Introduction

Cooking in a watery medium causes certain chemical and physical effects on food. I’ll show you what these are and what tricks you can apply to solve any problems that may arise.

Eggs

You’ve probably experienced eggshells cracking during boiling. This usually happens due to a temperature difference between the water and the egg, and also because the air inside the shell tends to expand.

Tip: You can avoid this by making a small hole in the blunt end of the egg or by adding salt or vinegar to the boiling water.

The cooking temperature of eggs greatly influences the final result.

At 60°C, the proteins in the egg white coagulate, absorbing energy and stabilising, while the yolk remains soft and does not coagulate.

To make a soft-boiled egg, the temperature should be 63°C for 3 minutes, resulting in a runny centre.

At 71°C, a thermal setting of the vitellins in the yolk occurs, causing coagulation.

Between 85–90°C, the yolk sets at the same time as the white without the latter becoming rubbery, resulting in a softer hard-boiled egg.

At 100°C, the weak hydrogen bonds that hold water in the protein network break, the water evaporates, and the proteins bind more tightly, producing a rubbery egg.

If cooking time is prolonged, the yolk hardens and a greenish layer forms. Proteins coagulate, forming water, carbon dioxide, and hydrogen sulphide.

If the egg cools slowly, the hydrogen sulphide (H₂S) reacts with iron (Fe) in the yolk to form a compound, iron sulphide (FeS + H₂), which is blackish-green. This happens when eggs are left to cool in the hot water.

If the cooling is quick—by running cold water over them—this chemical reaction doesn’t occur and the yolk remains yellow.

Tip: To avoid this chemical reaction, always cool boiled eggs immediately under cold running water.

Vegetables

When boiling vegetables, their colour is enhanced during the first minutes of cooking because the air bubbles inside plant cells act like magnifying glasses, intensifying the green of the chloroplasts. That’s why some vegetables are only cooked for a few seconds (blanched), to achieve vivid colours.

As cooking continues, some cells burst and release organic acids and chlorophyll. The vegetable may turn brown or greyish-green due to a substance called pheophytin.

Tip: For high-moisture vegetables like spinach, celery, pumpkin, onion, turnip, etc., always boil them in open containers so the volatile substances responsible for unpleasant odours can escape. Use plenty of salted water and leave the lid off. This reduces cooking time and results in less tough vegetables. Pre-blanching is also advisable to eliminate odours. And never throw away vegetable cooking water—it’s perfect for enriching stews or preparing rice.

For starchy vegetables like potatoes, steaming is ideal. (link to the article on steaming)

Legumes

When soaked, legumes rehydrate and increase in volume, which continues during cooking.

In hard water, the cell walls toughen. In alkaline pH, they soften as starch and proteins are released. As the temperature rises, the starch tends to gelatinise, thickening the broth.

In acidic and calcareous water, legumes often remain tough.

Adding sodium bicarbonate (NaHCO₃) softens legumes but causes greater loss of thiamine.

Tip: Long cooking is essential for legumes due to their low moisture content—they need plenty of time to rehydrate.

Meat

Cooking meat also causes various physical changes, mainly related to colour, texture, and flavour.

Colour changes are noticeable in meats such as chicken, which turns whitish; beef and lamb become brown; pork and veal turn greyish.

Texture changes refer to loss of juiciness due to exuded liquids. Meat becomes tougher at 40–60°C and more tender above 60°C (or 50°C in young animals).

Boiled meat often has a bland taste.

Tip: Poultry is juicier and more flavourful when roasted. If boiling, submerge in cold stock to start.

Fish and Shellfish

When cooked, fish turns whitish. Dark fish like tuna tend to brown. Shellfish take on red tones.

If overcooked, fish falls apart and loses juiciness due to exuded liquid.

Salmon contains a carotenoid called astaxanthin, which gives it its distinct colour. As it’s fat-soluble, it resists heat.

Shellfish also contain astaxanthin, bound to proteins, giving them a blackish colour. Heat releases the astaxanthin, turning them red.

Pasta

Pasta absorbs water as it cooks, swells, softens, and can stick together.

With brief cooking, heat coagulates the gluten at about 60°C, thickening the pasta and increasing its volume.

Prolonged cooking dissolves starch into the water, giving it a milky appearance, and some proteins are solubilised—resulting in sticky pasta.

Cheaper pasta contains more starch, which increases stickiness.

How can we prevent pasta from sticking?

• Tip 1: Don’t drain pasta through a colander with the water—starch that settles at the bottom of the pot gets redistributed when poured.
• Tip 2: Always add pasta to boiling water—this rapidly coagulates proteins and prevents starch from escaping.
• Tip 3: Use plenty of water and avoid cooling it by adding too much pasta at once.
• Tip 4: Adding olive oil to the cooking water is useless—it doesn’t mix and floats, doing nothing to prevent sticking. Better to add it afterwards for flavour and quality.
• Tip 5: Pasta requires shorter cooking times than legumes—stir it (or rice) to prevent clumping.