Selected Results of OSU Post-Packaging
Pasteurization of RTE Meat Products Project
C. Lynn Knipe, V. M. Balasubramaniam and Juhee Ahna
Departments of Food Science and Technology
And Animal Sciences
The Ohio State University
Columbus, OH 43210
aKangwon National University
ChunCheon, Republic of Korea
Research at OSU has evaluated the potential for post-packaging pasteurization of ready-to-eat (RTE) meat products, using hot water (185°F), steam (190°F) and dry heat cooking in a smoke house. Steam cooking may have some potential, however, the processing time takes longer than in a hot water tank. Dry heat cooking in a smoke house is not recommended due to the very long processing time, and expected negative effects on the product quality.
Cooked, cured pork and beef sausages and cured hams were produced in the OSU Meat Laboratory, which is a very small, state (Ohio Department of Agriculture) inspected establishment on the OSU campus. These products represented small (20 mm) and large (5 in.) diameter products that are typically made in small and very small operations. The small diameter sausages will be made with a lean (10% fat) and a fat (30% fat) formula, and the large diameter ham product will be made with and without an external fat layer, as fat does impact the heat penetration times.
L. innocua was used as a surrogate for L. monocytogenes, and was inoculated to the surface of cooked cured sausages and hams (7 log CFU/g), after cooking to 160F, but before packaging. Products were vacuum packaged, then given one of the post-pastuerization treatments and held for 14 days at 39ºF (4ºC), prior to being tested for L. innocua. The small-diameter sausages were packaged with four links to a package, whereas the large diameter product was packaged as one piece per package.
Time-temperature profiles were identified for lethality of L. innocua on the surface of these packaged products, using a one-cage smokehouse and a steam-jacketed kettle. The time-temperature profiles for the products in the smokehouse were identified with and without the use of added humidity, as many smaller processors are not able to control the humidity in their smokehouses.
Whereas a 2 log reduction of surface contamination was targeted in the RTE Final Rule, for post-package pasteurization, there is also a concern with making time-temperature recommendations for only a 2 log reduction, based upon the OSU equipment, that would be expected to apply to the many styles and sizes of cooking equipment that might be used by smaller meat processors. What would be most important for processors is that they achieve the surface temperature (between the product and the package, see Figure 1) that is needed for adequate surface lethality. Using the time-temperature profiles, and the American Meat Institute’s Lethality Equation, temperatures were calculated to reach 2, 5 and 7 log reductions of L. monocytogenes in the OSU study. Surprisingly, there was little difference in temperatures required to reach 2 and 7 log reduction of L. monocytogenes (3-5 degrees depending upon the product). In the OSU research, times to reach a 7 log reduction (calculated using the AMI Lethality Equation) of L. monocytogenes under the surface of the product package were as follows:
Product and Cooking Method to Reach 7 log Reduction
Lean sausages – water tank (185°F) 8 minutes
Fatter sausages - water tank (185°F) 8.4 minutes
Lean hams – water tank (185°F) 6.3 minutes
Fatter hams –water tank (185°F) 5.8 minutes
Lean sausages – smokehouse (without humidity) 30-35 minutes
Higher fat sausages – smokehouse (without humidity) 30-35 minutes
Hams - smokehouse (without humidity) did not reach 170ºF within 45 minutes
Lean sausages – smokehouse (steam cook) (190°F) 20 minutes
Higher fat sausages – smokehouse (steam cook) (190°F) 18 minutes
Lean Hams - smokehouse (steam cook) (190°F) 23 minutes
Fatter hams - smokehouse (steam cook) (190°F) 19 minutes
How can a smaller processor validate their post-packaging pasteurization process?
To properly validate such a process, processors would need to monitor the heating time, the water temperature, and the product surface temperature during reheating. But how does a smaller processor monitor the product surface temperature (between the product surface and the packaging material)?
One system was developed at OSU that should allow smaller processors to check the product surface temperatures, using an infra-red thermometer (MiniTemp, by Raytek, Santa Cruz, CA), to determine if they have accomplished sufficient post-packaging lethality. This method requires some training, as the reading must be taken immediately after raising product from the hot water or from the smoke house. However, errors in reading would most likely be on the conservative side, since the product surface chills very rapidly once the product is removed from the hot water or steam environment. Examples of target package surface temperatures (using the infra-red thermometer) include:
Lean sausages – water tank 173ºF 171-180ºF
Fatter sausages - water tank 174ºF 180ºF
Lean hams – water tank 169ºF 160-170ºF
Fatter hams –water tank 171ºF 160-170ºF
Lean sausages – smokehouse (steam cook) 153ºF 160ºF
Higher fat sausages – smokehouse (steam cook) 153ºF 160-170ºF
Lean Hams - smokehouse (steam cook) 154ºF 160-170ºF
Fatter hams - smokehouse (steam cook) 154ºF 160-170ºF
An additional method for small processors to use would involve the application of self-adhesive temperature indicators to the surface of products, so that after the processors had followed the validated time-temperature recommendations from this study, that they could verify that their products had reached the targeted surface temperature (see Figure 2). These temperature indicators could be removed from the product surface before selling the products. Realizing the temperature difference between the outside of the package (where temperature indicators would be applied commercially) and the more critical product surface temperature (under the package), the required outside surface temperature was determined to provide the required lethality to the product surface. In other words, if the product surface (under the package) temperature needs to be 157ºF to achieve adequate surface lethality, the outside surface temperature indicator would need to show a minimum of (e.g.) 172°F (See above Table).