Solar Drying of Natural and Food Products: A Review

International Journal of Agriculture and Food Science Technology. ISSN 2249-3050, Volume 5, Number 6 (2014), pp. 565© Research India Publications http://www.ripublication.com/ ijafst.htm

Solar Drying of Natural and Food Products: A Review Sagar Kapadiya1 and M.A. Desai2 1

Research Student, Chemical Engineering Department, Sardar Vallabhbhai National Institute of Technology, Surat, Gujarat–395007, India 2 Chemical Engineering Department, Sardar Vallabhbhai National Institute of Technology, Surat, Gujarat–395007, India

Abstract Natural and food products are dried to improve self-life, reduce packaging cost, increase shipping capacity, enhance appearance, encapsulate original flavor and maintain nutritional value. The main objective of drying is to withdraw moisture from the food so that bacteria, yeast and mold cannot grow and spoil the food. Mostly fossil fuel is used for heating air for drying purpose. Due to exponential rise in the price of fuel and depletion of fossil fuel, there is a need to look for other alternatives like nonconventional energy resources viz. solar energy. India is blessed with good sunshine hours. A review is made to use solar energy for drying of agricultural and food products with different dryers available and with various parameters affecting the drying process and the product. Effect of drying on texture and oil content of the natural products are discussed. A comparison is made between conventional drying and solar energy assisted drying. Work done on different solar dryers with different natural agricultural products along with parameters, result obtained are compared from different literatures.

1. Introduction A drying operation essentially requires phase change and production of a solid phase as final product. During drying operation, moisture will get evaporated within the surface of product by heat and subsequently from inside the product. Thus, drying is combined process of simultaneous heat and mass transfer. Initially the moisture inside the product is brought to the surface and dried at a constant rate then falling drying rate with the help of hot air and this process is related to the properties of the material to be dried [1].

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In the different process industries like agricultural, food, chemicals like fertilizer, cement, polymer, medicinal, paper, mineral and wood processing, drying is considered as a vital operation. Natural foodstuffs are dried to improve self-life, reduce packaging cost, increase shipping capacity, enhance mien, capture unique flavor and maintain nutritional values. According to the Food and Agricultural organization, India produces 27.8 million metric tons (8.1%), while China has a production capacity of 21.5 million metric tons (6.3%) of the total world production [2]. Due to the lack of proper and timely processing, approximately 33% of the global food production is lost yearly [3]. This loss is even more in the developing countries like Bangladesh, where 30–40% of fruits and vegetables are wasted due to inappropriate handling and lack of post-harvest techniques. [4, 5]. Hence, it is imperative that drying should be implemented as postharvest operation than employing reliable storage system. Drying converts the perishable products into more stabilized products that can be kept under a minimum controlled atmosphere for an extended period of time. Drying reduces the action of enzymes, which cause fruits ripen. Drying is one of the ancient, modest and extensively used methods of preserving food. Quality of dried food is another important issue in food drying. Improper drying cause changes in the food properties including discoloring, aroma loss, textural changes, nutritive value, and changes in physical appearance and shape. The most common drying method, especially for agricultural products, is thermal drying. It is used frequently in industrial and agricultural processes. The energy consumption in such industries is high, on average 12% of total energy consumption by the industries. Use of fossil fuels has lead to environmental issues [6]. Also exponential rise in the price of fuel and depletion of fossil fuel, have pave a path to look for alternatives like nonconventional energy resources viz. solar energy. Solar drying technology represents a new way to process the food and natural products in clean, hygienic and sterile conditions to satisfy national and international standards without energy usage [7]. Solar energy saves conventional energy as heating source, time required for drying, reduces area requirements, improves product quality and makes the product more efficient with zero harm to environment. Solar energy for drying can be implemented as complete drying process or as an add-on to conventional drying systems, which reduces the fuel energy requirements [8]. Solar drying in agriculture, especially in the rural areas in developing countries, is not a possibility but a compulsion because most of these areas do not have access to grid-connected electricity and cannot afford fuel for heaters for drying crops [9]. Thus, they use traditional open-sun drying. However, this method suffers from many problems. For example, crops are lost because of inadequate drying. Moreover, birds, insects, and rodents encroach on them. Other problems such as fungal attacks, unexpected rain, and adverse weather conditions are common. Hence, the use of solar crop dryers is increasing nowadays, and they have special economic attractions [10, 11]. In this article, a review is made on different solar dryer available and different parameters affecting the process of drying. A comparison is made between conventional drying and solar energy assisted drying with results reported in various

