Year 8 Design & Technologies · Food and Fibre / Food Specialisations · 9m read

Food Science — Chemical Reactions in Cooking

Ever wonder why an egg can't go back to being raw after you cook it? That's chemistry at work. When you cook, heat triggers chemical reactions that transform ingredients—making bread rise and brown, turning egg whites from clear to white and firm, and creating totally new flavors. Learning how these reactions work helps you cook smarter, design better meals, and understand what's actually happening to the food you eat every day.

Learning objective: Explain chemical reactions in cooking and how they affect food properties and nutrition.

By the end of this lesson you can:

• I explain the Maillard reaction and caramelisation.
• I describe how heat affects proteins and starches.
• I evaluate how cooking methods affect nutritional value and food safety.

Cooking is chemistry you can eat. Bread rises, eggs set and sugar caramelises — all chemical reactions you control with heat and time.

Food science — chemistry in cooking

THE MAILLARD REACTION:
The most important reaction in cooking. Produces browning, flavour and aroma.
Occurs between AMINO ACIDS and REDUCING SUGARS when heated (typically above 140–165°C).
Creates hundreds of different flavour compounds.
Where you see it: bread crust, seared meat, toasted nuts, roasted coffee, chocolate.
Different from caramelisation (sugar only).
Requires DRY HEAT (not boiling/steaming — too much water inhibits the reaction).

CARAMELISATION:
Browning and flavour change caused by SUGARS alone (no amino acids needed).
Occurs above 160°C.
Sucrose (table sugar) → complex toffee/caramel flavours.
Used in: caramel, toffee, onion caramelisation, crème brûlée crust.

PROTEIN DENATURATION:
When PROTEINS are heated, they unfold (denature) and coagulate (solidify).
Irreversible (you can't un-cook a hard-boiled egg).
Why eggs cook: egg proteins denature at 60–80°C → set solid.
Why meat browns and firms: myosin (muscle protein) denatures at ~50–65°C.
DENATURATION by acids and alcohol (no heat): ceviche "cooks" fish in citric acid.

STARCH GELATINISATION:
Starch granules absorb water when heated → swell → thicken liquids.
The basis of: sauce thickening, gravy, custard.
Occurs at 60–80°C for most starches.
Full gelatinisation requires both sufficient HEAT and WATER.

LEAVENING (rising in baking):
CHEMICAL: baking powder/soda → CO₂ released when heated → bubbles expand → bread/cake rises.
BIOLOGICAL: yeast produces CO₂ through fermentation of sugars.
PHYSICAL: steam from butter/water → creates puff pastry layers.

COOKING METHODS AND NUTRITION:
HIGH HEAT, LONG TIME: destroys most heat-labile vitamins (B, C).
BOILING in lots of water: maximises vitamin leaching (discard water = discard nutrients).
STEAMING, MICROWAVING, STIR-FRYING: shorter time, less water → better vitamin retention.

Food science rules

• Maillard needs dry heat: The Maillard reaction is inhibited by water (boiling keeps temperature at 100°C). That's why you pat meat dry before searing — to get it hot enough for Maillard browning.
• Denaturation is irreversible: Once proteins denature and set, you cannot reverse it. Over-cooked eggs cannot be un-cooked. This is why precision of temperature and time matters in cooking.
• Nutrition vs palatability trade-off: Raw foods retain maximum nutrition but are less digestible and less palatable. Cooking improves digestibility and palatability at the cost of some nutrients. Balance both factors.

Quick reference

| Reaction | Example |
|---|---|
| Caramelisation | Sugar browns |
| Maillard reaction | Toast, seared meat |
| Coagulation | Egg sets |
| Gelatinisation | Sauce thickens |
| Fermentation | Yeast makes bread rise |

Worked example (Medium)

Explain why searing a steak creates a brown crust with intense flavour, while boiling it doesn't.

