Effect of Fermentation, Blanching, and Drying Temperature on the Fuctional and Chemical Properties of Cassava Flour

Abstract

Cassava (Manihot esculenta crantz) was processed into flour by fermentation, blanching, and drying. Portions of the tuber were naturally fermented for 2, 3, and 4 days, and each was oven dried at temperatures of 60, 80, and 90°C and then milled to obtain naturally fermented cassava flours. Other portions were blanched for 3, 6, 9, and 12 min, and each similarly dried at 60, 70, 80, and 90°C and milled to produce blanched cassava flour. The last portion, which formed the control, was simply sun-dried. The bulk density, gelation capacity, water absorption, swelling index, and viscosity were investigated. Blanching was found to improve the functional properties of the flour. Residual cyanide was more commonly found in blanched samples than in both the fermented and control groups. Increasing the fermentation period was found to reduce both viscosity and residual cyanide, whereas increasing the drying temperature led to more residual cyanide in the fermentation and blanched samples. Fermented samples had lower pH (higher total titrable acidity) than both control and blanched samples. The blanched samples had lower moisture content than control and fermented samples.

Introduction

Cassava is a staple food in the developing world.[1] It is the most productive root in the tropics in terms of yield per dry matter per acre.[2] Cassava is processed in many forms[3] aimed primarily to convert the highly perishable food crop into dry, storable form as well as to reduce its toxicity.[4] Some of the processing methods include crushing, grating, soaking or fermentation, drying, and cooking. IITA[5] reported that cassava flour could be used in biscuit- and bread-baking either alone or in a composite flour.[6] The absence of fibre in cassava starch accounts for its good baking quality. Cassava starch also has good gelling property.

The food uses of cassava flour are limited by its low protein content, low energy, and its potential toxic effect from the naturally occurring cyanide-yielding compounds, Linamarin and Lotaustraline.[7] Rosling[8] suggested that the low protein and energy derived from cassava can be improved by enriching it with protein and energy rich food. Its toxic substance, according to Rosling, can also be removed by processing. Published work on the effect of processing methods on the functional properties of the flour is very scanty. Since cassava flour can be used in many food formulations, its functional and chemical properties as affected by the processing methods require investigation. The aim of this study is, therefore, to evaluate the effect of fermentation, blanching, and drying temperature on the functional and chemical properties of cassava flour.

Matherials and Methods

Materials

Freshly harvested IITA variety of TMS 4(2) (1425) cassava roots (12 months old) obtained from the National Root Crop Research Institute (NRCRI), Umudike Abia State Nigeria, were used.

Preparation of Samples

The cassava tubers were peeled and sliced into 10 mm thickness. They were washed in running tap water and divided into three portions. One portion was fermented; a second was blanched, while the last was the control.

Fermentation was achieved by soaking three portions of the sliced cassava tuber for 2, 3, and 4 days, respectively. The water was changed every 6 h. The samples were washed after fermentation with clean water. Blanching was done by heating the samples in boiling water for 3, 6, 9, and 12 min, respectively. Both the blanched and the fermented samples were dried in an electric oven maintained at 60, 70, 80, and 90°C for 10 h. The dried chips were milled in an attrition mill (Thomas Willey Lab. Mill, Model EDS). The flour produced from each sample was sieved (1 mm pore size) and packaged in polythene bags before analysis. The control sample was sun-dried, milled, and packaged as before.

Analysis

The moisture content and pH were analysed using AOAC[9] and Nout, Rombouts, and Houst[10] methods, respectively. The bulk density was determined using the Okaka and Potter[11] method while the swelling index (SI) and water absorption capacity were measured following Eniola and Delarosa[12] and Beuchet[13] procedures, respectively. The Coffman and Garcia[14] method was adopted for a determination of gelation capacity, and viscosity was evaluated using the Mosha and Svaberg[15] method. The method of Cooke[16] was adopted for hydrogen cyanide (HCN) analysis.

