Nutritional Value of Cassava for Use as a Staple Food and Recent Advances for Improvement

Authors

  • Julie A. Montagnac,

    1. Author Montagnac is with SupAgro Montpellier, Ecole Nationale Supérieure Agronomique of Montpellier, 02 Place Pierre Viala, 34060 Montpellier Cedex 1, France. Authors Davis and Tanumihardjo are with Univ. of Wisconsin-Madison, Dept. of Nutritional Sciences, 1415 Linden Drive, Madison, WI 53706, U.S.A. Direct inquiries to author Tanumihardjo (E-mail: sherry@nutrisci.wisc.edu).
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  • Christopher R. Davis,

    1. Author Montagnac is with SupAgro Montpellier, Ecole Nationale Supérieure Agronomique of Montpellier, 02 Place Pierre Viala, 34060 Montpellier Cedex 1, France. Authors Davis and Tanumihardjo are with Univ. of Wisconsin-Madison, Dept. of Nutritional Sciences, 1415 Linden Drive, Madison, WI 53706, U.S.A. Direct inquiries to author Tanumihardjo (E-mail: sherry@nutrisci.wisc.edu).
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  • Sherry A. Tanumihardjo

    1. Author Montagnac is with SupAgro Montpellier, Ecole Nationale Supérieure Agronomique of Montpellier, 02 Place Pierre Viala, 34060 Montpellier Cedex 1, France. Authors Davis and Tanumihardjo are with Univ. of Wisconsin-Madison, Dept. of Nutritional Sciences, 1415 Linden Drive, Madison, WI 53706, U.S.A. Direct inquiries to author Tanumihardjo (E-mail: sherry@nutrisci.wisc.edu).
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Abstract

ABSTRACT:  Cassava is a drought-tolerant, staple food crop grown in tropical and subtropical areas where many people are afflicted with undernutrition, making it a potentially valuable food source for developing countries. Cassava roots are a good source of energy while the leaves provide protein, vitamins, and minerals. However, cassava roots and leaves are deficient in sulfur-containing amino acids (methionine and cysteine) and some nutrients are not optimally distributed within the plant. Cassava also contains antinutrients that can have either positive or adverse effects on health depending upon the amount ingested. Although some of these compounds act as antioxidants and anticarcinogens, they can interfere with nutrient absorption and utilization and may have toxic side effects. Efforts to add nutritional value to cassava (biofortification) by increasing the contents of protein, minerals, starch, and β-carotene are underway. The transfer of a 284 bp synthetic gene coding for a storage protein rich in essential amino acids and the crossbreeding of wild-type cassava varieties with Manihot dichotoma or Manihot oligantha have shown promising results regarding cassava protein content. Enhancing ADP glucose pyrophosphorylase activity in cassava roots or adding amylase to cassava gruels increases cassava energy density. Moreover, carotenoid-rich yellow and orange cassava may be a foodstuff for delivering provitamin A to vitamin A–depleted populations. Researchers are currently investigating the effects of cassava processing techniques on carotenoid stability and isomerization, as well as the vitamin A value of different varieties of cassava. Biofortified cassava could alleviate some aspects of food insecurity in developing countries if widely adopted.

Introduction

Cassava is to African peasant farmers as rice is to Asian farmers, or wheat and potatoes are to European farmers (Dixon A; Intl. Inst. of Tropical Agriculture in Nigeria; personal communication). Because cassava (also called manioc or yucca, with various spellings) is drought-tolerant and its mature roots can maintain their nutritional value for a long time without water, cassava may represent the future of food security in some developing countries.

Cassava originated in the New World. Today it is a staple food and animal feed in tropical and subtropical Africa, Asia, and Latin America, with an estimated total cultivated area greater than 13 million hectares, of which more than 70% is in Africa and Asia (EL-Sharkawy 2003). Approximately 500 million people depend on it as a major carbohydrate (energy) source, in part because it yields more energy per hectare than other major crops (Table 1). Cassava is grown predominantly by small-scale farmers with limited resources in marginally fertile soils; it is resistant to adverse environments and tolerates a range of rainfall (El-Sharkawy 2003). Tapioca, a commercially important starch product common in the United States, is produced from cassava roots. Figure 1 illustrates the widespread use and daily consumption of cassava and its products.

Table 1—.  Maximum recorded yield and food energy of important tropical staple crops.a
CropAnnual yield (tons/hectare)Daily energy production (kJ/hectare)
  1. aAdapted from EL-Sharkawy (2003).

  2. bAll grains reported as dry.

Fresh cassava root711045
Maize grainb20 836
Fresh sweet potato root65 752
Rice grain26 652
Sorghum grain13 477
Wheat grain12 460
Banana fruit39 334
Figure 1—.

Cassava utilization (A) and consumption per day (B) throughout the world (produced by the Intl. Inst. for Tropical Agriculture Geospatial lab [Ibadan, Nigeria]) based on FAOSTAT 2003 data at http://faostat.fao.org/site/609/default.aspx.

Cassava is grown in areas where mineral and vitamin deficiencies are widespread, especially in Africa. A marginal nutrient status increases the risk of morbidity and mortality. Therefore, improving the nutritional value of cassava could alleviate some aspects of hidden hunger, that is, subclinical nutrient deficiencies without overt clinical signs of malnutrition. The relationship between hidden hunger and food insecurity has been reviewed elsewhere (Tanumihardjo and others 2007). The most common micronutrient deficiencies worldwide are those of vitamin A, iron, and iodine. The process of adding nutritional value to a crop is called biofortification (Tanumihardjo and others 2008). Micronutrients present in staple crops that are being targeted for biofortification include vitamin A, iron, and zinc. Cassava has been targeted for biofortification because of its unique geographical distribution and its importance as a staple food. This review describes the nutritional value and improvements that researchers have achieved in the cassava plant.

Nutritional Value of Cassava Roots

The composition of cassava depends on the specific tissue (root or leaf) and on several factors, such as geographic location, variety, age of the plant, and environmental conditions. The roots and leaves, which constitute 50% and 6% of the mature cassava plant, respectively, are the nutritionally valuable parts of cassava (Tewe and Lutaladio 2004). The nutritional value of cassava roots is important because they are the main part of the plant consumed in developing countries. In Table 2, the proximate, mineral, and vitamin compositions of cassava roots and leaves are reported. In Table 3, the nutrient composition of raw cassava is compared to other staple crops, such as wheat and corn, and some vegetable and animal foods.

Table 2—.  Proximate, vitamin, and mineral composition of cassava roots and leaves.
 Raw cassavaaCassava rootsb,c,dCassava leavesb,c
  1. aValues were obtained from the USDA Natl. Nutrient database for standard references (http://www.nal.usda.gov/fnic/foodcomp/search/). Nutrient values and weights are for the edible portion.

  2. bBradbury and Holloway (1988).

  3. cWoot-Tsuen and others (1968).

  4. dFavier (1977).

  5. eAsh refers to essential minerals as well as toxic elements such as heavy metals.

  6. fLancaster and others (1982).

  7. gOn a fresh weight (dry matter) basis (adapted from Gil and Buitrago 2002).

