STUDY AND ANALYSIS OF A REFRIGERATED AND CONTROLLED ATMOSPHERE ONION STORAGE SYSTEM

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1 STUDY AND ANALYSIS OF A REFRIGERATED AND CONTROLLED ATMOSPHERE ONION STORAGE SYSTEM U.A.Garba and D.Mohammed Department of Agricultural Engineering, and A. Ja afaru Department of Mechanical Engineering, Kaduna Polytechnic, Kaduna usmang9669@gmail.com or , Abstract This work is a study of a refrigerated and controlled atmosphere onion storage that was developed and constructed. The system uses a vapor compression refrigeration unit to reduce and maintain the temperature inside the system at 1% and 87% relative humidity. Soda lime and silica gel were used to control moisture and carbon dioxide production rate of onions in storage. Refrigerated unit components were sized and selected based on heat load calculated.the study was carried for a period of 60 days using 10kg of fresh onions placed inside the storage system and an exact quantity was exposed to open air. Results obtained shows that carbon dioxide production rate of onions in the storage system is significantly slower than normal respiration rate of onions exposed to open air while weight loss of onions in the storage system was reduced by 0.027% weight loss after thirty days, as compared to 0.100% weight loss for onions exposed to open air. It was concluded that combination of refrigeration and controlled atmosphere storage is a very efficient method for onions. Keywords:Onions, respiration rate, storage, weight loss, carbon dioxide Introduction Onions (Alliums cepa L) are highly valued worldwide for their nutritional value in supplying minor constituents such as minerals and trace elements. The bulbs are boiled and used in soups and stews, fried, or eaten raw. They are also preserved in the form of pickles. Onion leaves, especially from the spring onions are also used in salads. Edible Onions are important vegetables worldwide. Pre-eminent among them in terms of volume grown and traded is the common onion grown for bulbs. In terms of global weight of vegetables produced, at nearly 28 million tons per annum only tomatoes and cabbages exceed bulb Onions (Brewster, 1994). International trade is estimated at about 2 million tons annually worth about 400 million US dollars (Brewster, 1994)equivalent to about 5.6 billion Nigerian Naira. Onions are a major crop in the tropics, which account for nearly 30% of global production (Thompson et al, 1972). Nigeria being a tropical country also contributes to this account. According to Natural Resources Institute (NRI) bulleting of 1997, Nigeria which is located on latitude 4-14 o N produces the (Kano Red), (Kano White), (Gindin tasa), and (wuyan mokorowa) varieties of onion. Despite the economic viability of Onion local farmers and people who deal in the whole sale and retail of the crop are faced with high losses of their crop due to poor storage (Thompson et al, 1972). Estimated loss of total crop in tropical countries is high and can reach 20% to 95% (Linus and Martin, 1999). Losses between whole sale and retail of over 9% have being reported for spring onion (Amuttiratana and Parsonssiri, 1992). Proper storage environment is critical to minimize bulb softening, shriveling, weight loss and development of storage rot decay. Different cultivars have variable storagelife. In general poor-keeping cultivars are less pungent (Van kampen, 1970) and have a low dry matter content a low refractive index and high relative rate of water loss especially in periods immediately following harvest. Poor storing cultivars are also more susceptible to storage rots, and sprout more readily.during dormancy at lower and higher temperatures sprouting are depressed. The rate of elongation of sprouts within the bulb and the rate of leaf initiation were much faster at 15 o C than 0 or 246

