Understanding Biofilms
In plain terms, a biofilm is a result of the adherence of microorganisms to a surface. More specifically, a biofilm occurs when bacteria and/or yeast begin to build up inside or on equipment. Microorganisms secrete a sticky material, known as exopolysaccharide, which is responsible for the gooey film that makes biofilms so hard to remove. As the biofilm enters phase 3, shown below, it becomes more likely that microorganisms from the original biofilm may slough off and contaminate your product or another area of the equipment. Biofilms can also cause reduced flow of liquids, corrosion of equipment, cleaning chemical resistance issues and more. Also note, a recent Harvard University study showed that biofilms may increase in size by five fold in 24 hours making prevention through cleaning and sanitation essential to keeping any problems at bay. Given the many adverse effects of biofilms, it is important to know the warning signs that your plant may have a biofilm problem.
The stages of biofilm formation are outlined below.
How to identify and isolate biofilms
Biofilms can contain lactobacillus, streptococcus or other damaging microorganisms which can include gas producing bacteria, a major concern for cheesemakers. In order to maintain a quality product and save your equipment from the issues outlined above, it is important to
know the signs of biofilm formation.
Several “red flags” are listed below:
- High numbers of bacteria in pasteurized milk may
indicate:- High initial raw milk numbers
- Biofilm formation in regen section of pasteurizer, piping and gaskets
- If the standard plate count enumerated at 32°C, after pasteurization, is greater than 200 cfu/ml milk or if Streptococcus sp counts enumerated at 42°C are greater than100 cfu/ml, you may have a biofilm problem.
- Lactobacillus counts after pasteurization should be less than 10 cfu/ml on Rogosa SL agar at 32°C. If they are higher, you may have a biofilm.
- If you see a glistening surface or film on your equipment, test that area immediately for biofilm contamination.
In order to test for a biofilm, you can use a simple swab or you can use one of the many sampling devices on the market.
You have a biofilm, so where did it come from?
Biofilms are the natural state of microorganisms in the environment, so it can be difficult to pinpoint the source of your biofilm and eliminate it once you have a problem.While organisms can stem from biofilms on manufacturing equipment, biofilms that start on the farm can be found on bedding material or milking equipment.
Research conducted by Pam Ruegg, who is now the David J. Ellis Chair in Antimicrobial Resistance at the College of Veterinary Medicine at Michigan State University, has focused on biofilm formation on the farm and specifically the microbial quality of pre-harvest milk. Her research shows that cows may be exposed to manymore pathogens on the farm than previously thought and
this may be affecting the type of bacteria causing mastitis.
This is important because mastitis caused by Staphlococcus
aureus for example, may form biofilms. Further research
shows, however, that these bacteria shed in low numbers
and that pasteurization will kill them. Ruegg also
investigated coliforms on the farm, which is a more serious
issue. This can come from biofilm build up in the milking
equipment, plate coolers or non-hygienic milking practices,
but again it can be hard to trace. More research is needed to
identify all of the possible factors and determine which are
of consequence. In particular, Ruegg would like to look at
the biofilms in dried manure for bedding in relation to the
increased use of digesters on the farm.
How to reduce a biofilm:
The number one way to reduce a biofilm is to use proper
cleaning and sanitation practices. This includes cleaning
and sanitizing at 8-10 hour intervals, using heat, alkaline,
enzyme cleaners or molecular brushes. Keep in mind that
Many cheeses are generally considered to be an inherently safe food: if they have reduced moisture, a significant salt content, a higher acidity and contain an active starter culture. These characteristics help keep pathogens from growing. The dairy industry is also highly regulated with strict food safety and environmental controls in place. Considering the high volume of cheese that is produced across the globe, there are not many outbreaks or food safety issues with cheese. Therefore, some cheeses, produced under good hygienic and manufacturing process, may be safely held outside of temperature control if they meet certain requirements. So, how do you determine if a cheese qualifies?