Solar Drying of Natural and Food Products: A Review

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literature for the natural and food products. Thus this review highlights the extensive opportunity in application of solar drying for rural areas.

2. Solar Dryers 2.1 Importance of Solar Energy in Context to India: As per the International Energy Outlook 2006, world marketed energy consumption grows on an average rate by 2.0% per year from 421 quadrillion Btu in 2003 to 563 quadrillion Btu in 2015 and 722 quadrillion Btu in 2030 and India will become the fifth leading energy consumer as per British Petroleum [12]. Sun, the biggest available carbon free energy source offers the living beings with more energy in 1 hour than is consumed on the earth in an entire year. Achieving sufficient supplies of clean energy for the next generation is a prodigious societal challenge. Good sunshine is bless to India. Most of all parts of the India receive solar radiation in the range of 5–7 kWh/m2 on average, and delivers more than 275 sunny days in a twelve months [13]. Hence, solar energy as drying medium has a great prospective of distribution throughout the country, and offers a viable opportunity in the domestic sector [14]. It is identified as an appropriate technology for Indian agricultural proceedings and possesses numerous advantages such as high durability, high dietetic value of food, less recurring costs and potential to reduce drudgery etc., [15]. 2.2 Types of solar dryers In the traditional use of solar drying, the product to be dried is exposed directly to the sun in open environment. This process is known as open sun drying. This process possesses so many drawbacks like degradation by windblown rubbishes, rain and pest infestation. Humanoid and animal interference will results in rottenness of the product. There is no control over drying temperature, speed of drying and quality of dried product will be reduced. Intermittent sunshine, interruption and wetting by rain also take part into reducing the quality [16]. Numerous mechanical solar dryers using natural convection or forced circulation have been studied to overcome these difficulties.

Direct solar dryers Based on mode of drying Indirect solar dryer Solar dryer Based on auxiliary energy source requirement

Active type solar dryer Passive type solar dryer

Fig. 1: Classification of solar dryers.

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Solar dryers have been classified as shown in Fig. 1. There are almost 83 type of solar dryers with different design and configuration made by different researchers. Here only the most commercialized solar dryer are discussed. Enormous amount of research is going on throughout the world to use nonconventional energy source in drying operation. 2.2 1 Direct solar dryer: In this direct type of solar dryer, solar radiation directly influences the sample material which is to be dry. A part of radiation replicates back and some pass through glass glazing which comes in contact with sample. Sample absorbs some of radiation and part of it reflects back from material surface. Due to this phenomena of radiation, temperature inside dryer rises such that material will emanates long wavelength radiation but due to glass cover they are not allowed to escape to environment. However, this method faces some limitations like small capacity, discoloration due to direct exposure, higher moisture content inside product reduces transitivity etc., [1, 14, 17]. 2.2 2 Indirect solar dryer: In this case, a separate unit for heating of air is connected to main drying chamber. This unit is known as solar collector, comprising of parabolic troughs. Cold air passes through solar collector, gets heated and goes to drying chamber. Convective heat transfer take place during this operation. A better control over drying is achieved in indirect type of solar drying systems and the dried product is of good quality. Air velocity, dryer temperature, solid loading can be controlled easily compared to direct type dryer [1, 14, 17]. 2.2 3 Passive type solar dryer: Due to the effect of wind pressure variance or buoyancy forces, ambient cold air is drawn inside the dryer. Due to direct or indirect effect of solar radiation, cold air is heated to a reasonable temperature and then it removes the moisture from the feed samples. This type of dryers are often known as natural convection solar dryers [17, 18]. 2.2 4 Active type solar dryer: This type of dryer combines the solar energy with electrical or fossil fuel based thermal energy. Air is being circulated using motorized fans, pumps or both. These types of dryers are also known as forced convection dryers [17, 18]. 2.2 5 Tunnel type solar dryer: A multipurpose solar tunnel dryer consists of a small centrifugal blower, solar collector and tunnel drying chamber. Sometimes it is integrated with biomass furnace or heat exchanger to heat the drying air during cloudy and rainy days. It is useful for drying in tropical regions. Drying of cocoa, coffee and coconut using tunnel solar dryer and reduction in time up to 40 % are reported [19]. 2.2 6 Distributed type natural circulation solar energy dryers: This type of solar dryer contains trays to place sample material inside the opaque solar drying chamber. Due to thermosyphonic solar collector, a pressure drop is generated between ambient air which is at lower temperature and heated air inside the collector. This heated air passes


