Properties of Food: Fats & Oils (AQA GCSE Food Preparation & Nutrition): Revision Note

Properties of food: fats & oils

• Fats and oils provide food with several important characteristics

Shortening

• The use of fats and oils when making dough allows the production of firm and crumbly baked foods, e.g.

  • shortcrust pastry

  • shortbread biscuits

• Such products are described as having a 'short' texture, hence the fats and oils are said to have a shortening effect

• The shortening effect of fats and oils is achieved by rubbing fat into the flour at the start of the dough-making process; this means that:

  • the flour is given a waterproof coating of fat

  • when water is added the proteins in the flour do not come into contact with the water and no gluten network can develop

  • rather than being stretchy and strong the resulting dough is crumbly and 'short'

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Aeration

• Beating together fat and sugar, e.g. with a whisk or a wooden spoon, produces a mixture that contains many small air bubbles; this mixture is said to be aerated

• Baking an aerated cake mixture results in a cake with a light and spongy texture

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Plasticity

• Fats are made of many molecules known as triglycerides, which can be saturated or unsaturated

  • Saturated triglycerides have a high melting point, so fats that are high in saturated triglycerides will be solid at room temperature, e.g. butter

  • Unsaturated triglycerides have a low melting point, so fats that are high in unsaturated triglycerides will be liquid at room temperature, e.g. olive oil

• The different melting points of different triglycerides mean that fats soften over a range of temperatures; this property is known as plasticity

• Fats can be soft at room temperature without being fully liquid, making them useful for:

  • spreading

  • shortening

  • creaming, i.e. beating together with sugar to form an aerated mixture

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Emulsification

• In most situations fats and water do not mix; an attempt to mix oil with water will usually result in water that contains large drops of oil, or in a layer of oil floating on top of the water

  • The two liquids are said to be immiscible

• When oil and water are enabled to mix they form a mixture known as an emulsion

  • An oil in water emulsion contains tiny droplets of fat spread out in water, and will be a liquid, e.g.

    • milk

    • salad dressing

    • mayonnaise

  • A water in oil emulsion contains tiny droplets of water spread through fat or oil, and will be a solid, e.g.

    • margarine

    • butter

• Emulsions can form in the presence of a substance known as an emulsifier; emulsifiers prevent fats and water from separating

• The molecules within an emulsifier contain a hydrophilic and a hydrophobic region:

  • Hydrophilic = attracts water

  • Hydrophobic = repels water

• To form a stable emulsion:

  • the hydrophilic region of an emulsifier forms an attraction with water and

  • the hydrophobic region forms an attraction with fat droplets

Emulsification diagram

• Lecithin is an example of an emulsifier found in egg yolk; it allows mayonnaise to be produced

  • Mayonnaise is an emulsion formed from egg yolk, oil and vinegar