Two measurements help us determine if a cheese can be safely held outside of temperature control: water activity and pH. Water activity (aw) is the measurement of free or unbound water available in a food for microbial growth. It utilizes a scale of 0 to 1.0 where pure water equals 1.0.
A cheese with a high water activity (i.e 0.99 aw) has a lot of water available to be used by microorganisms and therefore is at a greater risk for growth of bacteria, yeast and molds. Conversely, a cheese with a low water activity (0.92 aw or less) would preclude the growth of some types of microorganisms yet allow the growth of the more tolerant. In short, the higher the water activity, the more potential there is for biological growth to occur.
Different styles of cheese have different water activity values. For instance, Cottage cheese, which has a high moisture content, is typically around 0.99 aw, while Parmesan, an aged cheese with a lower moisture content, can have a water activity as low as 0.67 aw. Based on this data, we know that Cottage cheese has a much higher risk of microbiological growth than Parmesan because more water is available for microorganisms to grow.
The other important factor to consider when evaluating whether a cheese can be held out of temperature control is the pH of the cheese. Generally, the lower the pH
(higher acidity) of the cheese, the more likely it will qualify to be held out of refrigeration. A high-acid environment is more hostile to pathogens. Cheese pH is achieved using an active starter culture that produces lactic acid and brings down the pH of the cheese. The use of an active starter culture is a requirement for cheese to be safely held out of temperature control.
Thus, the interaction of low pH and low water activity can be used to determine if a cheese can be safely held out of refrigeration. The Food and Drug Administration (FDA) includes a table (see Figure 1) in the Food Code that lays out the parameters of when a food product can be safely held out of refrigeration. Essentially, those cheeses with a lower water activity and a lower pH are more likely to be a non-potentially hazardous food (non-PHF) and thus don’t require temperature control.
A couple of important caveats should be mentioned before we go into determining what cheeses are safe to hold outside of refrigeration.
1. As mentioned at the beginning of the article, these guidelines are for cheeses that are assumed to have been produced safely (GMP, food safety plan, heat treated milk).
2. This applies only to cheese that has been uncut, has no cracks, breaks or punctures and/or retains the original, undisturbed wax or cloth packaging.
3. The maximum storage/display temperature can’t exceed 86°F (30°C).
4. Finally, storing a cheese out of temperature control is not ideal for the quality of the cheese.
With that said, let’s use the Interaction Table (Figure1) to determine if a cheese can be safely held out of refrigeration. Let’s say that we have a Parmesan with a water activity in the range of 0.68 - 0.76 aw and a pH of 6.5. The cheese milk was heat treated and the cheese was cured for 2-3 years and then packaged. Can this cheese
be safely held out of refrigeration? First, we would locate the cheese’s water activity (0.68 - 0.76 aw) in the correct line and then locate the pH (6.5) in the correct column. You will see that they intersect at “Non-PHF/Non-TCS,” which tells us that no temperature control is required.
Additionally, there is a list of cheeses that, when produced to the Code of Federal Regulations (CFR) standards, should be considered exempt from refrigeration requirements during aging, storing, shipping and display (maximum temperature of 86°F). The list includes:
- Asiago (medium and old)
- Aged Cheddar
- Colby
- Feta
- Monterey Jack
- Muenster
- Parmesan
- Pasteurized Process Cheese
- Provolone
- Romano
- Swiss/Emmentaler
In Wisconsin there are also special considerations for cheese curds. With the popularity and prevalence of cheese curds in the state, a special dairy inspection policy was drafted. The policy states that cheese curds must be refrigerated at all times with one exception. Cheese curds can be offered for sale outside of refrigeration only on the day they are manufactured. Any cheese curds that are not sold must be disposed of at the end of the day.
As mentioned at the beginning of the article, cheese is an inherently safe product. Cheese is a fermented product and fermentation is an age-old preservation method that is antagonistic to foodborne pathogens and will inhibit their growth or inactivate them. Research has also shown that active starter cultures are detrimental to pathogens. Given these characteristics of cheese, some cheeses can be safely held out of temperature control.
This article originally appeared in the Summer 2019 issue of the Dairy Pipeline.