Baroprotective effect of increased solute concentrations on yeast and moulds during high pressure processing

Abstract

The baroprotective effect of increasing solute concentrations on yeast cells and fungal conidia subjected to high pressure processing (HPP) was studied. Suspensions of yeast cells (Saccharomyces cerevisiae, Pichia anomala and Hanseniaspora uvarum) or fungal spores (Penicillium expansum, Fusarium oxysporum and Rhizopus stolonifer) in citrate phosphate buffer formulated with sucrose at 40, 50 and 60 °Brix, or with glycerol and NaCl at equivalent water activity (aw) values (0.925, 0.903 and 0.866 aw) were subjected to 600 MPa pressure for varying times, and then were enumerated by spread plate technique to assess survival. There was an increasing resistance to inactivation by high pressure with an increase in solute concentration. The two moulds with easily wetted spores, R. stolonifer and F. oxysporum, showed strongest resistance to HPP at 0.866 aw. Differing responses to the three solutes were observed among the fungal species tested, indicating that the chemical nature of the solute may also be important in protecting yeasts and moulds during and after pressure treatment. Sucrose had a stronger baroprotective effect for S. cerevisiae than the other solutes, at two of the three investigated aws. For P. expansum at 0.903 aw, NaCl gave the best protective effect. Scanning electron microscopy of HPP treated cells showed the protective effects of increased sucrose concentration. The results reported here have practical implications for the food industry in the application of HPP for production of fruit preparations or syrups, and should be taken in account in establishing efficient process design. As high concentrations of sugar, salt and glycerol provide protection for yeasts and moulds during high pressure processing, foods containing high levels of solutes may need longer processing times or higher pressures to achieve inactivation of these fungi.