Identifying Ghanaian Cassava Leaves with Safe Cyanide Levels for Use as Vegetable


Master's Thesis, 2011

107 Pages, Grade: pass


Excerpt

TABLE OF CONTENTS

Declaration.

Abstractiii

Dedication

Acknowledgements

Table of Contents

List of Figures

List of Plates

List of Tables

List of Abreviationsxiii

CHAPTER ONE
.1.0 INTRODUCTION
1.1. Background
1.2. Problem Statement
1.3. Rationale for the Study
1.4. Research Questions/Hypotheses
1.5. Objectives
1.5.1. Main Objective
1.5.2. Specific Objectives

CHAPTER TWO
.2.0. LITERATURE REVIEW
2.1. Cassava Cultivation in Ghana
2.2. Prevalence of Micronutrient Deficiencies in Ghana
2.2.1. Impacts of Micronutrient Deficiencies
2.3. Nutritional Value of Cassava Leaves
2.3.1. Micronutrient Potential of Cassava Leaves
2.3.1.1. The Effects of Vitamin C on Iron Status
2.3.1.2. The Effects of Carotenoids on Vitamin A Status
2.3.1.3. Dietary Protein as Enhancer of Zinc Absorption
2.3.1.4. Nutrient Requirements: The Case of Vitamin A
2.3.2. Amino Acid Profile of Cassava Leaves
2.3.3. Effects of Processing on the Nutrient Contents of Cassava Leaves
2.4. Cassava as a Cyanogenic Plant
2.4.1. Synthesis, Transport and Contents
2.4.2. Factors Affecting Cyanogenic Glycoside Contents
2.5. Pathological Effects of Cassava Cyanide Exposure
2.5.1. Metabolism/Detoxification of Cyanide in the Body
2.5.2. Pathological Effects of Cassava Cyanide Consumption
2.5.2.1. Evidences of Chronic Cassava Cyanide Intoxication

CHAPTER THREE
3.0. METHODOLOGY
3.1. Field Survey
3.1.1. Selection and Description of Study Sites
3.1.2. Sample Size Calculation and Selection of Respondents
3.2. Sample Collection
3.2.1. Sampling Technique
3.2.2. Exclusion/Inclusion Criteria
3.3. Chemical Analysis
3.3.1. Materials
3.3.2. Preparation of analytical Samples
3.3.3. Determination of Cyanide Concentration
3.4. Statistical Analysis

CHAPTER FOUR
4.0. RESULTS
4.1. Field Survey
4.1.1. Consumption of Cassava Leaves among Ghanaian Farmers
4.2. Chemical Analysis
4.2.1. Cyanide Content of Leaves of Most Commonly Consumed Cassava Varieties
4.2.2. Predictors of Cyanide Concentration
4.2.3. Identifying Safe Varieties by Wet weight Cyanide Content Only
4.2.4. Identifying Safe Varieties by Dry Weight Cyanide Content Only

CHAPTER FIVE
5.0. DISCUSSION
5.1. Field Survey
5.1.1. Background Characteristics of Respondents
5.1.2. Consumption of Cassava Leaves among Ghanaian Farmers
5.1.3. Commonly Consumed Cassava Varieties
5.2. Chemical Analysis
5.2.1. Cyanide Contents of the Leaves Popular Ghanaian Cassava Varieties
5.2.2. Environmental Impacts on Improved Varieties as Opposed to Unimproved Ones
5.2.3. Identifying Leaves with Safe Cyanide Levels
5.2.4. Reasons for Difference in the Effects of Processing Methods

CHAPTER SIX
6.0. CONCLUSIONS AND RECOMMENDATIONS
6.1. Conclusions
6.2. Recommendations

.REFERENCES

.APPENDICES

ABSTRACT

Cassava leaves have the potential to address protein and micronutrients deficiencies but the downside is the presence of cyanogenic glycosides. Consumption of a cyanogenic plant has been implicated in many pathological disorders including goitre. No study had been done in Ghana between cassava leaves consumption and goitre which was reported endemic in the northern belt. The northern belt of Ghana widely consumes cassava leaves, unlike the southern and the middle parts. The main objective of the study was therefore to identify Ghanaian cassava leaves with safe cyanide levels for use as vegetable. The study was conducted in three communities each in the southern, transition, and the northern belts of Ghana, on the basis that an identified agricultural research station closest to those communities in the respective belts, was the most active in the root and tuber improvement and marketing programme in Ghana. Cyanide analysis was done using the standard Chloramine-T/Pyridine barbituric acid colorimetric method, at the Société Générale de Surveillance laboratory in Tema. The cyanide content of Ghanaian cassava leaves from most commonly consumed varieties ranged from 72.79 – 203.50 ppm, dry weight. This is relatively lower than what have been reported by some researchers in other countries. Ghanaians in general, therefore, grow relatively low-cyanide cassavas. Two-factor analysis of variance revealed a significant interactive effect (p=0.024) between type of variety (genotype) and the geographical location (environment), of which the impact was greater in the unimproved varieties. By adequate processing involving pounding followed by boiling, all cassava leaves considered in this study were all safe for consumption. It is recommended that Ghanaian cassava leaves be promoted for use as vegetable, especially in the south. This must however be accompanied with proper education on the potential toxicity and on adequate processing techniques.

DEDICATION

This Thesis is dedicated to my Heavenly Father.

ACKNOWLEDGEMENTS

I gratefully acknowledge the International Development Research Center (IDRC, Canada) and the Double Fortified Salt Project for the sponsorship.

I am highly indebted to my supervisors, Prof. E. Asibey-Berko and Prof. Anna Lartey for seeing to the success of this work.

I also want to appreciate Mr. Salifu, Mr. Nyamekye, Mr. Boateng, Mr. Lambert, Mr. Appiah, Mr. Abass, Mr. Akyia, Mr. Arthur and Mr. Shiras – all of the root and tuber improvement and marketing programme (RTIMP), Ministry of Food and Agriculture (MoFA) - Ghana. Their assistance in identifying potential places for sample collection was invaluable.

To the Lectures of the dept. of Nutrition and Food Science, University of Ghana, I want to say thank you very much for your constructive criticisms.

