Journal of the Science of Food and Agriculture
J Sci Food Agric 84:581–590 (online: 2004) DOI: 10.1002/jsfa.1699
Bitter taste in cassava roots correlates with cyanogenic glucoside levels† Linley Chiwona-Karltun,1,2∗ Leon Brimer,3 John D Kalenga Saka,4 Albert R Mhone,4 Jonathan Mkumbira,5 Lisbeth Johansson,6 Mpoko Bokanga,7 Nzola Meso Mahungu8 and Hans Rosling1 1 Division
of International Health, Karolinska Institute, SE-171 76 Stockholm, Sweden of Plant Biology and Forest Genetics, Swedish University of Agricultural Sciences, Box 7080, SE-750 07 Uppsala, Sweden 3 Department of Pharmacology and Pathobiology, The Royal Veterinary and Agricultural University, 13 Bulowsvej, DK-1870 Frederiksberg ¨ C, Denmark 4 Department of Chemistry, Chancellor College, University of Malawi, PO Box 280, Zomba, Malawi 5 Bvumbwe Agricultural Research Station, Box 5748, Limbe, Malawi 6 Department of Domestic Sciences, Uppsala University, Dag Hammarskjolds ¨ ¨ 21, SE-752 37 Uppsala, Sweden Vag 7 International Institute for Tropical Agriculture, Oyo Road, PMB 5320, Ibadan, Nigeria 8 IITA/SARRNET, Chitedze Research Station, Box 30258, Lilongwe 3, Malawi 2 Department
Abstract: Cassava roots contain cyanogenic glucosides. Malawian farmers classify cultivars into two groups based on the perceived danger of eating raw roots that they associate with bitterness. In the vernacular, cultivars that produce roots with bitter taste are called vyakubaba (bitter), whereas those yielding non-bitter roots are called vyakuzizra (cool). In the scientific literature they are distinguished as ‘bitter’ or ‘sweet’. Roots from ‘bitter’ cultivars are processed prior to consumption. We studied the ability of farmers to predict the cyanogenic glucoside levels of 492 roots from the 10 most commonly grown cultivars. Twenty-eight farmers predicted the taste of each of the cultivars that they grew, and scored bitterness on a five-point scale by tasting the root tip. Thereafter cyanogenic glucosides were determined on half of the root, while a taste panel scored the taste of the other half. The mean cyanogenic glucoside level in 132 roots from ‘cool’ cultivars was 29 mg HCN eq kg−1 fresh weight (CI 25–33, range 1–123) and in 360 roots from ‘bitter’ cultivars was 153 mg HCN eq kg−1 fresh weight (CI 143–163, range 22–661). Farmers’ distinction of ‘cool’ and ‘bitter’ cultivars predicts glucoside levels. The tasting of the tip of the root improved the farmers’ prediction of toxicity. Scoring of bitterness by a trained taste panel showed a stronger correlation with glucoside levels (r2 = 0.67). This suggests that cyanogenic glucosides confer the bitter taste, notwithstanding the probability of additional modifying intrinsic factors. 2004 Society of Chemical Industry
Keywords: bitter; cassava; cyanogenic; farmers; glucosides; HCN; linamarin; Malawi; perceptions; taste; women
INTRODUCTION Several plant species are used as food or feed although they contain palatability-reducing compounds or toxins.1 Instead of excluding such plants from the diet, mankind has developed processing methods that reduce the levels of these compounds prior to consumption.2 – 4 Such compounds include the cyanogenic glucosides linamarin and lotaustraulin found in cassava roots, an important staple crop in the tropics. When the Amerindians domesticated cassava cultivars that yield bitter and toxic roots, they also invented processing methods that reduced the
cyanogen content, thereby enabling them to consume the starchy products from cassava as their main staple food.5,6 In many cassava farming and food systems, cassava cultivars are classified into two main groups. In South America, where cassava originated and is the mainstay of the Amazonian Indians, this classification also prevails. For example, the term kii refers to ‘bitter’‡ and makasera refers to ‘non-bitter’ cassava7 cultivars. ‡ The punctuated terms bitter and cool are used to denote differences in usage and meaning as follows: ‘bitter’ or ‘cool’
∗ Correspondence to: Linley Chiwona-Karltun, Department of Plant Biology and Forest Genetics, Swedish University of Agricultural Sciences, Box 7080, SE-750 07 Uppsala, Sweden E-mail:
[email protected] † The preliminary findings on the correlation between taste and cyanogenic glucoside content were presented at the International Society of Tropical Root Crops symposium in Trinidad as a preliminary communication in the proceedings: Trop Agric (Trinidad) 75:169–173 (1998) Contract/grant sponsor: Swedish International Development Agency (Sida/SAREC) Contract/grant sponsor: International Science Programme (ISP), Uppsala University
