Structural and functional characteristics of optimised dry-heat-moisture treated cassava flour and starch

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Abstract

The flexibility of hydrothermal conditions gives rise to an array of effects on the properties of starch-containing systems. This study examined and optimised moisture, temperature and time conditions with regards to functional properties of cassava flour and starch. The derived optimal conditions for cassava flour were 25%, 90 °C and 45 min, respectively, while cassava starch were 27%, 120 °C and 21 min, respectively. Consequently, effect of the optimal conditions on structural, physicochemical, rheological and melting characteristics of cassava flour and starch were studied. Optimal heat-moisture treatments changed both morphology and size distribution of cassava flour and starch granules. Relative crystallinity of cassava flour increased (34 to 40%) and cassava starch decreased (32% to 23%) following optimal-heat-moisture treatment. Both cassava flour and starch upon optimal dry-heat-moisture treatment exhibited enhanced; swelling power (15.57 to 21.62 g/g and 17.24 to 19.64 g/g, respectively), peak viscosity (3555 to 5493 cP and 6377 to 8701 cP, respectively), elasticity (tan δ) (0.31 to 0.24 and 0.58 to 0.47, respectively) and melting onset temperature to conclusion temperature range (14.66 to 15.75 °C and 23.26 to 30.69 °C, respectively). This research presents hydrothermal conditions that can effectively be applied in industry to improve the bakery potentials of cassava flour and starch.

Introduction

Increasing awareness of celiac disease and the high cost of wheat in the global market has led to on-going research for low-cost and sustainable substitutes to wheat. Cassava (Manihot esculenta Crantz) also referred to as ‘tapioca’, is a tuberous root shrub widely cultivated in most parts of Sub-Saharan Africa, Latin America, Asia and the Caribbean [1,2]. It is prized for its ability to thrive under harsh climatic conditions [3] and a cheap source of flour and starch.

Cassava flour and starch have exceptional quality attributes which could be exploited in bakery applications, however, their innate structures are not suited for the conditions during baking. Compared to wheat flour, cassava flour has a comparable starch composition, water absorption, melting onset and lower retrogradation tendency [4]. However, cassava flour possesses an inferior protein composition (lacks gluten and deficient in sulphur containing amino acids; methionine and cysteine) and has a low diastatic activity which hampers its bakery performance [5,6]. Cassava starch, on the other hand, has a bland flavour and excellent thickening and gelling attributes [7,8]. However, native cassava starch exhibits a weak structure that is incapable of effectively trapping gas during baking consequently producing products with structural defects [9].

Hydrothermal modification of gluten-free flours and starches has been reported to improve their physicochemical and bakery potentials [8,10,11]. In this study, cassava flour and starch were hydrothermally modified using heat-moisture treatment. This hydrothermal method is typically used to control the molecular mobility of starch functional groups and their inter- and intra-chain interactions [[12], [13], [14], [15], [16], [17], [18]]. These molecular alterations consequently influence the physicochemical, structural, rheological and thermal attributes of the modified starch-containing system as demonstrated by the above-mentioned authors.

Typically, thermal applications employed in heat-moisture treatment of starches are mainly from dry-heat sources derived from convection ovens [15,16] and microwave systems [19], and steam-heat derived from autoclave systems [20,21]. Based on existing knowledge, there are no peculiar distinguishing characteristics between dry-heat- and steam-heat-moisture treated starch-containing systems. Thus, regardless of heat-source, intensities of the thermal conditions confer different changes in flour or starch systems and this vary with specie and or botanical source of the flour or starch. However, dry-heat from convection oven systems provides a cheaper source of heat as opposed to the moisture and pressure requirements of steam-heat systems.

There are few studies that have presented optimisation variables for heat-moisture cassava flour and starch suitable for bakery application. Recently, a study comprehensively examined the impact of steam-heat-moisture treatment conditions on structural and functional properties of cassava flour and starch. The study also presented optimised steam-heat-moisture treatment conditions that were effective in altering the structural, physicochemical, rheological and melting characteristics of cassava flour and starch [21].

In line with the above-mentioned study, this present study employed response surface methodology to investigate and optimise dry-heat-moisture treatment conditions (i.e. moisture temperature and time) in relation to structural and functional characteristics of cassava flour and starch. The rationale for this optimisation study is to establish optimal-dry-heat-moisture treatment conditions for cassava flour and starch that would enhance their bakery functionality.

Section snippets

Materials

Commercial cassava flour was obtained from Zhongshan Hongyi Industrial Materials Co. Limited, China. Commercial cassava starch was procured from Guangxi Hongfeng Starch Co. Limited, China. All chemical reagents utilised in the experimental study were of analytical grade.

Chemical composition

Native cassava flour and starch were analysed for moisture (AOAC, 2005, 925.10), protein (AOAC, 1995, 960.52), lipids (AOAC, 2005, 2003.05), ash (AOAC, 2005, 923.03) and fibre (AOAC, 1990, 962.09) [[22], [23], [24]]. Total

Chemical composition of native cassava flour and starch

Chemical compositional data showed that total carbohydrate was the major component in cassava flour and starch, which was very similar (approx. 85%) (Table 1). Cassava flour had low levels of protein (1.55%), lipids (0.84%), crude fibre (1.73%), and 1.64% ash (Table 1). Similar proximate compositions have been reported on cassava flours [28,29]. Cassava starch showed low levels of ash (0.66%) and fibre (0.43%) and contained no lipid and protein suggesting the starch is relatively pure. Cassava

Conclusion

Optimal variables for the dry-heat-moisture treatment for cassava flour and starch have been derived and structural and functional properties of the outputs have been presented in this study. The optimal dry-heat-moisture treatments altered the granule morphology and size distribution of cassava flour and starch. The A-type crystal patterns of cassava flour and starch were resistant to heat-moisture treatment, however, the relative crystallinity of cassava flour increased and cassava starch

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