Comparative Studies on Nutrient and Anti– nutrient Composition of Carrot ( Daucus carota L.) and Cucumber ( Cucumis sativus L.)

: Carrot ( Daucus carota L.) and cucumber ( Cucumis sativus L.) are underutilized root vegetable and fruit belonging to the Apiaceae and Cucurbitaceae family, respectively. A comparative study was carried out on proximate composition, amino acid profile and anti–nutritional factors of Daucus carota and Cucumis sativus . The proximate composition values (%) for Daucus carota and Cucumis sativus were found to be as follows: Moisture (5.06 and 4.39), ash (7.75 and 15.26), crude fat (6.09 and 4.83), crude fibre (13.04 and 18.25), crude protein (9.39 and 14.39) and carbohydrate by difference (58.67 and 42.90). The calculated fatty acids and metabolizable energy values were 4.87 and 3.86%; 1382.35 and 1152.64 kJ 100/g, respectively. The amino acid profiles revealed that Daucus carota and Cucumis sativus contained nutritionally useful quantities of most of the essential amino acids. The total amino acid (TAA), total essential amino acid (TEAA) (with His), total sulphur amino acid (TSAA) and essential aromatic amino acid (EArAA) for the Daucus carota and Cucumis sativus samples were 82.36 and 64.14; 22.93 and 30.11; 1.26 and 1.71; 2.13 and 2.66, respectively. However, supplementation of essential amino acids is required in a dietary formula based on the flour samples of Daucus carota and Cucumis sativus when comparing the EAAs in this report with the recommended FAO/WHO provisional pattern. The first limiting EAA in both samples was Met and Cys (TSAA). The antinutrient contents of Daucus carota and Cucumis sativus were also found to be as follows: Oxalate (241.67 and 142.45 mg/100 g), saponin (0.22 and 0.91%), alkaloids (2.85 and 2.23%), tannins (329.03 and 254.45 mg/100 g), cyanide (4.01 and 3.03 mg/100 g) and phytate (616.41 and 349.62 mg/100 g). These antinutritional factors have been shown to be deleterious to health or evidently advantageous to human and animal health if consumed at appropriate amounts.


Introduction
Vegetables are parts of plants that are consumed by humans or other animals as food. The original meaning is still commonly used and is applied to plants collectively to refer to all edible plant matter, including the flowers, fruits, stems, leaves, roots, and seeds. The alternate definition of the term vegetable is applied somewhat arbitrarily, often by culinary and cultural tradition. It may exclude foods derived from some plants that are fruits, nuts, and cereal grains, but includes fruits from others such as tomatoes, scourgettes and seeds such as pulses [1].
Originally, vegetables were collected from the wild by hunter-gatherers and entered cultivation in several parts of the world, probably during the period 10,000 BC to 7,000 BC, when a new agricultural way of life developed. At first, plants which grew locally would have been cultivated, but as time went on, trade brought exotic crops from elsewhere to add to domestic types. Nowadays, most vegetables are grown all over the world as climate permits, and crops may be cultivated in protected environments in less suitable locations. China is the largest producer of vegetables, and global trade in agricultural products allows consumers to purchase vegetables grown in faraway countries. The scale of production varies from subsistence farmers supplying the needs of their family for food, to agribusinesses with vast acreages of single-product crops. Depending on the type of vegetable concerned, harvesting the crop is followed by grading, storing, processing, and marketing [2].
Vegetables can be eaten either raw or cooked and play an important role in human nutrition, being mostly low in fat and carbohydrates, but high in vitamins, minerals and dietary fiber. Many nutritionists encourage people to consume plenty of fruit and vegetables, five or more portions a day often being recommended. They supply dietary fiber and are important sources of essential vitamins, minerals, and trace elements. Particularly important are the antioxidant vitamins A, C, and E. When vegetables are included in the diet, there is found to be a reduction in the incidence of cancer, stroke, cardiovascular disease, and other chronic ailments [1]. Research has shown that, compared with individuals who eat less than three servings of fruits and vegetables each day, those that eat more than five servings have an approximately twenty percent lower risk of developing coronary heart disease or stroke. The nutritional content of vegetables varies considerably; some contain useful amounts of protein though generally they contain little fat [2] and varying proportions of vitamins such as vitamin A, vitamin K, and vitamin B6; provitamins; dietary minerals; and carbohydrates. However, vegetables often also contain toxins and antinutrients which interfere with the absorption of nutrients. These include α-solanine, α-chaconine [3], enzyme inhibitors (of cholinesterase, protease, amylase, etc.), cyanide and cyanide precursors, oxalic acid, tannins and others.
Carrot (Daucus carota L.) is a root vegetable, usually orange in colour, though purple, black, red, white and yellow cultivars exist [4]. Carrots are a domesticated form of the wild carrot, Daucus carota, native to Europe and southwestern Asia. The plant probably originated in Persia and was originally cultivated for its leaves and seeds. The most commonly eaten part of the plant is the taproot, although the stems and leaves are eaten as well. The domestic carrot has been selectively bred for its greatly enlarged, more palatable, less woody-textured taproot. Cucumber (Cucumis sativus L.) is a widely cultivated plant in the gourd family, Cucurbitaceae). It is a creeping vine that bears cucumiform fruits that are used as vegetables. The fruits are eaten raw with the seeds and peel. There are three main varieties of cucumber: slicing, pickling and seedless. Within these varieties, several cultivars have been created. In North America, the term ''wild cucumber'' refers to plants in the genera Echinocystis and Marah, but these are not closely related. The cucumber is originally from South Asia, but now grows on most continents. Many different types of cucumber are traded on the global market [5]. A lot of investigations have been carried out on the chemical composition, effects of processing, lipid profiles and biochemical functions of carrot (Daucus carota) and cucumber (Cucumis sativus) which are available in the literature [6][7][8][9]. Therefore, the purpose of this work is to comparatively study the proximate, amino acids and antinutrients of Daucus carota and Cucumis sativus grown in Nasarawa State, Nigeria as contributors to the availability of nutritionally available amino acids as food.

