Cassava root (Manihot Esculenta Crantz) characterisation and evaluation of process-induced changes on functional of its flour

Abstract

Cassava (Manihot esculenta Crantz) is the world’s most important root crop, highly perishable, mostly grown and consumed in the tropics and subtropics of Asia, South America and Africa. Despite the popularity and utilisation of cassava in tropical and sub-tropical regions of Africa, it is not well known in South Africa. In this study, morphological, physicochemical, structural, elemental, and metabolic characterisation was conducted on two South African cassava landraces red (RCLR) and white (WCLR) highlighting their potential end-use properties. Response surface methodology (RSM) was employed in determining the linear, interactive and quadratic effect of varying concentrations of pre-treatment (0.6 – 3.4%w/v) and drying temperature (45 – 74⁰C) on thermal, functional and physicochemical properties of cassava flour (CF) from both landraces. Calcium chloride and citric acid were separately applied in pre-treatment of the flours. Four experimental groups: CF from red landrace pre-treated with citric acid (CAR); CF from red landrace pre-treated with calcium chloride (CCR); CF from white landrace pre-treated with citric acid (CAW); and CF from white landrace pre-treated with calcium chloride (CCW) were comparatively evaluated. In each experimental group, the experimental samples were compared with control samples by One-way ANOVA and separation of means using Duncan Multiple Range Test in SPSS statistics software Version 25 (IBM Corp., New York, USA). Experimental design, analysis, response plots, one-way analysis of variance (ANOVA) of model parameters and process optimisation was done with Stat-Ease design expert software (Version 12). Visual assessment of the root parenchyma showed no distinct features as both landraces appeared cream in colour. However, colorimetric analysis revealed that L* (87.42), a* (0.06), b* (14.17), whiteness (89.45) and brownness index (4.15) of WCLR were significantly different (p < 0.05) from the L* (83.30), a* (0.67), b* (16.07), whiteness (86.50) and brownness index (5.17) of RCLR. Peel thickness of the RCLR (2.27 mm) was significantly higher (p < 0.05) than that of WCLR (1.85 mm) while the percentage flour yield of RCLR (36.15) was significantly lower than WCLR (37.03). Flour from the roots showed significant variance (p < 0.05) in a*, b* and brownness index but the lightness and whiteness index were not significantly different (p > 0.05). Cyanide content (RCLR – 3.62 mg/kg; WCLR – 3.51 mg/kg) of the root was not significantly (p > 0.05) different, but the flour (RCLR – 2.92 mg/kg; WCLR – 1.83 mg/kg) was significantly (p < 0.05) different. Cyanide content of the root and flour were below WHO recommended safe consumption level of 10.00 mg/kg. Scanning electron micrographs of both flours showed spherical and truncated starch granules clustered and dispersed in no regular pattern. Pattern and main peaks (2θ = 43⁰, 23⁰, 17⁰ and 15⁰) of X-ray diffractometry spectra of the flours were the same, exhibiting A-type starch crystallinity. Metabolic profiling, with the aid of gas chromatography-mass ii spectrometry, revealed that phenolic acids identified were higher in WCLR than RCLR. A reverse trend was observed with identified FAMEs in the landraces. Of all sugars identified, sucrose had the highest concentration in both landraces. X-ray fluorescence spectrometry of CF revealed that both landraces contained potassium (RCLR - 26.10 mg g−1; WCLR – 30.30 mg g−1), magnesium (RCLR – 23.40 mg g−1; WCLR – 16.80 mg g−1), calcium (RCLR – 11.50 mg g−1; WCLR – 5.60 mg g−1), aluminium (RCLR – 1.50 mg g−1; WCLR – 1.50 mg g−1), phosphorus (RCLR – 0.80 mg g−1; WCLR – 1.50 mg g−1), iron (RCLR – 0.50 mg g−1; WCLR – 0.50 mg g−1), chromium (RCLR – 0.20 mg g−1; WCLR – 0.20 mg g−1), and titanium (RCLR – 0.20 mg g−1; WCLR – 0.20 mg g−1). Differential scanning calorimetry showed that pre-treatment had an increasing effect on the gelatinisation temperatures and enthalpy of CF. Citric acid treatment had a decreasing effect on water holding capacity of CF when compared to calcium chloride. Loose bulk density (LBD) ranged between 0.34 – 0.41 g/cm3 (CAR), 0.37 - 0.45 g/cm3 (CCR), 0.35 – 0.43 g/cm3 (CAW) and 0.37 – 0.44 g/cm3 for CCW respectively. The LBD increased with an increase in DT. Packed bulk density (PBD) of CF treated with calcium chloride and citric acid were similar with the least and highest PBD of 0.62 and 0.73 g/mL respectively. An increase in drying temperature and concentration increased ash content. Calcium chloride and citric acid pre-treatments improved the lightness and whiteness index of CF. The L* values of CF were between 91.37 and 93.65 with the control (not pre-treated) samples significantly lower (p < 0.05) than the experimental samples in all four groups. The study reveals that the pre-treatments have a mitigating effect against enzymatic browning associated with cassava root processing. An increase in thermal properties indicates that the processing conditions confer on CF more stability in the presence of heat and water. Chemical characterisation shows that both landraces are of the sweet type, with low cyanide content which makes them safe for human consumption. The flours contain minerals that are useful for proper body function and metabolism. The A-type starch crystallinity of flour exhibited, positions the flours as a suitable wheat replacement in flour-based food applications. Therefore, the roots of these landraces can be processed into minimally processed foods such as chips. The flours can be utilised in food applications such as baked products, gels and stabilisers.