Food Protection Trends - May 2009 - 269

FIGURE 1. Schematic of thermocouple and button data logger placement on an experimental oyster specimen. Note: any single oyster specimen was fitted with only one probe. Adapted from Martin et al. 2007. INTRODUCTION Temperature control is an important aspect of food safety. Thermocouples are widely used for temperature sensing because they are inexpensive ($0.50/0.3 m for the thermocouple plus cost approximately $200.00), for the multimeter readout display are interchangeable, and can measure temperatures over a wide range (10). They are, however, less suitable for applications where smaller temperature differences need to be measured with high accuracy (2). For example, T-type thermocouples have a standard limit of error of 1oC (3), and small temperature differences are defined as increments between whole degrees. Furthermore, using thermocouples for temperature monitoring in certain circumstances, for instance in molluscan shellfish, is difficult because of concerns about portability, slippage, and sensitivity to moisture. Taken together, this means that temperature data collected by use of thermocouples may not always be as accurate or reliable as one might wish. Improvements in time-temperature indicators have resulted in miniaturization, waterproofing, and ease and security of data retrieval (1). These new devices, termed button data loggers, are relatively inexpensive ($35.00 for each button data logger and $45.00 for the interface cable between button data logger and the computer) and durable in moist environments. For example, the Smartbutton data logger measures temperature in 0.5oC increments, holds 2,048 consecutive temperature readings, can be programmed to take temperature readings at intervals of 1 to 255 min and/or with alarms at upper or lower temperature settings, and has a 10-year battery (1). An appealing application for these button data loggers is monitoring the temperature of seafood, particularly molluscan shellfish, during harvest, handling, shipment, and storage. The purpose of this study was to compare traditional thermocouples and button data loggers with respect to their ability to accurately record the temperature of shellstock oysters under simulated commercial storage conditions. MATERIALS AND METHODS Cooling curves of oysters During the summer of 2007, three commercial size burlap bags of oysters (approximately 120 kg/ bag) were obtained on three different days from a commercial oyster harvester. The bags of oysters were transported to the Food Microbiology Laboratory, Louisiana State University (LSU) AgCenter (Baton Rouge, Louisiana) in covered trucks. Immediately upon receipt of the product at LSU, temperature recording devices were put in place. For each commercial bag, 9 oysters were obtained. Replicates (3 oysters each) were fitted with temperature probes representing (i) internal thermocouple placement; (ii) external thermocouple placement; and (iii) external button data logger placement. For internal thermocouple placement, a 0.64 cm hole was made by drilling into the shell approximately 1.27 cm from the bottom center of the oyster. These holes were cleaned of any drilling debris and blotted dry of water and oyster liquor with absorbent paper, consistent with the method described by Martin et al. (7). A T-type thermocouple (copperconstant) (TMQSS-032U-6, OMEGA Engineering Inc., Stanford, CT) was inserted approximately 2.54 cm into the oyster meat and the hole was then sealed with modeling clay (Crayola, PA). Thermocouples were secured with duct tape to the outside of the shell to measure external oyster temperature. The thermocouple data were collected by use of an OM-3000 portable datalogger (OMEGA Engineering Inc, Stanford, CT). SmartButton data loggers (ACR Systems Inc., Surrey, B.C., Canada) were taped to the outside of a third set of oysters (Fig. 1). Sets of three oysters (thermocouple-external; thermocouple-internal; and button data logger-external) were placed side by side at different locations in the burlap sacks (top, middle, and bottom) in an effort to record temperatures representative of different product locations. In addition, a thermocouple wire was placed inside and outside of the burlap bags to monitor ambient temperature. The commercial oyster bags were allowed to equilibrate to room temperature (18 to 20oC) and then placed in a walk-in cooler (5oC) for 360 min (6 h). Although commercial shellstock oysters may be stored under refrigeration conditions for up to 21 days prior to consumption (4), this abbreviated time period was chosen simply for the purpose of comparing data loggers and thermocouples with respect to temperature monitoring capabilities. The data loggers and thermocouples were programmed to record temperatures every two minutes. The study was repeated three times for each type of temperature recorder and placement location. Statistical analysis The 2005 Vibrio parahaemolyticus risk assessment uses the work of Cook and Ruple (5) to justify the assumption that growth of this organism does not occur at temperatures <10oC. Therefore, temperature data were stratified for statistical analysis, using this biologically relevant cut-off value. The Wilcoxon signed-rank test for paired data was used to compare button logger with internal probe measurements, and to compare button logger and internal probe measurements with concurrent ambient external thermocouple temperature readings (8). The Bonferroni MAY 2009 | FOOD PROTECTION TRENDS 269

Food Protection Trends - May 2009

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