Effects Of Processing Methods On The Physico-Chemical Properties Of Sweet Potato And Sorghum

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ABSTRACT

This study evaluated “the effects of processing methods on the physico-chemical properties of sweet potato and sorghum flour”. Sweet potato (Ipomoea batatas) is an important food crop in the tropical and sub-tropical countries and belongs to the family convolvulaceae. Sweet potatoes are rich in dietary fiber, minerals, vitamins, and anti oxidants such as phenolic acids, anthocyannins, tocopherol and β-carotene. The proximate composition of sweet potato was determined and these include moisture, lipids, ash, protein, carbohydrates and fiber. In carrying out the analysis practically, methods used vary according to the food material. The anti oxidants were also determined alongside with phenol oxidase, pasting properties, minerals and sugar contents. Sorghum is a tropical plant belonging to the family of poaceae. More than 35% of sorghum is grown for human consumption. The analyses carried out in sweet potatoes are same with sorghum with the exclusion of phenol oxidase

CHAPTER ONE

INTRODUCTION

Sweet potato (Ipomoea batatas) is an important food crop in the tropical and sub tropical countries and belongs to the family convolvulaceae. It is cultivated in more than 100 countries. ( Woolfe, 1992). Nigeria is the third largest producer in the world with china leading, followed by Uganda. Sweet potato ranks seventh among the world food crops, third in value of production and fifth in caloric contribution to human diet (Bouwkamp, 1985). Sweet potatoes are rich in dietary fibre, minerals, vitamins and anti oxidants such as phenolic acids, anthocyanins, tocopherol and ß- carotene. Besides acting as anti oxidants, carotenoids and phenolic compounds also provide sweet potatoes with their distinctive flesh colours ( cream, deep yellow, orange and purple). Sweet potato blends with rice, cowpea and plantain in nigerian diets. It is also becoming popular as a substitute to yam and garri. It can be reconstituted into fofoo or blended with other carbohydrate flour sources such as wheat ( Triticum aestivum) and cassava ( Manihot esculenta) for baking bread, biscuits and other confectioneries (Woolfe, 1992)

The leaves are rich in protein and the orange flesh varieties contain high beta carotene and are very important in combating vitamin A deficiency especially in children.

Sorghum (sorghum bicolor (S. bicolor) is a tropical plant belonging to the family of poaceae, is one of the most important crops in Africa, Asia and Latin America. More than 35% of sorghum is grown directly for human consumption. The rest is used primarily for animal feed, alcohol production and industrial products ( FAO, 1995). The current annual production of 60 million tons is increasing due to the introduction of improved varieties and breeding conditions. Several improved sorghum varieties adapted to semi-arid tropic environments are released every year by sorghum breeders. Selection of varieties meeting specific local food and industrial requirements from this great biodiversity is of high importance for food security. In developing countries in general and particularly in West Africa demand for sorghum is increasing. This is due to not only the growing population but also to the countries policy to enhance its processing and industrial utilization.

More than 7000 sorghum varieties have been identified, therefore there is a need of their further characterization to the molecular level with respect to food quality. The acquisition of good quality grain is fundamental to produce acceptable food products from sorghum. Sorghum while playing a crucial role in food security in Africa, it is also a source of income of household . In West Africa, ungerminated sorghum grains are generally used for the preparation of “to”, porridge and couscous. Malted sorghum is used in the process of local beer “dolo” (reddish, cloudy or opaque), infant porridge and non fermented beverages. Sorghum grains like all cereals are comprised primarily of starch.

The aim and objective of this work is to obtain diet low in sugars, with enriched nutrients intended for diabetics.

