Rheology and microscopy of low calorie mayonnaise-type emulsions made with stearic acid- modified maize and teff starches

Abstract

Excessive consumption of diet rich in fat has been identified as a risk factor for lifestyle diseases. Fat replacers which can imitate the functional properties of fats with lower or no calorie contribution have been developed to produce low-calorie foods. Starch (mostly modified) can be used as fat replacer in low fat foods. Teff (a cereal indigenous to Africa, which is underutilized and underresearched) and maize starches modified with stearic acid (a naturally occurring fatty acid), has resulted in a paste with reduced gelling ability and higher viscosity in previous research. This suggests that these modified starches may have potential to be used as fat replacers. This project investigates the effect of teff and maize starch pastes modified with stearic acid as fat replacer on the rheological properties (flow properties and viscoelastic behaviour), microstructure (using light and confocal laser scanning microscopy), and freeze-thaw and high temperature stability of low-calorie mayonnaise type emulsions (LCMTE). Starch suspensions (10% w/v of starch in water) containing stearic acid (1.5%) were pasted for an extended holding time of 2 hr at 91 °C in a Rapid Visco Analyser (RVA) and used to prepare LCMTE with the oil replaced at 50%, 80% and 100% in comparison to a full fat mayonnaise. All samples exhibited a shear thinning behaviour and yield stress in terms of flow properties. Modification of teff and maize starches with stearic acid decreased the yield stress and viscosity of the LCMTE compared to unmodified starches. LCMTE with maize starch had higher yield stress and viscosity compared to LCMTE with teff starch. The yield stress value decreased on increasing the level of oil replacement from 50% to 80% and then increased when the oil replacement was 100% except for LCMTE with modified teff starch. The yield stress value for LCMTE with modified teff starch decreased with increasing level of oil replacement. Storage for 8 days was found to increase the yield stress and viscosity except for LCMTE with modified teff starch. Viscoelastic properties of mayonnaise were characterized using dynamic oscillatory shear test and it was observed that all samples behaved like solid with storage modulus (G') greater than loss modulus (G") and tanδ < 1. Generally LCMTE with modified starches had lower G', G" and tanδ than LCMTE with unmodified starches, suggesting less gelling behaviour of teff and maize starch pastes modified with stearic acid. The optical and laser scanning microscopy images showed all samples to consist of oil droplets dispersed in aqueous phase except the LCMTE at 100% oil replacement level. LCMTE had oil droplets of smaller size (1-10 μm) compared to the full fat mayonnaise (5-10 μm). LCMTE with modified starches had larger size oil droplets compared to LCMTE with unmodified starches. LCMTE with maize starches seemed to have smaller oil droplets compared to LCMTE with teff starches especially with the modified starches with stearic acid. The smaller size oil droplets of LCMTE with unmodified starches can be related to the higher yield stress and viscosity values compared to the LCMTE with modified starches which have larger size oil droplets. All the LCMTE with modified and unmodified starches were found to be more stable to freezethaw cycles and high temperature storage compared to full fat mayonnaise. Generally, LCMTE could be produced with modified and unmodified teff and maize starches, the properties being dependent on the level of oil reduction. At 50% oil replacement modified and unmodified teff and maize starch with stearic acid could produce LCMTE. When the oil content was further decreased to 80% and 100% only the LCMTE with modified starches were similar to the full fat. The LCMTE with unmodified starches were found to have non-pourable gel-like behaviour with higher yield stress and viscosity values than the full fat mayonnaise. Teff and maize starches modified with stearic acid (1.5%) could be used as fat replacers in mayonnaise replacing up to 100% oil and this can substantially decrease the calorific value of mayonnaises. The reduction in the calorie may contribute to weight loss, and prevention of cardiovascular diseases. The starch pastes modified with stearic acid may also have a potential to be used as emulsifiers.