Rheological characteristics of wheat-cassava composite flour and quality of the resultant bread

Abstract

Uganda is a major producer of cassava in Africa. Cassava has great potential as a raw material for agro-industry. Currently, utilization of cassava in Uganda is limited to semiprocessed products through the informal sector. The overall objective of the study was to use rheological properties in assessing the potential of wheat-cassava composite flour as an industrial raw material for bread production in Uganda, and thus enhance cassava utilisation. Consequently, the study assessed the chemical composition of High Quality Cassava Flour (HQCF) from five selected cassava cultivars (NASE 3, NASE 14, NASE 19, Nyamatia, and Nyarukeca) used in the study. Proximate composition, starch content, minerals and anti-nutritional composition were also determined using standard methods. Minerals were determined by Atomic Absorption Spectrophotometry (AAS) and cyanogenic glucoside by Picrate Paper Kit. The HQCF from NASE 14 cassava cultivar was prepared and utilised as partial wheat substitute for product formulation. Rheological characterisation of the composite flour/dough was carried out using the Mixolab, Consistograph and Alveograph. Physical and sensory evaluation of bread produced from the composite flours was performed under standard procedures. Bread quality attributes were correlated with rheological characteristics of flour/dough in order to provide knowledge about the existence of relationships among properties and define groups of rheological parameters that could characterise and discriminate between dough samples, since rheological parameters might influence the quality of the final product. Quantitative data were subjected to analysis of variance (ANOVA) using the IBM Statistical Package for Social Sciences (SPSS), version 23. Results were presented as Means ± standard deviations. Least significant difference (LSD) test was used to separate means for the different cassava cultivars. Differences between means were considered significant at p < 0.05. The Mixolab Profiler indices were systematically rounded to the nearest unit. A difference of 1 point on the Profiler was regarded not a significant difference. The relationship between proportion of cassava addition (%), dough rheology and bread physical and sensory properties was evaluated by principal component analysis (PCA) using XLSTAT (v.2.2, 2019). Moisture content of the HQCF produced from the selected cassava cultivars was determined by weight difference before and after drying of the samples in a hot-air oven based on the AOAC official methods of analysis. It ranged from 5.43 to 10.87 %, with significant (p < 0.05) differences recorded except between NASE 3 and NASE 19; ash ranged from 1.05 to 2.39 %, with significant (p < 0.05) differences between improved and local cultivars; crude fibre from 1.06 to 1.18 %, with NASE 19 and Nyamatia significantly (p < 0.05) differing from the rest; crude protein ranged from 0.74 to 1.51 % with significant (p < 0.05) differences between some cultivars; crude lipid ranged from 0.39 to 0.63 %, with significant (p < 0.05) differences between some cultivars; and starch contents ranged from 66.72 to 84.42 %, with significant (p < 0.05) differences among the improved cultivars and between the local cultivars. The mineral contents (mg/kg): calcium ranged from 13.15 to 16.56; iron ranged from 0.002 to 0.01; zinc ranged from 0.56 to 0.87; magnesium ranged from 3.58 to 3.88; and copper ranged from 0.002 to 0.14. Content of minerals differed significantly (p < 0.05) among cassava cultivars. The mineral content of HQCF was generally low implying that cassava is a poor source of minerals. The contents of anti-nutrients (mg/kg): cyanogenic glucosides ranged from 30 to 800, and were significantly (p < 0.05) higher in the local cultivars; phytates ranged from 661.33 to 984.64, and were significantly (p < 0.05) higher in improved cultivars; oxalates ranged from 90.6 to 227.8 and were significantly (p < 0.05) different in all the cassava cultivars, though the levels were generally higher in improved cultivars; and tannins ranged from 0.18 to 0.33, with significant (p < 0.05) differences among some cultivars, with levels generally higher in improved cultivars. HQCF from all the five cassava cultivars contained higher levels of cyanogenic glucosides than recommended by Ugandan and East African Standards of 10 mg/kg, making them unsafe in the primary form for direct utilization as food and food raw materials for industries. Proper methods to detoxify cassava roots have to be designed in order to utilise highcyanogenic cassava roots. The standards on HCN should be revised, since Indonesia has 40 mg/kg as her standard without any detrimental consequences on the population. Content of tannins were lower than the permissible level of 1.72 mg/kg in HQCF from all cassava cultivars in the study. The low content of essential minerals, iron, zinc and copper necessitates fortification of HQCF used as human food. Fortification of HQCF with zinc and iron is recommended if it is to be used in bread, as is already done with wheat flour as a policy in Uganda. The high content of phytates and oxalates in cassava requires some form of processing before consumption to reduce their chelating effects on minerals. The high starch content in the cassava cultivars make them valuable raw materials for starch and starch-related industries. Bread quality, expressed as overall acceptability, was positively correlated with Mixolab parameters amylase activity (r = 0.957), dough stability (r = 0.749), C4 (r = 0.941), and C5 (r = 0.945); and Consistograph parameter PrMax (r = 0.913) and WAC (r = 0.890). Crumb texture was positively correlated with Mixolab parameters DDT (r = 0.880), C3 (r= 0.819), C2 (r = 0.807), viscosity (r = 0.904), mixing (r = 0.843), retrogradation (r = 0.881); Alveograph parameter P (r = 0.803); and bread volume (r = 0.956). Results showed that bread of acceptable quality can be processed using wheat-cassava composite containing not more than 30 % HQCF. There is thus, need for further research to design processes for higher inclusion levels of HQCF in the composite for bread making. Economic value of cassava can therefore, be realised through industrial processing of cassava-based products. Based on Mixolab parameters, DDT of not less than 1 min and dough stability of not less than 9 min are the parameter levels for wheat-cassava composite flour which are likely to produce bread of acceptable quality. Alveograph parameters P of not less than 78 mm H2O and W of not less than 124 x 10-4 J; and Consistograph parameter PrMax of not less than 1675 mb are as well parameter levels for processing of high quality cassava-wheat composite bread of acceptable quality. Rheological properties of flour/dough can thus be used to assess the suitability of flour for bread making. Rheological parameters (dough stability, amylase activity, starch gelling, maximum pressure and water absorption capacity) can be used to predict specific volume, taste, and crust colour of the resultant bread. However, further research is required to assess the shelf-life of the composite bread, the suitability of flour from other cassava cultivars in composite with wheat for baking bread and the acceptability of the resultant bread by the general public/consumers. Key words: composition, anti-nutrients, cassava cultivars, composite flour, bread, wheat, sensory, physicochemical, rheology, Mixolab, Alveograph, Consistograph, correlation