​​Levels of Metals and Nutritional Composition of Garden Cress (Lepidium sativum) Seed, in Southern Ethiopia

Garden cress (Lepidium sativumin) is a member of the family Brassicaceae cultivated throughout many countries in the world (Tiwari et al., 2004). Major secondary metabolites in Garden cress’s alkaloids, flavonoids, tannins, glucosinolates, sterols, triterpenes, saponins, glycosides, carbohydrates and anthracene proteins phenolics (Manohar et al., 2012) which are responsible for its various ethnopharmacological activities. The seeds possess appropriate levels of protein, fat, carbohydrate, dietary fiber, phosphorus, magnesium, calcium, iron and (Mohite et al., 2012) but the lee of the is not well reported for specific countries.

In Ethiopia, food insecurity has been a severe problem for decades. A series of production failures have resulted in chronic food insecurity (Kaluski et al., 2001). This indicator demonstrates that a large proportion of the population has been undernourished over the last one and half decades. Malnutrition has been a severe obstacle to economic development in Ethiopia. A major cause is lack of access to nitrous food products The Ethiopia Demographic and Healthy survey carried out in 2005 showed that 47% of children under five in Ethiopia were stunted. Similarly, 27% of all women of childbearing age were found to suffer from chronic energy deficiency (FDRE, 2008). To alleviate this problem, the consumption of garden cress seed is among the options.  

However, there was no research conducted on levels of metals and nutritional composition of garden cress in the study area. Therefore, the objective of this study was to determine the level of essential and non-essential metals and nutritional composition in garden cress seed grown in Wolaita zone in southern Ethiopia, to ensure that all individuals secure adequate nutritional status in a sustainable manner which is essential for health and productive life.

Sample collection and preparation
 
27 triplicate samples were collected from nine selected sites, namely; Jage, Wandara, Taba, Hilena, Siyara, Demba Gale, Himbecho, Afama and Gara Godo of Wolita zone  during the dry season in 2021. These collected samples were dried by the use of sunlight to be constant in their weight. The dried samples grind by the use of an electronic blending device and stored in a plastic bags and freeze below -25°C till digestion (Yu et al., 2005).
 
Analysis of metal
       
Metals of K, Ca, Mg, Fe, Cu, Pb and Zn were determined by FAAS equipped with the air-acetylene flame system using an external calibration curve from the digested sample (Demirel et al., 2008). All the experimental works are conducted in the laboratory of Arba Minch University, Ethiopia.
 
Analysis of proximate composition
 
Estimation of moisture content
 
Two grams of the powdered samples of seeds and aerial parts of garden cress are taken in three replications and dried initially at 80-90°C. Weights of dried samples were noted until constant weights were obtained. The percentage of moisture content was calculated per (AOAC, 2000).
 
Estimation of fat
 
The crude fat (%) contents are calculated using the following formula (AOAC, 2005):
 

 

 
Estimation of ash
 
The ash contents (%) were calculated using the following formula (AOAC, 2005): 
 

 

       
Estimation of protein
       
Nitrogen and crude protein content in the powdered samples of garden cress was estimated by following the conventional micro Kjeldahl,s method. The protein content of samples was determined by the Kjeldahl method as follows. Two gram of sample was digested with 5 gram of digestion mixture (10 parts potassium sulphate and 1 part copper sulphate) and 20 ml of concentrated sulphuric acid in Kjeldahl flask until the contents were carbon free. The digested sample was made up of 100 ml. An aliquot of 10 ml was distilled with 20 ml of 30 per cent sodium hydroxide and liberated ammonia was collected in 20 ml of 2 per cent boric acid containing 2-3 drops of mixed indicator [0.1 % methyl red and 0.1% bromo-cresol green of 95 per cent ethyl alcohol separately and mixed in the ratio of 1:5 respectively]. The entrapped ammonia was titrated against 0.1N hydrochloric acid.

