High Pressure Process Boosts Oyster Safety and Quality
Virginia Tech’s high pressure processing laboratory has several projects underway that deal with improving the safety of oysters as well as increasing their shelf life. High pressure processing treatments (HPP) have the capacity to disrupt the function of bacteria, viruses, parasites, enzymes, and other common contributors to food spoilage, while leaving the food in an essentially raw state.
For oysters in particular, being able to be marketed raw and fresh is of key importance. In the marketplace, a cooked oyster is worth only a fraction of what a raw oyster is worth, making the complexity of live oyster marketing well worth the effort. However, oysters, because they are benthic filter feeders, can accumulate a number of different microorganisms that have the potential to cause harm.
Vibrios, for example, are ubiquitous in oyster growing waters, and a few strains of them are harmful to humans. Vibrios are the single biggest microbiological hazard that consumers can encounter when eating raw oysters. HPP treatment of oysters has been shown to drastically reduce the prevalence of vibrios in the treated samples. Interestingly, while the HPP treatment produced an immediate and marked drop in vibrio populations, the die-off continued for days after processing. After five days, levels of vibrios were undetectable, probably due to a combination of cellular injury and cold storage.
HPP treatments can also extend the period of time that oysters smell and taste fresh. Studies have shown that starting at 15 days post harvest, untreated oysters start to smell ‘off’. HPP treated oysters, however, retain a pleasant, fresh smell well past 20 days post harvest. How much is an extra five days of shelf life worth to an oyster processor? What if that extra five days is combined with lower costs of production, because the treatment also ‘pre-shucks’ the oyster? Only the industry can answer those questions.
One problem encountered with HPP of oysters is that the treatment seems to increase drip loss, especially in summer oysters. Apparently the same force that inactivates pathogens and spoilage organisms also damages the cell walls of the oyster, leading to loss of fluid from the oyster meats. Further work is currently underway at the Virginia Tech High Pressure Processing Laboratory to determine the economic ramifications of this problem, balanced with the positive effects of reduced shucking costs and increased shelf life and safety. Vital funding and support for HPP research has been provided by the Fisheries Resource Grant Program administered by Virginia Sea Grant, with the goal of expanding seafood-industry awareness and utilization of this remarkable new technology.
The use of HPP treatments, combined with proper storage, can have significant effects on the oyster industry worldwide. We are committed to fully exploring these potentials, and then sharing our findings with industry and regulatory groups.
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