Purpose: The purpose of this study was to determine if HPP is effective means for achieving a 5-log reduction of Listeria monocytogenes in wheatgrass juice, and observe any physical changes from the processing.
Methods: Wheatgrass juice was harvested from 7-10 day old plants and inoculated with a 3-strain L. monocytogenes inoculum to achieve a ~7 log CFU/ml inoculation. All treatments and controls were packaged (5.0 ml) in duplicate into separate sterile Type 402 polyester pouches and heat sealed with minimal headspace. HPP treatments were applied at 586 MPa for 15 and 30 seconds. No HPP treatments were applied to the inoculation and non-inoculated controls. All samples were diluted and plated on Modified Oxford Agar (MOX) for 48 hours at 35°C. Sample enrichment was performed with Universal pre-enrichment broth (UPB), incubated at 35°C for 48 hours and plated onto MOX agar. Color, texture, and mouth feel was determined on non-inoculated samples. Experiment was replicated three times.
Results: Following both HPP treatments no L. monocytogenes was observed at a detection level of 3 CFU/ml, but enrichments showed presence in 15 second treatment (P < 0.01). No notable differences in odor, texture or mouth feel were identified through sensory and organoleptic analysis when compared to untreated samples.
Significance: HPP at 586 MPa was effective at reducing Listeria monocytogenes by 7 log CFU/ml at both 30 and 15 seconds in wheatgrass juice. Further analysis should be conducted to determine if HPP processing affects textural or flavor aspects of wheatgrass juice.