The 5 Core Reactions Behind Every Bake

When you bake, five things are quietly doing all the work: protein, starch, fat, sugar and gas. Each one changes in its own way when it meets heat, water or air , some through real chemical reactions, some through simple physical changes , and together they decide whether your bake rises, sets, stays tender and turns golden. Learn what these five do and you’ll understand what’s actually happening inside the oven, not just what the recipe tells you to do.

This guide is your one-page map to those five players. We’ll keep it friendly, explain every term as we go, and show you exactly what each one is up to.

New to all this? It helps to read What Is Baking Science? first for the big picture. This article zooms in on the chemistry , the specific reactions and changes behind each ingredient.

Key takeaways

• Every bake is driven by five players: protein, starch, fat, sugar and gas.
• Some of what happens is a true chemical reaction (browning, leavening, fermentation) and some is a physical change (melting, swelling, trapping air) , knowing the difference helps you troubleshoot.
• Heat, water, acid and air are the triggers that set these changes off.
• Once you can name what each player is doing, baking problems stop being mysteries and start having clear causes.

What Chemical Reactions Happen When Baking?

The main chemical reactions in baking are: protein coagulation (proteins set with heat), the Maillard reaction and caramelisation (browning and flavour), the acid–base reaction in chemical leavening (baking soda + acid making carbon dioxide) and fermentation (yeast producing gas). Alongside these run important physical changes , starch swelling, fat melting, sugar dissolving and air expanding , that don’t create new substances but are just as vital to the result.

Now let’s meet the five players one by one.

The one-page reaction map

Here’s the whole thing on a single page. Skim it now, then we’ll walk through each row in plain English.

Five players. That’s the entire cast. Everything else in baking is these five interacting.

Player 1: Protein , the structure-builder

Proteins are what hold a bake together. They come mainly from eggs, from flour (the proteins that form gluten) and from dairy.

Protein goes through two steps when heated, and it’s worth knowing both:

1. Denaturation , the protein “unfolds.” Raw proteins are curled up like tiny balls of wool. Heat, acid or vigorous whisking makes them relax and unravel.
2. Coagulation , the unfolded proteins then link arms with each other and set into a solid network. This is a genuine chemical change: you can’t un-cook it.

You watch this happen every time you fry an egg , clear, runny white turns firm and opaque. The same thing sets your sponge cake, firms your custard and holds a meringue together.

Proteins have a second trick, too: they take part in the Maillard reaction (more on that under sugar), which is what browns your crust and builds savoury, toasty flavour. So protein is doing double duty , building structure and helping create colour and taste.

Where it goes wrong: heat protein too hard or too fast and it bonds too tightly, squeezing out liquid , that’s a curdled custard or a dry, rubbery bake. Gentle, even heat keeps the network soft and silky.

Player 2: Starch , the quiet thickener

Most of the flour in your recipe isn’t protein , it’s starch, packed into tiny granules that behave like microscopic sponges. Starch is responsible for turning a wet batter into a set, sliceable crumb, and it’s the reason a custard or sauce thickens on the stove.

The process is called gelatinisation . When starch is heated with water , usually around 60–70 °C , the granules soak up that water, swell up and thicken everything around them. As the bake cools, the swollen starch firms further and holds the shape.

Here’s a useful, slightly surprising fact that most articles skip: gelatinisation is a physical change, not a chemical reaction. No new substance is created , the starch simply absorbs water and swells. Knowing this matters, because it tells you the fix when things go wrong: a gummy, undercooked middle usually means the centre didn’t get hot enough for long enough for the starch to finish swelling. The answer is heat and time, not a different ingredient.

(Bonus: when a bake goes stale, the starch is slowly re-crystallising , a process called retrogradation. That’s why warming a day-old muffin briefly makes it taste fresh again.)

Player 3: Fat , the tenderiser

Fat , butter, oil, or plant-based spreads , is the player that makes things tender, rich and moist. And here’s something helpful to understand: almost everything fat does is a physical change, not a chemical reaction. That’s a genuinely useful thing to know, because it reminds you that “baking science” isn’t only chemistry.

Fat has several jobs:

• Melting. Solid fat melts in the heat, which affects how a bake spreads and how tender it feels. Cold butter behaves very differently from soft butter , that’s why recipes are fussy about butter temperature.
• Shortening. Fat coats the flour proteins and physically gets in the way of gluten forming. Less gluten means a shorter, more crumbly, tender texture , which is exactly what you want in pastry and shortbread. (This is literally why baking fat used to be called “shortening.”)
• Trapping air. When you cream soft butter with sugar, the sugar crystals cut tiny air pockets into the fat. Those pockets become bubbles that help your cake rise.
• Emulsifying. Fat and water don’t normally mix, but with the help of an emulsifier (like the lecithin in egg yolk), they can blend into a smooth, stable batter. That’s an emulsion , think of the silky texture of a good cake batter.

Where it goes wrong: butter that’s too warm can’t hold air (flat cake); too much fat can make a bake greasy or crumbly to the point of falling apart.

Player 4: Sugar , far more than sweetness

Sugar is the multitasker of the group. Yes, it sweetens , but it also softens, browns and helps bakes keep. Some of what it does is physical, and some is genuinely chemical, so it sits in both camps.

• Dissolving (physical). Sugar dissolves into the liquid in your batter, which affects texture and spread.
• Holding water (physical). Sugar is hygroscopic , a fancy word meaning it grabs and holds onto water. That keeps your crumb soft and your bakes fresher for longer.
• Tenderising (physical). Sugar competes with flour for water and gently interferes with gluten and protein setting, giving a softer, more tender result.
• Caramelisation (chemical). Heat sugar on its own to around 160 °C+ and it breaks down and browns into hundreds of new flavour compounds , those deep, sweet, toffee notes.
• The Maillard reaction (chemical). This is the big one for flavour. When sugars and proteins meet high heat together, they react to create the rich, savoury, golden-brown crust on bread, cakes and pastry. It’s why the outside of a bake tastes so much more interesting than the inside. Caramelisation and Maillard often happen side by side.

