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Principles of Unit Operations in Food Processing

Author: Rachel Tan Choon Hui (Lecturer, UCSI University)

A wide range of more convenient and value-added food products are produced every day to meet consumer demands. For many foods, the product shelf life is limited by intrinsic or extrinsic factors or the effects of both factors. Intrinsic factors are the properties resulting from the make-up of the final product and include water activity, total acidity, presence of natural microorganisms, availability of oxygen, and product formulation. Extrinsic factors are a result of the environment that the product encounters during the life for instance product packaging, composition of gas atmosphere within packaging, consumer handling, and types of processing. Relative humidity, temperature control and exposure to light during storage or distribution also contribute to extrinsic factors that will affect the shelf life of food products.

Food manufacturers apply various unit operations to improve product quality. Unit operations imply a set of methods used during food processing. This article will cover some of the unit operations involved in the food industry including raw materials handling, cleaning, separating, disintegrating, forming, pumping, and mixing. Besides these seven operations, there are other unit operations like application methods (formulations, additives, heat, cold, evaporation, drying, fermenting etc.) or combined methods. Food manufacturers typically combine unit operations to create hurdles for microorganisms’ growth and thereby ensure food safety. The number of operating steps for each food varies from one food to the other, and the exact procedure vary from country to country.

Raw materials handling
Raw materials handling refers to the manner in which raw commodities (crops or animal) are harvested and transported to a processing facility. The rationales of this operation are to minimize product loss, guarantee proper sanitation, enhance product quality, and minimize bacterial growth. Shelf stable raw ingredients like cereal grains or nuts are non-perishable at room temperatures. Some of the materials have a rigid or strong shell that protects the content. These raw materials do not deteriorate or spoil easily. However, there are some raw materials that are perishable or fragile, and hence precaution steps are necessary. For instance, proper material handling is crucial during the gathering and transporting of eggs to a processing facility. Handling of sugar also poses great challenges. The sugar will cake (become compact due to absorption of moisture) if the storage time, temperature, and humidity are not appropriate. Improper transfer of sugar may result in dusting and buildup of static electricity, which can cause an explosion since sugar particles are highly combustible. Proper pest control during storage of raw materials is also necessary to reduce loss.

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.

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.

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.

Many categories of the food industry require forming operation, which is a size enlargement operation to obtain desirable shapes and sizes. Meat and poultry patties are gently compacted into a disk shape with the aid of a patty-making machine. Uniform pressure is applied to produce patties with minimal variation in weight. Molds and special tableting machines are used to form various shapes in the confectionery industry for the production of fondants, candies, chocolate, and jellies. Forming is also essential for the manufacturing of pasta. Dough is formed by forcing through extrusion dies of various forms and shapes before it is dried in an oven.

Pumping is an operation that moves foods from one location to another by mechanical action. The foods can be liquid, semisolid, paste or solid. There are many types of pumps available in the market for general transfer processes. Food manufacturers often choose pumps that are able to move large quantities of foods, easy to maintain, a lower cost, and able to preserve the physical properties of foods. Some pumps are employed for clean-in-place (CIP) purposes whereby the inner parts of pipes or instrument can be cleaned without dismantling.

When we cook food at home, we beat eggs in the air before frying to produce a more fluffy scrambled egg. We also mix flour, sugar, and butter using a blender for the making of cookies. Commercially, manufacturers apply mixing operation to blend food ingredients to create a food product. Mixing has no preservative effect, but it can alter the eating quality of foods. The operation of mixing includes kneading, agitation, homogenizing, emulsifying, dispersing, whipping and blending. Carbonated beverage production begins with careful measurement of the formula quantities of each ingredient into a syrup blending tank. Syrup is then pumped to a mix processor, where the syrup is diluted to finished beverage level with chilled, carbonated, treated water. In addition, mixing is crucial to create an emulsion (a mixture that contains immiscible liquids e.g. water and oil) like mayonnaise, ice cream, and margarine.

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