The present study designed, constructed and undertook the performance evaluation of a post harvest passive solar crop dryer for drying agricultural produce. The solar crop dryer consists of a solar collector with dimensions 110 x 61 x 10cm, a drying chamber measuring 102 x54cm, a movable heat storage unit with the dimensions 40 x 35 x 13cm and drying trays each having an area of 1806cm2. The performance evaluation was conducted using three crops- tomatoes, pepper and okra which are locally produced. The test performance was done in both rainy and dry seasons to conform to the two major seasons in Nigeria and to determine the functionality of the crop seasonally. Open sun drying of the above crops was undertaken simultaneously as a control measure. Results of the performance evaluation showed that the highest chamber and ambient temperatures without the heat storage system incorporated was 53.3 and 32.8 oC respectively during the rainy season while the chamber and ambient temperatures with heat storage unit incorporated were 65.6 and 36.6 oC respectively. This indicates that the incorporation of heat storage unit improved the quality of heat output of the dryer temperature which was able to increase the drying process hours after the sun ceased shining. During the dry season (December), there was a little rise in ambient temperature as compared with rainy season. The maximum ambient and chamber temperature obtained during the dry season was 68.2 and 33.2oC respectively. As a result of the relative rise in temperature during the dry season, the rate of moisture lose was faster than during the raining season. As part of the performance evaluation the physicochemical properties of the crops were evaluated before and after drying. Physicochemical properties evaluated includes: moisture, protein, fat, fibre, ash, carbohydrate and vitamin C, contents. The fresh, open and solar dried samples were analysed for their proximate composition using the recommended method of Association of Official Analytic Chemists (AOAC). Also, statistical analysis of the data was conducted using analysis of variance (ANOVA) using Completely Randomize Design (CRD) and means were separated by Duncan’s New Multiple Range test (DNMRT). Proximate analysis showed that solar dried vegetables had significantly (P < 0.05) higher protein, fibre, ash, carbohydrate and vitamin C except for the fat content that was significantly (P < 0.05) higher for all the open sun dried samples than the solar dried and fresh products. The nutrient which is highly affected by sun drying is vitamin C. Result indicates that moisture loss in solar dried vegetables was faster than the open dried samples and as such makes the solar dried products of lesser tendency to mould and bacterial growth. The open sun dried samples had to be carried into the sheltered place each time it rains. The observation is that the solar dried produce is of high quality and hygienically good for human consumption. Further processing of the dried crops will involve packaging for commercial purposes. This will also help in making these agricultural produce available in a relatively cheap prices in off season and also avert micronutrient deficiencies in diet especially among the low income groups in Nigeria.
1.1: Background of the Study
Vegetables and their products are of great nutritional and health importance since they make significant contribution in supplying essential vitamins, minerals, antioxidants, fibers and carbohydrates that improve the quality of the diet and also contains constituents that have health benefits and anti-disease factors, such as antioxidants and polyphenols. These components are known to scavenge harmful free radicals that are associated with incidence of cancer and heart diseases (Cao et al., 1996; Velioglu et al., 1998).
Unfortunately, fresh fruits and vegetables are not only seasonal but highly perishable since the moisture content is more than 80%, they are classified as highly perishable commodities (Orsat et al., 2006). As a result they deteriote very fast few days after harvesting, losing almost all their required quality attributes and some could likely result to total waste. The post harvest losses in vegetables has been estimated to be about 30-40% due to inadequate post harvest handling, lack of infrastructure, processing, marketing and storage facilities (Karim and Hawlader, 2005). It has been shown that as high as 50% of these produce are lost between rural production and town consumption in the tropical areas. Studies have also recorded that 20 to 40% of harvested vegetables are not eaten because they are made unavailable through some forms of spoilage (Orsat et al., 2006).
FAO (2000) estimated that the world production of fruits and vegetables over a three-year period (1993 – 1995) was 489 million tons for vegetables and 448 million tons for fruits. This trend increased as expected, reaching a global production of 508 million tons for vegetables and 469 tons for fruits in 1996. This trend of production is expected to increase at a rate of 3.2 percent per year for vegetables and 1.6 percent per year for fruits. However, little considerations and attention are given to preservation aspect of these important agricultural produce in Nigeria. The increase in production of these vegetables usually results in gluts at harvest time and very low price, while few months after, scarcity sets in resulting in high prices.
