Stearic acid addition to tef and maize starches and its influence on their functional properties

Abstract

Starch is applied in food products as a thickener, gelling agent and fat replacer. The modification of native starch using a naturally derived chemical like stearic acid could bring about improved starch properties, for example reduced retrogradation and stronger gels, without the risk associated with the use of synthetic chemicals. Tef is considered to be underutilized and under-researched. This project investigated the microstructure of tef and maize starch pastes as affected by stearic acid addition using light microscopy, scanning electron microscopy (SEM) and X-ray diffraction. The thermal properties of tef and maize starches were also investigated with Differential Scanning Calorimetry (DSC). Starch suspensions (10% w/v) containing stearic acid (0.25% and 1.50%) were subjected pasting with an extended holding period of 2 or 4 hours at 91 °C in a Rapid Visco Analyser (RVA). As expected, a large second viscosity peak was observed for both tef and maize starches modified with stearic acid. This second pasting peak increased in size with increased concentration of stearic acid. No second viscosity peak was observed with tef control and only a small second peak was observed with maize control. This suggests that the formation of amylose-stearic acid complexes was responsible for the second peak formation. Additionally, tef starch modified with 0.25% stearic acid did not reach a first peak viscosity and increased in viscosity throughout holding, while for 1.50% stearic acid there was decreased peak viscosity and increased time to peak viscosity. At predetermined points during pasting of tef and maize starches (controls and modified with stearic acid), starch paste samples were taken and immediately plunge frozen in liquid nitrogen (-180°C). The freshly thawed samples were analysed using light microscopy. For SEM, X-ray diffraction and DSC analysis, frozen samples were freeze dried. Light microcopy and SEM showed that both tef and maize starch pastes contained large proportions of intact granules up to second peak formation. This was suggested to be due to the interaction of stearic acid with the starch granules. A few crystalline structures (1-10μm) were observed for maize starch modified with 1.50% stearic acid and sampled at the second peak. However, there was not sufficient microscopical evidence suggesting the presence of micron sized crystalline amylose-stearic acid complexes. X-Ray diffraction supported the presence of amylose-stearic acid complexes because of the formation of 0.44 nm, 0.68 nm and 1.2 nm peaks These were observed for both tef and maize starch pastes sampled at the first viscosity peak, the second viscosity peak and at the end of pasting. The size of the X-ray diffraction peaks, however, was largest for tef and maize starch (modified with 1.50% stearic acid) pastes sampled at the second pasting peak. DSC showed endothermic changes in heat flow for tef and maize starches modified with 1.50% stearic acid and sampled at the end of pasting at temperatures of 99 – 120 °C, which corresponds to the melting transitions of amylose-lipid complexes. Both the X-ray Diffraction and DSC data support the hypothesis that amylose-stearic acid complexes have a role to play in the second pasting peak formation of tef and maize starches. While microscopy has not provided evidence for the existence of such complexes, their presence possibly as nano structures cannot be disregarded and should be investigated in future by use of more sensitive techniques such as Atomic Force Microscopy (AFM) and Small Angle X-ray Scattering (SAXS). The effects of stearic acid on the microstructures and thermal properties of tef and maize starch pastes are attributed to the formation of amylose-stearic acid complexes. These fatty acid modified starches may be useful as fat replacers due to their high viscosities and low tendency to retrograde.