David Achulo was a true friend in this work, I thank him.

Last, but not the least, I want to acknowledge the assistance of the following relatives in the entire programme: Capt. Ampomah (of blessed memory), Capt. Thompson, Mr. Osei and Kwasi Dua.

LIST OF FIGURES

Fig. 1.1: Global Share of Cassava Production Compared with Proportion of Population of Undernourished and Micronutrient Deficient

Fig. 1.2: Intensity of Cassava Production by Region in Ghana

Fig. 1.3: Intensity of Cassava Production in the World

Fig. 2.1: Agro-Ecological Zones of Ghana

Fig. 2.2: Consequences of Micronutrient deficiencies during the Life Cycle

Fig. 2.3: Biosynthesis of Cyanogenic Glycosides

Fig. 2.4: Scheme of Cyanogenesis in Plants

Fig. 2.5: Pathways of Cyanide Metabolism

Fig.3.1: Surveyed Areas of the southern, middle and northern Ghana

Fig. 4.1: Most Commonly consumed Ghanaian Cassava Varieties

Fig. 4.2: Identifying Safe Varieties by Coursey and Bolhuis Cut-off

Fig. A1: Calibration Curve for Cyanide Determination

Fig. A2: Correlation Matrix of Variables

LIST OF PLATES

Plate 1: Afisiafi (Improved Variety – Southern Belt)

Plate 2: Afisiafi (Improved Variety – Transition Belt)

Plate 3: Afisiafi (Improved Variety – Northern Belt)

Plate 4: Bankyehemaa (Improved Variety – Southern Belt)

Plate 5: Bankyehemaa (Improved Variety – Transition Belt)

Plate 6: Bosomensia (Unimproved Variety – Southern Belt)

Plate 7: Bosomensia (Unimproved Variety – Transition Belt)

Plate 8: Afosa (Unimproved Variety – Transition Belt)

Plate 9: Bensere (Unimproved Variety – Transition Belt)

Plate 10: Capevas Bankye (Improved Variety – Transition Belt)

Plate 11: Nyerikogba (Improved Variety – Northern Belt)

Plate 12: Eskamaye (Improved Variety – Northern Belt)

Plate 13: Gbanfuful (Unimproved Variety – Northern Belt)

LIST OF TABLES

Table 2.1: Nutritional Composition of Cassava Leaves Compared to other Leafy Vegetables

Table 2.2: Amino Acid Profile of Cassava Leaves Compared to Egg White and Soybean

Table 3.1: Varieties Available where Samples were obtained for Analysis

Table 3.2: Reagents and their Sources

Table 4.1: Background Characteristics of Respondents

Table 4.2: Consumption of Cassava Leaves and Knowledge of their Nutritional Value

Table 4.3: Comparing Mean Cyanide by Type of Variety

Table 4.4: Multiple Regression Analysis

Table 4.5: Identifying Safe Varieties by the WHO Cut-off of 10 ppm of Residual Cyanide after Processing

Table A1: Varieties Provided by Respondents and Age of Plant

Table A2: Raw Data on Chemical Analyses

Table A3: Intensity of Temperature in the Three Geographical Locations

Table A4: Rainfall Patterns in the Three Geographical Locations

Table A5: Relative Humidity in the Three Geographical Locations

Table A6: Altitudes of Areas where Cassava leaves were sampled in the Southern Belt

Table A7: Altitudes of Areas where Cassava leaves were sampled in the Transition Belt

Table A8: Altitudes of Areas where Cassava leaves were sampled in the Northern Belt

LIST OF ABBREVIATIONS

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CHAPTER ONE

1.0. INTRODUCTION

1.1. BACKGROUND

Cassava (Manihot esculenta Crantz) is a perennial, woody shrub belonging to the family Euphorbiaceae. It is found in the tropics and sub-tropics of Africa, Latin America and Asia, where micronutrient deficiencies and undernutrition are coincidentally widespread (Fig. 1.1). It is mostly adapted to latitudes 30º north and 30º south of the equator; at altitudes up to 2,300 m above sea level; optimal temperature between 25-29 ºC; annual rainfall from < 600 mm (in the semi-arid tropics) to > 1,500 mm (in the sub-humid and humid tropics); and within soil pH of 4-9 (Alves, 2002).

The nutritional value of cassava leaves as rich source of protein and micronutrients has long been recognised. For example, some authors have recommended cassava leaves be made into acceptable edible forms through industrial means and backed up by educational programmes (Lancaster and Brooks, 1983). This underscores their wide use as vegetable in some countries like Brazil, Democratic Republic of Congo (DRC), Guinea, Indonesia, Liberia, Nigeria, Sierra Leone, the Philippines and other countries where cassava is widely cultivated. Bianchini (2003) defines vegetable as edible plant part including stem, stalk, root, tuber, leaf, flower and fruit; generally consumed raw or cooked with a main dish, in a mixed dish, as an appetizer or in a salad. Unlike those cassava-growing areas, the nutritional potential of cassava leaves is under-appreciated in Ghana, as a survey ranked them the least important among all cassava-based foods: “Gari > Fufu > Kokonte > Ampesi > Agblema > Yake Yake > Akple/Banku > Roasted/Fried > Tapioca > Cassava Leaves” (Anonymous, 1986).

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Fig. 1.1: Global Share of Cassava Production Compared with Proportions of Population Undernourished and Micronutrient Deficient

Data Source: FAO/WFP (2010), IFPRI (2010), WHO (2009a, b), FAOSTAT (2008)

Despite the little recognition that most Ghanaians accord cassava leaves, the country is classified among the few, that have drastically reduced undernutrition by more than 30% points between 1979 and 1998; a drop from 62% to 10% (FAO, 2000). Available data now show that, the proportion of the population that are undernourished is 5% (FAO/WFP, 2010). The country is declared to have achieved the World Food Summit goal: “Reduce the number of hungry people by half between 1990-1992 and 2015”; and the Millennium Development Goal 1, target 1C: “Reduce the proportion of this number by half between 1990-2015” (FAO/WFP, 2010). According to the FAO (2000), an important underlying factor to this achievement was the rapid increase in cassava products supply during those periods; production tripled between 1961 and 1999. The FAO (2000) attributed this phenomenal growth in cassava production to implementation of government programmes and economic policies that favoured the promotion and the marketing of roots and tubers, and by so doing encouraged the spread of cassava products in the country.