One major means of preservation of vegetables is by drying. Drying is an excellent way to preserve food and solar food dryers are appropriate food preservation technology for sustainable development. Drying was probably the first ever food preserving method used by man, even before cooking. It involves the removal of moisture from agricultural produce so as to provide a product that can be safely stored for longer period of time (Scalin, 1997).
Fruits and vegetables are dried to enhance storage stability, minimize packaging requirement and reduce transport weight. Drying is a suitable alternative for post harvest management especially in developing countries where exist poorly established low temperature distribution and handling facilities. It is noted that over 20% of the world perishable crops are dried to increase shelf-life and promote food security (Grabowski et al., 2003).
Moreover, products with low moisture content can be stored at ambient temperatures for long period of time due to a considerable decrease in the water of the material, reduced microbiological activity and minimized physical and chemical changes (Vlachos et al.2002). Dried vegetables are more concentrated than any other preserved form of foodstuffs and are tasty, nutritious, light weight and easy to prepare, store and use (Yaldyz et al.2001).
Sun drying is the earliest method of drying farm produce known to man and it involves simply laying the agricultural products in the sun on mats, roofs or drying floors. This has several disadvantages since the farm produce are laid on the open sky and there is greater risk of spoilage due to adverse climate conditions like rain, wind, moist and dust, loss of produce to birds, insects and rodents (pests); totally dependent on good weather and very slow drying rate with danger of mould thereby causing deterioration and decomposition of the produce. The process also requires large area of land, takes time and highly labour intensive (Gujarat Energy Development Agency, GEDA, 2003).
With cultural and industrial development, artificial mechanical drying came into practice, but this process is highly energy intensive and expensive which ultimately increases product cost (GEDA, 2003). Recently, efforts to improve sun drying have led to solar drying.
Solar dryers are specialized devices that control the drying process and protect the agricultural produce from damage by insects, pests, dust and rain. In comparison to natural sun drying solar dryers generate higher temperatures, lower relative humidity, and lower product moisture content and reduce spoilage during the drying process. In addition it takes less time to dry produce and is relatively inexpensive compared to artificial mechanical drying method. Thus, solar drying is a better alternative solution to all the drawbacks of natural drying and artificial mechanical drying (GEDA, 2003).
The Nigerian stored product Research Institute (NSPRI) has developed techniques for the storage of fruits, vegetables and tubers. These methods are not strictly solar dependent, may in some cases require high-energy like refrigeration. It is believed that these inventions cannot be practically transferred to the rural-poor farmers in the society, since they may require financial inputs which are not within their reach.
The revival of solar drying of the more perishable agricultural products (like fruits and vegetables), appears to be a promising method of reducing post-harvest losses, improving rural incomes and contributing to self-sufficiency, even of reducing some imports through substitution products.
However, there is an objection over sun-drying as a preservative method that considerable amount of nutrients are lost during this process due to heat. Therefore, this study is aimed at evaluating the effect of the sun-drying on the nutritive properties of Tomatoes, Pepper and Okra.
1.2: Statement of the Problem
The production of agricultural products such as fruits and vegetables is seasonal in Nigeria. There are periods when there is a glut and others when there is acute shortage. The periods of glut clearly indicate that relatively large quantities of fresh agricultural products are produced in Nigeria. Hence, there exists a possibility of having the product round the year. The obvious acute shortage of agricultural products at some periods of the year in Nigeria is attributed to poor storage facilities. The immediate effect of poor storage facilities is that the farmer is robbed of benefits accruable from his hard labour, as he hurriedly disposes off his product at give-away prices, to save him the situation of helplessly watching his produce rot away. This serious lack of opportunity for adequate economic return reduces the farmer’s incentive and consequently his desire to produce more and as such there is need to device a measurement that could encourage and enhance the preservation and storage of agro-produce at least for some period of time before usage. Solar energy drying technology appears the best option for such measure.
However, the concept of solar drying to mitigate the above challenge is nonetheless without some major problems such as inability to undertake drying process over the night or during off sunshine hours. A solar dryer that could dry agricultural products during the off sunshine periods could have an advantage over an ordinary dryer and will be of great benefits to farmers. Such a solar dryer would incorporate energy storage device for drying purposes when needed for all day round (Okonkwo and Okoye, 2005). There are many methods of solar energy storage materials available (Duffie and Beckman, 1979). One of such method is storage of solar energy as sensible heat using materials such as pebbles (rocks). Pebble is an inexpensive material locally available in Nigeria. Its utilization could pose no burden to farmers.