Abstract: In this study, the drying efficiency of potato chips processed using open sun drying and a solar dryer is reported. Equal quantities of potato chips were subjected to Open Sun Drying (OSD) and also dried using a solar dryer, constructed using locally available materials. The experiment lasted for 3 weeks and the drying efficiency of the chips from the two methods were computed. The results show that there is a significant improvement in the drying efficiency of the potato chips using the solar dryer compared to the OSD method. The average drying period for the solar dryer was two weeks while that of the open sun drying lasted longer. The drying rate of the potato chips and the system efficiency (solar dryer) were found to be 0.75 kg h–1 and 68.5%, indicating the potentials and effectiveness of the constructed solar dryer compared to the OSD method.
INTRODUCTION
Effective preservation of agricultural products is one of the most ubiquitous problems faced by local farmers especially in third world countries. In the developing nations, most farmers encounter serious post harvest losses due to the utilization of very poor and primitive preservation techniques in preserving their farm produce. It has been established recently that renewable energy have a significant role to play in extending technology to farmers in developing countries in order to increase their productivity. According to Ogwueche (2000), sweet potato (botanical name; Ipomoea batatas) is a dicotyledonous plant which belongs to the family "Convolvulaceae". Sweet potatoes are large, starchy, sweet-tasting, tuberous roots and the crop is mostly propagated vegetatively from small slips or shoots growing from the tubers or vine cuttings. Sweet potatoes are highly adaptable to relatively marginal soils and erratic rainfall, offers high productivity per unit land and labour and guarantees some yield even under the most adverse weather conditions. The origin of sweet potatoes (Ipomoea batatas) is mostly associated to the tropical regions of the North and South America (Williams, 1968). The maturity period of sweet potato tubers is typically ≤4 months depending on the species (Anyanwu et al., 1982; Anyanwu and Komolafe, 2003). Owing to this short maturation period, sweet potato can be grown two to three times in a year with supplementary irrigation thereby increasing food abundance in the nation and the world at large.
Sweet potatoes are useful to man in so many areas which include; (1) As vegetables in some West African countries such as Guinea, Sierra Leone, Liberia and Nigeria and in North Eastern Uganda, East Africa, (2) As good source of vitamins A, C and B2 (riboflavin), (3) As staple food in many continents including Africa, Asia, Europe, North America and South America, The Caribbean and New Zealand, (4) For biofuel production, (5) The leaves and vines are important source of hay and fodder to livestock farmers and (6) As medicines (Caroll and Melvi, 2004; Dincer et al., 2011; Hartemink et al., 2000; Rowan et al., 2008; Verrill and Barrett, 1937; Chukwu, 1999; Woolfe, 1992). In Nigeria, sweet potato is not just an established staple crop but also serve as an important source of raw materials for the manufacture of wide range of industrial products including starch, liquid glucose, ethanol and as a potential substitute for wheat flour in bread production. The Federal Government of Nigeria, recently released its plan early this year to improve the productivity of sweet potato in the country from the six tonnes per hectare to 25 t ha–1 by 2015 and national production from 2-6 million t per annum by 2016 (http://dailyindependentnig.com). Although, some of the production constraints facing potatoes farmers in Nigeria include land fragmentation, lack of improved agricultural practices, insufficient credit facilities, poor agricultural extension services, poor road transportation and high cost of productivity, the major production threat has remained that of poor preservative and storage facilities. This ugly trend has been highlighted by other research scholars (Tewe et al., 2003; Odebode et al., 2008; Fawole, 2007; Mbanaso et al., 2012; Babatunde et al., 2005; Egbe, 2012; Adewumi and Adebayo, 2008; Egbe and Idoko, 2009). Sweet potato has the potentials for food security, job creation, as well as serving as revenue generation through export especially this time that variety of approaches to poverty alleviation are being considered by the Nigerian government and their development partners in order solve the problems of food inadequacy in Nigeria.
Just as other developing countries in the tropical regions, one of the major method of removing moisture from agricultural products in most parts of Nigeria is by open sun drying. This method is not only very primitive and barbaric but also exposes the products to dust, stains, rodents, birds and flies, such that the product becomes a potential vector for different ailments. Another serious disadvantage of the OSD method is that it is very unhygienic and more labour intensive since the crops are being moved frequently in and out during the day and night and from unfavourable weather conditions e.g. increased humidity in the atmosphere or rain. This method also prolongs the drying period and could result in the deterioration of the quality/decay of the crops. According to Kurtbas and Turgut (2006), solar air heaters are simple devices that heat up the air by tapping solar energy from the sun and are mostly applied in solar thermal devices that requires low to moderate temperatures ≤80°C. Such requirements are suitable for crop drying purposes.