2 30 o C (Brewster, 1994). Therefore sprout development in onion bulbs unlike most physiological processes does not increase in rate progressively as temperature increases (Brewster, 1994). Once sprouting has occurred in rooted bulbs like onions sprout growth rate increases progressively with temperature (Brewster, 1994). Respiration rates of produce indicate the degree of metabolic activity and provide useful insights in the design of storage systems for environmental and atmospheric control. The rate of post-harvest (spoilage) of produce generally is proportional to the rate of respiration (Robinson, 1975). Bulb onions have low respiration rates (3-4mgCO 2 kg- 1 h- 1 at 5 o C (Robinson, 1975) and this increases with corresponding increase in temperature. Green onions have higher respiration rates, comparable to leafy vegetables at the same temperature a low oxygen level in the storage chamber halves the respiration rate (Robinson, 1975). In storage as time progresses, the rate of respiration increases. If bulbs are wounded, their rate of respiration increases and reaches a maximum after about 12 hrs. The higher respiration level is measurable over the whole storage period. If the dry outer skin of onions is removed, the respiration rate of bulbs increase nearly two fold and the rate of water loss also increases. Bulbs with the skin removed also sprout more rapidly than those with intact skin. Because of geographical location of Nigeria and variety of onions produced in the country storage beyond 23 to 30 days of the crop is the difficult. There are various methods used in storage of onions one of which is refrigeration or low temperature storage. Ranges of temperature and relative humidity were recommended for storage of onions, where onions can be stored for a period of weeks at a temperature range of 0 to 2 o C and relative humidity of 80 to 85 %.(Oberbanscheidt, 1996).Fresh fruits and vegetables are live products and they continue to respire for days and even weeks after harvesting at varying rates (Cengel and Boles, 1998). During respiration a sugar like glucose combines with oxygen to produce carbon IV oxide and water. The rate of respiration varies strongly with temperature; an increase in temperature represents the overall effect of temperatureon the different chemical reaction of the respiration process. Keeping the product quality throughout a storage period as near as possible to the quality at harvest time requires that the metabolic process be slowed down as much as possible. This is best achieved by storage at low temperature, provided that no other adverse effects such as cell membrane damage occur at low temperature (Cengel and Boles, 1998). Respiration, as known, is the taking in of oxygen and releasing of carbon dioxide. In a publication based on a data from U.S. department of Agriculture, the carbon dioxide production of products can be expressed as a function of temperature, by a least square regression fit of the form x 9Tm 1 1 mco f 32 mgkg h (1) Where m CO 2 = The carbon dioxide production per unit 1 1 mass of a product mgkg h T m = the mass average temperature ( o C) f and x are respiration coefficients given in table 1 below Table 1: Coefficient for carbon dioxide production by commodities. Skin mass transfer coefficient Ks (g/m 2.s.mpa) Respiration coefficients Low Mean High VPL f x Carrots Onions x Potatoes Sugar beets x Source: (Becker and Frickle,1996) This work is a study of a developed and constructed refrigerated and controlled atmosphere Onions storage system that was designed and operated at a temperature between 0 and 2 o C with 247

3 the objective of determining and using the carbon dioxide production rate of onions in storage to assess the storage system. Materials and method The storage system is a container of length 60cm (2ft), Breadth 50cm (1ft 20 ) and height 150cm (4ft 59 ).The container is fitted with insulation between the inside and outside surfaces of thickness 2.5cm. Onions are stored inside the system at recommended temperature and relative humidity of 1 o C and 87 % relative humidity. In order to achieve and maintain this temperature a vapor compression refrigeration unit comprising of compressor, condenser, expansion valve, and an evaporator was incorporated to the system.the size of the refrigeration components was determined from the total heat load that was calculated in the design. The source of power utilized is electricity. The system is also fitted with dry and wet bulb thermometer. Readings from the thermometer were used to determine relative humidity of the storage area. The system was also fitted with small perforated containers in which there is silica gel and soda lime, inside the storage area. Their purpose is to control respiration rate of stored onions thereby preventing them from shrinking and losing their quality. Plates I and II shows the picture of the storage system. Plate I: Front view of the storage system. 248