Therefore, cassava as a staple food crop is extremely important in Ghana. This is further reflected in its contribution to as much as 22% to Ghana’s Agricultural Gross Domestic Product (MoFA, 2004), making cassava the highest agricultural commodity in the country, with cultivation spreading throughout the country (Fig. 1.2). Today, the country is recognised among the top producers of cassava in the world; it is the 6th leading producer, after Nigeria, Brazil, Thailand, Indonesia, and Democratic Republic of Congo (DRC) (Fig. 1.3).

Source: Produced from SRID-MoFA (2010)

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Fig. 1.2: Intensity of Cassava Production by Region in Ghana

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Fig. 1.3: Intensity of Cassava Production in the World

1.2. PROBLEM STATEMENT

Cassava is a cyanogenic plant and consumption has been implicated in both acute and chronic cyanide intoxication. This makes cassava (leaves) consumption an important issue in food safety and/or public health. Of particular mention is the observation that consuming a cyanogenic plant is an aetiological factor to the persistence of residual goitre in the post salt iodisation phase (Marwaha et al., 2003), to which Ghana is liable to be a victim. This is because a biological impact assessment study on salt iodisation programme in Ghana revealed a high prevalence (mild) of iodine deficiency (10.6-18.2% total goitre rate) in severely endemic zones after more than a decade of mandatory consumption of iodised salt by an Act of Parliament (Asibey-Berko and Armah, 2007).

1.3. RATIONALE FOR THE STUDY

According to the Copenhagen Consensus (2008), consumption of local foods deserves promotion because among resource-poor and hard-to-reach populations with fortified foods, local foods seem to be the only accessible option for alleviating micronutrient deficiencies. Therefore, promoting the consumption of cassava leaves (readily accessible and cheap source of micronutrients) is therefore consistent with this recommendation. Furthermore, in an interview with the MediaGlobal (2009) on the link between cassava consumption and iodine deficiency, it was reported that no study had been done in Ghana between cassava consumption and iodine deficiency. Meanwhile, widespread consumption of cassava leaves has been linked to high goitre rates in Sri Lanka (Priyadarshani, et al., 2004).

1.4. Research Questions/Hypotheses

1. Should there be promotion of cassava leaves consumption in Ghana and on what basis should this be done? It is hypothesised that Ghanaian cassava leaves would have ≤ 10 ppm dry weight cyanide after processing.
2. Preliminary survey revealed that cassava leaves are widely consumed in the northern belt and with respect to Ghana’s IDD history (endemic in the north), could consumption of the leaves of the northern cassava varieties be a contributory factor and is that geographical zone prone to chronic cyanide intoxication? It is hypothesised that the northern belt would have elevated risk to chronic cyanide intoxication from the consumption of a cyanogenic plant, compared to the middle and south of Ghana.

1.5. OBJECTIVES

1.5.1. Main Objective

Identify Ghanaian cassava varieties with safe cyanide levels in their leaves.

1.5.2. Specific Objectives

1. Identify most commonly consumed cassava varieties in Ghana
2. Survey the consumption of cassava leaves among Ghanaian farmers and determine their knowledge of the nutritional value of cassava leaves.
3. Determine the cyanide contents of the leaves of the most commonly consumed varieties.
4. Investigate the effects of some ecological factors on cassava cyanogenesis.

CHAPTER TWO

2.0. LITERATURE REVIEW

2.1. CASSAVA CULTIVATION IN GHANA

Geographically, Ghana is situated in the Southern Coast of West Africa, between Latitude 4° 44’N and 11° 11’N; and Longitude 3° 11’W and 1° 11’E (MoFA, 2010). The total land area is about 23,853,900 H (238,539 km2). Of this, 57.1% constitute agricultural land area, out of which 53.6% is under cultivation as at 2009 (MoFA, 2010).

Agriculture contributes the highest share (34.1%) of Ghana’s Gross Domestic Product (GDP). The highest contribution to Ghana’s Agricultural GDP is from crops (61.8%), excluding cocoa. Of this about 22% is from cassava, making it the highest contributor to Ghana’s Agricultural GDP, among all other crops grown; maize and cocoa respectively contribute 5% and 11.5% (MoFA, 2010).

There are 7 agro-ecological zones in Ghana namely, Sudan Savannah; Guinea Savannah; Transitional Zone; Moist Semi-deciduous Forest; Evergreen Forest; Coastal Savannah; and Strand and Mangrove Swamp. These respectively cover 1%, 46%, 15%, 32%, 3%, 2%, and 1%; of total land area (Fig. 2.1). Cassava cultivation is documented on all the agro-ecological zones in Ghana. The contribution of the Sudan Savannah Zone, which covers only 1% of the total land area (Fig. 2.1), is however negligible (Fig 1.2).

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Among all the documented Ghanaian cassava varieties, it is worth mentioning that Eskamaye, Filindiakong, and Nyerikogba, popularly known as the Northern improved varieties are only recommended for cultivation in the Guinea Savannah Zone (RTIMP, 2004). In other words such varieties can thrive only in that area. Cassava is widely cultivated in the Country; in terms of area planted, it ranks second (8,858 km2) to maize but has the highest production level (12,230,600 tonnes), among all the major staples (MoFA, 2010). Available data further reveal that cassava has the highest per capita consumption (152.9 kg/head/year) among all the major food crops (MoFA, 2010).

From the foregoing, it is evident that cassava is a very important staple food crop in Ghana. It is high time their leaves were widely patronised as vegetable, given their micronutrient richness in order to help combat some nutritional deficiencies like vitamin A, iron and zinc, which are widespread in the country.

2.2. PREVALENCE OF MICRONUTRIENT DEFICIENCIES IN GHANA

Estimates show that over 2 billion people worldwide are at risk of vitamin A, iodine and/or iron deficiencies, with extremely high prevalence in Southeast Asia and sub-Saharan Africa (Ramakrishnan, 2002).

