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
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.