Thermal resistance and high-performance microwave decontamination assessment of Bacillus endospores isolated from food-grade herbal extracts.

Armin Tarrah,Antonella Cirillo,Alessio Giacomini,Shadi Pakroo,Viviana Corich,Angiolella Lombardi,Milena Carlot,Camilla Nesto,Peter Setlow

doi:10.1371/journal.pone.0261988

Abstract

Generally, endospore contamination can occur from different sources during product manufacturing in many industries and therefore lower its quality by affecting physicochemical properties and shelf-life. Bacterial endospores can germinate inside the product and produce several enzymes, which can cause several undesirable changes. This study assessed the spores thermal resistance and applied a microwave decontamination technique toward herbal extracts (Tilia tomentosa and Centella asiatica) containing ethanol or glycerol. Based on 16S rRNA analysis, the detected contaminant endospores belonged to different Bacillus species, namely B. subtilis, B. zhangzhouensis, and B. pumilus. The thermal resistance assessment using inoculated endospores in the actual products revealed B. pumilus T2 as the most resistant endospore to the heat treatments tested in both T. tomentosa and C. asiatica extracts. Finally, a high-performance microwave technique was used to decontaminate T. tomentosa extract against the mixture of Bacillus spores. Results from the microwave technique indicate that the increase of temperature from 100°C to 105°C not only decontaminated the product but also could dramatically decrease the effective thermal treatment time (10 times), which can benefit the product quality. The results provided in this study considerably contribute to improving an original decontamination method for products containing glycerol and ethanol with the most negligible effect on product quality.

Highlights

Plants that are used for medicinal or aromatic properties are defined as medicinal and aromatic plants (MAPs) [1]
Microbiological analysis of herbal pharmaceutical extracts (Tilia tomentosa and Centella asiatica) was performed by plate count analyses for the following microbial categories: total mesophilic microorganisms were counted on PCA (Plate Count Agar, Difco, MD, USA) medium, incubated at 30 ̊C for 72 h; Enterobacteriaceae were enumerated on VRBG agar (Violet Red Bile Glucose Agar, Difco, MD, USA) after incubation at 37 ̊C for h; yeasts and molds were counted on DRBC (Dichloran-Rose Bengal Chloramphenicol Agar, Difco, MD, USA) incubated at ̊C for 72 h
High-performance microwave-assisted equipment Minilabotron 2000 (SAIREM, DecinesCharpieu, France) with 2-kW output power and an operating radio frequency heating at 2450 MHz was used for treating the Tilia tomentosa extract, which was inoculated with approximately 104 endospores of B. subtilis C5, B. zhangzhouensis M1, and B. pumilus T2 (104 endospore mixture)

Summary

Introduction

Plants that are used for medicinal or aromatic properties are defined as medicinal and aromatic plants (MAPs) [1]. The present study aimed to assess the thermal resistance of Bacillus endospores isolated from aqueous or alcoholic plant extracts and to investigate the potentiality of high-performance microwave techniques to inactivate them.

Results

Conclusion