Improving the Safety of Hispanic-Style Fresh Cheeses

Non-Technical Summary:<br/>
Most cheese styles have not been associated with foodborne illness in the U.S. However, Hispanic-style fresh cheeses present a unique problem as evidenced by their numerous Listeria monocytogenes outbreaks. Favorable growth conditions in Hispanic-style fresh cheeses like queso fresco, including high moisture, low salt content, and near neutral pH, along with the ability of L. monocytogenes to grow under refrigeration, are primary factors contributing to increased L. monocytogenes-associated food safety risk. To this date, there are few L. monocytogenes control measures during the manufacturing of these types of cheese; as a consequence the contamination of cheese with L. monocytogenes is likely to affect consumers. As U.S. population demographics shift, the demand for these products has risen. Outbreaks of listeriosis from illicit production of queso fresco make headlines and highlight the growing market for these products, but this has been met with hesitance to meet market demands by many manufacturers due to liability concerns. Strategies to make Hispanic-style fresh cheeses safer are needed to help meet market demand and prevent Listeria outbreaks.
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Approach:<br/>
We have already started development of a laboratory-scale fresh cheese model for testing small quantities of antimicrobial compounds. To minimize the amounts of potentially expensive or difficult to obtain compounds, as well as for rapid screening of multiple test conditions and manageability in a laboratory environment, cheeses of less than 1 g can be prepared to closely resemble conventionally produced cheese. This smaller-scale production has been previously shown to have little impact on the quality and microstructure of cheese. We will investigate the bacteriostatic and bactericidal activity of various bacteriocins, organic acids and novel food ingredients against Listeria monocytogenes, in vitro and using the model cheese developed in our lab. The efficacy of antimicrobial compounds can be compared in vitro by testing their ability to inhibit or kill target organisms. The smallest amount of a compound needed to prevent the growth of an organism, its minimum inhibitory concentration (MIC), can differ from the amount required to kill the organism, its minimum bactericidal concentration (MBC), to varying degrees depending on the compound's mode of action. Bacteriostatic inhibition by antimicrobial compounds in a food may prevent spoilage or pathogen proliferation, but still-viable pathogens may remain infective. If an antimicrobial is not just bacteriostatic, but is bactericidal and kills the pathogens, the product would be safer, so the MIC and MBC both provide important measures of a compound's efficacy, particularly when its stability in a food matrix may be in question. The antimicrobial must be maintained at its MIC to inhibit an organism, but may be consumed during pathogen death and preserve its efficacy. Bacteriostatic and bactericidal effect varies by both antimicrobial compound and its target organism. As such, it is important to compare the effects of different compounds on the same target organisms. Fortunately, Listeria innocua is a well characterized non-pathogenic model organism for Listeria monocytogenes. Our laboratory has already validated the inhibitory effects of a few bacteriocins on L. innocua in comparison to L. monocytogenes and found their resistance to be very similar. We plan to carry out preliminary antimicrobial analyses in vitro using L. innocua, followed by validation with the most effective compounds on L. monocytogenes strains. Screening numerous antimicrobials in vitro via MIC and MBC testing in microtiter plates will be used to identify the compounds most promising for use within our cheese model, which will follow.