THE EFFECT OF PROCESSING ON THE NUTRIENT AND ANTI-NUTRIENT CONSTITUENTS OF TWO SPECIES OF POTATOES: SOLANUM TUBEROSUM AND IPOMEA BATATAS

THE EFFECT OF PROCESSING ON THE NUTRIENT AND ANTI-NUTRIENT CONSTITUENTS OF TWO SPECIES OF POTATOES: SOLANUM TUBEROSUM AND IPOMEA BATATAS

 

 

ABSTRACT

Effect of two cooking methods (boiling and frying) on the nutrients and anti-nutrients of Irish potato and two varieties of Sweet potatoes (white flesh (Wf) and orange flesh (Of)) were evaluated in this work using standard techniques. The percentage proximate analysis of the raw samples revealed that the moisture (85.44 ± 0.78%), fats (1.04 ± 0.03%) and protein (2.10 ± 0.03%) contents of the Irish potato sample were significantly (p ≤ 0.05) higher compared to that of the sweet potatoes. While the sweet potatoes had a higher carbohydrate (25.99 ± 1.22%: Wf and 29.08 ± 1.46%: Of) and fibre (1.28 ± 0.03%: Wf and 1.39 ± 0.02%: Of) contents compared to the Irish potato. The white flesh sweet potato had the highest ash content (1.71 ± 0.03%) which was significantly (p ≤ 0.05) higher than others. The mineral analysis of these potatoes showed that the calcium (144.04 ± 1.69mg/100g) and magnesium (64.12 ± 1.75mg/100g) contents of the Irish potatoes were significantly (p ≤ 0.05) higher compared to the two varieties of sweet potato. The sweet potatoes had significantly (p ≤ 0.05) higher iron content (4.91 ± 0.03mg/100g: Wf and 3.97 ± 0.05mg/100g: Of) than the Irish potato (2.98 ± 0.04mg/100g). The orange flesh sweet potato sample had the highest potassium (127.74 ± 2.66mg/100g) and sodium (137.28 ± 0.53mg/100g) contents, while the white flesh sample had the highest selenium content (0.91 ± 0.03mg/100g) compared to the others. The Irish potato had the highest zinc content although it was not significantly (p ≥ 0.05) higher than that of the orange flesh sweet potato sample. The result of the vitamin analysis revealed that these potatoes have an appreciable amount of vitamin C. The orange flesh sweet potato had the highest contents of vitamin A (0.614 ± 0.017mg/100g), B1 (0.172 ± 0.006mg/100g), B9 (0.043 ± 0.005mg/100g), C (24.840 ± 1.721mg/100g) and K (0.152 ± 0.004mg/100g) compared to the other two potatoes, while the white flesh variety had the highest content of B2 (0.527 ± 0.005mg/100g) and E (0.884 ± 0.005mg/100g). Result of the anti-nutrient constituents analyses showed that the Irish potato had the highest oxalate content (2.11 ± 0.04g/100g), the white flesh sweet potato had the highest phytate content (1.81 ± 0.03mg/100g) and the orange flesh variety had the highest hydrogen cyanide content (0.76 ± 0.02mg/100g) compared to the other two potatoes. Boiling and frying had varying effects on the different nutrients and anti-nutrients of these potatoes. Boiling increased the moisture content of the potatoes while frying decreased it. Comparing the fried and boiled samples, the fried potato samples had a significantly (p ≤ 0.05) higher fat and carbohydrate contents while the boiled samples had significantly (p ≤ 0.05) lower contents. The fibre content of all the potatoes showed a significant (p ≤ 0.05) decrease after processing. The protein content of the processed Irish potatoes and the fried white flesh sweet potato sample were significantly (p ≤ 0.05) higher, while those of the orange flesh and boiled white flesh sweet potato were significantly (p ≤ 0.05) lower. Cooking had a decreasing effect on the potassium, calcium, iron and selenium contents of the potatoes and an increasing effect on their magnesium and sodium contents, while the zinc content was hardly affected. Frying had a more retaining effect on most of the minerals. The cooked potatoes had a significantly (p ≤ 0.05) higher vitamin A content compared to the raw samples with the fried potatoes having significantly (p ≤ 0.05) higher content compared to the boiled samples. The boiled and fried potatoes had a lower vitamin B1, B2, B9, C, and K contents compared to the raw samples. Vitamin E was significantly (p ≤ 0.05) higher and lower in the fried and boiled potatoes respectively. The anti-nutrient results of the boiled and fried tubers showed that their oxalate and phytate contents were significantly (p ≤ 0.05) lower compared to the unprocessed potatoes.

CHAPTER ONE

INTRODUCTION

In recent years, there has been an increased interest in the role of food in health, and effect of processing on the nutritive value of food. Together with government, households play a central role in ensuring that the food consumed is healthy, nutritious, and safe (Howard et al., 2013). Cooking is the art of preparing food for consumption with the use of heat which transform an unpalatable, or hardly edible raw material into a safe, flavourful, nutritious, stable, and enjoyable food (Floros et al., 2010). Nutrient loss is a consequence of nearly every cooking process (Zhang et al., 2011). Exposure to heat, light or oxygen will alter the nutrients found in food, and methods that involve water often reduce the amounts of nutrients as these get ‘washed out’ and left behind. However, cooking foods also has its advantages, including a reduction of the number of possible pathogenic microbes, increase in digestibility and increased availability of certain nutrients (Shahnaz etal., 2003; Miglio et al., 2008).

Root crops and tubers are used in the preparation of the major staples eaten in Nigeria and are estimated to contribute about 15% of the total calories and 8% of the total protein in the daily diet of the average Nigerian (Odebunmi et al., 2007).

