How do raising agents make food rise and how do emulsions hold oil and water together?
How chemical, mechanical and biological raising agents work to introduce gas into mixtures, and how emulsifiers form and stabilise emulsions of oil and water.
A focused answer to AQA GCSE Food Preparation and Nutrition on how chemical, mechanical and biological raising agents introduce gas to make mixtures rise, and how emulsifiers form and stabilise oil-in-water emulsions.
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What this dot point is asking
AQA wants you to explain the three types of raising agent and how each introduces gas into a mixture, and to explain how emulsifiers allow oil and water to mix and stay mixed. You should name the gas and link it to food examples.
Raising agents
There are three types:
In all cases the trapped gas (carbon dioxide, air or steam) expands on heating, and the mixture sets through coagulation of protein or gelatinisation of starch to hold the risen shape. The choice of raising agent suits the product: yeast gives bread its open, chewy crumb but needs time to prove; baking powder is quick and reliable for cakes and scones; mechanical aeration alone gives the light, fat-free structure of a whisked sponge; and steam raises high-water, high-temperature mixtures such as choux pastry and Yorkshire puddings, where rapid steam expansion pushes the structure up before it sets.
Bicarbonate of soda alone releases carbon dioxide but leaves an unpleasant soapy taste, so it is used where there is an acid to react with it (such as buttermilk or in gingerbread). Baking powder is bicarbonate of soda already combined with a dry acid (cream of tartar) plus a starch, so it works in any recipe.
Emulsions and emulsifiers
The best-known emulsifier is lecithin, found in egg yolk, which is why egg yolk is used to make mayonnaise and hollandaise and to enrich sauces. The emulsifier surrounds tiny droplets of oil and holds them suspended in the water, giving a smooth, stable mixture that does not split. A permanent emulsion (such as mayonnaise) stays mixed, while a temporary emulsion (such as an oil-and-vinegar salad dressing) separates again on standing and must be shaken. Manufacturers add emulsifiers such as lecithin (often listed as E322) to margarine, chocolate, ice cream and salad cream to keep them smooth and stable.
Try this
Q1. Name the gas produced by yeast that makes bread rise. [1 mark]
- Cue. Carbon dioxide.
Q2. Explain why egg yolk is added when making mayonnaise. [2 marks]
- Cue. It contains lecithin, an emulsifier that holds the oil and water together so the mixture does not separate.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20196 marksExplain how the raising agent in bread works and how it differs from the raising agent in a Victoria sponge. (Paper 1, Section B)Show worked answer →
Bread uses a biological raising agent, yeast. Given warmth, moisture and the sugar in the dough, yeast ferments and produces carbon dioxide gas, which is trapped by the stretchy gluten network and makes the dough rise. Baking then kills the yeast and sets the structure.
A Victoria sponge relies on a chemical raising agent (baking powder), which releases carbon dioxide when it reacts with liquid and heat, and on mechanical aeration, where creaming the fat and sugar and beating eggs trap air that expands on baking.
Top-band answers (5 to 6 marks) identify yeast as biological producing carbon dioxide by fermentation, versus chemical and mechanical raising in the sponge.
AQA 20214 marksExplain how an emulsifier allows oil and water to mix when making mayonnaise. (Paper 1, Section A)Show worked answer →
For 4 marks, explain the structure and action of the emulsifier.
Oil and water do not normally mix. An emulsifier, such as lecithin in egg yolk, has a molecule with one end that attracts water (hydrophilic) and one end that attracts oil (hydrophobic). When whisked in, the emulsifier surrounds tiny droplets of oil and holds them suspended in the water.
This forms a stable emulsion that does not separate, giving smooth, thick mayonnaise. Markers reward the two-ended molecule (water-loving and oil-loving) and the link to holding droplets suspended so the mixture does not split.
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Sources & how we know this
- AQA GCSE Food Preparation and Nutrition (8585) specification — AQA (2016)