EGGS

Nearly all animals produce eggs, but only some of them lay eggs outside of the body. Birds' egg shells are one of nature's great design solutions: thin, porous, yet amazingly strong. They are shaped and structured to provide protection and nutrients for the developing embryo.

Birds' eggs have five important parts. The most important is the germinal disc. This is found inside the yolk and is the cell nucleus from which the young bird would have developed if the egg had been fertilised. Commercially produced eggs for eating are never fertilised. The bulk of an egg consists of a thin white solution of proteins, mineral elements, carbohydrate and water called albumen. In hens' eggs this makes up 58% of the total egg mass. Egg yolk makes up the remaining 31% and is held in place by two dense cord-like strands which are called chalazae. The shell of an egg is made from a type of calcium carbonate called calcite. This is the same material that also occurs in marble, limestone, coral and chalk! Eggs shells vary naturally in colour, depending on the breed and age of the hen. Inside this thin shell are two semi-permeable membranes. These act as filters to protect the egg's contents from dirt and bacteria. The outer membrane is the one upon which the shell is built. The inner membrane surrounds the white and the yolk. A third membrane, the vitelline membrane, supports the yolk.

The structure of a hen's egg.

air sac albumen chalazae germinal disc membrane shell thin white thick white yolk

A hen's egg is designed to meet the nutritional needs of the developing chick. However, the eggs we eat are not fertilised. It is probably not surprising, therefore, that it contains many of the nutrients that are essential in a human diet: proteins, fats, vitamins and minerals.

Egg protein is the highest quality food protein available. The quality of a protein is measured by the protein content, the quantity of essential amino acids (which enable the body to grow and repair itself) and how easily it is absorbed and digested by the human body. 12.3% of the weight of a hen's egg is made up of protein.

Eggs are a rich source of fats. Fats provide the body with concentrated stores of energy and aid the absorption of soluble vitamins. Carbohydrates provide the body with energy for rapid use. Eggs contain very little carbohydrates and no fibre, but combine well with other foods that do.

Vitamins are important in keeping the body healthy. Eggs contain most of the recognised vitamins, except Vitamin C. Humans also require at least 20 mineral elements to function normally, and most of these can be found in eggs.

Nutrients of whole egg

Animal protein

Fat in the yolk (this is emulsified and easily digested)

Minerals: 1. Iron – a rich source 2. Calcium – useful amounts 3. Sulphur 4. Phosphorus

Vitamins: 1. vitamin A 2. Some of the B group 3. Vitamin D – sunshine vitamin

Water about 75% of the whole egg.

Testing for freshness 1

Eggs can be checked for freshness in the home, by placing them in a jug of slated water (brine).

If the egg sinks, it is fresh. The air sac is very small and the water content is high.

If the egg floats to the surface, it is stale. The air sac is much bigger and water has evaporated from the egg making it lighter.

Fresh egg Partially stale egg Stale egg

Testing for freshness 2

You can also see the difference when you crack open an egg.

If the egg is fresh, the yolk is quite tall and plump and is surrounded by a firm, thick layer of egg white, then a smaller thin layer of white.

As the egg gets older the yolk flattens out and the thick white becomes thinner and the thin white becomes larger and more watery.

If using stale eggs for frying, the yolk is more likely to break as the white protecting it is much thinner and watery. When whisking the white is too thin to hold the air.

Fresh egg Not so fresh egg Stale egg

A stale egg has a strong smell.

Have you ever examined an egg box and wondered why it is such an unusual shape? Like most packaging, egg boxes have been carefully designed. Each box is strong, yet fairly lightweight. This makes transportation and storage of the eggs easier. The individual compartments inside the box prevent the eggs from rolling around and smashing, while the flat tops mean that the boxes can be stacked easily.

Most egg boxes are made from a lightweight fibre which is cheap to produce and easy to work with. It is also biodegradable, which means that it is better for the environment.

Eggs are usually packaged in batches of 6 or 12.

Packaging is an important source of consumer information. It not only identifies the brand but also tells the customer important details such as 'best before' date and cooking and storage instructions. European Union regulations mean that every egg box must tell the customer: the quantity of eggs in the box, the size of the eggs in the box, give an indication of the quality of the eggs and list the registered number of the appropriate packing station.

Many eggs in the UK also carry the Lion Quality mark which guarantees that the eggs have been produced in accordance with The Lion Code of Practice, which sets higher standards of food safety and animal welfare than required by law.

Egg quality gradingClass A eggs are the highest of these and must be naturally clean, fresh, internally perfect and with intact shells. These are the eggs we buy in the shops for eating.Class B eggs are used by food and other manufacturers. Eggs that do not reach either of these two standards are classified as unfit for human consumption but are used by industry in products such as shampoo.

How to store eggs

The tiny holes in the eggshell make it porous. This means that it will absorb liquids and smells.

Eggs are best stored in a cool place, away from strong smelling foods like onions. The broad end of the egg should be at the top so that the yolk does not rest near the shell. This prevents the yolk from drying up.

As most kitchens are very warm eggs should be kept at the bottom of the fridge in their box. Eggs should always be eaten before the best before date, and dirty or cracked eggs should not be eaten.