Sweet potato (Ipomea batatas) is an important root crop grown all over the world and consumed as a vegetable (Oke and Workneh, 2013). Sweet potato currently ranks as the most important food crop on a fresh-weight basis in developing countries after rice, wheat, maize, and cassava (Ezeano 2010). It is a household food security crop for many people (Lyimo et al., 2010). It’s associate, Irish potato (Solanum tuberosum) is an edible tuber which is actually native to South America, not Ireland. They are also known as white potato in some places. Irish potatoes are one of the major sources of starch worldwide. It is a major vegetable throughout the world (Gumul et al., 2011).

Like many other foods, roots and tubers are rarely eaten raw. They normally undergo some form of processing and cooking before consumption (Lyimo et al., 2010). The basic purpose of these processing and cooking is to make tubers and their products more palatable and digestible, safe for human consumption, extend shelf life and provide a variety of products, which are more convenient to prepare, cook and consume than the raw tuber (Stephen et al., 2005). In Nigeria, there is little sweet potato processing, more than half of the sweet potato produced is either boiled or fried (Odebode et al., 2008). Research studies have revealed that these two root crops (Irish and sweet potatoes) possess a lot of nutrients (Camire et al., 2009; Mitra, 2012). However, the rate at which cooking processes reduce or enhance the availability of these nutrients is not well understood and so deserves investigation.

1.2       SWEET POTATO

1.2.1    Description

The sweet potato (Ipomea batatas) is a dicotyledonous plant that belongs to the family Convolvulceae. It is a root vegetable. The plant is an herbaceous perennial vine with trailing or twining stems up to 4 m long, which sends roots into the soil at the nodes. The general form of the leaf is heart-shaped. The flowers have purplish throats and white margins, resembling those of the morning glory. The roots are large, fleshy, edible storage roots formed on the underground stem nodes. The tubers are also variable in shape, size and colour. Some are long and cylindrical, others short, thick and rounded at the ends. Latex occurs in all parts of the plant. There are many varieties of sweet potato. They differ in skin colour and flesh colour. The skin can be white, light yellow, red, orange or purple while the flesh can be white, lavender, orange or purple depending on the variety (Huaman 1992). The different varieties are not equally nutritious, some are more nutritious than others (Lyimo et al., 2010; Ingabire and Vasanthakaalam, 2011). Orange flesh sweet potatoes are rich in β-carotene the precursor for vitamin A (Mitra, 2012).

 

 

 

Plate 1: Leaves, flowers and roots of sweet potato plant

 

1.2.2    Classification

Family:            Convolvulaceae

Tribe:               Ipomoeae

Genus:             Ipomoea

Sub-genus:      Quamoclit

Section:           Batatas

Species:           Ipomoea batatas (L.) Lam.

1.2.3    Origin and Natural Habitat

Sweet potato originated from tropical Central America. It is cultivated as a perennial in tropical and subtropical lowland agro- ecologies, although it is well adapted to other zones and can be grown over widely different environments. Among the root and tuber crops, it is the only one that has a positive per capita annual rate of increase in production in sub-Saharan Africa (Bashaasha and Mwanga, 1992). Its ease of cultivation and ability to thrive even under harsh conditions promote its spread in Africa.

Sweet potato has the following advantages over other root crops.

• Low demand on soil nutrient.
• Tolerance of drought
• Capability of providing reasonable yields in agro-ecological zones where other crops would fail.
• Low requirements for external inputs such as fertilizer
• Flexibility in planting and harvesting period (Odebode et al., 2008)

1.2.4    Sweet Potato in Nigeria

Sweet potato is considered as an important food security crop, especially in Nigeria and is also identified as the least expensive, year round source of dietary vitamin A, especially the orange-fleshed type among the poor (Adepoju and Adejumo, 2015). The production, marketing and utilization of sweet potato have expanded in the last decade to almost all ecological zones of Nigeria, presently between 381,000 and 510,000ha of land area under sweet potato cultivation in Nigeria (FAO 2008).  Yields have increased from farmers’ pre-research era of 2-3 tones/ha to 30-40 tones due to the availability of improved varieties (Odebode, 2004).

Table 1: Sweet potato growing states in Nigeria and various forms of utilization

• = Sweet potato Growing States

Source: (Odebode, 2004).

1.2.5    Nutritional and Health Benefits

Sweet potato is an extremely versatile and delicious vegetable that possesses high nutritional value. It is also a valuable medicinal plant having anti-cancer, antidiabetic, and anti-inflammatory activities (Mohanraj and Sivasankar, 2014). The tubers are packed with many essential vitamins such as pantothenic acid, pyridoxine, thiamine, niacin, riboflavin, vitamin A, vitamin C and also high in minerals such as potassium, calcium, zinc, magnesium, sodium, phosphorus, manganese and iron (Aywa et al., 2013, Oke and Workneh, 2013). They are also an excellent source of fibre, which is important in reducing blood cholesterol and aid digestion (Adepoju and Adejumo, 2015). Several recent studies have shown the superior ability of sweet potatoes to raise blood levels of vitamin A (Anderson et al., 2007; Burri, 2011; Egbe, 2012; Mitra, 2012). Sweet potatoes are now being used in Africa to combat widespread vitamin A deficiency in 250,000 – 500,000 children. About two-thirds of the children developing xerophthalmia, resulting from lack of vitamin A, die within a year of losing their sight. The strategy of increasing orange flesh sweet potato consumption helps to alleviate vitamin A deficiency (Anderson et al., 2007; Egbe, 2012). According to Panda and Sonkamble (2012), sweet potatoes have been reported to possess antioxidant, anti-diabetic, wound healing, anti-ulcer, anti-bacterial and anti-mutagenic properties. They are also used as an immune booster and for relief of gastrointestinal and upper respiratory symptoms.

