What kind of science is involved as you make bread? Here I go through the bread making process, and how enzymes, your techniques, the Maillard reaction, starch gelatinization & starch retrogradation all affect your bread! The references are referenced in [brackets] and the links are a the bottom of the page.
The embedded video is here (the video link is below)
Feel free to check out the other bread science videos and information:
Part 1 - Introduction to Bread Science: Citations and References ; YouTube video
Part 3 - The Science of Sourdough Bread: Citations and References ; YouTube Video
Part 4 - The Science of Rye Flour and Rye Bread: Citations and References ; YouTube Video
Part 5 - The Science of Salt-Rising Bread: Citations and References ; YouTube Video
Part 2: The Science of the bread making process (yeast bread)
Amendments to the last video:
Slide 2:
Enriched vs. unenriched doughs
Slide 3:
More on gluten:
Glutenin provides strength and elasticity to gluten, while gliadin provided its stretchiness.[27]
Slide 4: How sugar and fat affects gluten
Sugar molecules attract water because they are hydrophilic (water-attracting), so the sugar competes with gluten for water. if sugar is mixed with the flour and liquid, then the sugar can attract water molecules that would otherwise help bind glutenin and gliadin into gluten[28]. So essentially, a bread with too much sugar can be dense.
Fats influence gluten development. Fat repels water because it is hydrophobic (water-repelling). In a dough, if you add fat to flour before adding liquid and kneading, the fat molecules surround and coat the flour particles, allowing less water to penetrate and access the glutenin and gliadin that form gluten. Fat also binds with the gluten that does form, making it hard to form long, strong gluten strands.
So, if you mix in the fat into the flour before hydrating, you can get a more cake-like bread.[30]
But if you mix in the fat after kneading (e.g., in a brioche), you can get a more bread-like structure because the gluten structure is still disturbed, but after it has already formed, as opposed to before.[30]
So essentially, a good amount of sugar and fat can reduce gluten formation and shorten gluten strands, giving a softer and more tender bread. This bread also has more flavor than a lean dough.
Slide 5: Whole wheat flour and gluten
Whole grain flours do contain more nutrients than "white" flours, but not all these nutrients are bioavailable, so we may not be able to extract them during digestion.
Whole wheat flours tend to contain the most protein, but do not form gluten as well as bread or all-purpose flours because the shards of germ and bran break gluten strands.[27]
Slide 6:
More on starch:
Only damaged starch is processed (or used) by enzymes like amylase, not intact starch granules. However, as you’ll see in today’s video, intact starch granules play a huge role in the bread making process during baking.
Slide 7: Outline
Slide 8:
I am using active dry yeast so first I activate it with warm water and sugar.
Add half the flour and the butter, plus salt
Mix in
Mix in the milk
Then mix in the rest of the flour
Hydration to form gluten, adding salt and water to aid in this process of gluten development.
Some milk and butter are used, but not a lot, which makes a more flavorful and tender bread.
Mix the ingredients together in different steps so they are combined before kneading and also to give the flour time to hydrate.
Slide 9:
9. Kneading to develop gluten by stretching and pulling it, dispersing the water and salt so more gluten can form and align. Once the gluten is not tearing and it passes the windowpane test it is finished kneading.
Slide 10:
10. First Fermentation: Adding sugar speeds up process. Some people call this proving, or proofing. Microscopic kneading occurs too, so gluten may still be forming. Gluten is also relaxing during fermentation, which allows the dough to stretch and grow.
Slide 11:
11. “Punching down”, or knocking the air out of the dough after the first fermentation
Pushes out CO2 from the dough, reduces CO2 buildup that can inhibit yeast
Equilibrates temperature
Remixes the sugars and redistributes the yeast, which can reactivate the yeast that may have otherwise been running out of resources in their area2
12. Shaping:
a. Re-Strengthens gluten prior to baking. Forms a good shape that traps gas, helping the loaf stay tall.
Slide 12:
13. Second rise before baking
Ferments again
Gluten relaxes
Longer and multiple rises allow more starch molecules to be broken down and more flavor and gluten to develop.[3]
Slide 13:
14. Baking
Slide 14: Baking: What's happening at 115°F (46°C)
Room Temperature to 115°F:
CO2 gas bubbles expand, and more CO2 is produced during baking once 104°F (40°C) is reached.[16] Remember the last (second) rising period. This is the last chance the gluten has had to relax. That strong, but relaxed gluten network helps trap those bubbles, reducing their release from the bread.
