ABSTRACT
Cyanide contents of locally processed cassava based products (CBP) in Ilorin-West urban markets, Nigeria was estimated and compared to established safety levels. Hence, 29 locally processed cassava products samples were randomly collected from retail outlets and tested for hydrogen cyanide content, pH, titratable acidity (TTA), water absorption capacity (WAC) and least gelation capacity (LGC) following standard procedures. Results from the survey showed that hydrogen cyanide contents of the CBP ranged from 3.36 to 37.73 mg/kg. Titratable acidity ranged between 0.22 and 1.79 × 10-3 (% w/w); the pH of the samples was between 4.55 and 6.75; WAC ranged from 1.46 to 5.82 g/ml, while the LGC was from 8.07 to 38.08%. Twenty five out of the 29 CBP samples collected had hydrogen cyanide content above the maximum safe level (10 mg/kg), the TTA was low, while the pH was high. These problems might be due to poor processing methods adopted by the processors. Continuous exposure of consumers to sub-lethal dose of hydrogen cyanide may lead to serious health hazards.
Key words: Cassava, cyanide, Ilorin, market, survey.
Cassava (Manihot esculenta Crantz) belongs to the family Euphorbiaceae and it is a perennial woody shrub, producing enlarged tuberous roots (Oghenechavwuko et al., 2013). Cassava being the second important Africa’s staple food after maize in terms of calories consumed, is a reliable and inexpensive source of food for more than 700 million people in the world, with Nigeria being the largest producing country (FAO, 2003; Eleazu et al., 2011).
However, the presence of cyanogenic glucosides constitute a major limitation to the utilization of cassava as human food (Asegbeloyin and Onyimonyi, 2007; CAC/RCP, 2013). The leaves and roots of cassava plant contain the cyanogenic glucosides called linamarin and small amount of lotaustralin (Orjiekwe et al., 2013). Linamarin is readily hydrolysed to glucose and acetone cyanohydrin in the presence of enzyme linamarase, which is released when the cells of cassava roots are ruptured (CAC/RCP, 2013; Omotioma and Mbah, 2013; Orjiekwe et al., 2013). The acetone cyanohydrin decomposes rapidly in neutral or alkaline conditions to liberate hydrogen cyanide and acetone (Orjiekwe et al., 2013). The levels of the cyanogens vary considerably between tissues depending on many factors such as cultivar, climatic conditions, age of cassava at harvesting and postharvest practices (Ojo et al., 2013).
Hydrogen cyanide is a volatile compound which evaporates rapidly in the air at temperature over 28°C, dissolves rapidly in water, may be toxic to humans and animals, and the severity of the toxicity depend on the quantity consumed (CAC/RCP, 2013). According to Orjiekwe et al. (2013), there are several health disorders which have been associated with regular intake of sub-lethal quantities of cyanogens, some of which have resulted into outright poisoning and death due to cyanide intake from consumption of poorly processed cassava products. The authors further stated that health risks caused by ingestion of these chemicals (cyanogens) include the exacerbation of goiter, cretinism and cardiovascular diseases such as encephalopathy and neuropathy, while severe cyanide poisoning can lead to heart, brain and optic nerve degeneration. The Nigerian Industrial Standards (NIS) and the Codex Alimentarius Commission (CAC) standards for maximum safe level of hydrogen cyanide in most cassava products meant for human consumption is 10 mg/kg (Sanni et al., 2005; Ubwa et al., 2015).
Monitoring of the cyanogenic potentials of cassava is therefore of utmost importance due to the following reasons: determination of safety of different cassava products, evaluation of the efficiency of different existing methods of cassava processing on the removal of cyanogenic glucosides and determination of level of cyanogenic potentials of newly released cassava varieties in breeding programmes (Tivana et al., 2014). Literature evidences have shown that hydrogen cyanide contents of cassava products from eastern and western parts of Nigeria were above the acceptable limit and there were reported cases of occasional deaths resulting from consumption of cassava meal (Adindu et al., 2003; Odoemelam, 2005; Adindu and Aprioku, 2006; Komolafe and Arawande, 2011). Hence, the need to survey and compare the other parts of the country.
