Changing properties of proteins

Proteins have a complex structure. When food is cooked the proteins denature (this is when the chemical bonds holding the structure together break down). The proteins unravel and their shape changes and in most cases this process is irreversible.  Proteins can be denatured in different ways including physical agitation such as whisking, kneading or beating, temperature changes such as heating or cooling and acids for example lemon juice and marinades. Acidic marinades denature the protein in meat before cooking making the meat more tender before you start cooking it.

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After they have been denatured, protein molecules collide with other protein molecules and coagulate (they join together). Water becomes trapped between protein molecules during this process. Coagulation changes the texture and appearance of food (egg whites become solid and steaks become firmer and browner). If food is overcooked the protein tightens forcing the water out of the molecules and making food dry and chewy.

Foams are formed when air is trapped (aeration) inside liquid, when liquids containing proteins are agitated (such as egg whites being whisked) the proteins inside the liquid denature causing them to stretch and air becomes trapped inside the liquid. When proteins coagulate this air becomes trapped creating a foam. Over whisking can cause the new protein bonds to break, the air escapes and the foam collapses. Foams include whipped cream, cappuccino foam and chocolate mousse. Some foams form a solid structure when they are cooked (egg whites become meringues).

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Gluten is a protein found in wheat flours (wheat, rye and barley). Gluten is formed when water is mixed with the flour to make a dough and can be found in pasta, bread, cakes and pastries. All doughs have elasticity as the molecules of gluten are coiled making them able to stretch and bend. Doughs need to be kneaded to work the gluten this causes gluten strands to get longer, stronger and stretchier. Gluten coagulates when it reaches high temperatures and the dough stays stretched, giving foods like bread a light texture.