There are two types of browning that we are all familiar with – browning that occurs during the cook of bread, pastry and meats and the browning of fruits and vegetables when they are cut, torn or bruised. The first, Maillard or non-enzymic browning, is a reaction between certain sugars and proteins and the second is an enzymic reaction which is part of the plant defence system.
Make some simple short-crust pastry biscuits and coat them with different sugars before you cook them. Try ordinary table sugar (sucrose), glucose, lactose and starch (use corn flour). Mix these into water, milk and egg white. Brush each onto a separate pastry; have a control which is only brushed with water. Then bake all in the same oven at the same time and temperature.
Take them out and compare the differences in browning. If you're really into the chemistry, you can compare the structures of the sugars to see which cause the most browning. Browning should be strongest with glucose in egg white. Glucose is a so-called reducing sugar because its structure is able to react with the amino acid lysine. This is an essential amino acid and so the browning reaction can reduce the nutritive value of the food.
Get some fresh fruit and vegetables – lettuce, apple, potato are good. Have two containers of water, one of sugar solution (say 10%) and a bottle of lemon juice (or some lemons).
Cut up each fruit or vegetable into pieces about 5 cm long. Leave one sample of each on a plate and quickly place a sample of each into the water and sugar solutions. Sprinkle another sample generously with the lemon juice.
Leave the experiment running for about 1 hour and then observe the colour changes, if any, in the different samples. You should see browning in the samples left on the plate and compare this with the others. What conditions prevented browning most? Think of food preparation and cooking where similar conditions are used. Why do chefs often tear rather than cut up lettuce leaves?
Enzymes are biochemical catalysts and speed up reactions between molecules. In the intact fruit and vegetables the enzymes are kept separate from their substrates. When the tissues are damaged they get together and the reaction occurs. The reaction needs oxygen, so the water and sugar solution prevent oxygen getting in; lemon juice is very acid (citric acid) and changes the pH so that the conditions for the enzyme are not optimal.