The nitrogen content in the sample was calculated by the following expression:
 

 

       

Protein content (%) in sample = Nitrogen content in sample x 6.25.

 
Estimation of total carbohydrates
       
The per cent carbohydrates were calculated by subtracting the sum of moisture, protein, fat, ash and fiber from 100. Total carbohydrates content is calculated by difference as follows (AOAC, 2005).
 

Total carbohydrates content (%) = 100- [Moisture (%) + Fat (%) + Ash (%) + Protein (%) + Crude fiber (%)]  

 
Pure carbohydrate in food was evaluated as the difference between total carbohydrates and the amount of total crude fiber (USDA, 2017).
 
Estimation of total energy
 
The energy value in kilocalories (kcal) was calculated by the sum of four times the value of total carbohydrate and protein content and nine times fat content according to (AOAC, 2000):
 

Calorific value (kcal/g) = (4 x % Protein) + (9 x %Fat ) + (4 x % carbohydrates) 

Statistical analysis
 
The determined data were analyzed by one-way ANOVA (analysis of variance) to determine whether there is a significant concentration difference within Garden Cress or not. Mean separation between sample categories was computed using least significant difference (LSD) at a 95% confidence interval.

The concentration of essential and non-essential metals in garden cress seed
 
 Statistical analysis results showed that there is a significant difference at (p<0.05) confidence level in the mean concentration of K metal in all sample sites except, the sites of Jage, Wandara, Taba, Hilena and Siyara. As recorded in Table 1, the concentration of K in garden cress seed range from 1179 to 1226.79 mg/100 g. The highest level of K was observed at the Jage site and the least value was observed at Afama site. The source of this significant difference between K concentrations may be the difference in mineral composition of soil or pH of the soil which predicts the degree of mineral absorption by plants. According to USDA national nutrient database, the reported level of K was 1246 mg/100 g (USDA, 2001) which is higher than the K level present study indicating a lower level of K in the edible plant.



Also, the concentration of Ca in garden cress seed range from 249.52 to 291.62 mg/100 g. The highest level of Ca was observed at Taba this could be due to the highest amount of Ca in soil which results from the nature of the parent material. and the least value was observed at Afama. The result of this study showed that the mean concentration of Ca was statistically significantly different between sites. The source of this significant difference between sample means may be the difference in mineral composition of soil or pH of the soil which predicts the degree of mineral absorption by plants. According to USDA national nutrient database, the reported level of Ca was 132 mg/100 g (USDA, 2001) which is higher than the present study.

The concentration of Mg in garden cress seed varied between 314.84  and  341.44 mg/100 g. The highest level of Mg was observed at Jage and the least value was observed at Himbecho which may be due to different levels of Mg-containing minerals in the soil of the study area. As shown in Table 5, one can see that there is a significant difference at (p<0.05) confidence level in the mean concentration of Mg metal in all sample sites except, it was not significantly different at Jage, Taba and Demba Gale. The source of this significant difference between sample means may be the difference in mineral composition of soil or pH of the soil which predicts the degree of mineral absorption by plants. According to USDA national nutrient database, the critical level of Mg is 189 mg/100 g (USDA, 2001) which is lower than the concentration of  Mg in the study crop.

As indicated in Table 2, the concentration of Fe in garden cress seed range from 7.29 to 8.48 mg/100 g. The highest level of Fe was observed at Afama and the least value was observed at Jage. According to USDA national nutrient database, the reported level of Fe was 6.74 mg/100 g (USDA, 2001), which was higher than the present study. The permissible limit of Fe set by WHO/FAO in edible plants was 425.5 mg/kg (FAO/WHO, 2001). The comparison of Fe level with the present study was lower than the permissible level of Fe in an edible plant.