CHAPTER TWO

LITERATURE REVIEW

ORIGIN AND DISTRIBUTION OF SWEET POTATO

Sweet potato (Ipomoea batatas) is a member of the convolvulaceae family (purseglove, 1972). Approximately 900 different species of convolvulaceae in 400 genera have been identified around the world. Yen, (1974) and Austin (1978,1988) recognized 11 species in the batatas, which includes sweet potato. The closest relatives of the sweet potato appears to be ipomoea trifida that is found wild in maxico, and ipomoeto tabascana. Sweet potato has a chromosomes number for the genus ipomoea is 15, sweet potato is considered to be a hexaploid. Most sweet potato cultivars are self-incompatible, which means that when self pollinated, the cannot produce viable seed. It is accepted that cultivated sweet potato originated in central America or tropical south America. Sweet potatoes are cultivated where ever there is enough water to support their growth: optimal annual rainfall for growth range between 750-2000mm. sweet potato is a warm season annual,

requiring 20-25°C average temperatures and full sunlight for optimal development. Sweet potato thrives in well drained loamy soils with high humus content that provides warm and moist environment to the roots.

2.1.1. DESCRIPTION OF SWEET POTATO

THE ROOT SYSTEM

When sweet potato is planted from stem cuttings, adventitious roots arise from the cutting in a day or two. These roots grow rapidly and form the root system of the plant. Research has shown the roots of sweet potato can penetrate the soil to a depth of over 2m, the exact depth attained being dependent on the soil condition (Onwueme, 1978 and Kays, 1985). Based on its origin, the root system of sweet potato is divided into the adventitious roots arising from subterranean nodes of a vine cutting and lateral roots arising from existing roots. Kays (1985) subdivided the adventitious roots into storage, fibrous and pencil roots. The lateral roots are subdivided into primary, secondary and tertiary roots.

During the early ontogeny of young adventitious roots emerging from the stem, they are often separated into two classes’ namely thick and thin roots (Togari, 1950). According to Wilson (1982) and Kays (1985), thin roots are typically tetrarch in the arrangement of their primary vascular tissue, i.e. four xylem and phloem poles found within the vascular cylinder. The most important functional differences between these root types are their capacity for storage root initiation in a specific region of the thick roots. Several factors such as exposure of potential storage roots to long photoperiod (Bonsi et al, 1992), water logged soil conditions (Kays, 1985), high levels of nitrogen supply (Chua & Kays, 1981), gibberellic acid application (McDavid & Alamu, 1980), as well as exposing the plant to long days (McDavid & Alamu, 1980; Du Plooy, 1989) encourage lignification and inhibit storage root development. Alternatively high potassium supply (Isuno, 1971, Hah

  • Hozyo, 1984), the absence of light (Wilson, 1982), as well as well aerated soil conditions, low temperature and short days been demonstrated to encourage storage root formation (Du Plooy, 1989).

Storage Roots

Storage roots arise from pentarch or hexarch thick young roots if the cells between the protoxylem point and the central metaxylem cell do not become lignified, or if only a slight proportion of these cells are lignified (Togari, 1950). The increase in storage root size is attributed to the activity of the vascular cambium as well as the activity of the anomalous cambia (Wilson, 1982). The initial sign of storage root formation is the accumulation of photosynthetic consisting predominantly of starch (Chua & Kays, 1982). Storage root initiation is reported to occur between the periods of 35 to 60 days after planting (Agata, 1982, Wilson 1982). But the work of Du Plooy (1989) indicated that storage root initiation might occur as early as 7 days after planting. These conflicting results suggest the need for further research on the storage root formation in sweet potato.

Agata (1982) reported that storage root formation started about 30 to 35 days after planting and the roots dry weight increased linearly until harvest

