I was watching one of those “2020 in Review” TV specials and it was talking about how a lot of people took up baking in 2020. You might even remember when there was a run on yeast. But are you wise to how yeast makes bread rise? Or how chemical leavening agents (like baking powder and baking soda) work? And how does yeast lead to the wine at your feast?
Yeast are microorganisms. They’re like bacteria in the sense that they’re small and single-celled. But unlike bacteria, they’re eukaryotic – they store their DNA in a membrane-bound compartment – like animals. In a process called fermentation, yeast digest the sugar in grains to produce energy, and let off carbon dioxide (CO₂) gas in the process. (as a side product, they also produce ethanol, the “alcohol” part of alcoholic beverages).
Gas molecules, by definition, have a lot of energy and, unlike the lower-energy states of matter (solids and liquids), the molecules in gases aren’t “tied down” by attractions to other molecules. Therefore, gas molecules try to explore and get as much personal space as possible (well, they really just move around randomly, but if there’s nothing to bump into they’ll just keep going and if they bump into things they’ll move away so they end up spread out). When gases form within the dough, they push out trying to escape, but they’re trapped in a web of bread goo, with its structure coming from proteins called glutenin and gliadin which together make gluten (yep, if you had always wondered what “gluten” is, there’s your answer – it’s a family of plant storage proteins!). This goo is flexible, so it can expand, but the air can’t get out. And when you heat it up it “gelates” and solidifies, holding in that expanded structure.
But in order for the yeast to do their job, that sugar has to be “pre-chewed” because it comes in the form of STARCH – long chains of individual sugars. Starch is broken down by enzymes (reaction facilitators) called AMYLASES. Thankfully, flour provides these for the yeast – it has them “for themselves” because the flour’s grain kernels need to be able to break down the starch in their kernels when they want to germinate.
Once the starch is broken down into smaller pieces, the yeast can take over, using their maltase and sucrase enzymes to break those smaller pieces into individual glucose units (single molecules of blood sugar). And then they can ferment that glucose to make gas from.
The yeast aren’t trying to make gas, instead they want the energy that comes from fermentation – the gas is just a byproduct that happens to be handy for us. Another byproduct produced is ethanol, which is why fermentation is also useful for making alcohol (more on this later in the post).
Energy doesn’t actually come from the fermentation step. Instead, it comes from the glycolysis step – the initial steps by which glucose is broken down to get building blocks for other molecules and cellular energy in the form of ATP (2 ATP from glycolysis per glucose). More on ATP here: https://bit.ly/atpenergymoney basically it has 3 phosphate groups (phosphorus surrounding by 4 oxygens) stuck together – and each of those phosphates is negatively charged. Since opposites charges repel, they want to get apart from each other so, like clamping together a stiff spring, it takes energy to keep them bonded together (we call this chemical potential energy)- if you let one go (split ATP into ADP + Pi) you release energy, and you can use that energy to do things (like build proteins and stuff).
Your cells can take lots of different sources of fuel (carbs, fats, proteins, etc.) and break them down (metabolize them), “cashing them in” for ATP molecules which act a bit like biological arcade tokens in that they can be “spent” anywhere regardless of where they came from. But different fuels have different structures, so they have to go through different breakdown pathways. When it comes to the sugar glucose, the first pathway is that “glycolysis” thing we were talking about (other sugars can join the pathway at different places after some slight adjustments).
But glycolysis is only a small part of the energy-making picture. After glycolysis you’re left with big “1/2 chunks” of glucose called pyruvate molecules, which still have energy-making potential. In our cells, therefore, these glucose parts then usually go on to “aerobic respiration,” a process that requires oxygen (hence aerobic) and produces even more energy (for a final yield of over 30 ATP per glucose).
But what if that aerobic option isn’t available? You still get those 2 ATP from glycolysis (Yay!) BUT you get stuck because, in the process of making that ATP, they had to “spend” a different kind of “cellular money” – “electron accepting money.”
Quick overview – atoms (like individual carbons, oxygens etc.) are made up of positively-charged protons & neutral neutrons surrounded by a “cloud” of negatively-charged electrons. Atoms can link together to form molecules (like water, glucose, etc.) by sharing electrons through covalent bonds. Additionally, some molecules can take electrons (become reduced) and others can give up electrons (become oxidized). Remember OIL RIG: Oxidation Is Loss (of electrons); Reduction Is Gain (of electrodes). more on redox reactions: http://bit.ly/2yBOeDi
And that’s important because electrons are the really energetic parts – so by passing electrons from one to another you can kinda build up enough energy to make ATP. The ATP-making in the glycolysis step requires another molecule called NAD⁺ to accept electrons (get reduced) and become NADH. But then they have to regenerate the NAD⁺ if they want to make more ATP. In aerobic respiration, the NADH pass-off happens through a chain-reaction process called oxidative phosphorylation which makes a lot of energy http://bit.ly/metabolismbb
But when that aerobic route isn’t available, you need a different way for NADH to pass off its electrons. And this is the situation faced by the yeast. So they have to find something else to pass their electrons off to, and decide that a molecule of acetaldehyde (a decarboxylated pyruvate) will do. When they pass an electron to acetaldehyde, that reduces acetaldehyde to ethanol (and oxidizes NADH back to NAD⁺ in the process). No energy is generated in this fermentation part of the cycle, it only serves to regenerate the NAD⁺ so glycolysis can make more.