(Received 1 June 2003; revised version received 5 January 2004; accepted 14 January 2004) Published online 4 March 2004
2004 Society of Chemical Industry. J Sci Food Agric 0022–5142/2004/$30.00
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In the English scientific literature the two groups of cassava cultivars are mostly referred to as ‘‘bitter’’ and ‘‘sweet’’. However, the meaning of the designation of the two groups of cultivars may be quite different in the many local languages within cassava-growing communities. Our previous studies from northern Malawi revealed that cassava is classified as vyakubaba (‘bitter’) and vyakuzizira (‘cool’)8 respectively for the two types of cultivars. This classification is deemed critical in discerning the need for processing in order to detoxify the roots prior to consumption. Farmers’ classification into these two groups of cultivars may have been underestimated in some studies owing to the discrepancy between scientific and local terminology. For the purpose of correct reporting of our interview findings, we have chosen in this paper to use the cultivar terminology of farmers in northern Malawi,8 namely ‘bitter’ and ‘cool’. The local farmers define the term ‘cool’ as referring to the absence of a discernible taste and the absence of risk of poisoning. As farmers stated that the roots of ‘cool’ cultivars do not have any sweet taste, the use of the term ‘‘sweet’’ cultivars was out of context. Studies in Africa have also shown that, where cassava is predominantly used as a staple food, the ‘bitter’ cultivars predominate. On the other hand in areas where cassava is consumed in the fresh raw form or used to supplement other staple crops, the ‘nonbitter’ varieties, referred to as ‘‘sweet’’, tend to excel.9 The methods for processing cassava that have evolved in Africa have been adapted to suit the local preferences of the desired food products.10,11 Roots from ‘bitter’ cultivars are preferred for obtaining flour,11 as has also been shown in South America.5,7 Furthermore, some studies show that farmers prefer ‘bitter’ cultivars because the products produced from the roots have superior taste and texture qualities.5,7,9 In many parts of Africa, farmers who are prone to food insecurity preferentially grow ‘bitter’ cassava cultivars that yield toxic roots. The reasons are that toxicity improves food security by conferring protection from theft, animal destruction and unplanned harvesting by family members.8,12 – 15 Depending on the availability of water and on end-product taste preference, the preferred method of processing entails a wet or heap fermentation or simple sun drying. Interview studies with women farmers in Malawi revealed that farmers use the ‘‘bitter’’ taste of cassava roots as an indicator of toxicity. They also considered it safe to eat processed products from ‘bitter’ roots, since their experience had shown that processing removed bitterness and that kondowole, the dumpling-like staple dish made from flour obtained from soaked and welldried roots, did not induce any acute poisoning.16 The relationship between ‘‘bitter’’ taste and the level of cyanogenic glucosides of cassava roots is unclear. While some studies have shown that ‘‘bitter’’ taste in raw cassava roots correlates positively with levels of when referring to the ethnoclassification of a cultivar; ‘‘bitter’’ or ‘‘cool’’ when referring to the taste of the root 582
cyanogenic glucosides,17 – 20 others have failed to find this association.21,22 In northern Malawi it is common practice for cassava farmers and consumers to predict toxicity of single raw roots by tasting the tip of the root parenchyma. To our knowledge, the ability of cassava farmers to predict the levels of cyanogenic glucosides on the basis of local knowledge and taste has never been studied either in Malawi or elsewhere. The first aim of the study was to elucidate to what degree cassava farmers could predict the potential toxicity of raw cassava roots using their local knowledge and practices. We assessed the precision with which farmers could predict cyanogenic glucoside levels in raw roots by using (1) their local classification of cultivars into two main groups called ‘bitter’ and ‘cool’, (2) their knowledge about the average degree of bitterness of roots from each cultivar, (3) their knowledge regarding the moderating effect of the environment in a specific field on the bitterness of roots, and (4) their practice of tasting the root parenchyma of the tip of a single root. The second aim was to determine the correlation between taste and levels of cyanogenic glucosides in raw cassava roots using a trained taste panel.
