Impact of postharvest UV-C and ozone treatments on microbiological properties of white asparagus (<em>Asparagus officinalis</em> L.)

Abstract

To meet the increasing demand for safe and high quality fresh white asparagus and the recent food safety regulations, optimization of postharvest handling, processing and storage is essential. Modern sanitation techniques relying on physical methods and/or Generally Recognized As Safe (GRAS) compounds are desired for reducing microbiological spoilage. To evaluate the effects of aqueous ozone and UV-C on the microbial load of spears, samples were UV-C irradiated (254 nm, 1 kJ m-2) and/or washed with ozonated water (approx. 3 ppm or 4.5 ppm at 10 °C), and analyzed at three times during a four day storage. Also, the potential effects of initial natural microbial loads, and precondition of the spears in terms of water and sugar contents on the responsiveness of asparagus to these treatments were determined in detail over four growing seasons. The initial microbial loads (mould and yeasts, and aerobic mesophilic total bacterial counts) of white asparagus spears varied considerably during the different harvest seasons of this four-year study. This variability could not be explained by the variance of climatic conditions nor by the respective water and sugar content. Furthermore, there was never a clear cut relation of the initial microbial load and the growth of pathogens during four-day storage at 20 °C in nearly water vapour saturated atmosphere. Neither washing the spears with ozonated water (3 or 4.5 ppm) nor treating them with UV-C radiation (1 kJ m-2) systematically and significantly affected their microbial loads during storage. In addition, the assumption that a combination of both treatments could synergistically improve the effect of each treatment could not be verified during this long-term study. In conclusion, microbial load and pathogen development in asparagus spears are highly persistent and, thus, to meet hygienic requirements further investigations will be necessary.