What jobs do fats and oils do in cooking, and how do emulsions hold together?
The functional properties of fats and oils: shortening, aeration and plasticity, and emulsification, with the conditions that cause each, the role of emulsifiers such as egg yolk lecithin, and food examples.
A focused answer on the functional properties of fats and oils for Eduqas GCSE Food Preparation and Nutrition (C560), covering shortening, aeration and plasticity, and emulsification, the role of emulsifiers such as egg yolk lecithin, and food examples in pastry, cakes and sauces.
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What this dot point is asking
Eduqas wants you to explain the jobs fats and oils do in cooking, using the correct terms (shortening, aeration, plasticity, emulsification) with food examples, and to explain how emulsions are held together by an emulsifier such as the lecithin in egg yolk.
Shortening
With less gluten, shortcrust pastry, shortbread and crumbles are tender and crumbly rather than tough. Solid fats such as butter, lard and block margarine work best because they coat the flour well; this is why a recipe for short pastry uses a solid fat, not oil.
Aeration and plasticity
Aeration is the basis of the creaming method for cakes such as a Victoria sponge: the trapped air, helped by any raising agent, lifts the cake. Plasticity matters because a fat that is too hard cannot trap air or rub in, while one that is too soft (or an oil) will not hold air, so block fats at the right temperature are chosen for creaming and rubbing in.
Emulsification
An emulsifier has one part that attracts water and one part that attracts oil, so it sits at the surface of each oil droplet and stops the droplets joining back together. The classic natural emulsifier is lecithin in egg yolk, used to make mayonnaise and hollandaise. To make mayonnaise, oil is whisked very slowly into egg yolk and vinegar, breaking the oil into tiny droplets; the lecithin keeps them suspended so the sauce stays thick and does not separate.
Try this
Q1. Name the emulsifier found in egg yolk. [1 mark]
- Cue. Lecithin.
Q2. Explain why fat is rubbed into flour when making shortcrust pastry. [2 marks]
- Cue. Fat coats the flour and limits gluten formation, giving a short, crumbly, tender texture (shortening).
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20194 marksExplain the term 'shortening' and describe how it gives a short, crumbly texture to shortcrust pastry.Show worked answer →
A 4-mark structured question.
Shortening is the functional property of fat that gives baked goods a short, crumbly, tender texture. When fat is rubbed into flour, it coats the flour particles with a waterproof layer of fat.
This fat layer stops the flour proteins absorbing water and forming long gluten strands, so little gluten develops. With less gluten, the baked pastry is tender and crumbly rather than tough and chewy. Solid fats such as butter, lard and block margarine work best because they coat the flour well.
Markers reward defining shortening, explaining that fat coats the flour and limits gluten formation, and linking this to the short, crumbly texture.
Eduqas 20216 marksExplain how mayonnaise is made as an emulsion and why egg yolk is added.Show worked answer →
A 6-mark extended-response question on emulsification.
An emulsion is a mixture of two liquids that do not normally mix, oil and water (here oil and vinegar or lemon juice). To make mayonnaise, oil is added very slowly to egg yolk and vinegar while whisking hard, which breaks the oil into tiny droplets spread evenly through the water-based liquid.
Egg yolk is added because it contains an emulsifier, lecithin. An emulsifier has one part that attracts water and one part that attracts oil, so it sits at the surface of each oil droplet and stops the droplets joining back together. This keeps the oil and water mixed and gives a stable, thick mayonnaise that does not separate.
Top-band answers (5 to 6 marks) define an emulsion, describe whisking oil into the liquid as tiny droplets, and explain that egg yolk lecithin is the emulsifier that stops the mixture separating.
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Sources & how we know this
- WJEC Eduqas GCSE Food Preparation and Nutrition specification (C560) — WJEC Eduqas (2016)