Hi, I’m Tristan, this is Magic Maillard Maple, and today we are thinking about the
science behind bread.
[Music]
So, we are still on Covid19 lockdown, and home baking has become something of a national
pass time here. Unfortunately, this means that people are struggling to get hold of
small bags of flour, and people are finding it very tough to get hold of bakers yeast.
So, as an alternative to leaven their bread, some people are turning to sourdough cultures.
So as well as learning about the science behind bread, we are going to also try and make sourdough
cultures of our own.
Specifically, we are going to compare two of the most popular sourdough recipes from
the BBC, and BBC goodfood websites. The first relies on the traditional method of mixing
flour and water, leaving in a warm place so that it can be colonised by wild yeasts and
bacteria, and then feeding with more flour and water at intervals. Because this method
is reliant on microbes present in the flour, environment, and also on whomever is making
the starter, we would expect some variability between starters made in different kitchens.
Our second method uses natural yogurt as a source of live bacteria – in this case a
mixture of lactobaccili and bifidobacteria. Whilst our recipe calls for warm skimmed milk,
we must be careful not to add milk that is above 45oC to our yoghurt, lest we start to
kill off the bacteria.
I’m also going to try to make my own “beer bread starter”. This follows the traditional
starter recipe, but I’ve added the sediment from a bottle of my homebrew, as a source
of live yeast. Whether the result will count as a “sourdough” loaf remains to be seen,
however this is certainly an interesting way to kickstart a leavening culture.
Growing the starters takes 5 days, and during this time we would expect the pH to decrease
slightly as the lactic acid bacteria metabolise and multiply, and for bubbles of carbon dioxide
to form. The yogurt starter was the slowest to develop, whilst, our beer starter began
bubbling after the first day. All the starters developed their own distinct tangy, sour and
sometimes boozy aromas, that should add real character to the bread.
Now that we have some viable starters, its time to make the bread itself. This process
can be broken down into 4 key stages
Stage 1 is the mixing of dry ingredients with water. A few interesting things are happening
here. Firstly, starch granules will absorb the water and start to swell – this increases
the viscosity of the dough. Second, this water will wet or “hydrate” the proteins in
the flour, so they can uncoil and get to work. This includes amylases that will break some
of the starch into simple sugars – (I’ve explained how some of these work in episode
one) and also gluten, which gives bread dough its ability to capture air.
Stage 2 is where we knead the dough. Traditionally this is done by hand, but today we will use
a food mixer with a dough hook attachment. By folding the dough over again and again,
we will incorporate air pockets, that can be enlarged during proving. Most importantly
however, we will also provide the energy needed to form a gluten network, which allows the
dough to trap air and expand without splitting apart.
Gluten is sometimes referred to as the “muscle of the flour” – it provides dough with
extensibility, elasticity and expandability. Gluten network development will influence
the final volume of our dough, the firmness of the bread and how coarse our final crumb
will be. Gluten is actually a group of proteins, that can be divided into gliadins and glutenins.
The end of each glutenin protein is especially rich cystine amino acid residues,
which contain sulfur.
As we knead the dough we add energy to the system, and we keep bringing the terminal
ends of different glutenin chains closer together. This energy, the extra air we have incorporated,
plus any bread improvers we might add all contribute to the oxidation of these cysteine
residues, which results in the formation of disulphide bonds.
Therefore as kneading progresses, we in increase the number of glutenin strands that are joined
end to end, like springs, through the system. We will also form some hydrogen bonds between
these chains, and taken alone, the glutenins would form a very stiff, inelastic dough.
Luckily, we also have the gliadins. These chains are less able to form disulphide bonds,
and will instead sit between long strands of glutenins. Hence the gliadins act as ball
bearings, enabling sheets of glutetnins to move past each other and back again, giving
true elasticity to the dough.
