The functions and properties of eggs

Lesson starter Eggs!!! Complete the following task on your post-it note – you will have 3 minutes (write the answer without looking it up!!) Egg property How does it work One example Aeration coagulation emulsification

Food science: The changes that take place in protein Food science: The changes that take place in protein. The role of Eggs in food preparation AGO To know how proteins change during food preparation and to understand how the working properties of eggs can perform different functions. You will be able to: Explain how the 3 main properties of eggs perform a range of functions in food preparation. Explore recipes which demonstrate the functions of eggs. Key terms lightening coating raising agent setting thickening enriching binding lecithin glazing ovalbumin coagulation denature

What does denature or denaturation mean? This is the term used to describe the change in the chemical structure of protein. Protein molecules can easily be denatured. The chemical bonds that hold protein molecules together can be broken – the molecule bundle will unfold and change its shape (see page 106) Denaturation of protein molecules can be caused by: Heat eg. frying or boiling an egg. Acids eg. adding lemon juice to cream or marinating meat in an acid like lime or tomato. Air bubbles eg. in a whisked sponge mixture Mechanical agitation eg. whisking whites for meringue. This is a permanent alteration and cannot be reversed. Add this key word to your post-it note and explain it!! So which functions do the 3 properties perform?

Coagulation Denatured protein molecules are larger and take up more space. They knock into other denatured protein molecules and join together in large groups - this is coagulation. As they coagulate, protein molecules trap and hold water from food in pockets between them. The appearance and texture of food changes as coagulation continues. Look at the stages of cooking eggs on page 107. If a food containing protein is overcooked, the coagulated protein molecules tighten up and squeeze out the water they were holding. This is called syneresis. Syneresis = the proteins shrink as they coagulate and separate from the watery liquid. This is why overcooked meat or fish is dry and chewy. Overcooked scrambled eggs becomes rubbery and watery. The process of coagulation is irreversible. The coagulation properties are used to: Thicken and set custards, sauces, soups, omelettes, quiches, lemon curd etc. In custards gentle heat should be used to prevent overcooking and curdling . Ovalbumin in the egg white starts to coagulate (set) at 60 oC. The protein in the yolk starts to coagulate at 70 oC. The yolk will eventually become dry and hard. If over cooked the protein becomes tough and difficult to digest.

Coagulation properties also perform: Binding: coagulation properties help ingredients stick together eg. burgers and fish cakes.. Coating: the egg coagulates setting the breadcrumbs to provide a protective coating which seals the product and holds it together. Batter also performs in this way. Glazing: Egg is brushed over the surface of baked items. This coagulates and gives a golden shiny finish.                                                                  

The aeration properties perform: Egg white can hold up to 7 times its volume of air because of the ability of the protein ovalbumin to stretch and hold air, which will form a foam. The aeration property of eggs is used in: Cakes, soufflés and choux pastry as a raising agent. In mousses and meringues as a lightening agent.                                                                              

How are foams formed? Gas (often air) is trapped inside a liquid to form a gas-in-liquid foam. Meringues and whisked sponges are a good example of gas-in-liquid foam. The action of a whisk rotating very fast traps lots of air bubbles to produce a gas-in-liquid foam. The protein bonds denature by breaking the bonds which hold them together, causing them to unfold. The denatured protein starts joining up and bonding with other protein molecules – this is C _ _ _ _ _ _ _ _ _ _. Protein forms a wall around the air bubbles so the water is held in place and the foam becomes stable. Egg yolk or traces of fat will prevent the foam from forming. Over whisking the egg whites results in the foam becoming watery and loose – can you explain why? What is this called? Fresh eggs are better as older eggs take up water through the tiny pores in the shells which makes the protein watery and weaker. Adding an acid like vinegar or cream of tartar will stabilise the protein and help prevent loss of water.

What happens when…….. Meringue is made? Heat of the oven coagulates the protein molecules in egg white and drives some of the water out. The foam sets and becomes solid. The meringue is now a solid foam. Whisked sponge is made? Whole eggs and sugar are whisked until a thick gas-in-liquid foam is produced. Air bubbles produce a light spongy texture which sets in the oven.

The emulsifying properties are used to: To prevent the separation of liquid and oil. Egg yolk contains Lecithin which acts as an emulsifier. Mayonnaise and hollandaise sauce are good examples where egg is used as an emulsifier, to produce an oil in water emulsion. This property is used in the creaming method of cake making to help the fat trap air and to prevent the cake mixture from separating before baking. How do emulsifiers work? Emulsifiers are molecules with two ends. One end is attracted to water (it is hydrophilic) and the other end is attracted to oil (it is hydrophobic – doesn’t like water). When an emulsifier eg. lecithin in yolk is added to a mixture of oil and water, its molecules arrange themselves so they prevent the oil and water from separating.

Further functions Enriching – eggs can be added to sauces, pastry, soups, milky puddings, stuffing's, pasta and mashed potato to add extra nutrients as well as colour and flavour. Garnishing – sliced, chopped or quartered eggs can be used to decorate and enhance the appearance of finished dishes e.g. in salads.

Functions of eggs – recipe analysis Title of dish Main components Properties and functions of eggs demonstrated Fish cakes   Fish Mashed potato Bread crumbs Coagulation Binding Coating Enriching  Trifle  sponge cake Custard Fruit cream  Emulsification – in the creaming method the fat and other ingredients will not separate. Aeration – lightening and raising agent Coagulation– the eggs will set the cake Coagulation – the eggs will thicken the custard Enriching – the eggs will add nutritional value to the custard  mayonnaise  eggs oil

Homework Complete the egg task sheet use www.egginfo.co.uk Bring ingredients to make fish cakes Complete the equipment section on the recipe sheet.

Further research https://www.theguardian.com/lifeandstyle/wordofmouth/2010/aug/19/how-to-make-perfect-meringue How to make perfect meringues. http://www.waitrose.com/home/recipes/recipe_directory/m/meringue.html Meringues http://saucydressings.foodzube.co.uk/guide-baking-ingredients/ Facts about baking ingredients