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DATE – 7TH FEB, 2022
Types of Unit operation
1)SIEVING - Sieving is a separation technique based on the difference in particle size.
Successive smaller mesh size is used which gives a series of particle size which can be
classified according to the size.
Agitation method – Oscillation, Vibration and Gyration
Mesh size- The mesh size range from 2.5 to 400 which correspond to 8.00mm to 0.037mm
Working principle – Material which need to be separated kept above the mesh of the sieve
and then the sieve is vibrated or agitated, particle which is smaller than the mess size passes
through and the bigger remain on the top, this is done under the influence of gravity. In this
was we get the different sizes of material at different sieve depending on the mesh size.
This operation is used in flour mills to separate flour from husk. Vibrating sieve is also used
in snacks and confectionaries companies as a part of production process. It is also used for
Milk powder, chocolate powder, Whey powder etc.
Changes – We only observed physical changes in this operation as particle are separated on
the basis of their size under the influence of gravity. No Biological or Chemical changes is
2)Osmosis – “By definition, Osmosis is the spontaneous floor of the solvent molecules
through a semipermeable membrane into a solution or from a dilute solution to a concentrated
Working Principle – “Osmosis is also observed when you were to solutions made from the
same solvent but of different concentrations are separated from each other by a
semipermeable membrane. In such cases solvent flows from less concentrated solution into
the more concentrated one”(Gosh,2021,p.1)
Osmotic pressure – “The hydrostatic pressure required to resist the movement of solvent
molecules in this way is osmotic pressure” (Gropper, 2020). According to an 't Hoff equation,
which states that osmotic pressure depends on the number of solute particles, temperature,
and how well a solute particle can move across a membrane.
Use in Food industry - concentration of fruits and vegetables juices (grape, pineapple, red
raspberry, orange, tomato, and red radish juices) and natural food colorants (anthocyanin and
betalain extracts).
Physical change – As solvent flow from region of low concentration to high concentration it
becomes less diluted and more concentrated solution. There is change in viscosity of the
3) Reverse osmosis – Movement of water molecule from the region of higher concertation to
the region of lower concentration through semi-permeable membrane when reverse pressure
is applied is called reverse osmosis.
Working Principle – Reverse Osmosis (RO) works by reversing the principle of osmosis,
the natural tendency of water with dissolved salts to flow through a membrane from lower to
higher salt concentration. This process is found throughout nature. Plants use it to absorb
water and nutrients from the soil. In humans and other animals, kidneys use osmosis to
absorb water from the blood (Ning, 2015)
Uses – It is used in RO filtration, it can filter chemical like arsenic, chlorine, lead, mercury,
sodium etc. It is also used in desalinating seawater. It is also used to remove bacteria and
brine in meat. The beverage industry uses RO in the production of beer, wine and spirits, as
well as soft drinks and juices. (Philip, 2021)
Changes During RO – Physical separation of chemical or ion took place through semipermeable membrane. It is also helps in collecting impurities on one side e.g., Water
desalination process.
4) Centrifugal separation - Centrifugation is a method of separating molecules having
different densities by spinning them in solution around an axis (in a centrifuge rotor) at high
speed (Stephenson, 2016)
Working principle - the driving force behind the separation is the difference in density
between the phases. By using centrifugal forces the separation process is accelerated. The
necessary centrifugal forces are generated by rotating the materials. The force generated
depends on the speed and radius of rotation. When the differences in density are large and
time is not a limiting factor separation can take place by gravity (known as sedimentation and
skimming) (Hyfoma, 2018).
Use in Food industry – It is used in dairy industry to separate cream, skim milk and whey,
concentration of cream, in cheese factory and also used in beverage technology, vegetables
and fruit juices, coffee, tea, beer, wine etc
Changes during process – There is physical separation take place during the process e.g. in
dairy fat crystal are separated and collected from milk to get skim milk as result. In Beverage
industry also pulp is filtered out to give clear juice.
Sieving- R. L. Earle. (n.d.). Unit operations in food processing - R. L. Earle. Retrieved February 4,
2022, from
Gosh, P. (2021, september ). Osmosis and Diffusion. T.E.M.S Journal, 3(1), 1.
Gropper, M. A. (2020). Perioperative Fluid and Electrolyte Therapy. Miller's Anesthesia, 1.
Philip, R. (2021, april 28). Retrieved from Watertechonline:
Ning, R. Y. (2015). Reverse Osmosis Chemistry — Basics, Barriers and Breakthroughs.
Stephenson, F. H. (2016). Calculations for Molecular Biology and Biotechnology (Third Edition)
(Vol. 3).
Hyfoma. (2018, March 8). Centrifugation/sedimentation. Retrieved from Safefoodfactory :