Proximate composition (100 g)
Food energy (kcal)160    110 to 14991
Food energy (KJ)667    526 to 611209 to 251
Moisture (g)59.6845.9 to 85.364.8 to 88.6
Dry weight (g)40.3229.8 to 39.319 to 28.3
Protein (g) 1.360.3 to 3.51.0 to 10.0
Lipid (g) 0.280.03 to 0.50.2 to 2.9
Carbohydrate, total (g)38.0625.3 to 35.77 to 18.3
Dietary fiber (g)1.80.1 to 3.70.5 to 10.0
Ashe (g) 0.620.4 to 1.70.7 to 4.5
Vitamins
Thiamin (mg)  0.0870.03 to 0.280.06 to 0.31
Riboflavin (mg)  0.0480.03 to 0.060.21 to 0.74
Niacin (mg)  0.8540.6 to 1.091.3 to 2.8
Ascorbic acid (mg)20.6 14.9 to 50 60 to 370
Vitamin A (μg)5.0 to 35.08300 to 11800f
Minerals
Calcium (mg)16   19 to 17634 to 708
Phosphorus, total (mg)27   6 to 15227 to 211
Ca/P0.61.6 to 5.482.5
Iron (mg) 0.270.3 to 14.00.4 to 8.3
Potassiumg (%)0.25 (0.72) 0.35 (1.23) 
Magnesium (%)0.03 (0.08) 0.12 (0.42) 
Copper (ppm)2.00 (6.00) 3.00 (12.0) 
Zinc (ppm)14.00 (41.00) 71.0 (249.0)
Sodium (ppm) 76.00 (213.00)51.0 (177.0)
Manganese (ppm)3.00 (10.00)72.0 (252.0)
Table 3—.  Nutritional composition of different kinds of foods (100 g) for comparison to cassava root.a
FoodWater (g)Energy (kcal)Energy (kj)Protein (g)Total lipid (g)Ash (g)Carbohydrate by difference (g)Dietary fiber (g)Sugars (g)
  1. aAll values were obtained from the USDA Natl. Nutrient database for standard references (http://www.nal.usda.gov/fnic/foodcomp/search/). Nutrient values and weights are for the edible portion.

Cassava, raw root59.68160 667 1.360.280.6238.061.81.7 
Potato, raw79.34 77 321 2.020.091.0817.472.20.78
Cereals
Wheat flour, unenriched11.92364152310.330.980.4776.312.70.27
Bread, wheat35.74266111510.913.642.2 47.513.65.75
Rice, white, unenriched12.893601506 6.610.580.5879.34
Corn, sweet, white, raw75.96 86 358 3.221.180.6219.022.73.22
Corn, yellow10.373651527 9.424.741.2 74.267.30.64
Sorghum9.2339141811.3 3.3 1.5774.636.3
Vegetables (raw)
Green beans90.27 31 129 1.820.120.66 7.133.41.4 
Carrots88.29 41 173 0.930.240.97 9.582.84.74
Spinach94    14  591.50.2 1.8 2.5
Lettuce, green leaf95.07 15  61 1.360.150.62 2.791.30.78
Soybeans, green67.5 147 61412.956.8 1.7 11.054.2
Animal products
Raw egg (white)87.57 52 21610.9 0.170.63 0.730  0.71
Cheese, Cheddar36.75403168424.9 33.14 3.93 1.280  0.52
Milk (whole)88.32 60 252 3.223.250.69 4.520  5.26
Raw fish (trout)71.42148 61920.776.611.170  0  0  

Macronutrients

Cassava root is an energy-dense food. In this regard, cassava shows very efficient carbohydrate production per hectare. It produces about 250000 calories/hectare/d, which ranks it before maize, rice, sorghum, and wheat (Okigbo 1980). The root is a physiological energy reserve with high carbohydrate content, which ranges from 32% to 35% on a fresh weight (FW) basis, and from 80% to 90% on a dry matter (DM) basis. Eighty percent of the carbohydrates produced is starch (Gil and Buitrago 2002); 83% is in the form of amylopectin and 17% is amylose (Rawel and Kroll 2003). Roots contain small quantities of sucrose, glucose, fructose, and maltose (Tewe and Lutaladio 2004). Cassava has bitter and sweet varieties. In sweet cassava varieties, up to 17% of the root is sucrose with small amounts of dextrose and fructose (Okigbo 1980; Charles and others 2005). Raw cassava root has more carbohydrate than potatoes and less carbohydrate than wheat, rice, yellow corn, and sorghum on a 100-g basis (Table 3). The fiber content in cassava roots depends on the variety and the age of the root. Usually its content does not exceed 1.5% in fresh root and 4% in root flour (Gil and Buitrago 2002).

The lipid content in cassava roots ranges from 0.1% to 0.3% on a FW basis. This content is relatively low compared to maize and sorghum, but higher than potato and comparable to rice (Table 3). The lipids are either nonpolar (45%) or contain different types of glycolipids (52%) (Hudson and Ogunsua 1974). The glycolipids are mainly galactose-diglyceride (Gil and Buitrago 2002). The predominant fatty acids are palmitate and oleate (Hudson and Ogunsua 1974). The protein content is low at 1% to 3% on a DM basis (Buitrago 1990) and between 0.4 and 1.5 g/100 g FW (Bradbury and Holloway 1988). In contrast, maize and sorghum have about 10 g protein/100 g FW. The content of some essential amino acids, such as methionine, cysteine, and tryptophan, is very low (Table 4). However, the roots contain an abundance of arginine, glutamic acid, and aspartic acid (Gil and Buitrago 2002). About 50% of the crude protein in the roots consists of whole protein and the other 50% is free amino acids (predominantly glutamic and aspartic acids) and nonprotein components such as nitrite, nitrate, and cyanogenic compounds. The presence of cyanogenic compounds, which predominate in bitter varieties, and processes to reduce them were recently reviewed by Montagnac and others (2009).

Table 4—.  Amino acid profile of cassava.a
Amino acidContent in rootsContent in leaves
% wet wt% dry wt% proteinb% wet wt% dry wt% proteinb
  1. aAdapted from Gil and Buitrago (2002).

  2. bContent of total protein (%).

Arginine0.10 0.2911.0 0.301.485.30
Histidine0.02 0.07 2.600.130.662.30
Isoleucine0.01 0.03 1.000.331.675.90
Leucine0.11 0.3111.700.542.729.70
Lysine0.02 0.07 2.600.371.876.70
Methionine0.01 0.03 1.000.070.361.30
Phenylalanine0.01 0.03 1.000.180.923.30
Threonine0.01 0.03 1.000.271.354.80
Tryptophan0.29 0.500.050.240.80
Valine0.01 0.04 1.500.200.993.50
Alanine0.05 0.15 5.700.341.706.10
Aspartic acid0.04 0.13 4.900.492.448.70
Cysteine0.0030.01 0.400.040.210.70
Glutamic acid0.05 0.15 5.700.401.997.10
Glycine0.0030.01 0.400.351.736.20
Proline0.01 0.03 1.000.180.883.10
Serine0.01 0.04 1.500.341.686.00
Tyrosine0.0030.01 0.400.180.893.20

Minerals and vitamins

Cassava roots have calcium, iron, potassium, magnesium, copper, zinc, and manganese contents comparable to those of many legumes, with the exception of soybeans (Table 5). The calcium content is relatively high compared to that of other staple crops and ranges between 15 and 35 mg/100 g edible portion. The vitamin C (ascorbic acid) content is also high and between 15 to 45 mg/100 g edible portions (Okigbo 1980; Charles and others 2004). Cassava roots contain low amounts of the B vitamins, that is, thiamin, riboflavin, and niacin (Table 6), and part of these nutrients is lost during processing. Usually the mineral and vitamin contents are lower in cassava roots than in sorghum and maize (Gil and Buitrago 2002).

Table 5—.  Mineral content of 100 g of various foods for comparison to cassava root.a
FoodCabmgFemgMgmgPmgKmgNamgZnmgCumgMnmgSe μg
  1. aAll values were obtained from the USDA Natl. Nutrient database for standard references (http://www.nal.usda.gov/fnic/foodcomp/search/). Nutrient values and weights are for the edible portion.