Available data have also shown that under appropriate postharvest handlings especially fermentation and/or retting, more than 60% of cyanogen compounds were being detoxified (Onwuka and Ogbogu, 2007; Oghenechavwuko et al., 2013). Cassava products are being processed largely in the rural areas where some of the processors are known to employ short practices that are less effective in removing cyanogen compounds
(Adindu et al., 2003). Therefore, the need to monitor these compounds in cassava products cannot be over emphasized. This study is an attempt to evaluate some selected quality parameters of cassava based food products obtained from urban markets within Ilorin-West Local Government Area of Kwara State, Nigeria, with a view to determine whether they are within the acceptable safe level of hydrogen cyanide concentrations.
Reagents and test samples
All used reagents were of analytical grade and most were products of SIGMA-ALDRICH, Germany and BDH, England. All cassava products displayed in each markets were sampled, these include white gari, yellow gari, ‘lebu’ (fine gari), ‘gari Ijebu’, ‘lafun’ I (edible cassava flour, whole root tubers), ‘lafun’ II (edible cassava flour, pellatized root tubers), tapioca, ‘fufu’ and ‘abacha’.
Sampling area
Five urban markets (designated as A, B, C, D and E) within Ilorin-West Local Government Area of Kwara State, Nigeria were used for sample collection because apart from the facts that most consumers in the area collect their food stuffs from these urban markets, they also serve as major sources of goods for other smaller markets in the city. At least 5 to 10 samples of each product were randomly selected from each market. After collection, each product was thoroughly mixed together to give a representative sample from each market. Finally, a total of 29 samples were obtained for further analyses.
Analysis of cyanide contents
Cyanide content determination was done using the alkaline picrate method (Onwuka, 2005). The method was partially modified and used as follows; 5 g finely ground sample was dissolved in 50 ml distilled water in a corked conical flasks and left to stay overnight for proper extraction of cyanide. The extract was filtered through Whatman number 1 filter paper. To 1 ml of the extract was added 4 ml alkaline picrate solution and incubated in the water bath at 37°C for 15 min for colour development. The absorbance was read with UV-Vis Spectrophotometer (SEARCHTECH: UV1902PC) at 490 nm against the reagent blank containing 1 ml of distilled water and 4 ml of alkaline picrate solution prepared at the same time. Cyanide concentrations of the food samples were extrapolated from the standard curve.
Preparation of the standard cyanide curve
Pure potassium cyanide (KCN) was used as the standard in this determination. From 5 to 50 ppm of the standard was prepared and to each concentration was added 25 ml of 1 N HCl in 500 ml conical flasks. Exactly 1 ml of each solution and 1 ml of distilled water as blank were taken into separate corked boiling tubes followed by addition of 4 ml alkaline picrate solution. The mixture was incubated in water bath at 37°C for 15 min to develop the colour and the absorbance of the standards with the blank was read at 490 nm. The results obtained were plotted into a graph as standard curve (Figure 1).
Physicochemical analysis
Total titratable acidity (TTA) and pH of cassava based food samples were determined following the method of Onwuka and Ogbogu (2007). Three grams each of the cassava based food samples was weighed into a conical flask and 30 ml of distilled water was added. The suspension was allowed to stand for 30 min after which it was titrated with a standard base (0.1 N NaOH) using 3 drops of phenolphthalein as indicator. Total titratable acidity was estimated according to the formula:
%TTA (w/w) = [N × V × Eqwt/W × 1000] × 100
where N is the normality of NaOH (MEqmL-1), V is the volume of 0.1 N NaOH used, Eqwt is the equivalent weight of predominant acid (mg mEq-1) which is lactic acid, W is the weight of sample, and 1000 is the factor relating mg to gram (mg g-1).
The pH was determined by dispersing 1 g of the cassava based food samples in small quantities of distilled water and then making it up to 10 ml. The dispersion was allowed to stand for 30 min after which the electrode of the pH meter (SEARCHTECH: PHS-3C) was inserted and shaken vigorously and allowed to stand till a stable reading was obtained, the value was recorded as the pH.
