Evidence of nuclear membrane damage in yeast cells treated with pulsed electric fields

The application of pulsed electric fields (PEF) as a process in the treatment of liquid foodstuff, although not fully understood, appears to have significant potential. This treatment technology represents a promising nonthermal technique that may supplement or replace conventional pasteurisation methods for some applications. The advantages of such an electrotechnology include the potential retention of fresh-food characteristics and organoleptic qualities such as flavour, aroma, and texture. Pulsed electric field inactivation of microorganisms is generally thought to be caused by irreversible structural changes in the cellular plasma membrane resulting in swelling or shrinkage and finally lysis of the cell. Previous evidence of this external membrane damage has not established what damage, if any, is caused by PEF treatment on the internal structure of the cell. This investigation using laser scanning transmission microscopy of the spoilage yeast Zygosaccharomyces bailii after PEF treatment shows evidence of internal damage to the nuclear membrane. It has been reported that electric field rise times in the 1-100ns range are required for intracellular electroporation. The laser images indicate that this range must be more extensive than previously reported since with the rise time of ∼160ns extensive nuclear membrane damage has been caused to the yeast cells. Such damage, in combination with plasma membrane damage, could contribute to the inactivation of Z. bailii by PEF.