In Ghana it is estimated that about 75% and 18.1% of preschool-age children and pregnant women respectively have serum retinol < 0.70 µmol/L (WHO, 2009a), which are respectively severe and moderate, by definition. There is also an estimated 0.4% (mild) prevalence of night blindness for the preschool-age population (WHO, 2009a). These make vitamin A deficiency a public health importance in Ghana.

Anaemia (i. e. Hb < 11.0g/dL) is estimated to be 76.1% (severe) among preschool-age children (WHO, 2009b). Among pregnant women and women of child-bearing age (15-49 years) the estimates are respectively 64.9% and 43.1%, interpreted severe in both cases (WHO, 2009b). About 50% of all anaemia cases are as a result of dietary iron deficiency. Zinc deficiency, like iron and vitamin A, is also widespread. A study demonstrated a prevalence of 46% (severe) in adolescents (Asibey-Berko and Abbey, 2005). Another study among pregnant women also found 54% (severe) prevalence (Ashong et al., 2006).

On iodine nutrition, the situation is not different; A study revealed high prevalence of iodine deficiency in two districts, with more than 20% of the inhabitants having unsatisfactory urine iodine level (<50 µg/L) (Asibey-Berko and Armah, 2007). Nearly one-and-a-half decades before the study, iodine deficiency in those sentinel districts were described to be critical with median urine iodine level <5 µg/L (Asibey-Berko and Orraca-Tetteh, 1994). A marginal micronutrient status increases the risk of morbidity and mortality and reduces work capacity and ultimately productivity; posing great barrier to socio-economic development, as illustrated below.

2.2.1. Impacts of Micronutrient Deficiencies

Global report on vitamin and mineral deficiencies (UNICEF/World Bank/USAID/GAIN/MI/FFI, 2009) depicts that 1.1 million preschool-age children die annually from vitamin A and zinc deficiencies. These two micronutrients alone contribute to 20-24% of deaths from measles, diarrhoea and malaria. In high-mortality developing regions, vitamin A and zinc deficiencies were respectively the 5th and 6th leading cause of infectious and parasitic disease burdens, which have been reported to be responsible for more than 50% of child mortality (Lopez et al., 2006; Ezzati et al., 2002). An estimated 1.6 billion people also suffer reduced productive capacity due to anaemia, and iron-deficiency anaemia is reported to cause 2% loss of annual GDP in worst affected areas (UNICEF/World Bank/USAID/GAIN/MI/FFI, 2009).

The report further revealed that babies who were born mentally impaired due to iodine deficiency were 18 million. Reduced intellectual capacity (the most devastating consequence of iodine deficiency) weakens educational investments and further perpetuates the poverty cycle in worst-affected areas. Iodine deficiency is known to have lowered intellectual capacity by as much as 15% points in many nations (UNICEF/World Bank/USAID/GAIN/MI/FFI, 2009). The World Bank (1994) estimated that, as much as 5% of the world’s economic output is lost because of micronutrient malnutrition like iodine deficiency. In Ghana it is estimated that, productivity loss due to iodine deficiency is worth US$ 22 million per year (MediaGlobal, 2009).

According to the FAO (2010), the total cost of protein-energy malnutrition, low-birth weight babies and micronutrient deficiencies, through a lifetime of one cohort of undernourished children is estimated to be between 5-10% of the GDP of developing countries (US $500 billion-US $1 trillion). The short- and long-term consequences of micronutrient malnutrition during the life cycle of a human are shown in Fig. 2.2. The nutritional potential of cassava leaves as demonstrated in section 2.3 may be promising in the combat of some of these problems.

Fig. 2.2: Consequences of Micronutrient Deficiencies during the Life Cycle.

Adapted from ACC/SCN (2000)

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2.3. NUTRITIONAL VALUE OF CASSAVA LEAVES

2.3.1. Micronutrient Potential of Cassava Leaves

The potential of cassava leaves in addressing micronutrient deficiencies is reflected in their nutritional composition. Cassava leaves are good source of minerals and vitamins. As shown in Table 2.1, most mineral and vitamin contents (especially vitamin A, iron and zinc), of cassava leaves are greater than those of cocoyam leaves (kontomire) and cabbage. These are well acclaimed leafy vegetables in Ghana.

From Table 2.1, cassava leaves have high contents of β-carotene (20,803 μg/100g) and vitamin C (mg/100g). The respective roles of these nutrients on vitamin A and iron stata are well established (sections 2.3.1.1 and 2.3.3.2). Lean red meat, whole grain cereals, and legumes are known to be the richest sources of zinc, with concentrations ranging from 25-50 mg/kg (25-50 ppm) raw weights (WHO/FAO, 2004). As seen in Table 2.1, cassava leaves could be classified as one of the good plant source of zinc as well (5.0 mg/100g ≡ 50.0 ppm).

2.3.1.1. The Effects of Vitamin C on Iron Status

Iron absorption, being heam (animal-source) or non-heam (plant-source) occurs in the duodenum and the jejunum. Factors affecting iron bioavailability include iron content of food, the source, dietary components and body requirement. It is known that only about 1-10% dietary load of non-heam iron is absorbed and the most potent enhancer of such iron is ascorbic acid (vitamin C) (Sharp et al., 2007).

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Table 2.1: Nutritional Composition of Cassava Leaves Compared to other Leafy Vegetables

Source: Stadlmayr et al. (2010)

The positive effects of vegetable consumption (especially when consumed with the diet) in the absorption of non-heam iron is well documented. For example, Thankachan et al. (2008) reported a strong enhancing effect of ascorbic acid from vegetables on iron absorption. Even though, Péneau et al. (2008) reported a positive significant association between hemoglobin concentration and vitamin C-rich vegetables, no association was reported between serum ferritin and vegetable consumption based on vitamin C content. Sirdah (2008) also reported a significantly higher hemoglobin concentration in adolescents who frequently consumed vegetables than those who did not. Davidson et al. (1998) observed a 3-fold increase in fractional iron absorption when they added ascorbic acid to a test meal in their study on the influence of ascorbic acid on iron absorption. Shu and Ogbodo (2005) also in correlating serum iron with serum vitamin C among pregnant women, in their bid to find the role of vitamin C in the prevention of iron-deficiency anaemia, recommended adequate vitamin C intake throughout gestation.