As the recorded value of the concentration of Cu in garden cress seed range from 3.72 to 5.44  mg/100 g. The highest level of Cu was observed at Himbecho and least value observed at Hilena. As shown in Table 2, there is a significant difference at (p<0.05) in the mean concentration of Cu metal in Taba, Hilena and Himbecho sample sites. Also, the concentration of Zn in garden cress seed range from 5.80 to 6.68 mg/100 g. The highest level of Zn was observed at Wandara and the least value observed at Siyara. According to USDA national nutrient database, the reported level of Zn was 2.68 mg/100 g (USDA, 2001), which was higher than it in the present study. The permissible limit of Zn set by WHO/FAO for the edible plant was 99.4 mg/kg (FAO/WHO, 2001) indicating the Zn level in the present study was higher than the permissible level of Zn in an edible plant.
 
Proximate composition determined in garden cress sample               
 
Table 2, showenthat the average protein content ranged between 19.68 and 22.87%. The observed variation in protein content of linseed could be a result of the differences in environment and the type of soil in which it grows. The higher protein percentage was recorded at Siyara and least at Damot Gale. It can see that there is a significant difference at (p<0.05) confidence level in the mean concentration of protein in most sites. But it was not significantly different at Jage, Demba Gale and Afama sample sites. The recorded protein percentage in the garden cress seed sample was related to the value of protein reported by other researchers e in garden cress seed (Gokavi et al., 2004) which was about  24.2% and 22.4, respectively.

The average fat content ranged between 20.7 to 24.17% in garden cress seed in this study. The higher fat percentages were recorded at Jage and least at Hilena sites. The mean concentration of fat was significantly different at all sites except Jage.

The data of crude fiber in Table 3, showed that the average fiber content range between 7.6to 10.27%. The higher fiber percentage was recorded at Hilena and least at Gara. The mean concentration of fat was significantly different at Wandera, Taba, Hilena, Siyara and Gara. And it was not significantly different at Jage, Demba Gale, Himbecho and Afama. The recorded fiber percentage in the garden cress seed sample was related to the researcher value of protein percentage in garden cress seed (Gokavi et al., 2004) which was about 7.01  and 11.9% respectively.



The result of this study showed that the average moisture content range from 1.37% to 2.59%. The higher moisture content was recorded at Afama and least at Jane. The mean concentration of fat was not significantly different at Jage, Wandara, Taba, Hilena, Siyara and Gara. And it was very different at Demba Gale, Himbecho, Gara and Afama. The recorded moisture percentage in the garden cress seed sample was related to the researcher’s value of moisture percentage in garden cress seed (Gokavi et al., 2004) which was about 2.9%.

Table 4 showed that the average ash content ranged between 3.44 and  4.63%. The higher ash percentage was recorded at Afama and least at Wandara. The mean concentration of ash was significantly different at Hilena, Himbecho and Gara. The recorded ash percentage in the garden cress seed sample was related  to the  value of protein percentage reported in garden cress seed by Abd El-Salam et al. (2019)  which was about 4.8% and 4.65%’ respectively.



The average Carbohydrate content ranged between 40.56  and  43.24%. The higher Carbohydrate percentage was recorded in Jage district and least at Siyara. The mean concentration of carbohydrates was significantly different at Jage and Demba Gale.

The energy values showed that the average energy content ranged from 332.96  to 344.19%. The higher energy percentage was recorded in Jage district and least at Wandara. The recorded energy in the garden cress seed sample was related to the researcher’s value of protein percentage in garden cress seed (Abd El-Salam et al., 2019; Gokavi et al., 2004).

Garden cress seed contained (1.37-2.59)% of moisture, (19.68-22.87)% of crude protein, (20.72-24.17)% of crude fat, (40.46-43.24)% of carbohydrate, (8.3-10.27)% crude fiber, (3.44-4.63)% ash and (332.96-344.19) of energy in Kcal/100 g. These results show that the macronutrients are considerably high and suitable for human nutrition.

From this study, it can be concluded that the Garden cress seeds grown in the study area were rich in proteins, fats, crude fiber and carbohydrates. Also, essential metals in it were found at a sufficient level and there was no risk of toxic metals.

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