  2. bCa = calcium; Fe = iron; Mg = magnesium; P = phosphorus, K = potassium, Na = sodium; Zn = zinc; Cu = copper; Mn = manganese; Se = selenium.

Cassava, raw root160.272127271140.340.1  0.3840.7
Potato, raw120.78235742160.290.1080.1530.3
Cereals
Wheat flour, unenriched151.172210810720.7 0.1440.68233.9
Bread, wheat1423.46481551845211.210.1591.12328.8
Rice, white, unenriched90.8 351088611.160.11 1.1  — 
Corn, sweet, white, raw20.523789270150.450.0540.1610.6
Corn, yellow72.71127210287352.210.3140.48515.5
Sorghum284.4 — 2873506— — — — 
Vegetables (raw)
Green beans371.04253820960.240.0690.2140.6
Carrots330.3 1235320690.240.0450.1430.1
Spinach580.8 39281301300.380.0930.6390.7
Lettuce, green leaf360.861329194280.180.0290.25 0.6
Soybeans, green1973.5565194620150.990.1280.5471.5
Animal products
Raw egg (white)70.0811151631660.030.0230.01120  
Cheese, Cheddar7210.6828512986213.110.0310.01 13.9
Milk, whole1130.031091143400.4 0.0110.0033.7
Fish, trout, raw431.5 22245361520.660.1880.85112.6
Table 6—.  Vitamin composition for 100 g of various foods for comparison to cassava root.a
FoodVitamin Cb mgThiamin mgRiboflavin mgNiacin mgPantothenic acid mgVitamin B6 mgFolate total μgVitamin B12 μgVitamin A μg RAERetinol μgVitamin E mgVitamin K μg
  1. aAll values were obtained from the USDA Natl. Nutrient database for standard references (http://www.nal.usda.gov/fnic/foodcomp/search/). Nutrient values and weights are for the edible portion.

  2. bVitamin C = total ascorbic acid; Vitamin A is represented as retinol activity equivalents (RAE) and retinol refers to vitamin A in the preform; vitamin E =α-tocopherol; vitamin K = phylloquinone.

Cassava, raw root20.60.0870.0480.8540.1070.088 270     1  00.19  1.9
Potato, raw19.70.08 0.0321.0540.2960.295 160     0  00.01  1.9
Cereals
Wheat flour, unenriched0 0.12 0.04 1.25 0.4380.044 260     0  00.06  0.3
Bread, wheat 0.20.3730.3115.19 0.82 0.119 850     0  00.19  4.9
Rice, white, unenriched0 0.07 0.0481.6  1.3420.145  90     0  0
Corn, sweet, white, raw 6.80.2  0.06 1.7  0.76 0.055 460     0  00.07  0.3
Corn, yellow0 0.3850.2013.6270.4240.622 190    11  00.49  0.3
Sorghum0 0.2370.1422.9270     0  0
Vegetables (raw)
Green beans16.30.0840.1050.7520.0940.074 370    35  00.41 14.4
Carrots 5.90.0660.0580.9830.2730.138 190   841  00.66 13.2
Spinach30  0.04 0.13 0.5  0.3120.304 150   220  0
Lettuce, green leaf18  0.07 0.08 0.3750.1340.09  380   370  00.29173.6
Soybeans, green29  0.4350.1751.65 0.1470.0651650     9  0
Animal products
Raw egg (white)0 0.0040.4390.1050.19 0.005  40.09  0  00     0  
Cheese, Cheddar0 0.0270.3750.08 0.4130.074 180.832652580.29  2.8
Milk, whole0 0.0440.1830.1070.3620.036  50.44 28 280.06  0.2
Raw fish (trout) 0.50.35 0.33 4.5  1.94 0.2   137.79 17 170.2   0.1

The protein, fat, fiber, and minerals are found in larger quantities in the root peel than in the peeled root. However, the carbohydrates, determined by the nitrogen-free extract, are more concentrated in the peeled root (central cylinder or pulp) (Gil and Buitrago 2002). Thus, cassava roots are rich in calories but low in protein, fat, and some minerals and vitamins. Their nutritional value is, consequently, lower than those of cereals, legumes, and some other root and tuber crops.

Processing effects on nutritional value

Processing cassava can affect the nutritional value of cassava roots through modification and losses in nutrients of high value. Traditional processing techniques and several edible forms of cassava roots are illustrated in Figure 2. Analysis of the nutrient retention for each cassava edible product (Table 7) shows that raw and boiled cassava root keep the majority of high-value nutrients except riboflavin and iron. Gari is a common root product that involves grating, fermenting, and roasting. Gari and products obtained after retting of cassava root with peel are less efficient than boiled root in keeping nutrients of high value but are better than products obtained after retting of shucked cassava roots. However, the latter is richer in riboflavin than sun-dried flour. Fufu, an important staple in Africa, is a mashed cassava root product that is allowed to ferment with Lactobacillus bacteria (Sanni and others 2002). Medua-me-mbong is a root product that requires only boiling and prolonged washing. However, medua-me-mbong has the poorest nutritional value compared to other cassava products with the exception of calcium content (Favier 1977).

Figure 2—.

Different processing techniques for whole cassava root. The edible forms of cassava root are shaded in gray (adapted from Favier 1977).

Table 7—.  Nutritional value after processing 100 g of cassava root.a
 Whole rootPeeled rootBoiled rootBâton orChikwangueGariFlour (retting and no peel)Flour (retting and peel)Washed cooked
  1. aAdapted from Favier (1977).

Wet root (g)10077.087.649.238.525.3 to 29.627.9 to 34.066.8
Dry matter (g)40.032.328.321.629.721.3 to 25.620.8 to 28.719.0
Calories1571271128611985 to 10283 to 11576
Protein (g)1.00.480.380.180.370.16 to 0.220.26 to 0.510.16
Fat (g)0.10.10.040.020.20.04 to 0.060.04 to 0.120.03
Carbohydrates (g)37.931.027.421.228.820.9 to 25.120.3 to 28.118.8
Fiber (g)1.30.60.50.40.60.40.3 to 0.60.3
Ash (g)0.900.570.460.210.340.16 to 0.190.24 to 0.500.06
Calcium (mg)2613127106.0 to 8.07.0 to 15.011
Phosphorus (mg)47393113189.0 to 11.010.0 to 21.07
Iron (mg)3.50.40.43.11.50.2 to 0.70.8 to 11.90.2
Thiamin (μg)72312010186.0 to 12.0133
Riboflavin (μg)341816211510.0 to 12.08.0 to 21.06
Niacin (mg)0.730.520.410.160.330.11 to 0.180.17 to 0.370.03
Vitamin C (mg)3320112000

In contrast to boiled cassava, processed root loses a major part of dry matter, carbohydrates, protein, and thus calories. Although raw cassava root contains significant vitamin C, it is very sensitive to heat and easily leaches into water, and therefore almost all of the processing techniques seriously affect its content. Boiled cassava, gari, and products resulting from retting of cassava root with peel, retain thiamin and niacin better than products obtained after retting of shucked cassava roots, smoked-dried flour, and medua-me-mbong. Riboflavin is well retained in boiled cassava, gari, and smoked-dried cassava flour obtained after retting of cassava root with peel. In contrast, the losses of vitamin B2 (riboflavin) are high during sun-drying of cassava flours (more than 50%) and during medua-me-mbong preparation (66% lost) (Favier 1977).

