INTRODUCTION
1.1. Background of Study
Water is responsible for changes in the physical, chemical and/or biological characteristics of foods. It is an important component of all biological or living systems. In fresh foods, the percentage of water varies from 60% to more than 90% (Davide and Salvatore, 2012). Water content in foodstuffs during processing, storage and distribution may lead to deterioration of aroma and degradation of nutritional substances because it supports the existence and development of micro-organisms and chemical reactions within the foodstuffs. Drying or dehydration of foods is the basic food-processing operation used to preserve foods for a long period of time. Although both terms are applied to the removal of water from food, using heat and mass transfer, drying usually refers to natural desiccation such as spreading fruit on racks in the sun; and dehydration designates drying by artificial means such as a blast of hot air.
The purpose of food dehydration is to lower the availability of water in the food to a level at which there is no danger of growth of undesirable micro-organisms. A secondary purpose is the lowering of the water content in order to minimize rates of chemical reactions and to facilitate distribution and storage. The availability of water for microbial growth and chemical activity is determined not only by the total water content but also by the nature of it’s binding to foods. Although there are various method of dehydration or drying such as flash drying, spray drying, drum drying, agitated drying, freeze drying etc., this project will give a detail description of how food dehydration is achieved using freeze drying principles.
Freeze drying may be defined as the process of removal of water (free water) from a frozen product by sublimation, working at low temperature and pressure. The water passes from the solid phase directly into the vapor phase without becoming liquid; this implies that, it is necessary that the temperature of the sublimation zone in a material being freeze dried be held below the triple point temperature (i.e subcritical temperature and pressure) of the water or aqueous solution in the material being dried.
The driving force of freeze drying process is the vapor pressure difference between the ice front and the surrounding environment. Vapor, water and ice coexist in equilibrium (triple point) at 00
C and 6.1mbar. Thus, at temperatures below 00
C and vapor pressure below 6.1 mbar, liquid is never present and sublimation of ice to water vapor (and from vapor to ice) is the only phase change that can occur.Freeze drying also called lyophilization, cryopreservation or cryodesiccation is regarded as the best method of water removal from heat and oxygen sensitive food products or food having special end use, because low temperatures is required for the process (Ratti, 2001). It is a high quality food preservation process. Most deterioration reactions and microbiological activities are prevented, which gives food product of excellent quality. The process requires three basic steps which include the freezing, primary drying and secondary drying. These processes will be discussed in subsequent chapter. The solid state of water during freeze drying protects the primary structure and shape of the products with minimal reduction in volume. Freeze dried products have a long shelf-life without refrigeration, 2years for a product with a 2% residual moisture content being usual (Ratti, 2001). This technique has been applied with success to diverse biological material such as meat, coffee, juices, diary products, cells, bacteria, blood plasma, penicillin, berry, banana, orange, peep, Apricot and citrus.
The application of freeze drying principles has a wide range which includes; pharmaceuticals and biotechnology, food and agriculture based industries, chemical synthesis, bio-separation, nanotechnology, as recovery method for water damaged documents and books, bacteriology and high altitudes environment and advanced ceramics process.
1.2. Statement of Problem
The high market and consumer demand for “natural” and “organic” products with minimal process and without preservatives is increasing strongly. Thus, the study will be carried out to create solution to problems regarding preservation of original and natural quality of food during drying and sustaining the stability and durability at all season of dried product with minimal addition of additives or preservatives. This will in turn satisfy the market and consumer demand.
1.3. Aim and Objectives of Study
The project is aimed at producing freeze dryer for drying some selected agricultural and biomaterial products. The specific objectives are:
1.4. Scope and Limitations
This study will be limited to the design and fabrication of a machine aimed at freeze drying Nigeria local fruits or vegetables such as plantain, potato and carrot, and characterizing the machine using weight loss and water analysis.
1.5. Significance of Study
The project will be centered at food preservation method without the presence of preservatives but with high quality; (“organic, original and natural characteristics of product”) and high durability. Thus, reduce the amount of toxic/ chemical intake to the human body while maintain quality and quantity of nutrient intake from food.
1.6. Justification of Study
Food is defined as substance that contain nutriments for nourishing and sustaining the body existence, but without proper processing and preservation it becomes toxic and waste to man over a long period. Thus, dehydration process as a method of preservation of food cannot be considered to be achieved except the original food quality is sustained after the process. This desirable characteristic can be attaining using freeze drying principles.