this application incorporates by reference the subject matter of u . s . application ser . no . 12 / 631 , 497 ( apparatus and method for searing , branding , and cooking , filed dec . 4 , 2009 ) and ser . no . 12 / 836 , 886 ( method , continuous apparatus , and burner for producing a surface - roasted product , filed jul . 15 , 2010 ). a pasteurization system and process made according to this invention preferably makes use of a flame pasteurizer . a continuous flame grill or roaster such as the unitherm ® flame grill ™ ( see fig2 ) or tunnel of fire ™ roaster ( see fig3 ) ( unitherm food systems , inc ., bristow , okla .) or its equivalent is a preferred flame pasteurizer . the flame pasteurizer should heat the surface of the root vegetable or product to a temperature sufficient to kill pathogens on the surface ( at least 145 ° f . or about 62 . 8 ° c . ), and the product should remain in the flame pasteurizer for a sufficient time to achieve at least a 1 log reduction listeria monocytogenes and alternate pathogens such as salmonella , e . coli , and other bacteria . preferably , the residence time for onions should be at least 30 seconds and no greater than 90 seconds ; with 45 to 60 seconds being ideal . some variance may occur based on onion size and temperature . the flame should make contact on the surface of the product and engulf a minimum of at least 65 percent of the total surface area . ideally , 100 percent of the surface area should be contacted and engulfed by the flame . preferably , the flame pasteurizer should have the ability to change the angle of flame from burner - to - burner to ensure heat - treatment of all exposed surfaces . for example , the burner could be arranged at an angle in the range of 5 ° to 60 ° from vertical . a first burner could be arranged at − x ° from vertical and a second burner could be arranged at + y ° from vertical , where x and y are the angle from vertical ( see fig2 ). preferably , x and y are the same magnitude . the surface of the product ( e . g ., an onion &# 39 ; s outer layer , a carrot &# 39 ; s or potato &# 39 ; s skin ) should be completely burned up to at least the outermost layer and controlled by time to leave the membrane under the first layer intact or mostly intact ( see e . g ., fig1 ). once the product is pasteurized , the membrane can then be removed mechanically or manually to leave a product that does not appear to have been flame pasteurized . once the product exits the flame pasteurizer it can be transferred as part of a continuous process without human intervention . this limits the possibly of re - contamination . at the discharge opening or exit of the flame pasteurizer , the surface of the product can be rinsed by spraying , deluging or submerging with clean , bacteria - free water to inhibit further heat transfer to the core , control the burn , and end the pasteurization process . product that is not subjected to water at the end of the flaming process can continue to burn . the core temperature of the product should be unaffected between the entrance point to the flame pasteurizer and the exit point from the water application . for example , product entering with a core temperature of 50 ° f . ( 10 ° c .) would exit from the water application with that same core temperature . in order for this process to work , and using onions as an example , the onions must be individually separated or have space around them ( e . g ., 10 or 20 onions evenly spaced across a belt , one - level deep ) as the onions pass through a ribbon flame or burner arrangement like that disclosed in the above patent applications . a continuous ribbon burner that encircles the cook belt is a preferred burner . in other embodiments of the invention , a gas infrared system that operates at surface temperatures of 1 , 000 ° f . or higher can ignite the skin of the onion for a similar effect . however , systems in the above patent applications are preferred because each makes use of a ribbon flame which fully engulfs the outer layer , ignites it , and burns the circumference sufficiently to achieve lethality . while infrared would work on a single - lane , the shadowing for multiple lanes could leave cold spots on which pathogens survive . as mentioned above , a product pasteurized by this system and process typically requires a quick wash to ready it for further processing . shrinkage is reduced relative to peeling processes . in tests conducted by the inventor , shrinkage was in the range of about 3 to 5 % by weight . 1 . in a gas flame - fired grill at unitherm food systems , inc . ( bristow , okla .) (“ unitherm ”) for microbial testing of process effectiveness on indigenous microorganisms ( aerobic plate count , yeast and mold ). 2 . in a gas flame - fired grill at unitherm for microbial testing of process effectiveness on non - pathogenic listeria innocua - inoculated onions . 