4 Plate II: Side view of the storage system. The study was carried out by using 10kg of onions which were placed unto the middle shelve of the storage system after allowing the system to stabilize at temperature of 1 o C. Another 10kg of onions were kept outside in open air. This arrangement was monitored for 60 days during which readings forthe following parameters were taken three times daily and their average value was recorded; (i) Mass of onions in control storage and open air. (ii) Dry and Wet bulb temperatures of storage area. (iii) Relative humidity of storage area was determined using the value of temperatures above from carrier psychrometric chart. (iv) The carbon dioxide production rate of onions in storage and open air using the regression fit given as equation (1) above where T m = Average dry bulb temperature. f and x were taken from table 1 above. Results and discussion Results obtained is presented in table 2 below. The symbols used on the table were defined as follows; A = Average mass of onions in control storage (kg) B = Average mass of onions in open air. (kg) C = Average dry bulb temperature of storage area ( o C) D = Average wet bulb temperature of storage area ( o C) E = Average relative humidity inside storage area (%) Q = Computed average carbon dioxide production rate of onions in storage system (mgkg -1 h -1 ) Z = Computed average carbon dioxide production rate of onions in open air (mgkg -1 h -1 ) T mo = Average temperature of onions in open air ( o C) T ms = Average temperature of onions in control storage ( o C) To reduce cumbersome values each data presented in table 2 is an average value of three days reading. 249

5 Wt loss for A Wt loss for B JORIND 12 (2) December, ISSN Table 2: Summary of data obtained. Days A B C D E T mo T ms Q Z Total

6 WEIGHT kg RESPIRATION RATES JORIND 12 (2) December, ISSN FIG 1.0: RESPIRATION RATE OF ONIONS IN STORAGE SYSTEM AND IN OPEN AIR Q Z DAYS Wt loss for A Wt loss for B FIG 2.0 : WEIGHT LOSS MEASUREMENTS FOR ONIONS IN STORAGE AND OPEN AIR. DAYS From the data presented in the table above two parameters of major interest are carbon dioxide production rate (respiration rate) of onions in storage system and their loss in weight. The data indicated that respiration rates of onions in storage system varies from 2.78 to 2.99 mgkg -1 h -1 at an average temperature between 1 to 2 o C over the period through which the study was carried out. Also from data presented respiration rate of onions in open air varies between 2.78 to 4.35 mgkg -1 h -1 at 251

7 room temperature. Comparison of these response as shown in figure 1.0 above suggest that onions in storage system respire more slowly than those exposed in open air. The respiration rate is slowed by a value about two times lower daily. Average loss in weight of onions exposed to open air was 0.27kg about 2.7% by weight while for onions in storage average loss in weight was 0.100kg about 1% by weight. Relative humidity in storage system remained constant at 80% until after some time before a slight increase of 1% was recorded it remained at 81% to the end of the study. With only 1% loss by weight after 60 days it was forecasted that only about 2.59% loss is expected after seven months of storage in the system. Conclusion Based on the results presented and discussed above it was concluded that with a forecast of 2.59 approximately 3% loss by weight of onions stored in the system for seven months about 97% will be available for marketing. The achievement of forecast may depend on the size of the storage system. A combination of refrigeration and control atmosphere storage is a very efficient method for onion storage. References Amuttiratana, D and Parssonsiri, W, (1992) Postharvest losses of vegetables A workshop held between 17 th and 22 nd October at Pakistan Agricultural research council Islamabad. Becker, R.B and Frickle, B.A, (1996) Simulation of moisture loss and heat loads in refrigerated storage of fruits and vegetables as In Refrigeration Science and technology proceedings, October 2 nd to 4 th Lexington KT USA. Brewster, J.L, (1994) Onions and other vegetables ALLIUMS Walling Ford U.K.: C.A.B international Publishers Cengel, Y.A, and Boles, M.A, (1998) Thermodynamics, an engineering approach Third edition,princeton Road U.S.A.: Mcgraw Hill Linus, U.O and Martin, G, (1999) Economic importance of onions In CIGR hand book of Agricultural engineering 4: 127,132, 138,346. Oberbanscheidt, B, (1996) Studies on respiration rates of onions in storage Gemiise 32: Robinson, J.E, (1975) Storage characteristics of some vegetables and soft fruits Bio source Tech 81: 339. Thomson, A.K, Booth, R.H, and Proctor, F.J. (1972) Onions storage in the tropics Tropical science Journal 14(1),