1.3       IRISH POTATO

1.3.1    Description

Solanum tuberosum is a herbaceous plant that grows to 0.4-1.4 m tall and may range from erect to fully prostrate (Spooner and Knapp, 2013). Stems range from nearly hairless to densely hairy and may be green, purple, or mottled green and purple. Leaves are pinnate with a single terminal leaflet and three or four pairs of large, ovoid leaflets with smaller ones in between (Struik, 2007; Spooner and Knapp, 2013). The blades range in size from 8-22 x 5-13 cm with the petioles ranging from 2-6 cm. They bear white, pink, red, blue, or purple flowers with yellow stamens depending on the variety. Potato plants produce rhizomes (often called stolons) that have rudimentary leaves and are typically hooked at the tip. They originate from the basal stem nodes, typically below ground, with up to three rhizomes per node (Struik, 2007). The flesh of the tubers varies in colour from white to yellow to blue and the skin varies from white through yellow to tan and from red through blue. The colour of the flesh may or may not correspond to the colour of the skin (Spooner and Salas, 2006). On the surface of the tuber are axillary buds with scars of scale leaves that are called eyes (Struik, 2007). When tubers are planted, the eyes develop into stems to form the next vegetative generation.

 

a: Tubers                                                          b: leaves and flowers

Plate 2: Tubers, leaves and flowers of Irish potato

1.3.2    Classification

Kingdom:        Plantae (plants)

Subkingdom:   Tracheobionta (vascular plants)

Superdivision: Spermatophyta (seed plants)

Division:          Magnoliophyta (flowering plants)

Class:               Magnoliopsida (dicotyledons)

Subclass:         Asteridae

Order:              Solanales

Family:            Solanaceae

Subfamily:       Solanoideae

Genus:             Solanum L.

Section:           Petota

Subsection:      Potatoe

Series:              Tuberosa

Species:           Solanum tuberosum L.

Solanum tuberosum belongs to the Solanaceae family. This family includes, among 2000 other species, tomato (S. lycopersicum L.), sweet pepper (Capsicum annuum L.), eggplant (S.melongena L. var. esculentum) and others.

 

1.3.3    Origin

Irish potato (Solanum tuberosum) is said to have originated from the highland of Bolivia in South America (Martin and Leonard, 1949; Ifenkwe, 1989). The spread of the crop outside its centre of origin was mainly by deliberate introduction. The crop moved out of South America to Spain in 1570; to England in 1585; then to Ireland by Spanish explorers from 1588. The crop was grown on a large scale in Ireland and became so popular such that it acquired the misnomer of “Irish Potato” (Zemba et al., 2013).

1.3.4    Irish potato in Nigeria

Irish potato was introduced into Nigeria early in the 20th Century by European miners in Jos Plateau (Okonkwo et al., 1995). Jos Plateau has high altitude and thus, cool climate, which is favourable for the development of the crop. Jos South Local Government Area accounts for 25% of the total Irish potato produced in Nigeria (Okonkwo et al., 1986; Wuyep 2012). Plateau State could be tagged the home of Irish potatoes because of the chilled and favourable weather condition that favours the cultivation of the root crop. It is also exported from Plateau State to neighbouring countries of Ghana, Ivory Coast, Cotonou and Niger Republic. A brief analysis of potato production in various countries of Africa reveals that Egypt is Africa’s number one potato producer, followed by Malawi. Although Nigeria is known to be the fourth biggest producer of potato in Sub- Saharan Africa, it is the seventh biggest producer of potato in Africa (Ugonna et al., 2013).

1.3.5    Nutritional and Health Benefits

Carbohydrates are the major constituents of potato. It also contains essential nutrients such as proteins and minerals like calcium, phosphorus and iron, and vitamins (Bembem and Sadana, 2013). Since potatoes are consumed as a main vegetable in the developing countries, they form an important source of antioxidants (Brown, 2005; Marwaha et al., 2007). Many studies have shown that potatoes exhibit antioxidant properties (Kaur and Kapoor, 2002; Helmja et al., 2007; Rumbaoa et al., 2009; Hesam et al., 2011). The antioxidant properties of fruits and vegetables have been associated with phytochemicals, vitamins and pro-vitamins such as ascorbic acid, tocopherols and carotenoids (Loliger, 1991; Pourmorad et al., 2006). Potato is a good source of vitamin C, it also contains a number of health promoting phytonutrients such as phenolics, flavonoids, folates, kukoamines, anthocyanins, and carotenoids (Ezekiel et al., 2013). Potatoes contain significant levels of carotenoids (Blessington et al., 2010). They are good source of other vitamins like some important B group vitamins, rich in minerals such as potassium and iron, a source of phenolics, compounds that have important roles in health. They are virtually free of fat, although they are easily turned into fatty foods and are almost free of soluble sugar. They are a source of high quality protein, although deficient in the essential amino acid methionine (Lister and Munro, 2000).

1.4       NUTRIENTS

Nutrients are substances needed for growth, energy provision and other body functions. There are two major types of nutrients: the macronutrients and the micronutrients.

• Macronutrientsare those nutrients required in large amounts that provide the energy needed to maintain body functions and carry out the activities of daily life. There are 3 macronutrients – carbohydrates, proteins and fats.
• Micronutrients are substances such as vitamins and minerals that are necessary dietary components. Although needed in only small amounts, they are essential for health and wellbeing. Micronutrient deficiencies are a major public health problem in many developing countries, with infants and pregnant women especially at risk (Batra and Seth, 2002). Infants deserve extra concern because they need adequate micronutrients to maintain normal growth and development (Rush, 2000).