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through the trays, come in contact with the sample and moisture get evaporate. Advantage of this solar dryer is that air is continuously moving from different parts of drying chamber leading to elimination or reduction of hotspot and caramelization. Fruits contains vitamins and mineral. Direct exposure of fruits to the sun may cause evaporation, and color and texture change. Also, loss of volatile components may occur. In this type of condition distributed type natural circulation solar dryer are recommended [20]. Operational difficulties during loading- unloading and variation in inlet air temperature to drying chamber are some drawbacks associated with this dryer. 2.2 7 Mixed mode natural circulation type solar dryer: By combining principles of direct and indirect heating solar dryers, mixed mode solar dryer is designed. Dryer arrangement contains solar air heaters with solar collector arrangement to preheat the air with combination of direct adsorption of solar radiation. Very much higher temperature than ambient can be attained which can be utilized to reach reasonable moisture level in food within short instant of time compared with other solar dryer [1, 14, 17]. 2.2 8 Forced convection greenhouse solar dryer: This type of dryer contains hut type of drying chamber with motorized pump for circulation of heated air. It is an active mode type solar dryer. For large scale of commercial drying purpose, solar air heating systems contains solar collector with fossil fuel driven pumps so that overall consumption of fossil fuel is being reduced [1, 21].

3. Parameters Affecting Drying Using Solar Energy 3.1 Solid loading Amount of solid loading in the dryer is important factor which affect the drying characteristics. As the amount of sample for drying increases amount of air required for drying also increases. This may led to poor product quality because of incapability of dryer to obtain required final moisture content for a specific air flow and drying time. According to Balladin(1996), as the loading of 0.15 cm longitudinally sliced ginger increased from 14.97 kg to 28.12 kg, effective surface area available for drying reduced, thus consuming more time (19 hours against 9 hours) to reach final moisture content.(10.4%) [22]. Amount of fresh product loaded on the trays is also known as loading density. As compared to other solar dryer, cabinet type solar dryer contains highest loading density because of arrangement of so many shelves [23]. 3.2 Type of solids Drying of different solids are depends on the nature of the solids [23]. Some heat sensitive material do not require higher temperature of drying as in the case of rose petal drying where only 31 C temperature is required [24]. In case of drying of copra above 60 C temperature is required with forced convection solar dryer and time required for drying is up to 82 hours [25].