  1. Pencil Roots: Pencil roots are generally beteen 5 and 15mm in diameter, they are the least well defined of the adventitious root emerging from the subterranean node of the culting. They develop mainly from young thick adventitious roots under conditions not conducive for the development of storage roots. In pencil roots lignification is not total, but result in uniform thickening of the entire root.
  2. Fibrous Roots: According to Chua and Kays (1981), fibrous roots develop mainly from tetrarch, thin adventitious roots. The fibrous roots are generally less than 5mm in diameter and are branched with lateral roots forming a dense network throughout the root zone constituting the water and nutrient absorbing system of the plants. Fibrous roots have heavily lignified steels and very low levels of vascular cambial activity. High nitrogen and low oxygen within the root zone favors their formation (Chua & Kays, 1981).
  3. Lateral roots: The lateral roots of sweet potato emerge from existing roots adventitious roots (storage, pencil and fibrous) have a profusion of lateral roots at varying densities along their axis. The primary lateral roots emerge from adventitious roots. Lateral emerging from the primary laterals are called secondary lateral and those emerging from the secondary laterals are named tertiary laterals (Kays, 1985).
  4. ABOVE GROUND PLANT ORGANS a. Vines: Sweets potato has long thin stems that trail along the soil surface and can produce roots at the nodes. Sweet potato genotypes are classified as either erect, bushy, intermediate, or spreading, based on the length of their vines (Yen, 1974, Kays, 1985). Stem length varies with cultivar, and highly variable, ranging from a few centimeters up to 10cm in length. Planting density has pronounced effect on the internode length as well as on vine length (Somda & Kays, 1990a). The stem is circular or slightly angular. Stem color is predominantly green, but purplish pigmentation is often present. Branching is cultivar dependent (Yen, 1974) and branches vary in number and length. Normally, sweet potato produces three types of branches, primary, secondary and tertiary, at different periods of growth. The total number of branches varies between 3 and 20 among cultivars. Spacing, photoperiod, soil moisture and nutrient supply influence the branching intensity in sweet potato plant (Kays, 1985, Somda & Kays, 1990a).
  5. Leaf and Petiole: The leaves of sweet potato occur spirally on the stem. The total number of leaves per plant varies from 60 to 300 (Somda et al, 1991). The number of leaves per plant increases with decreasing plant density (Somda & Kay’s, 1990b), increasing irrigation (Indira & Kabeerathumma, 1990). Peptide length varies widely with genotype and may range from approximately 9 to 33cm (Yen, 1974). The petiole retains the ability to grow in a curved or twisted manner so as to expose the lamina to maximum light. The petiole is swollen at its junction with the stem, and it bears two small nectarines at the junction. The lamina is extremely variable in size and shape, even for leaves on the same plant. The lamina is green in color, sometimes with purple coloration.
  6. Flower: The flowers of sweet potato are born solitarily or on cymose inflorescence that grow vertically upward from the leaf axis (Purseglove, 1972, Onwueme, 1978). Each flower has five united sepals, and fire petals joined together to form a funnel-shaped corolla tube. The stamens are five in number and are attached to the base of the style. The filament is white and hairy; the anther is also white and contains one locule. Each locule contains two ovules, so that there is a maximum of four ovules in each ovary (Onwueme, 1978). The physiology of the sweet potato flower is complex. Firstly, the formation of the flower is subject to environmental control, especially photoperiodic control. Secondly, the flower is open and receptive for an extremely short period of time. Thirdly, incompatibility complexes exist. Fourthly, the existence of variation in stamen length with respect to the style introduces a further morphological complication into the pollination mechanism. All these features make seed production difficult (Onwueme, 1978).
  7. Fruit and See: The sweet potato fruit is a capsule 5 to 8mm in diameter. A false septrum, formed during fruit development, may divide each of the two locules into two, thereby creating four chambers in the mature fruit. Each chamber may contain a seed, but usually only one or two chambers in each fruits contain a seed, but usually only one or two chambers in each fruit contain any seed.

The seed is black and about 3mm long. It is flat on one side and convex on the other. The micropyle is located in a hollow on the flattened side. Endosperm is present in the seed in addition to cotyledons. The testa is very hard and almost impermeable to water or oxygen. For this reason, the seeds germinate with difficulty. Germination can be improved by scarifying the seed either by mechanically clipping the testa, or by treating it with concentrated sulfuric acid for about 45minutes.

