What roles do fats and oils play in cooking, and why do cut fruit and vegetables go brown?
The functional and chemical properties of fats and oils (shortening, aeration, plasticity and emulsification) and of fruit and vegetables (enzymic browning and oxidation), with food examples and ways to control each.
A focused answer on the functional and chemical properties of fats and oils and of fruit and vegetables for OCR GCSE Food Preparation and Nutrition (J309), covering shortening, aeration, plasticity, emulsification and enzymic browning, with examples and control methods.
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What this dot point is asking
OCR wants you to explain the functional roles of fats and oils in cooking (shortening, aeration, plasticity, emulsification) and the chemical change in cut fruit and vegetables (enzymic browning), with food examples and ways to control each.
Fats and oils: four functional properties
- Shortening - fat coats the flour particles in pastry and biscuits and stops long gluten strands forming, giving a short, crumbly texture (shortcrust pastry, shortbread).
- Aeration - creaming fat with sugar traps tiny air bubbles, which expand on baking to give a light, risen cake (creamed Victoria sponge).
- Plasticity - fats soften over a range of temperatures rather than melting all at once, so they can be creamed, spread and shaped (spreading butter, piping icing). The right fat suits the job: firm butter for flaky pastry, soft margarine for an all-in-one sponge.
- Emulsification - oil and water do not normally mix, but an emulsifier such as the lecithin in egg yolk holds them together as a stable emulsion (mayonnaise, hollandaise). The emulsifier has one part that attracts water and one that attracts oil.
Fruit and vegetables: enzymic browning
To prevent or slow enzymic browning:
- Acid - coat the cut surface with lemon juice or vinegar; the acid slows the enzyme.
- Exclude air - cover the surface or put the cut food in water or sugar syrup so oxygen cannot reach it.
- Blanching - briefly heat the food in boiling water to destroy the enzyme (used before freezing vegetables).
- Chilling - storing in the fridge slows the reaction.
Enzymic browning is not always unwanted: it gives the colour and flavour of tea, coffee and dried fruit such as raisins and prunes. But for a fresh fruit salad or peeled potatoes it spoils the appearance, so the methods above are used to keep the food looking fresh.
Choosing the right fat for the job
Because plasticity and melting point differ between fats, the choice of fat changes the result. Firm, cold butter holds its shape and gives flaky pastry its layers and a rich flavour; soft block margarine or shortening rubs in easily for shortcrust pastry; soft tub margarine creams quickly for an all-in-one sponge; and liquid oils suit frying and some quick batters but cannot trap air by creaming. Choosing a fat that is too soft or too hard for the method is a common cause of a poor result, for example pastry that is greasy and tough or a cake that fails to rise.
Try this
Q1. Name the functional property of fat that gives shortcrust pastry its crumbly texture. [1 mark]
- Cue. Shortening.
Q2. Give one way to prevent a cut apple from browning, with a reason. [2 marks]
- Cue. Coat with lemon juice (acid slows the enzyme), or cover with water (keeps oxygen away), or blanch (destroys the enzyme).
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20186 marksExplain the functions of fat when making shortcrust pastry and a creamed Victoria sponge, using the correct terms.Show worked answer →
A 6-mark free-response question linking properties to two products.
In shortcrust pastry, the fat gives shortening: it coats the flour particles and stops long gluten strands forming, so the pastry has a short, crumbly texture rather than being tough. The fat is rubbed in to coat the flour evenly.
In a creamed Victoria sponge, the fat gives aeration: creaming the fat with sugar traps tiny air bubbles in the mixture, which expand on baking to give a light, risen cake. Plasticity (the fat being soft enough to cream and spread over a range of temperatures) lets the fat be creamed smoothly.
Top-band answers (5 to 6 marks) name shortening for the pastry and aeration (and plasticity) for the sponge, each explained with the effect on the product.
OCR 20204 marksExplain why a cut apple turns brown and describe two ways to prevent it.Show worked answer →
A 4-mark structured question.
A cut apple turns brown because of enzymic browning (oxidation): when the apple is cut, enzymes in the flesh are exposed to oxygen in the air and react with substances in the apple to form brown pigments.
Two ways to prevent it (any two): coat the cut surface with an acid such as lemon juice, which slows the enzyme; put the cut fruit in water or sugar syrup to keep air away; blanch the fruit in hot water to destroy the enzyme; or store it in the fridge to slow the reaction.
Markers reward enzymic browning (enzyme plus oxygen) as the cause and two valid prevention methods with a brief reason.
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