Nutritional Value of Cassava Leaves

Protein and carbohydrates

The nutrient composition of cassava leaves varies in both quality and quantity depending on the variety of cassava, the age of the plant, and the proportional size of the leaves and stems (Gil and Buitrago 2002). Cassava leaves are rich sources of protein, minerals, vitamins B1, B2, and C, and carotenoids (Adewusi and Bradbury 1993). Comparison of Table 2 and 3 shows that the crude protein content (5 to 7 g/100 g), the crude fat (1 to 2 g/100 g), and minerals (2 g/100 g) of cassava leaves surpass those of the legumes and leafy legumes, except for soybean. Cassava leaf protein ranges from 14% to 40% of DM in different varieties (Eggum 1970). The crude protein content is comparable to that of fresh egg (10.9 g/100 g) and the amino acid profile of cassava leaf protein is well balanced compared to that of the egg (Jacquot 1957) except for methionine, lysine, and maybe isoleucine. Indeed, Table 8 indicates a deficit in methionine, an excess of lysine, and a low content of isoleucine for cassava leaves in comparison with the egg. Futhermore, cassava leaves have an essential amino acid content higher than soybean protein and FAO's recommended reference protein intake (FAO/WHO 1973; Okigbo 1980; West and others 1988).

Table 8—.  Comparison of the amino acid profile of cassava with egg, on a 16 g N basis (adapted from Favier 1977).
Amino acidEgg proteinaCassava leavesbCassava rootc
JamaicaBrazil
  1. aMitchell and Block (1946).

  2. bRogers and Milner (1963).

  3. cBusson (1965).

  % on a basis of 16 g N 
Tryptophan1.51.52.1
Threonine4.92.84.94.7
Isoleucine8.05.04.81.8
Leucine9.28.98.82.9
Lysine3.97.26.37.2
Methionine4.11.71.71.0
Cysteine2.41.41.0
Phenylalanine6.35.85.52.1
Tyrosine4.54.23.91.6
Valine7.35.85.62.6
Arginine6.45.36.114.9 
Histidine2.12.22.61.7

The carbohydrate content in cassava leaves (7 to 18 g/100 g) is comparable to that of green-snap beans (7.1 g/100 g), carrots (9.6 g/100 g), or green soybeans (11.1 g/100 g), and it is higher than those of leafy vegetables such as green leaf lettuce (2.8 g/100 g) and New Zealand spinach (2.5 g/100 g). The carbohydrates in cassava leaves are mainly starch, with amylose content varying from 19% to 24% (Gil and Buitrago 2002).

Minerals and vitamins

Cassava leaves are rich in iron, zinc, manganese, magnesium, and calcium (Wobeto and others 2006). The following variations in mineral content for cassava leaf meal (CLM) have been reported: from 61.5 to 270 mg iron/kg DM, 30 to 63.7 mg zinc/kg DM, 50.3 to 263 mg manganese/kg DM, 6.2 to 50 mg copper/kg DM, 2.3 to 3 g sulfur/kg DM, 2.6 to 9.7 g magnesium/kg DM, 0.4 to 16.3 g calcium/kg DM, and 8 to 16.9 g potassium/kg DM (Barrios and Bressani 1967; Gomez and Valdivieso 1985; Nwokolo 1987; Ravindran and others 1992; Aletor and Adeogun 1995; Awoyinka and others 1995; Chavez and others 2000; Madruga and Câmara 2000). Cassava leaf meal is rich in iron in comparison with liver (121 mg/kg FW) and egg yolk (58.7 mg/kg FW), although the iron from plant origin is generally less bioavailable than iron from animal food sources. Iron and zinc content in CLM are comparable to those reported for sweet potato leaves and peanut leaves (Table 9). Calcium content is comparable to those of peanut and broccoli, and magnesium content surpasses that of broccoli but is below those of peanut and sweet potato. Thus, mineral content of CLM is comparable with that of other leaves (Wobeto and others 2006).

Table 9—.  Comparison of sweet potato leaf and peanut leaf nutrients with cassava leaf meal (CLM).a
Nutrient (100 g dry weight)CLMSweet potatoPeanut
  1. aAdapted from Wobeto and others (2006).

Protein (g)  28.1  30.6   26.6 
β-Carotene (mg)  88.0  75    113.3 
Vitamin C (mg)  90.2 141.7  293.3 
Iron (mg)  16.7  14.7   16   
Zinc (mg)   5.08  3.33   5.33
Manganese (mg)  14.1 
Magnesium (mg) 229.3 493.3 676.7 
Calcium (mg)1509.4 623.3 1236.7 

The vitamin content of cassava leaves (Table 2) is richer in thiamin (vitamin B1, 0.25 mg/100 g) than legumes and leafy legumes, except for soybeans (0.435 mg/100 g). The leaves have more thiamin than several animal foods including fresh egg, cheese, and 3.25% fat whole milk (Table 6). The riboflavin (vitamin B2) content of cassava leaves (0.60 mg/100 g) surpasses that of legumes, leafy legumes, soybean, cereal, egg, milk, and cheese (Table 6). The niacin content (2.4 mg/100 g) is comparable to that of maize (2 mg/100 g), and surpasses those reported for legumes and leafy legumes, milk, and egg (Table 6). The vitamin A content of cassava leaves is comparable with that of carrots and surpasses those reported for legumes and leafy legumes. The vitamin C content (60 to 370 mg/100 g) of cassava leaves is high compared to values reported for other vegetables (Table 6). Thus, the overall vitamin content of the leaves is comparable and in certain cases better than those reported for most legumes, leafy legumes, cereals, egg, milk, and cheese.

Fiber

The fiber content of cassava leaves is high (Table 2) compared to the fiber content of legumes and leafy legumes reported in Table 3 and ranges between 1 and 10 g/100 g FW. Dietary fiber is considered part of a healthy diet and can reduce problems of constipation. Although recent evidence is mixed, fiber may help prevent colon cancer (Rock 2007). The rich fiber of cassava may assist intestinal peristalsis and bolus progression (Favier 1977), but if fiber content from any source is too high, it will have negative effects in humans. Fiber can be a nutritional concern because it can decrease nutrient absorption in the body (Baer and others 1996). Excess fiber will increase fecal nitrogen, cause intestinal irritation, and reduce nutrient digestibility, in particular protein digestibility (Favier 1977; Baer and others 1996). It is important to optimize the utilization of nutrients from cassava because nutrient deficiencies are more prevalent in regions where cassava is used as a staple food.

Comparison of Nutritional Value of Cassava Roots and Cassava Leaves

Table 2 compares the proximate, vitamin, and mineral compositions of cassava leaves and roots. The roots are twice as rich as the leaves in carbohydrates, but the leaves contain more protein, lipid, minerals, vitamins, and fiber. The total protein content in cassava leaves is 5 to 10 times higher than in roots and is comparable with the protein content of egg (Jacquot 1957) based on grams of nitrogen. The protein content of cassava leaves is similar to those of sweet potato leaves and peanut leaves (Table 9) (Wobeto and others 2006). There is a significant deficit in methionine for both cassava leaves and roots. Cassava roots also have important deficits in cysteine and tryptophan, and have relatively low concentrations of isoleucine, leucine, phenylalanine, tyrosine, and valine (Jacquot 1957). The leaves have tryptophan concentrations comparable to those found in eggs, but the cysteine content in leaves is only about half that of the egg (Jacquot 1957).

Cassava roots have a large excess of arginine, while the arginine content of leaves is relatively low and comparable to that of eggs. Both leaves and roots have an excess of lysine, which is approximately twice as high as what has been observed in eggs (Jacquot 1957) on a basis of 16 g nitrogen. Although cassava roots do not have a well-balanced amino acid profile and are of lower nutritional value because of low protein quantity, cassava leaves have good protein content. Therefore, cassava leaf proteins could be used to improve the nutritional value of a diet primarily made up of cassava roots. However, methionine and cysteine would still be limiting amino acids (Gil and Buitrago 2002), so a methionine and cysteine source should be added to the diet to supplement these 2 sulfur-containing amino acids.