Germination of scarified seed occurs in 1 to 2days. The radical is first to elongate, and develops into the primary root system. Germination is epigeal, since the cotyledons are carried above the soil level. After emergence, the bi-lobal cotyledons expand, develop chlorophyll, and become photosynthetic (Onwueme, 1978).

USES OF SWEET POTATO

Sweet potato is an important crop in many parts of the world. The storage roots of sweet potato serve as staple food, animals feed (Posas, 1989) and to a limited extent as a raw material for industrial purposes as a starch source and for alcohol production (Collins, 1984). Sweet potato starch is used for the manufacture of adhesives, textile and paper sizing and in the confectionery and baking industries.

In most parts of the tropics, sweet potato is consumed boiled, baked, roasted or fried.

NUTRITIONAL PROFILE/VALUE

The orange-flesh sweet potatoes are exceedingly rich in beta-carotene. The purple-flesh varieties are outstanding sources of anthocyanins, especially peonidins and cyanidins. Both types of sweet potatoes are rich in unique phytonutrients, including polysaccharides related molecules called batatins and batatosides. Sweet potatoes also include storage proteins called sporamins that have unique antioxidant properties. Sweet potatoes are an excellent source of vitamin A (In the form of beta-carotene). They are also a very good source of vitamin C and manganese.

In addition, sweet potatoes are a good source of copper, dietary fiber, niacin, vitamin B5 and potassium. The sweet potato is rich in vitamin A, B and C and potassium. But in sub Saharan Africa it is estimated that 32% of the population suffers from vitamin A deficiency which can lead, amongst other illnesses, to blindness, (Collins, 1984). The sweet potato can therefore be a value added food staple particularly for children and pregnant women who are more often exposed to deficiencies (Posas, 1989).

2.1.4. ANTI-NUTRITIONAL FACTORS

Raw sweet potato tubers contain medium levels of trypsin inhibitors that are still sufficient to decrease protein digestibility in diets. Trypsin inhibitor content is very variable between varieties (2.6 to 32.0 Tul/g). Moist heat treatments (MHT) at temperature above 80°C are affective in eliminating trypsin inhibitor activity in sweet potatoes . Raw sweet potatoes also contain oxalate up to 1.2g/kg DM.

2..1.5.  NUTRITIONAL COMPOSITION OF SWEET POTATO

Recent studies show that sweet potato contains such functional components as polyphenols, anthocyaninins and dietary fiber, which are important for human health.

Sweet potatoes are a good source of carbohydrates, while sweet potatoes tops (leaves and stems) contain additional nutritional components in much higher concentrations than in many other commercial vegetables in many parts of the world. They are rich in vitamins B, β-Carotene, iron, calcium, zinc and proteins and the crop is more tolerant of diseases, pest and high moisture than many other leafy vegetables grown in the tropics. Because sweet potato tops can be harvested several times a year, their annual yield is much higher than many other green vegetables.

Sweet potato is one of the most important summer food crops in the southern united state. It is a versatile plant. For example, it is used as food, as livestock feed and for starch and alcohol production. Several researchers report that sweet potato leaves are an excellent source of antioxidative polyphenolics, among them anthocyanins and phonelics and are superior to other commercial vegetables. The nutritional value of sweet potato leaves is gaining recognition, as the understanding between diet and health increases. Sweet potato leaves with their high nutritive value and antioxidants may become an excellent leafy vegetable.

Depending on varieties and growing conditions, sweet potato leaves are comparable to spinach in nutrient content. The average mineral and vitamin content in a recently developed cultivar, suioh, is 117mg calcium, 1.8mg iron, 3.5mg carotene, 7.2mg vitamin C, 1.6 mg vitamin E and 0.56mg vitamin K. levels of iron, calcium and carotene rank among the top, compared with other major vegetables. Sweet potato leaves are also rich in vitamin B, β-carotene, iron, calcium, zinc, and protein. Studies have shown that sweet potato leaves contain as many vitamins, minerals and other nutrients as spinach. As a crop, sweet potato is more tolerant of diseases, pests and high moisture. Sweet potato leaves are an excellent source of antioxidative polyphenolics, among them anthocyanins and phenolic acids such as caffeic, monocaffenylquinic (chlorogenic), dicaffenylquinine and tricaffeoylquinic acids, and are superior in this regard to other commercial vegetables (Ishiguro et al, 2004).

