Gel Forming Property of Proteins
Tofu is made by coagulating soy milk and pressing the resulting curds. Although pre-made soy milk may be used, most tofu producers begin by making their own soy milk, which is produced by soaking, grinding, boiling, and straining dried soybeans.
According to McWilliams (1997), Coagulation of the protein and oil (emulsion) suspended in the boiled soy milk is the most important step in the production of tofu. This process is accomplished with the aid of coagulants. Two types of coagulants (salts and acids) are used a tofu that is tender but slightly brittle in texture. The coagulant itself has no perceivable commercially.
According to Fennema (1996), the coagulant used in the experiment is a salt coagulant which is Calcium sulfate. As such, many tofu manufacturers choose to use this coagulant to be able to market their tofu as a good source of calcium.
According to Fennema (1996), in beverages, the solubility of proteins and their functionality is based on numerous factors including protein structure, pH, ionic structure and salt content. Salts compete with protein for water. In the absence of salt or at low concentrations, there is reduced interaction between oppositely charged proteins. At high concentrations, proteins tend to stick together or agglomerate, and protein solubility decreases. The precipitation that occurs is called “salting out.”
According to McWilliams (1997), a gel is a continuous three-dimensional, solid-like, cross-linked network of protein molecules embedded in an aqueous solvent. Yogurt, hot dogs, cheese and custards benefit from gel formations. Protein gelation often is a result of the application of heat or mechanical mixing, which can denature proteins. Moreover, other food ingredients can influence the temperature of denaturation; as an example.
According to McWilliams (1997), Relative to most other gelling agents, gelatin is unique in that it has an elastic texture. There are two types of gelatin, Type A and B, and they perform differently in various applications. The two types of gelatin have a range of isoelectric points (IEP–the pH at which the charge of the protein changes and becomes neutral). The IEP for Type A gelatin is between 7-9. Below pH 8, Type A gelatin is positively charged. Above pH 8, it is negatively charged. The isoelectric point of Type B is in a pH range of 4.5-5.5. These different charges affect how the gelatin will perform in a food application system.
According to McWilliams (1997), Proteins have the least functionality when nearest their isoelectric point. Their viscosity and gel strength drop quite a bit at the isoelectric point.
According to McWilliams (1997), Manipulating the pH and ionic strength can affect flavor, translucency and gel strength. If, for example, a manufacturer wants a more acidic flavor while formulating a gummy bear, the addition of an acid buffer would meet the requirements. Adding an alkaline buffer like sodium citrate would help maintain a higher pH for a given acid level.
According to Belitz (1999), Protein-based gels can be either translucent or turbid. A turbid or cloudy gel is created by first formulating the pH close to its isoelectric point and then heating the solution, whereas clear gels are formed by moving the pH far away from the isoelectric point prior to heating.
According to Fennema (1996), unlike most polysaccharide gelling agents, gelatin gel formation does not require the presence of other reagents such as sucrose, salts and divalent cations.
Foaming Property of Proteins
According to McWilliams (1997), Proteins may endure changes when subjected to stresses, particularly heat, agitation, and ultraviolet light which may decrease solubility and loss of ability to catalyze reactions. Denaturation and coagulation are physical changes occur in the protein molecule. The physical changes said earlier are effects of introduction of energy into the protein containing system, energy that is provided by heating or beating. Protein in egg white denatures and coagulates which then result to foam formation. The energy available from the beating action causes denaturation of some of the protein, and it is this denatured protein that gives rigidity to cell walls, hence gives stability to the foam. These stable foams provide a light and airy product with large volume. And as stated by Lowe (1955), Surface tension is lowered with increased in temperature. This explains why eggs taken from the refrigerator and beaten while still cold do not whip up as quickly as when beaten at room temperature.
Effect of Heat on Proteins
According to McWilliams (1997), Heat can be used to disrupt hydrogen bonds and non-polar hydrophobic interactions. This occurs because heat increases the kinetic energy and causes the molecules to vibrate so rapidly and violently that the bonds are disrupted. The proteins in eggs denature and coagulate during cooking. Other foods are cooked to denature the proteins to make it easier for enzymes to digest them.
According to McWilliams (1997), Denaturation is a change in the nature of a protein – a breakdown in its structure. It generally occurs during food preparation, when a protein is heated, subjected to physical agitation or when chemicals are added to it.
Added to that, during denaturation, the molecule unravels, losing its normal zigzag or globular structure and its elasticity, so that it can no longer function as before. Denaturation is generally irreversible. It may be caused by:
- Heat: the protein coagulates and shrinks. For example, egg yolk and white solidify, and milk forms a skin on top.
- Addition of chemicals: Acids, alkalis, alcohol and enzymes may cause denaturation. For example, lemon juice or vinegar will curdle milk. Rennin clots milk in cheese making. Many poisons work by denaturing essential body proteins.
- Agitation such as whipping or shaking: When egg white is whipped it changes to a foam. Cooking coagulates the foam in its expanded state.
COAGULATION OF PROTEINS
According to Fennema (1996), Opalescence is a type of dichroism seen in highly dispersed systems with little opacity. The material appears yellowish-red in transmitted light and blue in the scattered light perpendicular to the transmitted light. The phenomenon is named after the appearance of opals.
According to Fennema (1996), there are different degrees of opalescent behavior. One can still see through a slightly opalescent phase. The more particles and the bigger the particles are, the stronger the scattering arising from them and the cloudier the particular phase will look. At a certain concentration the scattering is so strong that all light passing through is scattered, so that it is not transparent any more.
Effect of pH on Hydration of Meat Proteins
According to Deman (1999), in low and intermediate moisture foods, such as comminuted meat products, the ability of proteins to bind water is critical to the acceptability of these foods. The ability of a protein to exhibit a proper balance of protein and protein water interactions is critical to their thermal gelation.
According to Fennema (1996), the water binding capacity of proteins is defined as grams of water bound per gram of protein when a dry protein powder is equilibrated with water vapor at 90-95% relative humidity.
According to Fennema (1996), Proteins exhibit the least hydration at their isoelectric pH, where enhanced protein-protein interactions result in minimal interaction with water. Above and below isoelectric pH, because of the increase in the net charge and repulsive forces, proteins swell and bind more water. The water binding capacity of proteins is greater at pH 9-10 than at any other pH. This is due to ionization of sulfhydryl and tyrosine residues. Above pH 10, the loss of positively charged — amino groups of lysyl residue results in reduced water binding.