METHODS Study area The study was conducted in Nkhata Bay, the main cassava-consuming district in Malawi, where well over 70% of the farmed land is allocated to cassava.8 The agricultural extension service divides the district into 53 sections. Two of these, Lweya and Mgodi, were selected for this study as being representative of the lakeshore zone. Each section comprised eight blocks, each with about 100 farming households. One block from each section, ‘Thowolo-B’ and ‘Matyenda-1’ respectively, was selected on the basis of being typical of the agricultural and social variation within that section. The study was approved by the Research Division and Extension Service of the Ministry of Agriculture, by the District Health Commissioner in Nkhata Bay and by oral consent from the community leaders. Sampling Selection of farmers Thirteen households in ‘Thowolo-B’ and 15 households in ‘Matyenda-1’ were consecutively selected from the census list based on the following criteria: the cultivation of one or more of the 10 most frequently grown cultivars, having at least two plants of each cultivar with roots ready for harvest in the same field, and the presence of the female farmer previously interviewed to indicate the plants of each cultivar in her own field.16 These 28 farmers made general predictions on the taste of the roots of each cultivar that they grew when interviewed in their home. They made a second prediction on the taste of the roots of each cultivar while in the field, and finally they tasted the tip of the roots of the harvested cassava plants. J Sci Food Agric 84:581–590 (online: 2004)
Bitter taste and cyanogenic glucosides in cassava roots
Farmers’ prediction and observation of taste and harvesting of roots We sampled plants of the 10 cultivars that were reported to be grown by the highest proportion of households.16 Out of these, the farmers classified three as ‘cool’ and non-toxic and seven as ‘bitter’ and toxic cultivars. While in the field and prior to uprooting a pair of plants of each cultivar, the farmer was asked to predict their taste based on her experience of how roots of these cultivars usually taste in the particular field. In predicting taste, the farmer used a five-grade scale as follows: very cool (1), cool (2), intermediate (3), bitter (4) and very bitter (5); a slightly modified scale from the previous study.16 Thereafter the farmer identified two plants as belonging to one of the specified cultivars, and these plants were tagged before uprooting by the investigators. Sample handling prior to analysis Independent persons carried out the harvesting, and neither the farmer nor the principal investigator was present when the two biggest roots from each plant were packaged in labelled paper bags. While still in the field, the roots were given to the farmers one by one at the end of harvesting for them to grade the taste using the five-grade scale as per their usual practice. The roots were then repackaged and transported to Mkondezi Agricultural Research Station within 6 h of harvest. Upon arrival they were peeled and washed by a trained group of assistants, using sharp stainless steel knives and potable tap water. The peeled roots were finally split longitudinally on plastic chopping boards, and the two halves of each root were put into labelled polythene bags and immediately used for the sensory and chemical analyses respectively. A total of 492 roots from 246 plants stemming from 10 cultivars were collected for analysis. The numbers of plants and roots sampled from each cultivar are shown in Table 1 (see later). Sensory analysis From the staff households at the Agricultural Research Station, 30 self-reported healthy, non-smoking, nonhabitual cassava consumers with at least 8 years of education consented to participate in the taste panel. Following standard procedure,23 the candidates were assessed on their ability to distinguish the lowest levels (0.6 mg caffeine l−1 water) of the bitter-tasting compound caffeine. Four panel selection sessions were conducted and each prospective panellist was assessed twice. Twelve out of 15 assessors that could distinguish the caffeine solutions from the blank sample were selected for the taste panel.24 Roots from two cassava cultivars perceived by the farmers in the Mkondezi area to be very cool ‘fyoka’ and very bitter ‘gomani’ in taste were used as standards for the range of possible taste. These training sessions included discussions between the assessors and the food scientists. This was crucial for the distinction of the degree of bitterness along the five-grade scale. J Sci Food Agric 84:581–590 (online: 2004)