2.1.5.1.POLYPHENOLS COMPOSITIONS

Sweet potato leaves represent at least 15 anthocyanin and 6 polyphenolic compounds. These biologically active compounds posses multifaceted action, including antioxidation, antimutageneicity, anti-inflammation and anticarcinogenesis.

2.1.5.2.  ANTIOXIATIVE, ANTIMUTAGENICITY AND ANTICARCINOGENICITY

Cancers occur through such processes as initiation, promotion, and progression in body cells. Initiation is a kind of mutation that occurs in cancer and anti cancer genes.

Thus, controlling the gene mutation brought about by the carcinogens leads to cancer prevention. Sweet potato leaves are a good supplementary resource of anti oxidants and antimutagenic compounds.

2.1.5.3.ANTI-DIABETES

The latter stage of noninsulin-dependent diabetes mellitus (NIDDM), one of the major diseases of adults, is caused by the secretion of adults, is caused by the decrease in the secretion of insulin by the pancreatic langerhans cells. Diabetes contributes to the death of more than 213,000 Americans each year and is also a leading cause of heart diseases, blindness and kidney failure. Foods with anti-diabetes effect are desired for diet therapy. Several researchers report that sweet potato leaves have anti-diabetic compounds that reduce the blood glucose content significantly in model rats.

2.1.6.   ANTI NUTRIENTS IN SWEET POTATO

The anti-nutritional factors have been reviewed (McLean, 1970). However, the biochemical effects of the anti-nutritional factors will be briefly highlighted. Cyanogenic glucoside on hydrolyses yields toxic hydrocyanic acid (HCN). The cyanide ions inhibit several enzyme systems; depress growth through interference with certain essential amino acids and utilization of associated nutrients. They also cause acute toxicity, neuropathy and death

Alkaloids cause gastrointestinal and neurological disorders. The glycoalkaloids, solanine and chaconine present in potato and solanum spp. (Saito et al, 1990) are haemolytically active and toxic to fungi and humans. Some of the toxicological manifestations of potato glycoalkaloids disorders.

Saponins cause hypochollesterolaemia by binding cholesterol making it unavailable for absorption (Johnson et al, 1986). Trypsin (protese inhibitor) causes pancreatic enlargement and growth depression.

Haemagluttinins are proteins known for agglutinating red blood cells. Phytates bind makes them unavailable. Oxalates, like phytates, bind

minerals like calcium and magnesium and interfere with their metabolism. They also cause gastrointestinal tract irritation, blockage of the renal tubules by calcium oxalate crystals, development of urinary calculi and hypocalcaemia (Oke, 1969) .

2.1.7. ENZYME COMPOSITION OF SWEET POTATO

β-Amylase is an exoenzyme that releases successive maltose units from the non reducing end of a polysaccharide chain by hydrolysis of α-1,4-glucan linkages. The shortest normal saccharide attacked is maltotetraose (Myrback and Neumuller 1950). Since it is unable to bypass branch linkages in branched polysaccharides such as glycogen or amylopectin, the hydrolysis is incomplete and a macromolecular limit dextrin remains. Β- amylase is found primarily in the seeds of higher plants and sweet potatoes. It yields a single product: maltose.

The tuberous root of sweet potato is unusually rich in the enzyme, accounting for approximately 5% of the total soluble proteins. In contrast, other tuberous roots contain trace amounts of β-amylase activity.

2.2. ORIGIN AND DISTRIBUTION OF SORGHUM

Sorghum is the 5th most important grain crop after wheat, maize, rice and barley. It is indigenous to Africa. Globally, it produces approximately 70 million tons of grain from about 50 million ha of land. It is the dietary staple of more than 500 million people in more than 30 countries. For all that, however, sorghum now receives merely a fraction of attention of what it could. Not only is it inadequately supported for the world’s fifth major grain crop, it is under supported considering its vast untapped potential. Sorghum could contribute more to food supplies than at present, especially to those regions and peoples in greatest need.

