i
DESIGN DEVELOPMENT AND PERFORMANCE
EVALUATION OF SOLAR DRYER FOR DRYING OF
TOMATO AND ONION SLICES
M. Sc. Thesis
ABDULAHI UMAR
April 2011
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DESIGN DEVELOPMENT AND PERFORMANCE
EVALUATION OF SOLAR DRYER FOR DRYING OF
TOMATO AND ONION SLICES
A Thesis Submitted to the School of Graduate Studies through
Department of Food Science and Post Harvest Technology
HARAMAYA UNIVERSITY
In Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE IN FOOD ENGINEERING
By Abdulahi Umar
April 2010 Haramaya University
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SCHOOL OF GRADUATE STUDIES
HARAMAYA UIVERSITY
As Thesis Research advisor, I hereby certify that I have read and evaluated this thesis prepared, under my guidance, by Abdulahi Umar entitled “Design Development and Performance Evaluation of Solar Dryer for Drying of Tomato and Onion Slices”. I recommend that it be submitted as fulfilling the thesis requirement.
Solomon Abera (D. Eng.) _________________ _____________ Major Advisor Signature Date
As member of the Board of Examiners of the M.Sc. Thesis Open Defense Examination, We certify that we have read, evaluated the Thesis prepared by Abdulahi Umar and examined the candidate. We recommended that the Thesis be accepted as fulfilling the
Thesis requirement for the Degree of Master of Science in Food Engineering.
______________________ _________________ ____________ Chairperson Signature Date
______________________ _________________ _____________ Internal Examiner Signature Date
______________________ _________________ ____________ External Examiner Signature Date
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DEDICATION
I dedicate this thesis manuscript to my father UMAR AHMED, and my mother
ASHA ABDULAHI, for nursing me with affection and love and for their
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STATEMENT OF THE AUTHOR
First, I declare that this thesis is my bonafide work and that all sources of materials used for this thesis have been duly acknowledged. This thesis has been submitted in partial fulfillment for the requirements for M.Sc. degree in Food Engineering at the Haramaya University and is deposited at the University Library to be made available to borrowers under rules of the library. I solemnly declare that this thesis is not submitted to any other institution anywhere for the award of any academic degree, diploma, or certificate.
Brief quotations from this thesis are allowable without special permission provided that accurate acknowledgement of source is made. Requests for permission for external quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the School of Graduate Studies when in his or her judgment the proposed use of the material is in the interest of scholarship. In all other instances, however, permission must be obtained from the author.
Name: Abdulahi Umar Signature: Place: Haramaya University, Haramaya
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LIST OF ABBREVIATIONS
ANUB Annual Net Undiscounted Benefits DM Dry matter
FARC Fadis Agricultural Research Center GPS Global Positioning Satellite
II Initial Investment
MMSCD Mixed mode solar cabinet dryer NCSD Natural convention solar drying OARI
Oromia Agricultural Research Institute OASD Open- air sun drying
PC Polycarbonate
PP Payback Period
PVSD Photo voltaic ventilated solar drying
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BIOGRAPHY
The author was born in September 1967 in Haramaya town, Ethiopia. He attended his elementary and secondary school education at Bate Junior and Senior Secondary School, and Harar Junior and Secondary High School, Harar, respectively. He joined the then Alemaya University of Agriculture (AUA) and received B.Sc. in Agricultural Engineering in 1988.
Soon after leaving Alemaya University, he was employed by Ministry of Agriculture (1989-1995), Haramaya University (1996-2006) and Oromia Agricultural Research Institute (OARI) until joining the School of Graduate Studies of Haramaya University for his graduate studies since Oct. 2008.
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ACKNOWLEDGEMENTS
Praise to God, the Almighty who sustain my life in this world and in the hereafter. The Author is highly indebted to his advisor D. Eng. Solomon Abera without his encouragement, insight, guidance and professional suggestions, the completion of this work would not have been possible.
I also thank Dr. Geramew Bultesa, for my successes and who has encouraged me in this field. His advice and guidance for my research and contribution to my education has been invaluable. I thank Dr. Eng. Solomon Worku, for the inspiration and encouragements to complete this research work.
Great deal of thanks must be given to the sponsor, OARI and its staff for providing the funds for this research. Special thanks go to FARC and its staff for providing workshop services and sincere cooperation. Special thanks go to the FARC workshop staff in manufacturing the solar dryer and for their technical support and friendly assistance during the manufacturing work at FARC. Special thanks go to Haramaya University Food Science and Post-harvest Technology staff for providing me materials and services.
A very deep admiration and special thanks also go to my parents, family and friends for encouragement, financial support, and affection during my stay at SGS and immeasurable sacrifices they made to bring me to this stage.
