EFFECT OF UNIT OPERATIONS OF PRODUCTION ON THE CYANIDE CONTENT AND ACCEPTABILITY OF GARI

Transcription

1 EFFECT OF UNIT OPERATIONS OF PRODUCTION ON THE CYANIDE CONTENT AND ACCEPTABILITY OF GARI J.O. AKINGBALA,' G.B. OGUNTIMEIN and M.K. BOLADE Department of Food Technology University of Ibaah, Ibadan Nigeria Accepted for Publication July 13, 1593 ABSTRACT The effect of holding, pressing, initial moisture content, sunheating, and garifying, on ph, total titratable acidity (TXA) and residual cyanide content of cassava mash, cake and gari, were evaluated. A new method to make gari with very low residual cyanide was developed. Increasing holding time decreased ph, cyanide content, but increased TTA of mash and gari when initial moisture content of mash was greater or equal to 43-65%. Reducing the initial moisture content of mush below 60% increasedph, reduced the residual cyanide, but increased the P A in the mash. lle optimum holding period was 72 h beyond which there was no reduction in residual cyanide content. Pressing and garibing firther reduced the residual cyanide of cassava mash and gari, respectively. Sensory properties of gari prepared by the new laboratory method were similar to those of gari produced by the traditional method. INTRODUCTION Utilization of cassava root as food has been limited by the toxicity, on hydrolysis, of the inherent cyanogenic glucosides (Conn 1969). However, over the years, cassava consuming populations have devised processes, e.g., peeling, steeping, grating, fermenting, boiling, roasting, garifying and drying to reduce or completely eliminate cyanide in cassava before consumption (Coursey 1973). The unit operations involved in the production of gari, a granulous cassava product, are peeling, washing, grating of cassava roots, pressing and fermenting the mash, before sieving and garifying. Garifying is the process of frying sieved 'To whom correspondence should be addressed. Journal of Food Processing and Preservation 17 (1593) All Righrs Reserved. 0 Copyright 1993 by Food & Nutrition Press. Inc. I Tnunbull, Connecticut 331

2 338 J.O. AKINGBALA, G.B. OGUNTIMEIN and M.K. BOLADE cassava mash in shallow pans with constant stirring (Oke 1966; Collard and Levi 1959). These processes reduce the cyanide content of gari considerably (Collard and Levi 1959; McIlroy et al. 1981; Maduagwu 1983; Mahungu et al. 1987). Mahungu et al. (1987), reported that the existing traditional methods of gari production reduces only about 83% of the total cyanide content of the root. Thus a significant amount of cyanide remains in gari, especially since only the high cyanide cassava cultivars (Manihot utilisima Pohl) are traditionally used for gari processing. Diseases, e.g., goiter, spastic paraparesis and tropical ataxias, have been associated with long-term consumption of residual cyanide in cassava foods (Oshuntokun 1981; Bourdoux er al. 1982; Cliff ef a!. 1985). Therefore the Nigerian Food and Drug Administration, in 1980, restricted the permissible residual cyanide in gari to not more than 2 ppm (FDA 1980). A recent survey (Bolade 1990) of the cyanide content of commercial gari samples ( pprn) in Oyo State, Nigeria shows that the FDA standard was not strictly adhered to; possibly because of limitations in the traditional method of gari processing. The objective of this study was to evaluate the contribution of each of the unit operations involved in traditional gari production to cyanide reduction in the product, with the aim of evolving a method that will produce an organoleptically acceptable gari that meets the FDA standard of residual cyanide content of 2 ppm. Cassava Sample MATERIALS AND METHODS A high-cyanide cassava cultivar TMS from the International Institute of Tropical Agriculture (IITA) Ibadan, was used for this study. The cultivar was chosen on the belief that any processing technique that can effectively reduce cyanide content to very low levels in a high-cyanide cultivar will be suitable for producing gari from a low-cyanide cultivar. Chemical Analyses Cyanide content of cassava and cassava products were determined by the method of Ikediobi ef al. (1980). The ph of 5 g sample in 10 ml distilled water was determined using a Metrohm Herisau (Model E520) ph meter. Total titratable acidity (as lactic acid) was determined as described by Lees (1975), on 10 g sample in 50 ml distilled water, titrated with 0.1M NaOH using phenolphthalein as indicator. Proximate analysis of sample (protein, fat, total ash, crude fiber) was determined by the official methods of analysis (AOAC 1980). Soluble carbohydrate was estimated by difference.

