Foam is an outcome of agitation, fermentation or salivation. It is basically a mass of tiny bubbles in a liquid. Generally foams are defined as the gas bubbles dispersed in a liquid which is the continuous phase. The gas bubbles are segregated from each other by an elastic liquid wall.
The foam bubble’s diameter ranges from 1 micrometer to several centimetres. The foam density relies on the size of the bubble and thickness of foam wall.
“Few bigger size bubbles form a light foam while dense foam is formed by numerous small size bubbles”.
- If the number of the bubbles is less and bigger in size it forms a light foam
- If the size of the bubble is less and more in number it will form a dense foam.
Some common examples are whipped cream, ice cream, cake, bread, aerated drinks, etc.
PROPERTIES OF FOAM
1. Food foams carry a large quantity of entrapped gas. They have an extensive or large surface area among the gases & liquid phase.
2. The solute concentration is higher in the bubble wall as compared to bulk liquid.
3. The walls of each bubble is elastic and reflects light which leads to the opaque appearance of the foam.
FORMATION OF FOAMS
For foam formation foaming agent is mandatory in the continuous phase before the dispersion of gas. At the surface, the foaming agent must be adsorbed to facilitate the reduction in surface tension and provide a distinct surface layer for the formation of a gas bubble
Examples of foaming agents - surface active lipids, proteins, cellulose derivatives and glucosides. Mechanically FOAMS are formed by injecting gas into a foaming solution through an Orifice or by beating or whipping mechanically as in the case of eggs, etc.
The foam collapses due to diminishing of liquid in the bubble walls. These changes occur due to the following reasons:
- Gravitational force i.e due to pull of gravity liquid from the walls is drained out.
- The suction effect at the periphery of the walls of the container
- Due to the evaporation of the liquid from the walls of the bubble.
Sometimes small bubbles join to form large bubbles in foam and it is during this process that some liquid is drained off.
Both vacuums and high pressure have a destructive effect on the stability of foams.
HOW THE STABILITY CAN BE ENHANCED
Foam stability can be intensified/enhanced by the addition of certain solutes or particulate matters into the foaming solution e.g added sugar leads to an increase in the viscosity, which helps in increasing the strength and elasticity of the bubble wall, thus forming a more stable foam.
Foams are unacceptable or undesirable in certain food for example during processing of certain fruit juices, vegetable oils, syrups, heating of liquids, evaporation processes, drying of food, etc. During such processes, anti-foaming agents are used as excessive foaming results in a reduced rate of processing, packaging and losses in the product volume.
These anti-foaming agents collapse the foam and retard the formation of additional foam. A common anti-foaming agent used is Dimethyl Siloxane commonly known as silicon oil ( Manay S, 2010).
Proteins are macromolecules needed by humans and are made up of amino acids. Amino acids are of two types:
- Essential amino acids
- Non-essential amino acid
Essential ones are not synthesised in the body and hence need to be taken from the diet. Non-essential ones are synthesised in the body. In comparison with other macronutrients, physical properties of foodstuff are contributed by proteins. Proteins have some functional properties. They have special ability to form or stabilises the foams, gels, doughs, emulsions and fibrillar structures.
Milk is a good source of amino acids. They are considered to be complete proteins. They contain a good quantity of proteins which helps in maintaining the health of muscles. Milk proteins are a good source of nitrogen and amino acids that are needed by the human body to maintain various body functions. Milk proteins are categorised into 2 categories:
- Whey Proteins
- Casein Proteins
Milk contains 20% of whey protein, it is also known as fast releasing protein. 80% of milk protein constitutes of casein.
Whey proteins are a by-product of cheese making and casein manufacture in the industry. It is used as a value-added ingredient in various food products as it is rich in riboflavin. Having good functional properties makes whey protein a widely used food ingredient in many food industries (Morr C V and Ha EY,1993). Foaming capacity is one the most important functional property of whey proteins ( Kinsella J E and Whitehead M D,1989).
FOAMING PROPERTIES OF PROTEINS
In Comparison to other food macronutrients, proteins aid remarkably to the physical properties of foodstuffs. Their functional property varies from the capacity to build or strengthen (stabilise) gels, doughs, foams, emulsions and fibrillar structure.
Our focus is on the foaming property of gels:
Whey protein acts as both foam forming agent as well as foam stabilising agent.
As mentioned above dispersion of gases in the liquid is referred to as foams. Protein forms pliable, cohesive films throughout the gas bubble surface which is responsible for foam stabilisation. As the hydrophobic area of protein is adsorbed at interface followed by partial unfolding during whipping. Protein adsorption causes a decrease in surface tension which in turn further facilitates the formation of the gas bubble as well as the emergence of a new interface. Film formation associated with partially unfolded proteins.
The formability of molecules of a protein depends on two factors:
- Its diffusion rate
- Ease of denaturation
Above parameters depend on the following factors: molecular mass, the hydrophobicity of surface and stability confirmation.
“ Foam disintegration occur when large bubbles grow at the expense of smaller ones. Thus, the foam stability is determined by the stability of the protein film and on its gas permeability. Again, the film stability hinges on the quantity of adsorbed protein and on the coupling capacity of the adsorbed protein. Usually, surface denaturation uncovers the auxiliary amino acid side chains which can participate in intermolecular interaction. Film stability is directly correlated to the cross-linking i.e. the stronger the cross-linking, the more stable is the film. However, the system pH should be closer to the isoelectric points of the participated proteins as the association is initiated by small net charge” ( Weder J.K.P. and Belitz H-D, 2003)
According to a study conducted by Onwulata I C and Tomasula M P, smaller particle size is correlated with lower fat content and higher solubility.
Foams are formed by gas bubbles dispersed in a liquid in a continuous phase. Foams have different properties. They are opaque in appearance, bubble walls have higher solute concentration and they carry a large quantity of entrapped gas. Foams are formed by foaming agents and protein is one of the foaming agents responsible for foaming in proteins. There are many factors on which depends the stability of foams: Gravitational force, Suction effect and Evaporation. Stability of foams can be enhanced by adding solutes. There is an anti-foaming effect that collapses and retards the formation of additional foam. Proteins are macromolecules made up of amino acids. Whey proteins act as both foam forming and stabilising agent. The smaller size of whey protein molecules correlates with higher solubility. Whey proteins have good functional properties and foaming is one of the main functional property of whey proteins. There are various factors for ideal foaming in proteins and stabilisation, characterised by low molecular mass, high surface hydrophobicity, and good solubility.
So what do you think - Is Froth of your protein is good or bad?
- Manay, S., & Shadaksharaswamy, M. (2010). Foods Facts and Principles (third edition). 129-130.
- Functional Property of Proteins.Retrieved from Click here
- Morr, C.V. and Ha E.Y.(1993).Whey protein concentrates and isolates-processing and functional properties. Crit. Rev. Food Sci. Nutr., 33(6), 431-476
- Kinsella, J.E. and Whitehead, D.M.(1989).Proteins in whey: Chemical, physical and functional properties. Adv. Food Nutr. Res., 33,437-438
- Weder J.K.P and Belitz H.D.(2003).Encyclopedia of Food Sciences and Nutrition (Second Edition)