Abstract: The effect of sun, oven and solar drying on the nutrients and non-nutrients composition of leaves of Leptadenia hastata was investigated. All the drying methods were found to significantly decrease (p<0.05) magnesium content in sun (1.49±0.49 mg 100 g-1), followed by oven (1.27±0.06 mg 100 g-1) and the lowest in solar dried (0.93±0.07 mg 100 g-1) leaves compared to fresh sample. Drying methods with exception of oven drying did not significantly (p>0.05) lower lipids content. The drying processes employed significantly (p<0.05) decreased carbohydrate, crude protein, magnesium and moisture content. Potassium, ash and crude fibre were significantly (p<0.05) increased. Oven drying significantly reduced acid value (2.82±0.41%), followed by sun (2.12±0.65%) and solar drying (1.76±0.21%) in decreasing order. Tannins, saponins, volatile oils, saponin glycosides and alkaloids were detected in fresh and dried samples. These compounds with exception of saponins and saponin glycosides (sun and oven dried) decreased in trace amounts upon drying. The results reinforce the growing awareness that the leaves of Leptadenia hastata can contribute useful amounts of nutrients to human diets and reduction of toxic non-nutrients compounds upon drying.
INTRODUCTION
Drying has been used traditionally as a method of preserving leafy vegetables in Nigeria and other developing countries. In Nigeria, one of such plant vegetables is leaf of Leptadenia hastata. The rationale for drying is to reduce the moisture content to a level, which prolongs shelf life during storage, reduces colonization by microorganisms and as a source of food after rainy season (Eklou et al., 2006). Fruits and other foodstuffs with high moisture content, provide a favourable condition for the growth of micro-organisms, which lead to their spoilage and wastage (Ladan et al., 1997). Drying is one of the methods of food preservation adopted in order to decrease losses in quality and quantity, which will otherwise occur (Habou et al., 2003). Sun drying is the most widely used method of drying agricultural produce in most of the developing countries of the tropical region. Solar drying is an elaboration of sun drying and is considered to be an efficient system of utilizing solar energy. Generally, solar drying tends to be the most hygienic method of drying (Bala and woods, 1994).
Processing makes foods safe including the leafy vegetables for consumption and also destroys pathogenic microorganisms. The effect of food processing on nutrient content will depend on the sensitivity of the nutrient to the various conditions prevailing during the process, such as heat, oxygen, pH and light (Morris et al., 2004). In Sokoto, Nigeria, leaves of Leptadenia hastata serve as one of the most widely used vegetables in meals and are preserved traditionally using sun drying (open air). This has risk of possible contamination by microorganisms, lack of protection from dust, infestation by insects, rodents and the quality of the products are seriously degraded and sometimes inedible (Diamante and Munro, 1993). Two processes occur during drying; the addition of heat and removal of moisture from the food (Morris et al., 2004).
Leptadenia hastata (family Asclipiadaceae) is a twiner with a corky bark on the older stems. It has well developed, half-succulent and persistent petiole leaves with a thick greenish sap. The leaves are more abundant and fresh during rainy season (Aliero et al., 2001). Leptadenia hastata is commonly used in Niger republic in day to day cooking and is considered as hunger food due to its very important content of valuable nutrients (Freiberger et al., 1998). The plant has been reported to be safe with low LD50 of 1513 mg kg-1 (Tambaura et al., 2005). It contains triterpenes, fatty acids, amino acids, poly-oxypregnane, lutein and β-carotene (Aquino et al., 1996; Nikiema et al., 2001; Tambaura et al., 2005) and selenium and phosphorus (Freiberger et al., 1998).
The objectives of our study were to compare the traditional sun drying with other drying methods with regard to the effect on nutrient and non-nutrients compositions on the dried samples using fresh sample as basis for comparison. The study was conducted at Department of Biochemistry, Faculty of Science, Usmanu Danfodiyo University, Sokoto, Nigeria, from June to July 2006.
MATERIALS AND METHODS
Chemicals
Most of the chemicals used were of analytical grade.
Plant Material and Preparation
Leptadenia hastata was obtained from within Usmanu Danfodiyo University
Campus Sokoto, Nigeria. The plant sample was authenticated at the Herbarium,
Botany unit, of the same institution. Voucher specimen was deposited in the
Herbarium for reference. The leaves collected were first washed with large amount
of water to remove dust and some portions were dried to a constant weight using
different drying methods. Fresh sample was used as control.
Sun Drying
The leaves were kept under the sun (ambient temperature 35-41°C) in
June 2006, between 10.30 am-5.30 pm daily till the leaves attained constant
weight.
Solar Drying
A solar dryer designed by the Sokoto Energy Research Center, Usmanu Danfodiyo
University Sokoto, Nigeria was used as solar dryer for the sample. The dryer
is a combine direct and indirect rocked thermal storage passive cabinet dryer.
The dryer stores heat in rock bed, a device intended to overcome temperature
fluctuations during sun set hours. The temperature of the drying chamber ranged
between 42 and 63°C while that of the solar dryer of the leaves collector
was between 40 and 73°C. Solar drying took place between 10.30 am-5.30 pm
daily till the sample attained a constant weight (Matazu and Haroun, 2004).
Oven Drying
The leaves were oven dried (60°C) using hot air oven (Stuart Scientific
oven, England) for 24 h to obtain a completely dried sample.
Nutrients and Non-Nutrients
The dried and fresh leaves were analysed for moisture content using the
method of Oyeleke (1984) and crude protein content by modified Kjeldhal method.
