Food and Nutrition (0648) | Cooking of food

Study Notes: Cooking of Food (IGCSE Food & Nutrition 0648)

Hello future culinary expert! This chapter, Cooking of Food, is one of the most practical and scientific parts of your course. Cooking isn't just mixing ingredients; it's a science that uses heat to change food completely. Understanding this science helps you cook safely, deliciously, and healthily.
Don't worry if the scientific terms seem tricky—we will break them down using everyday examples!

1. Why Do We Cook Food?

Cooking is essential for several key reasons:

• Safety and Hygiene: Heat destroys harmful bacteria, viruses, and parasites, making food safe to eat.
• Improved Digestibility: Cooking softens tough fibres (like those in meat and vegetables) and breaks down starches, making food easier for our bodies to digest and absorb nutrients.
• Better Appearance and Sensory Properties: Cooking makes food look more appealing and enhances its flavour, taste, and texture.
• Variety: It allows us to combine ingredients and create diverse dishes (e.g., transforming raw dough into bread).

Sensory Properties of Food

These are the qualities we perceive when we eat:

• Flavour: The combination of taste (detected by the tongue: sweet, sour, salty, bitter, umami) and smell (aroma).
• Taste: What the tongue registers.
• Texture: How the food feels in the mouth (e.g., crispy, smooth, chewy, soft, crunchy).

Key Takeaway: Cooking is primarily about safety, digestion, and improving the sensory enjoyment of food.

2. How Heat Travels: Methods of Heat Transfer

Heat needs to move from the source (like the hob or oven) into the food. There are three main ways this happens:

a) Conduction (Direct Contact)

Heat transfer through direct contact between solids, or between a heat source and a pan. The heat travels slowly through the material.

• Analogy: Holding a metal spoon in a hot bowl of soup. The heat travels slowly up the handle to your hand.
• Example: Frying an egg. The heat moves from the stove element, through the metal frying pan, into the egg.

b) Convection (Moving Fluids)

Heat transfer through the movement of liquids (like water or oil) or gases (like air in an oven). Hot substances rise, cool substances sink, creating a current.

• Analogy: A washing machine cycle. The hot water moves around, mixing everything evenly.
• Example: Boiling water. The water at the bottom gets hot, rises, cools at the top, and sinks again. This circulating movement heats the food. Also occurs in ovens (air currents).

c) Radiation (Waves)

Heat transfer by infrared waves that travel directly from the source to the food without needing a medium (like air or water) in between.

• Analogy: Feeling the heat from a bonfire or the sun on your face, even when you aren't touching it.
• Example: Grilling, toasting bread, or using a barbecue. The intense heat waves hit the surface of the food directly.

3. Cooking Methods (Dry, Moist, and Other)

Cooking methods are broadly grouped based on whether they use moist heat (liquid/steam) or dry heat (hot air/fat).

Moist Heat Methods

These methods generally preserve vitamins well but can leach (dissolve) water-soluble nutrients if too much liquid is used.

• Boiling: Cooking food in a liquid (usually water) at a high temperature (100°C).
• Simmering: Cooking food gently just below the boiling point (around 85°C–95°C). Often used for stocks or tenderizing meat.
• Poaching: Cooking food gently in a small amount of liquid, below simmering point (71°C–82°C). Ideal for delicate foods like eggs and fish.
• Steaming: Cooking food suspended above boiling water, using the steam. This is a very nutritious method as nutrients are not lost to the cooking liquid.
• Stewing: Cutting food into pieces and cooking slowly in a small amount of liquid (which is usually served with the food).
• Braising: First, browning the food (e.g., meat) then cooking it slowly in a covered dish with a small amount of liquid.
• Pressure Cooking: Cooking food in a sealed pot that creates very high pressure, allowing water to reach temperatures above 100°C, significantly reducing cooking time.

Dry Heat Methods

These methods often create browning and crispy textures but can result in nutrient loss due to high temperatures.

• Baking: Cooking food in an oven using dry heat (convection and radiation). Common for cakes, breads, and casseroles.
• Roasting: Cooking large pieces of food (like meat or potatoes) in an oven, often using fat or oil.
• Grilling: Cooking food quickly using intense radiant heat from above (or below, as in a barbecue).
• Frying: Cooking food in hot fat or oil.
  • Shallow Frying: Using a small amount of fat (e.g., frying bacon).
  • Deep Frying: Submerging food completely in hot fat (e.g., chips).

Other Methods

• Microwave Oven Cooking: Uses electromagnetic waves to vibrate water molecules inside the food, generating heat internally. Very fast, uses little fuel.
• Slow Cooker: Uses low, steady heat over a long period. Excellent for tough cuts of meat and uses minimal fuel. (Uses conduction and gentle convection).