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3.3 Air flow rate Airflow is another important parameter that influences the drying process. As the air flow rate is increased, the conduction and radiation losses may be small due to the smaller temperature rise. Drying efficiency may suffer at high airflow rates since air may not have adequate contact time with the food to reduce its moisture content. Insufficient air flow can result in slow moisture removal as well as high dryer temperatures. However, the internal resistance to moisture movement in agricultural products is much greater when compared to the surface mass transfer resistance that the airflow rate beyond certain levels has no significant effect on the drying rate. In natural circulation systems, air flow is primarily determined by the temperature rise in collector. Higher flow may be used at the beginning of drying and lower flow when drying enters the falling-rate period [23, 26]. 3.4 Temperature Temperature inside dryer is parameter which affects much on drying time as well as on properties of final product. It is a function of temperature of heated air. As the temperature inside dryer increases the rate of drying increases and time required for drying decreases. At the higher initial moisture content, drying temperature plays a major role in drying. At lower moisture content, heat duty required to evaporate moisture is very high so at this stage temperature does not make significant effect. Drying with higher temperature air benefit to evaporation of the water on the surface that is unbound moisture [21, 23, 27]. 3.5 Relative humidity Relative humidity is a primary constraint for the study of drying performance. It is the characteristics of heated air. If air comprises relative humidity as 100% (fullysaturated air) then no more moisture can be carried over with the air. When air contains certain lower amount of humidity with an increase in temperature air contains great affinity to gain moisture from feed sample and drying rate becomes faster. Partial pressure difference of vapor between sample and heated air is the driving force for drying. It increases ramp of humidity. When humidity is too low then shrinking and stiffening will take place, so rotation is the key parameter to control dehydration [23, 26]. 3.6 Pretreatments Pretreatment is necessary to achieve desired product quality. Pieces of mango were dried by hot air drying after dipping in solution such as glucose. Nine saturated salt solutions (NaOH, CH3COOK, MgCl2, K2CO3, Mg(NO3)2, KI, NaCl, KCl and KNO3) in different closed containers were prepared to obtain constant relative humidity environments [7, 23]. Textural changes occur due to chemical changes of cellular components of fruits and vegetables such as degradation of pectin, gelatinization of starch and/or structural alterations in cellulose like crystallization. SEED (Society for Energy and Environment Development) developed methods for making solar dehydrated figs using pretreatments such as immersion into alkaline or acidic solutions of Oleate esters with good success [28].


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4. Comparison of Different Techniques Drying of different natural and food products are discussed by Chouicha et al., Li et al., Ozcan et al., Singh et al., Fudholi et al., Balladin et al., Janjai et al., Kaur et al., Hassanain, Almuhanna and Condori et al. [8-10, 14, 25, 29-34]. In Table 1 comparison is made among various raw materials as well as different types of dryers. This table contains various parameters like solid loading, air velocity etc., result obtained for the study and comment on the result obtained.

Table 1: Comparison of Different Techniques. Results Comment Ref. Sr Natural Type of Parameters . Product dryer no 1 Mango Solar mp= 4.8kg, Time required The final moisture [11] dryer dw/dt=0.9243, for drying 15 content and rate of with a SA =9 * 10-4, hour, drying were found to chimney Rsp=2.2 * 10- Wf=27.6, be different because 3 of difference in , structure surface area Air availability etc. velocity=0.95 Rewetting study m/s, showed that raw Wi=526 banana has the lowest gm/100 rewetting rate and gm(dry) 2 Banana Hurdle mp=6.7 kg, Time required thus it can be stored longer time (ripe) dryer dw/dt =0.3326, for drying 14 for compared to other SA =0.785 * hour, with three products. 10-4, Rsp=1.2 Wf=16.8% wire nesting * 10-3 Wi=169.1 gm/100 gm(dry) 3 Banana mp=7.1 kg, Time required (raw) dw/dt=0.2826, for drying 14 SA=0.785 * hour, 10-4, Rsp=1.04 Wf=9.2% * 10-3 Wi=154.1 gm/100 gm(dry)