2.2.1. DESCRIPTION OF SORGHUM PLANT

MORPHOLOGY, GROWTH AND DEVELOPMENT OF THE SORGHUM PLANT.

Sorghum belongs to the grass family, Graminea. It is essential that producers know the crop they are cultivating in order to develop the most effective production practices

ROOT SYSTEM

The roots of the sorghum plant can be divided into a primary and secondary root system. The primary roots are those which appear first from the germinating seed. The primary roots provide the seedling with water and nutrients from the soil. Primary roots have a limited growth and their functions are soon taken over by the secondary roots. Secondary roots develop from nodes below the soil surface. The permanent root system branches freely, both literally and downwards into the soil. If no soil impediments occur, roots can reach a lateral distribution of 1m and a depth of up to 2m early in the life of the plant.

The roots are finer and branch approximately twice as much as roots from maize plants.

LEAVES

Sorghum leaves are typically green, grasslike and flat and not as broad as maize leaves. Sorghum plants have a leaf area smaller than that of maize. The leaf blade is long, narrow and pointed. The leaf blades of young leaves are upright, however, the blades tend to bend downwards as the leaves mature. Stomata occur on both surfaces of the leaf. A unique characteristic

of sorghum leaves is the rows of motor cells along the mid rib on the upper surface of the leaf. These cells can roll up leaves rapidly during moisture stress. Leaves are covered by a thin wax layer and develop opposite one another on either side of the stem. Environmental conditions determine the number of leaves, which may vary from 8-22 leaves per plant.

STEM

The stem of the plant is solid and dry, succulent and sweet, under favourable conditions more internodes develop, together with leaves, producing a longer stem.

The stem consists of internodes and nodes. A cross section of the stem appears oval or round.

The internodes are covered by a thick waxy layer reduces transpiration and increases the drought tolerance of the plants.

SEED

The ripe seed (grain) of sorghum is usually partially enclosed by glumes, which are removed during threshing and / or harvesting. The shape of the seed is oval to round and the colour may be red, white, yellow, brown or shades thereof. If only the pericarp is coloured, the seed is usually yellow or

red. Pigment in both the pericarp and testa results in a dark brown or red brown colour. The sorghum grain consists of the testa, embryo and endosperm.

SEED COAT

The seed coat consists of the pericarp and testa

PERICARP

This is the outermost layer of the seed and consists of the epicarp, hypodermis, mesocarp and endocarp.

TESTA

The testa is situated directly below the endocarp and encloses the endosperm. Apart from the role of the testa in the colouring of the seed, it contains a tannin like substance with a bitter taste. The presence thereof results in less bird damage significantly increases. The bitter taste of sorghum with a testa however, makes it less acceptable as food for humans and animals.

EMBRYO

The embryo contains those parts which give rise to the new seedling. The new plant, which is already a complete unit, depends on the right moisture and temperature conditions to start developing.

ENDOSPERM

The endosperm consists of hard and soft endosperm. The endosperm supplies the seedling with nutrients until it can take up its own nutrients.

2.2.2. USES OF SORGHUM

In many parts of the world, sorghum has been used traditionally in food products and various food items, porridge, unleavened bread, cookies, cakes, couscous and malted beverages are made from this versatile grain. Traditional food preparation of sorghum is quite varied. Boiled sorghums are one of the most basic uses and small, corneous grains are normally desired for this type of food product. The whole grain maybe ground into flour or decorticated before grinding to produce either a fine particle product or flour, which is then used in various traditional foods. The seed is used as food, in brewing beer, sorghum malt and meal. It is fermented to make “Lleting” ( a sour mash), the pith is eaten and the sweet culm chewed.