The lipid content is 10 times higher in leaves than in roots. Although the lipids and lipid-soluble components such as chlorophyll, resin, and xanthophylls are much more concentrated in leaves, some of them, such as volatile fatty acids, chlorophyll, and resin, do not bring significant energy to the diet. Therefore, the energy density of the lipid is lower in leaves than in roots (Gil and Buitrago 2002).

The mineral content of cassava leaves is 2 to 5 times higher than that of the roots. The roots typically have more phosphorus, but the leaves have a greater concentration of calcium (Gil and Buitrago 2002). The calcium content in the leaves is 100 times higher than in roots and the phosphorus content is 2 to 3 times higher in roots than in the leaves. Cassava leaves are more concentrated than the roots in vitamins and the minerals iron, potassium, magnesium, copper, zinc, and manganese. Indeed, thiamin and niacin contents are 4 to 5 times higher in leaves than in roots, and riboflavin and vitamin C are 10 to 12 times higher in the leaves. Cassava leaves have a high quantity of vitamin A in the form of provitamin A carotenoids. Vitamin E, however, is low in both the leaves and roots (Gil and Buitrago 2002).

Thus, cassava roots are of lower nutritional value regarding mineral, vitamin, lipid, and protein contents, but the leaves are well provided in these, and should be added to a diet consisting mainly of roots. Cassava leaves are typically served boiled and mixed with other vegetables, such as okra and beans. The fiber content of leaves is greater than that of roots (3.5 times more), and so, it may limit the absorption of minerals, vitamins, and proteins by the body and may be a restricting nutritional factor.

Cassava and Selected Antinutrients

Analyzing the nutritional value of cassava, it appears that cassava roots are a good carbohydrate source and cassava leaves are good mineral, vitamin, and fiber sources for humans. However, cassava contains antinutrients and toxic substances that interfere with the digestibility and the uptake of some nutrients. Nevertheless, depending on the amount consumed, these substances can also bring benefits to humans.

Cyanide is the most toxic factor restricting the consumption of cassava roots and leaves. Indeed, cassava, particularly its bitter varieties, has a cyanide level higher than the FAO/WHO (1991) recommendations, which is < 10 mg cyanide equivalents/kg DM, to prevent acute toxicity in humans. Cassava leaves have a cyanide content ranging from 53 to 1,300 mg cyanide equivalents/kg of DW (Siritunga and Sayre 2003; Wobeto and others 2007), and cassava root parenchyma has a range of 10 to 500 mg cyanide equivalents/kg DM (Arguedas and Cooke 1982; Dufour 1988; Siritunga and Sayre 2003); both of these are much higher than what is recommended. Several health disorders and diseases have been reported in cassava-eating populations. Consumption of 50 to 100 mg of cyanide has been associated with acute poisoning and has been reported to be lethal in adults (Halstrom and Moller 1945). The consumption of lower cyanide amounts are not lethal but long-term intake could cause severe health problems such as tropical neuropathy (Osuntokun 1994), glucose intolerance, konzo (spastic paraparesis) (Ernesto and others 2002), and, when combined with low iodine intake, goiter and cretinism (Delange and others 1994).

In addition, the nitrate content in cassava leaves ranges from 43 to 310 mg/100 g DM (Corrêa 2000; Wobeto and others 2007). Cassava-eating populations ingesting cyanide and high amounts of nitrates and nitrites have the risk of developing stomach cancer. Cassava-eating individuals tend to have a high amount of thiocyanate in the stomach due to cyanide detoxification by the body, which may catalyze the formation of carcinogenic nitrosamines (Mirvish 1983; Maduagwu and Umoh 1988; Onyesom and Okoh 2006).

Phytate (inositol hexakisphosphate) is another compound found in high abundance in cassava, with approximately 624 mg/100 g in roots (Marfo and others 1990). Phytic acid is able to bind cations such as magnesium, calcium, iron, zinc, and molybdenum and can, therefore, interfere with mineral absorption and utilization which may affect requirements (Hambidge and others 2008). It may also bind proteins preventing their complete enzymatic digestion (Singh and Krikorian 1982). However, phytic acids also have antioxidant and anticarcinogenic properties. Indeed, phytic acids can reduce free ion radical generation and thus peroxidation of membranes by complexing iron, and phytate may protect against colon cancer (Graf and others 1987). Phytate was able to reduce serum cholesterol and triglycerides in an animal model fed a cholesterol-enriched diet (Jariwalla 1999). Dephosphorylation of phytate occurs during processing of cassava, especially during fermentation when > 85% of phytate is removed (Marfo and others 1990). The possibility of genetic modification of cassava to reduce phytate concentrations has not yet been investigated (Montagnac and others 2009).

The polyphenol content (tannins) in cassava leaves is increased with the maturity of the plant. Cassava leaf meal has a polyphenol content of 2.1 to 120 mg/100 g DM (Wobeto and others 2007). Polyphenols can form insoluble complexes with divalent ions such as iron, zinc, and copper. They can also inactivate thiamin, bind certain salivary and digestive enzymes, and enhance secretion of endogenous protein. Consequently, they inhibit nonheme-Fe absorption, reduce thiamin absorption and the digestibility of starch, protein (Silva and Silva 1999), and lipids, and also interfere with protein digestibility (Bravo 1998). However, tannins also have antioxidant and anticarcinogen properties that can benefit humans (Chung and others 1998; Chen and Chung 2000; Alessio and others 2002; Matuschek and Svanberg 2002; Nakagawa and others 2002). Catechins (catechin, catechin gallate, gallocatechin) and flavone 3-glycosides (rutin and kaempferol 3-rutinoside), suggested to have cardiovascular health benefits, have been identified in cassava roots (Buschmann and others 2000). Anthocyanidins (cyanidin and delphinidin) have been identified in cassava leaves (Reed and others 1982). Processing cassava leaves reduces the polyphenol content, but 50% to 60% is retained; again, it has been suggested that genetic modifications might well reduce total polyphenol content (Fasuyi 2005).

Oxalates are antinutrients affecting calcium and magnesium bioavailability (Massey 2007) and form complexes with proteins, which inhibit peptic digestion (Oboh 1986). Oxalate content ranges from 1.35 to 2.88 g/100 g DM for cassava leaf meal (Fonseca 1996; Corrêa 2000; Wobeto and others 2007). The negative effect of oxalates on humans depends on the level of both oxalate and calcium in the cassava leaves. Wobeto and others (2007) reported that the calcium-to-oxalate ratio of 5 cassava cultivars was greater than 0.44%, which means that oxalate levels found in cassava leaf meal do not diminish the uptake of calcium.

Saponins are plant glycosides that are studied for their potential health benefits, particularly those derived from ginseng and soy (Bachran and others 2008; Yin and others 2008). Saponins are considered the bioactive component of ginseng responsible for its metabolic and potential health effects (Yin and others 2008; Christensen 2009). They have antitumor properties and may offer synergistic effects when used in combination with drug therapy (Bachran and others 2008). Saponins can also act on the central nervous system of humans with potential therapeutic effects (Nah and others 2007). Cassava leaf meal has a steroidal saponin content ranging from 1.74 to 4.73 g/100 g DM (Wobeto and others 2007), which compares to those found for soybeans (0.07 to 5.1 g/100 g DM) (Fenwick and Oakenfull 1983; Ireland and Dziedzic 1985; Schiraiwa and others 1991), but is lower than those observed in alfalfa (5.6 g/100 g DM) and beet leaves (5.8 g/100 g DM) (Fenwick and Oakenfull 1983). Saponin content increases in cassava leaf meal with plant maturity (Wobeto and others 2007).