The major objective of this research is to develop a less expensive way of preserving potato chips through a modern, locally fabricated cost-effective drying equipment in order to reduce the post harvest wastes and food contamination that is usually encountered by farmers in the developing nations like Nigeria. In order to achieve this noble objective, a solar dryer that works on the principle of simultaneous crop drying was developed. The simultaneous drying of the potato chips are made possible through tapping of the direct solar radiation from the transparent walls of the solar dryer and the cabinet roof, coupled with the heated air from the solar collector. The results of the reserach will go long way to alleviating the post harvest challenges usually encountered by potato farmers in Nigeria and in other regions of the world with similar scenario and thus leads to increased global food availability.
MATERIALS AND METHODS
The solar dryer used in the study was constructed with cheap and locally available materials that were sourced from the local markets. The experiment lasted for 3 weeks (January 7-28, 2015). The most important parts of the constructed solar dryer include; the collector, drying cabinet and drying trays. The architectural design is relatively close to that reported by Bolaji and Olalusi (2008) but differs in the dimensions and properties of the materials used as well as the orientation and location of the study area.
| Fig. 1: | Solar dryer |
Figure 1 gives a picture of the solar dryer. Adegoke and Bolaji (2000), noted that the best stationary orientation for solar dryers is to face it due South in the Northern hemisphere and due North in Southern hemisphere and with a tilt angle>the local latitude of the study area. The study area (Abakaliki, Ebonyi State, Nigeria) is located on 6.3333° N, 8.1000° E (Menakaya, 1980), hence the solar collector is oriented south with a tilt of 20.33o for increased air circulation and performance of the fabricated solar dryer.
Design equations and theoretical considerations: In the literature, there a lots of different design equations that have been utilised in the construction of solar dryers. In the present investigation, the design equations presented in the works of Okonkwo and Okoye (2005) and that contained in Sabarez and Price (1999), were utilised. In this regard, the optimum angle of inclination of the collector was obtained to be 21.33° (local latitude+latitude of Abakaliki), determined using the relation described in Eq. 1:
| (1) |
where, θ is the tilt angle and L is the local latitude of the study area.
Declination (δ): The declination is the angle between the sun’s direction and the equatorial plane and is mathematically given as (Tiwari, 2002) in Eq. 2:
| (2) |
where, n is the day in the year which varies from n = 1 to n = 365.
Collector efficiency (η): The collector efficiency is given by equation defined as (Bolaji and Olalusi, 2008) in Eq. 3:
| (3) |
where, ρ is the density of air (kg m–3), T is the temperature elevation, Cp is the specific heat capacity of air at constant pressure (J kg–1 K–1), V is the volumetric flow rate (m3 sec–1), A is the effective area of the collector facing the sun (m2) and Ic is the insolation received by the collector in watts per meter squared (W m–2).
Dryer efficiency (ηD): The dry efficiency can be deduced from the relation as given in Eq. 4:
| (4) |
where, ΔM is the change in the mass of the crop, L is the latent heat of vaporization of water, t is the time of drying, ID is the insolation received by the dryer and A is the effective area of the dryer.
Minimum heat energy Qmin required for crop drying: The minimum quantity of heat needed for crop drying is given by the relation given in Eq. 5:
| (5) |
where, L is the latent heat of vaporization of water, Mw is the mass of crop before drying, ρ is the density of water, Tamb is the ambient temperature and Tdryer is the dryer temperature.
Moisture Content (MC): The moisture content from the crop is calculated using the formula given in Okonkwo and Okoye (2005), Adegoke and Bolaji (2000) and Bolaji and Olalusi (2008) as described in Eq. 6:
| (6) |
where, Mi is the mass of sample before drying and Mf is the mass of sample after drying.
