Eggs PDST Home Economics Nutritive value of eggs

Eggs © PDST Home Economics

Nutritive value of eggs § Protein: 13%, HBV, albumin & globulin in white, livetin & vitellin in yolk. § Fat: 12%, saturated, in yolk, in fine emulsion because of lecithin, easy to digest, high in cholesterol. § Carbohydrate: 0%, serve with complex carbohydrates, no fibre present.

§ Vitamins: A & D in yolk, B 2, Niacin and B 12 ( more in free range). Lack vitamin C. § Minerals: 1%, Calcium and phosphorus useful amounts, iron in yolk. § Water: 74%, more in white than yolk. § Energy: 147 kcal, mostly in yolk.

Dietetic value of eggs • Cheap & nutritious, low budgets • Versatile • Protein alternative to meat/fish • Source of HBV protein for lacto-vegetarians • Restrict on low calorie or low cholesterol diets • Easily digested therefore good for invalids, children, elderly • Should be served with food rich in complex carbohydrates, fibre and vitamin C

Egg structure • • Shell = calcium carbonate, shell is porous so air, bacteria, smells and flavours can get in and water vapour can get out Shell pores are covered by a natural varnish when laid 2 membranes - one stuck to the shell and one surrounding the white Air space at rounded end of the egg between shell and membrane White/ albumin - 2 layers thin and thick Yolk membrane Chalazae hold yolk in centre of white for protection Yolk

Fresh eggs • • Heavy for size Rough shell Domed yolk Thick white Sinks in water Date stamp Small air space No bad smell when cracked

Grading eggs • Graded by weight and quality • Weight from size 1 -7, size 1=70 g size 7=45 g • Quality decided by candling • Extra sticker = within 7 days of laying should be removed when 7 days is over. • Class A = best, small air space, poaching frying boiling • Class B = large air space, staler, yolk off centre, scrambling, baking, sauces. • Class C = similar to class B but sold only to food manufacturers.

Buying and storing eggs Storage Buying • Free range cost more. • Store at 7 -13ºC e. g. fridge door, for 1 month. • Quick turnover • Away from strong • Class A, B or extra smelling food • Size 1 -7 • Rounded end up • Check shells • Left over egg white, air tight container in fridge • Check date stamp • Heavy with rough shell • Left over yolk, covered with water in fridge • Remove from fridge 1 hour before use

Quality assured eggs • Ireland has an EU approved salmonella plan to maintain the health of the country’s laying stock. • Incoming hens must be certified as salmonella free. • Laying hens are checked for salmonella on a monthly basis. • All feed for hens is heat treated. • Management systems ensure full traceability of eggs. • Eggs carry best before date and house code and logo. • All producers and suppliers are inspected and approved. • All systems are independently inspected before QA mark is awarded.

Properties of eggs • • 1. Coagulation Egg protein coagulates and sets when heated. White protein @ 60 -65ºC. Yolk protein @ 65 -70 ºC. Coagulation causes the protein chains to untwist and straighten (denature) and bond together around small pockets of water. When overheated the protein clumps together squeezes out the water and this causes curdling. Coagulation is used when eggs are boiled, poached, scrambled, fried or turned into omelettes or custards. When eggs are used to thicken, bind, glaze, coat the principle of coagulation is used.

2. Aeration/Entrapping air • Whisking egg or just egg white brings bubbles of air into a mixture. • Whisking also causes heating of the egg protein (by friction) which slightly sets the protein chains and makes them unravel and line up around the air bubbles. • This causes a temporary foam to form. • To keep the foamy texture permanently in place the mixture must be further heated or a setting agent like gelatine must be added. • Aeration is used to make meringues and souffles.

3. Emulsification • Lecithin in egg yolk is an emulsifier. • An emulsifier is a substance that causes mixtures that would normally separate (e. g. vinegar and olive oil) to stay mixed together. • This property is used in making mayonnaise and hollandaise sauce.

Uses of eggs in food preparation • Binding: sticking ingredients together e. g. burgers, fish cakes (coagulation). • Coating: protects food when frying, egg is used to stick a layer on outside of food e. g. breaded fish (coagulation). • Glazing: beaten egg brushed on baked foods to make them brown and shiny e. g. scones (coagulation). • Thickening: e. g. custard sauce (coagulation). • Enriching: increasing the nutritive value of a dish e. g. brown bread. • Garnishing: hard boiled egg sliced or chopped for decoration e. g. salad or dressed crab. • Emulsifying: e. g. mayonnaise, hollandaise sauce. • Aerating: e. g. sponge cake, meringue. • Clarifying: egg whites used to make jelly, stock or wine clear (coagulation).

Effects of heat on eggs • Egg protein denaturates and coagulates causing it to set and harden, egg white becomes opaque • Shrink • Sulphur in egg white protein reacts with iron in yolk to form iron sulphide which causes the greenish colour on outside of the yolk of hardboiled eggs • Destroys pathogenic bacteria like salmonella. • Loss of B group vitamins especially vitamin B 1 • Too much heat causes curdling • If overcooked white becomes tough and rubbery, yolk becomes dry and crumbly • Egg albumin (white) which is soluble in cold water becomes insoluble

Pasteurisation of eggs • This involves slowly heating whole raw eggs to kill harmful bacteria without cooking the egg • Effects: White is cloudy Harder to whisk Cost more Kills harmful bacteria Allows children, pregnant women, elderly, invalids to eat raw egg products without risk of food poisoning