The halved roots for the taste analysis were cut into 2–3 cm pieces on plastic chopping boards and stored at room temperature in covered coded plastic bowls. The assessors sat separately in a room with good lighting and air conditioning, with no facial view of one another. Assessors were supplied with water for mouth washing and a bucket for disposing of the samples. Starting at 14:00, coded root samples were served one at a time in multicoloured plastic bowls. The individual panellist recorded the taste score for each sample on a pre-coded sheet. During the 11 days of the study, at 12:00, assessors were served a standard lunch made from local dishes that excluded cassava. Assessors followed a standardised procedure, ie rinsed their mouths with room-temperature water before and between samples. The samples were chewed but not swallowed. Between 48 and 50 samples were tasted each day. A mean taste score was calculated for each root from the scores recorded by the 12-member panel. To avoid mouth-numbing sensations and fatigue,25,26 a 30 min break was taken and assessors were served biscuits and orange squash halfway through the tasting session. Determination of glucoside levels The other half of the root was analysed for cyanogenic glucoside content. The root was cut into approximately 1 cm cubes. Approximately 50 g was weighed in a plastic cup, then mixed with 160 ml of 0.1 M orthosphosphoric acid and homogenised.27 Analysis was then performed using the previously described solid phase method.28 Quality control of assays The reproducibility of the chemical analysis28 and the taste assessments24,29 was assessed by double determinations of 11 roots. Each of these roots was divided into four equal longitudinal portions to obtain double determinations for both mean taste score and cyanogenic glucosides. Each longitudinal portion was labelled with a 4-digit sample number by an investigator not involved in the sensory or chemical analyses of the roots. Eleven double estimations of mean taste score and cyanogenic glucoside content were made and yielded correlation coefficients of 0.95 and 0.99 respectively. Statistical analysis Simple descriptive statistics, means, confidence intervals, ANOVA and regression using SPSS30 version 9.0 were used to analyse the data.
RESULTS Farmers’ ability to use bitter taste as an assay for cyanogenic glucoside level Local ethnoclassification into ‘bitter’ and ‘cool’ cultivars Table 1 shows a considerable gap between the mean glucoside levels of the seven ‘bitter’ and the three ‘cool’ cultivars respectively. The mean cyanogenic glucoside 583
L Chiwona-Karltun et al Table 1. Mean ± SEM taste scores and cyanogenic glucoside levels as mg HCN eq kg−1 fresh weight for roots of each of the 10 cultivars
General assessment of each cultivar while at homea
Assessment of plants of each cultivar in the field prior to harvesting
No of farmers
Mean taste score
No of plants
Mean taste score
No of roots
Farmers’ taste score
Panellists’ taste score
Cyanogenic glucosides
‘Bitter’ Nyamakozo Gomani Nyankhata Ngwenyani Depweti Nyaharawa Koloweki
10 21 8 7 22 9 12
4.9 ± 0.1 4.6 ± 0.1 4.8 ± 0.2 4.8 ± 0.2 4.3 ± 0.2 4.1 ± 0.2 4.1 ± 0.2
24 28 26 26 26 24 26
4.3 ± 0.1 4.1 ± 0.1 4.8 ± 0.1 4.5 ± 0.1 3.2 ± 0.2 4.1 ± 0.1 3.5 ± 0.1
48 56 52 52 52 48 52
4.2 ± 0.1 3.3 ± 0.1 3.7 ± 0.2 3.9 ± 0.2 3.4 ± 0.2 3.1 ± 0.2 3.2 ± 0.2
4.3 ± 0.1 3.7 ± 0.1 3.4 ± 0.1 3.4 ± 0.1 3.1 ± 0.1 2.9 ± 0.1 3.1 ± 0.1
242 ± 2 161 ± 1 160 ± 1 158 ± 1 120 ± 7 117 ± 9 114 ± 7
‘Cool’ Nyachikundi Chimpuno Mbundumali
7 8 20
1.3 ± 0.3 1.3 ± 0.3 1.0 ± 0.0
22 20 24
1.6 ± 0.1 1.2 ± 0.1 1.3 ± 0.1
44 40 48
1.6 ± 0.2 1.2 ± 0.1 1.1 ± 0.1
1.8 ± 0.1 1.6 ± 0.1 1.5 ± 0.1
31 ± 4 30 ± 4 25 ± 3
Cultivar
a
Assessment of roots as tasted by farmers, by taste panellists and by chemical analysis
Mean ± SEM of general taste scoring for each cultivar obtained from the same woman in an earlier field survey.8