Cassava leaf meal has a trypsin inhibitor content of 3.79 inhibited trypsin unit (ITU)/mg DM at the starch accumulation phase (17-mo-old plant) and of 11.14 ITU/mg DM at the leaf development phase (12-mo-old plant) (Corrêa and others 2004). Trypsin inhibitor has adverse effects on the pancreas. Indeed, Liener (1977) has demonstrated that for species with a pancreas comprising more than 0.3% of the body weight, trypsin inhibitor feeding in these species will produce an enlargement of the pancreas. Unheated soybean trypsin inhibitor decreases the activity of rat, monkey, human, bovine, porcine, and mink trypsins at a rate of 90% to 100% and rat, monkey, and human total proteolytic activity by up to 40% (Struthers and Macdonald 1983). However, protease inhibitors may suppress carcinogenesis (Park and others 2007).

Biofortification and Processing Methods to Improve the Nutritional Value of Cassava

Cassava is a target for biofortification because of its importance as a staple crop. The Bill and Melinda Gates Foundation has supported a global effort to develop cassava germplasm enriched with bioavailable nutrients since 2005 (BioCassava Plus[http://biocassavaplus.org/]). This initiative is called BioCassava Plus and has 6 major objectives: to increase the minerals zinc and iron, increase protein, increase vitamins A and E, decrease cyanogen content, delay postharvest deterioration, and develop virus-resistant varieties.

Biofortified cassava and protein value

Cassava roots, with a crude protein content of about 1.5%, are low in protein and some essential amino acids. To date, several different strategies have been investigated to improve the protein content and the amino acid composition of cassava ready-to-eat products. To engineer improved storage proteins with balanced amino acid composition in cassava tubers, Zhang and others (2003a) successfully transferred a 284 bp synthetic gene (ASP1) coding for a 11.2 kDa-storage protein rich in essential amino acids (80%) into embryonic suspensions of cassava using Agrobacterium. They observed stable integration and expression of ASP1 in cassava leaves and primary roots. However, contrary to the results obtained for transgenic sweet potatoes (Egnin and others 2001) and tobacco lines (Kim and others 1992) where ASP1 had been over-expressed, no significant differences in protein content and in the overall amino acid composition of cassava leaves were observed between transgenic and wild-type cassava lines (Zhang and others 2003a). Nonetheless, analysis of 1-y-old cassava plants from the 2nd vegetative generation grown under greenhouse conditions showed an increase in protein content and essential amino acid composition in storage roots of several transgenic lines (Zhang and others 2004). Two cassava root specific promoters related to vascular expression and secondary growth were identified (Zhang and others 2003b), which represent valuable candidates for targetting the protein ASP1 in storage roots for genetic improvement. Currently, studies to improve the levels of expression and accumulation of the ASP1 protein into cassava tubers are underway (Zhang and others 2004).

Researchers have also tried to improve the nutritional value of cassava by crossbreeding wild-type varieties. Two hybrids showed promising results regarding protein content compared to typical cassava cultivars. The interspecific hybrid of cassava (UnB 033) and Manihot dichotoma showed higher protein content (26.4%) in its leaves compared to cassava cultivars (24.25%) (Nassar and others 2004). This hybrid also resulted in a 5-times higher content of manganese and zinc than those of typical cassava cultivars (EB01). The leaf cyanide content was moderate, that is, 128.5 ± 11.7 mg cyanide/kg FW (Nassar and others 2004). A 2nd valuable hybrid was achieved by crossing with Manihot oligantha (Nassar and Dorea 1982). Protein content in the roots was twice that of typical cassava cultivars. For peeled tuber, the interspecific hybrid had a protein content of 4.5%, while the protein content observed in cassava cultivars ranged from 0.9% to 1.4%. Moreover, this cassava hybrid also had richer protein content in the peel (8.06%) than typical cassava cultivars (from 1.11% to 2.09%) (Nassar and Dorea 1982). Further research has continued to indicate the feasibility of selecting interspecific hybrids that are rich in both crude protein and amino acids to improve the protein value (Nassar and Souza 2007). This interspecific cassava hybrid has an improved amino acid profile with 10 times more lysine and 3 times more methionine than the common cultivar.

Transgenic approaches to reduce cyanogen in cassava have focused on suppressing cyanogen synthesis or accelerating cyanogen breakdown (Siritunga and Sayre 2007). One potential benefit of lowering cyanogen content is the facilitation of free cyanide assimilation into amino acids (Siritunga and Sayre 2007). Thus, reducing toxic cyanogens would have the added benefit of improving the protein value of the roots.

Postharvest processing to enhance protein

Another approach that increases the protein content and quality of ready-to-eat cassava products is the development of postharvest processing techniques. Crude protein of cassava root and leaf by-products can be increased by solid-state fermentation via Aspergillus niger (Iyayi and Losel 2001), while also decreasing cyanogen content by up to 95% (Birk and others 1996). Smith and others (1986) reported a significant increase in the protein content of cassava roots by solid-state fermentation via Sporotrichum pulverulentum. This fungus was able to produce 30.4 g of high-quality protein per 100 g of dry cassava in 48 h at 45 °C. The protein bioavailability of fermented cassava leaves was similar to that of soybean pressed cake diets delivered to ruminants, and, therefore, fermented cassava leaves can replace soybean as a source of protein (Bakrie and others 1996).

The development of cassava leaf protein concentrates with low fiber could enhance the protein value of cassava meals. Crude protein content of cassava leaf protein concentrate is twice (42% to 43%) that of cassava leaf meal (22%) (Castellanos and others 1994). Indeed, Fasuyi and Aletor (2005) reported that cassava leaf protein concentrates had crude protein, fat, and gross energy content higher than those of cassava leaf meals and lower crude fiber and ash contents. Table 10 shows the proximate composition of cassava leaf meal and cassava leaf protein concentrate. Although methionine (2.48 g/16 g N) and cysteine were limited in concentrates, the amino acid analysis determined that lysine (6.80 g/16 g N), leucine (9.65 g/16 g N), valine (6.30 g/16 g N), and tryptophan (2.31 g/16 g N) exceeded those of soybean, fish, and egg reported by the FAO/WHO (1973). In addition, the water absorption capacity (181.5%± 45.4%), the fat absorption capacity (19.2%± 1.2% to 40.8%± 1%), the emulsion capacity (32.5%± 8.3%) and stability (42.9%± 2.9%), the least gelation concentration (12.5%± 3.4%), the foaming capacity (32.1%± 7.7%) and stability (10.2 ± 4.1 cm3), and the solubility of cassava leaf protein concentrates in acid and alkaline media support the nutritive potential of cassava leaf protein concentrate (Fasuyi and Aletor 2005). These properties of cassava leaf protein concentrate allow formulation into viscous products such as soups, protein-rich carbonated beverages, and curds; or as additives for gel formation in food products. They also show the ability of cassava leaf protein concentrate to form and stabilize emulsions in food products (Fasuyi and Aletor 2005). Therefore, cassava leaf protein concentrate is a good alternative protein for human and animal nutrition, but due to the few limiting amino acids it should not be the exclusive source of protein. Growth bioassays in rats showed that cassava leaf protein concentrate should be fed along with another viable protein source, because the concentrate with and without dl-methionine supplements did not support growth (Fasuyi 2005).