The method of Yawas and Obi (2001) was employed in the analysis of carbohydrates.
Ash content was determined by the method of Samuel et al. (1997). Crude
lipid and fibre content were determined by the procedures of Association of
Official Analytical Chemists (AOAC, 1980). Acid value was determined by the
method of Chopra and Kanwar (1991). Phytochemical screening was done using standard
procedures of Harbone (1973), Trease and Evans (1978) and El-Olemyl et al.
(1994).
RESULTS
The results of proximate, mineral element and phytochemical compositions of fresh and dried leaves of leptadenia hastata are presented in Table 1, 2 and 3, respectively. The ambient temperature at the period of drying ranged from 35-41°C (mean 38°C). The time taken for the leaves to attain constant weight was 25, 9 and 17 h for sun, oven and solar drying, respectively. For sun and solar drying, the drying was done over a period of 5 and 4 days, respectively due to interruption by rainfall. The final moisture, carbohydrate and crude protein are presented in Table 1 and were significantly decreased (p<0.05) upon drying. There was no significant (p>0.05) increase and decrease of sodium and calcium content by all the drying methods.
Oven dried leaves had significantly lowest moisture content followed by solar with highest moisture content in sun (range 7.30-10.16%) dried leaves (Table 1) compared to fresh sample. Significant (p<0.05) decrease of carbohydrates and crude protein with a non significant (p>0.05) decrease of lipid content was observed upon drying when compared with fresh sample. The higher ash content of oven-dried followed by sun-dried with solar dried leaves having the lowest were observed.
Qualitative analysis of phytochemical compounds in leaves extracts of Leptadenia
hastata showed the presence of tannins, saponins, volatile oils, saponin
glycosides and alkaloids (Table 3). However, these compounds
were decreased with the drying methods. The acid value was significantly reduced
in oven (2.82±0.41%) followed by sun (2.12±0.65%) and solar-dried
samples (1.76±0.21%) in decreasing order (range 1.76-2.82%). This shows
that the acid value was reduced by the drying methods.
| Table 1: | Proximate composition of fresh and dried leaves of Leptadenia hastata |
Values are means±standard deviation.
Means in a column followed by same letter(s) are not significantly different
using LSD at 5% (n = 4), Analysis of variance, complete randomized design,
statistical analytical system (1988). SAS/STAT user’s guide. Release
(6,0.35. A. Cary, N.C, USA) was used to analyze the data. LSD = Least
Significant Difference |
|
| Table 2: | Mineral element composition of fresh and dried leaves of Leptadenia hastata |
Values are means±standard deviation.
Means in a column followed by same letter(s) are not significantly different
using LSD at 5% (n = 4), Analysis of variance, complete randomized design,
statistical analytical system (1988). SAS/STAT user’s guide. Release
(6,0.35. A. Cary, N.C, USA) was used to analyze. LSD = Least Significant
Different. Na = Sodium, Ca = Calcium, K = Potassium and Mg = Magnesium
|
|
| Table 3: | Phytochemical screening of fresh and dried leaves extracts of Leptadenia hastata |
+ = Trace amount, +++ = Presence, - = Absence |
|
DISCUSSION
Heating can have both beneficial and detrimental effects on the nutrients content of foods. It generally improves the digestibility of foods, making some nutrients more available (Morris et al., 2004). Moisture content of vegetables and fruits provides an enabling environment for the growth of microorganisms. Generally removal of moisture results in increased concentration of nutrients (Morris et al., 2004). For vegetables and fruits to be preserved or kept for long time to be used, the moisture content has to be reduced. This will inhibit the autolytic enzymes (Ladan et al., 1997).
The significant (p<0.05) decrease in the macronutrients content (Table 1) of the dried leaves could be attributed to stability of the bonds involved in the macronutrients. The decrease in protein content was found to be commensurate with the intensity of heat applied due to the efficiency of the dryers. The observed decrease could be attributed to the ability of the dryers to concentrate energy which could in turn cause some denaturation of protein in the dried samples. Nutritional losses during drying occur to great extent due to application of heat, there by decreasing the concentration of some nutrients especially protein (Morris et al., 2004). Loss of nutrients which occur during drying can be minimized by drying at low temperatures for shorter period (Morris et al., 2004).
Higher ash content from the results (Table 1) indicated probable high mineral elemental composition of the leaves of Leptadenia hastata. The significant (p<0.05) increase of potassium in the dried samples are added advantage and can be use for therapy (Dzomeku et al., 2006). The higher (p<0.05) calcium and phosphorus also in the dried samples are vital for bone. A similar observation was reported with tomatoes, though a different vegetable by Ladan et al. (1997). The increased ash and crude fibre content could also be due to the method of analysis and genetic changes. Non-nutrients in foodstuffs are known to be decreased by heat upon processing as indicated from the results obtained (Matazu and Haroun, 2004). This may explain the decrease of these non-nutrients in the dried leaves and probably increase the bioavailability of micronutrients such as calcium and magnesium, to the body.
From the results, the leaves of Leptadenia hastata have indicated valuable nutrients in fresh and dried samples and with reduction of some toxic non-nutrients by the drying methods. The solar drying could be the most preferred method of drying the leaves of Leptadenia hastata because it is hygienic faster and has less effect on the nutrients when compared with oven dried sample. The lost nutrients during drying should be complemented with other rich sources of the micronutrients and macronutrients.