This document summarizes two studies related to reducing microbial growth in fresh cheeses. The first study evaluated the effect of salt levels on microbial survival in mozzarella cheese over nine weeks of storage. It found that salt levels beyond 2% are needed to extend the shelf life. The second study examined using high-pressure processing to reduce Listeria monocytogenes in queso fresco cheese. It determined that 600 MPa at 20°C was effective at reducing surface contamination. The document compares and contrasts the two studies, and concludes that research into control factors like pH, water activity and hurdle technologies will continue improving the safety of dairy products.

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Microbial Survival in Fresh Cheeses Post-Pasteurization
Title and Contents………………………………………………………………………………... 1
Summary of Evaluation of Microbial Survival Post-Incidence on Fresh Mozzarella
Cheese………………………………………………………………………...….. 2-3
Summary of Effect of High-Pressure Processing on Reduction of Listeria monocytogenes in
Packaged Queso Fresco (Phase One Only)…...……………...…………………. .4-6
Compare and Contrast ………………………………………………………………………… 6-7
Conclusion……………………………………………………………………………………….. 8
References…………………………………………………………………………….……… 9-14

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Summary of Evaluation of Microbial Survival Post-Incidence on Fresh Mozzarella Cheese
Mozzarella Cheese is one of the most popular cheeses in the United States. This cheese
has little to no flavor but mostly resembles the flavor of fresh milk. The two factors that control
microbial growth in Mozzarella Cheese are refrigerated storage and having the low pH of 5.4.
Because of these limited controls the shelf life of Mozzarella Cheese 21-30 days. However, the
cheese will start to develop an off flavor and a textural compromise due to proteolysis. This is
most likely due to the curd’s high moisture content, and lack of salt and lactic acid bacteria in
production of the cheese. Although the milk used for cheese production must be pasteurized for
15 seconds at 162 degrees Fahrenheit, some heat resistant pathogenic or spoilage bacteria may
remain. There has been a study conducted about the shelf life of Mozzarella cheese. However,
this study was deemed insufficient because the cheese was not tested for a long enough duration.
This study encompasses the effect of salt on Mozzarella cheese, and that the lack of such does
not inhibit unwanted bacteria- pathogenic or spoilage- from growing in the cheese throughout
storage duration.
The milk used for this procedure was procured from Utah State University’s Caine Dairy
Research and Teaching Center, and the milk was pasteurized. The manufacturing process was
altered to include direct acidification. Tetra Scherping horizontal cheese vats along with white
vinegar and Maxiren double-strength rennet was used to make the Mozzarella cheese. The
cultures used in this experiment were a non-pathogenic E. coli strain (12) and Enterococcus
faecalis (ATCC 47077). These cultures were selected because of their representation of both
gram negative and gram positive status. A suspension of this culture was prepared before
inoculation. Trials one and two were inoculated with the bacterial suspension before packaging

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and storage. The brine was also inoculated using thawed cultures that were prepared on the same
day that bringing took place.
After vat, the curd was cut, allowed heal for approximately 5 minutes, and then stirred.
After draining on the table, the mozzarella curd was dry stirred. At this step the curd was
separated into three different trial groups. The first group had no salt added before cooking, 3%
salt in cooking brine, cooked at 60 degrees Celsius, and packaged in 1% salted brine. The second
group had no salt added before cooking, 3% salt in cooking brine, cooked at 60 degrees Celsius,
and packaged dry with no salt. The third group had 1% salt added before cooking, 5% salt in
cooking brine, cooked at 60 degrees Celsius, and packaged dry without salt.
Microbial activity was tested at day one of storage as well as week two, four, six, nine
week increments. Samples were analyzed based on aerobic plate counts, coliform counts, and
psychotrophic counts. Bacteria were randomly selected and were tested in triplicate.
Success was found in the three different trials of Mozzarella cheese that was
manufactured. After nine weeks of storage there was no presence of coliforms and psychrophiles
in the cheese, leading researchers to believe that these two groups were not the problem. Aerobic
plate count did increase over the storage period, however. Both inoculants that were placed in the
brine both attached to the curd. This interprets that the salt was not enough to inhibit these
microorganisms from growing. Mozzarella cheese has a relatively high water activity when
compared to other cheeses of its kind. This leads to a higher bacterial microorganism presence in
the cheese. Therefore, this study proves that salt levels beyond two percent must be used in to
extend shelf life of Mozzarella cheese. The salt level alone, however, will not be enough, so
additional mechanisms, such as antimicrobials, are suggested for a greater shelf life extension.