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STATEMENT OF THE AUTHOR
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LIST OF ABBREVIATIONS
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BIOGRAPHY
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ACKNOWLEDGEMENTS
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TABLE OF CONTENTS
ix
LIST OF FIGURES
xi
LIST OF TABLES
xii
LIST OF TABLES IN APPENDIX
xiii
ABSTRACT
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1. INTRODUCTION
1
2. LITERATURE REVIEW
4
2.1. Drying
4
2.1.1. Purpose of drying 4 2.1.2. Application of drying 5 2.1.3. Drying methods 52.2. Theory of Drying
6
2.2.1. Air properties
6
2.2.2. Drying mechanism 92.3. Thin Layer Drying Models
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2.4. Sun and Solar Drying
13
2.3.1. Classification of solar drying 15
2.3.2. Types of solar dryers 16
2.3.3. Major components of solar dryers 18
2.4. Drying Efficiencies
19
2.5. Drying of Tomato and Onion
20
2.5.1. Solar drying of tomato 20
2.5.2. Solar drying of onions 21
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TABLE OF CONTENTS(Contd)
3.1. Description of the Study Site
31
3.2. The Design of the Solar Dryer
31
3.2.1 Drying chamber 32
3.2.2. The collecting chamber 35
3.3. Performance Evaluation of Solar Dryer
39
3.3.1. Measuring instruments 39
3.3.2. Preliminary test of the solar dryer
40
3.3.3 Efficiency of solar dryer 40
3.3.4. Sample preparation 42
3.3.5. Moisture content determination of samples 42
3.3.6. Testing the solar dryer using tomato with natural convection current 44 3.3.8. Performance evaluation of solar dryer using tomato and onion in
forced ventilation 46
3.3.9. Kinetics of drying 46
3.4. Statistical Analysis
47
4. RESULTS AND DISCUSSION
49
4.1. Preliminary Test Data of the Solar Dryer
49
4.2. Collector Efficiency
51
4.3. Test of Solar Dryer Using Tomato Slice in Natural Convection
Current
52
4.4. Test of Solar Dryer Using Onion Slice in Natural Convection
Current
56
4.5. Characteristics of the Solar Dryer under Forced Ventilation
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4.6. Testing the Solar Dryer in Forced Air Circulation Using Tomato
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4.7. Testing the Solar Dryer in Forced Air Circulation Using Onion
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4.9. Economic Feasibility and Pay Back Analysis of the Solar Dryer
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5. SUMMARY, CONCLUSION AND RECOMMENDATION
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5.1. Summary
71
5.2. Conclusions
73
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TABLE OF CONTENTS(Contd)
6. REFERENCES
75
7. APPENDIX
82
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LIST OF FIGURES
Page
Figure 1 Framework of the solar Dryer ... 32
Figure 3. Drying chamber frame of the solar dryer ... 33
Figure 4. Drying chamber wall frame ... 34
Figure 5. The roof frame of drying chamber ... 34
Figure 6. The position of the shelves in the drying chamber ... 35
Figure 7. The collector plate of the solar dryer ... 37
Figure 8. The roof frame structure of the collecting chamber ... 38
Figure 9. Photo of solar dryer ... 39
Figure 10. Schematic diagram of solar dryer ... 40
Figure 11. The solar radiation, collector outlet & ambient air temperature ... 50
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LIST OF TABLES
Table 1. Treatment combination, replication and randomization... 48
Table 2. Preliminary test data at no load of the dryer at half open position of control
device ... 49 Table 3. Raw data of the collector efficiency analysis for solar dryer ... 51 Table 4. Weight of tomato, percentage moisture contents on wet basis, dry basis and
drying rate on dry basis on Tray1, Tray2, Tray 3, Tray 4, Tray 5 and open air sun trays during tomato drying using natural convection current and open-air
sun drying ... 54 Table 5. Weight of onion, percentage moisture contents on wet basis, moisture contents
on dry basis and drying rate on dry basis on Tray1,Tray2, Tray 3, Tray 4, Tray 5 and open air sun tray during onion drying using natural convection current
and open-air sun drying tests ... 58 Table 6. Weight of tomato, percentage, moisture content on wet basis and percentage
drying rate on dry basis on Tray1, Appendix Tray2, Tray 3, Tray 4 and Tray 5
and open air sun Tray4 and Tray 5 (Ventilated tomato drying) ... 62 Table 7. Weight, percentages of moisture content on wet basis and drying rate on dry
basis of onion samples in the dryer on trays 1,2.3.4 and 5 and open air sun
(Ventilated onion drying) ... 64 Table 8. Values of drying rate coefficients ‘k’(h-1) for tomato and onion slices dried in
the solar dryer and open-air sun drying. ... 66 Table 9. ANOVA of the drying rate coefficient... 66 Table 10. Payback period of the solar dryer used for drying tomato and onion ... 70
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LIST OF TABLES IN APPENDIX
Appendix Table 1: Whether parameters of Haramaya University ... 83
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DESIGN DEVELOPMENT AND PERFORMANCE
EVALUATION OF SOLAR DRYER FOR DRYING OF
TOMATO AND ONION SLICES
ABSTRACT
A solar dryer was designed and manufactured at Fadis Agricultural Research Center workshop of Oromia Agricultural Research Institute. The framework of all the parts of the dryer were built by joining perforated angle irons of 40 mm40 mm4 mm and 20 mm 20 mm4 mm by means of bolts and nuts. The dryer covers 3.0 m 3.0 m area of the ground of which the 1m2 was used for drying chamber while the rest was saved for collecting solar radiation. The drying chamber surrounded by the collector from three sides , had five shelves positioned one on the top of another with 10 cm clearance in between. The roofs and walls of the dryer were covered with the flexible transparent plastic leaving the three sides of the solar collector open to allow air in. Preliminary tests with no load to the dryer showed that the solar collector raised the ambient air temperature of 20°C to 41°C to a warm air of 28°C to 64°C between the morning and midday. This lowered the relative humidity of air from average 26% in the morning to 5% at midday. The dryer, loaded at 5 kg/m2, dried tomato slices of 8 mm thickness from initial moisture content of 93.3% (w.b) to final moisture content of 12% (w.b) in 13 hours and11hours when operated under natural convection current. Similarly, onion slices of 3 mm thickness, loaded at a rate of 4 kg/m2, dried from 87.10% (w.b) initial moisture content to 9.1% (w.b) final moisture content in 10 hours. Using forced ventilation, the slices of tomato and onion took 11 hours and 9 hours to reach their final moisture contents of 12% and 9.1% (w.b), respectively. The open air-sun drying tests conducted side by side with solar drying needed an average of 20 hours to reach the same final moisture contents for both tomato and onion slices. The maximum drying rate of tomato slices attained under natural convection and forced circulation were 3.1 and 2.8
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kg of water per kg of dry matter-hr, while those of the onion slices 2.6 and 1.5 kg of water per kg of dry matter-hr. For the open-air sun drying, the maximum drying rates for tomato and onion slices were 1.5 and 0.82 kg of water per kg of dry matter-hr. Drying tomato and onion slices to their final moisture contents took one-half, two & four days and one, two and three days in PVSD, NCSD and OASD, respectively. Drying rate coefficients ‘k’(-1hr) of Lewis model were statistically significantly different and could be used to describing solar and open-air sun drying characteristics of solar and open-sun dryings of tomato and onion slices. From economic feasibility and payback analysis of the solar dryer, the payback period was determined and was very small (1.20 months) compared to the life of the dryer, so the dryer will dry product free of cost for almost its life period of 15 years.
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1. INTRODUCTION
Vegetables and their products are of great nutritional importance since they make a significant contribution in supplying wealth of essential vitamins, minerals, antioxidants, fibers and carbohydrates that improve the quality of the diet. Vegetable production is seasonal in nature and during peak, harvest there is often a glut to the market and at unsafe storage moisture levels. That leads to drastic drop in the price of the produce as there are no facilities for long-term storage and that the commodity has to be sold out before it perishes. Ethiopia has different agro-climates and soil types that enable to produce various types of vegetable and fruit crops for both local consumption and export markets. However, growing and marketing fresh produce in Ethiopia is complicated by high postharvest loss, which reaches about 30% (EARO, 2000). Naturally, fresh produce needs low temperature and high relative humidity environment during storage and transportation.
However, the means of achieving these for long-term purpose is beyond the reach of the economy of the majority of the producers and local traders. Established system of cold chain consisting of packinghouses, cold storage and refrigerated transportation is needed to reduce this loss to acceptable level.