level of the 132 roots from the three ‘cool’ cultivars was 29 mg HCN kg−1 fresh weight (CI = 25–33, range 1–123). The 360 roots from the seven ‘bitter’ cultivars had a mean glucoside level of 153 mg HCN kg−1 fresh weight (CI = 143–163, range 22–661). Fig 1 (see later) shows that there is a minor overlap in cyanogenic glucoside levels between the roots from cultivars designated as ‘cool’ and ‘bitter’ respectively. General knowledge about the character of each cultivar Column three in Table 1 shows the mean scores of the farmers’ prediction of taste for each cultivar. In the initial interviews, farmers predicted ‘nyamakozo’ to be the most ‘‘bitter’’-tasting cultivar. We found that it had the highest mean cyanogenic glucoside level. Within the ‘bitter’ cultivars, farmers further predicted ‘depweti’, ‘nyaharawa’ and ‘koloweki’ to be less ‘‘bitter’’ than the other ‘bitter’ cultivars. The roots of these three cultivars had lower cyanogenic glucoside levels than the other ‘bitter’ cultivars. For the seven ‘bitter’ cultivars there was a statistically significant (p < 0.05) positive correlation (r 2 = 0.67) between the mean taste score for predicted bitterness and the mean glucoside levels of the roots. Since the ability to taste bitterness is not linear by nature,29 the cyanogenic glucosides were logarithmically transformed. With log transformation of cyanogenic glucoside levels the r 2 value increased to 0.79 (p < 0.01). Farmers’ ability to predict environmental effect Column five in Table 1 shows the mean taste scores of the farmers’ prediction for each cultivar when taking into account the modifying effect of the environment in each field. For the ‘bitter’ cultivars the general predictions for the taste of each cultivar were initially higher than those made in the field, and the correlation between glucoside level and predicted taste was higher when made in the home. There is a positive (r 2 = 0.21) but not statistically significant correlation between 584
mean predicted taste scores made in the field and mean cyanogenic glucoside levels (whether log transformed or not) for the seven ‘bitter’ cultivars. Table 2 shows the mean cyanogenic glucoside levels for roots of each cultivar when analysed by categories of predicted taste score as given by the farmers for each cultivar in a given field. The table shows that the farmers’ predicted taste score was statistically significantly associated with higher mean glucoside levels for only three of the seven ‘bitter’ cultivars, ie ‘koloweki’, ‘depweti’ and ‘nyankhata’. Farmers’ tasting of the tip of the cassava root Table 3 shows the man glucoside levels for each cultivar disaggregated by the taste score by the farmers for each individual root after tasting the tip of the root. The mean cyanogenic glucoside level for nine out of the 10 cultivars is statistically associated with farmers’ assessment of taste. There was a positive correlation (r 2 = 0.42) between the glucoside level and the taste score given by the farmers for all 492 roots. With logarithmic transformation of the glucoside levels the r 2 value increased to 0.53. However, there was a stronger positive correlation (r 2 = 0.69) between the farmers’ mean taste score and the mean glucoside level (linear and logarithmic) for each of the seven ‘bitter’ cultivars only (Table 1). When the cultivar ‘nyaharawa’ (48 samples) was excluded from the analysis, the correlation r 2 between glucoside levels and farmers’ taste scores for the remaining nine cultivars increased from 0.42 to 0.68. Correlation between taste and cyanogenic glucoside level Fig 1 shows a high correlation between the mean taste score from the sensory panel for each root and the cyanogenic glucoside level of the 492 roots on a logarithmic scale (r 2 = 0.75). The r 2 value without log transformation was 0.67. The mean taste score and J Sci Food Agric 84:581–590 (online: 2004)
Bitter taste and cyanogenic glucosides in cassava roots 800 600 500 400
Cyanogenic glucosides HCN eq. (mg/kg fresh weight)
300
200
100 80 60 50 40 30
20
10
Cool roots Bitter roots
5 1
2
3
4
5
Taste score Figure 1. Correlation between levels of cyanogenic glucosides (logarithmic) and mean taste score as determined by the sensory panel for each of the 492 roots.
