Briefing

Coagulation is the change in state from a solution (ie having the characteristics of a liquid) to a gel (ie having the characteristics of a solid). This occurs in liquid foods which contain proteins because the shape of the globular polypeptide molecules change when the liquid is heated. The energy supplied to the molecules by the heat causes the bonds which link different parts of the chain to break, resulting in a change in the protein structure. Because the protein changes from its state in nature, this process is called denaturation. Other bonds then form as a second process called coagulation occurs - the protein eventually becomes a three-dimensional network of molecules within which water is trapped. At this stage the food has solid characteristics: it has coagulated.

Coagulation of whole eggs starts at about 63°C but several factors can affect the quality of the finished product. The most common error is to cook the eggs too quickly; this investigation tests the effects of different methods of cooking eggs to produce a savoury egg custard, which is in fact comparable to a quiche filling.

The quality of the product must be assessed in order to evaluate the results. This is done by hedonic ranking, in which a number is given to the product which represents a phrase which describes how much the product is liked or disliked.

Effect of temperature on coagulation

Work in groups of 4. Each student will require the following materials:

Materials

• 1 egg
  
  
• 125ml milk
  
  
• Salt and pepper
  
  
• 25g mixed vegetables eg onions (chopped finely), carrots (in matchstick strips), frozen peas, canned sweetcorn
  
  
• 15ml cooking oil
  
  
• Basin
  
  
• Frying pan
  
  
• Tablespoon
  
  
• Slotted spoon
  
  
• Aluminium foil dish, about 12cm x 10cm
  
  
• Flameproof dish at least 15cm x 13cm
  
  
• Knife
  
  
• Fork
  
  
• Plate
  
  
• Labels and pens

Method

		Heat the oil in the frying pan.
		Add all the vegetables and stir-fry for two minutes. Drain off the fat.
		Put the fried vegetables in the foil dish.
		Beat the egg and milk in the basin. Add salt and pepper to taste.
		Pour the beaten egg over the vegetables. You will find that the vegetables float to the top. Arrange them so that they look attractive.
		Using a shelf near the top of the oven, cook in ONE of the following ways as assigned by your teacher: (a) In an oven at 160°c, gas mark 3 or (b) Place the foil dish in the flameproof dish containing water to a depth of approximately 1.5cm, then cook in an oven at 160°C, gas mark 3. or (c) In an oven at 200°C, gas mark 6. or (d) Place the foil dish in the flameproof dish containing water to a depth of 1.5cm, then cook in an oven at 200°C, gas mark 6. In each case, look to see if the custard is cooked after 20 minutes then at 5 minute intervals until you judge that it is firm. Note the time taken to cook.

Results

		When it is cooked, cut the custard in half lengthways and spoon half out on to a plate. Label the plate with a, b, c, or d as appropriate.
		Examine the foil dish carefully to see if any liquid is in the bottom, ie, if 'weeping' has occurred.
		Look carefully at the portion on the plate. Assess the TEXTURE of the custard on a five-point hedonic scale: 1 very smooth 2 quite smooth 3 neither smooth nor porous 4 quite porous 5 very porous
		Record your results on a table: Sample code Method of cooking Time to set Smell a       b       c       d

• When you have all results, look at the custards together and taste them. You may be familiar with a sweet egg custard - the dish you have made is more like a quiche filling. Discuss which method produces the most-liked custard, and why. Summarise the discussion here:
  
• The only words used to describe texture have been 'smooth' (assumed to be good) and 'porous' (assumed to be bad) - are these attributes always good or bad? What other words could be used to describe the texture of a custard? List them here: