Reactions in Food during Preparation and Cooking

Welcome to the Science of Cooking!

Ever wondered why bread turns brown in the toaster, or why egg whites turn into fluffy white clouds when whisked? Cooking isn't just about following a recipe; it is actually a series of chemical reactions! Don't worry if science seems a bit intimidating—we are going to break these reactions down into simple steps that happen every day in your kitchen.

By the end of these notes, you’ll understand how heat, air, and moisture change our food. Let's get started!

1. Carbohydrate Reactions: The "Three -ations"

Carbohydrates (like starch and sugar) react in three main ways when we cook them. A simple way to remember them is that they all end in "-ation".

A. Gelatinisation (Starch + Liquid + Heat)

This is how we thicken sauces! When starch granules (like flour or cornflour) are heated in a liquid, they act like tiny sponges. At about 60°C, they begin to absorb the liquid and swell up. Around 85°C, they burst and release starch into the liquid, making it thick and "jelly-like."

Example: Making a white sauce for pasta or a thick corn soup.

B. Caramelisation (Sugar + Dry Heat)

When sugar is heated on its own (without water), it melts and changes from white to a golden-brown color. It also starts to smell amazing! If you heat it too long, it will turn bitter and black.

Example: Making the brown top on a Crème Brûlée or caramel for popcorn.

C. Dextrinisation (Starch + Dry Heat)

This happens when starchy foods are cooked using dry heat (like baking, toasting, or grilling). The starch molecules break down into smaller pieces called dextrins. This makes the food turn brown and taste sweeter.

Example: The brown crust on a loaf of bread or the golden color of toast.

Quick Review:
• Gelatinisation: Thicken liquids (needs moisture).
• Caramelisation: Sugar turning brown.
• Dextrinisation: Starch turning brown (needs dry heat).
Common Mistake: Students often confuse Caramelisation and Dextrinisation. Just remember: Sugar = Caramelisation; Starch = Dextrinisation.

2. Fat Reactions: The Magic of Texture

Fats (like butter and oil) do more than just add flavor; they change how food feels in your mouth.

A. Shortening

Fats can give biscuits and pastries a "short," crumbly texture. When fat is "rubbed into" flour, it coats the flour particles. This acts like a waterproof raincoat, preventing the flour from absorbing water and forming long, stretchy gluten strands. This keeps the pastry "short" and crumbly instead of chewy.

B. Emulsion

Normally, oil and water do not mix. An emulsion is a mixture of two liquids that usually stay separate. To keep them together, we need an emulsifier (like the lecithin found in egg yolks). One end of the emulsifier loves water, and the other end loves oil, holding them together in a happy marriage!

Example: Mayonnaise is an emulsion of oil and vinegar held together by egg yolk.

C. Melting Point and Smoke Point

Melting Point: The temperature at which a solid fat (like butter) turns into a liquid. Different fats melt at different temperatures.
Smoke Point: The temperature at which an oil starts to break down and produce blue smoke. This is dangerous! You should always choose oils with a high smoke point (like peanut oil) for deep-frying.

Key Takeaway: Fats make things crumbly (shortening) and help mix oil and water (emulsion).

3. Protein Reactions: Changing Shape

Proteins are like long, tangled balls of yarn. Cooking "untangles" them.

A. Denaturation and Coagulation

Denaturation is the first step. Heat or acid causes the protein "yarn" to untangle and change shape. Think of it as the protein "relaxing."
Coagulation follows denaturation. The untangled proteins clump together to form a solid mass.
Example: A raw egg (liquid) becomes a fried egg (solid) because the proteins have coagulated.

B. Foaming

When you whisk egg whites, you are trapping air bubbles inside the protein. The protein denatures and wraps around the air bubbles to create a foam.

Example: Making meringues or sponge cakes.

C. Gluten Development

Gluten is a protein found in wheat flour. When you add water and knead (stretch) the dough, the gluten proteins become strong and elastic, like a rubber band. This traps air bubbles and allows bread to rise.

D. Maillard Browning

This is a special kind of browning that happens when protein and sugar (natural sugars in food) react together under dry heat. It creates a very savory, "meaty" smell and flavor.

Example: The brown crust on a seared steak or a roast chicken.

Did you know? Maillard browning is why a steak smells so much better than a piece of boiled meat!

4. Fruit and Vegetable Reactions: Enzymatic Browning

Have you noticed how an apple turns brown quickly after you take a bite? This is Enzymatic Browning.

When the cells of a fruit are damaged (by cutting or bruising), an enzyme inside the fruit reacts with oxygen in the air. This chemical reaction creates a brown color.

How to stop it:
• Acid: Squeeze lemon juice on the fruit (the acid stops the enzyme).
• Blanching: Dipping the fruit in boiling water for a short time (heat destroys the enzyme).
• Excluding Oxygen: Submerge the cut fruit in water or wrap it tightly.

5. Functions of Key Ingredients in Products

Let's see how these ingredients work together in real dishes.

The "Big Five" Ingredients:

1. Flour: Provides the structure (bulk) of the product. Contains starch (for dextrinisation) and protein (for gluten).
2. Sugar: Adds sweetness, helps with browning (caramelisation), and softens the texture by holding onto moisture.
3. Fat: Adds flavor, keeps products moist, and helps with shortening (for crumbly pastry) or aeration (when creamed with sugar).
4. Egg: Acts as a binder (holds things together), an emulsifier, or a raising agent (when whisked into a foam).
5. Raising Agents: (like baking powder or yeast) Produce gases (like \(CO_{2}\)) that make cakes and bread light and airy.

Putting it into Practice (Justifying Procedures):

• In Cakes: We cream butter and sugar together to trap air bubbles (aeration) so the cake is light.
• In Pastry: We rub in the fat to ensure it coats the flour (shortening), making it crumbly.
• In Sauces: We stir constantly while heating to prevent the starch granules from clumping together during gelatinisation.
• In Bread: We knead the dough to develop gluten so the bread can stretch and rise.

Don't worry if this seems tricky! Just remember that every step in a recipe is there for a scientific reason. If the recipe says "whisk until fluffy," it's trying to create a foam. If it says "rub in the butter," it's trying to achieve shortening!

Summary Checklist

Can you explain...?
• Why a sauce thickens? (Gelatinisation)
• Why toast is brown? (Dextrinisation)
• Why biscuits are crumbly? (Shortening)
• Why egg whites turn white and solid? (Coagulation)
• Why cut apples turn brown? (Enzymatic Browning)

Keep practicing and observing these reactions in your own kitchen—science is everywhere!