Theoretical considerations: In general, the three major modes of heat energy transfer in physics are radiation, convection and conduction. The sum of these and other processes taking place in the system is necessary to determine the energy balance of the device. The energy balance of the absorber can be adequately approximated to the heat equations by assuming that the total heat energy lost by the solar collector is equal the total heat energy gained and can be represented mathematically as given in Eq. 7:
| (7) |
where, IC and AC retain their usual meanings, QA in watts (W) is the rate of useful heat collected by air, QC in watts (W) gives the rate of heat loss from the absorber due to conduction, QV in watts (W) is the rate of heat loss from the absorber due to convection, QR in watts (W) gives the heat loss from the absorber due to re-radiation of long wavelengths and QRR in watts (W) is the heat loss from the absorber due to reflection losses. According to the literature (Gatea, 2011), the energy loss due to conduction, convection and radiation can be unified to a simplified form as is given in Eq. 8:
| (8) |
Research done by Sodha et al. (1985) and Sodha and Chandra (1994), indicate that Eq. 8 can be written in the form of Eq. 9:
| (9) |
RESULTS AND DISCUSSION
Figure 2 gives the variation of the temperature and the time of the day within the period of the research investigation. The temperatures, relative humidity of the dryer and ambient conditions were monitored with thermometers and a relative humidity sensor. The potato chips were carefully sliced into uniform dimensions of 2.25 cm2 by surface area and 200 g of the sliced chips was measured out. The dryer trays were then lightly filled with the potato chips. The control experiment contained similar quantity and size of potato chips but were spread outside under open sun drying. The result shows that a maximum temperature range of 49-50°C was recorded in the collector between 13.00-14.00 pm. This implies that the collector retains the heat sensibly, such that effective drying takes place longer. The difference between the ambient temperature and the dryer is very wide (20°C), indicating the effectiveness of the dryer. Research done by Gatea (2011), indicate that the drying efficiency of a product is a complex heat and mass transfer process which depends largely on other external variables such as temperature, humidity and velocity of the air stream and internal variables which depend on other parameters such as the surface characteristics, chemical composition, physical structure and size and shape of product. In this study, the difference in the dry kinetics was attributed to the more efficient drying rate of the solar dryer compared to the open sun drying approach. The observation reported herein is in agreement with the reports of other authors in the literature (Kurtbas and Turgut, 2006; Bolaji and Olalusi, 2008).
Figure 3 gives the result of mass loss in kilograms for both methods used for drying the potato chips. The results of the solar dryer show that the mass loss was more pronounced compared to the open sun drying method.
| Fig. 2: | Variation of temperature with time of the day |
| Fig. 3: | Variation of the mass loss with the day number |
| Fig. 4: | Variation of the moisture content with the time of the day |
The materials dried faster, resulting in zero mass loss from the 13th day while the materials subjected to the OSD technique exhibited zero mass loss on the 16th day. This wide difference is a clear indication that the drying done using the solar dryer is more efficient, less time consuming, less energy demanding and more cost effective compared to the open sun drying approach. The decreased drying rate that was observed in the open sun drying method was attributed to a variety of factors including; varying temperatures, humidity, velocity of the wind, wind direction, dust particles and shadowing effects from nearby buildings/trees amongst other factors.
Figure 4 shows the change in the moisture content with the time of the day. The moisture content removal for each day was computed on daily basis. It was observed that the potato chips dried from an initial moisture content of 85% to an average storage moisture content of 15.5% in 4 days of the drying process, using the solar dryer. However, for the open sun drying case, it took about 6 days for the potato chips to attain a moisture content of 18.5% from the same initial moisture content. Physical observation of the potato chips dried using the solar dryer, indicates that it maintained its colour while those dried under open sun drying conditions exhibited a colour change from white to milky-brown.
| Fig. 5: | Variation of the relative humidity with time of the day |
This was attributed to some dust particles and debris that it Some sign of contamination and smell was also noticed, indicating that the potato chips are gradually attaining stages of deterioration and decay. This is possibly due to flies and other vectors/rodents that perch on them as they were exposed to the open environment in the OSD method.
Figure 5 shows the variation of the relative humidity with time of the day. The relative humidity is usually least at noon as the atmosphere is clearest at noon. Similar behaviour has been reported by other research groups for other agricultural products (Bolaji and Olalusi, 2008; Kassali, 2011; Aloys and Angeline, 2009). However, the values observed for the solar dryer is relatively higher compared to that obtained for the open sun drying as shown in Fig. 5. Such behaviour can be attributed to the difference in the atmospheric conditions of the open environment compared to the solar dryer during the research period.
CONCLUSION
In this study, the comparative studies of the drying efficiency of potato chips using open sun drying and a locally fabricated solar dryer is presented. This results showed that the potato chips processed with the solar dryer, gives faster drying rate, yields high quality products and is more time saving compared to that of the open air sun drying. Post harvest losses is a serious issue to farmers especially in developing nations. This not only leads to wastage of food and agricultural products due to poor preservation techniques but also to food contamination and to the build-up of municipal solid wastes. In most third world countries, problems of unsustainable waste management is common and the negative consequences of poor waste management to man and to the environment to such nations including Nigeria is quite disturbing. Use of cost-effective solar dryer by farmers to replace the age-long solar drying method in Nigeria will add a significant quota towards increased food availability, increased revenue generation, job creation and environmental sustainability in Nigeria and other countries with similar challenges.