of January 14, 2010, I talked about drying flour in the oven and reported what I found the water content to be. I found that the amount of water was somewhere between 7.5% and 14.6%, and I couldn't narrow it down more unless I had done the experiment using much greater quantities of flour, because my kitchen scale simply wasn't precise enough, weighing only to the nearest gram. Since then I read something that said all flour has a moisture content of 12% when it leaves the mill. I thought that was cool - the real figure is right in the range I found, so I wasn't crazy. OK, it's good to know.
On that day, I started an experiment to see how much moisture flour can take in. I took 36 grams of the flour I had dried and put it in an open glass container. I put the container in a larger airtight container that had water on the bottom of it, and sealed it. I've attached a photo of the setup. The idea is that the air in the sealed container will become saturated with moisture, hastening the uptake rate of water into the flour. You'll notice the flour looks kind of bluish. For some reason it hadn't struck me that moist flour will do the same thing moist bread will do - get moldy. So there's a nice crop of mold that grew up on the flour, with the photo nicely preserving the image for all time. I don't think that invalidates the experiment, though.
After just over three weeks, the weight of the 36g of dry flour has grown to 45g, an increase of 9 grams. That happens to be a nice even 25% weight increase. That's huge! The flour lost about 12% moisture, then gained it back and gained yet another 13%. I didn't think flour would be affected that much, but not so - the weight of flour really does change with the humidity! Now, the mold might have drawn some part of that weight of water into the flour, but there just isn't that much of it there to account for much.
Here's what I get out of that. First and foremost, don't store flour in unsealed containers long-term. And second, don't leave the flour container half-full or less for long periods of time. And third, weigh a new bag of flour to see if its weight is about what it says on the bag. If it's much different, it's probably because of moisture lost or gained since it left the factory. One can adjust the amount of flour according to weight lost or gained, if you know how much to adjust by. Of course, the simplest thing is to assume flour you buy at the store is reasonably fresh and probably weighs about what it says on the bag. It doesn't hurt to confirm that, but usually it's a safe assumption and any adjustments in recipes will be so small you could easily just ignore it. But once you get the flour home, protect it from high or low humidity by putting it in an airtight container. If you keep the container full, there won't be much air for moisture to go to are be absorbed from, so the amount of moisture in the flour will remain fairly constant. Naturally you can't simply keep the container full - that would mean you never use the flour! But you can minimize changes by using as large a container as possible. If your container is large enough for two 5-pound bags of flour, for example, then as soon as it is half-empty you can get a new 5-pound bag, put that bag in the container with the old flour, and mix the new flour with the old so you don't continually have old flour getting older and older on the bottom. Or alternatively and probably better, as you use the flour, transfer what is left into smaller airtight containers to keep the volume of air in the container small relative to the amount of flour.
Now, my experiment was pretty extreme. Inside the airtight container with water on the bottom, the humidity would go up as high as it possibly could, and no matter how much water the flour absorbed, the moisture would be replenished in the air from the water on the bottom of the container. I should think that flour would not normally absorb so much water. Still, it's reasonable to minimally make sure your flour is always stored in airtight containers to limit the amount of moisture that can be gained or lost. And if you can conveniently move small quantities of flour to smaller containers, then it's wise to do it. Right now I'm using flour fast enough that I'm not worried about it taking in or giving off water while it's sitting waiting to be used, but when I eventually stop making cookies at such a rate, I will probably see what I can do to make the container size match the quantity of left-over flour.
To see how much weight varies under realistic circumstances, I have purchased two five-pound bags of flour which I plan on leaving unopened until perhaps June. I weighed both of them and wrote the weights on the bags. I placed one inside a plastic bag. The other is simply sitting out, unwrapped. I will weigh them again in June and see how much the weight has changed in the flour left out versus the flour I wrapped. That will give me a practical indication of whether or how much I need to adjust for moisture changes when weighing it out. More on that in a few months!
On that day, I started an experiment to see how much moisture flour can take in. I took 36 grams of the flour I had dried and put it in an open glass container. I put the container in a larger airtight container that had water on the bottom of it, and sealed it. I've attached a photo of the setup. The idea is that the air in the sealed container will become saturated with moisture, hastening the uptake rate of water into the flour. You'll notice the flour looks kind of bluish. For some reason it hadn't struck me that moist flour will do the same thing moist bread will do - get moldy. So there's a nice crop of mold that grew up on the flour, with the photo nicely preserving the image for all time. I don't think that invalidates the experiment, though.
