Ground beef is the most popular beef product consumed in the United States. While E. coli O157:H7 in ground beef has decreased significantly according to government data, there is still work to be done to prevent foodborne illness. The U.S. Department of Agriculture recommends that ground beef patties should be cooked to an internal temperature of 160°F, however consumers tend to rely more on the color and texture of cooked meat as a means of determining doneness. Unfortunately, premature browning in ground beef can lead to inadequate cooking by consumers, which can in turn lead to the survival of E. coli O157:H7. When compared to steaks and roasts, ground beef is especially problematic because of the accelerated oxidation of myoglobin that occurs as a result of grinding. The grinding process can also lead to a greater distribution of pathogenic bacteria. The infectious dose of E. coli O157:H7 is very low—two to 2,000 cells—emphasizing the importance of properly cooking ground beef. An antimicrobial hurdle applied at the end-user level would be prudent to inactivate E. coli O157:H7 and prevent foodborne illness.
Caprylic acid is a fatty acid present in breast milk, bovine milk and coconut oil and is a food- grade chemical approved by the Food and Drug Administration as “generally recognized as safe” (GRAS). Monocaprylin is a form of caprylic acid. Monoglycerides such as monocaprylin have been hypothesized to act as non-ionic surfactants, which penetrate and get incorporated into bacterial plasma membranes, thereby altering membrane permeability thus increasing their sensitivity to heat. Applying caprylic acid to ground beef might help ensure that consumers cook it to the appropriate level of doneness.
Fresh, coarse ground beef (90 percent lean and 10 percent fat) was purchased from a local meat supplier. The beef was fine ground and formed into patties (50 grams each). Each patty was inoculated with 500 µl of a five-strain mixture of E. coli O157:H7 to obtain a final concentration of 6 log10 CFU/g. The patties were randomly assigned into one of three treatments: 1) Control (0 mM monocaprylin), 2) 20 mM monocaprylin and 3) 25 mM monocaprylin. The patties were placed on foam trays, overwrapped with fresh meat film and stored at 4°C for five days. A batch of treatment and control patties were also frozen at -20°C and stored for 21 days. After this period, the patties were cooked directly from the frozen state.
Patties were cooked on an indoor electric grill to one of three internal temperatures (60°C, 65°C or 68°C). Upon reaching the desired internal temperature, patties were removed from the grill, chilled and homogenized. Samples were surface plated and plate counts of surviving E. coli O157:H7 were determined after incubation at 37°C for 24 to 48 hours.
Ground beef patties were also analyzed for pH, and a* color values (redness). Evaluations for sensory characteristics were conducted by an untrained panel of 25 individuals.
At 60°C, E. coli O157:H7 counts in control patties were decreased by 0.6 log colony forming units per gram (CFU/g), whereas significant reductions of 1.4 log CFU/g and 1.9 log CFU/g were achieved in patties containing 20 mM and 25 mM monocaprylin, respectively.
On the first day of storage, cooking patties to 65°C reduced the pathogen load by approximately 2.0, 4.0 and 4.1 log CFU/g in patties containing 0, 20 mM and 25 mM monocaprylin, respectively. Further, at 65°C E. coli O157:H7 was completely inactivated in the samples containing 25 mM monocaprylin.
Cooking the patties to 68°C reduced E. coli O157:H7 populations by 4.8 log CFU/g in control samples, whereas complete inactivation of the pathogen was observed at this temperature in patties treated with 25mM monocaprylin. A similar trend was observed in frozen patties, where monocaprylin significantly reduced the population of E. coli O157:H7 in comparison to that in control patties.
Results indicated monocaprylin significantly increased the heat sensitivity of E. coli O157:H7 at all three of the cooking temperatures, thereby killing the pathogen in ground beef. The increased heat sensitivity of E. coli O157:H7 in the presence of monocaprylin could be attributed to its effect on bacterial cell membrane. Monoglycerides such as monocaprylin have been hypothesized to act as non-ionic surfactants, that penetrate and get incorporated into bacterial plasma membranes, thereby altering membrane permeability.
The results from this study showed that the addition of monocaprylin killed substantial numbers of E. coli O157:H7 in undercooked patties, compared to patties containing no monocaprylin.
Moreover, it was found that the addition of monocaprylin did not adversely affect the color of ground beef compared, which would be an important attribute if this technology is adopted in commercial applications.