I Really Like Bread: Chemistry of Baking Joel Rael April 26, 2022 1 Introduction Every culture has their bread: some folks grew up with dinner rolls, or corn bread, or baguettes or tortillas. I grew up in New Mexico and aside from eating chili every other day, tortillas were there too. Primarily corn and wheat flour tortillas, depending on the dish. I once asked my grandmother, ”why is the corn tortilla so flat and why is the flour tortilla so fluffy?” and she then told me that I ask too many questions. This wonder remained pretty dormant for the rest of my life: until this week. I fall asleep best when there is some sort of noise like a fan, or pre-recorded rain sounds, or something on Netflix. I was introduced to ”The Great British Baking Show” and that was my sleep noise for the night. They men- tioned that a contestant was punching the dough on the counter to ”develop the gluten” and I was so confused: what is there to develop? Is the gluten not already made? What is so special about all the other ingredients? This commanded me down the rabbit hole that is this paper and I hope you find it informative and insightful. 1 2 Glory of Gluten Figure 1: Common Gliadin Structure In short, gluten is a com- plex protein that primarily has two constituents: gliadin and glutenin [1]. These constituents are umbrella terms that get more specific depending on the grain (rye, rice, etc.), though their interactions are grossly similar. One notable feature of glutenin’s is that they may be composed of several differ- ent protein subunits but overall have a combination of high and low molecular weight portions. Under aqueous conditions, gliadin and glutenin molecules are encouraged together by hydrogen bonding to create disulfide bonds [2]. In a baked good, these gluten complexes are relatively abundant but the fun starts when the oven turns on. These gluten molecules are able to somewhat anneal in an oven, promoting cross-linking when CO2 gas evolution expands the dough [3]. This high heat is not enough to fully detach the gluten complexes, in fact, the expansion allows for more of these complexes to meet. The eventual cooling of the baked goods overall leaves these expanded (and more inter-connected) complexes, permitting puffy pastries. As you may be aware, the addition of yeast catalyzes glycolysis (critically producing more gas to further expand). Below is a link to some little animation I made to depict contact points of hydrogen bonds in the gluten complex. 2 Figure 2: Gluten Conglomerate Bound to Human HLA-DQ2 (”Celiac Gene”, in green/purple), PDB entry ”4OZH”[4] Animation Link Perhaps one of my favorite things about baked goods is that you can enjoy them either salty or sweet, and so I began thinking about sugars: 3 Sweet Stuff The standard sugar we all know and (maybe) love comes in the form of sucrose, a disaccharide of one glucose and one fructose molecule. Something very interesting in sugars is that some may be classified as reducing or non-reducing depending on whether or not they have a free hemiacetal group, glucose is a reducing sugar. The sugar may tautomerize between its ring- form and linear form. This allows for potential reactions that the intrachain sugars cannot participate in. So, these hemiacetal areas are typically found on the end of a polysaccharide and allow for linkage with another sugar. This is possible under acidic or basic conditions, but let’s focus on protonation of glucose. The anomeric carbon is protonated and converts the hydroxyl into a good leaving group. This is where we have some interesting variation, the next sugar-to-be-added dictates the 3 Figure 3: Sugar Protonation Figure 4: Water Leaving identity of the new disaccharide. As aforementioned, glucose and fructose can join to create sucrose (table sugar). Somewhere in Season 9 of ”The Great British Baking Show”, the co-host of the show, Ms. Prue Leith, unveils a special malt loaf. The loaf recipe calls for 150g of malt extract during the bake and also after, when it is used as a glaze [5]. Figure 5: Prue Leith’s Malt Loaf [5] The malt extract is primarily composed of maltose [6], a disaccharide that has two glu- cose molecules. So, lets go on how to make it. After protonation of the first glucose, this now-deficient carbon is then nucleophilically at- 4 tacked by another sugar, allowing for glycosidic bond formation. Figure 6: Glycosidic Linkage We now have maltose. Figures Generated in pyMOL, ChemDraw 5 References [1] A. D. Blonstein and P. J. King. ”Endosperm Proteins” , page 210–211. Springer Science amp; Business Media, 2013. [2] Joel Markgren, Mikael Hedenqvist, Faiza Rasheed, Marie Skep ̈ o, and Eva Johansson. Glutenin and gliadin, a piece in the puzzle of their structural properties in the cell described through monte carlo simulations. Biomolecules , 10(8):1095, 2020. [3] Margaret Hill. Attack of the gluten. https://www.acs.org , 2012. [4] Jan Petersen, Veronica Montserrat, Jorge R Mujico, Khai Lee Loh, Dennis X Beringer, Menno van Lummel, Allan Thompson, M Luisa Mearin, Joachim Schweizer, Yvonne Kooy-Winkelaar, and et al. T-cell receptor recognition of hla-dq2–gliadin complexes associated with celiac disease. Nature Structural amp; Molecular Biology , 21(5):480–488, 2014. [5] Prue Leith. Prue leith’s malt loaf, Dec 2021. https://thegreatbritishbakeoff.co. uk/recipes/all/prue-leith-malt-loaf/ [6] Daniela F Seixas Chaves. Malt extract as a healthy substitute for refined sugar. American Journal of Biomedical Science amp; Research , 4(1):52–53, 2019. 6