1.4.1    CARBOHYDRATES

Carbohydrates are a major class of naturally occurring organic compounds. They are the macronutrients required in the largest amounts. When eaten and broken down, carbohydrates provide the major source of energy to fuel our daily activities. Carbohydrates provide fuel for the body in the form of glucose. Glucose is a sugar that is the primary source of energy for all of the body’s cells. Tubers, such as these sweet potatoes, yam, cassava, Irish potatoes are excellent food sources of starch. Grains that contain carbohydrates include rice, corn, wheat, barley, oats and buckwheat. Some fruits that contain carbohydrates too include banana, citrus fruits, watermelon, apple and berries. Not all of the carbohydrates found in foods are digestible. For example, cellulose is a non-digestible carbohydrate present in fruits and vegetables. Although unable to be used as an energy source, this type of carbohydrate plays a very important role in maintaining the health of the large intestine and assisting with the removal of body waste. It is often referred to as ‘Dietary fibre’. There are many types of dietary fibre: cellulose, lignin, hemicellulose, pectin, gums, mucilages, and others (Dhingra et al., 2012). Cellulose, a principal component of plant cell walls, is another polysaccharide composed of glucose; however, humans do not have the enzymatic ability to break the bonds connecting the glucose molecules in cellulose as they do for starch and glycogen, and thus cellulose passes through the digestive tract as roughage, largely unaltered. Fibre helps prevent constipation, appendicitis, lower blood cholesterol, as well as reduce the risk of certain types of cancers. Fibre foods are bulky and thus they offer a feeling of fullness, which aids in preventing obesity (Burton-Freeman, 2000). Fibre intake can be increased by consuming more fruits, vegetables, whole grain breads, brown rice, and oatmeal.

1.4.2    PROTEINS

Proteins are nutrients that are essential to the building, maintenance, and repair of body tissues such as the skin, the internal organs, and muscle. They are also the major components of the immune system and hormones. Amino acids are substances that make up the body’s proteins. There are 20 naturally occurring amino acids, which can be assembled in various combinations and numbers to make the thousands of different types of proteins. All 20 amino acids are necessary for protein synthesis, and cells in the human body have the ability to synthesize 11 amino acids from raw materials; the other nine cannot be synthesized by the body. These nine are called the essential amino acids which include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Foods that contain all essential amino acids that the body needs in proper amounts are called the complete proteins. These sources include animal products such as, fish, meat, poultry, eggs, milk, cheese, yogurt, and many soybean products. Incomplete proteins are foods that lack some of the essential amino acids. Such sources are foods derived from the seeds of plants, legumes, nuts, whole grains, and the seeds themselves. However, by eating a variety of plant sources, all the essential amino acids can be supplied.

• FATS

Fats are made up of compounds called fatty acids and the alcohol glycerol. Some of our daily energy requirement should be supplied through the consumption of fats and oils In addition to supplying energy, fats are needed to:

• Supply fatty acids that the body needs but cannot synthesize (such asomega-3)
• Assist with absorption of the fat-soluble vitamins A, D, E and K and carotenoids
• Provide foods with flavour and texture.

Depending on their chemical structure, these fatty acids are called monounsaturated, polyunsaturated, saturated and trans fats. Saturated and trans fats are the unhealthiest fats to eat. Unsaturated fats are found in foods like olive oil, avocados, nuts and canola oil (plant sources). Saturated fats are found in foods like meat, butter and cream (animal sources), while trans fats are found in commercially produced baked goods, snack foods, fast foods and some margarines.

1.4.4    MINERALS

Minerals are inorganic substances, present in all body tissues and fluids and their presence is necessary for the maintenance of certain physicochemical processes which are essential to life (Soetan et al., 2010). They have important roles to play in many activities in the body. Minerals may be broadly classified as macro (major) or micro (trace) elements. The macro-minerals include calcium, phosphorus, sodium and chloride, while the micro-elements include iron, copper, cobalt, potassium, magnesium, iodine, zinc, manganese, molybdenum, fluoride, chromium, selenium and sulphur (Soetan et al., 2010). The macro-minerals are required in amounts greater than 100 mg/dl and the micro-minerals are required in amounts less than 100 mg/dl (Murray et al., 2000).

1.4.4.1     Potassium

Potassium (K) is an essential macro mineral nutrient. Within the body, potassium is the principal cation in intracellular fluid and participates in acid-base balance, regulation of osmotic pressure, conduction of nerve impulses, muscle contraction, cell membrane function and more. Its metabolism is regulated by aldosterone (Yang and Ma, 2009). Hyperkalaemia is a medical condition where there is increased level of serum potassium and this occurs in Addison’s disease, advanced chronic renal failure, shock and dehydration. Symptoms include dilatation of the heart, cardiac arrest, small bowel ulcers. Hypokalaemia is low level of serum potassium and this occurs in diarrhoea, metabolic alkalosis and familial periodic paralysis. When lactating dairy cows have hypokalaemia, the milk production is markedly lowered (Soetan et al., 2010). It is a mineral found in most fruits like banana, apricots, orange, grapefruit, avocado, pineapple, berries, apple; in vegetables like pumpkin, spinach, artichoke, potatoes and some other foods like legumes, mushrooms, milk and milk products.

