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Protein separation techniques

Using protein separation techniques to understand protein performance in food systems


Introduction


The chemical composition of raw ingredients affects their functionality. Understanding how raw ingredients function in food systems is critical to controlling final product quality. In addition to their nutritional value, proteins can confer many functional properties, including elasticity, extensibility, viscosity, foaming, gelling ability, coagulation, film forming ability and emulsification. Wheat proteins can form gluten, which gives dough its elasticity and extensibility, and consequently the characteristic texture and structure of the wheat-based products. Egg proteins have a range of different properties including foaming, coagulation and gelation, and are commonly used in a wide range of products such as cake, meringues and custard.


Proteins are composed of chains of amino acids (polypeptides) with a specific spatial conformation that corresponds with biological function. There are four levels of protein structure. Primary structure is the sequence of amino acids in the chain; individual amino acids are joined together by covalent peptide bonds. Secondary structure is the areas of folding and coiling within a protein and is determined by hydrogen bonds between the amino acids on the same chain. The overall three dimensional structure of a protein is its tertiary structure and is driven by non-covalent interactions. Quaternary structure refers to the non-covalent interactions that bind multiple polypeptides into a single, larger protein.


Proteins can be classified by their chemical structure, biological function or solubility. Proteins can have multiple functionalities in a food system, including structural, foaming, gelling, coagulating, film forming, emulsification and browning, as well as their nutritional role.


Different methods of analysis can reveal different characteristics of proteins – their overall structure, size, solubility and interactions – all of which can help predict functionality. Therefore, it is important to choose a method which will best give you the information you need.


Polyacrylamide Gel Electrophoresis (PAGE)


Gel electrophoresis is a technique where proteins are separated according to their physical properties as they are forced through a polyacrylamide gel by an electric current. The gel matrix consists of a cross-linked polymer network which has a sieving effect. Different forms of this technique can be applied, which can provide different types of information, including composition, and evidence of denaturation or degradation. This information can be applied to track the effect of processing variables such as time, pH and temperature, and the use of processing aids such as reducing, oxidising or crosslinking agents and enzymes. This can help to determine which process is most appropriate. It can also help explain how a change in processing conditions has had a beneficial or deleterious effect on final product quality. For example, changing the pH of a formulation could lead to the presence or absence of haze (from suspended solids) – due to changes in protein solubility. Some commonly used PAGE techniques are described below.


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