Drying is a common method for preservation of food products. The main purpose of drying is the reduction of moisture content to a safe level for extending the shelf life of products. The removal of water from fruit and vegetables provides microbiological stability and reduces deteriorative bio-chemical reactions. In addition, the process allows a substantial reduction in terms of mass, volume and packaging requirement, which reflects on handling, storage and transportation costs with more convenience (Okos et al., 1992). It ensures their availability at all times of the year.
Drying kinetics is generally affected by air temperature, relative humidity of the air, air velocity and material size (Kiranoudis et al., 1992). Generally, the drying phenomena can be described using thin layer drying models mainly to estimate the drying times and
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moisture content of the food materials at any time after they are subjected to a known temperature and relative humidity (Torgul and Pehlivan, 2004). Many research studies have been reported on mathematical modeling and experimental studies conducted on thin layer drying process of various food products such as onion and pepper (Kiranoudis et al., 1992), chilli (Hussain and Bala, 2002), carrot (Doymaz, 2004) and tomato (Sacilik et al., 2006).
Use of dehydrated vegetables in various convenience foods is a common phenomenon all over the world. The application of dried potatoes, tomatoes, garlic, onion, carrot, mushrooms and sweet potatoes in various food products including bread, doughnuts, soups, stews, etc. is a practice of long history.
The introduction of solar drying system seems to be one of the most promising alternatives to reduce postharvest losses. Solar dried products have much better colour and texture as compared to open sun dried products. The justification for solar dryers is that they dry products rapidly, uniformly and hygienically. Since, they are more effective than open sun drying and have lower operating costs than mechanized dryers (Diamente and Munro, 1993; Condori et al., 2001); more importance is given now a day to the use of solar dryers. The open-air sun drying process is not very hygienic. It depends on weather conditions and there is a risk of deterioration (Bala et al., 2003). Some of the problems associated with open-air sun drying can be solved with a solar dryer, which can reduce crop losses and improve the quality of dried product significantly compared to traditional drying methods (Madhlopa and Ngwalo, 2007).
Use of solar dryers is a much-preferred alternative in view of its low initial capital and running costs, and free and ample supply of solar energy in the country. However, no information is available on solar drying of fruit and vegetables under Ethiopian climatic conditions in general and particularly under the local conditions of the eastern part of the country.
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Although a number of designs of solar dryers exist in various countries, there are no such dryers with proper design with adequate information on drying performance available on the market in Ethiopia. The very few attempts done in some places ended up in solar dryers that are not affordable by the farming communities, difficult to transport from place to place, and have no scientific information at all on the capacity, drying performance and utilization. Those which are imported from elsewhere are expensive, cumbersome, complicated and unavailable to the users.
One can clearly see the need for easily available and affordable appropriate drying technology as a means of tackling the unacceptably high postharvest loss of fruits and vegetables in Ethiopia. Development of solar dryer with all the necessary information on its performance and operation can be one aspect of the solution for the problems. Therefore, this research was initiated to design, develop and conduct performance evaluation of a solar dryer for drying of vegetables and fruits. Tomato and onion were considered as study crops, based on ease of supply during the test period. The dryer was intended for use with mainly natural convection air movement but also tested with photovoltaic powered fans for use (in the event) when the need arises to increase the drying efficiency.
The general objective of this work, therefore, is the development of economically affordable and simple to construct solar operated dryer for drying fruit and vegetables. The specific objectives include:
o To design, construct and evaluate the performance of solar dryer for drying fruits and vegetables.
o To compare the characteristics and performance of solar dryings to open air sun drying.
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2. LITERATURE REVIEW
This chapter deals with the review of research works carried out on drying and its theory, classification, types of dryers and general information about tomatoes and onion drying.
2.1. Drying
Drying is one of the oldest food preservation methods and it is defined as the application of heat under controlled condition to remove the majority of the water normally present in a food by evaporation. (Fellows, 2000).
2.1.1. Purpose of drying
The main purpose of drying is to extend the shelf life of food by reduction of water activity. This inhibits microbial growth and enzyme activity, but the drying air temperature is usually insufficient to cause their inactivation. Furthermore drying causes decrease in weight and volume of vegetables thereby reducing transport and storage costs. Since drying can lead to deterioration of both the eating quality and the nutritive value of the food, design of drying equipment and operation is aimed at minimizing these negative effects by selection of appropriate drying conditions for the food.
The basic essence of drying is to reduce the moisture content of the product within a certain period, to a level that prevents deterioration, normally regarded as the “safe storage moisture”. It was described by Ife and Bas (2003), that the moisture level of most vegetables is 10-15% so that the microorganisms present cannot thrive and the enzymes become inactive, that dehydration is usually not desired, because the products often become brittle and stored in a moisture-free environment, ,
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2.1.2. Application of drying
Drying operation is used for dehydration of various types of foods. The drying of fruit and vegetables is a subject of great importance. Dried fruit and vegetables have gained commercial importance and their growth on a commercial scale has become an important sector of agricultural industry (Karim and Hawlader, 2005). Examples of commercially important dried foods are coffee, milk, raisins, sultanas, and other fruits, vegetables, pasta, flours (including bakery mixes), beans, pulses, nuts, breakfast cereals, tea and spices. Important dried ingredients used by food manufacturers include egg powders, flavorings, colorings and lactose, sucrose, or fructose powder, enzymes and yeasts.
The advantages of dried foods were listed as follows:
Extended shelf life because of inhibition of microbial and enzymatic reactions. Providing consistent product and the seasonal variations are diminished. Substantially lower cost of handling, transportation and storage.
The dried products size, shape and form are modified and the price is constant throughout the year.
Dried foods can be packed in recyclable packages; this is not always done with fresh foods.
The dried foods can be used as snacks and other processed foods.
2.1.3. Drying methods
Several drying methods are commercially available and the selection of the optimal method is determined by quality requirements, raw material characteristics, and economic factors. There are three types of drying processes: sun and solar drying; atmospheric dehydration including stationary or batch processes (kiln, tower, and cabinet driers) and continuous processes (tunnel, continuous belt, belt-trough, fluidized-bed, explosion puffing, foam-mat, spray, drum, and microwave-heated driers); and sub-atmospheric dehydration (vacuum shelf, vacuum belt, vacuum drum, and freeze driers) (Chua, 2003).
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2.2. Theory of Drying
Dehydration involves the simultaneous application of heat and removal of moisture from food; the factors that control the rate of transfer are summarized and categorized as those related to the processing conditions, nature of the food and the drier design.
