With oysters, PHP is any process that has been validated using National Shellfish Sanitation Program validation procedures, according to Corinne Audemard, PhD, an associate research scientist with the Virginia Institute of Marine Science, College of William & Mary. “PHP aims to reduce the levels of pathogenic hazards to below the appropriate FDA action level or, in the absence of such a level, below the appropriate level as determined by the Interstate Shellfish Sanitation Conference (ISSC),” Dr. Audemard relates.
Along with reducing Vibrio and other bacteria to non-detectable levels, Dr. Audemard says PHP kills spoilage bacteria, thus extending shelf life.
Four PHP technologies are currently utilized by ISSC-approved firms for PHP: 1.) Individual quick freezing, which involves rapid freezing of half shell oysters on trays, then adding a thin glaze of ice to seal in the natural juices before storing them frozen; 2.) Heat-cool pasteurization, a process whereby live oysters are placed in warm water for a certain time period and then immediately dipped in cold water to stop the cooking process; 3.) High hydrostatic pressure, that subjects oysters to high pressures (35,000 to 40,000 pounds per square inch) for three to five minutes; and 4.) Low-dose gamma irradiation.
High Salinity Relay
A relatively unexplored PHP method called relaying holds promise as an alternative strategy for reducing V. vulnificus levels, Dr. Audemard says.
“High salinity relaying involves transferring oysters from salinity waters, 8 to 15 psu (practical salinity units), to higher salinity waters, 30 to 35 psu, to achieve a reduction in pathogenic bacteria, to less than 30 V. vulnificus per gram in as little as 14 days,” Dr. Audemard explains. “High salinity waters appear to negatively affect the survival of V. vulnificus.”
High salinity relay differs from previously approved PHP methods in that it is not a controlled process, Dr. Audemard points out. “That’s because the procedure typically relies on the exposure of oysters to natural high salinity waters for several weeks,” she says. “However, high salinity relaying is also used in molluscan shellfish transfer to more controlled environments, such as land-based tanks with similar results.”
In research published in August 2018, Dr. Audemard and several colleagues evaluated high salinity relay as a PHP for reducing V. vulnificus.
Dr. Audemard says the study was based on FDA validation guidelines, which specify, among other things, the initial Vibrio density before the process, the number of samples to be analyzed, the analytical methods to be used, and the endpoint concentration criteria to be reached for process validation, 30 per gram (g). (See page 203 of the National Shellfish Sanitation Program Guide for the Control of Molluscan Shellfish 2017 Revision.)
During each of three relay experiments, oysters cultured from three different Chesapeake Bay sites of contrasting salinities (10 to 21 psu) were relayed without acclimation to high salinity waters (31 to 33 psu) for up to 28 days. Overall, nine lots of oysters were relayed with six exhibiting initial V. vulnificus greater than 10,000 per g.
“As recommended by the FDA PHP validation guidelines, these lots reached both the 3.52 log reduction and the less than 30 perg densities requirements for V. vulnificus after 14 to 28 days of relay,” Dr. Audemard relates. “Densities of total and pathogenic V. parahaemolyticus in relayed oysters were significantly lower than densities at the sites of origin, suggesting an additional benefit associated with high salinity relay. While relay did not have a detrimental effect on oyster condition, oyster mortality levels ranged from 2 percent to 61 percent after 28 days of relay. Although the identification of the factors implicated in oyster mortality will require further examination, this study strongly supports the validation of high salinity relay as an effective PHP method to reduce levels of V. vulnificus in oysters to endpoint levels approved for human consumption.”