Bread
One question is, what happens, chemically and biologically, when one makes bread, and can length scales help to shed some light onto this process? 1
Overview
At a high level, the flour particles will break apart and form gluten chains. Yeast will ferment the sugar in-between the gluten chains, releasing $CO_2$ that pushes against the gluten chains and causes the bread to rise. The alcohol output of the fermentation will flavor the bread as it bakes. Baking dehydrates the bread, leaving the strong gluten chains behind, and air bubbles.
Ingredients
Size of particles for ingredients for bread are given below, ordered by size:
| Particle Type | Size (Order of $10^{x}m)$ |
|---|---|
| Flour Particles | $-5$ to $-4$ 2 |
| Yeast Cell | $-6$ (on average) |
| Oil | $-9$ (50-100 length atom-long chains) |
| Sugar (Sucrose) | $-9$ ($C_{12}H_{22}O_{11} \approx 40\text{ atoms}$) |
| Sugar (Glucose) | $-9$ ($C_6H_{12}O_6 \approx 20\text{ atoms}$) |
| Water | $-10$ ($H_20$ is $3 \text { atoms}$) |
Steps
Cooking the bread starts by activating 3 part (dry) yeast to 1 part sugar, diluted in 72 parts of water. Since the yeast is on the order of $1000\times$ larger than the glucose molecules, there is a lot of sugar here. On a human scale, this seems to me like one emaciated human (dry yeast) would be up to the knees (3:1) in a room full of candy bars (glucose), then having someone turn on the “anti-gravity” switch to make everything start floating around (water dilution).3 The yeast needs to put some meat on its bones; based on this conversion table, it will need to triple its mass. Interestingly, overall, the amount of sugar in this recipe is actually triple the amount of dry yeast added (however, it’s likely the water adds a lot of mass as well, so this ratio may be somewhat of a coincidence).
Next, one must mix the dry ingredients: sugar and flour. The flour particles are the largest order of magnitude, and will break apart to form structural bonds. Tucked in-between these large boulders that are flour particles rest small, valuable candy bars (surcrose/glucose).
Next, oil is added to the yeast mixture. This step seems controversial/unnecessary among bakers at thefreshloaf.com, but overall they seem to agree that it helps keep the bread from drying out, and makes the bread softer. One user, richawatt, provides some chemical insights:
using fat or any type of shortening in a wheat bread will soften the crumb. It actually “shortens” the gluten strands and makes it less chewy and softer. That is why it is called shortening. As far as the moistness aspect. The oil will make the bread seem to be moister longer because oil is liquid at room temperature and does not evaporate.
Adding to his answer, I hypothesize that the oil will make the bread softer not just because the gluten strands are less dense (first order), but also because fewer gluten strands for the yeast to navigate early on allows it to find more of the initial sugar, ferment it, and add air (second order). Particularly, if using dehydrated yeast, the oil’s blockade of gluten formation could enable the yeast to find these sugars when it is the most “hungry.”3 In any case, the 3TBs of sugar will provide attractants for the yeast to explore a giant, packed landslide of flour-particle boulders that the yeast now finds itself in.
Kneading will break the (giant!) flour particles apart 4. The newly formed starches and gluten proteins emerging from the flour particles will provide some important dynamics:
- The gluten will inhibit yeast motion, trapping it in one place. Even before the gluten can form chains, it will automatically pack much more tightly than the large flour particles, since gluten is smaller.
- The starch will begin to feed the yeast, even (and especially) when the yeast gets stuck in between gluten molecules.
These two steps happen gradually. When the yeast ferments the starch, the gas pressure and fermentation loosen the flour particle more, over a few-hour period. The loosening then makes it easier for future rounds of kneading to break apart the flour particles more, to form the gluten into longer chains, and break apart even more starch for the yeast to feed on.
The last step is baking. The fermentation process will also create alcohol residue. Once the bread is baking, the alchol will evaporate, leaving interesting flavoring behind. It is likely that the yeast will have “one last hurrah” in the initial baking phase, as the heat will accelerate their activity initially, until the internal temperature reaches 140 degrees, so I wonder if that hurrah contributes to the flavor as well. It is suggested to stop baking once the internal bread temperature reaches 200 degrees F; just below boiling, perhaps to prevent too much water from escaping.
In a literal sense, we will try to add some understanding to the recipe of bread, as depicted by Jim Keller on the Lex Fridman podcast. ↩︎
According to this study that measured laser diffraction of flour particles, most flour particles cause laser diffraction to occur around $10^{-5}m$ and $10^{-4}m$ orders. These numbers are relatively consistent with another study, that measures effects of whole-wheat flour on bread structure, who find medium- and fine-grained whole wheat to be on this order of magnitude. ↩︎
The fact that the rehydration is occuring also seems critical. If the yeast move with a process similar to chemotaxis, their attractant sensors may “saturate” once immersed in so much sugar, causing sensory adaptation. However, I find it unlikely to that the yeast attractant sensors will saturate, when they are in “rehydration mode” - they have to triple their mass quite rapidly. Yeast are eukaryotic, so they may have some more exotic ways of hunting nutrients than simple prokaryotic chemotaxis. ↩︎ ↩︎
Without fully understanding the chemistry, the length-scales analysis requires that the flour particles get broken apart. Proofing requires the dough to double in size; for that magnitude of change to occur, the yeast must begin to break down the starches in the flour particles. There simply isn’t enough sugar to power the proofing, and it’s unlikely that rehydrating yeast will be physically large enough to make a dent in the gigantic flour particles - it would be like a 150 pound human trying to sink his teeth directly a 1600 pound cow. It’s possible for a human to eat a cow, but there’s some preprocessing work to do beforehand to cut the cow into smaller pieces. ↩︎