Proteins are one of the key molecules of Ingredientia, and are the main material in animals. Proteins are very complex large molecules, made from lengths of amino acids, folded up into various three dimensional shapes, and are complex organic nitrogen molecules. Proteins "translate" the instructions of genes into everyday operations in all living things - animals, plants, microorganisms etc. An important sub-group is enzymes, which are proteins that mediate most of the chemical reactions in organisms.
From a food perspective, proteins are found in high proportions in fish and meat, cheese, eggs, nuts and beans especially soy beans. Proteins differ in their characteristics, with some easily mixed in water, e.g. albumin and others insoluble, e.g. casein found in milk.
Proteins and Life
Proteins are made from carbon, hydrogen, oxygen and nitrogen, with nitrogen being their key distinguishing chemical. Additionally, there might be some phosphorus and sulphur. Proteins are built up from amino acids, which are linked together by "peptide bonds", which are relatively weak in strength and can be easily cleaved.
Plants can make proteins from chemicals in the soil and the air. Unfortunately, plants cannot directly take in nitrogen, so work with bacteria via symbiotic relationships to "fix" the nitrogen out of the air, which is then absorbed into the root system. This interrelationship between plants and other organisms is heightened via close links with fungi in the form of "hyphae" which act to extend the root system both in terms of its size and also its concentration, i.e. it is made up of lots of long and very fine filaments that aid the absorption of chemicals from the soil via the hyphae into plants. Plants that cannot fix nitrogen must rely on chemical nitrogen in the soil, either naturally available or through nitrogen-based fertilisers added to the soil during farming.
Animals cannot directly manufacture proteins, yet need them to grow and repair their tissues. So animals must eat plants or other animals to get hold of proteins. Fortunately, animals do not need to eat the exact protein they require, because digestion breaks the proteins down into amino acids, then the body rebuilds them using enzymes into the actual proteins required.
For a human, an adult man needs roughly 45 g (1.6 oz) of protein each day. Any extra protein is burnt to provide energy, giving roughly the same value as carbohydrates - 18.8 kJ per gram (or 127.5 calories per ounce).
Proteins and the Diet
There are 20 amino acids that are required for human nutrition, but only 8 - 10 are essential in the diet, because the rest can be manufactured by the body from other amino acids.
Furthermore, from a nutritional perspective, all necessary amino acids are needed at the same meal to be useful and enable the body to fulfil all its building and repairing functions. Therefore, proteins that include all the required essential amino acids in the right amounts are the most valuable as foods, so foods of animal origin are regarded as the best - i.e. fish and meat. Some term these "complete proteins", although this is not strictly accurate.
In contrast, foods from plants generally do not contain all the necessary proteins and may be termed "incomplete". For example, grains, such as corn, rice and wheat, are low in lysine, one of the key amino acids for the diet. These grains are the primary source of protein for many diets across the world and for vegans and vegetarians. In contrast, pulses often are replete in lysine but lacking in methionine, which is abundant in cereals. Therefore, eating cereals and pulses together creates a "complete" set of proteins. In effect, this is all that's meant by a balanced diet, i.e. the right amounts of amino acids, together fats and carbohydrates needed by the body to carry out all its living activities.
Furthermore, many legumes are impacted by the "indigestibility" of their proteins, which is especially true of dried pulses. Therefore, they must often be treated to activate the proteins, e.g. soya might be processed to make it useful as in miso, tempeh and tofu.
Proteins are digested by enzymes, a class of active proteins, which break down the peptide bonds and releasing the amino acids. However, enzymes can only start to act of the proteins if they can access the peptide bonds, however in some cases these are not accessible in their natural state. For example, collagen and myosin are indigestible. In contrast, globular proteins like raw eggs are easily digestible and so an excellent source of protein in their natural state.
But heating with water or using acids can start the break down of proteins via a process of hydrolysis - the action of water on molecules. Therefore, cooking makes meat digestible and marinading steak with lemon juice before cooking also helps. Likewise, the acidic conditions of the stomach's digestive juices carry out hydrolysis on proteins to facilitate digestion further down the digestive tract.
Proteins and Cooking
There are two groups of protein relevant to cooking: globular and fibrous. In fibrous proteins, the chains of amino acids lie lengthways, e.g. collagen, the main substance in connective tissue, and myosin, a key protein in muscles. Globular proteins have the peptide chains are scrumpled up, e.g. ovalbumin, the main protein in eggs, and casein in milk. The water in fibrous proteins is held in the network of strands, while globular proteins are dispersed within water, forming a thick liquid. Globular protein are always soluble in water, but fibrous ones may be soluble or insoluble.
The unravelling of proteins occurs in cooking through "denaturation". How this process proceeds depends on the particular protein:
- Fibrous Proteins: when meat is cooked, hydrolysis frays and break the collagen. This starts at c. 60oC (140oF) and is most active at c. 70oC (160oF). Myosin behaves similarly, but if the meat reaches boiling point, myosin shrinks and coagulates and becomes really tough. That is why meat should be cooked slowly at a low temperature, which is useful in making stews. Furthermore, meat should not be allowed to dry out as this stops hydrolysis, and so the denaturation process.
- Globular Proteins: globular proteins partly unravel and become tangled, causing them to coagulate. These proteins start to coagulate at 50 - 60oC (122 - 140oF) and are completely coagulated at 65 - 70oC (149 - 158oF), but some take a higher heat like casein of 100oC (212oF). Proteins can harden to a solid substance if strongly heated, so egg white becomes solid on heating but if fried in very hot fat becomes very hard and brittle.
These coagulated proteins can be broken down by enzymes or acids into amino acids, as happens within human's digestive tracts. Globular proteins are easy to digest, whereas fibrous proteins are more or less indigestible.
As above, cooking - or heating - facilitates the breaking down of proteins to make them more usable for digestion. Changes can be made to proteins using acids, so marinading meat in lemon juice and red wine works to break down tough meats. Another method is to use enzymes themselves, e.g. rennet, which is an enzyme from the stomachs of calves. Some fruits contain enzymes that will break down proteins, e.g. figs and pineapples.
Other methods of processing proteins used in cooking are to beat them or knead them. So egg white is whisked to create a coaugulated mass. The whisking (or beating) process pulls out the globular albumin proteins, so developing a network of fibres that can support the froth that results. Similarly kneading dough develops fibres of gluten into a network of fibres that can support carbon dioxide if yeast has been used.