 

1.4.4.2     Calcium

Calcium is an essential nutrient that is necessary for many functions in human health. Calcium is the most abundant mineral in the body with 99% found in teeth and bone. Only 1% is found in serum. Research has shown that adequate calcium intake can reduce the risk of fractures, osteoporosis, and diabetes in some populations (Beto, 2015). The most obvious function of calcium is to provide rigidity to the skeleton by virtue of the insoluble salts it forms with phosphoric acid, but it is also used throughout the body in small amounts. Calcium is very essential in muscle contraction, oocyte activation, blood clotting, nerve impulse, transmission, regulating heart beat and fluid balance within cells. The requirements are greatest during the period of growth such as childhood, during pregnancy, and during breast feeding. Long term calcium deficiency can lead to osteoporosis in which the bone deteriorates and there is an increased risk of fractures (Pravina et al., 2013). Calcium deficiency is a condition in which the body has an inadequate amount of calcium. It must be ingested daily and absorbed effectively in order to maintain optimal health. Most people can get enough calcium by eating a variety of foods rich in calcium. Foods that naturally contain calcium include milk and other dairy products; green, leafy vegetables; seafood, nuts, and dried beans. Calcium is also added to orange juice, breakfast cereals, bread, and other fortified food products. High dietary calcium intake is necessary for infants, children and adolescents in order to promote bone growth and formation. Pregnant women also have higher calcium needs, because it is required for the normal development of fetal bones. In addition, women who have reached menopause need to ensure an adequate amount of calcium intake to reduce the risk of osteoporosis (Pravina et al., 2013).

1.4.4.3     Magnesium

Magnesium is one of the major mineral nutrients in the human body. It is required for many of the major organs to function and plays a crucial role in human and mammalian physiology. Magnesium is essential for the structure of bones and teeth, acts as a cofactor for more than 300 enzymes in the body, including binding to ATP for kinase reactions, and affects permeability of excitable membranes and neuromuscular transmission (Long and Romani, 2014). The adult Recommended Dietary Allowance (RDA) for magnesium is 350 mg per day for men and 280 milligrams for women. Magnesium deficiency may be a risk factor for postmenopausal osteoporosis. This may be related to the fact that magnesium deficiency negatively alters calcium metabolism and the hormone that regulates bone-calcium stores (Rude and Olerich, 1998). It also plays an important role in carbohydrate metabolism, influencing the release and activity of insulin, the main hormone that exerts control of blood glucose levels. Its deficiency has been found to allow for increased intracellular concentrations of sodium and potassium, which results in increased peripheral resistance and vasospasm leading thus to high blood pressure (Douban et al., 1996). Magnesium plays a key role in the activation of nervous system sympathetic activity (Long and Romani, 2014). High magnesium foods include dark leafy greens, nuts, seeds, fish, beans, whole grains, avocados, yogurt, bananas, dried fruit and dark chocolate.

1.4.4.4    Sodium

Sodium is the principal cation in extracellular fluids. It regulates plasma volume and acid-base balance, involved in the maintenance of osmotic pressure of the body fluids, preserves normal irritability of muscles and cell permeability, activates nerve and muscle function and is involved in Na+/K+-ATPase, maintenance of membrane potentials, transmission of nerve impulses and the absorptive processes of monosaccharides, amino acids, pyrimidines, and bile salts. The changes in osmotic pressure are largely dependent on sodium concentration (Soetan et al., 2010). Sodium is necessary for humans to maintain the balance of the physical fluids system, it’s also required for nerve and muscle functioning. Too much sodium can damage the kidneys and increase the chances of high blood pressure. Most of the sodium in the body (about 85%) is found in blood and lymp fluid, sodium levels in the body are partly controlled by a hormone called aldosterone (Constantin and Alexandra, 2011).

1.4.4.5    Iron (Fe)

Iron is a mineral that is important for good health. It functions as haemoglobin in the transport of oxygen. In cellular respiration, it functions as essential component of enzymes involved in biological oxidation such as cytochromes c, c1 and a1 (Malhotra, 1998). Fe is an important constituent of succinate dehydrogenase as well as a part of the heme of haemoglobin, myoglobin and the cytochromes. Iron is required for proper myelination of spinal cord and white matter of cerebellar folds in brain and is a cofactor for a number of enzymes involved in neurotransmitter synthesis (Soetan et al., 2010). Iron is involved in synthesis and packaging of neurotransmitters, their uptake and degradation into other iron-containing proteins which may directly or indirectly alter brain function (Beard, 2001). Iron exists in the blood mainly as haemoglobin in the erythrocytes and as transferrin in the plasma. It is transported as transferrin; stored as ferritin or haemosiderin and it is lost in sloughed cells and by bleeding (Murray et al., 2000). It can be obtained from different foods; animal sources include meat, fish and poultry, while plant sources include beans, peas, lentils and some fruits and vegetables.

1.4.4.6    Zinc

Zinc is involved in numerous aspects of cellular metabolism (Classen et al., 2011). It was estimated that about 10% of human proteins potentially bind zinc, in addition to hundreds which transport and traffic zinc. It is required for the catalytic activity of more than 200 enzymes and it plays a role in immune function, wound healing, protein synthesis, DNA synthesis and cell division (Osredkar and Sustar, 2011). Zinc is required for proper sense of taste and smell and supports normal growth and development during pregnancy, childhood, and adolescence (Maret and Sandstead, 2006). It is believed to possess antioxidant properties, which may protect against accelerated ageing and helps speed up the healing process after an injury; however, studies differ as to its effectiveness. Zinc ions are effective antimicrobial agents even at low concentrations (Osredkar and Sustar, 2011). A daily intake of zinc is required to maintain a steady state because the body has no specialized storage system of it (Rink and Gabriel, 2000). A wide variety of foods contain zinc, they include oysters, red meat, especially beef, lamb and liver, beans, nuts, seafood (such as crab and lobster), whole grains, cereals, almonds, pumpkin seeds and others (Osredkar and Sustar, 2011).

