Introduction: The Magic of Heat

Have you ever wondered why a clear, runny egg white turns solid and white when fried? Or why a piece of white bread becomes brown and crunchy in the toaster? These changes aren't magic—they are scientific reactions caused by heat!

In this chapter, we will look at how dry heat (like baking or grilling) and moist heat (like boiling or steaming) change the nutrients in our food. Understanding these changes helps us become better cooks and ensures we get the most nutrition from our meals.

Note: This chapter focuses on the chemical changes to nutrients. For more information on how heat travels, see the chapter on "Heat transfer and methods of cooking."

1. Effects of Heat on Proteins

Proteins are found in foods like meat, fish, eggs, and pulses. When we heat them, their structure changes significantly.

Coagulation

The most important term to remember for protein is coagulation. Imagine protein molecules as tiny, coiled-up balls of yarn. When you heat them, they "uncoil" (denature) and then clump together to form a solid structure.

  • What happens: The liquid or soft protein becomes firm and solid.
  • Example: A raw egg is liquid, but when heated, the proteins coagulate to make the egg solid.
  • Important Tip: If you heat protein for too long or at too high a temperature, it becomes tough, dry, and harder to digest. This is why overcooked meat can feel "rubbery."

2. Effects of Heat on Carbohydrates (Sugars and Starches)

Carbohydrates react very differently depending on whether the heat is dry or moist.

A. Dry Heat: Dextrinisation and Caramelisation

When you use dry heat (like toasting or baking), two main things happen:

1. Dextrinisation (Effect on Starch): When starchy foods (like bread or potatoes) are heated with dry heat, the starch molecules break down into smaller groups called dextrins.

  • The Result: The food turns brown, develops a slightly sweet taste, and becomes crunchy.
  • Example: The brown crust on a loaf of baked bread or the surface of toast.

2. Caramelisation (Effect on Sugar): This happens when sugar is heated by itself.

  • The Result: The sugar melts, turns from white to golden brown, and eventually becomes dark brown with a distinct aroma. If heated too far, it becomes bitter and carbonises (turns to ash).
  • Example: Making caramel sauce or the "burnt sugar" top of a Crème brûlée.

B. Moist Heat: Gelatinisation

When starch is heated in a liquid (moist heat), it undergoes gelatinisation.

  • How it works: Starch granules absorb the liquid and swell up. At about \(60^{\circ}C\), they start to absorb a lot of water. At about \(80^{\circ}C\), the granules burst, releasing starch into the liquid.
  • The Result: The liquid thickens and forms a gel.
  • Example: Thickening a white sauce (roux method), cooking rice, or making custard.

Quick Review: Remember Dry = Dextrinisation; Moist = Gelatinisation.

3. Effects of Heat on Fats and Oils

Fats and oils are used for frying and roasting. They behave differently than proteins and carbs because they can reach much higher temperatures.

Smoking Point

Every fat has a smoking point. This is the temperature at which the fat begins to break down and emit a blueish smoke.

  • When a fat reaches its smoking point, it develops an unpleasant smell and taste. It can also be dangerous as it is close to its flash point (where it could catch fire).

Rancidity

Rancidity is when fats and oils "go off" and develop a nasty smell and flavor. While this usually happens over time during storage, heat can speed up this process by helping oxygen react with the fat.

4. The Science of Browning

Food turns brown for different reasons. The syllabus requires you to know two main types: Enzymic and Non-enzymic browning.

Non-Enzymic Browning: The Maillard Reaction

This is a special type of browning that happens when protein and sugar (starch) are heated together.

  • What happens: A chemical reaction between amino acids (from protein) and reducing sugars.
  • The Result: It creates a wonderful savoury "browned" flavor and dark color.
  • Example: The brown surface of a roasted piece of meat or the crust of a cake.

Enzymic Browning

This is not usually caused by cooking heat, but heat can be used to stop it! Enzymic browning happens when enzymes in fruit and vegetables (like apples or potatoes) react with oxygen in the air after being cut.

  • How heat helps: We can "blanch" (briefly boil) vegetables to destroy these enzymes, which prevents them from turning brown.

5. Effects of Heat on Vitamins

Vitamins are the most "fragile" nutrients. They are easily damaged by heat and water.

Vitamins B and C (Water-Soluble):

  • These are very sensitive to heat. High temperatures can destroy them completely.
  • Because they dissolve in water, moist heat (boiling) causes them to "leach" out of the food and into the cooking water.
  • Practical Tip: To save these vitamins, use as little water as possible and cook for the shortest time (steaming is better than boiling!).

Vitamins A, D, E, and K (Fat-Soluble):

  • These are generally more stable when heated and do not dissolve in cooking water. However, they can be damaged by very high frying temperatures.

Summary Checklist

Before your exam, make sure you can explain these key terms:

  • Coagulation: Protein setting into a solid (e.g., eggs).
  • Dextrinisation: Starch browning in dry heat (e.g., toast).
  • Gelatinisation: Starch thickening in moist heat (e.g., sauce).
  • Caramelisation: Sugar browning and melting.
  • Maillard Reaction: Browning when protein and sugar react together (e.g., roast meat).
  • Smoking Point: The temperature where fat breaks down and smokes.
  • Vitamin Loss: Heat and water destroy Vitamin C and B-group vitamins.

Key Takeaway: Heat changes the texture, flavor, and nutritional value of food. While it makes food safer to eat and tastier, we must control the heat to avoid losing vitamins or making proteins too tough!