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Improving the Safety of Hard Clams, Mercenaria Mercenaria, Through Post Harvest Treatment with High Hydrostatic Pressure Processing (HHP)

Karwe, Mukund
Rutgers University
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End date
  1. Establish optimal physical processing parameters for applying HHP technology to hard clams.
  2. Assess the HHP processing through a microbiological shelf life study, including work on viruses.
  3. Carry out preliminary studies to evaluate the sensory characteristics of the HHP processed clams.
  4. Educate the public about this new food preservation process and evaluate their perceptions.
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NON-TECHNICAL SUMMARY: Hard clams are sometimes consumed raw but they may contain harmful bacteria and viruses. This project will evaulate the effect of high hydrostatic pressure on getting rid of these harmful bacteria and viruses.

APPROACH: High Hydrostatic Pressure (HHP) processing is a novel food processing method that destroys food borne bacteria and viruses, and inactivates deleterious enzymes which affect food quality, without using heat. During HHP processing, pressures up to 145,000 pounds per square inch (psi) may be applied on a food product. HHP processing offers a new way to produce safe, value-added, and high quality products, with minimum detrimental effects associated with heat processing such as loss of original flavor, color, texture, and nutritional value. In our proposed work, we will apply HHP to process hard clams Mercenaria mercenaria to improve the food safety aspects of these raw shellfish.

PROGRESS: 2007/09 TO 2008/08
OUTPUTS: Littleneck hard clams (Mercenaria mercenaria) harvested from special restricted and approved waters of the state of New Jersey were high pressure processed. A response surface (RS) methodology approach was used to optimize the pressure and time parameters for microbial inactivation. The total surviving microbial load (total plate count) in these hard clams was enumerated after each experimental condition. The results from this research were disseminated at the annual conference of the Institute of Food Technologists, at a non-thermal food processing workshop in Portland OR, and at an international conference on food innovation with emphasis on emerging science, technologies and applications. One masters thesis was published based on this research.
PARTICIPANTS: Mukund V. Karwe, George E. Flimlin, Shalaka Narwankar, Beverly Tepper, Don Schaffner
TARGET AUDIENCES: Clam harvesters, Supermarkets which sell clams, Restaurants which serve raw clams, Seafood industry people, Regulatory agencies

IMPACT: 2007/09 TO 2008/08
In this project we worked with New Jersey clam harvesters. The results from the response surface (RS) study showed that log reduction of total plate count (TPC) in the hard clams from restricted waters due to high pressure processing was primarily a function of pressure. At least 480 MPa pressure was needed to achieve one log reduction in TPC. Thus we showed that there is good potential for using high pressure processing technology as an alternative method to the conventional method of depuration. Other advatages of this process are that it saves labor, it is clean, and can work for a variety of clams. As a part of this project, we also evaluated the consumer acceptability of high pressure processed hard clams. A panel of 60 regular raw clam consumers tasted both raw and processed hard clams from approved waters. Results showed that consumers equally preferred the processed and the unprocessed hard clams. However, two sub-groups of consumers were revealed; one group preferred the plumpness of the HPP clam and the other group preferred the aroma of the unprocessed clam. Thus, plumpness and aroma may influence consumer acceptance of HPP clams.

PROGRESS: 2006/09/01 TO 2007/08/31
The objective of our project is to explore high pressure processing (HHP) as a post harvest technology to process market sized hard clams. We hypothesize that this technology would produce a value added safe product for raw consumption. The types of clams that are savored raw are usually littleneck, topneck, and "specials." The post harvest treatment currently employed for clams from Special Restricted waters are relaying to beds in approved waters (2 to 4 weeks) or depuration in a closed system (48 hrs). However these treatments are time consuming and also are not very successful in eliminating viruses. Littleneck clams from Special Restricted waters (JT White Clam Plant, Highlands, NJ ) were processed in a high pressure vessel, with pressure range of 40,000-56,000 pounds per square inch (psi) and processing time of 2-6 min. The processed clams were kept at refrigeration temperature (5 degrees C or 41 degrees F). The bacterial count of these clams was measured immediately and after 4 days of storage. These measurements were also carried for clams which were not high pressure processed, as control. It was found that the effect of pressure up to maximum of 59,000 psi for 6 min, on the bacterial count immediately after processing, was not substantial, i.e., the count was reduced to less than 1/10th when we expected it to be more than 1/10th. The results on baterial count on day 4 were not conclusive. To obtain better resolution of measurement of bacterial count, two different methodologies, namely spread plate and pour plate, were used to enumerate the bacterial colonies that were formed. It was found that pour plate technique gave a better resolution of the bacterial count. In another experiment, effect of salinity in the media (plate count agar) on enumeration of bacterial colonies in clams was investigated. Since clams require a particular level of salinity in water to grow, addition of sodium chloride (NaCl) salt in the media was expected to increase the number of colonies. However, it was found that addition of 3% salt to the media and followed by pour plate technique to enumerate the batcerial count did not show significant difference in the number of colonies. In order to find out whether size/type of clams has any effect on the bacterial inactivation, we conducted a set of experiments where we processed clams of different sizes (littleneck, topneck, cherrystone and chowder) at 60,000 psi for 8 minutes which is a severe condition for shellfish. It was found that the smallest size clam (littleneck) showed least inactivation among all four types of clams. To check whether the shell is having a protective effect we processed clams with no shell, with half shell and unopened clams at 60,000 psi for 8 minutes. No significant differences in the level of inactivation were found between the three variants.

IMPACT: 2006/09/01 TO 2007/08/31
Although it is reported that the high pressure processing conditions that we selected were more severe than those used for commercially processed oysters, clams taken from restricted waters would need higher pressures than 60,000 psi. Literature indicated that high pressure processing conditions do not depend on the size and shape of the objects being processed. However, the results obtained so far indicate that different size clams needed different high pressure processing conditions to achieve the same level of reduction in bacterial count.

Funding Source
Nat'l. Inst. of Food and Agriculture
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Natural Toxins
Viruses and Prions
Bacterial Pathogens
Chemical Contaminants