High Pressure Homogenization
Purely mechanical process, High Pressure Homogenization, is a type of non-thermal process working at pressure range of 100-300 M Pa. Homogenization process is mostly used in food industry specifically in dairy sector for stabilization of emulsion. Though homogenisation process is long established in food industry but is also evidently used in pharmaceuticals, cosmetic and chemical industries as well.
To understand how homogenisation process stabilizes the emulsion it is necessary to understand what emulsion is. Emulsions comprises of mixture of immiscible liquids, which are present in two different phase- dispersed phase (droplet) and continuous phase.
Need of high pressure homogenization was to produce more consistent suspension and to get smaller mean droplet diameter. Here the energy density equals the pressure drop in the disruption unit. In conventional homogenisation process the pressure used to break the droplet molecules are 5-50 M Pa and in HPH the used pressure is around 100-300 M Pa. To optimize result in accordance to the need of process force can be customized and it also provide option between hot and cold homogenization hence lowering the chances of product contamination. Desirable droplet sizing depends highly on flow pattern which in turn determines the droplet breakup. Apart from providing emulsion stabilization it also has applications in cell disruption, micro/Nano emulsions, dispersion, particle size reduction and many more.
High Pressure Homogenization: Components & Mechanism
High pressure processing uses high pressure pumps and a disruption unit for continuous homogenisation. Pump produces positive displacement in a pulsating flow pattern. To prevent any cavitation in the pipelines there are dampers attached to pipes. This pulsation on the outer pipes causes noise due to vibration for which outer pipes are also equipped with dampers. There are impingements designed as impact bands.
The high pressure pump forces cell suspension to pass through very narrow but adjustable orifice to break the suspension into smaller particles which helps to overcome the Laplace pressure, which holds up the suspended particle against deformation, and reduces the diameter of the droplet. The impact ring at the end of narrow pathway provide efficient breakup of suspended particles. After passing this narrow path, due to high pressure, particle tend to hit on the impact ring of homogenizer with high velocity. Combination of large pressure drop, turbulent eddies and strong shearing forces produced by high pressure piston pump causes the breaking of fat globules in milk.
Two theories working behind this are turbulent eddies and cavitation theory. Theory of turbulent eddies is based on the fact that high pressure forces formation of small high energy eddies in the stream which collides with the fat globules. These eddies are formed due to the turbulent flow of milk. Due to high energy, pressure and velocity of the generated eddies are of around similar size, they hit the globules and deformation takes place leading to finally breakup.
Cavitation theory states that there is formation of shock waves in the stream of milk when passed through adjustable gap has been claimed. These shock waves are formed as the result of pressure drop in the milk. Vapour bubbles are formed due to high pressure during their turbulent flow and when pressure is released the bubbles collapses, which break apart the dispersed droplet.
High Pressure Homogenization: Influential Parameter
Flow rate, pressure, temperature and equipment parameter like impingement design and nozzle geometry are process parameter affecting the efficiency of the operation. Apart from process parameter, product parameter like fat, water, protein composition also influences the mechanism.
High Pressure Homogenization: Application
HPH can be used for products like liquid foods for emulsification, homogenisation and to an extend for pasteurization as well. Apart from less cream line formation, they make milk more appetiser and give better mouthfeel, reduce sensitivity to fat oxidation, gives better stabilization to cultured product. There are chances of increase in temperature during process due to usage of such high pressure. The rate of heat generation san be 17-20 Degree C. per 100 M Pa. which can be undesirable in case of heat sensitive product and control may be required in such case.
Certain common phenomena that occur in dairy sector are creaming of fat globules, flocculation, coalescence, sedimentation and phase change. These all condition adversely effects the quality of the milk and its product. During storage of milk it is quite common to observe the floating of milk fat on the surface which is called creaming effect. It happens when the density of fat globules is less than that of milk and on other hand when the density is more they tend to settle down at the bottom of container. Flocculation happens when two or more fat particles tend to agglomerate but when stirred they get again separated as individuals but in case of coalescence it’s an irreversible process where the fat globules stuck with each other and forms a new separate individuality. All this undesirable events can be avoided by using high pressure homogenization process.
Homogenisation process cannot be considered as sterilization process and are not effective of products that are solids or liquid foods with large particles. The only environmental aspect is that it is relatively low efficient as per the used energy. The major part of energy is lost in form of heat which require to be optimized in future through improvised designing of the process and equipment.