1. Searing conditions. Pan surface 180–220°C. Contact with metal. Meat surface DRIES quickly. Surface temperature exceeds 140°C.
2. Maillard reaction. Amino acids in meat protein + reducing sugars → Maillard reaction at surface → browning, hundreds of new flavour compounds, characteristic "seared" aroma.
3. Boiling conditions. Water temperature limited to 100°C. Surface remains wet. Temperature never reaches Maillard reaction threshold.
4. Result. Boiled meat: no browning, no Maillard flavours, often described as "grey" and bland. Seared meat: brown crust, intense flavour, aroma. Both cook the protein, but only searing triggers Maillard.

Answer: Searing reaches 140°C+ (Maillard threshold) + dry surface → browning + flavour compounds. Boiling is limited to 100°C + water present → no Maillard reaction.

Worked example (Hard)

Explain the science behind making a white sauce (béchamel): why does flour thicken the milk, and what happens if you add flour directly to hot milk without making a roux first?

1. Roux method. Melt butter, add flour, stir over low heat (2 minutes). This coats each starch granule in fat, separating them. Then gradually add milk, stirring constantly. The fat coating prevents starch granules from clumping together.
2. Gelatinisation process. As the sauce heats (60–80°C), starch granules absorb water from the milk, swell and burst — releasing amylose molecules that form a network, thickening the liquid. This is STARCH GELATINISATION. Continued heating and stirring ensures even thickening.
3. Why direct addition fails. Adding flour directly to hot milk causes the outer starch granules to gelatinise instantly on contact, forming a sticky coating around CLUMPS of dry flour. The dry flour inside the clumps cannot absorb water → lumpy sauce. The roux method prevents this by separating granules before they encounter hot liquid.

Answer: Roux separates starch granules with fat → prevents clumping. Starch gelatinisation (60–80°C) thickens as granules absorb water and swell. Direct flour addition → instant surface gelatinisation → lumps.

Common mistakes

• Confusing Maillard reaction with caramelisation Both produce browning, but Maillard requires AMINO ACIDS + SUGARS (meat, bread). Caramelisation involves SUGARS ONLY (toffee, caramel). They are different chemical reactions. Fix: Maillard = amino acids + sugars + heat (bread crust, seared meat). Caramelisation = sugars only + heat (toffee, onion caramelisation). Identify the ingredients present to name the correct reaction.
• Saying heat "kills nutrients" Heat doesn't destroy all nutrients — some vitamins are heat-sensitive (B, C) while minerals and some vitamins (A, D, E, K) survive cooking well. Cooking also IMPROVES digestibility and bioavailability of some nutrients. Fix: Be specific: "Heat-sensitive vitamins (B and C) are partially lost during cooking, especially with prolonged boiling. However, cooking improves the digestibility and bioavailability of many other nutrients."

The Maillard reaction — the most important reaction in cooking

The Maillard reaction was first described by French chemist Louis-Camille Maillard in 1912, but its full complexity wasn't appreciated until decades later. It is not a single reaction but a cascade of hundreds of reactions between amino acids and reducing sugars, producing hundreds of different flavour, aroma and colour compounds.

What makes the Maillard reaction so important is that it creates flavour complexity that cannot be achieved any other way. The specific compounds produced depend on which amino acids and sugars are present, the temperature, the pH, and the water activity — which is why seared beef tastes completely different from toasted bread, even though both involve the same type of reaction.

Food scientists now use Maillard reaction control to design flavours in processed foods. Flavour enhancers, artificial aromas and the characteristic taste of roasted coffee, chocolate and baked goods are all Maillard products. Understanding this reaction also explains culinary techniques: patting meat dry before searing removes surface water (which limits temperature to 100°C), allowing the surface to exceed 140°C and trigger Maillard browning. The science explains the technique.

Practice questions

1. What reaction is responsible for the brown crust on baked bread?

   1. Caramelisation
   2. Gelatinisation
   3. The Maillard reaction
   4. Denaturation only

   (1 marks)

2. Explain what happens to an egg when it is heated, using the concept of protein denaturation.

   (2 marks)

Summary

• Explain the Maillard reaction and caramelisation.
• Describe how heat affects proteins and starches.
• Evaluate how cooking methods affect nutritional value and food safety.