Statistical Design and Analysis

A completely randomized design (CRD) was used in the experiment while analysis of variance (ANOVA) and Duncan new multiple range tests were used for mean separation.[17]

Results and Discussions

Effect of Fermentation and Blanching on the Functional Properties

Table 1 presents the results. The bulk density value ranged 450–870 kg/m³ within the processing conditions considered. The control sample had bulk density of 440 kg/m³ that was below the processed samples. Generally, the fermented sample had lower bulk density than the blanched ones. The bulk density of the fermented samples did not significantly differ (p < 0.05) with the control. They, however, were significantly different from the blanched samples. Blanching, therefore, affected the bulk density of the flours while fermentation did not. The bulk density did not significantly (p < 0.05) vary with fermentation and blanching times. Ekwu and Ugwu[18] reported similar values (510–750 kg/m³) for blanched and dried cassava flour and 430–550 kg/m³ for unblanched samples. This shows that blanching affected the bulk density of the cassava flour. Balagopalan et al.[19] reported that blanching confers harder consistency to chips due to the gelatinization of starch. This, ultimately, toughened the chips leading to the production of granulated materials, which had higher bulk density than unblanched flours.

Table 1 Effect of fermentation and blanching on functional properties of cassava flour

Fermentation blanching
Functional properties	2	3	4	3	6	9	12	Control	LSR
Bulk density kg/m^3	450^b	450^b	530^b	830^a	810^a	983^a	870^a	440^b	122
Gelation capacity (%)	22^ab	22^ab	24^a	22^ab	24^a	20^b	22^ab	22^ab	2.642
Water absorption Capacity (%)	1.51^b	1.49^b	3.5^a	2.55^a	2.55^a	2.55^a	3.53^a	1.50^b	1.01
Swelling index at 50°C (%)	1.33^d	1.22^e	1.20^w	3.5^a	3.93^b	4.43^a	3.68^c	1.12^f	0.03
Viscosity Nsm^−2	2.7081^a	2.068^b	2.041^c	2.864^d	2.872^e	2.883^f	2.896^g	2.773^h	0.003

*Values in the same row with different superscript are significantly different (p < 0.05).

The results of the gelation capacity indicate that all the flour samples gelled at relatively low flour concentration (20–24%). There was no significant difference (p < 0.05) between the control, the sample fermented for 2 or 3 days, and those samples blanched for, 3, 9, and 12 min. The samples fermented for 4 days, and the sample blanched for 6 min did not differ statistically (p < 0.05). They were, however, different from others. From these results, it may be concluded that fermentation and blanching did not affect significantly the gelation capacity of the flours.

The sample that had blanched for 12 min had the highest water absorption capacity of 3.53% while the value of 1.5% for the control sample was the lowest. The water absorption capacity of the fermented samples ranged from 1.49 to 1.51%, which was lower than 2.05–3.53% obtained for blanched flours. The fermentation did not affect the water absorption capacity of the cassava flours but blanching did (p < 0.05). There was no significant difference (p < 0.05) between the water absorption capacity of the fermented samples and the control (Table 1). It has been reported that blanching cassava chips and drying them between temperature range of 40 and 70°C resulted in higher water absorption capacity values than unblanched one.[18] This agrees with the results obtained here. Beuchet[13] also observed that processing techniques influenced the water absorption capacity of flours.

The swelling index (SI) of the blanched samples was generally higher (3.68–4.43%) than the fermented ones (1.12–1.33%). The swelling index (SI) of fermented samples differed significantly (p < 0.05) from the blanched samples. The viscosity (2.864–2.896 Nsm⁻²) of blanched samples were higher than the fermented samples which ranged from 2.041 to 2.081 Nsm⁻². The processing techniques, the blanching time, and fermentation period, significantly (p < 0.05) affected the viscosity of the samples. From the results, however, fermentation affected the viscosity of the cassava flours more than blanching. The lower viscosity values observed in the fermented samples may be due to the breakdown of the macromolecules such as polysaccharides and polypeptides by enzymes mobilized during the fermentation process.[15] From the result, it may be inferred that there is a direct relationship between the bulk density and the viscosity. This is because the blanched sample, which had higher bulk density, also produced higher viscosity than the fermented samples.

