Decimal reduction energies of UV-C-irradiated spoilage yeasts in coconut liquid endosperm

Abstract

The ultraviolet-C (UV-C) decimal reduction energy (DUV-C) values of 17 spoilage yeasts and their composited inoculum were determined in coconut liquid endosperm (pH 5.26, 5.8 °Brix, 0.04% malic acid, 0.17% w/v insoluble solids). Growth kinetic parameters of all the test yeast strains were first established to standardize the growth stage of the cells prior to inactivation studies. Approximately 4.0 to 5.0 log CFU/mL cells in the mid-stationary growth phase (30.3 to 39.9 h, 25 °C) were suspended in 4 mL turbulent flowing juice and subjected to UV-C irradiation at a surface irradiance range of 3.42 to 4.99 mW/cm2. Survivor populations after exposure to predetermined UV-C energy were enumerated, and were used to derive the DUV-C values using the linear regression and Baranyi and Roberts (1994) model fitting. Results show that the yeast strains exhibited either log-linear or biphasic inactivation behavior with inactivation lag. The most UV-C resistant spoilage yeast was found to be Cryptococcus albidus (LJY1) with DUV-C values of 122.72 and 214.89 mJ/cm2 determined from linear regression and model-fitting, respectively. The least UV-C resistant was Torulaspora delbrueckii (LYJ5) with a DUV-C of 17.34 (linear regression) and 17.35 mJ/cm2 (model-fitting). The DUV-C values determined from the model fitting were generally greater than those calculated from linear regression, although only those determined for C. albidus were significantly different. To the investigators' knowledge, this is the first report of the UV-C inactivation kinetic parameters of Kluyveromyces marxianus, Trichosporon cutaneum, Pichia anomala, and Meyerozyma guilliermondii and C. albidus in coconut liquid endosperm. The results of this study can be used in the establishment and validation of UV-C process schedules for coconut liquid endosperm and other similar commodities.