Effect of heat treatment and extraction pH on macroscopic and molecular properties of dromedary milk gel induced by Urtica dioica extracts

This study investigates the use of the nettle leaves (Urtica dioica) extract as a novel source of a protease for clotting dromedary milk. The aim of this work is to study the impact of heat treatment (60 and 70 °C during 30 min) applied to dromedary milk before coagulation process and extracting pH of Urtica dioica (pH 4 and pH 6) on the macroscopic and molecular properties of dromedary milk gel. Enzymatic coagulation was monitored by turbiscan, rheology and fluorescence spectroscopy measurements. Turbiscan results indicated a sedimentation phenomenon for milk heated at 70 °C, while for milk heated at 60 °C, particle migration showed flocculation. The results obtained by rheology showed an increase in G′ and G′′ moduli for all milk gel samples as a function of time, reflecting the formation of casein bonds and the hydrolysis of additional bonds. Fluorescence intensity of milk gel fluorophores exhibited changes with a similar shape during enzymatic coagulation, indicating variation in the structure of casein micelle integrity. Principal component analysis applied to fluorescence spectroscopy data showed a clear discrimination between heated and raw milk coagulated by Urtica dioica extract at pH 4 from those coagulated by extract at pH 6 and camel chymosin. Finally, it was found that the heat treatment of dromedary milk at 60 °C during 30 min and coagulation with Urtica dioica extract at pH 4 generated properties of the milk gel closer to that of chymosin.