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Effect of High-Pressure Processing on Reduction of Listeria monocytogenes in Packaged Queso
Fresco (Phase One Only)
Another popular fresh cheese is Queso Fresco. Queso Fresco is a Hispanic-style cheese
that is known for its white color, crumbly nature, salty, and non-melting characteristics. This
cheese also has a high pH and high moisture content, making it a perfect environment for
microorganisms to flourish. When microorganisms are allowed to flourish, the shelf life is
significantly reduced, and food illness risks increase. In fact, Queso Fresco has been subjected to
many recalls due to possible Listeria monocytogenes contamination. As far as sources have been
able to tell, this post pasteurization contamination is found on both the surface and the inside of
the cheese. This could be possible because some of the process involves handling the cheese by
employees.
High-Pressure Processing has been proven to be an effective method for inactivating
pathogens and other microorganisms at room temperature conditions in salsa, guacamole, meats,
and fruit juices. It inactivates the microorganisms by pressurizing the medium as well as the
sample and forcing through. This ensures that the sample is treated uniformly throughout the
entire sample body. This is a very economical system that has the ability to extend shelf life
without the addition of heat. Because of Queso Fresco’s already naturally occurring properties,
high-pressure processing is an ideal treatment for this cheese. Since high-pressure processing has
already been proven to be effective in cheese, the purpose of this experiment was to look at the
effects of high-pressure processing with temperature, hold time, and pressure parameters being
monitored for the total reduction of Listeria monocytogenes in and on slices of Queso Fresco that
has been packaged.

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Raw cow milk was procured from a local farm for use in this study. The milk was
standardized to 3.5% fat and then pasteurized and homogenized. This study was conducted in
five different trials. Starter cultures were not used and salt was added at three different additions
with a ten minute stir inbetween. This cheese was not pressed. This study was completed using a
High- Pressure processing unit entitled the Avure model 2L-700 high pressure food processor.
The inoculant in this study is Listeria moncytogenes. The curds were inoculated before molding
and slicing, or on the surface of the cheese slice. Non- inoculated samples served as the control
for this experiment. Before the study could be conducted the High-Pressure processing unit had
to be honed to the perfect temperature and pressure combination that would effectively reduce
microorganisms in the inoculated cheese. After several trials, it was determined that the trials
would be conducted at twenty and forty degrees Celsius and at pressures of two-hundred, fou-
hundred, and six-hundred Mega-Pascals. The trial that was conducted at twenty degrees Celsius
contained samples that were stored in a four degree Celsius ice bath before being high- pressure
processed. The trial that was conducted at forty degrees Celsius contained slice cheese samples
that were warmed in a water bath until the slices reached forty degrees Celsius which was
approximately two minutes. After the high-pressure processing was completed, the samples were
stored at four degrees Celsius until analysis that next day. It should be stated that the preliminary
trials of this technology showed that a combination of twenty degrees Celsius and six-hundred
Mega-pascals would show a decline in Listeria monocytogenes on the surface of the Queso
Fresco cheese.
Samples were tested after storage on day one, seven, twenty-eight, fifty-six, and eighty
four. In addition to reducing Listeria monocytogenes in Queso Fresco, High-pressure processing