At this temperature, ethanol dissipates and water dissipates, producing gases.
The yeast is still active at the beginning of baking, and up to 115°F (46°C) when they die, meaning that they are still producing sugars in the early stages of baking. However, once they die, those leftover sugars help with bread flavor and browning (Maillard reaction).
Slide 15: Baking: What's happening at 140°F (60°C)
140°F:
By 140°F, all the yeast are dead
At about 140°F (60°C) the starches start to gelatinize, meaning that the starch granules absorb water and explode, releasing a gel that then starts to set and solidify.[16] Think of gelatin or Jello. You add a gelatin packet to warm water and with time it hydrates and forms a gel, and when cooled it solidifies. That’s also what’s happening in bread, but in a different way of course.
Slide 16: What is starch made of?
What even is Starch?
Starch is mainly made up of 2 polymers of D-glucose (so basically glucose superstructures). These two polymers include 1) amylose: a lightly branched polymer with a small number of glucose chains, and 2) amylopectin: highly branched polymer with many clusters of short chains (Wang et al. 2015).
Starch’s natural form is usually crystalline, consisting of a core that contains mainly amylose and amylopectin chains surrounded by concentric semicrystalline growth rings that also contain amylose and amylopectin (in the figure, black lines are amylopectin, blue lines are amylose molecules (Wang et al. 2015)).
Slide 17: Baking: Starch Gelatinization in Detail
To go into more detail, during baking gluten releases water that the starch granules absorb. That water, along with heat causes 3 things to happen:
The crystallites melt
The double helices unwind
The hydrogen bonds holding the amylose and amylopectin together, break (Wang et al. 2015).
When the hydrogen bonds break, the absorbed water molecules bind to the open bonding sites; however, the water does not bind the starch molecules back together, instead it increases the size of the area taken up by the individual starch molecules.[25] When the water and hydrogen bond breaking gets to be substantial, the starch granules burst and release hydrated amylose and amylopectin that create a gel-like paste (referred to as gelatinization). These gelled starches start to firm up as the bread continues baking[23], which begins the stabilization of the bread structure and crumb structure, a.k.a. internal structure in the bread is forming.
Slide 17: Baking: What's happening at 165°F (75°C)
165°F:
Any residual amylase enzymes can keep working until 165°F (75°C) max[16]
Gluten starts becoming semi-rigid as it cooks, releasing water. Similar to other proteins, gluten hardens when it’s heated, which helps create the bread structure.[26]. That hardening will trap air bubbles and form the crust. However, although it's hardening, the gluten remains flexible (have you ever squished fresh bread?).
Slide 19: Baking: What's happening at 280°F (140°C)
280°F onwards:
Both the sugars produced from enzymatic breakdown and the amino acids play a role in the Maillard reaction: above 284°F (140°C) they add flavor to the dough and help create a brown crust[1]
So essentially the act of fermenting and baking the dough releases more flavor, which is why bread tastes so much more complex than just flour alone[2]
Slide 20: More on the Maillard Reaction
Biological compounds that participate in Maillard reactions are[1]:
Monosaccharides: pentoses and hexoses (xylose, ribose, arabinose, glucose, galactose and fructose).
Disaccharides: lactose and maltose.
Amino acids: lysine, arginine, histidine and tryptophan.
Good browning: Transformations caused by Maillard reactions include generation of melanoidins (light yellow to dark brown) while simultaneously generating flavor and aroma compounds.
Bad browning/burning: Under extreme reaction conditions (burning), mutagenic or potentially carcinogenic compounds, such as acrylamide may form.[1]
Factors that influence the Maillard reaction (see reference below in “more on Maillard reaction”):
Sugar type: The type of sugar and its amount can enhance browning.
Fat type: Baked goods formulated with milk solids (e.g., butter or butterfat) develop darker crust color due to added lactose and amino acids in milk. Olive Oil can yield a crispy crust. Egg wash enhances browning significantly.
