Water sorption characteristics of freeze-dried bacteria in low-moisture foods

Jie Xu; Yucen Xie; Narayan C Paul; M S Roopesh; Devendra H Shah; Juming Tang

doi:10.1016/j.ijfoodmicro.2021.109494

Water sorption characteristics of freeze-dried bacteria in low-moisture foods

Int J Food Microbiol. 2022 Feb 2:362:109494. doi: 10.1016/j.ijfoodmicro.2021.109494. Epub 2021 Nov 25.

Authors

Jie Xu¹, Yucen Xie², Narayan C Paul³, M S Roopesh⁴, Devendra H Shah⁵, Juming Tang²

Affiliations

¹ Department of Biological Systems Engineering, Washington State University, P.O. Box 646120, Pullman, WA 99164-6120, USA; Center for Nanotechnology and Nanotoxicology, Harvard T.H. Chan School of Public Health, Harvard University, Boston, MA 02115, USA. Electronic address: jie.xu4@wsu.edu.
² Department of Biological Systems Engineering, Washington State University, P.O. Box 646120, Pullman, WA 99164-6120, USA.
³ Texas A&M Veterinary Medical Diagnostic Laboratory, 483 Agronomy Rd, College Station, TCX 77843, USA.
⁴ Department of Agricultural, Food and Nutritional Science, University of Alberta, 3-16 Agriculture/Forestry Centre, Edmonton, AB T6G 2P5, Canada.
⁵ Department of Veterinary Microbiology and Pathology, College of Veterinary Medicine, Washington State University, Pullman, WA 99164-6120, USA.

PMID: 34895752
DOI: 10.1016/j.ijfoodmicro.2021.109494

Abstract

Water sorption isotherms of bacteria reflect the water activity with the change of moisture content of bacteria at a specific temperature. The temperature-dependency of water activity change can help to understand the thermal resistance of bacteria during a thermal process. Thermal resistance of bacteria in low-moisture foods may differ significantly depending on the physiological characteristics of microorganisms, including cell structure, existence of biofilms, and growth state. Previous studies demonstrated that the incremental change of a_w in bacterial cells during thermal treatments resulted in changes in their thermotolerance. In this study, a pathogen associated with low-moisture foods outbreaks, Salmonella Enteritidis PT30 (in planktonic and biofilm forms), and its validated surrogate, Enterococcus faecium, were lyophilized and their water sorption isotherms (WSI) at 20, 40, and 60 °C were determined by using a vapor sorption analyzer and simulated by the Guggenheim, Anderson and De Boer model (GAB). The published thermal death times at 80 °C (D_{80 °C}-values) of these bacteria in low-moisture environments were related with their WSI-derived a_w changes. The results showed that planktonic E. faecium and biofilms of Salmonella, exhibiting higher thermal resistance compared to the planktonic cultures of Salmonella, had a smaller increase in a_w when thermally treated from 20 to 60 °C in sealed test cells. The computational modeling also showed that when temperature increased from 20 to 60 °C, with an increase in relative humidity from 10% to 60%, freeze-dried planktonic E. faecium and Salmonella cells would equilibrate to their surrounding environments in 0.15 s and 0.25 s, respectively, suggesting a rapid equilibration of bacterial cells to their microenvironment. However, control of bacteria with different cell structure and growth state would require further attentions on process design adjustment because of their different water sorption characteristics.

Keywords: Biofilm; Enterococcus faecium; Salmonella Enteritidis PT30; Thermal resistance; Water activity; Water sorption isotherm.

MeSH terms

Food Microbiology*
Freeze Drying
Hot Temperature*
Salmonella enteritidis
Water / analysis

Substances

Water