1.4.4.7    Selenium

Selenium (Se) is an essential trace element having biological functions of utmost importance for human health. Its low status in humans has been linked to increased risk of various diseases, such as cancer and heart disease (Savitha, 2014). In recent years, Se research has attracted tremendous interest because of its important role in antioxidant selenoproteins for protection against oxidative stress initiated by excess reactive oxygen species (ROS) and reactive nitrogen species (NOS) (Tinggi, 2008). The synthesis of selenoproteins requires a unique incorporation of amino acid selenocysteine into proteins directed by the UGA codon, which is also a termination codon. Interest in Se research has led to the discovery of at least 30 selenoproteins (Tinggi, 2008). Foods are major natural source of Se, and its levels generally depend on soil Se levels. In general, seafoods, cereals and meat products contain relatively high levels of selenium, while low levels are found in milk, vegetables and fruits (Tinggi et al., 1992).

1.4.5    VITAMINS

Vitamins are essential organic compounds required in very small amounts (micronutrients) to maintain the fundamental functions of the body (Bennasir et al., 2010). Vitamins are catalysts for all metabolic reactions using proteins, fats and carbohydrates for energy, growth and cell maintenance. Eating fats, carbohydrates and proteins without enough vitamins means the energy from these nutrients cannot be utilized. Likewise, vitamins do not provide energy and they cannot be used without an adequate supply of fats, carbohydrates, proteins and even minerals (Ghosh et al., 2015). There are two different types of vitamins; the fat-soluble vitamins and the water-soluble vitamins. Fat-soluble vitamins; vitamins A, D, E and K dissolve in fat before they are absorbed in the bloodstream to carry out their functions. Excesses of these vitamins are stored in the liver, and are not needed every day in the diet. Water-soluble vitamins dissolve in water and are not stored by the body. Hence they are eliminated in urine, and a continuous daily supply in the diet is required. The water-soluble vitamins include the vitamin B-complex group and vitamin C (Bellows and Moore, 2012).

1.4.5.1    Vitamin A

Vitamin A also known as retinol is an essential nutrient needed in small amounts by humans for the normal functioning of the visual system; growth, development and maintenance of epithelial cellular integrity; immune function and reproduction. It also has an important role in the development of teeth, especially in the formation of ameloblasts (in enamel) and odontoblasts (in dentin) (Ghosh et al., 2015). Free retinol is not generally found in foods. Retinyl palmitate, a precursor and storage form of retinol, is found in foods from animals. Plants contain carotenoids, some of which are precursors for vitamin A (e.g., alpha-carotene, beta-carotene, and beta-cryptoxanthin). Yellow and orange vegetables contain significant quantities of carotenoids. Green vegetables also contain carotenoids, though the pigment is masked by the green pigment of chlorophyll. Some food sources of vitamin A include cod liver oil, eggs, butter, milk, sweet potato, carrot, spinach and broccoli (Bennasir et al., 2010).

1.4.5.2    Vitamin B1

Vitamin B1 also known as thiamine is a water-soluble vitamin. It functions as a coenzyme in metabolism of energy nutrients via the Krebs cycle to produce energy. This role makes it crucial for normal functioning of the brain, nerves, muscles, and heart. Thiamine is also a necessary component in the synthesis of niacin and helps regulate appetite. It is a constituent of enzymes that degrade sucrose to organic acids that can ultimately dissolve tooth enamel (Ghosh et al., 2015). Food sources of vitamin B1 include peas, pork, liver, and legumes. Most commonly, thiamine is found in whole grains and fortified grain products such as cereal, and enriched products like bread, pasta and rice (Bellows and Moore, 2012).

1.4.5.3     Vitamin B2

Vitamin B2 also called riboflavin functions as a coenzyme in the metabolism of carbohydrate, protein, and fat to release cellular energy. Riboflavin is also essential for healthy eyes and maintenance of mucous membranes. Along with thiamine, it is necessary for synthesis of niacin (Ghosh et al., 2015). Sources include liver, eggs, dark green vegetables, legumes, whole and enriched grain products, and milk. Ultraviolet light is known to destroy riboflavin, which is why most milk is packaged in opaque containers instead of clear (Bellows and Moore, 2012).

1.4.5.4     Vitamin B9

It is also known as folate, folic acid or folacin. This generic term folate encompasses several constituents that have nutritional properties similar to those of folic acid (Ghosh et al., 2015). Folic acid is the synthetic form of vitamin B9 which is found in supplements and fortified foods, while folate occurs naturally in foods. Folic acid is crucial for proper brain functioning and plays an important role in mental and emotional health. It helps in the production of DNA and RNA, the body’s genetic material, especially when cells and tissues are growing rapidly, such as during infancy, adolescence, and pregnancy. Folic acid works closely with vitamin B12 in making red blood cells and helps iron function properly in the body (Mahmood, 2014). Rich sources of folate include spinach, dark leafy greens, asparagus, turnip, beets, and mustard greens, Brussels sprouts, lima beans, soybeans, beef liver, brewer’s yeast, root vegetables, whole grains, wheat germ, bulgur wheat, kidney beans, white beans, lima beans, salmon, orange juice, avocado, and milk (Mahmood, 2014).

1.4.5.5     Vitamin C

Vitamin C, also known as ascorbic acid, abounds in nature and is highly labile (Walingo, 2005). It is a water-soluble vitamin. Vitamin C functions as an antioxidant in numerous bodily reactions. As a coenzyme, it also has numerous metabolic roles. It is important in the production of collagen (insoluble protein of connective tissue, cartilage, and bone), which plays a vital role in wound healing. Vitamin C strengthens tissue and promotes capillary integrity. Vitamin C facilitates development of Red blood cells by enhancing iron absorption and utilization. It also aids the body in utilization of folate and vitamin B12 (Ghosh et al., 2015). In human health, vitamin C has been associated with immunity, drug metabolism and urinary hydroxyproline excretion, tissue regeneration and reduction of the incidence of cancer and blood pressure (Walingo, 2005). Consuming vitamin C-rich foods is the best method to ensure an adequate intake of this vitamin. While many common plant foods contain vitamin C, the best sources are citrus fruits (Bellows and Moore, 2012). Other sources include asparagus, broccoli, mangoes, cabbage, guava, cauliflower, berries, peppers, spinach, sweet potatoes and tomatoes.

