The following information is taken from The science of
cooking, by Peter Barham, Springer, 2001 (I warmly
recommend the book, not only for bread but for cooking in
general)
Flour: Flour is made up from small "granules",
containing sugars and proteins. The difference between
granules, the amount of protenis, and other factors determine
the "type" of flour (strong, durum, brown, 00, etc.). The
most important factor of the flour for bread making is the
formation of gluten sheets as the dough is kneaded When water
is added to the flour, the proteins begin to stick together,
and they bind the granules to one another. When the proteins are
stretched, they interact, and they lead to the formation of
gluten (gluten is not a protein and does not occur in nature,
but it is formed when two different protein molecules, gliadin
and glutenin, are made to interact by kneading of a wet dough,
to form a protein complex).
Gluten is a highly elastic material; as it develops, it forms
thin sheets that behave like rubber balloons. These "balloons"
are blown up by the carbon dioxide generated by yeast as the
bread leavens. To make good bread you need these gluten
sheets to be robust, which you obtain by kneading a lot!.
("The science of cooking", cit., p. 108).
Types of flour: There are many different types
of flour, and unfortunately the names change a lot even within
a single country. Usually you are interested in wheat flour
for bread making, but you may use other cereals.
There are many kinds of wheat, and these can change with
climate, soil, etc. (similar to grapes and wines).
One difference between flours is the amount of protein in the
flour. Flours with high protein content are good for bread
making (above 12%), and these are called strong flours (flours
with less than 8-10% proteins are good for cakes).
Some flours are "wholemeal", brown, or whole wheat: these
flours contain some outer skin of the grains, have a brown
color and distinctive flavour which may be very good for
baking (but always mind the proteins). (Notice: in Italy we
differentiate the "purity" of the flour. A 00 flour is
particularly white, with no impurities. A 0 flour is less
refined. This distinction is not related to the amount of
proteins. When buyng flour in Italy, always check the protein
content, a 0 flour with high protein content being the best
for bread).
Yeast: "Yeast are single cell fungal
micro-organisms that metabolise sugars into carbon dioxide and
alcohol." Put more simply: yeast are small things whose
food is mostly sugar and whose waste products are CO2 and
alcohol. There exist hundreds of different yeasts (some for
baking, some for wine making, etc.). The sugars for the yeast
is provided by the sugars in the flour, and the CO2 causes the
dough to rise, because it is kept by the gluten sheets.
Yeasts are already active at 5 C, but the rate of production
of gas increases with temperature, with a top around 38 C. If
the temperature is greater than 40 C, the yeasts are
killed. Ideally, a temperature around 25C should be very good
for yeasts.
Types of yeast: There are different types of yeast
that can be found in shops:
Fresh yeast is a block of "compressed" yeast (lievito
di birra, in Italian). This kind of yeast is kept in the
refigerator, and it can be added directly to the flour before
kneading, or it can be dissolved in water (maybe with a pinch
of sugar), and then addedd to the flour when it bubbles.
Dried yeast: to use this kind of yeast, it must be
reactivated using water and sugar, and then it can be used
with the flour.
Fast-action dried yeast: this is similar to dried yeast,
but it does not require reactivation.
Additionally, the term natural yeast is used to denote a
yeast which is kept alive across each baking: a piece of dough
is kept before adding the salt, and it is re-used. This yeast
(or family of yeasts) has a very distinctive flavour,
completely different from the compressed yeast you can buy in
shops.
How much yeast?: Do not add too much yeast, or your
bread will smell and taste yeast! I usually do not use more
than 2% fresh yeast (over flour, i.e. no more than 20 g of
yeast for 1 Kg of flour), and no more than 1% dried yeast (in
spite of the instructions you can find on their boxes). If the
dough does not rise, it is not because of the yeast, but
because you did not knead enough!
A bit of theory...
The following information is taken from The science of cooking, by Peter Barham, Springer, 2001 (I warmly recommend the book, not only for bread but for cooking in general)
Flour: Flour is made up from small "granules", containing sugars and proteins. The difference between granules, the amount of protenis, and other factors determine the "type" of flour (strong, durum, brown, 00, etc.).
The most important factor of the flour for bread making is the formation of gluten sheets as the dough is kneaded When water is added to the flour, the proteins begin to stick together, and they bind the granules to one another. When the proteins are stretched, they interact, and they lead to the formation of gluten (gluten is not a protein and does not occur in nature, but it is formed when two different protein molecules, gliadin and glutenin, are made to interact by kneading of a wet dough, to form a protein complex).
Gluten is a highly elastic material; as it develops, it forms thin sheets that behave like rubber balloons. These "balloons" are blown up by the carbon dioxide generated by yeast as the bread leavens. To make good bread you need these gluten sheets to be robust, which you obtain by kneading a lot!. ("The science of cooking", cit., p. 108).
Types of flour: There are many different types of flour, and unfortunately the names change a lot even within a single country. Usually you are interested in wheat flour for bread making, but you may use other cereals. There are many kinds of wheat, and these can change with climate, soil, etc. (similar to grapes and wines).
One difference between flours is the amount of protein in the flour. Flours with high protein content are good for bread making (above 12%), and these are called strong flours (flours with less than 8-10% proteins are good for cakes).
Some flours are "wholemeal", brown, or whole wheat: these flours contain some outer skin of the grains, have a brown color and distinctive flavour which may be very good for baking (but always mind the proteins). (Notice: in Italy we differentiate the "purity" of the flour. A 00 flour is particularly white, with no impurities. A 0 flour is less refined. This distinction is not related to the amount of proteins. When buyng flour in Italy, always check the protein content, a 0 flour with high protein content being the best for bread).
Yeast: "Yeast are single cell fungal micro-organisms that metabolise sugars into carbon dioxide and alcohol." Put more simply: yeast are small things whose food is mostly sugar and whose waste products are CO2 and alcohol. There exist hundreds of different yeasts (some for baking, some for wine making, etc.). The sugars for the yeast is provided by the sugars in the flour, and the CO2 causes the dough to rise, because it is kept by the gluten sheets. Yeasts are already active at 5 C, but the rate of production of gas increases with temperature, with a top around 38 C. If the temperature is greater than 40 C, the yeasts are killed. Ideally, a temperature around 25C should be very good for yeasts.
Types of yeast: There are different types of yeast that can be found in shops:
Additionally, the term natural yeast is used to denote a yeast which is kept alive across each baking: a piece of dough is kept before adding the salt, and it is re-used. This yeast (or family of yeasts) has a very distinctive flavour, completely different from the compressed yeast you can buy in shops.
How much yeast?: Do not add too much yeast, or your bread will smell and taste yeast! I usually do not use more than 2% fresh yeast (over flour, i.e. no more than 20 g of yeast for 1 Kg of flour), and no more than 1% dried yeast (in spite of the instructions you can find on their boxes). If the dough does not rise, it is not because of the yeast, but because you did not knead enough!