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?
blog form (text old): http://bit.ly/fermentandleaven
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.”
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