Use of microbes in the production of cheeses


            Microbes are commonly thought of as dangerous pathogens, especially in the food service industry, however many microbes contribute to the preparation or flavor of popular consumables.  One of these products is cheese.  In 2012, americans consumed more than 33 pounds of cheese per capita (“USDA Economic Research Service - Dairy Data,” 2013) and its general enjoyment is due to the variety of bacteria that are added in the cheese making process.  Cheese has been produced for hundreds of years in many countries around the world and while modern techniques are being employed in its current production, the key to unique and flavorful cheeses continues to be in the selection and addition of bacteria.  Studying microbiology is important to continued production of excellent cheeses as it helps makers (and consumers) understand the science behind traditional cheese making and how modern biotechnology influences future production and food safety.
            Traditionally, cheese can be categorized by its production method; either acid coagulated, acid-heat coagulated, or rennet coagulated.  Rennet is a group of enzymes commonly sequestered from mammalian (bovine) stomach that hydrolyzes casein to an insoluble gel.  Rennet only works in acidified milk, so effectively all three types of cheese making requires the addition of acid which can be achieved by adding bacteria.  Different types of microorganisms create different types of cheese- each bacteria’s metabolic process creates a different flavor for the end product.  Bacteria that are commonly used are Lactococcus sp. and Lactobacillus sp., Streptococci sp. and famously, Propionic shermanii  for swiss cheese (the gas produced by the bacteria makes the holes). The bacteria’s fermentation causes the lactose in milk to turn into lactic acid.  The acidic environment into a solid curd while the liquid- the whey- is pushed out of the protein matrix while being gently heated.  Rennet addition at this step creates a rubbery texture.  Salt and flavoring can then be added and the curd is gathered and formed to the final shape.  Separation is crucial in the cheese making process, making microorganisms a key ingredient.  Certain strains will modulate the temperature, water retention and pH of the final product.  An accomplished cheese maker will know the usefulness of this natural ingredient and will select the right bacteria at the right proportion to make a unique and delicious cheese.
            Biotechnology has advanced the the cheese making process as well as clarify traditional techniques.  For example, sequencing the bacteria involved in traditional techniques is one of the most useful applications of molecular biology.  Cheeses from specific regions have signature flavor profiles.  Sometimes a known mixture of bacteria is used but in older, traditonal recipes, a single bacteria is knowningly added and additional bacteria is introduced in the process which adds to regional flavors.  Stilton cheese, for example, is known for its sharp flavor and dry texture, and is only produced in specific regions of Europe.  Lactococcus lactis, is added initially to milk but at the end of the aging process, more bacteria, including a non-pathogenic Staphylococcus, is present.  Sequencing the bacteria present will help identify the real cocktail of microbes  required for stilton and also predict the flavors of the end product. (Pearson, 2003)
            The usefulness of genetic engineering microbes has even penetrated the culinary world in the form of synthetic rennet.  Besides the bacteria directly added to milk in the early stages rennet, is used in many soft cheeses such as the popular mozzarella.  Traditional recipes call for the rennet, a group of proteolytic enzymes, to be collected from a mamalian (usually bovine, goat, or sheep) stomach.  To keep up with the high demand for cheese, some makers have opted for rennet, produced by genetically modified bacteria.  The term, rennet, is colloquial although still widely used.  When reffering to these enzymes but not of mamallian origin, they are often called chymosin.  Transformed bacteria are grown in large vats.  The bacteria secrete chymosin and the enzymes are collected, purified, and sold in liquid form to cheese producers.  Genetically modified bacteria are very efficient in rennet production and are a fraction of the cost to “farm” compared to housing animals to collect their stomach enzymes.  Also in the realm of alternate rennet sources, many home cheese makers will opt for the convenience of “vegetable” rennet tablets which is actually of bacterial origin, Mucor miehei, which naturally produces chymosin, no genetic modification required.
            Although bacteria is essential in the production of cheese, there are also pathogenic bacteria that are involved in causing foodborne illness. Staphylococcus aureus and Listeria sp. are common pathogens in dairy and dairy products.  In the United States there have been very few incidences of infection from cheese consumption- only six cases over 40 years- however most of these cases stemmed from faulty pasteurization process.  This makes eradicating bacteria from raw dairy crucial in preventing infections.  When pasteurization fails, biotechniology may have a hand in preventing foodborne illness. 
            The natural ability of bacteriophage to target and destroy specific bacteria can now be used to control pathogens.  A commercial product, Listex™ P100, is currently available to treat L. monocytogenes in dairy products.  Phage is specific to the bacteria they target; an introduction of phage to a mixed culture of bacteria will only affect the bacteria the phage is specific for.  Simillarly, additon of phage to cheese making process will eliminate the pathogens they are specific for- in this case, Listeria- while leaving crucial bacteria alone to characterize the cheese.  Furthermore, phage may still be active in the aging process, even after the “cooking” is completed.  This helps prevent contaminants from entering during the aging process. 
Research is currently being done to implement a phage against a strain of Staphylococcus aureus (Weiman, 2013) in dairy products.  This will have the same benefits as the current Listeria phage but the target will be to a common bacteria found on human skin.  Cheese making is a hands-on process that can introduce normal flora to the food product.  S. aureus, found commonly on skin, could be combatted by the introduction of phage without changing the bacterial cultures for changing the cheese flavors.  This is especially useful for cheese making because cheese are handled during the aging process after pasterurization.  S. aureus is more likely to be introduced at this stage than Listeria since Listeria  is not usually part of the normal skin flora.
            Cheese is a widely consumed food product whose origins reach back many years.  While the original production of cheeses may have been accidental, modern cheese making is a specific science.  The process uses specific bacteria species to control the outcome of the products.  This combination of new and old techniques yield a great variety of cheeses and perfection to the popular food.  The current direction of microbiology will enhance this process and make cheeses safer to eat.  Microbiology research is essential to the preservation of the art of cheese making.


Works Cited
Fankhauser, D. B. (2009). Rennewt. RENNET FOR MAKING CHEESE. Retrieved December 9, 2013, from http://biology.clc.uc.edu/fankhauser/cheese/rennet/rennet.html
Haddad, L. E., & Moineau, S. (2013). Characterization of a novel Panton-Valentine leukocidin-encoding staphylococcal phage and its natural PVL-lacking variant. Applied and Environmental Microbiology. doi:10.1128/AEM.03852-12
Kindstedt, P. S. (2013, September). The Making of Great Cheeses. Microbe, pp. 361–367.
Kindstedt, P. S. (2013, October). Making Great Cheeses, Part 2. Microbe, pp. 401–407.
Pearson, H. (2003). Sequencing reveals secrets of Stilton. Nature News. doi:10.1038/news030609-18
USDA Economic Research Service - Dairy Data. (2013). Retrieved December 9, 2013, from http://www.ers.usda.gov/data-products/dairy-data.aspx#.UqYnpbOhAy4

Weiman, S. (2013, September). Harnessing Microbes to Help Control Foodborne Pathogens. Microbe, 8(9), 344–345.

2 comments:

  1. cheese upon which I shall pass:
    http://www.theverge.com/2013/11/25/5142882/selfmade-cheese-from-human-microbes

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