Why’s matzoh flat, but matzoh balls are fluffy? Fluffiness comes from gas generation in dough that causes it to expand (leaven), which can come from biological fermenters (yeast) or chemical LEAVENING AGENTS. Matzoh is cooked before yeast have a chance to leaven it. This kills the yeast so they can’t ferment any more, but if you crush that matzoh up into “matzoh meal” you can make a dough with that and you can make it fluffy with chemicals (baking soda) instead of yeast

Tonight my family is holding a Passover Zeder (Seder over Zoom) & Snoopy & I are ready with some science! I’m not religious at all, but my dad’s side of the family is Jewish and Passover (Pesach) has traditionally been the one time of the year that our extended family gets together, catches up on one another’s lives, remembers the past, reflects on the present, and hopes for the future. We still want to catch up, but don’t want to catch (or spread) coronavirus, so this year we’ll be holding our ceremonial meal (Seder) together but apart.

Passover is a Jewish holiday that remembers the Jews’ flight out of Egypt. The Book of Exodus describes how the Israelites had to flee Egypt on such short notice that they didn’t have time to let their bread rise. So the observance of Passover involves avoiding letting certain grains ferment.

These days we usually purposefully add yeast to our foods, but wild yeast spores float around on dust and stuff, so if you leave dough out and wet, yeast can start to chomp on it and produce those gases. So for Passover you aren’t supposed to let wet certain grains stay exposed to air for longer than 18 minutes before you bake it (baking kills the yeast with their leavening ability). Hence the matzoh eating. Matzoh is unleavened bread. It looks kinda like a giant cracker. It’s made of flour & water like bread, but it’s baked before it has time to rise. 

This avoids biological fermentation that’s caused by the production of gasses by yeast. Yeast are microorganisms. They’re like bacteria in the sense that they’re single-celled. But unlike bacteria, they’re eukaryotic – they store their DNA in a membrane-bound compartment – like animals. Yeast digest the sugar in grains to produce energy, and let off carbon dioxide gas in the process. 

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. This goo is flexible, so it can expand, but the air can’t get out. And when you heat it up it “gelatos” and solidifies, holding in that expanded structure.

But that sugar has to be “pre-chewed” because it comes in the form of STARCH – long chains of individual sugars. It’s broken down by enzymes (reaction facilitators) called AMYLASES. Thankfully, flour provides these for the yeast – it has them 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. 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 a byproduct that happens to be handy for us. Another byproduct produced is ethanol 

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

In our cells, the glucose parts then usually go on to aerobic respiration, a process that requires oxygen (hence aerobic) and produces even more energy.

But what if that 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). 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 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 https://bit.ly/atpenergymoney 

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 acetaldehyde 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 just regenerates the NAD+ so glycolysis can make more.

Another thing it generates: carbon dioxide (CO₂) gas. When fermentation happens in bread dough, proteins in the dough trap the air inside. And when you bake the bread the alcohol evaporated 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. 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. 

Yeast were the “original” leavening agents Passover traditionally avoids, 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/or get swapped between molecules 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). 

So, back to our baking (or not baking) story… Let’s look at a couple common leavening agents we’re told to leave out for Passover: baking soda & baking powder

BAKING SODA (sodium bicarbonate) is a base (proton (H⁺) stealer). It steals protons and gives off carbon dioxide gas. 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.

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

Gas formation can come from mechanical sources – think RediWhip can – decompress compressed air

Bottom line: at Passover we eat matzoh which is bread that’s cooked before the yeast can make it rise. So it stays unleavened like a big thin cracker. So we avoid that useful yeastiness. But we drink lots of wine (I stick with water). So yeast still make their mark this holiday.

Happy Passover everyone!

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