the relationship of air to microbial growth
Write a one page summary of the following learning objectives:
Understand:
- the relationship of air to microbial growth
- vacuum packaging and OTR and their effects on microbial growth
- MAP (N2, CO, CO2) and its effects on microbial growth
- the antimicrobial relationship of CO2 vs H2CO3 based on food pH
- the three main refrigerated MAP, CAP, VP pathogens
1/31/2012 10:44 AM
© 2007 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries. The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION. 1
Let’s review the relationship of gases to microbes. The first test tube (A) represents growth of obligate aerobic microbes. These will only grow in the presence of oxygen. The next tube is “b”, the obligate anaerobes. These will only grow in the complete absence of oxygen. Test tube “c” represents facultative anaerobes. They usually will respire using oxygen when present, but will change to anaerobic respiration or fermentation when oxygen is absent. The last test tube “d” are aerotolerant anaerobes. They will grow anaerobically when oxygen is present or absent.
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Air is a mixture of 78% nitrogen and 21% oxygen. If we allow normal air to be present above the media, which of these will grow? (answer – a, c, and d).
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If we remove all gases (air included) from the intrinsic and extrinsic environment, which of these will grow? (answer – b, c, and d).
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Which test tube does not have catalase OR superoxide dismutase? (answer b).
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Vacuum sealing is used to remove air, including oxygen, from a food sample. http://www.youtube.com/watch?v=CQ43cYH5wys&feature=related
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The keys to successful vacuum packaging are the ability to create the vacuum or the strength of the vacuum pump and the ability of the packaging to maintain that vacuum. Basically commercial vacuum sealer products have more efficient pumps that remove greater levels of air. The home units do not do so well. Likewise, home plastic packaging does not prevent t transmission of oxygen through it. The key number to measure bags is the OTR or oxygen transfer rate. If under 10 ml of oxygen pass through a square meter in 24h, then that is considered an excellent OTR. Some of the metalicized bags such as the brand name Mylar have OTR rates in the one‐ hundreths of a ml.
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Vacuum sealing is used to remove air, including oxygen, from a food sample. The resulting reduced oxygen atmosphere prevents growth of obligately aerobic molds. It prevents growth of obligately aerobic bacteria. It will slow growth of facultative microorganisms. Pseudomonas is one of the most common spoilage microorganisms. Most of this genus are obligately aerobic and many are psychrotrophic. Most pathogens, like Salmonella are facultative. Salmonella will not grow at refrigeration temperature. Clostridium botulinum is obligately anaerobic and will also not grow at refrigeration temperature. What would happen to the microbial biota of a vacuum packaged chicken breast held at refrigeration temperature? What would happen if that package were temperature abused and allowed to warm? What would happen if that package were opened, allowing air in, then temperature abused and allowed to warm?
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Vacuum packaging has two obvious drawbacks. The first is that delicate products will be crushed. Check out what happens to the soft bread loaf during vacuum packaging. The second is that removing all oxygen will prevent any reactions with oxygen that might be needed. An example is in red meat color. Oxygen is needed to react with the heme proteins to change fresh meat from a bluish color to red. Vacuum packaged red meats are not appealing to consumers because they are expecting the true red color.
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Modified atmosphere packaging is a variation of vacuum packaging. The main difference is that after removing air, it is replaced with a mixture of gases that are most effective in preserving the food in question. There are three main gases used: nitrogen, oxygen, and carbon dioxide. We’ve discussed what the presence or absence of oxygen can do for a microbial flora. And, we discussed the need for oxygen in some foods like red meats. Nitrogen is fairly inert and does not possess any antimicrobial properties. That leaves carbon dioxide. Carbon dioxide dissolves readily in COLD water and forms carbonic acid (H2CO3) which is antimicrobial. Carbon monoxide is used in MAP for meats, since it can use its oxygen in the myoglobin red color reaction. However, carbon monoxide has relatively no antimicrobial properties.
