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United States Department of Agriculture

Agricultural Research Service


Location: Dairy and Functional Foods Research

2009 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.

3. Progress Report
Queso fresco is a high moisture, fresh, Hispanic-style cheese with a high pH, characteristics that can provide ideal conditions for outgrowth of pathogens. The first phase of this study characterized the growth of Listeria monocytogenes (Lm) that was inoculated in and on Queso fresco cheese slices, manufactured in the DPPRU cheesemaking facility using industry protocol, and stored under typical refrigeration conditions of 4°C in the MFSRU Challenge facility. Also, the chemical, functional, and textural properties of pathogen-free Queso fresco were measured after 1, 4, and 8 weeks of storage at 4°C to monitor changes as the cheese aged. The data from these experiments will be used as baselines for the rest of the study to ensure that new methods to control Lm growth will not affect cheese quality. The results of this study will help cheese manufacturers meet regulatory guidelines, expand our knowledge regarding the growth of LM in Queso fresco and other high moisture cheeses, and lead to better methods to control pathogen outgrowth. The ADODR used telephone and e-mail communications to coordinate and monitor research progress among all investigators, and used quarterly reports to keep the collaborator apprised of research progress.

4. Accomplishments

Last Modified: 2/23/2016
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