Sedimentation, Creaming and Centrifugation

The principle of creaming and centrifuging involves the application of Stokes Law [17]. In any dispersion, the dispersed particles cream or sediment under the influence of gravity and the velocity of creaming depends on several factors and is stated mathematically as ... 

The processing operations for fluid or manufactured milk products include cooling, centrifugal sediment removal and cream (a mixture of fat and milk semm) separation, standardization, homogenization, pasteurization or sterilization, and packaging, handling, and storing. 

Three types of driving forces can lead to coagulation. (1) The body forces, such as gravity and centrifugal force, cause sedimentation of the heavier particles in suspensions or creaming of the... 

Creaming and sedimentation result from external forces, usually gravitational or centrifugal. When such forces exceed the thermal motion of the droplets (Brownian motion) a concentration gradient builds up in the system, with the larger... 

Body forces, such as gravity and centrifugation causing creaming or sedimentation, depending on whether the mass density of the drops is smaller or greater than that of the continuous phase. 

Centrifugation. Centrifugal devices include clarifiers for removal of sediment and extraneous particulates, and separators for removal of fat (cream) from milk (see SEPARATION, CENTRIFUGAL). 

Another technique consists of submitting the emulsion to centrifugation and determining the droplet volume fraction < / at the top (bottom) of the cream (sediment). The centrifugation typically takes several hours until the equilibrium volume fraction is achieved. After equilibration, if the droplets occupy a distance much less than that of the centrifuge lever arm, the spatial gradient in the acceleration can be neglected, and the osmotic pressure can be determined (see Fig. 4.1) ... 

The ageing at 5°C of whippable emulsions such as ice cream mix will enhance the hydration of milk proteins in the system. This is due to a property of casein micelles in milk. At low temperatures, the hydration or voluminosity of casein increases. The voluminosity is the volume of hydrated protein per gram of protein. This can be studied by analyzing the protein and water content in the sedimented casein pellet after centrifugation of skimmed milk. 

This is 2500 times faster than with gravity alone, but the residence time in the centrifuge would have to be about 20 minutes, which is not practical. To speed up the separation, naphtha is added to the level of 25%. This lowers the viscosity to about 4.5 mPa-s and lowers the density of the continuous phase to 0.88 g/mL. Note that now the water drops would sediment rather than cream under gravitational force, and while the emulsion density is much reduced, the absolute value of the density difference changes very little Ap = -0.07 g/mL originally, and becomes Ap = +0.09 g/mL The overall effect is to lower the viscosity by about two orders of magnitude. The droplet velocity now becomes (dx/dt)" = 1.1 cm/s, which yields a satisfactory residence time of about 8 seconds. 

Sedimentation The settling of suspended particles or droplets due to gravity or an applied centrifugal field. Negative sedimentation, when droplets rise upwards, is also called flotation and is a part of the creaming process. 

Sedimentation centrifuges are commonly known as solid bowl systems, i.e., perforated bowls that are used to separate materials such as cream from milk, sludges from water in waste water treatment plants, and, of course, the biotechnology materials. 

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