Effect of Fermentation and Blanching on the Chemical Properties of the Flour

The results presented in Table 2 show that fermentation reduced the pH value of the samples. The pH of the blanched samples and the control were generally higher than the fermented samples. The pH and total titrable acidity (TTA) were inversely related. The higher the pH, the lower the TTA, irrespective of the processing methods used. Table 2 shows that the longer the fermentation period, the more acidic the samples become. Westby and Twiddy[20] reported that fermentation generally lowers pH. Rose[3] observed that during cassava fermentation, some organic acids, such as lactic and acetic acids, were produced. This did not only contribute to the characteristic flavour of fermented cassava products but lowered the pH. The pH of the blanched samples did not significantly differ from each other. They were statistically the same as the control sample (p < 0.05). Blanching did not, therefore, affect the pH of the cassava samples.

Table 2 Effect of fermentation and blanching on chemical properties of cassava flour*

Fermentation (days)			Blanching (min)
Chemical properties	2	3	4	3	6	9	12	Control	LSR
pH	4.79^a	4.38	4.16^a	7.25^b	7.32^b	7.23^b	7.20^b	6.80^b	1.653
Total titrable acidity (%)	0.18^c	0.24^a	0.22^b	0.04^d	0.03^d	0.04^d	0.02^c	ND	0.017
Moisture content (%)	8.856^a	7.55^b	4.15^d	3.78^c	4.53^c	4.53^c	4.62^c	ND	0.149
Hydrogen cynide (mg/kg)	6.82b^c	2.63^c	1.87^c	14.42^b	13.64^b	13.13^b	11.98b^c	27.35^a	10.153

ND = Not determined.

Values are means of triplicate.

*Values with the same superscript in the same row are statistically the same (p < 0.05).

The moisture content of the fermentation samples was generally higher than the blanched ones. The values ranged from 4.15 to 8.05% and 3.75 to 4.46% for fermentation and blanched samples, respectively. The moisture content of the samples is within the allowable rage for effective storage of flour. The hydrogen cyanide (HCN) content of the blanched samples was generally higher than the fermented samples. It was also observed that the longer the fermentation days or the blanching time, the lower the HCN value in the samples.

The HCN reduction was 71–93% during the 2–4 days of fermentation while blanching gave a reduction of 46–56%. Fish and Trim[21] reported 50% reduction when they blanched cassava chips in boiled water for 5–10 min and sun dried. They attributed low reduction of HCN to the blanching process which inactivates linamarase. Increasing the blanching time resulted in more reduction of HCN. Nambisan and Sundaresan[22] made a similar observation. The smaller value of cyanide content of the fermented samples may be due to the leaching of the released HCN into the soak water. Ekwu and Ugwu[18] also reported that soaking cassava chips achieved higher reduction of HCN.

Effect of Drying Temperature on the HCN value of the Fermented and Blanched Cassava Flour

Table 3 shows that fermented samples had lower HCN values than the blanched ones, irrespective of the drying temperatures. As the fermentation period and blanching time were increased, the HCN of the samples decreased at each drying temperature. It was further observed that increasing the drying temperature increased the HCN value in the samples, irrespective of the processing methods.

Table 3 Effect of drying temperature on the hydrogen cyanide value of the fermented and blanched samples

Hydrogen Cyanide Value mg/kg
Fermentation (days)			Blanching (min)
Drying temp. (°C)	2	3	4	3	6	9	12	Control	LSR
60	6.82^bc	2.63^c	1.87^c	14.42^b	13.64^b	13.13^h	11.96^bc	27.16^a	0.153
70	6.94^a	2.73^b	1.94^c	14.12^d	13.80^c	13.56^f	12.42^g	27.16^h	0.044
80	6.98^a	2.81^b	1.97^c	14.71^d	14.20^c	13.90^f	12.68^g	27.16^h	0.023
90	7.05^a	2.84^h	2.04^c	15.16^d	14.30	14.09^g	27.16^h	27.16^h	0.019

Values in the same row with the same letter are significantly the same (p < 0.05).

Fish and Trim[21] compared HCN reduction in sun-dried and oven-dried cassava chips and observed that high temperature drying resulted in low glycoside removal. This had been attributed to the combination of rapid depletion of moisture resulting in reduction of enzyme activity and inactivation of linamarase, which breaks down linamarin and release HCN, by high temperature drying.[21] [23]

Conclusion

Blanching improved the functional properties of the flour. Residual cyanide was found more in blanched samples. The viscosity and cyanide content were reduced by increasing the fermentation time. The fermented samples had lower pH values than the control and blanched samples.