Lesson 3. Methods and Principles of Food Processing (Part II)

The methods and principles can further classify based on food preservations. These are;

1. Control of Water Activity

Water activity of the food can be control the different methods which are explained hereunder.

 a. Evaporation: Frequently in the food industry a raw material or a potential foodstuff contains more water than is required in the final product. When the foodstuff is a liquid, the easiest method of removing the water, in general, is to apply heat to evaporate it. Evaporation is thus a process that is often used by the food technologist. The basic factors that affect the rate of evaporation are;

 - Rate at which heat can be transferred to the liquid

- Quantity of heat required to evaporate each kg of water

- Maximum allowable temperature of the liquid

- Pressure at which the evaporation takes place

- Changes that may occur in the foodstuff during the course of the evaporation process

 b. Dehydration: It is defined as the application of heat under controlled conditions to remove the majority of the water normally present in a food by evaporation. The main purpose of dehydration is to extend the shelf life of foods by reduction in water activity.

 c. Osmo-dehydration: In osmo-dehydration, the prepared fresh material is soaked in a thick liquid sugar solution and or a strong salt solution and then the material is dried. During osmotic treatment, the material loses some of its moisture. The syrup or salt solution has a protective effect on colour, flavor and texture. This protective effect remains throughout the drying process and makes it possible to produce dried products of high quality.

 d. Drying: Drying is the process to remove the moisture content from the food material and thereby reduce the water activity and extend the shelf life. Several types of dryers and drying methods are commercially used. This is depending upon the form of material and its properties, desired physical form and characteristics of dried products, operating cost, etc. Sun drying, solar drying, atmospheric drying (batch type) like, kiln, tower, cabinet and (continuous type) like tunnel, belt, fluidized bed, puff, foam mat, spray, drum and microwave, etc.         

 2. Cold Treatment

 Cold treatment is the treatment like, fermentation, irradiation, di-electric, ohmic, infrared heating, freezing, super cooling, refrigeration, etc. given to the food materials for increasing the shelf life.

 a. Fermentation: During food fermentations, the controlled action of selected micro-organisms is used to alter the texture of foods, preserve foods by production of acids or alcohol, or to produce subtle flavors and aromas which increase the quality and value of raw materials. Main advantages are;

 - The use of mild conditions of pH and temperature which maintain the nutritional properties and sensory characteristics of the food.

- The production of foods which have flavors or textures that cannot be achieved by other methods

- Low energy consumption due to the mild operating conditions

- Relatively low capital and operating costs

- Relatively simple technologies

 b. Irradiation: Ionizing radiation takes the form of ϒ-rays from isotopes or, commercially to a lesser extent, from X-rays and electrons. Main advantages are;

 - There is little or no heating of the food and therefore negligible change to sensory characteristics.

- Packaged and frozen foods may be treated.

- Fresh foods may be preserved in a single operation, and without the use of chemical preservatives

- Energy requirements are very low

- Changes in nutritional value of foods are comparable with other methods of food preservation

- Processing is automatically controlled and has low operation costs.

 Applications of Irradiation;

- Sterilization

- Reduction of pathogens

- Prolonging shelf life

- Control of ripening

- Disinfestations

- Inhibition of sprouting

c. Dielectric, Ohmic and Infrared heating: Dielectric energy and infrared energy are two forms of electromagnetic energy. They are both transmitted as waves which penetrate food and are then absorbed and converted to heat. In contrast, ohmic heating uses the electrical resistance of foods to directly convert electricity to heat. Dielectric and ohmic heating are direct methods in which heat is generated within the product, whereas infrared heating is an indirect method that relies on heat that is generated externally being applied to the surface of the food mostly by radiation, but also by convection and to a lesser extent conduction.

d. Freezing: Freezing is the reduction in temperature generally by super cooling followed by crystallization of water, nucleation and finely crystal growth.

 Methods of quick freezing

- Freezing by indirect contact with a refrigerant

- Freezing in a blast of cold air

- Freezing by direct immersion in a refrigerating medium

 Freezing by indirect contact with refrigerant:  Food may be frozen by being placed in a contact with a metal surface which is cooled by a refrigerant or packaged or packed in a can and cooled by immersion in a refrigerant. Also food packaged in paper boxes may be frozen by contact with refrigerated metal plate which may be moving or stationary.

 Air Blast freezing: To obtain very cold air, a blast of air is directed through refrigerating coil.  For greater effect, the cold air blast is confined in an insulated tunnel.  The material to be frozen may be placed on a moving belt within variable of moved countercurrent and the air blast.

 Freezing by direct immersion (FBDI): FBDI in low temperature drying was the beginning of quick freezing. Since liquid are good heat conductors, a product can be frozen rapidly by direct immersion in low temperature liquid for example brine and sugar solutions.

 Freezing time: The definition of freezing time is a function of two instances i.e. when freezing starts and when it stops. It is very difficult to determine the freezing time (q) since freezing will occur at different rate and at different point in a piece of food. The freezing will be faster at some point on the surface and in the body of the piece of food, there is a point which cools slowest. The highest temperature at which ice crystals have a stable existence in a food material is known as the freezing point of that material and this signals the starts of freezing time. Because of the nature of materials of food and the presence of water soluble constituents, all water does not crystallize at this temperature, this is known as cryoscopy effect.

e. Super Cooling: Occurs when temperature of water is lowered below the freezing point and crystallization does not occur.  The super cooling provides the means of determining the in depth effect of a reduction in temperature relative to the initial freezing point.

f. Refrigeration: This is the process by which heat is removed from a confined place and material for the purpose of maintaining a lower temperature. The standard unit of generating heat capacity is 1 tone of refrigeration. This is derived on the basis of removal of latent heat of fusion of 1 tone of water at 32o F or 0oC to produce 1 tons of ice.

  3. Control of Microbial Activity

 Food materials can be stored for longer period by controlling the microbial activities by pasteurization and sterilization methods.

 a. Pasteurization: Pasteurization is a relatively mild heat treatment, in which food is heated to below 1000C. In low acid foods it is used to minimize possible health hazards from pathogenic micro-organisms and to extend the shelf life of foods for several days. In acidic foods it is used to extend the shelf life for several months by destruction of spoilage micro-organisms and or enzymatic inactivation.

 b. Heat sterilization: It is the unit operation in which foods are heated at a sufficiently high temperature and for a sufficiently long time to destroy microbial and enzyme activity. As a result, sterilized foods have a shelf life in excess of six months at ambient temperatures provided they are aseptically packed after sterilization to avoid post sterilization contamination. Severe heat treatment of sterilization causes damage to the nutrition and it can be avoided by development of suitable processing technology.


Dhamsaniya, N. K. (2009). Guide to Post Harvest Unit Operations. Kalyani Publishers.

Fellows, P. (2000). Food Processing Technology, Principles and Practice (Second Edition ed.). Cambridge , England: Woodhead Publishing Limited.

Sahay, K. M. and Singh, K. K. (2009). Unit Operations of Agricultural Processing (Second ed.). India : Vikas Publishing House Pvt Limited.

Srivastava, R. P. and Kumar, S. (2002). Fruits and Vegetables Preservation: Principles and Practices. Lucknow, India: International Book Distributing Co.

Last modified: Saturday, 5 October 2013, 10:06 AM