3 UNIT OPERATIONS AND GARI QUALITY 339 Effect of Holding Period on Cyanide Content Freshly grated cassava (3 kg) each was weighed into 6 nonporous containers and held at room temperature (29 f 2C). At every 24 h, content of one container was analyzed for total cyanide content, ph and total titratable acidity (TTA). Then the sample was dewatered using a hydraulic press, to % residual moisture and garified. Moisture, ph, TTA and cyanide contents of the dewatered mash (cake) and garified product (gari) were determined. The experiment was replicated three times. Effect of Initial Moisture Content on Cyanide Content, ph and TTA Freshly grated cassava was divided into 6 lots of 3 kg each. Moisture contents of 4 of the lots were adjusted to 55, 60, 75 and 80% by pressing out moisture (55 and 60%) and by adding water (75 and 80%), respectively, before holding in nonporous containers for 72 h. The remaining two lots were held at the natural moisture content of the cassava root (65%), one in a nonporous container, and the other in a porous container as practiced in traditional gari-making process. The samples were held at ambient temperature (29 f 2C) for 72 h then dewatered, sifted and garified. Cyanide content, ph and TTA were determined at 24 h intervals during the holding period and after pressing and garifying. The experiment was replicated three times. Effect of Sundrying on Cyanide, ph, TTA Freshly grated cassava mash (3 kg) was kept in porous and nonporous containers at the natural water content of the root (65 %). The mashes were held 24 h at room temperature (29 f 2C), then sun dried in trays in 3 cm layer thickness, for 2 days before pressing and garifying. Another batch of mash was prepared by the traditional method (without sundrying) to serve as control. Total cyanide content, ph and TTA was determined for the mash at 24 h intervals and for the cake and gari. Traditional Processing of Gari Weights were placed on freshly grated cassava in a jute sack to express the juice during a 72 h holding at room temperature. Then the cake was sieved and garified.

4 340 J.O. AKINGBALA, G.B. OGUNTIMEIN and M.K. BOLADE RESULTS AND DISCUSSION Proximate chemical composition of the fresh cassava tuber (TMS 30572) of about 14 month maturity (Table l), compares favorably with that reported by others (Jones 1959; Ekpeyong 1984), for cassava. The moisture content of the several batches of cassava used in this study ranged from %. The batches were from the same farm and planted at about the same time. The differences in moisture may be due to differences in soil moisture content at the different locations of the farm. The Effect of Duration of Holding, Dewatering and Garifying on Cyanide Content of Cassava The effects of duration of holding, dewatering and garifying on cyanide content of grated cassava is presented in Fig. 1. Total cyanide content decreased from to mg/kg mash (23.8%) during 120 h of holding. The decrease might be due to volatile HCN released by hydrolysis of the glucosides by linamarase enzyme present in the mash or by the acidity developed during holding, or to both activities. Maduagwu (1983) reported that enzymatic hydrolysis is more effective than acid hydrolysis of the glucosides of cassava. Pressing the mash into cake appears to be the single most effective way of reducing the cyanide content of cassava mash (Fig. 1). Pressing reduced the moisture content of the fresh mash from 63.0 to 36.7% and the cyanide content by %. Pressing may have removed the cyanide as glucosides, which are water soluble, or as water soluble free cyanide after hydrolysis of the glucosides (McIlroy er al. 1981). The residual cyanide in the cake appears related to the efficiency of dewatering. Maduagwu and Oben (1981), reported a % reduction in free cyanide by quick removal of water from cassava pulp within 24 h of grating. TABLE 1. PROXIMATE COMPOSITION OF FRESH PEELED CASSAVA TUBERS Chemical Component Water 66.6 x Protein (N x 6.25') Fat Crude fiber Ash 0.55 Carbohydrate (by difference) 30.8