2.2.1. Air properties
The properties of the air flowing around the product are major factors in determining the rate of moisture removal. The capacity of air to remove moisture is principally dependent upon its initial temperature and humidity; the greater the temperature and lower the humidity is the higher the moisture removal capacity of the air. The relationship between temperature, humidity and other thermodynamic properties is represented by the psychrometric chart. The absolute humidity is the moisture content of the air (mass of water per unit mass of air) whereas the relative humidity is the ratio, expressed as a percentage, of the moisture content of the air at a specified temperature to the moisture content of air if it were saturated at that temperature.
Relative humidity is defined as the ratio of the amount of water vapor in the air (Nw) to the amount the air will hold when saturated at the same temperature (Nws).The partial pressure of water vapor at saturation (Pws) is a function only of temperature. However, this relationship is complex, involving multiple exponential and logarithmic terms (Wilhelm, 1976).
Another psychrometric parameter of interest is the humidity ratio (W). The humidity ratio is the ratio of the mass of water vapor in the air to the mass of the dry air.
The partial pressure of the dry air (Pa) is the difference between atmospheric pressure and the partial pressure of the water vapor, also called vapor pressure. The degree of saturation is another psychrometric parameter that is sometimes used. It is the humidity ratio divided by the humidity ratio at saturation.
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A final parameter that can be determined from the perfect gas law is the specific volume of the moist air. The specific volume is defined in terms of a unit mass of dry air.
The dew point temperature (tdp) is the temperature at which moisture begins to condense if air is cooled at constant pressure. The dew point temperature is directly related to partial pressure of the water vapor (Pw); however, that relationship is complex, involving several logarithmic terms (ASHRAE, 1997). Since Pw is also related to the humidity ratio W, this means that specifying any one of the three parameters tdp, Pw, and W specifies all three.
The wet-bulb temperature (twb) is the temperature measured by a sensor (originally the bulb of a thermometer) that has been wetted with water and exposed to air movement that removes the evaporating moisture. The evaporating water creates a cooling effect. When equilibrium is reached, the wet-bulb temperature will be lower than the ambient temperature. The difference between the two (the wet bulb depression) depends upon the rate at which moisture evaporates from the wet bulb. The evaporation rate, in turn, depends upon the moisture content of the air. The evaporation rate decreases as the air moisture content increases. Thus, a small wet bulb depression indicates high relative humidity, while a large wet bulb depression is indicativeof low relative humidity.
The enthalpy (h) of moist air is one of the most frequently used psychrometric parameters. It is a measure of the energy content of the air and depends upon both the temperature and the moisture content of the air. It is determined by adding the enthalpy of the moisture in the air (W hw) to the enthalpy of the dry air (ha):
h = ha +W hw = Cpa t +W(hfg + Cpw t) (Wilhelm, 1976).
h = 1.006t +W(2501+1.805t), based upon (Cpa) a specific heat for air of 1.006 kJ/kg K° and a zero value of h at t = 0°C. The enthalpy of water is based upon: a zero value of h at 0°C (liquid state); hfg = 2501 J/kg at 0°C; and an average specific heat for water vapor of 1.805 kJ/kg K. The above equation provides a good approximation for the enthalpy of moist air over a wide range of temperatures; however, the error increases rapidly at temperatures above 100°C. Empirical relationships, charts, or tables must then be used to determine.
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Psychrometric Charts
Properties shown on most psychrometric charts are dry bulb, wet-bulb, and dew point temperatures; relative humidity; humidity ratio; enthalpy; and specific volume.
Processes on the Psychrometric Chart
The psychrometric chart is used in many applications both within and outside the food industry. Drying with air is an extremely cost-effective method to reduce the moisture of a biological material, and the addition of a small amount of heat significantly improves the air’s drying potential. .
By a process, it means moving from one state point to another state point on the chart. Few simple processes, the paths of these processes can be displayed on small psychrometric charts. These are ideal processes assuming no heat transfer from the surroundings. In actual processes, there will be always some heat gain or loss.
These processes are:
Heating or cooling
These processes follow a constant moisture line (constant humidity ratio). Thus, temperature increases or decreases but moisture content and dew point are unchanged. Further cooling follows the saturation (100% relative humidity) line until the final temperature is reached. Moisture is condensed during the part of the process that follows the saturation line.
Moisture addition
While using only energy from the air, both drying and evaporative cooling follow this process. A constant wet-bulb line represents it. Temperature and moisture content change but the wet-bulb temperature remains constant. This can be verified by an energy balance analysis. Note that enthalpy increases slightly in this process. This is due to energy present in the water before it is evaporated.
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Heating and drying
This process is common in drying applications. Air is heated and passed over the material to be dried. A second stage of heating and drying is sometimes included.
Adiabatic mixing (no heat transfer) of air
Moist air from two sources and at different state points is mixed to produce air at a third state point. Relationships among the properties at the three state points are established from mass and energy balances for the air and water components.
Adiabatic saturation
The drying process was identified earlier as a constant wet bulb process. While this is the generally accepted approach, a review of the adiabatic saturation process is provided here for added clarification. An adiabatic saturation process occurs when the humidity of the air is increased as it flows through an insulated chamber. Water evaporates into the air as it passes through the chamber. If the chamber is long enough for equilibrium to occur, then the exit air will be saturated at an equilibrium temperature, t.
2.2.2. Drying mechanism
In the process of drying, heat is necessary to evaporate moisture from the material and a flow of air helps in carrying away the evaporated moisture. There are two basic mechanisms involved in the drying process: the migration of moisture from the interior of an individual material to the surface, and the evaporation of moisture from the surface to the surrounding air. The drying of a product is a complex heat and mass transfer process which depends on external variables such as temperature, humidity and velocity of the air stream and internal variables which depend on parameters like surface characteristics (rough or smooth surface), chemical composition (sugars, starches.), physical structure (porosity, density), and size and shape of products. The rate of moisture movement from the product inside to the air outside differs from one product to another and depends very much, on whether the material is hygroscopic or non-hygroscopic. Non-hygroscopic materials can be dried to zero moisture level while the hygroscopic materials like most of
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the food products will always have residual moisture content. This moisture, in hygroscopic material, may be bound moisture, which remained in the material due to closed capillaries or due to surface forces and unbound moisture, which remained in the material due to the surface tension of water.