1.4.5.6     Vitamin E

Eight different compounds are collectively called vitamin E; four tocopherol and four tocotrienols. Vitamin E is the major lipid-soluble antioxidant in the cellular defence system and is exclusively obtained from the diet (Ghosh et al., 2015). Vitamin E is an example of a phenolic antioxidant. Such molecules readily donate the hydrogen from the hydroxyl (-OH) group on the ring structure to free radicals, making them unreactive. On donating the hydrogen, the phenolic compound itself becomes a relatively unreactive free radical because the unpaired electron on the oxygen atom is usually delocalized into the aromatic ring structure thereby increasing its stability (Scott, 1997). The major biological role of vitamin E is to protect PUFAs and other components of cell membranes and low-density lipoprotein (LDL) from oxidation by free radicals. Vitamin E is located primarily within the phospholipid bilayer of cell membranes. It is particularly effective in preventing lipid peroxidation – a series of chemical reactions involving the oxidative deterioration of PUFAs (Duthie, 1993). Good sources of vitamin E are vegetable oils, nuts and nut seed oil, egg yolk, margarine, cheese, soya beans, wheat germ, oatmeal, avocados, olives, green leafy vegetables. Tocopherols are predominant in olive, sunflower, corn, soya beans oils, and tocotrienols are the major vitamin E components of palm oil, of barley and rice bran (Colombo, 2010).

1.4.5.7    Vitamin K

Vitamin K refers to a family of forms that share a common 2-methyl 1,4-naphthoquinone nucleus substituted at the 3-position with different polyisoprenoids (Berkner and Runge, 2004). It was originally identified as a fat-soluble nutrient required for coagulation. Later it was found to be an essential cofactor for post-translational modification of glutamic acid (Glu) residues to carboxyglutamic acid residues of vitamin K– dependent hepatic blood-coagulating proteins. Hence, vitamin K deficiency results in a bleeding tendency due to malfunction of vitamin K– dependent clotting factors. Other actions of vitamin K include potential protective effects against osteoporosis, hepatocarcinoma, and atherosclerosis.  (Kaneki et al., 2006). Vitamin K exists in two forms in nature: vitamin K1 (phylloquinone) and vitamin K2 (menaquinones). Vitamin K1 is produced by plants and algae and is widely distributed in green and leafy vegetables; vitamin K2 is of microbial origin and is contained in meats, eggs, curd, cheese, and fermented soybeans (Kaneki et al., 2006).

• ANTINUTRIENTS

Anti-nutrients are compounds or substances which act to reduce intake, digestion, absorption and utilization of nutrients and may produce other adverse effects in the body (Akande et al., 2010). They are chemicals which are produced in plants for self-defence, among other biological functions. They also reduce the maximum utilization of nutrients especially proteins, vitamins, and minerals, thus preventing optimal exploitation of the nutrients present in a food and decreasing the nutritive value. However, some anti-nutrients may exert beneficial health effects at low concentrations. Despite of this, the balance between beneficial and hazardous effects of plant anti-nutrients is a function of their concentration, chemical structure, time of exposure and interaction with other dietary components. Due to this, they can be considered as anti-nutritional factors with negative effects or non-nutritive compounds with positive effects on health (Gemede and Ratta, 2014).

1.4.5.1     Oxalate

Oxalic acid and its salts occur as end products of metabolism in a number of plant tissues. When these plants are eaten, they may have an adverse effect because oxalates bind calcium and other minerals (Popoola et al., 2014). It forms water-soluble salts with Na⁺, K⁺ and NH⁴⁺ ions; it also binds with Ca²⁺, Fe²⁺ and Mg²⁺, rendering these minerals unavailable. However, Zn²⁺ appears to be relatively unaffected. While oxalic acid is a normal end product of mammalian metabolism, the consumption of additional oxalic acid may cause stone formation in the urinary tract when the acid is excreted in the urine. Soaking and cooking of foodstuffs high in oxalate will reduce the oxalate content. The distribution of oxalate within plants is also uneven. In general, oxalate content is highest in the leaves, followed by the seeds (Noonan and Savage, 1999).

 

1.4.5.2     Phytate

Phytic acid (myo-inositol-l,2,3,4,5,6-hexakis dihydrogen phosphate, PA) is present in foods in varying concentrations. With its highly negatively charged structure, it is a very reactive compound and particularly attracts positively charged ions such as those of zinc and calcium. It may also react with charged groups of proteins, either directly or indirectly, via negatively charged groups of proteins mediated by a positively-charged metal ion such as calcium. Interaction of PA with starch molecules, directly via hydrogen bonding with phosphate groups or indirectly through proteins to which it is attached, is also possible. Such bindings may reduce the solubility and digestibility of protein and starch components of food (Shahidi, 1997). Consumption of phytate, however, seems not to have only negative effects on human health but positive effects also. The latter view is evidenced by the fact that dietary phytate was reported to prevent kidney stone formation, protect against diabetes mellitus, caries, atherosclerosis and coronary heart disease as well as against a variety of cancers (Greiner et al., 2006).

