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United States Department of Agriculture

Agricultural Research Service


Location: Dairy and Functional Foods

2010 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.

3.Progress Report

The ADODR used telephone and e-mail communications to coordinate research and monitor research progress among all investigators, and used quarterly reports to keep DMI apprised of research progress.

Queso fresco (QF) is a high moisture, fresh, Hispanic-style cheese with a high pH, characteristics that can provide ideal conditions for outgrowth of pathogens. During the FY, multiple batches of QF were manufactured in the cheese pilot plant from pasteurized milk, without starter cultures, using industry protocol. Slices of the cheese were inoculated on both sides with a three or five-strain cocktail of Listeria monocytogenes (Lm) to test the effects of process modifications and/or the addition of post-processing food grade chemical or biological interventions, in controlling the growth of Lm. The slices were stored at either 4 degrees C, representative of warehouse storage conditions, or 10 degrees C, representative of retail dairy case or home refrigerator conditions. Lm had very little sensitivity to the biological interventions - a slight initial lethality (<0.5 log reduction) and a 5-d lag period in the growth of Lm was shown. Although a slight initial lethality was shown, the chemical interventions did not prevent growth of Lm at 4 degrees C. Process modifications included determining if fine-milling of the curd affected Lm growth or attachment to the curd. Fine-milling did not appear to affect the growth of Lm compared to a control.

The growth of Lm and two other pathogens, inoculated at levels of 3.0 to 4.0 log per gram on low-salt/reduced sodium Cheddar cheese, is being monitored for up to 3 months at 6 and 12 degrees C to determine if reducing the sodium content of Cheddar cheese supports pathogen growth. This study is ongoing.

The results of these studies will help cheese manufacturers meet regulatory guidelines, expand our knowledge regarding the growth of Lm in QF and other high moisture cheeses as well as in low-salt/reduced sodium Cheddar cheeses, and lead to better methods to control pathogen outgrowth.

Last Modified: 8/1/2015
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