As shown in Table 2.1, cassava leaves have high ascorbic acid content (370 mg/100g) and therefore consumption may maximize iron absorption. Ascorbic acid acts by reducing the non-heam iron (mostly ferric, Fe3+) to the more soluble and readily absorbable ferrous state (Fe2+) (Han et al., 1995). This is believed to take place in the acid environment of the gastric lumen because in the context of achlorhydria (a condition of defective hydrochloric acid production in the stomach), iron deficiency anaemia is mostly present (Sharma et al., 2004; Poskitt, 2003). The pro-absorptive action of dietary vitamin C can also counteract the actions of phytates and polyphenols (the most potent inhibitors of non-heam iron absorption). When the effects of ascorbic acid on the inhibition of iron absorption by sodium phytate at different levels of phytate were studied, Hallberg et al. (1989) reported that, at all levels of phytate, ascorbic acid induced a dramatic increase in iron absorption, with the increase being more dramatic with higher inhibition from phytate.

2.3.1.2. The Effects of Carotenoids on Vitamin A Status

Vitamin A in foods occurs in two forms; preformed retinol as retinyl esters and provitamin A carotenoids, which are mostly β-carotene, α-carotene and β-cryptoxanthin (Hess et al., 2005). Preformed retinols are of animal origin and highly bioavailable, whereas, provitamin A carotenoids are of plant origin and less bioavailable but can however be hydrolyzed to release retinol in the intestinal mucosa. The major provitamin A carotenoid is β-carotene which can give rise to two retinal molecules upon central cleavage, and further reduction to retinol, followed by esterification (Casternmiller and West, 1998). In cassava leaves, trans-β-carotenes constitute the major carotenes (>76%), followed by cis-β isoforms (Nassar et al., 2005).

Bioavailability, which is the fraction of an ingested nutrient that is available for utilization in normal physiological functions or for storage, of β-carotene is known to range from 4-55%; whereas bioconversion, the proportion of bioavailable carotene converted to retinol, is between 60-70% (Van Vliet et al., 1996). These depend on several factors which De Pee and West (1996) have termed SLAMENGHI. That is, S pecies of carotenoids (trans isomers are readily absorbed); Molecular L inkage; A mount consumed in a meal; M atrix in which the carotenoid is incorporated (β-carotene dissolved in oil is readily absorbed than that in foods); E ffectors of absorption and bioconversion (presence of protein in the small intestine helps stabilize fat emulsions and enhances micelle formation and carotenoid uptake while absorption and bioconversion is markedly reduced when dietary fat intake is low); N utrient status of the host (Iodine deficiency impairs absorption); G enetic factors; H ost-related factors (e. g. worm infestation); and Mathematical I nteractions.

To buttress some of these factors with empirical evidence, Takyi (1999), for example observed that when Ghanaian Children consumed dark green vegetables (Cassava and kapok leaves) with added fat, there was a significant enhancement in their serum retinol from baseline. The author in his conclusion stated that increased consumption of cassava and kapok leaves could provide affordable and sustainable means of reducing or controlling vitamin A deficiency. His result was consistent with that of other investigators (Jalaal et al., 1998; Siqueira et al., 2007). Jalaal et al. (1998) reported that addition of β-carotene in meals significantly increased serum vitamin A levels, however dramatic increase occurred when the β-carotene was administered together with fat under dewormed conditions. The demonstration of Siqueira et al. (2007) was in laboratory rats. They reported that the β-carotene of cassava leaves was effective in enhancing the vitamin A status of deficient laboratory rats and also increased their liver stores of retinol. These researchers justified their choice of cassava leaves as vitamin A source in their protocol by saying that, the identification of new, safe and low-cost sources of vitamin A for hungry populations that do not consume other vitamin A sources is very crucial.

2.3.1.3. Dietary Protein as an Enhancer of Zinc Absorption

Cassava leaves are rich in protein with amino acid profile comparable to that of high quality proteins like egg white (Montagna, et al., 2009; Table 2.2). Even though this is mentioned with animal proteins, conferring a high zinc bioavailability of 30-40% (Sandström, 1995), a mixed diet of cassava leaves could be expected to confer moderate to high bioavailability of zinc. In a single-meal human studies that mixed diet with animal protein, and also vegetarian diet with moderate contents of unrefined cereals; there were increased zinc bioavailability to a moderate degree (20-30%) (Sandström, 1995).

It is also established through animal experimentation that soluble, low-molecular-weight organic substances like sulphur-containing amino acids and hydroxy-acids are enhancers of zinc absorption. They do this by acting as ligands or increasing zinc solubility (Sandström and Lönnerdal, 1989). From Table 2.2, though the sulphur-containing amino acids are limiting in cassava leaves, threonine (an essential and hydroxyl-containing amino acid) is among the amino acids with the highest chemical scores (1.0). This observation may reinforce cassava leaves’ role in enhancing zinc status.

Table 2.2: Amino Acid Profile of Cassava Leaves Compared to Egg white and Soybean

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*Essential Amino Acid EAAI = Essential Amino Acid Index a Ngudi et al. (2003a) b Zamora (2005)

2.3.1.4. Nutrient Requirements of Micronutrients Compared to Contents inCassava Leaves: The Case of Vitamin A

The WHO/FAO (2004) defines dietary requirement for a micronutrient as intake level that meets a specified criteria for adequacy, and therefore minimizes risk of nutrient deficiency or excess. It is granted that the biological activity of provitamin A may differ from one vegetable- or fruit-source to another. When a 50 g cooked carrot was included in a staple diet, it provided 500 µg RE (Oyarzun et al., 2001), which is the recommended nutrient density for vitamin A (WHO/FAO, 2004). In the double-blind randomized controlled trial of Takyi (1999), who documented significant enhancement in serum retinol from baseline, the test meal was made to provide 400 RE per 50 g stew of β-carotene from the dark-green leafy vegetable (cassava leaves). By calculation this means that, regarding the fact that he documented 1700 RE per 100 g of cooked cassava leaves, approximately 23.52 g of cooked cassava leaves was used in the stew, and therefore provided 80% of the recommended nutrient density of vitamin A. Using 50 g of cooked cassava leaves regularly in diets may therefore provide about 170% of the recommended nutrient density of vitamin A.

