Bran, Germ, & Endosperm - Gluten?
With the upcoming Community Bake featuring whole wheat bread there is an interest in gluten.
How do the following parts of the wheat berry contribute gluten?
- Bran
- Germ
- Endosperm
The question arose when I thought about commercial white flour compared to whole wheat flour (100% extraction). Lets take a kilo of white flour containing 14% protein and compare it to a kilo of Hard Red Wheat containing 14% protein also. The white flour is composed of 100% endosperm. On the other hand the HRW, consisting of bran, germ, and endosperm and both have an equal amount of 14% protein.
Question - if we consider white flour @ 70% extraction, leaving only the endosperm compared to whole wheat @ 100% extraction, how do both contain the same amount of protein? The WW has 30% of it’s weight in bran and germ.
Note - I realize that protein is not a definitive measure of gluten, but it is generally all we have to go by.
As always, I am interested to learn.
Danny
”inquiring minds want to know”
I cannot claim professional authority on this particular biochemical/botanical detail but as far as I understand the question and the issues it addresses: Gluten's component proteins are exclusively stored in endosperm, not in the 'germ' (embryo) or 'bran' (pericarp). The 'germ', being the embryo and thus a tiny fraction of the kernel's overall mass, is still relatively protein- (but not gluten-) rich at 25-35% protein, hence the popularity of wheat germ as a nutritional additive and as a source of macromolecules to support protein synthesis in the test tube (wheat germ extracts are historically invaluable for this). Bran OTOH is relatively protein-deficient, being a few layers of complex polysaccharide-coated cells that package the seed, both protecting it as well as enabling germination by readily absorbing water.
My guess is that the arithmetic yields the same protein percentage for 70% extraction flour as 100% extraction from the same seed because the amount of protein-rich germ in the 30% removed from 'white' flour is vanishingly small on an weight/weight basis, plus most of that 30% consists of bran which is very low in protein. So maybe a tiny amount of high protein germ + a large amount of low protein bran = a removed fraction (from white flour) that comes out to 9-15% protein, the range of commercially available white flours.
Make sense?
Tom
(retired botanist)
Tom, let me repeat my understanding of what I think you said and let me know if I’m on the right track.
You basically confirmed my thoughts that the germ and bran contributed very little gluten (protein). I think you are approaching this from the viewpoint of white flour. My focus is on the whole wheat flour that contains the 30% of non-protein (no gluten).
How do I say this?
I think this may clarify my concern. Hope so...
Danny
Dan, I think you're missing something about the protein in the germ.
These two things are true:
- all gluten (ie, gliadin and glutenin) is protein.
- not all protein in the wheat berry is gluten.
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The protein in the germ is not the gluten type of protein. That's how/why whole wheat has a high percent protein, but poor gluten performance -- the germ-protein is not contributing to the gluten-matrix when whole wheat dough develops. That fact is __in addition__ to the "problem" of bran and germ "getting in the way of" (formerly thought to be "cutting") the formation of the gluten strands/matrix.
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Two more examples: Durum and spelt have higher protein ratings than regular wheat (triticum aestivum), yet ... they have _poorer_ gluten performance. This poorer gluten performance is because of two things: a) some of their protein is neither gliadin nor glutenin, b) their gliadin and glutenin is not balanced/matched (there's more of one than the other) and it takes an _equal_ amount of gliadin and glutenin to "match up" and form gluten. If there is an over-abundance of either one (gliadin or glutenin) it sits by itself and never participates in the gluten matrix.
I don't have explanatory links handy so google this: gliadin glutenin gluten
Danny,
If you look here for example, you see that the pericarp (bran) and embryo (germ) are substantially richer in protein than the endosperm, on a per gram basis (that is, in their protein concentration). So even though the kernel has a lot less mass of germ and bran than endosperm, they make a disproportionately large contribution to whole wheat flour's total protein because they have a higher concentration of protein than the endosperm does.
That Wikipedia table gives a pretty pathetic value for endosperm protein concentration. That would be pastry flour for sure. Not surprisingly, the numbers one can find online for the relative masses of endosperm, germ and bran in the wheat kernel are all over the place. Same goes for the protein concentrations of each. Granted, there's plenty of biodiversity in kernel composition across wheat cultivars and relatives. I'm sure that's part of the variability. The folks at WSU probably have some pretty hard, reliable numbers.
Are we getting anywhere? Let me know.
Tom
“Now I see”, said the blind man.
So, if I understand the information above all 3 components of a wheat berry contain carbs, protein, fat, and fiber with the one exception being that the Endosperm contains no fat. Because of this the endosperm, which is 100% white flour has a much longer shelf life.
Even though the endosperm contains less protein per gram than the bran and germ, the fact that it is 83% by weight of the berry helps me to understand the question posed above.
NOW, how do we learn more about the gluten (Glutenin and Gliadin) quality of the protein as it relates to each of the three components of the berry?
Thanks, Tom!
Gluten quality (for baking) has nothing to do with the three tissues that comprise the wheat kernel. All the gluten is in the endosperm. Period.
Gluten is a heterodimer consisting of gliadin and glutenin polypeptides. Indy Dave said that the lower [baking] quality protein of durum wheat is attributable to its having different relative proportions of gluten's two constituent proteins. I don't recall that it's as simple as that but there's more and more that I don't recall these days ;-). It could also be that there are different versions of gliadin and glutenin (so-called 'polymorphisms' -- slightly different amino acid sequences from one wheat cultivar or species to the next) that contribute to different gluten chemistry and physics in doughs made from Triticum aestivum versus Triticum durum. Seek and ye shall find that info. But it's all in the endosperm, no searching required to be assured of that.
Tom
“All the gluten is in the endosperm.“
So now we come to the crux of the matter. We typically use percentage of protein as an indicator of gluten content. As far as I know there is no simple way to know the actual gluten, and definitely no way to know the content of Gliadin and Glutenin. I get that spelt , for example, has a high percentage of protein. And is high in Gliadin, but low in Glutenin. As far as I understand this accounts for it’s characteristic super extensibilty.
So white flour that contains 14% protein will have much more gluten available than Whole Wheat (100% extraction) containing the same 14% protein. This is because the bran and germ contribute to the total protein percentage in whole wheat, but as Tom explained the bran and germ contribute no gluten to the dough.
Did the student get that correct?
Dan
.
“ Gluten quality (for baking) has nothing to do with the three tissues that comprise the wheat kernel.”
Not sure I get that statement. The quality or type of gluten seems very important to bread baking. The example of spelt above should indicate.
Do I need another lesson?
I appreciate your patience with me. I really would like to understand.
I only meant that gluten "quality" has to do with the proportion (and possibly biochemical structure -- the polymorphisms I referred to above) of gliadin and glutenin in a particular endosperm or flour derived therefrom. It has nothing to do with the three tissues that comprise the wheat kernel because it's always all in the endosperm and thus never an issue of or difference in where the gliadin and glutenin are located. All is always in the endosperm.
"So white flour that contains 14% protein will have much more gluten available than Whole Wheat (100% extraction) containing the same 14% protein."
All good except possibly the words "much" and "available". If a white and a whole wheat flour are both 14% protein, then the ww will indeed have a lower gluten percentage than the white since some of the ww's protein will be non-gluten embryo and pericarp (germ and bran) proteins. But I wouldn't say the white would have "much" more. Some, yes, but probably not "much" more. And "available" gets into (literally) sticky issues about how proteins are physically disposed: tethered, complexed, modified, compartmentalized etc. Any molecule's "availability" is fraught with cellular-biochemical context and the issue of "available to whom?" (i.e., bioavailability). So use that word carefully in this context.