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Thermal Processing Methods

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

Malaysia is one of the developing countries that show continual growth in food production ever since its independence in 1955. Palm oil, indigenous chicken meat, rice, hen eggs, indigenous pork and vegetables are the top productions in this country. Under ordinary storage conditions, it is inevitable that food starts to deteriorate once the living cells in the food (plant and animal origins) are dead. The exposure of food and damaged cells to the environment attracts microorganisms (e.g., bacteria, molds and virus) and insects, which in turn further increase the rate of decomposition of the food. Therefore, food processing is necessary to transform raw ingredients into final products that are safe for human consumption. Food preservation serves to stop or greatly slow down spoilage to prevent food-borne illness while enhancing the nutritional value, texture, appearance and flavour of food products. Proper food packaging and storage are also essential to prevent oxidation, microbial contamination and loss of moisture.

Heat application is a common procedure used in the manufacture of processed food products. As far as food industry is concerned, the reasons for heating include destruction of microorganisms, preservation of food, removal of moisture, and development of flavours. Blanching, pasteurization, and sterilization are the common heat applications. The main function of blanching, which is a heating method by immersing food ingredients in hot water for a short time, is to inactivate enzymes which some are responsible for the deterioration of food products. Blanching can also enhance the colour of foods and soften vegetable tissues to facilitate filling into containers. However, blanching of foods is not a sole treatment because it only reduces the population of microorganisms on the surface of foods. Hence, this method is always combined with other methods like freezing, sterilization and dehydration. The combined methods can extend the shelf life of food products.

We sometimes wonder why some milk products are stored in chilling temperature; while some are stored in room temperature. Are the products pasteurized or sterilized? Understanding pasteurization and sterilization is essential. Pasteurization is a mild heat treatment with the range of 60-90 degree Celsius. This process was named after a French chemist, biologist and microbiologist, Louise Pasteur. Pasteur discovered that spoilage organisms could be inactivated in wine by applying heat at temperatures below its boiling point. The process was later applied to milk and remains the most important operation in the processing of milk.

One of the purposes of pasteurization is to minimize health hazards from pathogenic microorganisms. For low acid food (pH > 4.5) such as milk, pasteurization can extend the shelf life of this product for several days. It has to be chilled and kept at 4 degree Celsius after pasteurization because microorganisms cannot be destroyed completely through pasteurization. On the other hand, pasteurized high acidic food (pH < 4.5) like bottled fruit can last up to several months. Pasteurized milk can be treated by Holding or HTST (high temperature short time) method. Raw milk is heated at about 63 to 65oC for at least 30 minutes in Holding method. Alternatively, HTST method can be used to pasteurize raw milk at about 73oC for at least 15 seconds. This is applied using a heat exchanger and the pasteurized milk is clean filled into glass bottles, plastic bottles or cartons. Pasteurized milk is immediately cooled to 4oC and maintained at the temperature with protection from contamination.

Pasteurization is used extensively in the production of many different types of food like fruit products, pickled vegetables, jams and chilled ready meats. Pasteurization causes some changes towards food. For instance, colour deterioration may occur in fruit juice due to enzymatic browning. Vitamin losses are unavoidable especially heat sensitive vitamin such as vitamin C. Nonetheless, sensory characteristics of pasteurized food do not change significantly. It is of concern to note that pasteurized foods are not sterile, and pasteurization will usually rely on other preservative mechanisms to prolong the shelf life of foods. Other than chilled temperatures, some foods have a sufficiently high salt, sugar or acid content to render them stable at room temperature (e.g. pasteurized canned ham).

Sterilization is a unit operation in which foods are heated at a sufficiently high temperature and for a sufficiently long time to destroy microbial and enzyme activity. Canning is one of the most common applications of sterilization. The concept of canning is to heat food in a hermetically sealed container so that it is commercially sterile at ambient temperatures. In other words, no microbial growth can occur in the food under ambient storage conditions until the package is opened. The shelf life of sterilized food ranges from months to a few years without the need of refrigeration. Sterilized or UHT (ultra high temperature or ultra heat treatment) milk, for example, can last for more than 6 months because sterilization destroys all of the microorganisms.