Slight modifications to our bread recipe can cause big changes in the end product. Small
amounts of salt will improve the flavour, but it will also decrease electrostatic repulsion
between gluten proteins, increasing the non-covalent bonding between these chains, and thus improving
the overall strength of the gluten network. If we add fat, we will prevent the hydration
of some of the gluten proteins, and so will weaken the gluten network. This practice of
“shortening” is the basis of several crumbly baked goods from “shortbread” biscuits
to flaky puff pastry.
Now that the bread has passed our window pane test, we can move to the next stage
In stage 3, we allow our dough to prove. This is a process where we simply place our dough
in a warm place, and allow our microbial leavening agents to create gas, and expand the existing
air pockets in the dough.
In episode one, we covered the way that yeasts work, taking the glucose and maltose liberated
by amylases, and fermenting them to ethanol and carbon dioxide. When making a traditional
bread, we would be using Saccharomyces cerevisiae, just as for brewing – albeit a different
strain. However in a soughdough, we can find a range of other wild yeasts.
For example, In a study of 14 sourdoughs sampled in France, Urien et al. found the genus Kazachstania
dominated the yeast profile, finding the Humilis, Bulderi, Sulgeensis, and Unispora species
in these doughs. Interestingly, some of these wild yeasts, such as Kazachstania Humilis,
cannot directly use maltose for fermentation, and this is where some species of bacteria
can come in really useful.
Of course, sour dough also contains a wide range of friendly bacteria, which can out-number
yeasts somewhere between one hundred to a thousand fold. The majority of these bacteria
are lactic acid producers - that is bacteria that convert glucose to lactic acid in order
to generate energy. Depending on the species involved, this will be done via either the
homo or heterolactic fermentation pathway, shown here.
Lactobaciilus sanfranciscensis is probably the most common lactic acid bacteria found
in sourdough. This organism can convert simple sugars from flour, to a mixture of lactic
acid and carbon dioxide – giving a sour flavour, whilst helping the dough to rise.
However, this bacteria also has another neat trick that allows it to help out other microorganisms
in the dough. Specifically, it can use an enzyme called maltose phosphorylase, to directly
use maltose as a food source. However, for every molecule of maltose it uses, it also
liberates a molecule of glucose. This glucose can then be used by yeasts that are otherwise
unable to use maltose as an energy source, making the dough rise even faster.
Despite all this, sourdough takes a lot longer to prove than regular yeast leavened bread.
For our loaves, we will knock back after 3 hours of proving…. shape and place into
bowls covered with a flour lined tea towel then leave to prove for another 3 hours before
the final stage
After 6 hours of proving, we are finally ready to bake our bread. As the water and ethanol
in the dough is vaporised, we get a second rapid rise in dough volume,
called “oven spring”.
Soon the heat will cause the gluten network to set, this forms the scaffolding of the
bread. This scaffold is filled with a body of partially gelatinised starch granules,
together giving us a soft but firm crumb.
As the surface of the bread is much drier than the centre, and is rich in sugar and
proteins, baking gives us crispy crust, as well as a golden brown colouring and cooked
flavours through the Malliard reaction.
After a 35 minute bake, all three recipes have risen in the oven and have a beautiful
brown colouring. The yogurt and traditional starter are similar in shape and volume, though
using a rough volumetric calculation, we see the traditional bread has risen about 10% more.
My breads have a fine crumb, but to be honest they are a little densefor my taste
– I think a few more hours of proving in a warmer room will help my results next time.
By comparison, the beer bread is very well risen, and has approximately 40% greater volume
than either the yogurt or the traditional starter sour loaves. It also has a more coarse,
open crumb, that we would typically associate with a yeast leavened farmhouse loaf. All
three loaves have a tangy, sour flavour quite unlike a typical bought sliced loaf. It will
be interesting to see whether these flavours will change over the next few baking days,
as the starters are re-fed and mature further. We can also use our established starter in
a wide range of other bakery projects, from pancakes to pizza.
Well, there we are – I hope you enjoyed learning some of the science behind bread.
We are off to enjoy a slice, slathered in butter and some fresh homemade jam.
See you all next time!