2.3.2. Amino Acid Profile of Cassava Leaves

Besides micronutrients, cassava leaves also contain protein, especially in the young apical leaves (Hock-Hin and Kalanethee, 1989). They are significantly high in alanine, arginine, aspartic acid, glutamic acid, glycine, leucine, lysine and phenylalanine (Table 2.2). The amino acid profile of cassava leaves is comparable to high-quality proteins like egg white and soybean, as demonstrated in their chemical scores, even though, as in soybean, they are limiting in the sulphur-containing amino acids (Table 2.2). Amino acid score is defined as mg of amino acid in 1 g of a test protein divided by the mg of the same amino acid in 1 g of a reference protein (egg white) (FAO/WHO/UNU, 1985). The Essential Amino Acid Index (EAAI), which is the scores of all the essential amino acids relative to the reference protein, of whole soybean is 84%. This does not differ so much from that of cassava leaves (EAAI=80%). As is evident from Table 2.2, Ngudi et al. (2003a) have documented that the total essential amino acid composition of raw cassava leaves is higher (35.7 – 40.1 g/100g) than what the FAO/WHO (1991a) recommends (33.9 g/100g).

The protein in cassava leaves being compared to egg white and soybean is justified by the fact that evaluating protein quality by Protein Digestibility Corrected Amino Acid Score (PDCAAS), which is the preferred method for routine prediction of protein quality, rates egg white and soy protein scores of 1.0 (the highest possible score) and 0.99 respectively (FAO/WHO, 1989). This PDCAAS is the official method by the World Health Organization (WHO), the United States Food and Drug Administration (FDA) and the United States Department of Agriculture (USDA) for evaluating protein quality, even though Sarwar (1997) has questions about its validity.

It is therefore expected that when cassava leaves gravy or sauce is eaten with cereals such as rice, sorghum, millet, wheat, etc. (which are rich in the sulphur-containing amino acids) but deficient in lysine and threonine; a good composite protein would result in the diet, and this would upgrade the protein quality of the diet by optimizing the balance of essential amino acids. Getahun et al. (2003) observed the consumption of cereals to be protective against neurolathyrism, a paraparetic condition that has been associated with cassava cyanide consumption.

2.3.3. Effects of Processing on the Nutrient Contents of Cassava Leaves

Studies on the effects of processing on the nutritional contents of cassava leaves have presented mixed results. For example, in a study to find the effects of some African processing methods of cassava leaves: heat-treated; pounded and cooked; crushed, grounded and cooked; on nutrient contents; Achidi et al. (2008) found that the processing methods had no effects on the contents of ash, lipids, protein, carotene, zinc, copper, magnesium, phosphorus sodium, calcium and manganese; there was even a 3-5 – fold increase in iron content (with grinding but not pounding), but a decrease in the contents of ascorbic acid and thiamine. Awoyinka et al (1995) have reported that blanching increased protein content and in vitro protein digestibility, but decreased ash and mineral contents.

Bradbury and Denton (2011) in their recent study on mild processing methods of cassava leaves to remove cyanogens and conserve key nutrients revealed that, the most preferred method involves pounding the leaves for at least 10 minutes until well macerated and washing the macerated leaves twice in twice their weight of water at ambient temperature, discarding the water after each wash. According to these investigators, this technique reduces the total cyanide to 8% of its original concentration and still retains nearly all the essential nutrients like protein, sulphur-containing amino acids (which is important for cyanide detoxification in the body), and vitamins. Two further washes reduce the cyanogens even further to 3% Bradbury and Denton (2011).

The second method, according to the researchers, involve treating the intact leaves in ten times their weight of water at 50 °C for 2 hours (blanching), followed by a change of water and another treatment under the same conditions as the first. Essential nutrients remained intact and cyanogens levels reduced to 7% (Bradbury and Denton, 2011).

The nutrients in cassava leaves are of plant origin and are therefore limited in digestibility and uptake by anti-nutritional factors and toxic substances among which are alkaloids, cyanogenic glycosides, nitrates, nitrites, oxalates, phytates, polyphenols, saponins, and toxic proteins. Among all these factors, the greatest barrier to the culinary use of cassava leaves is the cyanogenic glycosides.

2.4. CASSAVA AS A CYANOGENIC PLANT

2.4.1. Synthesis, Transport and Contents

The endogenous presence of cyanogenic glycosides in cassava (leaves, petioles, stem, roots, etc.) is well documented (Wobeto et al., 2007; McMahon et al., 1995; Bradbury et al., 1991). In cassava, the cyanogenic glycosides exist predominantly as linamarin (95% of total content of cyanogens) and lotaustralin (methyl linamarin, 5%) (Bradbury et al., 1991). The precursors for their synthesis are L-valine (for linamarin) and L-isoleucine (for lotaustralin). The amino acids are hydroxylated to form L-hydroxyl amino acids, and then converted to aldoxime, and in turn to nitrile (Fig. 2.3). Hydroxylation of the nitrile forms hrodroxynitrile leading to the formation of the corresponding cyanogenic glycoside, following glycosylation.

When cassava tissues are disrupted by any mechanical means such as cutting, grating, bruising, etc., the endogenous enzymes (linamarase) come into contact with their substrates (cyanogenic glycosides) and hydrolyse them into glucose and cyanohydrins (Fig. 2.4). This is because the cyanogens (cyanogenic glycosides) and the cyanogenic enzymes are found in different compartments in the cell (Whyte et al., 1994). This presupposes that cassava (leaves), when intact, cannot generate free hydrocyanic acid (Teles, 2002). At pH > 5 and temperature >30˚C, cyanohydrins break down spontaneously into ketones and hydrocyanic acid (HCN) (Fig. 2.4). In the leaves, cyanohydrins can also be decomposed by a second enzyme hydroxynitrile lyase; hence giving a higher linamarase catalytic efficiency in cassava leaves than in the roots (Whyte et al., 1994). Bokanga (1994) has reported that the enzymatic catalytic activity in cassava leaves is over 200 times greater than in the roots.

