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#### Leavening
There are all kinds of ways the leaven breads. Here, we’ll be talking
about two leavens: commercial yeasts and sourdoughs.
##### Commercial Yeast
The three most common types of commercial yeast are:
**Fresh yeast:** Truth be told, fresh yeast, which is also known as cake
yeast, is hard for home bakers to find these days. They are sold as
little “cakes” that must be kept refrigerated, and they go bad after a
few days. Many professional bakeries, however, still use fresh yeast,
and so many bread formulas are written with fresh yeast in mind.
Typically, breads that rise for 1.5 to 2 hours in bulk and 1 to 1.5
hours shaped call for fresh yeast at 2% of total flour weight.
Many recipes use much, much less yeast than that, however, and they
ferment for much longer, which gives the bread more flavor.
**Active dry yeast:** This is the kind of yeast that you’re most likely to
find in your grocery store. Typically, you’ll want to use about 1 to 2
tsp per loaf and, if the formula calls for fresh yeast, you’ll need to
measure out 40% of that weight to convert to active dry.
Active dry yeast needs to be “proofed” before using, which means it
needs to be dissolved in about ¼ to ½ cup of lukewarm water (about 90
\degree F or so).
**Instant yeast:** This is what many amateur bakers prefer to use. In
grocery stores you'll often find it labels "Rapid Rise Yeast" or
"Bread Machine Yeast." It looks like active dry yeast, but it retains
many more living yeasts in each grain. As a result, there's no need to
proof it – just add the yeast directly to the dry ingredients. If the
formula calls for fresh yeast, measure out 1/3 of that weight for
instant yeast. If the recipe calls for active dry yeast, cut the
measurement by about 25 percent.
##### Sourdough
Before the 19th century, sourdough was really the only leavening
available (unless you’re talking about salt rising bread, which uses
bacteria alone – a leavening with which this book does not deal).
Sourdough is really not that hard to work with – as some Internet sage
once wrote, “People who thought the earth was flat made bread like
this for thousands of years.”
First, what is this stuff? Sourdough starter is a stable symbiotic
culture of wild yeasts and bacteria. The yeasts break down starches
into sugars, which the bacteria eat. The bacteria, on the other hand,
create an acidic environment that kills off competitors to the yeasts.
The yeasts were almost certainly already living on the grains when
they were out in the field. As for the bacteria, that’s a trickier
question, but the consensus seems to be that they come from us –
studies have failed to isolate Lactobacillus Sanfranciscensis anywhere
except in long-lived sourdough starters and on human teeth. The
individual mix of yeasts and bacteria varies from starter to starter,
and region to region. It's part of their charm; every starter is
unique, and produces bread that tastes somewhat different from those
produced with other starters.
Sourdough starters work more slowly than commercial yeasts, which are
much more concentrated that starters and have been carefully selected
for their gas production. Typically, a sourdough loaf will rise for at
least three to four hours in the bowl and will then need
another two to three hours as a shaped loaf before it will be
ready to bake.
Not all sourdough breads are sour. The French pain au levain and
Flemish desem breads are typically not very sour at all, while San
Francisco-style sourdoughs and many German ryes are very sour, indeed!
Different starters will produce different levels of sourness, but by
far the most important factor in a sourdough bread's flavor is
temperature. If the dough is allowed to ferment at 80 to 85 degrees or
is allowed to rise slowly overnight in the fridge or in a cool room
(35 to 50 degrees F), the bread will have a markedly stronger flavor
than a sourdough that rose at room temperature (65 – 70 degrees F).
Sourdough breads generally keep well, because their acid content slows
down the staling process. In addition, the acid in sourdough both
reduces the impact of bread on one's blood sugar and also neutralizes
phytic acid in whole wheat breads. Phytic acid prevents the body from
absorbing many nutrients.
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