Table 10—.  Comparison of cassava leaf protein concentrate and cassava leaf meal on a nutritional basis.a
Proximate compositionVariety
Cassava leaf mealaCassava leaf protein concentratesb
1212
  1. aAdapted from Fasuyi (2005).

  2. bAdapted from Fasuyi and Aletor (2005); Fasuyi (2006).

Crude protein (g/kg DM)343.0363.0500.0493.0
Crude fiber (g/kg DM)127.0115.018.014.0
Ether extract (g/kg DM)75.070.0216.0224.0
Ash (g/kg DM)72.069.093.079.0
Nitrogen free extract (g/kg DM)383.0382.0115.0141.0
Gross energy (MJ/kg)47.147.253.650.1
Digestible energy (MJ/kg)46.747.6

Regarding techniques to produce protein concentrates, ultrafiltration is more efficient than that of acidic thermocoagulation (Table 11) (Castellanos and others 1994). Indeed, ultrafiltration provides a better protein efficiency ratio (1.81), protein digestibility in vitro (85%), and the availability of lysine (90%). It also increases the amino acid content, especially of lysine (5.05 g/100 g protein), threonine (4.15 g/100 g protein), tryptophan (0.7 g/100 g protein), and sulfur-containing amino acids. However, sulfur-containing amino acids and tryptophan are still deficient after processing compared with FAO's recommendations.

Table 11—.  Comparison of ultrafiltration and thermocoagulation processes on nutritional value of cassava leaf protein concentrates.a
Proximate analysisCasein referenceCassava leafUltrafiltrationAcidic thermocoagulation
  1. aAdapted from Castellanos and others (1994).

  2. b(N * 6.25).

  3. cLyophilized cassava leaf.

  4. dFresh cassava leaf.

Crude protein contentb (%) 22.0043.9242.92
Lipids (%) 16.0012.9913.03
Ash (%) 5.746.008.74
Crude fiber (%) 13.191.061.72
Carbohydrates (%) 43.0736.0333.59
Cyanide content (ppm) 3751245
Protein digestibility (%)928580
Available lysine (%)92c9083
Protein efficiency ratio (%)2.501.811.60
Protein efficiency ratio (% of casein)10072.460.0
Carotene content (ppm)50d235180

Biofortified cassava and energy density

Although cassava root has one of the highest rates of CO2 fixation (43 μmol CO2/m2/s) and of sucrose synthesis for a C3 plant, carbohydrate yield is below its full potential. Therefore, it has been hypothesized that cassava plants can be engineered to enhance their starch yield. The catalyst of the first dedicated, rate-limiting step of starch synthesis is ADP-glucose pyrophosphorylase (AGPase). Its activity is a key factor for the enhancement of starch production. Muller-Rober and others (1992) observed severely reduced starch production and accumulation of sucrose and glucose in potato tubers as a result of inhibiting AGPase expression. Because bacterial AGPase activity is several hundred-folds higher than that of the cassava plant, Ihemere and others (2006) hypothesized that AGPase activity of cassava roots could be increased with addition of a modified bacterial AGPase. This would theoretically enhance cassava root starch production. Under greenhouse conditions, Ihemere and others (2006) observed an AGPase activity 70% higher than that of wild types (pH 7.5, 25 °C) and a 2.6-fold increase in total tuberous root biomass for transgenic cassava plants. Although the density of starch was not modified between the transgenic and wild-type lines, an increase in tuberous root size and number (biomass) resulted in an increase in total starch.

Postharvest technique to enhance energy density

Another strategy to enhance the energy density of cassava by-products is to add amylase. In the case of weaning food products, Treche and others (1994) demonstrated that the energy density of cassava gruels would double, and would maintain an acceptable consistency when plant amylase sources such as flour from malted cereals are added. The alpha-amylases hydrolyze the starchy component of cassava gruel into maltose and dextrins of low molecular weight and of low water-binding capacity, which allows a reduction in the viscosity of cassava liquid gruel.

Biofortified cassava and vitamin A

Vitamin A is a fat-soluble vitamin playing an important role in vision, bone growth, reproduction, and in the maintenance of healthy skin, hair, and mucous membranes (FAO/WHO 2002). Identified as a widespread public health problem in 37 countries worldwide, vitamin A deficiency is the most common cause of childhood blindness. It is estimated that 228 million children are affected and 500000 children become partially or totally blind every year as a result of vitamin A deficiency (WHO/FAO 2003). The geographical areas most affected by vitamin A deficiency are tropical areas where cassava is a staple crop, for example, Brazil, Africa, and Asia (Shrimpton 1989). Biofortification of staple crops with provitamin A carotenoids is an emerging strategy to address the vitamin A status of the poor (Tanumihardjo 2008; Tanumihardjo and others 2008).

Cassava root contains small amounts of β-carotene, a provitamin A carotenoid, which can be converted as needed into retinal, reduced to retinol, and stored in the liver esterified to fatty acids. The bioconversion of β-carotene to vitamin A in the body is naturally regulated and therefore β-carotene has little potential for toxicity compared with high intake of vitamin A-fortified foods (Tanumihardjo 2008). Fortified foods, such as sugar in Guatemala (Dary and Mora 2002) and Nicaragua (Ribaya-Mercado and others 2004), and supplements contain highly bioavailable preformed vitamin A and uptake is not regulated like bioconversion. Utilizing biofortified cassava with enhanced β-carotene (Figure 3) would be a sustainable strategy to reduce the prevalence of vitamin A deficiency in areas where cassava is a staple food. In addition, β-carotene can act as an antioxidant to protect cells and tissues from the damaging effects of free radicals and singlet oxygen species (Paiva and Russell 1999). For example, in Mongolian gerbils fed biofortified carrots or vitamin A supplements, the antioxidant status of the carrot groups was higher than the supplement group (Mills and others 2008). However, β-carotene's role in cancer prevention is not completely understood; smokers taking daily β-carotene supplements had a greater risk of lung cancer and cardiovascular disease than those not taking supplements (Goodman and others 2004).

Figure 3—.

Boiling the cream-colored biofortified cassava reveals the β-carotene through the deepening of color to yellow. The outer brown peel was removed before boiling. The underlying white peel, which is sometimes used in livestock feed, was removed after freeze-drying and before flour preparation for an animal study (Howe and others 2009).

Cassava also contains other interesting carotenoids that are not provitamin A, such as the carotene lycopene and the xanthophylls lutein and zeaxanthin. Lycopene appears to be particularly efficient at quenching the destructive potential of singlet oxygen (Di Mascio and others 1989). Red cassava with substantial lycopene is currently being distributed to small-scale farmers throughout Brazil (Nassar 2007). Lutein and zeaxanthin might act as antioxidants in the macular region of the human retina (Snodderly 1995; Pauleikhoff and others 2001). The possible role of lutein in preventing age-related eye disease is currently under investigation.

