Relationship between growth of food‐spoilage yeast in high‐sugar environments and sensitivity to high‐intensity pulsed UV light irradiation

The relationship between prior growth of food-spoilage yeast in high-sugar environments and their subsequent survival postpulsed UV (PUV) irradiation was investigated. Test yeast were separately grown to early stationary phase in YPD broth containing increasing concentrations of glucose (150% w/v) and were flashed with <= 40 pulses of broad-spectrum light at lamp discharge energy settings of 3.2, 7.2 and 12.8 J (equivalent to UV doses of 0.53, 1.09 and 3.36 mu J cm-2, respectively) and their inactivation measured. Findings showed that prior growth in high-sugar conditions (>= 30% glucose w/v) enhanced the sensitivity of all nine representative strains of Zygosaccharomyces bailii, Z. rouxii and Saccharomyces cerevisiae yeast to PUV irradiation. Significant differences in inactivation amongst different yeast types also occurred depending on amount of UV dose applied, where the order of increasing sensitivity of osmotically stressed yeast to PUV irradiation was shown to be Z. rouxii, Z. bailii and >S. cerevisiae. For example, a 1.2-log order difference in CFU mL-1 reduction occurred between Z. bailii 11 486 and S. cerevisiae 834 when grown in 50% w/v sugar samples and treated with the uppermost test UV dosage of 3.36 mu J cm-2, where these two yeast strains were reduced by 3.8 and 5.0 log orders, respectively, after this PUV treatment regime compared to untreated controls. The higher the UV dose applied the greater the reduction in yeast numbers. For example, a 1.0-, 1.4- and 4.0-log order differences in CFU mL-1 numbers occurred for S. cerevisiae 834 grown in 15% w/v sugar samples and then treated with PUV dose of 0.53, 1.09 and 3.36 mu J cm-2, respectively. These findings support the development of PUV for the treatment of high-sugar foods that are prone to spoilage by osmotolerant yeast.