Characterisation of nano-scale amylose-lipid complexes isolated from teff and maize starches during pasting, and their application as a fat replacer in margarine

Abstract

The amylose component of starch can form complexes known as V-amylose with amphiphilic or hydrophobic ligands such as fatty acids. The main objective of the present research was to isolate and characterize amylose-lipid complex nanomaterials from tef and maize starch through biphasic pasting (two peak paste viscosities at short and prolonged pasting times of 11.5 and 130 min, respectively), and assessing their potential as fat replacers in margarine. First, occurrence of amylose-lipid complexes in tef and maize starch biphasic pastes (two pastes at short and prolonged pasting times of 11.5 and 130 min) was assessed. Maize and tef starches were pasted with or without added stearic acid followed by thermo-stable alpha-amylase hydrolysis in a rapid visco-analyser (RVA). Wide angle X-ray scattering (WAXS) before and after hydrolysis showed crystalline V-amylose diffraction patterns for the starch pasted for a prolonged time with added stearic acid while less-distinct V-amylose patterns with non-complexed stearic acid peaks were observed with a short pasting time. Differential scanning calorimetery (DSC) before and after paste hydrolysis showed that Type I amylose-lipid complexes are present in the paste formed at the short wet-heat processing times, while Type II complexes are present in the second paste formed after wet-heat processing for a prolonged time. In order to isolate amylose-lipid complex nanostructures in the second biphasic peak viscosity paste, the maize and tef starches were wet-heat treated for 130 min with added stearic acid. The paste was then hydrolysed using thermo-stable alpha-amylase in an RVA for 0, 5 and 10 min of hydrolysis and the resultant starch residues were isolated before and after acetate buffer treatment (0.05 M, pH 3.5, 95 ºC, 20 min). XRD (X-ray diffraction) patterns showed that the isolated residues consisted of V6I and V6II-amylose-lipid complexes for the residues isolated before and after acetate buffer treatment, respectively. DSC results showed the tef and maize starch residues isolated both before and after acetate buffer treatment to consist of Type II V-amylose-lipid complexes. Dynamic laser scattering particle size distribution (DLPSD), atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM) showed that the isolated tef and maize starch residues consisted of non-distinct materials which showed more physically separated (distinct) nanomaterial with increased hydrolysis. The distinct nanomaterial appeared at 5 and 10 min of hydrolysis for the samples isolated after and before acetate buffer treatment respectively. The tef and maize starch nanomaterial with distinct particles that was isolated before and after acetate buffer treatment had yields of 24-30% w/w (based on initial starch weight) for both tef and maize starch based on the total starch initially pasted. The isolated tef and maize distinct nanomaterial obtained before acetate buffer treatment had distinct particles of about 3-10 nm and 2.4-6.7 nm in diameter, respectively. The distinct nanomaterial isolated after acetate buffer treatment had diameters of about 6.1-94 nm and 6.2-64.7 nm for tef and maize, respectively. The effect of fat replacement using aqueous dispersions of isolated tef and maize starch nanomaterial on the textural, viscoelastic and microscopic properties of the resultant low-calorie spreads was examined. Textural, viscoelastic, and microscopic assessments indicated that the spreads fat-replaced with the tef or maize starch nanomaterial were similar (p > 0.05) to commercial full fat or low fat spreads at 25%w/w fat replacement. Beyond 25%w/w (based on total fat in full fat margarine) fat replacement, the properties of the resultant fat-replaced spreads significantly (p > 0.05) deviated from those of the commercial samples. Modifications in the preparation protocols and ingredients (especially emulsifier types and concentrations) may be required for fat replacement beyond 25% w/w (based on total fat in full fat margarine) without significant deviation in properties from the commercial spreads. The application of the distinct nanomaterial that have been isolated from the second biphasic peak viscosity pastes in the present research, as fat replacers, may assist in the fight against life-style diseases that associated with high fat diets such as obesity, diabetes, and cardiovascular disease. Further research on the nanomaterial isolated from the second biphasic pastes could involve: up-scaling of the isolation process, chemical modification of the nanomaterial through polymer grafting, application of the nanomaterial for improved properties in food packaging material and optimization of fat replacement in margarine or other high-fat products. Restricted until April 2018