When the hygroscopic material is exposed to air, it will absorb either moisture or desorbs moisture depending on the relative humidity of the air. The equilibrium moisture content (EMC = Me) will soon be reached when the vapor pressure of water in the material becomes equal to the partial pressure of water in the surrounding air (Garg, 1987). The equilibrium moisture content in drying is therefore important since this is the minimum moisture to which the material can be dried under a given set of drying conditions. A series of drying characteristic curves can be plotted. The best is if the average moisture content, “M” of the material is plotted versus time. Another curve can be plotted between drying rate i.e. “dM/dt” versus time. But more information can be obtained if a curve is plotted between drying rate “dM/dt” versus moisture content.
For both non-hygroscopic and hygroscopic materials, there is a constant drying rate terminating at the critical moisture content followed by falling drying rate. The constant drying rate for both non-hygroscopic and hygroscopic materials is the same while the period of falling rate is little different. For non-hygroscopic materials, in the period of falling rate, the drying rate goes on decreasing until the moisture content become zero. While in the hygroscopic materials, the period of falling rate is similar until the unbound moisture content is completely removed, then the drying rate further decreases and some bound moisture is removed and continues till the vapor pressure of the material becomes equal to the vapour pressure of the drying air. When this equilibrium reaches then the drying rate becomes zero (Garg, 1987).
The period of constant drying for most of the organic materials like fruits, vegetables, timber and the like is short and it is the falling rate period in which is of more interest and which depends on the rate at which the moisture is removed. In the falling rate regime moisture is migrated by diffusion and in the products with high moisture content, the diffusion of moisture is comparatively slower due to turgid cells and filled interstices. In most agricultural products, there is sugar and minerals of water in the liquid phase which
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also migrates to the surfaces, increase the viscosity hence reduce the surface vapour pressure and hence reduce the moisture evaporation rate. Drying is done either in thin layer drying or in deep layer drying. In thin layer drying, which is done in case of most of fruits and vegetables, the product is spread in thin layers with entire surface exposed to the air moving through the product and the Newton’s law of cooling is applicable in the falling rate region (Garg, 1987).
There were many research reports, where the drying took place only in the falling rate period and constant stage was not observed during the drying experiments. These characteristics for tomato slices were reported by Hawlader et al. (1991), Akanbi et al. (2006) and Sacilik et al. (2006) . Krokida et al. (2003) reported similar characteristics for some different vegetables.
For thin carrot, mulberry fruits and figs (Cui et al., 2004 and Doymaz ,2005) indicating non exist ant water film at the surface of the crop and transfer of moisture could be effectuated by liquid diffusion or vapor diffusion or capillary forces which complicated mechanism that could change during the drying process. Most probable mechanism controlling the mass transfer in agricultural products are diffusion (Diamente and Munro, 1993). Such similar observations were also reported by (Togrul and Pehlivan, 2004)
2.3. Thin Layer Drying Models
Thin layer drying models have gained wide acceptance to design new or simulate the existing system or for the analytical drying solutions. Many researchers have used the exponential drying model in describing the drying behavior of the food materials. The solution of the Fick’s equation, with the assumptions of diffusion based moisture migration, negligible shrinkage, constant diffusion coefficients and temperature, is simplified to get the simple exponential model (Lewis, 1921) as:
Moisture Ratio (MR) = e kt M M M M e i e t (1)
Where, Mi = initial moisture content, dry basis (decimal)
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Me = equilibrium moisture content, dry basis (decimal)
k = drying rate constant (min-1) t = drying time, min
The Henderson and Pabis (1961) model is also the general series solution of Fick’s second law. The following thin layer drying equation (Henderson and Pabis model) was successfully used by Doymaz, (2004); Sacilik et al., (2006) for the prediction of drying time and for generalization of drying curves.
kt e t Ae Me i M M M (2)
If the constant ‘A’ in the above equation is equal to unity, the equation is reduced to the same form as Newton’s law of cooling for highly conductive materials.
Another model which has been widely used to fit the thin layer drying data is the Page equation (Hossain and Bala, 2002; Wang, 2002). It is a simple modification of the exponential law using moisture ratio with additional drying parameter. Page (1949) proposed a thin layer drying equation:
n qt e Me Mi Me Mt (3)
Where, ‘q’ and ‘n’ are drying constants that depend on the air temperature and type of material.
The empirical equation used to describe the thin layer drying characteristics of food materials (Akpinar et al., 2003; Doymaz, 2004):
2 1 at bt Me Mi Me Mt (4) Where, ‘a’ and ‘b’ are drying constants.
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A mathematical model for drying kinetics is normally based on the physical mechanisms of internal heat and mass transfer and on the heat transfer conditions external to the material being dried that control the process resistance, as well as on the structural and thermodynamic assumptions. Modeling of drying is usually complicated by the fact that more than one mechanism may contribute to the total mass transfer rate and the contribution from the different mechanisms may change during the drying process (Cui et al., 2003). The effect of air conditions (air temperature, air humidity and air velocity) and characteristic sample size on drying kinetics of various food materials such as tomato, potato, carrot, pepper, garlic, mushroom, onion, leek, pea, corn, celery, pumpkin during air drying was examined by Krokida et al., (2003). They found that the parameters of the model considered were greatly affected by the air conditions and sample size during drying and in particular, the temperature increment increased the drying constant and decreased the equilibrium moisture content of the dehydrated products.
2.4. Sun and Solar Drying
Open-air sun drying (without drying equipment) is the most widely practiced agricultural processing operation in the world; in some countries, food is simply laid out on roofs or flat surfaces and turned regularly until dry. More sophisticated methods of solar drying collect solar energy and heat air, which in turn is used for drying the food.
The term ‘sun drying’ is used to describe the process whereby some or all of the energy for drying of foods is supplied by direct radiation from the sun. The term ‘solar drying’ is used to describe the process whereby solar collectors are used to heat air, which then supplies heat to the food by convection. For centuries, fruit, vegetables, meat and fish have been dried by direct exposure to the sun. The fruit or vegetable pieces were spread on the ground, on leaves or mats while strips of meat and fish were hung on racks. While drying in this way, the foods were exposed to the variability of the weather and to contamination by dust, insects, birds and animals. Drying times were long and spoilage of the food could occur before a stable moisture content was attained. Covering the food with glass or a transparent plastic material can reduce these problems. A higher temperature can be attained in such an enclosure compared to those reached by direct exposure to the sun. Most of the incident radiation from the sun will pass through such transparent materials. However, most radiation from hot surfaces within the enclosure will be of longer
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wavelength and so will not readily pass outwards through the transparent cover. This is known as the ‘greenhouse effect’ and it can result in shorter drying times as compared with those attained in uncovered food exposed to sunlight. A transparent plastic tent placed over the food, which is spread on a perforated shelf raised above the ground, is the simplest form of covered sun-drier. Warm air moves by natural convection through the layer of food and contributes to the drying.