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* R1 and R2 are CH3 for liminarin and the amino acid is L-valine while they are respectively C2H5 and CH3 for lotaustralin and the amino acid is L-isoleucine

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Fig. 2.3: Biosynthesis of Cyanogenic Glycosides. Adapted from Conn (1973)

From literature, the cyanide content in leaves with petioles could be 5-20 times higher than that in the root parenchyma (Bokanga, 1994). Available data indicate that the cyanide content of cassava leaves can range from 53 – 1, 300 ppm (Wobeto et al., 2007; Siritunga and Sayre, 2003). Hidayat et al. (2002) reported a range of 9-779 ppm, whereas Bradbury and Denton (2011) reported 60-1,860 ppm total cyanide in cassava leaves.

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2.4.2. Factors Affecting Cyanogenic Glycoside Contents

Cyanide content of cassava (both leaves and roots) varies from one variety to another and even for the same varieties; there is influence of both endogenous and exogenous factors. In an experiment to determine anti-nutrients in cassava leaf powder, Wobeto et al. (2007) found that cyanide levels increased with maturity of plant. Teles (2002) has reported that young leaves contained the most hydrocyanic acid; an observation consistent with that of Bradbury and Denton (2011); Hidayat et al. (2002); and Nambisan and Sundaresan (1991). Furthermore, Bradbury and Denton (2011) have documented that even in cassava leaf blade, cyanide content increases from the tip towards the stalk end. Linamarin content of cassava root (both low and high cyanogenic cassava) is also known to increase to a peak level at 3 months after which it decreases to a constant level at 6 months and throughout the subsequent growth period until harvest. According to Nambisan and Sundaresan (1991), these are true when all conditions are stable.

Ecological factors known to affect cyanide content of cassava include soil fertility, climate and altitude. For example, stress on the plant due to insect attack or drought is reported to increases the cyanide content (Bokanga et al., 1994). Research in the US on white clover (Trifolium repens), another cyanogenic plant revealed greater cyanogenetic frequency for collections at low altitudes, high winter temperature, low summer precipitation, greater spring cloudiness, and less snow (Pederson et al. 1996). Caradus and Ford (1996) have also demonstrated a negative correlation between altitude and the population of cyanogenic plants. Under varying levels of potassium nutrition in the soil, a study found a significant variation in the cyanide content of cassava roots with the lowest content recorded for the cultivars with the highest potassium while the highest cyanide content was recorded for the cultivars with no potassium (control) (Endris, 2008).

2.5. PATHOLOGICAL EFFECTS OF CASSAVA CYANIDE EXPOSURE

2.5.1. Metabolism/Detoxification of Cyanide in the Body

Cyanide metabolism in the body occurs through several minor routes (Fig. 2.5), but the major route occurs in the liver involving the mitochondrial enzyme, rhodanese. Rhodanese catalyses the transfer of sulphane sulphur of thiosulphate to the cyanide radical to form thiocyanate (SCN–), which is less toxic (Fig 2.5). The primary source of the sulphur is a sulphur-containing amino acid (methionine). About 80% of cyanide in the body is metabolised this way (ATSDR, 1997). At high serum levels (250-450 µmol thiocyanate/L), SCN– is excreted in the urine, and can therefore serve as a biochemical indicator for one’s dietary exposure to cyanide (Rosling, 1994; Tylleskär et al., 1992).

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Fig. 2.5: Pathways of Cyanide Metabolism. Adapted from ATSDR (1997)

2.5.2. Pathological Effects of Cassava Cyanide Consumption

Consumption of poorly processed cassava (leaves, roots and their products) with high amount of cyanide has been associated with both acute and chronic cyanide intoxication (Milingi et al. 1992; Akintonwa and Tunwashe, 1992; Banea et al., 1992). Acute cyanide intoxication is due to cyanide’s high affinity for the ferric haem form of cytochrome C oxidase, the oxygen-reducing component of the mitochondrial electron transport chain. Cyanide inhibits this enzyme by first penetrating into the protein crevice and binding to the iron ion in the terminal electron acceptor of cytochrome C oxidase (Gupta et al., 2010). This forms a stable cytochrome C oxidase – CN complex and blocks electron transfer from cytochrome C oxidase to molecular oxygen. As a result there is a shift from aerobic to anaerobic respiration because of low tissue oxygen (hypoxia). The consequence is depletion of energy-rich molecules like glycogen, phosphocreatinine, adenosine triphosphate (ATP) and accumulation of lactate thereby reducing blood pH. Hypoxia together with lactic acidosis depresses the central nervous system and results in respiratory or cardiac arrest (Eisler, 1991).

In acute cyanide intoxication, cyanide absorption rate is greater than detoxification rate; free cyanide therefore accumulates in body tissues and fluid and this results in rapid onset of signs of acute cyanide poisoning beginning from headache, dizziness, nausea, vomiting, diarrhoea, stomach pains, confusion, lethargy, cyanosis (bluish skin), palpitation; leading to seizures, respiratory depression, cardiovascular collapse, coma and finally death (EPA, 2010; Akintonwa et al., 1994; Eisler, 1991; Milingi, et al., 1992). Acute cyanide intoxication from cassava consumption is very rare. Reported cases from Mozambique (in 1986), Tanzania (in 1988) and Brazil (in 1990) have been described to be more or less accidental (Teles, 2002). In Nigeria for instance, Akintonwa et al. (1994) reported that, eight people complained of abdominal pain, vomited, and went into coma with acute renal failure, and finally died from cardio-pulmonary arrest; few moments after the consumption of poorly processed cassava meal.

