Hydrogen Cyanide Content of Manihort Utilissima , Colocasia Esculenta , Dioscorea Bulbifera and Dioscorea Domentorum Tubers Found in Benue State

The hydrogen cyanide contents of Manihort utilissima (Cassava), Colocasia esculenta (Coco yam), Dioscorea bulbifera (Aerial yam) and Dioscorea domentorum (Trifoliate yam) Tubers found in Benue State were determined in their fresh and dried forms. The amount of hydrogen cyanide found in the fresh state was 304.60 mg·100g, 34.10 mg·100g, 49.70 mg·100g and 79.40 mg·100g for Manihort utilissima, Colocasia esculenta, Dioscorea bulbifera and Dioscorea domementeroum respectively; while the amount observed in the dry form was 249.50 mg·100g, 17.30 mg·100g, 32.50 mg·1200g and 55.10 mg·100g for Manihort utilissima, Colocasia esculenta, Dioscorea bulbifera and Dioscorea domementeroum respectively. Higher hydrogen cyanide contents were observed in the fresh samples than the dried samples. Manihort utilissima was found to have the highest values both in the fresh and the dried forms. Statistical analysis of the mean values of hydrogen cyanide content of the tubers indicates a significant difference between the fresh and the dried forms of the tubers. The study revealed that tubers of these plants have relatively high content of hydrogen cyanide content.


Introduction
Hydrogen cyanide is a colourless liquid or gas with a faint, bitter almond odour.It has a molecular formula HCN.It is readily soluble in water.Its solution in water is known as hydrocyanic acid or prussic acid.Its other name is formonitrile.Salts of HCN are called cyanides.Formonitrile is a poisonous substance; as such it is often regarded as an anti-nutritional factor (ATSDTR, 1993).
According to Kamalu (1995), cassava root, a dietary staple in many tropical regions contains cyanogenic glycosides, such as linamarin, which release cyanide (CN -) when metabolised endogenously.Jantz et al. (1997), reported that, people eating food that contain low level of cyanide for a long time develop damage to the central nervous system (CNS) and thyroid gland.To buttress this, Kamalu (1995) stressed that long time ingestion of food containing cyanide can lead to deafness, vision problems, and loss of muscle coordination.The effect of this on thyroid gland is cretinism (retarded physical and mental growth in children), or enlargement and over activity of the gland.Other effects of cyanide include.
Pancreatic diabetes, vitamin B 12 deficiency, and decrease in iodine uptake (Makkar et al., 1998).Liver inflammation and haemorrhage, kidney, adrenal, myocardial and testicular lesion have been observed in dogs that consumed cassava containing low amounts of cyanide (Abeye, 1998).Also, it has been reported that consumption of food substances containing hydrogen cyanide may cause death within few hours (Fina, 2004).This is however dependent on the amount consumed in the food material.Most importantly, the recommended safe (tolerable) limit of cyanide for human consumption is 10 mg•kg -1 (Coursey, 1983).Therefore, this studies report the levels of HCN in cassava, cocoyam, aerial yam, and trifoliate yam tubers obtained from Gboko local Government area of Benue State, Nigeria, hence these food materials form a staple diet among the people of this area.

Materials and Methods
The tubers of cassava, aerial yam and trifoliate yam were collected from Gboko local Government area in Benue state.The samples were identified by a Botanist of the biological science department, Benue State University, Makurdi.The peels were removed and the inner parts of the samples were sun-dried to a constant weight.The dried samples were ground, then sieved to fine particles, while the fresh samples were also ground to fine particles for analysis.

Hydrogen Cyanide Determination
Alkaline titration procedure was adopted (Anhwange, 2004).Ten grams of each of the ground samples were soaked in the mixture of 200 cm 3 of distilled water and 10 cm 3 of orthophosphoric acid.The mixture was kept for 12 hours to release all the bonded cyanide.The mixture was then distilled until 150 cm 3 of the distillate was collected.20 cm 3 of the distillate was taken into a conical flask containing 40 cm 3 of distilled water, 8 cm 3 of ammonia solution (6 mold•m -3 ) and 2 cm 3 of potassium iodide (5%) solution were added.The mixture was titrated with silver nitrate (0.02 mold•m -3 ) to faint but permanent turbidity (1 cm 3 0.02 mold•m -3 AgNO 3 ≡ 1.08 mg HCN).Replicates determination were done for each of the samples.

Statistical Analysis
Means of the fresh and dried samples were compared using student t-test and the level of significant difference determined at p < 0.05 (Parker, 1979).

Results and Discussion
Result of the assessment of cyanide (mg•100g -1 ) in Cassava, Cocoyam, Aerial yam, and Trifoliate yam both fresh and dried tubers are presented as follows: From the result, the content of hydrogen cyanide; mg•100g -1 , in fresh Cassava, Cocoyam, Aeriel yam, and Trifoliate yam tubers were 304.60, 34.10, 49.70, and 79.40 respectively.The cyanide content of the dried samples was found to be 249.50mg•100g -1 , for Manihort utilissima, 17.3 mg•100g -1 , for Colocasia esculenta, 32.4 mg•100g -1 , for Dioscorea bulbifera and 55.1 mg•100g -1 for Dioscorea domentorum.Generally the cyanide content was higher in the fresh tubers than the dried tubers.
The amount of hydrogen cyanide was found to be highest in cassava tuber for both the fresh and dried samples; 304.60 mg•100g -1 and 249.50 mg•100g -1 respectively.Conversely, least levels of cyanide were obtained, in cocoyam tubers, 34.10 mg•100g -1 (fresh) and 17.30 mg•100g -1 (dried).In nutshell, the result showed that the order of the amounts of hydrogen cyanide (mg•100g -1 ) in the tubers was Colocasia esculenta < Dioscorea bulbifera < Dioscorea domentorum < Manihort utilissima.Abaye et al. (1998), reports a high concentrations of cyanide (86 mg•kg -1 ) in fresh cassava tubers, while the amounts for the dried sample is low (15.70 mg•kg -1 ).The levels of HCN in cassava are largely due to environmental factors and duration of the drying.The normal range of cyanide content in cassava tubers is 15 -400 mg•100g -1 (Anhwange, 2004).
Statistical analysis of the means of fresh and dried samples indicate that there was significant difference (at p < 0.05) between the fresh and the dried samples for all the tubers studied.

Conclusion
The literature values of cyanide contents of aerial yam, cocoyam, and trifoliate yam were not found.Although the analysis indicated that the tubers of these crops also contained this antinutritional factor (cyanide).Relatively, the amount of cyanide obtained from aerial yam, cocoyam, and trifoliate yam tubers were lower compared to that of cassava tubers in both fresh and dried samples.
Results of this study are in agreement with earlier works as it is being pointed out.Therefore, further antinutritional and chemical analysis should be done on these tubers, especially aerial yam, cocoyam, and trifoliate yam.Food crops found to contain high level of cyanide should be efficiently processed before consumption.