Cooking and frying cause chemical changes in food and lead to the formation of new substances. One obvious change is that many foods like meat, eggs or potatoes turn brown when being fried. This is caused by a chemical process called the Maillard reaction. This reaction takes place between an amino acid and a reducing sugar. In the reaction, the carbonyl group of the sugar reacts with the amino group of the amino acid and forms a new substance with a brown colour and a better taste. There are many different sugars and amino acids in food which is why the number of different brown, well-tasting products is very large. The Maillard reaction requires the addition of heat and does not occur before 140 degrees Celsius. This reaction is responsible for the colour and taste of a great variety of foods. Besides the well-known colour of fried (or grilled) meat, eggs, potatoes and other vegetables, it is also resposible for the brown colour and taste of toasted bread as well as roasted coffee beans.
Another process which happens during both frying and cooking is the denaturation of proteins. Large protein molecules are normally folded in a very specific way. During denaturation they unfold. This process can be compared to unfolding a piece of Origami art until only a sheet of crumpled paper is left. These crumpled proteins can easily become entangled with each other in a process called coagulation. In addition, the cell walls of vegetables like potatos and carrots are broken down by high temperatures when being cooked or fried. This makes them easier to digest for us humans as compared to eating them raw.
Like for frying, the Maillard reaction plays an important role for baking too and causes the brown colour cakes take on when they are placed in an oven. Other browning reactions like the caramalization of sugars take place as well. Caramalization is the oxidation of sugars which turns them into brown products. It should be noted that caramalization happens at different temperatures for various sugars. Fructose, for example caramalizes at only 110 degrees Celsius, while 160 degrees are necessary to caramalize glucose. The denaturation and following coagulation of proteins is essential for baking too because it results in the stiffening of the dough.
Today, the chemical reactions caused by so-called raising agents are important as well. Baking powder and self-raising flour normally contain a substance called sodium bicarbonate (also known as baking soda) and a solid acid. When water and heat are added this mixture releases the gas carbon dioxide which gives cakes their fluffy textures. Evaporating water further contributes to this process which is called expansion.
Fluffy cakes as we know them today first entered the kitchen in the 18th and 19th century. Before that, cakes were generally flat and rather heavy, like fruited tea cakes for example. During the time of the first fluffy cakes sodium bicarbonate had not been discovered yet. Instead, yeast was often used as a raising agent and it is still used today in some countries to bake bread. Another method was to beat eggs before adding them to the dough until they contained enough air bubbles, a method that could take a long time and required strong arm muscles.
The reactions taking place during deep-frying resemble in parts the ones during cooking and frying. The Maillard reaction, the denaturation and following coagulation of proteins as well as the break-down of cell walls happen here too. But due to the higher temperatures even more reactions like oxidations and polymerizations can take place, even in the frying oil itself. Another problem is that while the frying oil extracts nutriants like vitamins and minerals, it also creeps into the food and adds high numbers of calories. For this reason, deep-fried food is normally less healthy than its regularly fried or cooked counter parts. This can easily be shown by the example of a large baked potato which normally contains about 220 calories and 1 g of fat. If the same potato is cut up and turned into chips (or French fries), it will contain almost 700 calories and 34 g of fat. In conclusion, deep-frying creates very well-tasting foods, but it is important not to consume them on a regular basis.