Materials and Methods Collection of samples
Fresh samples of carrot (Daucus carota L.) and cucumber (Cucumis sativus L.) were collected from a farm in Wukari local government area of Taraba State, Nigeria. These samples were identified in the Biology laboratory of Federal University Wukari, Nigeria.

Preparation and treatment of samples
The fresh samples of both carrot (Daucus carota L.) and cucumber (Cucumis sativus L.) were washed in clean water and sliced with a kitchen knife. The samples were then placed on two separate trays and sun-dried for five days. During this sun-drying, the samples were covered with fine meshes to trap particles of dirt or foreign bodies that may contaminate them. The dried samples of Daucus carota and Cucumis sativus were further oven-dried (between 75 to 105 o C) in the laboratory to a constant weight for three days in order to completely get rid of any moisture. These moisture-free samples were first pounded using pestle and mortar, then ground with a grinder, sieved through a sieve of size 0.5 µm and the powdered samples were stored in two separate well labelled air-tight plastic containers before finally taken for analyses (Fig. 3).

Proximate Analysis
The ash, crude fat, moisture, crude protein (N x 6.25), crude fibre and carbohydrate (by difference) were determined in accordance with the standard methods of AOAC [10]. All proximate analyses of the sample flours were carried out in triplicate and reported in percentage. All chemicals were of Analar grade.

Amino Acid Analysis
The amino acid analysis was by Ion Exchange Chromatography (IEC) using the Technico Sequential Multisample (TSM) Amino Acid Analyzer (Technicon Instruments Corporation, New York). The period of analysis was 76 min for each sample. The gas flow rate was 0.50 mLmin -1 at 60 o C with reproducibility consistent within ± 3%. The net height of each peak produced by the chart recorder of the TSM (each representing an amino acid) was measured and calculated. Amino acid values reported were the averages of two determinations. Norleucine was the internal standard. Tryptophan was determined after alkali (NaOH) hydrolysis by the colorimetric method.

