Modification of the paste properties of maize and teff starches using stearic acid

Abstract

Starch is used in many food applications as thickeners, texturisers and fat substitutes. Native starches, although useful, have low stability to conditions such as high shear, extreme pH and high temperatures encountered during food processing. Starches are modified to make them more suitable for processing conditions. The modification of starch by the use of a naturally occurring compound (for example stearic acid) may produce desirable properties and also removes the risk of a chemical residue in the starch. Starch can be from several grain sources. Teff grain is highly underutilized and underresearched. The work conducted in this project investigates the pasting properties, gelling tendencies, clarity and flow properties (using a rheometer) of teff starch pastes treated with stearic acid, in comparison to maize starch pastes. X-Ray Diffraction (XRD) and Confocal Laser Scanning Microscopy (CLSM) were also used to investigate the possible impact of stearic acid on the structure of the starch granules and pastes. Starch suspensions containing stearic acid (0.25% - 4%) were pasted in a Rapid Visco Analyser using a short pasting cycle of approx. 30 min (held for 5 min at 91°C). Maize starch (treatments) showed a reduced peak viscosity within the holding period, while teff starch (treatments) did not. Teff starch showed increasing viscosity without reaching a peak during the holding time. The pasting cycles were then extended (holding time extended to 2 hr) to investigate the pasting behaviour of teff starch. The extended pasting cycle resulted in a reduced first viscosity peak for maize starch with added stearic acid. Teff starch with added stearic acid showed a large increase in viscosity without the formation of the first viscosity peak. However, both starches displayed a second pasting peak. The addition of stearic acid resulted in an increase in the viscosity of the second pasting peak from about 175 Rapid Visco Units (RVU) to 228 RVU for maize starch, and from 113 RVU to 250 RVU for teff starch. The final viscosity of maize starch increased from 186 RVU to 227 RVU, while that of teff starch increased from 194 RVU to 261 RVU. The second viscosity peak was not observed with waxy maize starch (approx. 97% amylopectin). This suggests that amylose-stearic acid complexation might have been responsible for the formation of this peak. Complexation Index (CI) values increased as the concentration of stearic acid was increased. This further suggests that some interaction between amylose and stearic acid had taken place. The pastes of maize and teff starches modified with stearic acid were more opaque and showed reduced gelling compared to their non-modified counterparts. Maize and teff starches and their stearic acid-treated counterparts followed the Power-Law Model and were shear thinning (n < 1). However, teff starch pastes (control and treatment) seemed to be less shear thinning than their maize starch paste counterparts. An increase in consistency, k, after the extended pasting cycle was used (compared to the short pasting cycle) for the treated starches, reflects the increased viscosities obtained during extended pasting. XRD further suggested that amylose-lipid complexes may have been present in the starch pastes (after extended pasting) due to the occurrence of the 4.4 Å and 12 Å peaks (characteristic of V-type starches). CLSM showed that stearic acid diffused into maize starch granules but not into teff starch granules. This was probably due to the pores of the surfaces of maize starch granules which may have facilitated the diffusion process. In contrast, teff starch granules do not have pores on their surfaces. This structural difference may be attributed for the pasting differences between teff and maize starches. The effects of stearic acid on the pasting (effect on first and second peaks and final viscosity), and functional properties (reduced gelling and increased opacity of pastes) of maize and teff starches have been attributed to the formation of amylose-lipid complexes. These high viscosity and low gelling starches may be extremely useful as fat replacers. Teff starch has the added benefit of its small starch granules which may add to its ability mimic the mouthfeel of fat globules.