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1200b Long Technical Post 2




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This article is from the collection of recipes from the Sourdough Mailing List, by David Adams with numerous contributions by others.

1200b Long Technical Post 2

Rye is second to wheat as a bread grain


Comment No2: If discussing rye, it may be of importance, that rye quality
depends heavily on the weather conditions during the harvest: if is is
very humid before and during the harvest, sprouting starts, leading to
increased amylase activity. In a dry year, the amylase activities may be
rather low, so that the problem with the acidification is not very
prominent. It may also be noted that rye not only has a higher amylase
activity,a but also a higher protease activity, which is important for the
flavor development.


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In all, about 72 percent of the original kernel is left in most of the
white flour produced in the United States.


In Germany, the most common bread flour is wheat type 1050 (1.050 g ash
per kg) or rye type 1180. Many breads are "Vollkornbrot". Type 550 (or 55)
is used only for white wheat bread, not a very big share in the market.


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Rye flour is commercially ground to a range of colors and particle sizes,
and the nomenclature is confusing.

If very coarsely groung flour is used, it should be swollen in water
before the dough mixing, Otherwise, it will take up water during dough
fermentation and proofing, and result in too stiff doughs. This is called
"Quellstueck" by German bakers. As far as the enzyme activities go, see
comment No2 above. The difference in enzyme activities is also important
for the microorganisms: in rye flour they always have enough sugar
available due to the high enzyme activities, whereas in white wheat
flours, glucose (but not maltose) may be depleted during the fermentation.



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Rye flour contains a great deal of amylase. Rye amylase resists
inactivation by heat to a greater extent than wheat amylase. It is so
resistant to inactivation that it is still active when the gelatinization
temperature of rye starch is reached in the oven

This is a very nice explanation. Also: see above (Comment No2)


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No laboratory test assesses taste, even though there are real differences
in the taste and texture of bread baked from otherwise similar flours.


This would be impossible, since the flour has no flavor whatsoever. I've
been preparing a research proposal recently with Markus Brandt and Prof.
Hammes in our lab, and Prof. Schieberle, probably the best expert in
flavor chemistry of bread, so I'll go into some detail (and refer to it as
comment No3 later on). It may be useful to distinguish between taste and
aroma. Taste happens on the tongue, where only salty, bitter, sweet and
sour can be evaluated. Aroma is percieved in the nose: during chewing, the
volatile compounds diffuse to the receptors (mind that acetic acid, but
not lactic acid is volatile. Thus, the latter is just sour, while acetic
acid has aroma). There are about 15 compounds each (about 10 of them are
the same for wheat and rye bread, furthermore, crust and crumb have
different aroma volatiles) with which the impression of rye or wheat bread
is given (This is work of Prof. Schieberle in Munich).

To group the compounds according to their generation in dough, one may say
that:

i), they are produced by fatty acid oxidation by cereal enzymes upon dough
mixing (several baking aids contain soy flour with additional lipoxygenase
activity, and prolonged storage of whole flour leads to rancidity as
well). These compounds have a "green", "bitter" "tallowy" or "metallic"
taste - not very pleasant. Lactic acid bacteria and yeasts do inactivate
these compounds in part, thus, fermentation reduced the "rancidity" of the
bread.

ii) aroma compounds are produced by yeasts and lactobacilli. More of them
by yeasts, probably, though acetic acid also plays an important role.
These compounds often give a "flowery", "yeasty" or "malty" flavor.

iii) The Maillard reaction is extremely important, especially for the
crust aroma compounds. However, the precursor chemicals for this type of
reactions are amino acids, and the levels of amino acids in flour is verym
low. In wheat, there is little, if any proteolytic activity (proteases
degrade protein to amino acids), so, whatever amino acids there are
produced by enzymes of lactic acid bacteria (there has been nice work done
on proteolysis in wheat dough by Dr. Marco Gobbetti at the University of
Perugia). In rye, the proteolytic activity of the flour is much higher,
but the proteases need acidification to a pH below 5 to have their optimum
activity (and, of course, a long fermentation time gives the enzymes more
time to work). Sourdough yeasts are consuming amino acids, meaning a
sourdough with a high yeast count has fewer amino acids than a dough
containing only lactobacilli. Addition of excess amounts of baker's yeast
(>4%) also leads to an increase of Maillard compounds, but that may not be
the aroma a sourdough baker is looking for. The most important flavor
compound in rye crust, methional, as well as in wheat crust,
2-acetyl-pyrroline, are Maillard products of the amino acids methionine
and ornithine, respectively.

As I mentionned, we've been preparing a research proposal to figure out
which of the aroma compounds or aroma precursors (meaning chemicals
converted to aroma compounds during baking) are formed by which
microorganisms. In other words, other than acetic acid production, we
don't know whether or not aroma is produced by yeasts and lactobacilli of
sourdough. There are a few good working hypotheses: Some, but not all
strains of L. sanfranciscensis convert arginine to ornithine (MOST
important flavor precursos in wheat), so this metabolic activity may be of
importance. Several other compounds are produced by yeasts (but we don't
know whether the sourdough yeasts are more active than baker's yeast), and
all L. sanfranciscensis does convert the fatty acid oxidation products to
chemicals with less or no aroma intensity - but how this activity compares
to straight, baker's yeast dough, we don't know.

 

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