Food Microbiology

Write a one to two page summary of the following learning objectives:

Understand:

the effects of reduced temperature on microbial growth

the typical reservoir for psychrotrophs

the relationship of cell membranes to psychrotrophy

the relationship of refrigeration temperatures to shelf life (e.g. milk)

refrigerated meat sliming

psychrotrophic pathogens

time to toxin formation of Clostridium botulinum in refrigerated reduced oxygen packaged foods

the potential for consumer food storage temperature abuse

the effects of freezing on microbial growth (including parasites)

Welcome to module 16. This section will review food protection using low temperatures, such as refrigeration or freezing.

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The effects of low temperatures on microorganisms are rooted in enzyme systems. Most metabolic reactions in microorganisms are enzyme catalyzed. The rate of enzyme catalyzed reactions is temperature dependent. As the temperature rises, there is an increase in enzyme reaction rate. Conversely, as the temperature lowers there is a reduction in enzyme reaction rate.

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Most microorganisms prefer ambient temperatures with optimum growth rates from 25-40oC. These are termed mesophiles. Psychrophiles are cold loving microorganisms able to grow from subzero to 200C. They usually have an optimum growth rate from 10-150C. Psychrotrophs are capable of growth at the same low temperatures, but their optimal growth rates are similar to mesophiles.

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From an evolutionary standpoint, many of these organisms probably evolved in large cold bodies of water (lakes and oceans). Over 90% (deeper waters) of the ocean remains at a near constant 0-4oC. The surface 10% ranges from 0-20oC depending on the surface weather. Larger freshwater lakes will have a similar temperature range to oceans in their lower 90%.

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At the cellular level psychrophiles and psychrotrophs have evolved enzyme systems that still function, albeit slowly, at low temperatures. Conversely, as psychrophiles and psychrotrophs adapt to lower temperatures, they tend not to function as well or tolerate higher temperatures. Most have very low thermal death temperatures. An exception is the thermoduric, psychrotrophic, lactic acid bacteria.

Psychrophiles and psychrotrophs also tend to have cellular membranes that remain more fluid at low temperatures. There are greater numbers of polyunsaturated fats in their lipid membranes. Imagine transporting water and nutrients through butter versus olive oil in the refrigerator.

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There are three distinct temperature ranges of low temperature storage of foods: frozen, refrigeration, and sub-ambient.

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There are many genera that have species that are psychrotrophic. These include Vibrio where some strains can grow at minus 5 ºC. Bacillus is another common genera with psychrotrophs. And, of course our favorite spoilage genera Pseudomonas has many psychrotrophic strains. Lastly, the majority of lactic acid bacteria are psychrotrophic, including Lactobacillus, Leuconostoc, Microbacterium, and Micrococcus.

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There is an important group of psychrotrophs related to milk called thermodurics. Whereas, most psychrotrophs are easily killed using heat, these genera can tolerate heat for short periods of time. They then can live on to slowly grow in the milk at refrigeration temperatures. This is why normal pasteurized fluid milk has a short refrigerated shelf life. The most common genera include Bacillus and Lactobacillus.

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Note some of the psychrotrophic growth rates in refrigerated milk. Remember that many mesophiles growing at their optimum temperature can have a doubling time of 0.33 hours. Note the differences between just 3ºC and 7ºC. What affect will that have on milk shelf life in the retail chain and in home refrigerators. Remember the generality that it takes 6 logs of a spoilage organisms to effect organoleptic qualities. How long will it take to spoil milk for these microbes?

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Many psychrotrophs overproduce polysaccharides such as dextrans and glucans. These are just collectively called slime. Note the slime formation on colonies of Leuconostoc at left and Xanthomonas at right. How might this fact affect refrigerated meats?

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There are several pathogens capable of psychrotrophic growth at or below refrigeration temperatures ≤5oC. Keep in mind that refrigeration is not always efficient and frequently rises above 5oC, especially consumer refrigerators.

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Listeria monocytogenes is the most cold loving pathogen. It can grow down to ‐ 0.4ºC. The graph depicts the change in growth rate based on incubation temperature. Note that the change is quite linear until it reaches is maximum and then quickly drops off. The generation time scale in not depicted, but it will require from 30‐40 hours for L. monocytogenes to double at 4.5 ºC in different foods.

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Foods destined for refrigerated storage must be protected from the potential growth of psychrotrophic pathogens along the entire food chain from manufacturer to consumer. With few exceptions packaging materials will limit the exposure to oxygen creating a potential hazard for psychrotrophic Clostridium botulinum. The FDA has extensively studied the time to toxin formation based on storage temperature. This is depicted in the figure. A food stored at 4°C would remain safe for 9 days. However a food stored at 3oC would have an indefinite shelf life.

To increase the shelf life manufacturers have to add barriers or hurdles to pathogen growth. A barrier will prevent growth and a hurdle will minimize growth. Several hurdles can become a barrier. For example, a pH of 5 or water activity of 0.95 prevents growth of psychrotrophic Clostridium botulinum. Once barriers and hurdles are created to one pathogen, the remaining must be assessed and addressed. Products formulated to these parameters would have much greater refrigerated shelf lives.

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Consumers are the last link in the farm-to-fork food chain. And by nature, they are a large variable. Each household operates a refrigerator. The USDA recommends home refrigerator temperature ≤ 40°F (≤ 4.5°C). However, many refrigerators are actually above that limit. Approximately 30% are above 5°C and 5% above 8°C. Only 20% use a thermometer to actually measure temperature. Knowing the potential for temperature abuse requires planning and adaptation by manufacturers. The best method is to build in barriers to pathogen growth. Sometimes spoilage flora is desired. If a product is temperature abused, then the food will spoil and more likely not be consumed.

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One newer development to address temperature abuse is time-temperature indicators. These are special chemical mixtures that provide a color reaction related to the exposure to warmer temperatures. So, if a food is placed at warm temperatures the indicator will change colors.

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The effects of freezing on most microorganisms are related to the eventual ice crystal formation. During freezing the majority of ice crystals form just at and below the freezing point. This is where approximately 0-10% of a microbial population will die off. Certain food components can act as cryoprotectants and can negate this die off. Once microorganisms are frozen, there is relatively little reduction in cell numbers.

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While growth is very important at refrigeration temperatures, survival is important as well. Since low temperatures do not have a bacteriocidal effect on microorganisms their presence in foods will be maintained. Note that cell death during frozen storage in the data above is negligible to no more than 1 log.

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The higher order parasites (nematodes) are an exception and many cannot survive freezing. Freezing to an internal temperature of -20°C for at least 24 hours will kill nematodes in seafood. Trichinae (the nematode that causes trichinosis) can be destroyed by freezing for -17.8°C (0°F) for 106 hours, -20.6°C (-5° F) for 82 hours, or -23.3°C (-10° F) for 63 hours. Various other standards and guidelines exist for other known parasites. For example the Food Code lists freezing times and temperatures for seafood that are destined to be eaten raw.

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