glucoside level overlap between roots from ‘bitter’ and ‘cool’ cultivars. There is a large range of cyanogenic glucoside concentrations occurring between the mean taste scores of 4.5 and 5.0, ie in roots observed as being very bitter in taste by most panellists. Table 4 shows the mean glucoside levels for the roots of each cultivar analysed in the five taste categories as scored by the taste assessors. In contrast to the results of the farmers’ tasting in Table 3, the results in Table 4 show that there is a statistically significant association between taste and glucoside level for all 10 cultivars. A summary of correlation analysis showed that the farmers’ tasting of the tip of the roots could predict all cyanogenic glucoside levels (r 2 = 0.42) except that for ‘nyaharawa’. When compared with the results of the taste panel, it appears that the taste scoring of the longitudinal half of each root by the taste panellists was more robust in predicting cyanogenic glucosides (r 2 = 0.67, linear).
DISCUSSION Farmers’ ability to use taste in predicting toxicity The mean cyanogenic glucoside levels of roots for each cultivar fell into two groups that coincided with J Sci Food Agric 84:581–590 (online: 2004)
the farmers’ ethnoclassification of cultivars into two groups (Table 1). All seven ‘bitter’ cultivars had much higher mean glucoside levels than the three ‘cool’ cultivars. Earlier interviews16 revealed that farmers regarded ‘bitter’ and ‘cool’ cassava cultivars as being two different food crops having different functions within the food system. The differences in mean cyanogenic glucoside levels in the two groups strongly confirm that the farmers’ ethnoclassification of cassava cultivars into two groups is based on distinct potential differences in the toxicity of roots. This is in agreement with findings among Amazonian Indians in South America,7 who also perceive cassava cultivars as belonging to two different groups. In the study from South America it was observed that the distinction of cultivars into two groups corresponded to a severalfold difference in the levels of cyanogenic glucosides. To our knowledge, the present study is the first one to confirm this difference within an African cassava farming system. The farmers’ knowledge of the degree of bitterness of roots from each cultivar was measured by the mean taste score for each cultivar as predicted by each farmer when interviewed in the home. Interview studies in the same area revealed that farmers equated the degree of bitterness of roots to their potential toxicity if 585
L Chiwona-Karltun et al Table 2. Cyanogenic glucoside levelsa for roots from each cultivar grouped according to predicted taste scoring by farmers while standing at the edge of the field
Cultivar ‘Bitter’ Nyamakozo Gomani Nyankhata Ngwenyani Depweti
Number of roots (n = 492)
Taste score category of plants as predicted by farmers prior to harvesting Very cool
Cool
(0)
(0)
(0)
(0)
(0)
(0)
(0)
(0)
(0) 111 ± 8 (16) 109 ± 9 (4) 66 ± 11 (16)
23 ± 3 (24) 31 ± 5 (32) 23 ± 3 (36)
30 ± 5 (16) 28 ± 9 (8) 30 ± 4 (12)
48 56
52 52 48 (0)
Koloweki ‘Cool’ Nyachikundi Chimpuno Mbundumali
Bitter
Very bitter
195 ± 19 (16) 152 ± 22 (20) (0) 125 ± 20 (8) 107 ± 12 (20) 115 ± 14 (8) 102 ± 6 (16)
170 ± 23 (4) 140 ± 16 (8) 120 ± 9 (12) 120 ± 31 (8) 147 ± 23 (4) 120 ± 13 (16) 127 ± 10 (32)
279 ± 29 (28) 173 ± 15 (28) 172 ± 15 (140) 174 ± 18 (136) 145 ± 17 (12) 117 ± 17 (20)
89 ± 13 (4)
(0)
(0)
(0)
(0)
(0)
(0)
(0)
(0)
52
(0) Nyaharawa
Intermediate
52
44 40 48
ANOVA sig test p value 0.033 0.393 0.065 0.120 0.036 0.834 0.008
(0)
