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Milk Centrifugation

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Introduction
Centrifugation is a process used to separate or concentrate materials suspended in a liquid medium. The theoretical basis of this technique is the effect of gravity on particles (including macromolecules) in suspension. Two particles of different masses will settle in a tube at different rates in response to gravity. Centrifugal force (measured as xg, times gravity) is used to increase this settling rate in an instrument called centrifuge.
Centrifuges are devices used in a variety of application which spin carrier vessels (centrifuge tubes) at high rotation speeds and very high centrifugal force. The centrifugal force generated is proportional to the rotation rate of the rotor (in revolution per minute, rpm) and the distance between the rotor center and the centrifuge tube. Therefore, a given centrifuge may use multiple rotor sizes to give flexibility in choosing centrifugation conditions. Each centrifugation has a special graph, a nomograph, or a table which relates rotation rate (rpm) to centrifugal force (xg) for each size of rotor it accepts.
Centrifuges come in three general classes: low speed, up to about 5000 rpm, high speed machines of up to about 25000 rpm and ultracentrifuges which will turn up to 1000000 rpm. Another form of centrifuge very common in laboratories is the microcentrifuge. These are simple machines used with 0.5 or 1.5 mL disposal plastic tubes. Most of these machines generates between 10000 and 13000 rpm.
Certain procedures necessitate precise centrifugation conditions, which must be specified in terms of relative centrifugal force (rcf) expressed in units of gravity (times gravity or ×g). Many micro centrifuges only have settings for speed (rpm), not relative centrifugal force. Consequently, a formula for conversion is required to ensure that the appropriate setting is used in an experiment. The relationship between rpm and rcf is as follows: g = (118 ×10-5 ) rs2 . Where g is the relative centrifugal force, r is the radius of the rotor in cm, and s is the speed of the centrifuge in rpm. As an example, centrifugation of a sample at 5000 RPM in a micro centrifuge that has a motor with a radius of 7 cm will deliver a centrifugal force of 1957 × g.
Rotors for a centrifuge are either fixed angle or swinging buckets. Fixed angles generally work faster; substances precipitate faster, or they have an increased relative centrifugal force for a given rotor speed and radius. In a fixed angle rotor, the materials and forced against the side of centrifuge tube, and then slide down the wall of the tube. This action will lead to abrasion of the particles along the wall of the centrifuge wall. For a swinging bucket, the materials must travel down the entire length of the centrifuge tube through the media within the tube. Since the media is usually a viscous substances, the swinging buckets appears to have a lower relative centrifugal force, and it takes longer to precipitate anything contained within. If, however, the aim is to separate molecules or organelles on the basis of their movements through a viscous field, then the swinging bucket is the choice. (UTAR, 2016)
Objectives: 1. to learn the process of centrifugation
2. to determine the relationship between the coagulation of milk that will separate from solution and speed of centrifugation.
Materials: Centrifuges, Centrifuge tube (15 mL), micro centrifuge tubes (1.5 mL), Micropipette, fresh milk, acetic acid.
Methods: 1. 10 mL of fresh milk was drawn into centrifuges tube which contains 50% of acetic acid.
2. Inverted gently few times to mix.
3. Milk sample was centrifuged at 1500 rpm for 10 minutes to pellet the coagulated milk solid.
4. One mL of milk was transferred to the centrifuge tube and was centrifuged in 5000 rpm for 15 minutes.
5. One mL of milk was transferred to the micro centrifuge tube and was centrifuged in 13000 rpm for 15 minutes.
6. The differences between these tubes was observed at the end of procedure.
Discussion
After first centrifugation at speed of 1500rpm for 10 minutes, 1 fine layer of white precipitate, cloudy solution and clear solution formed. The fine precipitate is known as casein. However, only little casein was separate and present of cloudy of solution is due to low speed of centrifugation.

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