The capacity of such a drier may be increased by incorporating a solar collector. The warm air from the collector passes up through a number of perforated shelves supporting layers of food and is exhausted near the top of the chamber. A chimney may be fitted to the air outlet to increase the rate of flow of the air. The taller the chimney, the faster the air will flow. If a power supply is available, a fan may be incorporated to improve the airflow still further. Heating by gas or oil flames may be used in conjunction with solar drying. This enables heating to continue when sunlight is not available. A facility for storing heat may also be incorporated into solar driers. Tanks of water and beds of pebbles or rocks may be heated via a solar collector. The stored heat may then be used to heat the air entering the drying chamber. Drying can proceed when sunlight is not available. Heat storing salt solutions or adsorbents may be used instead, water, or stones. Quite sophisticated solar drying systems, incorporating heat pumps, are also available (Brennan 1994, Barbosa-Canovas and Vega-Mercado, 1996, Salunkhe, 1982, Imrie, 1997).
In solar drying, solar-energy is used either as the sole source of the required heat or as a supplemental source. The airflow can be generated by either natural or forced convection. The heating procedure could involve the passage of preheated air through the product or by directly exposing the product to solar radiation or a combination of both (Ekechukwu and Norton, 1998). The major requirement is the transfer of heat to the moist product by convection and conduction from the surrounding air mass at temperatures above that of the product or by radiation, mainly from the sun and to a little extent from surrounding hot surfaces (McLean, 1980). In a direct radiation drying, part of the solar radiation may penetrate the material and be absorbed within the product itself, thereby generating heat in the interior of the product as well as at its surface, and thereby enhancing heat transfer (Basunia and Abe, 2001). During drying, there is a tendency of the food to form dry surface layers which are impervious to subsequent moisture transfer, if the drying rate is
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very rapid. To avoid this effect, the heat transfer and evaporation rates must be closely controlled to guarantee optimum drying rates (Arinze et al., 1979).
2.3.1. Classification of solar drying
2.3.1.1. Natural convection and other solar dryings
All drying systems can be classified primarily according to their operating temperature ranges into two main groups of high temperature dryers and low temperature dryers. However, dryers are more commonly classified broadly according to their heating sources into fossil fuel dryers (more commonly known as conventional dryers), electric powered and solar energy dryers. Further, solar-energy drying systems are classified primarily according to their heating modes and the manner in which the solar heat is utilized (El-Sebaii et al., 2002).
passive solar-energy drying systems (conventionally termed natural-convection solar drying systems); and
active solar-energy drying systems (most types of which are often termed hybrid solar dryers).
Although, for commercial production of dried agricultural products, forced convection solar dryer might provide a better control of drying air; natural convection solar dryer does not require any other energy during drying operation. Hence, natural convection solar dryer is highly preferred for drying food products especially when in thin layers of drying (Pangavhane et al., 2002).
Natural convection solar drying depends for its operation entirely on solar-energy in which, solar-heated air is circulated through the product by buoyancy forces or as a result of wind pressure, acting either singly or in combination. It is reported that the dryer is superior operationally and competitive economically to natural open sun drying. The advantages of natural convection solar drying over open sun drying are reported by Ekechukwu and Norton (1998) as follows:
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It requires a smaller area of land in order to dry similar quantities of product.
It yields a relatively high quality of dry food because fungi; insects and rodents are unlikely to infest the food during drying.
The drying period is shortened compared with open-air sun drying, thus attaining higher rates of product throughput.
Protection from sudden down pours of rain.
2.3.2. Types of solar dryers
Solar dryers are also classified into direct natural circulation driers (a combined collectors and drying chamber), direct driers with a separate collectors and indirect forced convection driers (separate collectors and drying chamber) Ekechukwu and Norton (1998).
2.3.2.1. Direct natural convection solar dryers
These dryers do not use any fans and/or any blower; low cost and easy to operate. In the simple design, they consist of some kind of enclosure and a transparent cover. The food product gets heated due to direct sunlight, due to high temperature in the enclosure and therefore moisture from the product evaporates, and goes out by natural circulation of air. These dryers are mostly on use in developing countries (1982, Imrie, 1997).
a) Solar cabinet dryer
The main characteristic of simplest solar cabinet dryers is that the heat needed for drying gets into the material through direct radiation. The drying material is spread in a thin layer on a bottom perforated tray through which air flows by natural convection and finally leaves through the upper part of the cabinet. Its design is simple, low in cost, suitable for drying small quantities (10-20 kg) of granular materials (e.g., for individual farmers). Drying of the material can be made more even by periodic turning over of the material. The usual size of the drying area is 1-2 m2 (Imrie, 1997 and Garg, 1987).
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b) Green house type solar dryer
This dryer appears to look like a small greenhouse where there are two parallel long drying platforms made of wire mesh and are covered with slanted long glass roof with long axis along the north-south direction. There is a metallic cap at the top of the glass roof does not allow rain. The inside of the dryer as well as the trays are painted black. Solar radiation penetrates through the glass roof, heats the product directly and absorbed within the dryer increasing the inside temperature (Garg, 1987, Bala et al., 2002).
2.3.2.2. Indirect type solar dryers a) Shelf type solar dryer
In a shelf dryer, the material to be dried is placed on perforated shelves (trays) built one above the other. Shelf type solar dryer was tested by Best (1979) in which the movement of air around produce was further facilitated by drying on perforated trays rather than on solid platforms. The front wall of the case faces south, its top and sides, are covered by transparent walls (glass or sheet), and the back wall is heat insulated and painted black. A flat-plate collector, which is, situated below and besides the drying chamber heats the ambient air that flows up to the space under the lowest shelf. Moist air exits to the open through the upper opening of the casing. The chimney effect is ensured by the increased height of the dryer. The experiments indicated that separation of the collector is only justified with a high efficiency collector. The suitability of such dryer for drying fruits and vegetables were described by Imrie, (1997).
Exell and Kornsakoo (1978) developed a simple mixed mode solar dryer consisting of a separate solar collector and a drying unit, both having a transparent cover on the top. Solar radiation is received in the collector as well as in the dryer. The dryer was initially designed with a bed of burnt rice husk as the absorber and clear UV stabilized polyethylene plastic sheet as transparent cover.
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2.3.2.3. Indirect forced convection driers
The indirect forced drier consists of a separate flat plate air-heating collector, a tunnel-drying unit and a small fan to force or to provide the required airflow over the product to be dried. These are connected in series. Both the collector and the drying unit are covered with ultraviolet (UV) stabilized plastic sheet. Black paint is used as an absorber on the collector. The products to be dried are placed in a thin layer in the tunnel.
