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Population Dynamics of Postharvest Decay Antagonists Growing Epiphytically and Within Wounds on Grapefruit

Coating formulations of shellac, sucrose ester, and cellulose were tested that support populations of bacterial and yeast antagonists of postharvest Penicillium decay of grapefruit. Surface populations of Pseudomonas syringae, P. fluorescens, and P. putida were stable between 10³ and 10⁴ CFU/cm² on shellacked fruit over 4 months at 13°C, but numbers of the yeast Candida oleophila rose steadily from 2.35 × 10³ to nearly 10⁵. Through the first 15 days of cold storage, surface populations of P. syringae and C. oleophila were greater when applied to fruit within the pH 7.2 shellac than when applied by dipping fruit into an antagonist suspension and drying the fruit prior to shellacking; numbers were generally equivalent thereafter. Within wounds of shellacked fruit, populations of P. syringae were equal regardless of the method of application and increased from 10⁴ to more than 1.2 × 10⁷ CFU within 2 weeks. In contrast, numbers of C. oleophila in wounds over the first 2 weeks of storage were greater in fruit dipped and coated with shellac than when the antagonist was incorporated in this material; populations stabilized at ≈6 × 10⁶ CFU after 2 weeks. Incorporation of P. syringae and C. oleophila into ester and cellulose coatings, however, initially fostered greater numbers of both antagonists in wounds (≈10⁶ CFU) than did the preliminary dip application, and significant population differences persisted for several weeks. In cold storage, although application method did not affect the efficacy of P. syringae, C. oleophila was more efficacious after 3 and 4 months when applied in the shellac than when applied by a preliminary immersion and subsequent drying of fruit prior to shellacking.