From Bread to Toast

In a way, toasting is reviving stale bread. The simplest bread only needs three ingredients: flour, yeast, and water. The two main components of wheat flour are protein and starch. Technically, wheat flour does not contain gluten; it contains two proteins: glutenin and gliadin. In the presence of water, these proteins bond to each other and form gluten networks. The growth of the network just requires water. That’s why no-knead bread recipes work. There is a fascinating picture in Modernist Bread that shows the proteins growing branches in water. Kneading speeds up the development of the gluten network and strengthens it. Other flours (rye, rice) don’t have the same kind of proteins, so gluten networks do not form when they are mixed with water, no matter how much they are kneaded.

Another result of kneading is the even distribution of yeast. Yeast cells are living, single-celled fungi. They serve two functions in the making of bread:

1. They generate carbon dioxide as a product of their metabolism. This carbon dioxide is trapped by the elastic gluten network. That’s why dough size grows during proofing. In the oven, yeast activity accelerates with the higher temperature, and a last burst of carbon dioxide is generated, resulting in what’s known as the “oven spring.” Eventually, the yeast dies at around 60⁰C.
2. Yeast cells also generate flavor. As a byproduct of the microbes’ metabolic processes, they produce many aromatic compounds. The longer the dough ferments, the more complex flavors are produced. However, you cannot ferment the dough forever because the yeast will run out of food and stop generating gas. The existing gas will slowly leak out of the dough, and you will not have an airy crumb. Most commercial baker’s yeasts are a single strain of Saccharomyces cerevisiae, while levains typically contain a mix of different species of yeasts and lactic acid bacteria. So, in theory, sourdough bread has a richer flavor. But it’s harder to achieve consistency, and some people are turned off by the sour taste.

The food for yeast is sugar, which comes from the hydrolysis of the starch in wheat flour. Yeast cells cannot consume starch directly. They depend on the enzyme amylase to catalyze the hydrolysis (the splitting of compounds by the addition of water) of starch molecules. The resulting simple sugar not only feeds the yeast but also reacts with proteins during the Maillard reactions that brown the crust during baking. The aromatic compounds generated by the Maillard reaction and the yeast cells are why fresh bread smells so good.

To make the dough easier to handle before putting it into the oven, people spread dry flour. These flour particles don’t mix enough with water, and the starch molecules in them do not decompose into sugar. That’s where the white dust on top of the brown crust of baked bread comes from—they are cooked flour that did not participate in the browning Maillard reactions.

It takes time for the flour to absorb water. Novice bakers often make the mistake of thinking their dough is too wet and rush to add flour. They should trust the recipe and give it time. When starch is heated up in the presence of water, a process called gelatinization happens. The starch granules absorb water and swell. Water molecules break into the crystalline structure, and intermolecular bonds between starch molecules break down. The whole thing becomes viscous. Eventually, new bonds form between starch molecules. When the bread cools down, the new network becomes a gel that sets, which prevents the crumb from collapsing.

During baking, water moves both inwards and outwards. On the surface of the dough, water boils first and evaporates. This allows the surface temperature to rise above 100⁰C and accelerates the Maillard reactions. The browned outer layer is the crust of the bread. In professional deck ovens, steam is injected at the beginning of baking. The steam condenses on the surface of the cold bread, dumping a lot of heat (the specific latent heat of water vaporization). This gelatinizes the starch and causes the formation of a thin skin, the pellicle. This thin skin seals the moisture below. The bread ends up with a thinner and more crispy crust. If steam is present throughout the process, as in steamed Chinese bao, we will see the pellicle as a shiny, peel-able layer.

In the other direction, as heat propagates into the dough, water boils in the high-temperature region on the outer layer and condenses in the low-temperature region on the inner layer, acting like a heat pipe. That’s why bread bakes so fast even though the airy crumb actually makes it a nice insulator. When freshly baked bread is taken out of the oven, the moisture in the middle of the bread starts migrating out. Like steaks, you should wait for the bread to cool down before tasting it.

As the bread cools, the gelatinized starch molecules rearrange themselves and return to the crystalline form. This is called retrogradation. Retrogradation is the reason stale bread gets hard. Retrogradation is slower at room temperature than at low temperature (fridge temperature), but completely stops under -20⁰C. Putting bread in the fridge is the worst way to preserve it. You should either leave it out and eat it quickly or freeze it.

The retrogradation process is partially reversed in the temperature range between 60⁰C and 80⁰C. Another effect of the reversal is that more water molecules are bonded to the starch molecules at the hydrogen bonding sites, so the bread tastes moist.

Toasts are sliced bread where:

1. The top and bottom sides used to be the internal crumb of bread.
2. It’s slightly stale. Retrogradation has happened.
3. The surface-to-volume ratio is large, which means there is a relatively small window of time to maintain a temperature gradient.

So this is what we need to get our ideal toast:

1. The surface needs to dry out and reach a high enough temperature to accelerate Maillard reactions, which give us the golden crust and the aroma.
2. The interior should reach around 75⁰C to reverse retrogradation, but not so high that too much water evaporates. If retrogradation is not reversed, the interior will not become soft.

This concludes the second step of our investigation. Let’s move on to the third step: the cooking process.