Interfacial Properties of Protein: Emulsion and Foam

Emulsion: Mix of water and oil. Water and oil normally cannot be mixed. 2 kinds

a. Mixtures Were water is the continuous phase: oil in water emulsion (eg. simple sauce, mayonnaise )

b. Mixtures were oil is the continuous phase: water in oil emulsion (eg. margarine)

Why is it not stable? In milk for example there is cream at the top, the fat separates from the milk. What happens here? Small droplets on surface. In milk there are small casein micelles. In raw milk what happens is that the fat droplets are bigger and because there is a density difference between fat and water the droplets go upwards. The bigger the droplets the higher the density difference and the faster the speed at which the droplets go up.  The higher the density difference, the faster the droplet goes up. When we have creaming or a layer of oil on the top it means that it is unstable.

The important is what happens in the interphase between the water and the oil. The interphase must be stable. To make the interphase between the water and oil stable, we need to the polar and apolar part of the molecule together (anphiphilic part). The apolar par will be in the fat phase and polar par in the water phase. The oil droplet will be covered by water, the polar head will be attracted water, and it means that the water won’t feel the oil. Same with the oil, it will be only attracted by oil phase. In this way the interphase will be stabilized. To let this stable interphase happen we need an emulsifier. So for this we can use either a random coil protein (casein, don’t have tertiary structure) or globular.

When we have casein (random coil), some parts will be in the polar phase, others in the apolar phase, and other right on the interphase. This happens because the R group (leucine, phenil alanine are apolar), (cerine, aspartic acid, glutamic acid, this group is polar group). When we see at the primary structure we will se that there are parts were there is polar concentration, other apolar concentration , and in other a mix of polar and apolar. This is because the protein doesnt have a tertiary structure, it is a random coil, it is extremely flexible. In the primary structure there are this local zones of apolar, polar amino acids, then the caseins are good emulsifiers.

Globular protein is normally present in an aqueous environment and is not normally good emulsifier. So the surphase of the protein is in majority oil, because it is soluble in water.  If we bring this protein in an interphase between water and oil,  we will have a problem because they won’t like to be in the oil (if whoe surface is polar). The protein be defolded, it can undergo denaturation. When It is denatured, then it can enter in the interphase an d act as emulcifier. So normally globular proteins (with 3 rd structure) must to undergo denaturation (or partial denaturation). Before they become good emulsifiers. It can be done when we have the interphase and we mix water and oil, by adding mechanical energy, the protein can go out of each other , the dimer will be lost. The apolar and poalr zone will be separated and the emulsifier will be stabilized. Also in the globular protein there can be some more apolar zones and some more polar zones. It can act as an emulsifier depending on the distribution of the apolar zones, so sometimes we need denaturation and sometimes not.

Foams: Mixture of water and air. Composed of air bubles and they are surrounded by water. There is an interphase between air and water, the air is the apolar matrix and water is the polar matrix. It is chemicallya problem in protein solutions because they are foeaming when they have surface active properties (casein solutions, ovalbumine).