Did you know? Microwaving is very good for preserving heat-sensitive vitamins (like Vitamin C) because the cooking time is so short.

4. The Science of Heat: Changes in Food Components

When food is heated, chemical changes occur. You must understand the effect of dry heat and moist heat on proteins, fats, sugars, starches, and vitamins.

A. Effects on Carbohydrates (Sugars and Starches)

• Gelatinisation (Starch): This happens when starch is heated with liquid (moist heat). The starch granules absorb the liquid, swell up, burst, and thicken the liquid.

  Example: Making a white sauce (roux method) or gravy.

• Dextrinisation (Starch): This happens when dry heat is applied to starch. The long starch chains break down into smaller molecules called dextrins, which causes the food surface to brown and become slightly sweet.

  Example: Toasting bread.

• Caramelisation (Sugar): This happens when sugar is heated (dry heat) above its melting point. It changes chemical structure, turning brown and developing a rich, slightly bitter flavour.

  Example: Making caramel sauce or browning sugar on a crème brûlée.

B. Effects on Proteins

• Coagulation (Protein): Proteins are long, folded chains. When heated (by moist or dry heat), they unwind (denature) and then join together (coagulate), changing from a liquid or soft state to a solid, set mass.

  Example: An egg white turning solid white when boiled or fried. Meat firms up when cooked.

• Non-Enzymic Browning (Maillard Reaction): This complex reaction occurs when heat is applied to food containing both proteins (amino acids) and reducing sugars. It creates hundreds of flavour compounds and the characteristic brown crust.

  Example: The rich brown crust on roasted meat, baked bread, or seared steak.

• Enzymic Browning: Although not strictly caused by cooking heat, cooking *stops* this process. It occurs when cut fruit/vegetables (like apples or potatoes) are exposed to oxygen, causing enzymes to react and turn the surface brown.

C. Effects on Fats and Oils

• Melting: Solid fats turn liquid when heated.
• Smoking Point: This is the temperature at which a fat or oil begins to break down, producing a bluish smoke and irritating fumes. This means the fat is degrading and producing substances that can give food an unpleasant flavour.

  Common Mistake: Heating oil past its smoking point makes the food taste burnt and reduces the nutritional quality of the oil.

• Rancidity: This is the spoilage of fats and oils, causing unpleasant flavours and smells. Heat and light speed up this process (called oxidation). Deep-frying oil used repeatedly can quickly become rancid.

D. Effects on Vitamins

Heat generally causes nutrient loss, especially for water-soluble vitamins (B group and C).

• Vitamin C and Thiamin (B1) are particularly sensitive to heat and oxygen.
• To preserve vitamins, use minimal water, steam food, and cook for the shortest possible time.

5. Practical & Economic Considerations

a) Preparation and Cooking to Preserve Nutritive Value

As cooks, we need to balance safety and sensory appeal with nutrient retention.

• Reduce Preparation Time: Wash vegetables whole and chop them immediately before cooking to minimise exposure to oxygen.
• Use Minimal Water: Steam, microwave, or pressure cook rather than boiling, as water-soluble vitamins leach into the cooking liquid.
• Keep Skins On: The skin and just beneath it often contain high concentrations of vitamins and fibre (NSP).
• Use Cooking Liquid: If you do boil vegetables, use the leftover liquid (e.g., in a soup or stock).

b) Economical Use of Resources (Food, Equipment, Fuel, Labour)

We should aim to save money, time, and energy when cooking.

• Fuel Economy:
  • Use the correct size pan for the hob element (less heat waste).
  • Use lids on pans (traps heat, speeds up boiling).
  • Use pressure cookers or microwave ovens for rapid cooking.
  • Cook multiple items in the oven simultaneously (batch cooking).
• Equipment Economy: Choose multipurpose equipment (e.g., a food processor) to save costs and storage space. Ensure equipment is clean and well-maintained to work efficiently.
• Food and Labour Economy:
  • Plan meals to use leftovers (less food waste).
  • Use cheaper cuts of meat and tenderise them using slow methods like stewing or pressure cooking.
  • Prepare ingredients in advance or freeze portions (saves labour later).

Key Takeaway: Efficient cooking involves choosing methods that protect nutrients (like steaming) and maximise the use of fuel and time (like using a pressure cooker or a slow cooker).

Congratulations! You've covered the core science behind heating food. Remember these principles when you step into the kitchen, and your cooking will not only taste better but will also be healthier and more efficient!