Porridge and muffins can be made using sorghum meal. Parched seeds are used as coffee substituents or adulterants.

2.2.3. ENZYME COMPOSITION OF SORGHUM

SORGHUM PHENOLIC ENZYMES BIOCHEMISTRY

Role of phenylalanine ammonia lyase in the biosynthesis of phenolic compounds;

The biosynthesis of phenolic compounds in plant is initiated by the shikimic acid pathway ( Tomas Barberan and Heldth, 2005). This pathway continues with the production of phenylalanine, which is subsequently deaminated by the enzyme phenylalanine ammonia lyase (EC 4.3.1.5, PAL).

PAL can deaminate both L- phenylalanine and L- tyrosine into cinnamate derivatives ( Heldt 2005).

PAL is inhibited by its products e.g trans-cinnamates ( Heldt, 2005). PAL activity has been detected in the green shoots and leaves ( Staffford, 1969) of sorghum.

In sorghum, the infection of the plant with pathogen involved a very rapid accumulation of PAL mRNA (Cui et al., 1996). The presence of PAL activity in sorghum grain and its activation upon germination was assessed .

2.2.4. NUTRITIONAL COMPOSITION OF SORGHUM

Sorghum has nutritional composition similar to or better than rice and wheat in some aspects. The grains contain high fiber and non starchy polysaccharides and starch with some unique characteristics. There is a considerable variation in sorghum for levels of proteins, lysine, lipids, carbohydrate, fiber, calcium, phosphorous, iron, thiamine and niacin .

Protein quality and essential amino profile of sorghum is better than many of the cereals and millets. Sorghum in general is rich source of fiber and B-complex vitamins.

Grain sorghum is rich in fiber and minerals apart from having a sufficient quantity of carbohydrates (72%), proteins (11.6%) and fat (1.9%).

Starch is the main constituent of the grain. There is a need to popularize sorghum foods as sorghum with its high mineral and fiber content and with low or slow starch digestibility makes an ideal food for diabetic and obese population in the urban as well as rural society.

2.2.5. ANTI NUTIENTS IN SORGHUM

CYANOGENIC GLYCOSIDES

Cyanogenic glycosides are mainly present in germinating seeds, sprouts and the leaves of immature sorghum plants. Traore et al. (2004) showed that malted red sorghum that had been dried contained an average 320ppm cyanogens. The most abundant cyanogens is dhurrrin, which may comprise three to four percent of the leaves of germinating seeds (Waniska and Rooney, 2000). Stressors such as drought, frost, heavy insect infestation, or overgrazing can result in increased levels of these compounds, which along with tannins, are part of the plants’ defence mechanisms. The use of potassium nitrate fertilizer was also shown to increase cyanogens production in sorghum (Busk and Moller, 2002). In the stomach of livestock, cyanogenic glycosides may be converted into hydrogen cyanide, which is toxic, and at low level chronic exposure may result in poor growth or reduced milk production.

Although sprouted sorghum can contain high levels of cyanogens, typical methods of processing sprouted sorghum grain for human

consumption, such as manual degermination (removal of roots and shoots) removes most of the toxin (Traore et al., Dada and Dendy, 1987).

TANNINS

Early literature identifie annic acid as an anti nutritional factor in sorghum grain. However, more recent research indicates that tannic acids is not a sorghum component (Dykes and Rooney, 2006). Some but not all sorghum varieties have pigmented testa containing condensed tannins, polyphenolic compounds that possibly give the seed a bitter taste and have been known to reduce intake, digestibility (particularly protein), growth and feed efficiency of livestock (Gilani et al., 2005; Waniska and Rooney, 2000) tannins act as a plant defence against consumption by birds and also provide some resistance to mold.

PHYTIC ACID

Like all grain species, sorghum contains phytic acid which binds minerals and reduces their availability to the consumer. Since

sorghum grain is usually low in mineral content and the presence of phytic acid likely rendering its low mineral content unavailable, supplementation with other mineral sources is necessary where sorghum is a major component of the diet.

 

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