Screening, selecting, and crossbreeding varieties of cassava with high content of carotene is currently underway and shows potential as a dietary source (Nassar and others 2005). A broad distribution of carotene concentrations in cassava leaves and roots has been observed. Carotene content is 100 times higher in cassava leaves (ranging from 12 to 97 mg/100 g FW) than in roots (ranging from 0.102 to 1.069 mg/100 g FW) (Iglesias and others 1997; Chavez and others 2003). The color intensity of the cassava root and the carotene concentration are positively correlated (Iglesias and others 1997). The carotene concentration was 0.13 mg/100 g in white cassava roots, and 0.39 mg/100 g, 0.58 mg/100 g, 0.85 mg/100 g, and 1.26 mg/100 g in cream, yellow, deep yellow, and orange cassava roots, respectively. Moreover, 5 orange cassava genotypes have been found with β-carotene contents ranging from 2.04 to 2.55 mg/100 g FW in the Amazonian region of Brazil and Colombia (Iglesias and others 1997). These results are encouraging because using conservative conversion factors of 12 μg β-carotene to 1 μg vitamin A proposed by the Inst. of Medicine, Food and Nutrition Board (2001), this level of β-carotene would result in 170 to 210 μg vitamin A. This range of vitamin A includes the estimated average requirement for a young child 1 to 2 y old (that is, 210 μg vitamin A) and represents about 40% of the estimated average requirement for a woman of childbearing age (that is, 485 μg vitamin A). Yellow and orange cassava roots are a viable alternative for delivering provitamin A carotenoids to vitamin A-deficient populations that consume cassava. Considering the high daily intakes of cassava in several African countries (FAO 2006) (Figure 1B), cassava biofortified with β-carotene could readily impact the prevalence of night blindness due to vitamin A deficiency in women if widely adopted (WHO 2008) (Table 12).

Table 12—.  The impact of biofortified cassava with 2 mg β-carotene/100 g fresh weight on intake of vitamin A in several cassava-eating African countries. Considering that the estimated average requirement of women is 485 μg/d, biofortified cassava could positively impact the prevalence of night blindness.
CountryPrevalence of night blindnessaConsumption g/dbβ-Carotene content mgβ-Carotene retainedc mgEstimated retinol equivalentsdμg
  1. aThe range of prevalence in different communities of night blindness (a serious sequela of vitamin A deficiency) in women of childbearing years (WHO http://www.who.int/vmnis/vitamina/data/database/countries/en/index.html).

  2. bFAOSTAT agricultural data (database) Rome, Italy: FAO http://faostat.fao.org/site/609/default.aspx#ancor.

  3. cAn optimistic retention factor of 66% was used as reported for boiled cassava (Iglesias and others 1997).

  4. dEstimated retinol equivalents were calculated using conversion factors of 6 μg β-carotene (FAO/WHO 2002) and 12 μg β-carotene to 1 μg retinol (Inst. of Medicine 2001) to encompass a range of predicted equivalencies.

Congo, Democratic Republic5.4% to 9.5% 82016.410.8900 to 1800
Mozambique1.6% to 8.2% 68013.6 9.0750 to 1500
Ghana4.6% to 12.8%60012   7.9660 to 1300
Benin4.1% to 14.7%430 8.6 5.7480 to 950 
Guinea7.9% to 21.3%360 7.2 4.8400 to 800 
Rwanda4.8% to 11.5%3507  4.6380 to 770 
Madagascar1.7% to 15.3%320 6.4 4.2350 to 700 
Nigeria4.9% to 11.1%310 6.2 4.1340 to 680 

However, the stability of β-carotene during different processing techniques needs to be further evaluated. Carotene stability is genotypically dependent and cassava genotypes with the highest carotene content in fresh roots are not the ones with the highest carotene content after processing (Iglesias and others 1997). For example, comparing cream and yellow cassava root clones after processing, the yellow roots lost more β-carotene (Chavez and others 2004). Carotene concentration and availability is affected by heat-processing treatments. Iglesias and others (1997) reported that boiling, oven-drying, and sun-drying fresh cassava root parenchyma reduced carotene content by 34%, 44%, and 73%, respectively. Chavez and others (2004) pointed out that lyophilization and production of gari from fresh cassava root reduced the initial β-carotene content by 25% and 80%, respectively. Therefore, lyophilization and boiling were most effective at retaining β-carotene, and sun-drying and gari production the least.

Processing cassava increases the cis-β-carotene isomer content significantly (Thakkar and others 2007; Howe and others 2009). The percentage of cis content after processing was 30% to 52%, which was observed for 10 genotypes of processed cassava (Thakkar and others 2007). The vitamin A value of cis-β-carotene is generally accepted to be less than the trans isomer and is reported to be 23% to 61% for 9-cis-β-carotene and 48% to 74% for 13-cis-β-carotene in gerbil and rat models (Deming and others 2002). However, the bioconversion factor for β-carotene was similar in Mongolian gerbils fed different percentages of cis-β-carotene, that is 48% and 2.5% of total β-carotene as cis from cassava (Howe and others 2009) and carrots (Mills and others 2007), respectively. Bioefficacy studies in humans fed biofortified cassava need to be done to determine the influence of cis-β-carotene content formed during processing on the vitamin A value.

Measures of β-carotene absorption efficiency and conversion to vitamin A from cassava are important parameters to study to support the development of carotenoid-biofortified cassava. Carotenoid-biofortified cassava root (yellow cassava) was as efficient as β-carotene supplements in maintaining vitamin A status in a Mongolian gerbil model (Howe and others 2009). Indeed, no differences were observed for total vitamin A content in the livers of gerbils fed white cassava with daily β-carotene supplements or 45% high β-carotene cassava flour. Moreover, β-carotene was also found in the livers of gerbils supplemented with β-carotene in oil or fed 45% high β-carotene cassava, which implies that the gerbils had adequate vitamin A status. In addition, vitamin A-depleted gerbils fed about 15% and 30% high-β-carotene cassava flour had similar vitamin A liver content, but more β-carotene was stored in the livers of the group fed 30% high-β-carotene cassava (Howe and others 2009).

Regarding the bioavailability of carotene from cassava leaves, Wistar rats fed synthetic β-carotene or cassava leaf powder had similar growth and tissue weight (Siqueira and others 2007). However, β-carotene absorption was lower from cassava leaf powder than from synthetic β-carotene. This might be due to complexation of β-carotene with cassava leaf proteins and to the storage form of β-carotene within the cassava leaf matrix (Siqueira and others 2007).

Conclusions

The cassava plant is a valuable source of carbohydrate, protein, and vitamins. However, these macro- and micro-nutrients are not well distributed in the plant. Cassava roots are rich in carbohydrates but poor in vitamins and protein, while cassava leaves are an excellent source of protein and vitamins. Because some strains of cassava produce substantial quantities of cyanide, which makes them toxic for humans and animals, processing cassava into ready-to-eat products is necessary to remove cyanogens and other antinutrients. However, processing reduces cassava's nutritional value, especially when the peel is removed. In addition to genetically engineering and traditionally breeding crops to contain higher amounts of macronutrients, protein content and energy density of cassava can be increased through processing. Research to improve the provitamin A content of cassava cultivars is currently underway, especially on β-carotene stability after processing. Carotenoid-biofortified cassava is effective in maintaining vitamin A status in an animal model. Continued efforts to improve its nutritional value are important because cassava is a staple food for many people in developing countries.

Acknowledgments

This study is in partial fulfillment of the requirements for JAM to obtain the Diplôme d'Ingénieur Agronome (equivalent to a master's degree of Agronomic Engineering) from the Ecole Nationale Supérieure Agronomique of Montpellier SupAgro, France. The authors thank Julie Howe (Auburn Univ., Ala., U.S.A.) while at the Univ. of Wisconsin-Madison for encouraging JAM during the thesis preparation stages and Harold Furr for assistance in editing the manuscript. We also thank Bussie Maziya-Dixon (IITA, Ibadan, Nigeria) for providing the maps of utilization and consumption; Bonnie McClafferty (HarvestPlus, Washington, D.C., U.S.A.) and JV Meenakshi (HarvestPlus, New Delhi, India) for encouragement and coordination; and Christine Hotz (HarvestPlus, Ottawa, Canada) for helpful insights. This review was sponsored in part by HarvestPlus contract nr 8037 and Hatch Wisconsin Agricultural Experiment Station WIS04975.

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