When eggs are very cold it is difficult to whisk them, so for best results in cooking, take eggs out of the fridge about 30 minutes before using them.

Eggs are an extremely versatile food. Boiled, fried, used in an omelette, poached, scrambled or baked, eggs are a quick and simple meal solution - the ultimate in fast food! Eggs also play an important role in food preparation. Due to their three inherent properties - coagulation, aeration (foaming) and emulsification - eggs contribute to the structure and texture of many well-known dishes.

Baking is one of the oldest methods of cooking eggs. Baked eggs are cooked in an oven by convected heat and by heat conducted through the container - usually a small ramekin dish. Eggs served this way are called 'en cocotte'.Boiling involves the transfer of heat from the water through the shell. In frying, heat is conducted through the bottom of the pan to the egg. Fat is used as a lubricant to stop the egg sticking to the pan.

Scrambling and making omelettes involves whisking the whole egg then gently heating the mixture so that it coagulates. Poaching is cooking by the application of heat from a container of gently simmering shallow water. Eggs can be poached in water or in a poaching pan. Using a pan actually steams the eggs, as there is no direct contact between the eggs and the water.

Eggs can also be microwaved - although eggs in their shells should never be placed in a microwave, as pressure builds up inside the egg causing it to explode.

of eggs in cooking

As a main dish in a mealBinding dry ingredients so that they stick together when cooked

Coating to protect foods during frying

Thickening liquids

Beaten to trap air

Glazing to make the surface shinyGarnishing to make dishes look attractive

Enriching dishes by adding extra nutrientsMashed potatoes, milk puddings, soups

Sausage rolls, scones, breads

Omelette, eggs mornay

Meringues, swiss roll, cakes, mousses, soufflés

Fish cakes, scotch eggs

Egg custard, sauces

Beef-burgers, meat balls

Sliced egg in a salad

Emulsificationmayonnaise

Coagulation (setting)Quiche, sauces, lemon curd, soups, coating fish and scotch eggs

Functions and uses

Aeration – The action of whisking egg whites stretches the protein and incorporates air bubbles into the mixture. The air bubbles are then broken down into smaller bubbles, each of which is surrounded by a thin film of egg white. A foam has been created.The foam remains stable because the heat generated by whisking partially coagulates it.

Emulsification – oil and water mixed together will form an emulsion. Usually they will only stay in suspension for a short while. Lecithin found in egg yolks acts to keep the emulsion stable.

Thickening – egg white coagulates at 60°C and yolk at 70°C, so when these temperatures are reached it sets and stays ‘thickened’ (like a custard or quiche mixture).

Binding – coagulation sticks the ingredients together as they cook.

Glazing (for pastry etc) – during baking, the egg glaze turns a golden brown.

Coating – foods are brushed with egg and then dipped in breadcrumbs. During cooking, the egg coagulates forming a strong, protective ‘jacket’. This holds the product together and prevents oil from being absorbed during frying.

Effects of over heating

Overheating eggs causes toughness.

If added too quickly to hot liquids egg will curdle.

If boiled, custards, sauces etc containing eggs will curdle.

If boiled to be eaten cold and boiled for too long or left warm in the shell once cooked a grey ring will form around the yolk.

SALMONELLA

Raw eggs may contain the bacteria salmonella, which causes severe food poisoning.It’s very important that eggs are cooked thoroughly so that all bacteria are destroyed.Extra care should be taken when eggs are to be eaten by pregnant women, babies, the elderly and frail people.Manufacturers often use dried or pasteurised egg to be on the safe side, especially in mayonnaise.

How are eggs used in the food industry?

About 15% of eggs produced in the UK are sold to the food manufacturing and processing industry. Most egg products are available in liquid, frozen or spray-dried form. Whole egg is used for cakes, egg whites are used for meringues and sponges. Egg blends are used as a premix. The eggs are combined with other ingredients to help keep the product the same quality. Eg: egg white (albumen) is mixed with sugar to give the product a longer shelf life.Egg products may have a short shelf life or extended shelf life (ESL). The egg is pasteurised at higher temperatures for a shorter time (eg: 68° for 90 seconds) and then packed. ESL products have a 30-day shelf life.

ADVANTAGES OF USING PROCESSED EGG PRODUCTS

Eggs are pasteurised and made safer to eat.There is no wastage as the eggs are ready to use.Processing extends the shelf life of the products.Eggs can be stored and so are ready when needed.

How is a whole egg processed?

The egg shells are broken by a machine.

The egg quality is checked to identify and remove problems.

The liquid whole egg is pumped through filters to remove any shell and chalazae.

The egg is cooled to less than 4°C to slow down bacterial growth.

The egg is pasteurised at 64.4°C for a minimum of 2 minutes.

The time/temperature combination destroys pathogens and reduces the number of bacteria present.

The egg is cooled to below 4°C before packing or spray drying.

Uses Whole egg Yolk Whites frozen liquid dried frozen liquid dried frozen liquid dried

Cakes

Dairy products

Frozen desserts

Biscuits

Drinks

Baby food

Soup

Salad dressing

Noodles

Meat binder

Uses of eggs in food products.