Different types of solar dryers such as solar tunnel, roof-integrated and greenhouse type solar dryers have been demonstrated for drying fruits, vegetables, spices, medicinal plants and fish in the tropics and subtropics (Lutz et al., 1987 and Schrimer et al., 1996).
2.3.3. Major components of solar dryers
Many designs of solar dryers have the following major components: solar collector, drying unit or chamber where the materials to be dried comes in direct contact with the hot air from the collector, connecting ducts, the transparent cover and ventilations.
2.3.3.1. Solar collector
The solar collector plays the part of primary energy source for a solar dryer. Essentially, it has functions of energy conversion and energy transfer. Use of blackened surface as a collector is required because matt black surfaces absorb solar radiation more efficiently than others, and so the improvements can be enhanced by use of such surfaces. It has been demonstrated for example by Thanh et al., (1978) that the time required to dry cassava chips on a concrete floor is reduced by about 15% if the floor is painted black.
2.3.3.2. Transparent cover
The most common use of plastics in solar collectors and dryers is as a transparent cover allowing incident radiation to pass through and impinge on an absorber surface - or on the materials being dried and must be able to withstand elevated temperatures, high levels of
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insolation, high humidities, wind loading and the effects of heavy rain over long periods of time. Low cost, low density and good optical properties make some plastics very suitable for use in solar collectors and dryers.
The physical effects of photo-degradation vary from loss of transmissivity and discoloration to crazing of the surface and embrittlement of the plastics resulting in a lowering of the efficiency of a collector or drier will render the plastic more prone to damage by wind and rain. Degradation of plastics occurs more rapidly at higher temperatures and thus deterioration is often worst at hot-spots such as points where the plastic is supported or attached to the framework (White, 1977).
A wide range of clear plastic sheet and film with properties suitable for use in solar energy applications, which also have good resistance to weathering, is now available. Plastics commonly used for glazing in solar collectors include PMMA, polycarbonate (PC), glass-fiber reinforced polyester (GRP), polyvinyl fluoride (PVF), fluorinated ethylene propylene copolymer and polyester film (FEP) (White, 1977).
Specifying the polymer will not always be sufficient. In order to achieve the length of service of which UV resistant plastics are capable, methods of attaching the plastic to the framework, commonly used in simple agricultural systems, such as stapling or nailing are unsatisfactory as they create point of stress where the material is likely to fail. When attaching plastic sheet to the framework a method should be chosen which will distribute any stresses on the sheet as evenly as possible over its whole length or width.
2.4. Drying Efficiencies
The efficiency of solar drying can be studied under two contexts: Collection efficiency (
c) and the system efficiency (
s).Collection efficiency (
c) measures how effectively the incident energy on the solarcollector is transferred to the air flowing through the collector and is given as the ratio of the useful energy output (over a specified time period), to the total solar radiation energy, G, available during the same period.
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The thermal performance of the solar collector is determined by obtaining values of instantaneous efficiency using the measured values of incident radiation, ambient
temperature, and inlet air temperature. This requires continuous measurement of incident solar radiation on the solar collector as well as the rate of energy addition to the air as it passes through the collector, all under steady state or quasi-steady state conditions (Imrie, 1997).
2.5. Drying of Tomato and Onion 2.5.1. Solar drying of tomato
Tomatoes are the world’s most commercially produced and used vegetable crop (Ensminger, 1988). The annual worldwide production of tomatoes has been estimated at 125 million tons in an area of about 4.2 million hectares. The global production of tomatoes (fresh and processed) has been increased by 300% in the last four decades and the leading tomato producers are in both tropical and temperate regions (Dhaliwal et al., 2003). Ethiopian climate is suitable for the production of tomato and with an annual production of about 338,380.91 quintals only on small-scale farms in Maher season and mostly used for fresh fruit consumption (CSA, 2008).
Over the last few years, tomato products have aroused new scientific interest due to their antioxidant activity. Tomatoes and tomato products are rich in health-related food components as they are good sources of carotenoids (in particular, lycopene), ascorbic acid (vitamin C), vitamin E, folate and flavanoids (Davies and Hobson, 1981). They also provide potassium, iron, phosphorus and some B vitamins and are a good source of dietary fiber. They have around 90% water and the large amount of water also makes the fruit perishable. In a ripe fruit, solids form about 5-7% of the fruit, mainly sugars in the form of glucose and a small portion of acid in the form of citric acid (Wills, 1998). The chemical composition of the tomato fruit depends on factors such as cultivar, maturity and environmental conditions, in which they are grown (Davies and Hobson, 1981). It is a short duration crop, giving high yield. However, the excess production results in a glut in the market and reduction in tomato prices. In addition, it is highly perishable in the fresh state leading to wastage and losses during the peak harvesting period. The prevention of
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these losses and wastage during peak harvest is very much important to avoid imbalance in supply and demand during off-season and for economic consideration (Karim and Hawlader, 2005). Therefore, there is a need to increase the shelf life of tomatoes either in fresh or in processed form using food preservation techniques such as drying.
In the guidelines of preparation, drying conditions and information given by Ife and Bas (2003), tomatoes are washed in water and sliced 7-10 mm thick with a loading rate of 5 kg per square meter of a tray. A 100 kg fresh tomato yields 70- 90 kg when prepared for drying and mostly becomes 4-5 kg when dried. Maximum permissible drying air temperature is 65°C and a 5% moisture content of final product, which is tough and brittle, was given in the literature.
Sacilik et al., (2006) reported on the thin layer solar drying experiments of organic tomato using multi-purpose solar tunnel dryer under the ecological conditions of Ankara, Turkey. They reported that organic tomatoes could be dried to the final wet basis moisture content of 11.5% from 93.3% in four days of drying in the solar tunnel dryer as compared to five days of drying in the open sun drying.
2.5.2. Solar drying of onions
Onion, Allium cepa L., is considered as one of the most important crops in all countries. Domestic onion is the round, edible bulb of Allium cepa, a species of the lily family, and one of the world’s oldest cultivated vegetable crops. Red, white and gold onions represent the most known varieties of this species, but growers distinguish them also between freshly consumer onions and onions for industrial transformation, on the basis of sowing time and technique, harvesting time, bulb size, among others characteristics (Bonaccorsi et al., 2008).
Onion has a universal appeal in the Ethiopian diet and dehydrated onion is well accepted by consumers. The technique for sun drying onion is a simple one and the dry product has good storage life. There is a good export market for dehydrated onion.
