Rheological and microstructural characterisation of lotus seed milks and their glucono-δ-lactone induced acid-set milk gels: 1. Effect of protein content

Abstract

The physicochemical and acid gelation properties of lotus seed milks (LSMs), total solid concentrations varying from 5 to 20 wt%, were investigated in comparison with bovine skim milk (10 wt%). LSM was prepared by soaking, blending and milling, filtering followed by α-amylose treatment to hydrolyse the starch in order to reduce its viscosity. The particle size of LSM revealed a multimodal distribution with two main size distributions, a smaller one (∼0.06 and ∼1.5 μm) from protein aggregates as a result of the heat treatment and a larger one (∼1.5 μm and ∼250 μm) likely due to the presence of undissolved protein and cellular materials. A slight shift toward higher sizes, due to further protein denaturation, can be observed after heating at 80 °C for 30 min. As expected, the viscosity of LSM depended on the total solid concentration, and was found to be similar to that of the skim milk when the LSM concentration was around 5 wt%. Addition of glucono-δ-lactone resulted in the formation of LSM gels, and the gelation of LSM occurred when the pH decreased to ∼6 while that of skim milk occurred at pH 5.2. Small deformation rheological measurements showed that it is possible to obtain a LSM acid-gels similar in viscoelastic behaviour (complex modulus G*≈300 Pa after 5 h acidification) to 10 wt% skim milk gel when the concentration of LSM is about 11.1 wt% (same dietary energy as skim milk). Confocal laser scanning microscopy showed that a similar microstructure to skim milk gel is observed for 5 wt% LSM gel, while a denser protein network with voids were observed at high LSM concentrations. The colour of these milk gels appeared white under the naked eye and their syneresis decreased sharply with the increase in concentration with a syneresis similar to that of skim milk when LSM concentration was ≥15 wt%. This study suggested for the first time the potential of LSM to be a strong alternative for the manufacture of non-animal acid milk gels.