cassava slice

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ISSN: 1579-4377

COOKING TIME AND STEEPING TIME EFFECTS ON THE

CYANIDE CONTENT AND SENSORY QUALITY OF CASSAVA

SLICES

Christian Agatemor*

*Department of ChemistryFaculty of Physical Sciences, University of Benin, Benin City, Nigeria

[email protected]

ABSTRACT

The effect of different cooking times and steeping times on the cyanide content, colour andflavour of cassava slices were evaluated. The cassava slices were prepared in accordance

with conventional method used by consumers. The cyanide content determined with the aidof picrate paper kit B2 and the colour and flavour evaluation assessed on a 5-point hedonic

scale by a 20-member panel. The preparation of the slices were carried out under differentcooking times (20 minutes, 30 minutes, 40 minutes, 50 minutes, 60 minutes) and steeping

times (12 hours, 15 hours, 18 hours, 21 hours, 24 hours). The results showed that increasingcooking and steeping times reduced cyanide content and adversely affected colour. The

results also suggested that 40 minutes and 18 hours of cooking and steeping, respectively,were the optimum cooking and steeping times.

KEYWORDS

Cassava, cooking time, cyanide, colour, flavour, slices, steeping time.


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INTRODUCTION

Cassava (Manihot esculenta, Crantz) root is an important staple food for 0.5- 1.0 billion

people in the tropics (FAOSTAT). It is a great contributor of calories as 60% of the dailycalorific needs of populations in tropical Africa, Central America, and Asia is provided by

cassava root (Kobawila et al., 2005) Cassava root is usually processed into various products

in the tropics. These products include garri (Adindu and Aprioku, 2006), cassava fufu(Oyebanji, 2004) cooked cassava slices or chips (Adindu, 2007) Cooked cassava slices isprocessed by peeling the root, washing the peeled root in water and cooking the washed

root for 20-30 minutes (Adindu, 2007). The cooked root is cut into thin slices and steeped inwater over night. The slices are washed with water to removed slimy material on slices and

then sun-dried before eaten. The cassava slices is popular among populations in tropicalAfrica, where it is eaten as snack.

The cassava root is rich in cyanide in the form of cyanogenic glucoside known aslinamarine and lotaustraline (Dunstan et al., 1996). The cyanogenic glucosides can be

hydrolysed by endogenous or microbial linamarase. This hydrolysis releases cyanhydricacid, which is toxic. Besides, unhydrolysed linamarine in cassava root and products of the

hydrolysis constitute a health threat to consumers (Akintonwa et al., 1994). In fact, chronic,continuous ingestion of cyanogens either in the form of nondetoxified cassava products is

associated with diseases such as acute intoxication, goiter, konzo and tropical ataxicneuropathy (TAN) (Cliff, 1994). Most of the processing technologies for the production of

cassava products eliminate some amount of the cyanogenic glucosides in the cassava roots.The residual cyanogenic glucosides could be present at health-relevant concentrations,

which could pose a serious health threat to consumers. It is on this ground that this studywas conducted. This present study seeks to investigate the total residual cyanide in cassava

slices as a function of cooking and steeping time. Apart from the levels of residual cyanideinvestigated, sensory characteristics such as colour, aroma and taste were also monitored

with respect to cooking and steeping time. The results of the study will provide the

consumers of cassava slices information on the optimum cooking and steeping periods asliterature search reveals the absence of such information.

MATERIAL AND METHODS

Preparation of cassava slices

12-month-old freshly harvested cassava roots were obtained from a local farm in Benin City,

Nigeria in July 2007. The roots were transported to the laboratory in a jutes bag. Damagedroots were eliminated. The roots were peeled and washed in cleaned tap water to removed

dirt. The washed cassava roots were boiled in tap water for 20 minutes. At the end of thecooking time, the cooking water was removed and the roots were cut into thin slices. The

slices were steeped in water for 12 hours before analyses. Different cassava slices wereprepared by varying the cooking time (30 minutes, 40 minutes, 50 minutes, and 60 minutes)

and the steeping time (15 hours, 18 hours, 21 hours, and 24 hours).

Analyses

Cyanide content was determined on dried powdered cassava slices with the aid of a picrate

paper kit B2 (Bradbury et al., 1999; Egan et al., 1998). The method involved theimmobilization of linamarase in a small filter paper disc. Phosphate buffer at pH 8 was also


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loaded on the filter paper. The disc was placed in a small flat bottom bottle. A 100 mg of

the test sample was added to the filter paper and 0.5 ml of distilled water was added. Thiswas followed immediately by the insertion of a yellow picrate paper attached to a plastic

strip to the bottle. Care was taken to ensure that the picrate paper did not touch the testsample. The bottle was closed with a screw lid immediately after insertion of picrate paper

and allowed to stand for 16 24 hours at room temperature. The picrate paper was removed

and immersed in 5 ml distilled water in a test tube with occasional stirring for 30 minutes.An unreacted picrate paper was taken as blank. The absorbance of the solution wasmeasured against the blank with a Bausch and Lomb spectronic 20 spectrophotometer at 510

nm. The cyanide content, in ppm, was obtained by multiplying the absorbance by 396.The analysis was also carried out by replacing the test sample with a standard

linamarine discs prepared by adding a amount of linamarine solution to a small 3 mm filterpaper and allowing the filter paper to dry (Bradbury et al., 1999; Egan et al., 1998) and the

remainder of the method followed that given above.Sensory attributes (colour and flavour) were evaluated on a 5-point hedonic scale

(where 1 = dislike extremely and 5 = like extremely) by 20 panelists selected from theDepartment of Chemistry, University of Benins student and staff population who consume

cassava slices. The sensory evaluation was performed in the mid-morning under white lightin an airy room. The samples were served in polypropylene transparent plates, which had

been labeled with a 3 digit random number. Questionnaires and clean tap water to rinse themouth between each tasting were provided for the panelists. Prior to evaluation, a pre-

evaluation session was held in which the meaning of each attribute (colour and flavour) wasexplained to the panelists to avoid misinterpretation (Kilcast and Subramanian, 2000). The

panelists were not allowed to discuss their scores with one another during the evaluationsession. Another set of cassava slices were evaluated as second and third replicates on the

second and third day, respectively. Sensory evaluation data were presented as means ofpanelists scores.

Statistical analysisExperimental data were tested by ANOVA (Minitab 10.0, Release 7.1, Minitab, Inc., StateCollege, PA, USA) and mean separation was achieved by using Duncans multiple range

test.

RESULTS AND DISCUSSION

The results of the total cyanide content determination and sensory evaluation are presentedin Tables 1 3. The total cyanide content of the raw, uncooked cassava was also determined

and found to be 580 15 ppm. The three major processing techniques employed in theprocessing of cassava slices are cooking, soaking in water (steeping) and sun drying

(Oluwole et al., 2004). Cooking of cassava reduces the cyanide content. Cooke andMaduagwu (1985) reported a 90% reduction of free cyanide and a 55% reduction of bound

cyanide by the cooking of cassava (Cooke and Maduagwu, 1999). It was also reported thatcooking destroyed the enzyme, linamarase, at about 72

0C thus leaving a considerable portion

of linamarine and lotaustraline in the cooked cassava (Cooke and Maduagwu, 1999). Highlevels of cyanide in cassava have been implicated in a number of health disorders (Oyebanji,

2004; Bradbury, 2004) The results in Table 1 show a statistically significant reduction (P

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