Ingredients

Flour
Flour is made from finely ground cereal, such as wheat, barley, oats, rye, rice and maize (corn).
In Britain, the word 'flour' usually refers to flour produced from wheat. Wheat flour contains gluten, a protein that forms an elastic network that helps contain the gases that make mixtures (such as doughs and batters) rise as they bake.
Different types of flour are needed for different products. Bread flour, or strong flour, for example, has a high protein content and good gluten strength. Plain flour is usually a soft flour and is best for cakes and pastries. Self-raising flour has a standard amount of raising agent (usually a mixture of bicarbonate of soda and cream of tartar) already added to it.
Varying degrees of processing in the milling of the grain give wholemeal or wholewheat, brown and white types of flour. Spelt flour is made from an ancestor of modern wheat and, although it contains a small amount of gluten, some people who are intolerant to wheat flour can cope with it.

Baking powder
A raising agent used in cakes, biscuits and breads. Commercial baking powder contains bicarbonate of soda and tartaric acid (with a dried starch or flour to absorb any moisture during storage). When these chemicals become moist and warm they react and give off carbon dioxide, which causes food to rise.
It has a limited shelf life so check the sell-by date when using it; otherwise your cakes might literally be a flop! Make your own baking powder by combining 15ml/1tbsp bicarbonate of soda with 30ml/2tbsp cream of tartar. Measure carefully because too much or too little can upset a recipe's balance.

Zest
The outer rind of citrus fruit containing aromatic essential oils. Remove the zest carefully using a grater, potato peeler or zester, depending on the intended use. Take care not to remove any of the white pith with the zest, because it can be very bitter.
The zest of citrus fruits can be used to add flavour to sweet or savoury dishes, or as a decoration or garnish. Lemon zest is a key ingredient in gremolata.

Lemon
Very sharp, acidic citrus fruit with a shiny yellow skin and sour but zingy flavour.
Lemons are rich in vitamin C but with a low sugar content. They're available all year round and used in both sweet and savoury dishes.
The aromatic zest or outer rind and juice can be used in marinades, to flavour drinks such as lemonade, in marmalade, chutneys, pickles and a wide variety of desserts, such as mousses, syllabubs, soufflés, cakes, pies and tarts - lemon meringue pie and tarte au citron, for instance.
Lemon is a good accompaniment to fish and can also be used in place of vinegar as a salad dressing. The zest is often incorporated into stuffings for meat and Moroccan preserved lemons can be used to flavour stews.
Lemons can be bought waxed or unwaxed. Choose unwaxed lemons if you're using the zest or adding slices to drinks or using as a garnish. Waxed are fine if you're just using the juice. If you can't buy unwaxed then a good scrub with a vegetable brush will remove most traces of wax. Use a potato peeler or a zester to remove the rind, taking care not to remove any of the white pith with the zest, because it's very bitter.

Double cream
Cows' milk contains butterfat which is removed from milk using a centrifuge system. The longer the milk is centrifuged, the thicker the cream becomes.
Double cream is very rich, with a fat content of 48 per cent, making it the most versatile cream because it withstands boiling, whips and freezes well. Take care not to whip it too much though, because it goes grainy and separates. And if you keep whipping you'll end up with butter! It will keep for up to five days in the fridge.
Serve it with desserts for pouring or spooning over fruit salad, cake or puddings, or use it as the basis for desserts - whipped up in a trifle, on top of a pavlova, mousse, crème brûlée, soufflé or cheesecake.
Decorate cakes with stiffly whipped cream or use it to sandwich cakes together. Stir double cream into savoury dishes such as risotto or soup to add richness and flavour. In the US double cream is known as heavy cream. You can also buy extra-thick double cream in the UK, which is more of a spooning cream - the thickness is a result of it being homogenised.