1.4.5.3     Hydrogen Cyanide

Cynogens are glycosides of 2-hydroxyl nitriles and widely distributed among plants. In the case of emergency, when plants are wounded by herbivores or other organisms, the cellular compartmentation breaks down and cynogenic glycosides come into contact with active B-glucosidase, which hydrolyses them to yield 2-hydroxynitrile. In addition to the toxic effects, cynogens can serve as mobile nitrogen storage compounds in seeds which are important during germination (Bora, 2014). Various ailments from cyanide toxicity have been reported in human among which are: vomiting, diarrhoea, dizziness, headache, stomach pains, rapid respiration, drop in blood pressure, mental confusion, rapid pulse rate, twitching, convulsion and tropical ataxic neuropathy (Ubwa et al., 2015).

1.6       PROCESSING

Food processing is the alteration of foods from the state in which they are harvested or raised to other forms for better preservation and feed consumers (Weaver et al., 2014). Food processing improves food nutritionally by removing toxic substances, extending shelf-life and making it more palatable. It provides more value to crops as compared to the sale of the raw materials themselves (Ohiokpehai, 2003). Processed food contributes to both food security (ensuring that sufficient food is available) and nutrition security (ensuring that food quality meets human nutrient needs) (Weaver et al., 2014). Certain food preparation and processing method need to be promoted at the house-hold level to reduce the level of absorption inhibitors or increase the content of absorption enhancers and thus improve the bioavailability of Fe, Zn and provitamin A as well as eliminate the toxic component in roots and tubers (Ugwu, 2009).

 

1.6.1    COOKING

Cooking has been defined as the art of preparing food for consumption with the use of heat which transform an unpalatable, or hardly edible raw material into a safe, flavourful, nutritious, stable, and enjoyable food (Floros et al., 2010). It is a thermal method of processing food. There are different ways of cooking food. These include:

• Boiling: It is a method by which food is cooked in adequate quantity of water. It is a safe and simple method of cooking food but water soluble nutrients present in food dissolve in water in which the food is being boiled. If this water is poured away, nutrients will be lost. Therefore this nutrient rich water can be used to make soup or gravy for other vegetables.
• Frying: It is the process of cooking food in hot oil. Food can be cooked either by shallow frying or by deep frying. Shallow frying means frying in little oil and deep frying means immersing food fully in hot oil. Fried food has longer life than food cooked using other methods but usually contains more calories.
• Steaming: This is when food is cooked with the heat from water vapours. The food is kept in a pan in such a way that it comes in contact with steam from the boiling water. There is a special pot used for this cooking process. Steaming shortens the duration of cooking and helps to conserve nutritive value, colour, flavour and palatability of food.
• Baking: It is the method by which food is cooked by placing it inside a heated closed box called an oven. The air inside the oven gets hot due to fire lit at its base or with electricity and the food gets cooked by hot air, when placed inside.
• Roasting: This is the method where food is put directly on a hot sand or fire and cooked. Food is tastier when cooked this way. It also adds variety to a meal. It is a relatively slow method of cooking and sometimes the food becomes too dry.
• Grilling: It is the cooking of food over a glowing fire. It uses more indirect heat and is slower than roasting. The food is supported on an iron grid over the fire, or between electrically heated grill bars. Grilling like roasting also gives nice flavour to the food.
• Pressure cooking: It is a process of cooking in a special utensil which allows cooking with a lot of steam under pressure. Pressure cookers are made of steel or from a mixture of aluminium and other metals and can withstand high pressure. The steam produced is trapped inside the cooker thus increasing the pressure and temperature above 100°C. Pressure cooking kills all bacteria and hence the food is safe and hygienic to eat. The food gets cooked faster too.

1.6.2    Effect of Boiling and Frying on Nutrients and Anti-nutrients of Potatoes

Two most common ways of processing potato in Nigeria are boiling in water and frying in refined vegetable oil or palm oil (Ikanone and Oyekan, 2014). These methods of processing potatoes for consumption make them palatable but also have adverse effects on the nutrients. Cooking can be detrimental to the micronutrients but beneficial to macronutrient contents of food (Chukwu et al., 2010). Cooking of foods leads to the improvement of microbiological and organoleptic qualities, destroys toxins and antinutritional factors, increases digestibility and nutrients bioavailability (Ikanone and Oyekan, 2014). Toxic factors in potatoes can be removed by boiling (Ugwu, 2009). In a study carried out by Ikanone and Oyekan, (2014) on the effects of boiling and frying on the carbohydrate, vitamin C and mineral contents of potatoes, they reported that boiling retains more carbohydrate than frying while frying retains more vitamin C and minerals than boiling. Also in a study carried out by Lyimo et al., 2010, It was reported that processing of sweet potato varieties by boiling, roasting and sun drying did not have any significant effect on the carbohydrate, protein, fat, ash, calcium, iron and magnesium contents. However, significant (p≤0.05) losses were observed for the reducing sugars and total carotenoids. They concluded that there was a need to choose processing methods that cause minimum nutrient losses, and the selection of sweet potato varieties with high nutrient content could compensate for likely nutrient losses occurring during processing. Fillion and Henry, (1998) on checking the effect of frying on some nutrients of Irish potato found out that frying has little or no impact on the protein or mineral content, whereas the dietary fibre content increased after frying due to the formation of resistant starch. Moreover, the high temperature and short transit time of the frying process cause less loss of heat labile vitamins than other types of cooking.

1.7       Rationale of Study

Research has confirmed that these two root crops possess a lot of nutrients (Camire et al., 2009; Mitra, 2012). However the rate at which cooking processes reduce or enhance the availability of these nutrients is not clear. Hence, the aim of this study was to investigate the effect of processing on these nutrients.

1.8       Specific Objectives of Study

• Determination of the proximate composition of the raw and processed Irish potato, white flesh and orange flesh sweet potatoes.
• Determination of the vitamin compositions of the raw and processed potatoes.
• Determination of the mineral constituents of the raw and processed potatoes.
• Determination of the anti-nutrients of the raw and processed potatoes.

 

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