Where it goes wrong: too little sugar and bakes are pale and dry; too much and they can spread, sink or brown too fast on the outside before the middle cooks.

Player 5: Gas , the reason things rise

A cake or loaf is light because it’s full of gas bubbles that grow in the heat. Gas is the one player that can be produced three completely different ways, and it’s worth knowing all three because they explain almost every “why won’t it rise?” problem.

1. Chemical leavening (a real chemical reaction). This is baking soda (bicarbonate of soda) meeting an acid , like lemon juice, yoghurt or buttermilk , and reacting to release carbon dioxide gas. Baking powder is simply baking soda with a dry acid already mixed in, so it works with just water and heat. This acid–base reaction is one of the clearest chemical reactions in all of baking.
2. Biological leavening , fermentation (also chemical). Yeast is a living organism that feeds on sugars and produces carbon dioxide and a little alcohol as it grows. That’s what makes bread dough puff up over hours. The alcohol bakes off in the oven.
3. Physical leavening. No reaction here , just air you beat in (creaming, whisking eggs) and steam from water turning to vapour in the heat. Both expand as the bake warms and push it upward. Steam is the hidden hero behind puff pastry and choux.

Whatever the source, the gas expands in the oven while the protein and starch set a structure around it , locking the risen shape in place.

Where it goes wrong: old baking powder, forgetting the acid for your bicarb, or an oven that sets the crust before the gas has finished expanding , all leave you with a flat or dense bake.

Chemical change vs physical change, the shortcut for troubleshooting

Here’s a clarifying idea that makes you a sharper baker. Some of what happens in the oven is a chemical reaction (a new substance forms and you can’t reverse it) and some is a physical change (the ingredient changes form but no new substance appears).

• Chemical reactions: protein coagulation, Maillard browning, caramelisation, the acid–base leavening reaction, yeast fermentation.
• Physical changes: fat melting, starch swelling (gelatinisation), sugar dissolving, air and steam expanding, water evaporating.

Why does this help? Because it points you to the fix. A browning or rise problem is usually a chemical issue , check your leavening, your acid, your oven heat. A gummy middle or a greasy crumb is usually a physical one , check your heat, timing, temperatures and ratios. Same framework, two useful branches.

How all five work together in one bake

None of these players acts alone. Picture a simple cake going into the oven and watch them hand off to each other:

1. Air and steam (gas) start expanding, and the batter begins to puff.
2. Sugar is holding moisture and keeping things tender while the crumb heats through.
3. Starch reaches its temperature, swells and begins setting the crumb around the growing bubbles.
4. Protein coagulates, firming the structure so the risen shape holds.
5. Sugar and protein at the surface run the Maillard reaction and caramelisation, and the top turns golden.
6. Fat, melted throughout, keeps the whole thing tender and rich.

Time it right and you get a light, set, golden, tender cake. Every one of those steps is a lever you can adjust , and that’s the real power of understanding the reactions.

Why this matters for inclusive baking

This five-player map becomes genuinely powerful the moment you bake gluten-free, dairy-free or egg-free. When you remove an ingredient, you’re really removing a player and the job it was doing.

• Remove eggs and you lose protein’s setting and binding , so you need another way to coagulate and hold the structure.
• Remove wheat and you lose the gluten proteins and change how starch behaves , so gluten-free blends lean on extra starches and binders.
• Remove dairy butter and you change how fat tenderises and browns , so the swap has to do the same physical jobs.

When you understand which player you’ve taken out, you can replace its function instead of just guessing at a substitute. That’s the difference between a gritty, crumbly gluten-free cake and one nobody can tell apart from wheat.

Want to go deeper than a single article can? The Baking Science Academy turns these five players into structured, hands-on lessons , so you can read any recipe, see the reactions at work, and adapt it confidently for any diet. It’s the “why” behind every bake, taught step by step.

Frequently asked questions

Question: What chemical reactions happen when baking? 

Answer: The main chemical reactions are protein coagulation (proteins setting with heat), the Maillard reaction and caramelisation (browning and flavour), the acid–base reaction in chemical leavening (baking soda plus an acid producing carbon dioxide), and fermentation (yeast producing gas). Physical changes like starch swelling, fat melting and steam expanding happen alongside them.

Question: What are the five key players in baking? 

Answer: Protein, starch, fat, sugar and gas. Protein builds structure, starch sets the crumb, fat tenderises, sugar sweetens and browns, and gas makes the bake rise. Almost every baking result comes from how these five interact.

Question: Is baking a chemical or physical change? 

Answer: Both. Baking involves true chemical reactions (browning, protein setting, leavening, fermentation) and physical changes (melting, swelling, dissolving, evaporating). Knowing which is which helps you work out how to fix a bake that goes wrong.

Question: What is the Maillard reaction in baking? 

Answer: The Maillard reaction is what happens when proteins and sugars react together under high heat, creating the golden-brown colour and rich, savoury flavour of a crust. It’s different from caramelisation, which is sugar browning on its own, though the two often happen at the same time.

Question: Why is understanding baking reactions useful? 

Answer: Because every baking problem is really a question about which reaction or change went wrong. Once you can name what’s happening , a rise issue, a setting issue, a browning issue , you can fix it and adapt recipes with confidence instead of trial and error.