Determination of Isoelectric Point (pI), Quality of Dietary Protein and Predicted Protein Efficiency Ratio (P-PER)
The predicted isoelectric point was evaluated according to Olaofe and Akintayo [11]: The quality of dietary protein was measured by finding the ratio of available amino acids in the sample protein compared with the needs expressed as a ratio. Amino acid score (AAS) was then estimated by applying the formula [12]: The predicted protein efficiency ratio (P-PER) of the seed sample was calculated from their amino acid composition based on the equation developed by Alsmeyer et al. [13] as stated thus;

Anti-nutritient Content Determination
The contents of tannin, alkaloid, saponin, phytate, oxalate and cyanide were determined on each of the sample flours by methods described by some workers [14].

Statistical Analysis of the Samples
The fatty acid values were obtained by multiplying crude fat value of each sample with a factor of 0.8 (i.e. crude fat x 0.8 = corresponding to fatty acids value. The energy values were calculated by adding up the carbohydrate x 17 kJ, crude protein x 17 kJ and crude fat x 37 kJ for each of the samples. Errors of three determinations were computed as standard deviation (SD) for the proximate composition.

Results and Discussion
The proximate composition of Daucus carota and Cucumis sativus is displayed in Table 1. The moisture content of Daucus carota (5.06%) and Cucumis sativus (4.39%) were all within the recommended dietary allowance (RDA) (3 -10%) [15]. These contents are higher when compared with some legumes; bambara groundnut (1.7 ± 0.51%) and kersting's groundnut (1.7 ± 0.12%) [16]. High moisture content in food is important to act as a solvent to aid in all biochemical reactions and physiological activities during digestion. However, foods with high moisture contents are prone to easy microbial spoilage and subsequent short shelf life [17][18][19]. Moderate moisture content of < 12% is preferred for shelf stability of food on long storage [20]. Ash content is a measure of mineral content of food. The results indicate that there were more minerals in Cucumis sativus (15.26%) than in Daucus carota (7.75%); the values of ash content are high compared to those reported for some leafy vegetables such as Solanium nodiflorum (ogumo) (2.67%) [21] and Basella albs L. (Indian spinach). Both samples have ash values higher than the lowest RDA value of 6%. Crude fat content of Daucus carota and Cucumis sativus were 6.09 and 4.83%, respectively. This does not qualify the samples as oil-rich vegetables, since their crude fat content are low compared with soybean (22.8 -23.5%) [22,23] and pumpkin [24,25] grown in Nigeria. The crude fat content obtained in this report are fairly high when compared with values reported in some leafy vegetables such as bush buck (3.51%) and scent leaf (4.02%) [26]. Crude protein values for Daucus carota and Cucumis sativus were 9.39 and 14.39%, respectively. The value obtained for Cucumis sativus is more than what was reported for some leafy vegetables such as Momordica balsamina (11.29%) and Lesianthera africiana leaves (13.10%) [26]. Plant foods that provide more than 12% of their calorific value from protein have been shown to be good sources of protein [27]. This shows that Cucumis sativus is more of a good source of protein than Daucus carota L. Crude fibre is a significant component in the body. It increases stool bulk and decreases the time that waste materials spend in the gastrointestinal tracks [28]. Crude fibre in the diet consists mostly of the plant polysaccharides that cannot be digested by human dietary enzymes such as cellulose, hemicelluloses and some materials that make up the cell wall [29]. The crude fibre content values obtained in Daucus carota (13.04%) and Cucumis sativus (18.25%) exceed that of T. triangulare (2.40%), T. occidentalis (1.7%) and C. argentea (1.8%) [30]. Therefore, the consumption of Daucus carota and Cucumis sativus may be advantageous since high fibres content of foods help in digestion, prevention of colon cancer and in the treatment of diseases such as obesity, diabetes and gastrointestinal disorders [31]. High fatty acid value in oil indicates that the oil may not be suitable for use in cooking (edibility), but however, be useful for industrial purposes [33]. The high metabolizable energy values obtained showed that the samples have energy concentration more favourable than cereals [34][35][36]. However, the values are lower than those reported for some legumes such as bambara groundnut (1691.3 kJ/100 g) and kersting's groundnut (1692.9 kJ/100 g) [16], and red kidney bean (1678.4 kJ/100 g) [37].