Determination of functional properties
Water absorption capacity (WAC) and least gelation capacity (LGC) of cassava based samples were determined by following the methods described by Onwuka (2005). One gram of each sample was weighed into a conical graduated centrifuge tube and mixed thoroughly with 10 ml of distilled water for 30 s. The setup was allowed to stand for 30 min at room temperature and then centrifuged at 4800 × g for 30 min using a centrifuge (CENTURION SCIENTIFIC: K2202). The volume of the free water was read directly from the graduated centrifuge tube and expressed as grams of water absorbed per gram of sample. Varying amounts from 2 to 40% (w/v) of sample were prepared in 5 ml of distilled water in glass test tubes. The sample test tubes were then heated in a boiling water bath for 1 h followed by rapid cooling under running tap water. The test tubes were further cooled at 4°C and the gelation capacity was taken as the least gelation concentration determined as the concentration when the sample from the inverted test tube did not fall or slip down.
Data analysis
Results were expressed as mean ± standard deviation (SD) of replicate determinations. All data generated were subjected to statistical analysis by one-way analysis of variance (ANOVA) using the SPSS statistical software package version 20.0.0 (IBM SPSS Statistics, IBM Corporation 2011). The means were separated using New Duncan Multiple Range Tests (DMRT) as described by Duncan (1955). Significance was accepted at 5% probability level (p=0.05).
Cyanide contents of cassava based food samples sold in urban markets in Ilorin-West, Nigeria
The standard calibration curve of absorbance against the concentration of KCN is as shown in Figure 1. Hydrogen cyanide contents of cassava based food samples sold in Ilorin-West urban markets (Figure 2) varied from as low as 3.36 mg/kg (tapioca, market C) to as high as 37.73 mg/kg (‘fufu’, market B). Data collected from each market differ significantly (p=0.05) in their cyanide contents except in few cases; especially in white ‘gari’ where there was no significant (p=0.05) difference between market B and market E, lafun I where there was no significant (p=0.05) difference between market C and market E; also in ‘lafun’ II where there was no significant difference (p=0.05) between market A and market B. Only one market each displayed ‘abacha’ sample (market D) and ‘gari Ijebu’ (market C) during the survey exercise.
The data obtained in this present study showed that fufu had the highest significant (p=0.05) hydrogen cyanide contents. This might be attributed to poor processing of the product. Similar results were also obtained in previous studies. For instance, Orjiekwe et al. (2013) observed that out of three commodities (gari, fufu and tapioca) sampled in Okada area of Edo State Nigeria, fufu had the highest significant (p=0.05) value of cyanogenic glycosides.
The NIS and CAC standards for maximum safe level of hydrogen cyanide in most cassava products meant for human consumption is 10 mg/kg (Sanni et al., 2005; Ubwa et al., 2015). Only four out of the 29 samples collected had hydrogen cyanide contents below the maximum safe level of hydrogen cyanide concentration in foods meant for human consumption. This again might be due to poor processing and/or handling by the processors. Similar results have been published in respect of other locations in Nigeria. For instance, field evaluation conducted in Nigeria on three cassava products (garri, fufu and tapioca) revealed that total cyanogen content exceeded the safe level (10 mg/kg) recommended for cassava products (Adindu et al., 2003). The cyanide levels of cassava products reported for eastern and western parts of Nigeria were also above the acceptable limit and there have been reported cases of occasional deaths resulting from consumption of cassava meal (Odoemelam, 2005; Adindu and Aprioku, 2006; Komolafe and Arawande, 2011). Epidemiological studies have shown that exposure to small doses given over a long period of time can result in increased blood cyanide levels with the following symptoms; dizziness, headache, nausea and vomiting, rapid breathing, restlessness, weakness and even severe cases of paralysis, nerve lesions, hyperthyroidism and miscarriage (Orjiekwe et al., 2013).
Physicochemical properties of cassava based food samples sold in urban markets in Ilorin-West, Nigeria
Physicochemical properties of cassava based food products sold in Ilorin-West urban markets were presented in Table 1. Total titratable acidity (TTA) of cassava based food samples from five different urban markets within Ilorin-West Local Government was between 0.22 and 1.79 × 10-3 (% w/w). There was no significant (p=0.05) difference between the TTA of yellow gari from markets A and B. For white gari, no significant (p=0.05) difference in the TTA of samples from market A, B and E, as well as those samples from markets B, C and D. Similarly, no significant (p=0.05) difference in the TTA of lafun I from markets A, B and C, as well as between those from markets B and E. For lafun II samples, no
significant (p=0.05) difference in the TTA of samples from markets A, C and D, while those from markets B and E also showed no significant (p=0.05) differences in their TTA values. Fufu samples from markets A and D with those from markets B and C had no significant (p=0.05) difference in their TTA values.
