Size reduction in liquid foods generally involves homogenization and emulsification. Homogenization is the process of breaking down food components e.g. fats into smaller particles and dispersing them in the food system. The dispersed particles form the dispersed phase and the liquid that the particles are dispersed in forms the continuous phase.
Homogenization may be done using high-speed mixers, hydroshear homogenizers, membrane emulsifiers, pressure homogenizers, rotor-stator homogenizers, and ultrasonic homogenizers. Once dispersed, particles can move back to where they were. Therefore emulsifiers are used to keep them apart. Emulsifiers bind with dispersed components to form an emulsion. There are generally two types of emulsions. There is oil-in-water (o/w) and water-in-oil (w/o) emulsion. In an o/w emulsion, the oil is dispersed in water such as in milk and mayonnaise. In a w/o emulsion, the water is dispersed in the oil as in butter and mayonnaise. Emulsifiers are able to keep water and fat apart because they are amphiphillic. That means they have both a polar and a nonpolar part. Take lecithin that is found in egg and soybean for example.
The molecule consists of two fatty acids which are non-polar (able to interact with fat), and a phosphate group that has a negative charge and is therefore polar and able to interact with water. In an o/w emulsion, the non-polar part of each emulsifier attaches itself to the oil while the non-polar end orients itself to the water. With many emulsifiers in the food system, they completely surround each oil drop, preventing them from aggregating with other oil droplets. Hence, the emulsifier isolates and trap the oil droplets preventing them from reconnecting.
In a w/o emulsion, the opposite happens. The non-polar part of the emulsifier orient itself to the oil and the polar part attaches to the water droplets, surrounding them and preventing them from aggregating.
Type of Emulsions
Apart from lecithin, there are many types of emulsions being used commercially. Your choice of emulsion will be based on its hydrophillic/lipophillic balance or HLB value. Emulsifiers with a high HLB value (12-18) are highly hydrophillic and are therefore very soluble in water. These are therefore recommended for o/w emulsions which has mostly water. Emulsifiers with a low HLB value (0-6) are highly lipophillic (fat loving) and are therefore soluble in oil. These are recommended for w/o emulsions which contain mostly oil. Examples of high HLB emulsifiers include: lecithin (18-20), polyoxyethylene sorbitan monostearate (Tween 60) (14.9), and polyoxyethylene sorbitan monolaurate (16.7). Examples of low HLB emulsifiers include: Sorbitan trioleate (Span 85) (1.8), Sorbitan tristearate (Span 65) (2.1), and Sorbitan monooleate (Span 80) (4.6).
Stability of Emulsions
The stability of emulsions can be determined based on the velocity (v) of separation of the emulsion phases. Velocity of separation is given by the following formula:
Where d(m) = diameter of droplets in the dispersed phase; g = acceleration due to gravity (9.81 ms-2); ρp (kgm-3) = density of dispersed phase; ρs (kgm-3) = density of continuous phase and µ (Nsm-2) = viscosity of continous phase.
As you can see from the equation, the smaller the size of the droplets, the slower the emulsion will separate. Slower separation will also occur when the density of the dispersed phase and the continuous phase are close and the viscosity of the continuous phase is high.
Application of Emulsifiers
Common used of emulsifiers in food include:
- Slowing down retrogradation and maintaining softness in bread
- Interacting with gluten to form a stronger network
- Reducing surface tension in bakery and dairy products to allow more incorporation of air and hence greater volume and softer texture
- Preventing migration and crystallization of fat in chocolate
- Improving solubility of fat-coated solids in instant powders such as soup mixes
Reference: Fellow, PJ. 2009. Food processing technology, 3rd edition. Boca Raton FL: CRC Press.