The result of amino acid composition of Daucus carota and Cucumis sativus is shown in Table 2.
(together make up 13.18 g/100 g cp) and those for Cucumis sativus (together make up 17.79 g/100 g cp) are the most abundant amino acids in the two samples. Some workers [16,33,39,41,43,44] had similar observation. Arginine constituted the highest single essential amino acid (EAA) in both the Daucus carota and Cucumis sativus samples (3.61 and 5.33 g/100 g cp). Arginine is an AA acid for children growth [33]. The least amino acid was tryptophan (0.37 and 0.73 g/100 g cp) in Daucus carota and Cucumis sativus, respectively. The calculated isoelectric points (pI) were 2.98 for Daucus carota L. and 3.76 for Cucumis sativus. This is useful in predicting the pI for protein in order to enhance the quick precipitation of protein isolate from biological samples [11]. The predicted protein efficiency ratio (P -PER) is one of the quality parameters used for protein evaluation [12]. The P -PER in this report for Cucumis sativus (1.76) is higher than the reported P -PER values of Lathyrus sativus L. (1.03) [23], but lower than those reported by [37] (2.5) for red kidney bean. Moreover, the P -PER value obtained for Daucus carota (0.95) in this study is lower than that of Phaseolus coccineus (1.91) [45]. Chemical, biochemical and pathological observations in experiments conducted in human and laboratory animals showed that high leucine in the diet impairs the metabolism of tryptophan and niacin, and it is responsible for niacin deficiency in sorghum eaters [46]. High leucine is also a factor contributing to the pellagragenic properties of maize [47]. This study suggests that the leucine/isoleucine balance is more important than dietary excess of leucine alone. The nutritive value of a protein depends primarily on the capacity to satisfy the needs for nitrogen and essential amino acids [48].  [41] and tiger nut (41.21 g/100 g cp) by [40]. Nevertheless, the TEAA contents (%) of Cucumis sativus L. in this report are well above the 39% considered to be adequate for ideal protein food for infants, 26% for children and 11% for adults [49]. The concentrations of total sulphur amino acids (TSAA) were 1.26 and 1.71 g/100 g cp are lower than the 5.8 g/100 g cp recommended for infants [49]. The values of essential aromatic acids (EArAA) (2.13 -2.66 g/100g cp) are also lower than the ideal range suggested for infant protein (6.8 -11.8 g/100 g cp) [49]. The total acidic amino acids (TAAA) (13.18 and 17.79 g/100 g) are found to be greater than the total basic amino acids (TBAA) (7.44 and 11.58 g/100 g) in both samples indicating that their protein is probably acidic in nature (Aremu et al., 2012). The percentage ratios of TEAA with His to TAA in the samples were 27.84 and 46.94% for Daucus carota and Cucumis sativus, respectively. These values are low compared to that of egg (50%) [12], scarlet runner bean (48.31%) [45], Anarcadium occidentale (47.19%) [33].  The essential amino acid contents in this report are lower than the [12] recommended pattern. Thus by implication, dietary formula based on the flour samples of Daucus carota L. and Cucumis sativus L. will require all the essential amino acids supplementation for both samples. It has been reported that EAAs most often acting in a limiting capacity are Met (and Cys), Lys and Try [49]. The first limiting EAA in Daucus carota and Cucumis sativus was Met + Cys (TSAA) with values of 0.36 and 0.49, respectively.  Table 4.2); AAS = Amino Acid Score  [50]. Saponin has been shown to possess both deleterious properties and to exhibit structure-dependent biological activities [51]. Saponins in high concentrations, impart a bitter taste and stringent in dietary plants. The bitter taste of saponin is the major factor that limits its use. Saponins cause hypocholesterolaemia by binding cholesterol, making it unavailable for absorption. They also cause haemolysis of red blood cells and are toxic to rats [52]. Saponins from Bulbostermma paniculatum and Pentapamax leschenaultii have also been demonstrated to have anti-spermal effects on human spermatozoa [53,54]. They significantly inhibited acrosine activity of human sperms and the spermicidal effect was attributed to strong damage of the spermal plasma membrane [52].