In low acid food, there is a potential for Clostridium botulinum to survive the canning process. This heat-resistant pathogen is able to grow in anaerobic conditions (i.e. no oxygen) and thus, all canning processes target this organism if no other effective hurdle to its growth is present. The thermal process must reduce the probability of a single spore surviving in a can of low acid product to one in one trillion (i.e. 1 in 1012). Commercially, the standard process is 3 minutes at 121.1oC or equivalent. The severe heat treatment during the older process of canning may produce substantial changes in nutritional and sensory qualities of foods.

The rate of heat penetration during canning is influenced by a few factors: type of product, size and type of container, and temperature of the retort. Liquid or particulate foods (e.g. peas in brine) in which natural convection currents are established, heat transfer is faster than solid foods whereby heat is transferred by conduction1 (e.g. meat pastes and corned beef). Heat penetration to the centre of the container is faster in small containers than in large containers. Agitation of the container increases the effectiveness of natural convection2 currents and thereby increases the rate of heat penetration. The major types of heat-sterilizable container are metal cans, glass jars or bottles, flexible pouches and rigid trays. Heat penetration is faster through metal than through glass or plastics owing to differences in their thermal conductivity3. In addition, a higher temperature difference between the food and the heating medium causes faster heat penetration.

Higher processing temperatures for a shorter time are possible if the product is sterilized before it is filled into a pre-sterilized container in a sterile atmosphere. This forms the basis of UHT or aseptic processing. It is widely used in the production of liquid foods such as milk, fruit juices or concentrates, wine, salad dressing and ice cream mix. It can also be used to process foods which contain small discrete particles e.g. baby foods, tomato products, soups and vegetables. During the UHT processing, food is normally heated in relatively thin layers in a continuous heat exchanger with close control over the sterilization temperature and holding time. Agitation is necessary during the processing of viscous foods to increase the rate of heat transfer and aid temperature distribution.

In UHT processing, meat pigments change colour, but there is little caramelization or Maillard browning. Natural pigments like carotene and chlorophyll can be retained. Besides, natural flavours of milk, fruit juices and vegetables are better retained compared to canned foods. In canned meats, changes in texture like shrinkage and stiffening of muscle tissues may occur. Canning also causes the hydrolysis of carbohydrates and lipids, but these nutrients remain available. Moreover, in canned fruits and vegetables, significant losses may occur in all water-soluble vitamins, particularly vitamin C. Conversely, aseptically processed meat and vegetable products may lose thiamine and pyridoxine, but other vitamins are largely unaffected. This is because the processing time is largely reduced.

Thermal processing undoubtedly can delay food perishability and inhibit food spoilage by microorganisms. Blanching and pasteurization are able to destroy enzymes and spoilage microorganisms and thus, the shelf life of food products is prolonged. Aseptic process causes the death of microorganisms before the nutrients are destroyed and results in products of superior quality. Food canning ensures seasonal products are available all year round and facilitates distribution or exports. Therefore, developments in processing technology are important to reduce the damage to nutrients and sensory components during canning.

Notes:

1 Conduction is heat that is transferred between solid food molecules inside the can via molecular collisions.

2 Convection occurs when heat is transferred through a liquid according to density differences and it is unique to liquid foods or foods packed in liquid.

3 Thermal conductivity is the amount of heat that will be conducted per unit time through a unit thickness of the material in a unit temperature gradient exist across that thickness.

References:
Coles, R. and Kirwan, M. 2011. Food and Beverage Packaging Technology. UK: Blackwell Publishing Ltd.

Fellows, P.J. 2000. Food Processing Technology Principles and Practice. UK: Woodhead Publishing Limited.

Hui, Y.H., Lim, M. H., Nip, W.K., Smith, J.S. and Yu, P.H.F. 2004. Principles of Food Processing. In Food Processing Principles and Applications, ed. J. S. Smith and Y.H. Hui. USA: Blackwell Publishing.

Murano, P.S. 2003. Understanding Food Science and Technology. USA: Thomson Learning, Inc.