Doctoral Dissertations

Date of Award

12-1996

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Food Science and Technology

Major Professor

F. Ann Draughon

Committee Members

D. A. Golden, S. P. Oliver, J. L. Collins

Abstract

Growth and survival of spoilage microflora and pathogenic Yersinia enterocoliticawere investigated on fresh pork and minimally processed fresh broccoli packaged in modified atmospheres (MA). Phenotypic characteristics of Y. enterocolitica were used to detect presence of the virulence plasmid after exposure to each packaging and storage regime. These included Congo red uptake, calcium dependence, and autoagglutination in methyl red-Vogues Proskauer broth and tissue culture medium.

Fresh broccoli was inoculated with 3 to 4 log CFU/g Y. enterocolitica immersed in a salt solution, packaged in air, MA and under vacuum and stored at 4 and 10°C. The packaging material used was PD941 (Cryovac), a low barrier multilayered film. Y. enterocolitica populations were monitored over 15 days at 4 °C and 12 days at 10 °C. Loss of the virulence plasmid in Y. enterocolitica isolates was examined using phenotypic markers at the completion of each storage period. Populations of Y. enterocolitica increased in untreated and salt treated broccoli during storage at 4 and 10 °C. Salt treatment of broccoli did not inhibit growth of Y. enterocolitica in each packaging treatment. The virulence plasmid of Y. enterocolitica was detected in 13-22% of isolates following salt treatment and storage on minimally processed broccoli.

The microbiological quality of salt-treated minimally processed broccoli packaged in PD941 (Cryovac) film and stored at 4 and 10 °C in MA was studied by examining the fate of spoilage bacteria. Packaged broccoli, stored at 10 °C, spoiled more rapidly than broccoli stored at 4°C. Fresh untreated broccoli packaged in air and MA maintained a crisp green appearance for 9 days at 4 °C and 6 days at l0 °C. Vacuum packaged fresh broccoli had an extended shelf-life of up to 15 days at 4 °C. Vacuum packaged salt-treated broccoli showed signs of deterioration within 3 days of packaging, while salt-treated broccoli packaged in air and MA began to deteriorate within 3 to 6 days at 4 and 10 °C.

Slices of high (pH>6.0) and normal (pH<5.8) pH pork striploin were inoculated with Y. enterocolitica, vacuum packaged or packaged under 100% CO₂ in B900 (Cryovac) barrier bags, and stored at 4 °C. Numbers of Y. enterocolitica on the lean surface of high pH pork slices increased approximately 3.0 log CPU/cm² when vacuum packaged and stored at 4 °C for 30 days. Storage of inoculated normal pH pork in 100% CO₂ resulted in Y. enterocolitica remaining in lag phase throughout the storage period. The virulence plasmid of Y. enterocolitica was maintained in 10-21% of isolates for the duration of storage on vacuum and CO₂ packaged pork.

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