After just over three weeks, the weight of the 36g of dry flour has grown to 45g, an increase of 9 grams. That happens to be a nice even 25% weight increase. That's huge! The flour lost about 12% moisture, then gained it back and gained yet another 13%. I didn't think flour would be affected that much, but not so - the weight of flour really does change with the humidity! Now, the mold might have drawn some part of that weight of water into the flour, but there just isn't that much of it there to account for much.
Here's what I get out of that. First and foremost, don't store flour in unsealed containers long-term. And second, don't leave the flour container half-full or less for long periods of time. And third, weigh a new bag of flour to see if its weight is about what it says on the bag. If it's much different, it's probably because of moisture lost or gained since it left the factory. One can adjust the amount of flour according to weight lost or gained, if you know how much to adjust by. Of course, the simplest thing is to assume flour you buy at the store is reasonably fresh and probably weighs about what it says on the bag. It doesn't hurt to confirm that, but usually it's a safe assumption and any adjustments in recipes will be so small you could easily just ignore it. But once you get the flour home, protect it from high or low humidity by putting it in an airtight container. If you keep the container full, there won't be much air for moisture to go to are be absorbed from, so the amount of moisture in the flour will remain fairly constant. Naturally you can't simply keep the container full - that would mean you never use the flour! But you can minimize changes by using as large a container as possible. If your container is large enough for two 5-pound bags of flour, for example, then as soon as it is half-empty you can get a new 5-pound bag, put that bag in the container with the old flour, and mix the new flour with the old so you don't continually have old flour getting older and older on the bottom. Or alternatively and probably better, as you use the flour, transfer what is left into smaller airtight containers to keep the volume of air in the container small relative to the amount of flour.
Now, my experiment was pretty extreme. Inside the airtight container with water on the bottom, the humidity would go up as high as it possibly could, and no matter how much water the flour absorbed, the moisture would be replenished in the air from the water on the bottom of the container. I should think that flour would not normally absorb so much water. Still, it's reasonable to minimally make sure your flour is always stored in airtight containers to limit the amount of moisture that can be gained or lost. And if you can conveniently move small quantities of flour to smaller containers, then it's wise to do it. Right now I'm using flour fast enough that I'm not worried about it taking in or giving off water while it's sitting waiting to be used, but when I eventually stop making cookies at such a rate, I will probably see what I can do to make the container size match the quantity of left-over flour.
To see how much weight varies under realistic circumstances, I have purchased two five-pound bags of flour which I plan on leaving unopened until perhaps June. I weighed both of them and wrote the weights on the bags. I placed one inside a plastic bag. The other is simply sitting out, unwrapped. I will weigh them again in June and see how much the weight has changed in the flour left out versus the flour I wrapped. That will give me a practical indication of whether or how much I need to adjust for moisture changes when weighing it out. More on that in a few months!
Unfortunately, the mold does invalidate the experiment because it can take up much more moisture than plain flour (as well as transforming the flour into other chemical components, some of which are likely to be gaseous and will no longer be weighable using this procedure). Can you get irradiated flour?
ReplyDeleteI know what you mean, but it all depends on what level of rigor I'm applying. With the mold, it's totally unsuitable for publication in a scientific journal, even if there weren't a lot of other reasons to disqualify it. But here's what I thought. What percentage of the weight of stuff in the glass was mold? I have to say a pretty small percentage. I'm going to guess 1% of the total weight was mold. Probably less. So the flour gained 25%, of which some of that was mold. If 1% of the total weight was mold, then the flour itself only gained 24%. But 24%, 25%, or only 15%, the conclusion is the same - flour can take on a lot of water weight, and you need to keep it in an airtight (dry) container. :-) And of course you're right about gaseous products, but that would mean the weight gain was even greater. Same conclusion. Good point, but I still think the experiment was valid. It all depends on what you're trying to do. I can't pursue these experiments like it's a scientific lab, because it's only a kitchen and I have little money to spend on them. That means there are some pretty stringent limitations on what I can do. Food scientists go the irradiated flour route, I imagine. :-) On the other hand, I really want to do things anybody can do. Not that anybody would want to grow a crop of mold. But many of my results will depend on the equipment I'm using, and if people want to see how the results are with their own equipment, I think it's probably helpful not to get too elaborate. Great thoughts, though!
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