There are some pathological conditions that have been recognized from chronic cyanide intoxication, these include: Aggravation of protein deficiency; Increased susceptibility to goiter, cretinism and rickets, first reported by Delange et al. (1982); Tropical ataxic neuropathy (TAN), a neurological degenerative disease found in the tropics where there is large consumption of cassava, documented first by Monekosso (1964); Konzo (spastic paraparesis), an epidemic disease reported in rural areas of sub-Saharan Africa (Rosling and Tylleskär, 1996); Diabetes (McMillan, 1979); and Stomach cancer (Boyland, 1973).

2.5.2.1. Evidences of Chronic Cassava Cyanide Intoxication

There are quite a number of studies supporting chronic cyanide intoxication from cassava consumption, and the occurrence of the above-mentioned disorders.

Protein Deficiency

The essential amino acid, methionine is needed for cyanide detoxification in the body. Therefore under marginal protein status, cassava consumption could worsen the situation. It has been demonstrated that when dogs were fed cassava-containing diet, it increased their urinary excretion of nitrogen (proteinuria) and lowered their serum albumin (Kamalu, 1993; Ibebunjo et al., 1992). Kamalu (1993) reported swelling and vacuolation (cellular oedema) of the liver and kidneys of the cassava-fed experimental animals. According to him, linamarin from cassava-diets inhibited the Na-K-ATPase, and resulted in electrolyte imbalance from potassium loss. The loss of the potassium caused the cellular oedema and ruptured the proximal convoluted tubules of the kidney, allowing protein to filter through the glomerulus and leading to hypoalbumenaemia.

Iodine deficiency

On iodine nutrition, the main cyanide metabolite, SCN is a known goitrogen. SCN is known to competitively inhibit iodide uptake in the Na+/I- symporter (NIS) in the thyroid gland, at 100 μmol/L of blood (Preedy et al., 2009). At this level, SCN disrupt homeostatic feedback mechanisms that control biosynthesis, secretion and transport of thyroid hormones, ending in iodine deficiency in the form of goiter or hypothyroidism (Dohan et al., 2000; Adewusi and Akindahunsi, 1994). When weaned laboratory mice were treated with SCN for 25 days, Ghorbel et al., (2008) reported decrease in body weight, femur growth, thyroid iodine content, and plasma thyroid hormone levels; and increase in thyroid weight and thyroid stimulating hormone (TSH) levels; in SCN-treated mice compared to controls. There were improvements in all these variables when SCN was withdrawn in a different group after 15 days even though they did not reach control values.

According to Rao and Lakshmy (1995), even adequate intake of iodine may not ensure thyroid function in the presence of goitrogens. In the context of adequate iodine supply, taking goitrogen-rich diet is known to induce goitre (Chandrajith et al., 2005; Jooste et al., 1999). However, Wennberg (2009) is of the view that, the effects of cassava on thyroid gland is only likely in the context of already existing iodine deficiency.

Konzo

The disease Konzo is an upper motor neuron disease which causes irreversible and non-progressive paralysis of both legs in children between 4-12 years and women of child-bearing age. Victims of this disease have very poor socio-economic status and live in remote cassava-growing areas (Ngudi et al., 2011) This disease has been associated with famine and wars, where victims fall on poorly processed cassava with minimal protein supplementation (Adamolekun, 2010a). There have been reports of epidemic and endemic forms of this disease in the eastern, southern and the central parts of Africa. For example epidemics have been reported in Mozambique (Cliff et al., 1997) and Tanzania (Howlett et al., 1990). Both epidemic and endemic cases have also been reported in DRC since 1928 (Nhassico et al., 2008; Eisler, 1991; Tylleskär et al., 1991). Today, Konzo is still reported in DRC (Ngudi et al., 2011).

Several hypotheses have been proposed to describe the mechanisms involved in the aetiology of konzo from chronic cassava cyanide consumption. The most convincing mechanism seems to involve thiamine deficiency. It is postulated that konzo is a state of thiamin deficiency that results from thiamin inactivation arising from the absence of sulphur-containing amino acids. In the absence of sulphur-containing amino acids, the sulphur in thiamin is used for the detoxification process (Adamolekun, 2010a). Symptoms of konzo, such as spastic paraparesis, optic neuropathy, peripheral sensory neuropathy (characterized by impaired sensation to mild touch and pinprick in the extremities), encephalopathy and cerebral ataxia, have been shown to be compatible with thiamin deficiency (Adamolekun, 2010a).

Tropical Ataxic Neuropathy (TAN)

TAN is described to be a disease of the poor. Victims are undernourished and have very low dietary protein intake (Adamolekun, 2010b). The disease has been reported in both epidemic and endemic forms. For example endemic forms have been reported in Kenya Nigeria, Sierra Leone, Tanzania and Uganda. Epidemic of TAN has also been reported in Cuba and India (Adamolekun, 2010b). Chronic cassava cyanide consumption has been implicated in TAN in epidemiological studies. For example in Nigeria, a study reported that intake of cassava foods, dietary cyanide load, and serum thiocyanate levels, were higher in TAN-endemic areas compared to controls (Onabulu et al. 2001; Osuntokun, 1971). However, the occurrence of TAN in areas like Cuba, where cassava is not a staple food makes some authors wonder whether cassava intake is the aetiological cause of TAN (Adamolekun, 2010b). Because of this, Adamolekun (2010b) proposes that it is chronic thiamine deficiency which can explain the aetiology of TAN. Evidence supporting his hypothesis was from erythrocyte transketolase test on the red blood cells of TAN patients which depicted marginal but significant thiamine deficiency compared to controls and also abnormal pyruvate metabolism which was reversed when TAN patients were supplemented with thiamine.

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Title
Identifying Ghanaian Cassava Leaves with Safe Cyanide Levels for Use as Vegetable
College
University of Ghana, Legon  (School of Graduate Studies)
Course
Nutrition
Grade
pass
Author
Year
2011
Pages
107
Catalog Number
V278572
ISBN (eBook)
9783656719427
ISBN (Book)
9783656719403
File size
5005 KB
Language
English
Tags
identifying, ghanaian, cassava, leaves, safe, cyanide, levels
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William Opoku-Nkoom (Author), 2011, Identifying Ghanaian Cassava Leaves with Safe Cyanide Levels for Use as Vegetable, Munich, GRIN Verlag, https://www.grin.com/document/278572

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