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Controlled atmosphere packing or storage implies the addition or removal of gases from storage rooms, transportation containers or packages in order to manipulate the levels of gases such as oxygen, carbon dioxide, and nitrogen to achieve an atmospheric composition different to that of normal air around the food. In the case of controlled atmosphere storage, the gas composition around the product is continuously monitored and controlled, whereas in modified atmosphere packaging it is modified just once.
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In this slide you can see the effects of the different gases on microorganisms in a modified atmosphere packaged pork product. Note that the numbers of spoilage organisms quickly rises in both air and nitrogen packaged pork. A general rule is that when spoilage organisms reach 6 logs the food is most likely organoleptically spoiled. Thus the product would be spoiled in 6‐8 days. When packaging this same pork product in carbon dioxide it takes more than a month to reach 6 logs. This is a financial no‐brainer for the food industry.
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Carbon dioxide dissolves readily in COLD water and forms carbonic acid (H2CO3). Carbonic acid is antimicrobial. It is soluble in lipids and can affect the membrane of susceptible microbes. It will also enter the cell and result in lowering the internal pH. Lastly, it can interfere with some enzymatic reactions resulting is slow growth or no growth. Carbonic acid does not usually get to a level to be lethal. The antimicrobial effect of carbon dioxide occurs at or near a 10% level, and increases with higher concentrations. Using 20‐30% carbon dioxide is the most optimum usage levels. Higher levels are not usually ore beneficial and when too high there is a concern for growth of the anaerobe Clostridium botulinum.
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Here is a chart on the sensitivity to CO2 for some common microorganisms. Note how Pseudomonas, one of the most common spoilage microorganisms, is the most sensitive. The Enterobacteriaceae are not very sensitive to CO2. What implications does that fact have on Salmonella and E. coli pathogens? Lactic acid bacteria are also somewhat resistant to the effects of CO2. These are the spoilage organisms that will finally spoil foods after a longer shelf life resulting in a cleaner lactic acid spoilage. Finally, Clostridium is very resistant to the effects of CO2 and in fact CO2 may promotes it growth.
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The usage of MAP or CAP requires careful considerations of product safety, spoilage, and quality. Each product needs to b optimized for the appropriate gas mixture. For example, Baked products may have high CO2 levels to minimize molds and a balance of nitrogen. Strawberries and lettuce have mostly nitrogen, and only a small proportion of carbon dioxide. Since they are still respiring they also have a small proportion of oxygen.
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Red meats on the other hand have high oxygen levels to promote color and have 20% carbon dioxide to minimize spoilage. Poultry uses a very high level of carbon dioxide as an antimicrobial and no oxygen. Part of the higher level of CO2 is the more neutral pH of poultry meat. Lastly, fish is similar to poultry using high levels of carbon dioxide as an antimicrobial and no oxygen. Note that the temperatures of storage of these meats and seafood are quite low. In seafood this temperature MUST be below 3°C to prevent the growth of the anaerobic and psychrotrophic C. botulinum typically found in seafood. Let’s look at Clostridium botulinum a little closer.
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There are three pathogens of greatest concern when storing foods in modified atmospheres: Clostridium botulinum, Clostridium perfringens, and Listeria monocytogenes. Both Clostridia are anaerobes and are a concern due to the reduced level of oxygen in the atmosphere around the food. Listeria monocytogenes is a microaerophile and prefers a slightly reduced oxygen level for growth. It is also somewhat resistant to the antimicrobial effects of carbon dioxide. Another concern is that spoilage bacteria normally out‐compete all of these pathogens for growth. As spoilage bacteria are minimized, so too is that competitive inhibition. Food processors must use careful planning to provide control measures that prevent these pathogens from growing. An example was seen earlier where MAP packaged seafood was held at below 3ºC to prevent growth of Clostridia. However, it would take storage at below 1ºC to prevent the growth of Listeria monocytogenes.
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