Microencapsulation of Acidithiobacillus thiooxidans in a biopolymeric matrix of gum arabic and whey protein using complex coacervation and freeze drying

Abstract

Acidithiobacillus thiooxidans is a microorganism of great interest in biohydrometallurgical processes, as it is potentially capable of treating complex, low-grade sulfur minerals. However, there are concerns in the metallurgical industry regarding microbial manipulation due to problems related to cellular integrity, metabolic activities, genetic stability, and long-term maintenance of microbial cultures. In this study, the microencapsulation of Acidithiobacillus thiooxidans in a biopolymeric matrix composed of whey protein isolate (WPI) and gum arabic (GA) was carried out by complex coacervation and freeze-drying, with the goal to improve the above-mentioned factors. Results revealed that the highest level and quality of microencapsulation was obtained at a pH of 4.0 with a WPI: GA ratio of 2:1. The physicochemical properties of the powders (freeze-dried cells encapsulated in the WPI-GA matrix) revealed water activity ($a_w$) of 0.089 ± 0.001, encapsulation efficiency ($\mathrm{EE}$) of 88.88 ± 3.75 %, coacervation yield ($\mathrm{YC}$) of 37.71 ± 0.063 %, and moisture content ($\mathrm{MC}$) of 4.0 ± 0.14 %. Additionally, scanning electron microscopy (SEM) showed that the morphology corresponds to amorphous particles like crystals, with a polydisperse size distribution. Fourier transform infrared spectroscopy (FTIR) analysis confirmed the presence of electrostatic interactions between WPI and GA. Finally, growth kinetics and the bacteria metabolic activity remained intact after a freeze-drying process, reaching a growth of 1.64 x 10⁹ cells/mL in 16 days. This microencapsulation process of the Acidithiobacillus thiooxidans may be an innovative method with applications in the mining and metallurgical industry regarding the storage, handling, and use of this microorganism in bioleaching processes.