Shellac coatings for grapefruits that favor biological control of Penicillium digitatum by Candida oleophila

Shellac coatings that support significantly high populations of the yeast Candida oleophila for biocontrol of Penicillium digitatum were formulated for postharvest application to grapefruits. An aqueous shellac latex was not toxic to the yeast and was suitable for testing constituents of commercial shellac coatings. Alteration of this product to contain 2% ethanol was nontoxic, but 4, 6, 8, and 10% ethanol killed 90 of the yeast cells within 24, 4, 2, and 0.5 h, respectively. Above a pH of 8.25, cell death was rapid when amine bases were incorporated. This level of toxicity resulted from addition to the shellac latex of more than 1.5% morpholine or 0.3% ammonia; a similar death rate developed nearer to pH 9.0 with incorporation of approximately 1.5% KOH. Although higher concentrations of base were required for the preparation of experimental formulations derived from bleached shellac, shellac ester, and wood resin ester, the relationship between the pH of the liquid coating and yeast survival persisted. Particularly with wood resin ester, KOH solutions maintained higher populations of yeast. When applied to grapefruits postharvest, a shellac latex formulation and a dissolved shellac/shellac ester formulation supported surface populations of yeasts between 10(4) and 10(5) CFU/cm(2) throughout 4 months of storage at 13 degrees C, These preparations significantly improved shelf-life when compared to fruits coated with a commercial shellac formulation. (C) 1996 Academic Press, Inc.