572 4 Cassava


mp=13.3 kg, Time required dw/dt=0.3344, for drying 14 SA=23.76 * hour, -4 10 , Wf=14% Rsp=0.72* 103 , Wi=63% 5 Coriander Natural Temperature Time required Though the results [31] convecti 30-40 C, for drying 11 were almost identical on Wi=86% hour, for all the dryer, covered Wf=9.47% quality and tray rehydration characteristics of dried dryer Natural Temperature Time required leaves are very close convecti 30-40 C, for drying 9 to fresh leaves in case of mini multi rack on Wi=86% hour, solar dryer. uncovere Wf=9.64% d tray dryer Natural Temperature Time required convecti 30-40 C, for drying 9 on mini Wi=86% hour, multi Wf=9.25% rack dryer Portable Temperature Time required farm for drying 30-40 C, type Wi=86%, 11hour, solar Wf=9.58% dryer 6 Ginger Wire mp=11.34 kg, Time required Solid loading of 14.97 [22] (0.15 cm basket Bed thickness for drying 16 kg with 2 cm bed thick dryer 1 cm, hour, thickness was found Slice Packing Wf=10.4% to be the optimum longitudi density 700 amount. 0.67 gm per nal) kg/m3, 100 gm sample (dried) of pungent principles Wi=80.3% mp=14.97 kg, Time required was found. Bed thickness for drying 9 Higher amount of oleoresin was 2 cm, hour, obtained from ginger Packing Wf=10.20% which can be further density 462.04 exploited in oleoresin kg/m3, industry. Wi=78.66%


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mp= 28.12 kg, Time required Bed thickness for drying 19 7.4 cm, hour, Packing Wf=10.4% density 231.44kg/m3, Wi=78.5% 7 Rosella flower

Lemon grass

8 Thyme

Roof mp=200 kg, Time required Green house was [30] integrate Bed depth 50 for drying 4 designed in such a d solar to 80 cm, days, way that 16 solar dryer Air velocity Wf =16% collectors were placed 0.1 m/s, Wi= at roof. Rate of return 90%, and payback period mp=200 kg, Time required were 70.3 % and 3.9 respectively. 2 mm thick for drying 3 years Economically feasible stems, days, at commercial scale. Wi=70% Wf=6% Wire comparing [29] Temperature Time required By basket 50 C, for drying 12 conventional and solar Wi=75.155, hour, essential drying, time required solar Specific drying oil content 0.6 for drying in case of dryer rate 0.27 (hr-1) %, solar drying is higher Wf=14%, but essential oil content is high in case Air oven Temperature Time required of solar drying. drying 50 C , Wi= for drying 9 75.155, hour, essential content Specific drying oil rate 0.433 (hr 0.5%, 1 ) Wf= 10%,

mp=Initial mass of product, dw/dt=Moisture per minute, SA=Surface area available for drying, Rsp=Specific rate of drying (1/(kg s C)), Wi=Initial moisture content in % in wet basis, Wf=Final moisture content in % in dry basis

5. Conclusion In this article, solar drying characteristics of various natural and food products were reviewed from different literature. Solar dryers working on active mode and passive mode were also discussed. The drying rate is higher in forced convection mode than in natural convection mode.

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Study of various parameters affecting drying showed that air velocity, initial moisture content, temperature inside dryer and loading of samples played major role. Higher initial moisture content requires higher amount of energy to reach acceptable level and so the time required for drying is increases. Increase in air velocity increases drying phenomena. Small amount of loading causes increased rate of drying but sometime unusual loss of energy may take place. As temperature increases drying rate increases but while dealing with heat sensitive materials care should be taken on temperature control. In essential oil recovery from natural products solar drying is used as pretreatment method. Experiment has shown increase in oil recovery by solar drying compared to open sun drying. The review suggests that solar dryer for natural and food products can be an attractive method for food preservation as post harvesting technology and also for a commercial proposition. Solar drying can be considered a better alternative for natural and food products with no harmful effect to environment, cost effectiveness and elimination or reduction of fossil fuel use.

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