Another thing it generates: carbon dioxide (CO₂) gas, which is released when converting the pyruvate to acetaldehyde with the help of pyruvate decarboxylase. When fermentation happens in bread dough, proteins in the dough trap the air inside. And when you bake the bread the alcohol evaporates out. The same fermentation reactions happen when you make wine, but the gas escapes and the ethanol stays. Another difference is that, instead of flour, wine starts with grapes. The leaves and other green parts of plants like grape plants make sugar from sunlight & CO₂ in a process called photosynthesis. They make a double-sugar called sucrose, and then an enzyme called INVERTASE cuts that sucrose into its 2 component single sugars (monosaccharides) glucose and fructose, which the yeast can go to work on.
In our cells, instead of making acetaldehyde and ethanol, if oxygen isn’t available or energy is needed really quickly (such as in overworked muscle cells), we reduce pyruvate directly into lactate (with the help of lactate dehydrogenase) in order to regenerate NAD⁺. Since you’re bypassing the decarboxylation step (which yeast use to make acetaldehyde) you don’t produce CO₂ and your muscles don’t bubble (though they might bulge…) The lactate that’s made travels through your bloodstream to the liver where the liver can convert it back to pyruvate and use it “normally.” (note: this is the “same thing” as lactic acid – lactic acid refers to the protonated form, which can thus act as an acid (more below) and lactate refers to the deprotonated form (lactic acid’s conjugate base), but the terms are often used interchangeably because you never really know what form any one molecule will be in). Lactic acid is blamed a lot for muscle soreness, but research has shown that, although it might contribute to pain during the actual exercise, it’s not the cause of delayed onset muscle soreness. Instead, that’s thought to be caused by minor muscle damage and imbalances of other metabolites. http://bit.ly/3hBPGvJ
Yeast were the “original” leavening agents, but they’re not the only way to get gas. You can use CHEMICAL LEAVENING AGENTS to produce CO₂ bubbles in your dough by utilizing acid/base chemistry. Before you get confused, let me briefly review something that can be confusing (I know it confused me when I was learning chemistry!)
Before I said that a proton is a positively-charged subatomic particle – and it is – when a proton is part of an atom it’s just one part of that atom (chilling with neutrons and electrons). And, it’s the number of protons that actually defines an atom (e.g. oxygen is defined as an atom with 8 protons and carbon’s an atom with 6 protons). But protons can also hang out by themselves – and, since hydrogen is defined as an atom with a single proton, if you have hydrogen without an electron, you just have a proton. So proton = H⁺. If something donates a proton, we call it an acid and if something accepts a proton, we call it a base. pH is a measure of how many free protons are hanging out in a solution (the more H⁺, the more acidic, but the lower the pH because it’s an inverse log). http://bit.ly/phacidbase
So, back to our baking story… Let’s look at a couple common leavening agents: baking soda & baking powder
BAKING SODA (sodium bicarbonate) is a base (proton (H⁺) stealer). It steals protons and gives off carbon dioxide gas. But to do this, it needs something to give up the protons (an acid). So you have to give it one (some common ones are buttermilk, brown sugar, yogurt, lemon juice, vinegar, cream of tartar, molasses, applesauce, or honey). You want there to be the right ratios or you’ll have to much acid -> sour, or too much base -> metallic/soapy.
BAKING POWDER is baking soda with acid(s) included as well as an “inert” (nonreactive) part (often cornstarch) that keeps the reaction from starting (and using up its usefulness) before you’re ready. The “usefulness” in terms of gas generated is determined by how much baking soda there is. But the real usefulness in baking comes from the type of acid, which determines how quickly that gas will be released – you don’t want it all to be released before the dough’s ready to set.
The base (baking powder) needs the acid in order to make the gas, but they can’t “meet” until they’re both dissolved in water because in their solid forms they’re salts, meaning that they have oppositely-charged “counter ions” bound to them & thus “hiding them”. So baking powder isn’t activated until it gets wet. The cornstarch is there to absorbs moisture from the air so the active components don’t absorb it.
Most commercial baking powders are “double-acting” – they have 2 bursts of activation – one when you get it wet and a second when you heat it up. for example, in Clabber Girl baking powder, you have 4 ingredients: sodium bicarbonate (baking soda); monocalcium phosphate (MCP); sodium aluminum sulfate (SAS); and cornstarch
- MCP is “fast-acting” – it reacts with the baking soda when you get them wet – this sets up a network of mini gas bubbles and when you knead the bread you help spread them out evenly so that when the second acid, SAS, starts producing gases those gases can expand those existing bubbles which are nicely spread out
- since SAS is less eager to react, it won’t do so until you heat it up. Heating helps it dissolve -> gives the molecules more energy to wiggle loose from their countering shields
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