LEMON FOAM

1. LEMON FOAM • If you are interested in making a light foamy dessert - great for those unexpected visitors - get ready to learn the Lemon Foam recipe. • It’s quick, easy and has a very interesting chemistry to it. • RECIPE • - Briskly beat the contents of a can of evaporated milk and a can of condensed milk until the mixture is thick and creamy. • In the meantime, squeeze the juice from 3 lemons. • When the foam mixture is ready, add the lemon juice. • Place the mixture in the fridge and, if you like, sprinkle it with some powdered cinnamon, roasted ground almonds or crushed biscuit. • - What do you think? It’s cool and delicious with a very sticky consistency, right? • What has happened in this very simple mixture? • The characteristics of the milk • Let’s begin with the components of the milk. Both types of milk are concentrated, which means that about 50% of the water has been removed (60% for evaporated milk). • In the case of condensed milk, a large amount of sugar - around 44% - has been added. This prevents the growth of micro-organisms and makes the milk last longer. • Evaporated milk/condensed milk • When we want to make a foam (a dispersal of air bubbles in a liquid), we need to use a liquid that is a little sticky. Otherwise, the air bubbles (added by the mixer) will escape. Unlike normal milk, both condensed and evaporated milk are quite sticky. One of the other features of condensed milk is that there is no need to add sugar to make the dessert. • As you are beating the mixture, keep an eye out for the numerous air bubbles that appear and also note how the mixture increases in size. It may even look like beaten egg whites to you, although the specific characteristics of egg whites make a much firmer foam.

2. ESPUMA DE LIMÃO (cont.) Lemon juice in the foam When you pour the lemon juice into the foam, you will notice that it immediately becomes thicker and stickier. In order to understand how this happens, we need to talk about a certain part of the milk protein – casein – which makes up more than 80% of the total protein content of milk. Let’s take a look at the average composition of cow’s milk. We often say “you can’t judge a book by the cover” when we’re talking about people. The same is true for many substances. In the case of milk, the smooth white appearance hides a very complex composition and organisation of its component parts. It contains an emulsion (a mixture which appears homogenous), composed of fat globules, proteins (casein) and a solution, composed of sugars (lactose), proteins, vitamins and minerals salts. Casein is composed of negatively-charged units which because of this negative charge repel each other and move freely around the milk. When you add an acid, such as lemon juice, the pH of the milk will fall (acidity increases) and this environment causes the milk proteins – the casein – to attract each other and bond – that is, to join together, making a lump (or curd) which sinks to the bottom giving the final mixture a thick sticky texture. Maria Margarida Guerreiro