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has also been effective at reducing yeasts and molds at three-hundred Mega- pascals for five
minutes at a temperature range of five to twenty-five degrees Celsius.
Compare and Contrast
Both of these studies were aimed at reducing microorganism activity within a fresh
cheese. Using critical control points found in the HAACP plans for these products is a great
starting point to begin to look for ways to improve the product. Some factors looked at to do this
included pH, water activity, moisture content, and salt content. All of these factors have been
proven to be effective at reducing microorganisms in cheese. The problem is that significantly
altering one or more of these characteristics can also significantly change the quality, texture,
and taste of the cheese. It could also change the melt ability, color, crumbliness, elasticity, and
aging properties of the cheese. Because of this researchers must be very careful that one or more
properties do not alter these characteristics of the cheese. Changing these characteristics could
lead to a deviation from standard of identity. This deviation could cause a breach of consumer
trust, and ultimately lead to revenue loss in this dairy sector. Although changing one or more of
these control points may ultimately lead to a safer product, consumers would be turned off by the
quality differences.
These studies are a great example of the advances the dairy industry is working toward to
make safer products for consumers. These advances in technology are being brought about by
different hurdle technologies. Combining more than one hurdle technology is the best way to
improve a product without changing its characteristics. Pasteurization is one hurdle technology
that has seen a lot of attention as of recent.

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One of the biggest differences in this study was the way in which the milk was
pasteurized. In the Mozzarella study, the milk was conventionally pasteurized using heat. In the
Queso Fresco study, the milk was pasteurized using a high-pressure processing unit. The study
was conducted using a High- Pressure processing unit entitled the Avure model 2L-700 high
pressure food processor. High-Pressure Processing has been proven to be an effective method for
inactivating pathogens and other microorganisms at room temperature conditions in salsa,
guacamole, meats, and fruit juices. It inactivates the microorganisms by pressurizing the medium
as well as the sample and forcing through. This ensures that the sample is treated uniformly
throughout the entire sample body. This is a very economical system that has the ability to
extend shelf life without the addition of heat. This is not a very popular system yet, but I predict
that in the future more companies will transition to this technology. I think that this because
high-pressure systems are more sustainable since they do not use water to generate steam. They
also, in my opinion, are more effective at reducing microorganisms in the sample.
Tried and true conventional pasteurization has been around since the nineteenth century,
and has proved to be very effective at keeping the food supply practically free of pathogenic
microorganisms. Because of heat application the milk does have a slightly cooked flavor that
experts can pick up, but consumers can rarely taste the difference. Since the invention of
pasteurization food illnesses due to raw dairy products has steeply declined. This is due to the
Federal Drug Administration mandating that pasteurization be required to sell in the marketplace
with few exceptions in areas. Some states allow for the sale of raw dairy products, but those
states also tend to have higher incidents of food illness from the raw dairy sector as well.
Pasteurization, however, is only one example of hurdle technology that is being used to improve
the safety of food products.

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Conclusion
As stated previously, these studies are a great example of the advances the dairy industry
is working toward to make safer products for consumers. I think that as time goes on there will
be more and more studies of this kind to continue to try to find ways to improve the quality and
safety of dairy products. Using critical control points to reduce microbial growth will continue to
be a trend in the dairy research field. Some of these control factors are pH, water activity,
moisture content, and salt content. Pasteurization technique is hurdle technology that is also
being heavily researched. I think in the future more companies will utilize the high-pressure
processing systems as they become more researched and developed. They are also very
expensive systems to put in place, so as they become more popular the price may decline,
making companies more willing to install them. As long as the quality of these products does not
deviate too much, consumers should not be shocked and respond well to the improved safety of
their dairy products.
Microbial reduction is a hot topic in the dairy industry, especially as food safety is
becoming a global concern. These studies are some of the many research projects going on.
Dairy technologists and researchers are working hard to reduce the risk of food contamination to
keep the food supply safe.
As more research is conducted, and more information is gathered about microorganisms
and their role in dairy products is further examined, the safety and quality of the food supply is
improved. As consumers start to demand safer products for their families, and federal regulations
become stricter and more enforced, the dairy industry will have to rise to the occasion and
exceed consumer and federal agency expectations.

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