Cleaning
There are many methods or devices that can be used for cleaning: water, high velocity air, brushes, magnets, steam, ultraviolet light, ultrasound, and vacuum. Cleaning is an essential unit operation that serves to remove dirt, soil, bacteria, debris and foreign bodies. Moreover, cleaning is vital to eliminate pesticide residues that may be detrimental to human health. Eviscerating poultry and removing fish scales are also considered as cleaning operations if water is used. For example, tuna is usually gutted; its skin, bones and honeycombed tissues are removed before filling into cans. The irregular surfaces of a product like pineapples are usually cleaned by the scrubbing action of high-pressure water jets.
Other than raw materials, the equipment used in a food processing plant is required to be cleaned after each use. The formation of fouling layers is common during the thermal treatment of products. These fouling layers cause severe problems for the food industry that can lead to a drastic increase in resistance to heat transfer, thereby decreasing the thermal efficiency of equipment like heat exchangers. Therefore, cleaning is essential, and sanitizers are often used for this purpose.
Separating
Generally, separating or sorting is done on the basis of a measurable physical property such as size, shape, colour and weight. The purpose of this operation is to isolate desirable part of a raw material from another. By doing so, one can ensure uniformity among the food products before subsequent processing. We can isolate a solid part from another solid part like potato skin removal. In the production of sugar cane juice, liquid (sugar cane juice) can be removed from solid (fiber). A liquid can also be separated from a liquid, as in separation of cream and milk.
Mechanical separation is divided into four groups - sedimentation, centrifugal separation, filtration and sieving. In sedimentation, two immiscible liquids, or a liquid and a solid, differing in density, are separated by allowing them to come to equilibrium under the action of gravity, the heavier material falling with respect to the lighter. We can speed up this process by applying centrifugal forces to increase the rate of sedimentation; this is called centrifugal separation. Filtration causes a mixture to flow through fine pores, which are small enough to stop the solid particles, but large enough to allow the liquid to pass. Sieving is done by interposing a barrier through which the larger elements cannot pass.
Size-reduction
Raw materials often occur in sizes that are too large to be used, therefore they must be reduced in size. Size-reduction is an operation whereby large masses of foods are subdivided into smaller units or particles. It can be accomplished by cutting, grinding, crushing and pulping. Examples include automatic dicing of vegetables, milling of flour, grating of cheese, and cutting of tuna before canning. Homogenization is essential especially in the dairy processing. This produces disintegration of large globules and clusters of fat in milk or cream to minute globules. The milk or cream is forced under high pressure through a valve with very small openings.
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