The levels of tannin in the Daucus carota and Cucumis sativus samples were 329.03 mg/100 g and 254.45 mg/100 g, respectively. The tannin content in Cucumis sativus is low when compared to the value for Daucus carota (329.03 mg / 100 g) in this work ( Table 5). The nutritional effects of tannins are mainly related to their interaction with protein due to the formation of complexes [53]. Tannin-protein complexes are insoluble and protein digestibility is decreased. Tannin acid may decrease protein quality by decreasing digestibility and palatability. Other nutritional effects which have been attributed to tannin include damage to the intestinal tract, interference with the absorption of iron and a possible carcinogenic effect [56]. The alkaloid content in this study was 2.85% in Daucus carota. and 2.23% in Cucumis sativus. These values are higher than those obtained by [57] for black turtle bean (1.6, 1.8%). Alkaloids are considered to be antinutrients because of their action on the nervous system, disrupting or inappropriately augmenting electrochemical transmission. For instance, consumption of high tropane alkaloids will cause rapid heartbeat, paralysis and in fatal case, lead to death. Uptake of high dose of tryptamine alkaloids will lead to staggering gate and death. Indeed, the physiological effects that alkaloids have on humans are very evident. Cholinesterase is greatly inhibited by glycoalkaloids, which also cause symptoms of neurological disorder. Other toxic action includes disruption of the cell membrane in the gastrointestinal tract [58]. Phytate (a salt form of phytic acid) content was 616.41 mg/100 g in Daucus carota and 349.62 mg/100 g in Cucumis sativus. These values obtained for both samples are higher than the value obtained for sesame seed (25.96 mg/100 g) [59] but that for Cucumis sativus L. ( 349.62 mg/100 g) is lower than values reported for soybean (4050 mg/100 g), pigeon pea (1170 mg/100 g) and cowpea (2040 mg/100 g) [60] and black turtle bean (11250 mg/100 g) [57]. Phytic acid is an important storage form of phosphorus in plant, it is insoluble and cannot be absorbed in human intestine and it has 12 replaceable hydrogen atoms with which it could form insoluble salts with metals such as calcium, iron, zinc and magnesium. The formation of these insoluble salts renders the metals unavailable for absorption into the body. Phytate can also affect digestibility by chelating with calcium or by binding with substrate or proteolytic enzyme. Phytate is also associated with cooking time in legumes [57,60]. Oxalate is an anti-nutrient which under normal conditions is confined to separate compartments. However, when it is processed and/or digested, it comes into contact with the nutrients in the gastrointestinal tract [61]. When released, oxalic acid binds with nutrients, rendering them inaccessible to the body. If food with excessive amounts of oxalic acid is consumed regularly, nutritional deficiencies are likely to occur, as well as severe irritation to the lining of the gut [62]. Oxalate values presented in this report were 241.67 mg/100 g in Daucus carota and 142.45 mg/100 g in Cucumis sativus. Both samples of Daucus carota and Cucumis sativus have oxalate values lower than 254 mg/100 g for soybean and 286 mg/100 g for pigeon pea [63], 225740 mg/100 g found in red kidney bean and 166890 mg/100 g found in black turtle bean [57]. Oxalate is produced and accumulated in many crop plants and pasture weeds. Cyanogens are glycosides of a sugar, sugars and cyanide containing aglycone. Cyanogens can be hydrolyzed by enzymes to release a volatile cyanide gas. Excess cyanide inhibits the cytochrome oxidase, the final step in electron transport, and thus blocks ATP synthesis and so tissues suffer energy deprivation and death follows rapidly. Prior to death, symptoms include faster and deeper respiration, a faster irregular and weaker pulse, salivation and frothing at the mouth, muscular spasms, dilation of the pupils, and bright red mucous membranes [64]. High level of HCN has been implicated for cerebral damage and lethargy in man and animal. The cyanide values found in this work were between 4.01 -3.03 mg/100 g for Daucus carota and Cucumis sativus, respectively.

Conclusion
The proximate, amino acid and anti-nutrient compositions of Daucus carota and Cucumis sativus are presented in this study. The study showed that Daucus carota and Cucumis sativus have moderate fat and protein contents, and useful amino acids expected for infants. There are also potential health beneficial constituents to be derived from the incorporation of these vegetables into diets and this indicates the need for their exploitation in seeking optimum health benefits of the fruits for the populace. The study indicates that Daucus carota and Cucumis sativus may be better sources of some of the essential nutrients than some of the members of the family to which they belong. However, some of the antinutrient contents may pose nutritional problems in their consumption.