Fermentation: Amount of yeast, its activity, length of fermentation time and type of dough system - are all detrimental to the rate and extent of browning reactions because they consume sugar.
Enzymatic activity from hydrolases: Activity from amylases and exo-proteases can also affect non-enzymatic browning (NEB). The higher the enzymatic activity the higher the number of free sugars and amino acids that can participate in Maillard reactions.
Presence of free water in the system: Higher water activity (aw) enhances molecular mobility of sugar and amino acid solutes in the dough, thus increasing NEB. At too high of hydration Maillard reactions tend to slow down.
pH: Maillard reactions occur under alkaline conditions. Optimal browning takes place at pH 6–8.
Temperature and time: Temperature (frying and baking) and time of exposure can determine the rate and extent of NEB reactions[1].
Slide 21: The bread after baking (cooling)
After baking:
The gelatinized starches begin to cool and firm up, meaning that the gelled amylose and amylopectin start to firm up and crystallize, also known as the process of retrogradation.
This is why you should not cut into bread to early! Because the starch is too soft, and you can get a gummy texture![2]
Slide 22:
18. Staling:
Bread stales because the starch crystallizes and hardens overtime[1] which is continuation of the retrogradation process.[22,23] This is thought to be due to amylopectin retrogradation.
Starch retrogradation is essentially an attempt by the gelatinized amylose and amylopectin molecules to re-crystallize of starch.[24] The longer the bread sits after baking, the more it crystallizes and stales.
But that reformation isn’t perfect, things are still good, but they just can’t go back to the way they used to be. Think of starch retrogradation as sort of like a band from the 1980s or 1970s trying to get back together, but the band members are kind of old and crusty and not as good as they used to be, so it works but it’s not the same.
As the bread cools, the amylose crystallization happens sooner than amylopectin crystallization, therefore amylose forms crystals first, followed by linear parts of amylopectin.[22] The amylose (linear molecule), and linear parts of amylopectin begin to re-align.
The amylose and amylopectin release their water and re-form Hydrogen bonds. [22,24]
That water that is expelled (this process is known as syneresis) can evaporate, causing the bread to dry out, and contributes to its stale texture.[24]
Retrogradation and staling:
So, if we think of retrogradation from a broader scope, if it starts happening as the bread cools from the oven right after baking and continues to stale until it’s completely re-crystallized. We like a level of retrogradation so that we can easily slice the bread into firm, not squishy slices, but not so much retrogradation that it tastes old and stale. That's another reason why it's good to wait until the bread cools, but not too much.
Slide 23-24: More on staling, what to do with stale bread
Starch retrogradation is accelerated at temperatures between -8 – 8°C (-22 to -13°F) which is why it can be better to freeze your bread or keep it at room temperature rather than refrigerate it.
Additives like fat, glucose, and other additives can reduce starch degradation in your bread[22], for several reasons. For example, lipids can interact with starch during baking, altering the structure to hinder recrystallization (retrogradation when it cools) (Wang et al. 2015). Lipids and sugar also absorb moisture during baking[24] and moisture released from the retrograding starch molecules, thus prolonging the freshness of bread essentially.[23] This is one of the reasons why adding sugar and fats like oil, butter, eggs etc. can enhance bread softness and taste.[23]
Salt (at 2% or less) can also affect gelatinization and slow down retrogradation if the starch is stored between 4-25°C (Wang et al. 2015).
Some ways to combat retrogradation and staleness (but not fix/reverse it), is to heat the bread back up. By heating it up the starch crystal structure softens and can reabsorb some moisture from the air (a bit like a sponge) and become soft again without tasting as stale. But for this to work the bread can’t be too stale and hard to begin with.[23]
Slide 25: Summary
Extra info on starch digestion:
Well, when we consume starch, easily digestible forms of starch are digested rapidly by the human digestive enzymes in the upper gut. This causes a rapid release of glucose into the bloodstream, giving us a source of energy (Wang et al. 2015), kind of like how glucose is a source of energy and fermentation for yeast.
Cooking and enhanced gelatinization/breakdown of starch makes it easier to digest because it breaks up the starch and makes it easier for enzymes to access the molecules. Partially gelatinized starch can contain easily digested portions and less easily digested portions.