5 UNIT OPERATIONS AND GARI QUALITY 341 Frying or garifying further reduced the cyanide content of the cakes to 9.2, 6.6 and 5.9% for mashes at 0, 24 and 48 h holding, respectively. From 72 h, residual cyanide in gari remained unchanged up to 120 h of holding the mash (Fig. 1). Thus, it may not be necessary to hold cassava mashes longer than 72 h before pressing and frying. Differences in the residual cyanide contents of gari from the fresh (0 h) cake and cake from mash held for 72 h may be due to enhanced hydrolysis to free cyanide, of the glucosides, by linamarase at near optimum ph (Wood 1966). The free cyanide was volatilized by heat during frying of the cake. Effect of Holding on ph and TTA The ph decreased from 6.2 in the fresh mash (0 h) to 4.4 at 48 h holding and thereafter remained constant; and TTA from 0.04 (0 h) to 1.39% at 48 h holding (Fig. 2). Fermentation occurs during holding of cassava mash for gari production (Collard and Levi 1959; Akinrele 1964; Ngaba and Lee 1979; Okafor 1977). The fermentation, described as a two-stage anaerobic exothermic process (Collard and Levi 1959; Akinrele 1964), produce acids that hydrolyze the glucosides into free cyanide during holding of the mash. Other studies (Wood 1966; Maduagwu 1983; Vasconcellos el al. 1990) have shown that fermentation con- Holding period (h) FIG. 1. EFFECT OF DURATION OF HOLDING ON CYANIDE CONTENT OF GRATED CASSAVA MASH (O), CAKE (0) AND GARI (A)

6 342 J.O. AKINGBALA, G.B. OGUNTIMEIN and M.K. BOLADE 7 I I I I 1.5 r 0., TI V.- U d a9 u s 0.6 $ Holding period (h) FIG. 2. EFFECT OF DURATION OF HOLDING ON ph (0) AND TTA (A) tributes very little to cyanide detoxification during gari production and that certain phs might produce a reverse reaction when free cyanide is converted to cyanohydrin. From this study it appears that fermentation contributes to detoxification indirectly by lowering ph of the medium to around optimum ph (5.5) for linamarase activity and probably through acid hydrolysis of the residual glucosides in the cake during garifying. Effect of Mash Moisture Content, on the Cyanide Content of Mash, Cake and Gari Total cyanide content of mash at 0 h ranged from to mg/kg (dwb) for mashes at 75 and 55% moisture contents, respectively (Fig. 3). The cyanide contents of mashes at 75 and 80% moisture were greater than that of the mash at the natural moisture content (65 I) probably due to increased enzymic hydrolysis or increased solubilization of the cyanogenic glucoside at the higher rnoistures. The increased enzymic hydrolysis may have been responsible for the greater loss of cyanide during holding and the lowest amounts of residual cyanide in the cakes and gari (Fig. 3) of mashes with higher moisture content. Sokari (1992) also reported greater cyanide content for cassava mash of greater moisture content than mash at ~ t umoisture. d Cyanide content was lower in mashes prepress-

7 UNIT OPERATIONS AND GARI QUALITY 343 I I I I I C G Holding period (h) FIG. 3. EFFECT OF MASH MOISTURE CONTENT ON THE CYANIDE CONTENT OF MASH, CAKE (C) AND GARI (G) ed (55 % and 60%) before holding because some of the glucosides may have been expressed with water during prepressing. The same reason may be given for the lowest cyanide content of the traditional processed mash, which had the lowest moisture contents of all the mashes between 24 and 72 h holding; caused by continuous expression of moisture through pressing. Effect of Moisture Content of Mash on ph and TTA Reducing the moisture content below 65 %, the natural moisture content, increased both ph and TTA of the mash during holding (Fig. 4 and 5). ph of mash with 60 and 55% moisture increased from 6.1 in the fresh mash (0 h) to 8.4 and 8.9, respectively, at 72 h holding. TTA increased from 0.03 to 0.29 and 0.22, respectively, for 60% and 55% moisture mashes at 24 h and thereafter remained constant. In the traditional method of pressing, where moisture loss was more gradual, and in mashes of high moisture contents, ph decreased and TTA increased with longer holding time. The differences of ph and acidity patterns due to initial moistures of the mashes may be due to changes in the microflora of the mash. Bacteria, molds and yeasts have been isolated from fermenting gari cassava mashes (Collard and Levi 1959; Akinrele 1964; Okafor 1977; Ngaba and Lee 1979). It is possible that one or more of these microorganisms, under

8 344 J.O. AKINGBALA, G.B. OGUNTIMEIN and M.K. BOLADE gt L== :.i C G Holding period (h) FIG. 4. EFFECT OF MASH MOISTURE CONTENT ON ph (0) 8096, (A) 7596, (V) 6596, ). ( 60%. (0) 55%, (A) traditional sample; cake (C), gari (G) 2.0 I I I A.- U u 0.- u c u ap v x c.-.- U I I I c G Hoiding period (h) FIG. 5. EFFECT OF MASH MOISTURE CONTENT ON ACIDITY (0) 8046, (A) 7596, tm 6596, 60%, (0) 55% (A) traditional sample; cake (C), gari (G)