The pH values (Table 1) of cassava based food samples from urban markets in Ilorin-West Local Government ranged from 4.55 to 6.75. Fufu sample from market D was significantly (p=0.05) higher in the pH value than all other food samples, while Tapioca from the same market D had the least pH reading and the value was significantly (p=0.05) lower.
The NIS limit for total acidity of gari and edible cassava flour was set at 1.0% w/w maximum, while that of pH was set at 5 to 7 units (Sanni et al., 2005). Research works have shown that TTA increases as fermentation time increases (Onwuka and Ogbogu, 2007; Oghenechavwuko et al., 2013). In addition, 48 h fermentation caused increased acidity of processed fufu samples (Onwuka and Ogbogu, 2007). Low TTA values in the present study might be an indication of poor processing which means that they were partially or inadequately fermented.
Functional properties of cassava based food samples sold in urban markets in Ilorin-West Local Government Area
The results of WAC of cassava based food samples sold in Ilorin-West Local Government urban markets was as presented in Table 1. The WAC of cassava based food samples ranged from 1.46 g/ml (Lafun II, market D) to 5.82 g/ml (Tapioca, market B). The WAC of tapioca (market B) was significantly (p=0.05) higher than that of all the other food samples collected. There was no significant (p=0.05) difference found in the WAC of yellow ‘gari’ from markets A, B, and D. For white gari samples, no significant (p=0.05) difference was found in the WAC of samples from markets A and D. The lafun I samples from markets A, B, and E had no significant (p=0.05) difference in their WAC values. Similarly, lafun II from markets A, B, and D had no significant (p=0.05) difference in their WAC readings. Also, there was no significant (p=0.05) difference in WAC of lebu samples from markets A and C and fufu samples from markets A, B, C, and D.
The results of least gelation capacity (LGC) of cassava based food samples sold in Ilorin-West Local Government urban markets was as displayed in Table 1. The LGC of these food samples ranged from 8.07% (yellow gari, market B) to 38.08% (lafun I, market C). There was no significant (p=0.05) difference in the LGC of yellow gari samples from markets A and D. For white gari, no significant (p=0.05) difference was found in the LGC of samples from markets A, C and D, then B and E. For lafun I, there was no significant (p=0.05) difference between the LGC of samples from markets A and B, also between markets D and E. Similarly, no significant (p=0.05) difference was found in the LGC of lafun II samples from markets B and C, also between markets D and E. The two lebu gari samples (markets A and C), two tapioca samples (markets B and C) and three fufu samples (markets A, B and D) had significantly (p=0.05) the same LGC values. It was generally observed in the present study that values from different markets in many of the parameters analyzed were significantly (p=0.05) the same, this might simply suggests that some sellers from different markets under the survey probably got their goods from a common source.
It has been evidently shown that two days’ (48 h) fermentation is enough to detoxify up to 60% of hydrogen cyanide in most species of cassava. The present study revealed that hydrogen cyanide concentrations of cassava food products from these urban markets in Ilorin-West Local Government, Nigeria were as high as 4 times (in some cases) the maximum safe level in food meant for human consumption. In addition, low TTA observed in the majority of the samples under this study is an indication of poor processing. Since cassava based products are major staple in most Nigeria societies and many of them are ready-to-eat (RTE) with minimal or no further processes, continuous exposure of unsuspecting consumers to sub-lethal dose of hydrogen cyanide may pose serious health hazards. Therefore, introduction of new breeds of cassava with minimal cyanogenic glycosides and guidelines for processors in sourcing and handling their raw materials will be of tremendous help to reduce the level of cyanide in cassava products. Also, as part of CAC/RCP 73 recommendations, food safety authorities and public health monitoring bodies may consider the introduction of scientific kits such as picrate kits to monitor cyanide concentrations in cassava products at the point of sale or use and the urinary thiocyanate concentration in the population.
The authors have not declared any conflict of interests.
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