Shellac Formulations To Reduce Epiphytic Survival of Coliform Bacteria on Citrus Fruit Postharvest†

Survival of the coliform bacteria Enterobacter aerogenes and Escherichia coli was monitored in a neutral carboxyme-thylcellulose formulation and in shellac formulations with various pH and concentrations of ethanol and the preservative paraben; populations were subsequently measured from the surface of citrus fruit coated with these formulations. Numbers of the two bacteria increased over 24 h from 106 CFU/ml to approximately 108 CFU/ml in the carboxymethylcellulose solution, but over this time numbers remained little changed in the neutral solution of shellac. The Enterobacter was more tolerant of alcohol over a 3-h period; although its numbers in a shellac solution with 10% ethanol dropped from more than 106 CFU/ml to just over 103 CFU/ml, E. coli and a third species, Klebsiella pneumoniae, declined toward the limit of detection (5 CFU/ml) during this time. The addition of morpholine to increase the formulation pH to 9.0 caused numbers of bacteria to plummet to an undetectable level within 30 to 60 min. On Ruby Red grapefruit and Valencia oranges in storage at 13°C numbers of E. aerogenes and E. coli declined over 2 weeks from 105 CFU/cm2 to less than 2.5 × 101, but most of the loss in numbers occurred within 1 day. Numbers remained significantly less on shellacked fruit compared with those applied in the carboxymethylcellulose coating, and a shellac coating prepared from a pH 9 solution was more toxic to these species than one in which 12% ethanol had been added to the neutral formulation. The addition of the preservative paraben in the basic shellac was further inhibitory.