9 ~ ~~~~ ~ UNIT OPERATIONS AND GARI QUALITY 345 TABLE 2. MEAN ph, TTA AND CYANIDE CONTENTS OF CAKE AND GARI FROM MASH IN POROUS AND NONPOROUS CONTAINERS Moisture Residual DH TTA Cyanide Samples P Q P Q P Q P Q Cake Gari P = porous container; Q = nonporous container C G Holding period (h) FIG. 6. EFFECT OF HOLDING IN POROUS (0) AND NONPOROUS (A) CONTAINER ON TOTAL CYANIDE CONTENT OF MASH the low moisture condition of the mashes, catabolized the glucocyanide compounds in the mash, producing alkaline end products that increased the ph of the mash. Acid tolerant yeasts have been reported to increase the ph of some food media through catabolism of protein and the production of alkaline end products (Potter 1973).

10 TABLE 3. EFFECT OF SUN-HEATING AND HOLDING IN POROUS AND NONPOROUS CONTAINERS ON MASH, P * CAKE AND GARI PROPERTIES E z Parameters i? Batch Moisture % PH!l-l-A (%I HCN (mg/kg dry wt.) F P Q P Q P Q P Q F Mash Oh h C 48h h m Cake a n Gari z m k W E!2

11 UNIT OPERATIONS AND GARI QUALITY 347 Effect of Holding in Porous Versus Nonporous Containers on Cyanide Content Cyanide contents of the cakes of mashes held in porous and nonporous containers were similar. Also the cyanide content of gari from these cake samples were similar (Table 2). Differences in the cyanide content of these mashes before pressing (Fig. 6) may have been due to glucoside losses in the expressed juice of mashes stored in the porous containers. The ph and TTA of cake and gari of cassava mash held in porous and nonporous containers were also similar. CASSAVA ROOT WASHING GRATING CASSAVA MASH 1 65% M.C. ADDITION OF WATER CASSAVA MASH 80% M. C. HOLDING FOR 24h AT C FERMENTING MASH OUT IN THE SUN FOR 2 DAYS FERMENTING MASH PRESSING BREAKING AND SIEVING CASSAVA MASH GRANULES GAR IFYlNG FIG. 7. FLOW DIAGRAM FOR THE LABORATORY PRODUCTION OF VERY LOW CYANIDE GARl

12 ~ ~~ ~~ 348 J.O. AKINGBALA, G.B. OGUNTIMEIN and M.K. BOLADE Effect of Exposure of Mash to Direct Sun Heat Direct heat from the sun reduced moisture of mash in porous container from 55.2% at 24 h to 43.3% at 72 h of holding and in the nonporous container from 61.2 to 50.8% during the same period (Table 3). Similar samples in the shade had 62.1 and % for mashes held in nonporous and porous containers, respectively, at 72 h of holding. There was a greater cyanide loss of 66 ppm compared with 22 ppm in mashes stored in nonporous containers in the sun and in the shade, respectively. An increase in the temperature of the mash under direct heat from the sun may have increased the rate of chemical reaction resulting in greater loss of cyanide as volatile HCN from the mash in the sun. Preparation and Sensory Evaluation of Low Cyanide Laboratory Gari Gari was prepared as in Fig. 7 using the findings of this study. The gari has a moisture content of 6.8%, ph 4.0, TTA 0.45 and HCN concentration of 2.6 mg/kg dry weight, an overall reduction of 99.3%. Sensory quality of the low cyanide gari compares favorably with the traditional gari quality (Table 4). Color TABLE 4. MEAN SCORES OF SENSORY EVALUATION OF GARI SAMPLES VLC FLC TPS 6.44a 6.44a 3-89 b Taste 5.0a 5.44a 3.44 b Aroma 5.33a 6. OOa 4.11 b Appearance 6.00a 5.89a 1.22 b Overall Acceptability a 3.89 b VLC = FLC = TPS = Very low cyanide gari Fairly low cyanide gari (Traditional) Traditional processed gari with sundrying CONCLUSION An acceptable gari has been prepared by increasing the mash moisture content to 80% and sun heating the mash for 2 days. Though the gari had greater cyanide content than the 2 ppm permissible by the Nigerian FDA (1980), the FDA (1980) standard for gari appears attainable by this method of production, using a low cyanide cassava cultivar.

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