3 . in a pilot - plant scale oven at the robert m . kerr food & amp ; agricultural products center ( stillwater , okla .) (“ fapc ”) to mimic the commercial process and test its effectiveness against listeria monocytogenes . indigenous bacteria on the surface of onions were plated on nonselective tryptic soy agar ( tsa ). a non - pathogenic , antibiotic resistant strain of listeria innocua will also be used to inoculate onions ( 24 - hrs in advance of use ) for processing through commercial ovens at unitherm food systems manufacturing facility . for processing in the pathogen - processing lab at the robert m . kerr food & amp ; ag products center ( stillwater , okla . ), four strains of pathogenic l . monocytogenes ( scott a - 2 , serotype 4b , clinical isolate ; v7 - 2 , serotype 1 / 2a , clinical isolate ; 39 - 2 retail hotdog isolate ; 383 - 2 ground beef isolate ) were used for inoculation of whole onions for processing on pilot plant scale equipment that mimics the commercial process . the listeria strains are all resistant to streptomycin ( 100 μg / ml ) and rifamycin s / v ( 10 μg / ml ). samples were plated on general - purpose agar ( tsa ) containing these antibiotics for selective enumeration of our inoculum on non - sterile product ( i . e ., this precludes the enumeration of other contaminating bacteria ). natural , whole onions ( with their indigenous microflora ) as well as surface - inoculated onions were inoculated with either listeria innocua ( at unitherm ) or a 4 - strain cocktail of l . monocytogenes ( at fapc ). plating was performed on potato dextrose agar for enumeration of yeast and mold . the process was evaluated by microbial enumeration of pre - process and postprocess levels of organisms . processing included preliminary trials to determine approximate processing times that would be used for microbial process testing . a gas - fired flame oven at unitherm was used for testing for lethality on indigenous organisms on onions and onions inoculated with nonpathogenic listeria innocua . testing of onions in a mimic system at the food pathogen processing lab at fapc was performed on indigenous organisms ( for comparison of processes ) as well as pathogen - inoculated onions using 4 - strain cocktail of listeria monocytogenes . data from the microbial recovery trials at different processing times were analyzed by one - way analysis of variance using the holm - sidak test for pairwise comparisons for significant difference in comparison with unprocessed onions . onions ( see fig5 ) were obtained locally ( sunset brand , white onions , # 4663 ) and were approximately 0 . 6 - 0 . 7 lbs / each ). they were held at room temperature prior to processing . a flame oven with continuously moving conveyor belt transported onions through the gas flame grill ( see fig4 & amp ; 6 ). a clean - in - place system was in place but not used . workers wore nitrile laboratory gloves to place onions on the grill ; other workers removed onions and placed them two - to - a - bag for microbial processing ( see fig9 & amp ; 11 ). one additional set of onions were rinsed by hose / water spray to remove the black ash prior to microbial sampling ( see fig1 ). a 50 - ml rinse was added to each bag of two onions , hand massaged for 3 - 5 min , and then approximately a 10 - ml sample was removed for microbial testing ; the sample was placed on ice and transported to fapc for microbial sampling later the same day . an additional set of onions were inoculated by rolling the onion in a white plastic tray lined with absorbant paper towels to which 60 mls of non - pathogenic listeria innocua ( 10 ml test tube of l . innocua + 50 ml 0 . 1 % bpw diluent ) was added ; this was replaced with a fresh inoculum solution after processing 4 onions ( see fig1 ). the inoculated onions were placed on the conveyor belt and processed in a similar fashion as the prior onions . the equipment was sanitized by a hypochlorite bleach solution . the flame grill does an effective job at ‘ burning off ’ the outer paper - like layer of the onions without damaging the onion ‘ meaty ’ layers ( see fig7 - 9 ). this will very likely reduce waste and increase yield of the process compared to mechanical peelers that remove a considerable amount of the onion “ meat .” the process achieved a nearly 5 - log reduction of indigenous bacterial contamination , 4 - log reduction of yeast and mold ( below level of detection after processing ), and a 6 - log reduction of inoculated listeria innocua ( see fig1 ). although some residual contamination was obtained after heating . this could be due to non - aseptic handling by gloved workers and by the ‘ hairy root ’ region of the onion which may be more difficult to eliminate indigenous / inoculated organisms .
a system and method for pasteurizing the surface of a food product includes a flame pasteurizing means arranged to transfer heat to an outer layer of the food product as it is carried by a cook belt . the flame pasteurizing means provides a medium that engulfs more than 50 % of the surface area of the skin or outer layer and raises the temperature of the outer layer to at least 145 ° f . . a washing means , arranged at an exit point of the flame pasteurizing means , removes the raised temperature outer layer and stops heat transfer to an adjacent inner layer of the food product . the pasteurized and washed product retains the same color and flavor as that of the unpasteurized product .