Cooking of food and heat transfer

Welcome to the Science of Cooking!

Ever wondered why a raw egg turns solid when fried, or why a potato gets soft when boiled? That is food science in action! In this chapter, we are going to explore why we cook food and exactly how heat travels from your cooker into your dinner. Don't worry if science feels a bit heavy sometimes—we will break it down into simple, bite-sized pieces.

1. Why Do We Cook Food?

We don’t just cook food because it tastes better (though that is a big reason!). According to the AQA syllabus, there are five main reasons why we put food through the cooking process:

1. To make food safe to eat: Many raw foods, like chicken or red kidney beans, contain harmful bacteria or toxins that can make you very ill. High temperatures kill these "bugs."
2. To develop flavours: Cooking changes the chemical structure of food. Think of the difference between a raw onion and a sweet, brown caramelised onion!
3. To improve texture: Cooking softens tough meat fibres or crunchy vegetables, making them easier to chew and swallow. It can also make things crispy (like chips).
4. To improve shelf life: Cooking destroys bacteria and enzymes that cause food to spoil, helping it last longer.
5. To give variety in the diet: You can cook a potato in dozens of ways—boiled, fried, roasted, or mashed. This stops our meals from becoming boring!

Quick Review: Can you remember the 5 reasons? Use the mnemonic "S.F.T.L.V" (Safe, Flavour, Texture, Life, Variety).

2. How Heat Moves: The Three Methods

Heat doesn't just "happen"; it has to travel from the heat source (like a hob or oven) to the food. There are three ways this happens. Think of these as the "transport modes" for heat.

A. Conduction (Direct Contact)

This is heat traveling through solid objects or by direct touch. Imagine a metal spoon sitting in a hot pan of soup; the handle gets hot even though it isn't touching the liquid. That is conduction.

• How it works: Particles vibrate and pass energy to their neighbours.
• In the kitchen: Heat moves from the hob to the base of the metal pan, and then directly into the food touching the pan.

B. Convection (Liquid and Gas)

This happens in liquids (water/oil) or gases (air). It works on the principle that "hot things rise."

• How it works: When air or water is heated, it becomes less dense and rises. Cooler air/water sinks to take its place, creating a convection current (a circular moving loop of heat).
• In the kitchen: This is how an oven heats up or how water boils in a saucepan.

C. Radiation (Waves)

This is heat traveling in rays or electromagnetic waves. There is no direct contact needed between the heat source and the food.

• How it works: The heat travels through the air until it hits the food and is absorbed.
• In the kitchen: Using a grill or a toaster. The glowing red element sends heat waves directly to your bread.

Did you know? Microwaves also use radiation! They send waves that make water molecules inside the food vibrate incredibly fast, which creates heat through friction.

Key Takeaway: Conduction = Touching. Convection = Moving liquids/air. Radiation = Waves/Rays.

3. Selecting the Best Cooking Method

Depending on what you are cooking, you need to choose the right method. This affects the nutritive value (how healthy it is) and the palatability (how good it tastes/looks).

Water-Based Methods

These use water or steam to transfer heat (mostly via convection).

• Boiling: Cooking food in bubbling water at \(100^{\circ}C\). Warning: This can wash away water-soluble vitamins (B and C).
• Steaming: Cooking food in the steam rising from boiling water. Best for health because the food doesn't touch the water, so it keeps its nutrients and colour.
• Simmering: Like boiling but at a lower temperature. Great for stews.
• Blanching: Part-cooking food briefly in boiling water then "shocking" it in cold water. This stops enzymic browning.
• Poaching: Gentle cooking in liquid below boiling point (e.g., eggs).
• Braising: Searing meat in a pan then slow-cooking it in liquid in the oven.

Dry Methods

• Baking: Cooking in the dry heat of an oven (convection).
• Roasting: Similar to baking but usually at higher temperatures with a little added fat.
• Grilling: Using radiation to cook food quickly. Great for browning and adding flavour.

Fat-Based Methods

• Stir-frying: Cooking quickly in a wok with a tiny bit of oil. This is a very healthy way to fry because it's fast and uses little fat.
• Shallow frying: Cooking in a pan with a layer of oil (e.g., sausages or eggs).

Don't worry if this seems like a lot! Just remember: Water-based = gentle/healthy. Dry = crispy/browned. Fat-based = fast/tasty.

4. Changing the Properties of Food

When we cook, we change the sensory properties (appearance, colour, flavour, texture, and smell).

• Appearance & Colour: Meat changes from red to brown. Bread turns golden through dextrinisation. Vegetables can become brighter or duller.
• Flavour: Smells are released (aroma). Flavours become more intense as water evaporates (reduction).
• Texture: Proteins denature and coagulate (set firm). Starches gelatinise (thicken sauces).

Using Marinades

We often use marinades (acidic liquids like lemon juice or vinegar) before cooking. The acid helps to denature (unfold) the protein in meat, making it more tender and adding moisture and flavour.

Quick Review Box

Common Mistake to Avoid: Don't confuse Conduction and Convection! Remember D for Direct contact (Conduction) and V for Vapour/Liquid (Convection).

Key Factors for Choosing a Method:
1. The type of food (is it a tough cut of meat or a delicate fish?)
2. Health (steaming vs. deep frying).
3. Time available (stir-fry vs. slow-roasting).
4. Equipment available (do you have an oven or just a hob?).

Summary: Cooking makes food safe and tasty. Heat moves via conduction, convection, or radiation. We choose our cooking method to balance health, speed, and flavour!