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In the chemical composition of onions, carbohydrates are source of food energy reserves and make up much of the structure framework of cells. Shallot contain higher levels of fats and soluble solids, including sugars, than bulb onion with 16-33% dry weight vs. 7-15% dry weight, respectively (Currah and Proctor, 1990; Messiaen, 1992) which, together with sulphur-containing compounds, make shallot an essential component in cooking.
Onion is a strong-flavored vegetable used in a wide variety of ways, and its characteristic flavor (pungency) or aroma, biological compounds and medical functions are mainly due to their high organo-sulphur compounds (Mazza and LeMaguer, 1980; Corzo-Martínez et al., 2007).
In the manufacture of processed foods such as soups, sauces, salad dressings, sausage and meat products, packet food and many other convenience foods, dehydrated onion is normally used as flavor additive, being preferred to the fresh product, because it has better storage properties and is easy to use (Rapusas and Driscoll, 1995; Kaymak-Ertekin and Gedik, 2005). In addition, the preservation of vegetables, such as onion, in the dried form is commonly practiced to reduce the bulk handling, to facilitate transportation and to allow their use during the off-season. However, in the drying process of shelf-stable vegetables it is essential to preserve their desired quality attributes.
The moisture removal during drying processes is greatly affected by the drying air conditions as well as the characteristic dimension of the material, whereas all other process factors have a practically negligible influence (Kiranoudis et al., 1997). The effect of drying parameters on moisture removal, expressed by kinetic models have been studied for different varieties of onion (Krokida et al., 2003; Sarsavadia et al., 1999; Kiranoudis et al., 1992; Yald ´yz and Ertek ´ yn, 2001; Wang, 2002). However, the drying conditions, such as temperature and moisture content, have a great influence on the food properties, such as flavor or colour and nutritional composition during processing or storage (Kaymak-Ertekin and Gedik, 2005).
Kaymak-Ertekin and Gedik (2005) studied the kinetics of non-enzymatic browning and thiolsulphinate loss in onion slices during drying at different temperatures and air
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velocities and the corresponding quality losses. Kumar et al. (2007) dried onion slices under different processing conditions applying infrared radiation assisted by hot air, varying the drying temperature, slice thickness, inlet air temperature and air velocity, and tested different thin layer models. Kumar and Tiwari (2007) studied the open sun and greenhouse drying of onion flakes to evaluate the effect of mass on convective mass transfer coefficient. Sarsavadia (2007) developed a solar-assisted forced convection dryer for the drying of onion slices and studied the effect of airflow rate, air temperature, and fraction of air recycled on the total energy requirement. Sharma et al. (2005) developed an infrared dryer and studied the infrared radiation thin layer drying of onion slices at different infrared power levels, different air temperatures and air velocities.
It is not uncommon to preserve onion by drying. Various studies have been made on different aspects of onion. All these studies aimed at facilitating the onion drying and improving the quality of the dried product. Thus drying of onion is a widely used preservation techniques.
In the guidelines of preparation, given by Ife and Bas (2003), onion is cleaned, washed, peeled and sliced 3 mm thick for drying at a loading rate of 4 kg/m2 of a drying tray. A 100 kg fresh onion yields 90 kg when prepared for drying and mostly becomes 9 kg dried product at a 60°C maximum permissible drying air temperature and 5-7% moisture content of final product which is brittle that could be ground to powder.
Summary
The eastern region of the country is capable of producing large quantities of fruits and vegetables for local consumption and export. Many of these fruits and vegetables contain a large quantity of initial moisture content and are therefore highly susceptible to rapid quality degradation, even to the extent of spoilage, if not kept in thermally controlled storage facilities. Therefore, it is imperative that, besides employing reliable storage systems, post harvest methods such as drying can be implemented hand-in-hand to convert these perishable products into more stabilized products that can be kept under a minimal controlled environment for an extended period.
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Drying is the application of heat under controlled condition to remove the majority of the water normally present in a food by evaporation and extend the shelf life of food by reduction of water activity. The decrease in weight and volume reduce transport and storage costs. Design of drying equipment and operation is aimed at minimizing these negative effects by selection of appropriate drying conditions for the food.
“Safe storage moisture” the moisture level of most vegetables is 10-15% so that the microorganisms present cannot thrive and the enzymes become inactive, that dehydration is usually not desired, because the products often become brittle and stored in a moisture-free environment, ,
Commercially important dried foods are coffee, milk, raisins, sultanas, and other fruits, vegetables, pasta, flours (including bakery mixes), beans, pulses, nuts, breakfast cereals, tea and spices.
Drying methods
Several drying methods are commercially available and the selection of the optimal method is determined by quality requirements, raw material characteristics, and economic factors.
Types of drying processes: sun and solar drying;
atmospheric dehydration including stationary or batch processes (kiln, tower, and cabinet driers) and
continuous processes (tunnel, continuous belt, belt-trough, fluidized-bed, explosion puffing, foam-mat, spray, drum, and microwave-heated driers); and sub-atmospheric dehydration (vacuum shelf, vacuum belt, vacuum drum, and freeze driers) (Chua, 2003).
The factors that control the rate of heat transfer and removal of moisture are related to the processing conditions, nature of the food and the drier design.
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Properties air the major factors in determining the rate of moisture removal. Air capacity depend upon its initial temperature and humidity; thermodynamic properties is represented by the psychrometric chart.
In the drying process are the migration of moisture from the interior of an individual material to the surface, and the evaporation of moisture from the surface to the surrounding air depends on external variables such as temperature, humidity and velocity of the air stream and internal variables. These in turn influenced by parameters like:
surface characteristics (rough or smooth surface), chemical composition (sugars, starches.),
physical structure (porosity, density), and size and shape of products.
The equilibrium moisture content (EMC = Me) will soon be reached when the vapor pressure of water in the material becomes equal to the partial pressure of water in the surrounding air (Garg, 1987). The equilibrium moisture content in drying is therefore important since this is the minimum moisture to which the material can be dried under a given set of drying conditions.
Drying is done either in thin layer drying or in deep layer drying. In thin layer drying, which is done in case of most of fruits and vegetables, the product is spread in thin layers with entire surface exposed to the air moving through the product and the Newton’s law of cooling is applicable in the falling rate region (Garg, 1987).
A mathematical model for drying kinetics is normally based on the physical mechanisms of internal heat and mass transfer and on the heat transfer conditions external to the material being dried that control the process resistance, as well as on the structural and thermodynamic assumptions. The effect of air conditions (air temperature, air humidity and air velocity) and characteristic sample size on drying kinetics of various food materials such as tomato, potato, carrot, pepper, garlic, mushroom, onion, leek, pea, corn,