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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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