A comparative study on the structure of Saccharomyces cerevisiae under nonthermal technologies: high hydrostatic pressure, pulsed electric fields and thermo-sonication.

Nonthermal technologies are becoming more popular in food processing; however, little detailed research has been conducted on the study of the lethal effect of these technologies on certain microorganisms. Saccharomyces cerevisiae is a yeast related to spoilage of fruit products such as juices; novel technologies have been explored to inactivate this yeast. Three nonthermal technologies, high hydrostatic pressure (HHP), pulsed electric fields (PEF) and thermo-sonication (TS), were used to evaluate and to compare the structural damage of yeast cells after processing. Processing conditions were chosen based on previous experiments to ensure the death of cells; HHP was conducted at 600 MPa for 7 min (room temperature, 21 °C); for PEF, 30.76 kV/cm at 40 °C and 21 pulses (2 μs each), and finally for TS the conditions were 120 μm, 60 °C and 30 min in continuous and pulsed modes; all treatments were applied in apple juice. Cells were prepared for electron microscopy using an innovative and short microwave assisted dehydration technique. Scanning electron microscopy showed the degree of damage to the cells after processing and illustrated the important and particular characteristics of each technology. Cells treated with high hydrostatic pressure showed a total disruption of the cell membrane, perforation, and release of the cell wall; scars were also observed on the surface of the pressurized cells. PEF treated cells showed less superficial damage, with the main changes being the deformation of the cells, apparent fusion of cells, the formation of pores, and the breakdown of the cell wall in some cells. Finally, the thermo-sonicated cells showed a similar degree of cellular damage to their structure regardless of whether the TS was applied continuously or pulsed. The main characteristics of cellular death for this technology were the erosion and disruption of the cellular membrane, formation of orifices on the surface, lysis of cells causing the release of intracellular contents, roughness of the cell membrane, and displacement of cell debris to the surface of other cells. This study confirms some theories about cell inactivation and presents new and detailed results about nonthermal technologies, but also shows that after using the above mentioned conditions, recovery of cells, specifically those that are pressurized and thermo-sonicated, it is not possible to do it following the high extent of damage observed in the entire population. Furthermore, a faster methodology that was used in sample preparation for electron microscopy provided high quality resolution images, allowing closer study of the detail of structural lethal effects on treated cells.