Retrogradation influences starch digestibility. For example, after it’s baked the rapid retrogradation of amylose makes the starch more digestible. But as it cools and rests longer the amylopectin retrogradation renders the starch gradually less digestible. Being less digestible isn’t necessarily a bad thing. As mentioned early, easily digestible starch is quickly converted to glucose in the upper GI tract. But starch that is less digestible can be digested slowly to even be resistant to breakdown and reaches the lower GI tract, e.g., the colon, which means it’s not going to spike blood sugar levels as much (Wang et al. 2015). Kind of like having a bowl of sugary cereal as opposed to a bowl of unsweetened oatmeal.
Never forget that there’s many uses for stale bread, you just have to catch it before it gets moldy (which is less of an issue for sourdough). You can make croutons, breadcrumbs, stuffing, bread pudding, French toast, etc.
More on the Maillard reaction:
Biological compounds that participate in Maillard reactions are:1
Monosaccharides: pentoses and hexoses (xylose, ribose, arabinose, glucose, galactose and fructose).
Disaccharides: lactose and maltose.
Amino acids: lysine, arginine, histidine and tryptophan.
Transformations caused by Maillard reactions include generation of melanoidins (light yellow to dark brown) while simultaneously, flavor and aroma compounds are also generated. Under extreme reaction conditions, mutagenic or potentially carcinogenic compounds, such as acrylamide may form.[1]
Three primary stages are critical for the Maillard reaction[1]:
Initial condensation of a carbohydrate carbonyl group with an amine, followed by a series of reactions leading to formation of the Amadori product.
Rearrangement, dehydration, decomposition, and/or reaction of Amadori intermediates to form furfural compounds, reductones and degradation products.
Reaction of Maillard intermediary products to form heterocyclic flavor compounds and red/brown to black colored, melanoidin pigments of high molecular weight.
Factors that influence the Maillard reaction:
8. Sugar type: The rate and extent of browning vary with the type and amount of sugars in formula. Liquid sweeteners such as HFCS, invert syrup, honey or 42 dextrose equivalent (DE) corn syrup, for example, are rich in reducing sugars, and thus can enhance Maillard reactions. The higher the DE of liquid sweeteners (the higher the amount of simple reducing sugars), the higher the extent of Maillard reactions (more intense brown color). Baked goods formulated with milk solids develop darker crust color due to added lactose and amino acids in milk.
9. Fermentation: Amount of yeast, its activity, length of fermentation time and type of dough system, all are detrimental to the rate and extent of browning reactions. As sugars are consumed by yeast and bacteria, less substrates are available for non-enzymatic browning (NEB).
10. Enzymatic activity from hydrolases: Activity from amylases and exo-proteases can also affect NEB. The higher the enzymatic activity the higher the amount of free sugars and amino acids that can participate in Maillard reactions.
11. Presence of free water in the system: Higher water activity (aw) enhances molecular mobility of sugar and amino acid solutes in the dough, thus increases NEB. While maximum activity is obtained at aw of 0.60–0.80; at aw of 0.95 or higher, Maillard reactions tend to slow down.
12. pH: Maillard reactions occur under alkaline conditions. Optimal browning takes place at pH 6–8.
13. Temperature and time: Temperature (frying and baking) and time of exposure can determine the rate and extent of NEB reactions. Typical reaction temperature range is 150-160oC; browning can still occur at lower temperatures, although at a slower rate.
Maillard Reaction Source: https://bakerpedia.com/processes/maillard-reaction/
References:
1: Compound Chem: https://www.compoundchem.com/2016/01/13/bread/
2: Stella Culinary: https://stellaculinary.com/audio-podcasts/stella-culinary-school/scs-019-twelve-steps-bread-baking
3: Exploratorium: https://www.exploratorium.edu/cooking/bread/bread_science.html
16: GRDC: https://grdc.com.au/__data/assets/pdf_file/0027/227745/grdc-science-behind-dough-fa.pdf.pdf
Wang et al. 2015: Wang, S., Li, C., Copeland, L., Niu, Q., and S. Wang. Starch Retrogradation: A Comprehensive Review. Comprehensive Reviews in Food Science and Food Safety. 14(5): 568-585.
26: The Spruce Eats: https://www.thespruceeats.com/what-is-gluten-995123
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