2012 Annual Report
1a.Objectives (from AD-416):
1) Identify the specific processing step(s) in the manufacture of Queso fresco that may favor survival/outgrowth of target pathogens and/or spoilage microbes;.
2)Develop and validate interventions, used either alone or in combination with other processes, at a specific processing step and/or post processing, to enhance shelf-life of Queso fresco cheese; and (3) Test the efficacy of high pressure processing in combination with other interventions as a post-packaging step to reduce or eliminate pathogens such as LM from cheese.
1b.Approach (from AD-416):
Listeria monocytogenes (LM) will be added at various steps of the cheese making process to examine the effects of pH through addition of food grade acids, temperature adjustment, starter cultures, adjunct cultures, and processing technique on reduction or elimination of the pathogen through processing. Queso fresco will be manufactured according to a generic protocol. Initially, we will monitor the fate of LM during manufacture and storage of Queso Fresco using the general protocol. It is anticipated that the pathogen will be added to the raw milk, to the curd at the cutting, salting, and milling stages, and/or onto the surface of the finished product prior to vacuum packaging to mimic likely points of contamination in the industry. LM viability, pH, proximate composition of cheese and levels of indigenous lactic acid bacteria will also be monitored. We will investigate the effects of process adjustments in the cheese making process and also evaluate the feasibility of using high pressure processing (HPP), considered a non thermal process, at pressures in the range from 200 to 800 MPa, at temperatures from 10 to 30 degrees C, and for holding times ranging from 1 to 20 min, in the manufacture of the cheese as a step following packaging of the cheese. The fate of LM will be monitored throughout shelf life for a minimum of 30 d at both refrigeration (4 degrees C) and abuse temperatures (10 degrees C). At least two trials will be conducted for each treatment and three samples will be ananlyzed at each sampling interval. We will also expand the study as appropriate to monitor and/or to evaluate other pathogens.
The fate of LM will also be determined on low-salt Cheddar Cheese of various salt contents for comparison to Queso Fresco and the study expanded to monitor growth of other pathogens.
Improving the quality and safety of Queso Fresco cheese. Queso Fresco is one of the most popular of the Hispanic-style cheeses in the U.S. but, because of its high moisture content and pH, has been implicated as the source of several food borne illness outbreaks. A high-pressure post-packaging treatment was developed to determine if Listeria monocytogenes, a bacterium implicated in most of these illnesses, would be killed while preserving the expected quality traits of the cheese. Listeria cultures were added prior to treatment at various pressures, temperatures, and lengths of time, and combinations were found that initially killed the bacteria. QF were then treated at the optimum conditions and aged to determine long term effects. Listeria proved to be very resilient to high pressure and enough bacteria survived the treatment and recovered after a 7-28 day lag time to begin growing again. In parallel studies conducted with pathogen-free QF, the extreme conditions negatively affected the quality of the cheese; most noted was the accumulation of water, cheese whey, in the packaging and increased hardness of the cheese; flavor and mouth feel was different but not objectionable. High pressure processing is a very promising deterrent to pathogen growth but needs to be paired with other interventions to ensure the prolonged safety of the cheese. Results from this study help cheese manufacturers develop new options to make safe Queso Fresco cheeses.
Pathogen growth on reduced-salt, low-fat Cheddar cheese. Health conscious American consumers are demanding lower salt levels in all foods, including a favorite in the U.S., Cheddar cheese. Because salt is critical in limiting the growth of bacteria in cheese, it is essential to determine if altering the salt level will alter the growth of bacteria responsible for food borne illnesses. Low-fat Cheddar cheese made with four different levels of salt by collaborators at the Western Dairy Center, Utah State University, Logan, Utah, were received at the ARS Center for Excellence for Pathogen Validation, Wyndmoor, Pennsylvania, and exposed to three pathogens most often implicated in food borne illness from dairy products. After aging the cheese for six months, cell counts of two of the three pathogens (E. coli O157:H7 and Staph aureus) had decreased significantly in cheeses at all salt levels, while the third pathogen (Listeria monocytogenes) showed reduced numbers only at the two higher salt levels. The results from this study confirm that salt level in Cheddar will be a significant factor in maintaining the safety and quality of the cheese and provide guidance in developing safe, low-fat, low-salt cheese that meets the demands of the health conscious American consumer.