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3. Growth Characteristics
| Temperature | Listeria prefer temperatures between 300C and 470C but grow in the range 1-450C. | ||
| Note: Listeria will grow at normal refrigeration temperatures, with numbers increasing | |||
| pH | Listeria prefer to grow at pH 7-8 but they will grow in the range pH 5-10. Recent studies have shown that Listeria may survive and grow in material with a pH as low as 4.4 | ||
| However, it seems that the type of acid used to
acidify the material and the storage temperature have a marked effect on the
ability of Listeria to survive and grow at low pH. (Week organic acids, such as citric, acetic, and lactic acids seem to have antibacterial properties which are related not only to pH but also to their degree of dissociation, with the undissociated form being mist inhibitory. When determining conditions that would best inhibit Listeria it is important to consider not only pH but also the type and concentration of acidulant used). |
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| Salt | Listeria can survive in high salt concentrations ... up to 20% under ideal conditions (neutral pH, low temperature). Even at a low pH the ability of Listeria to survive in the presence of salt is significant. For example, Listeria may survive a combination of a pH of 4.2 and a salt concentration of 18%. However, the ability of Listeria to grow under these conditions is restricted. | ||
| Survival | Listeria can survive in wet conditions for many months ... and up to 2 years in dry soil or dust. | ||
| Heat Resistance | Listeria are killed by the heat treatment in most normal
processing operations. Appropriate time and temperature combinations need to be established for each heat process step. This will depend on the heat source, the size of the item being heated, and the ability of heat to penetrate that particular item. A reported thermal death time for Listeria monocytogenes in crab meat is 2.6 minutes at 600C i.e., at 600C there is a ten-fold die-off of Listeria monocyogenes every 2.6 minutes. |
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New Zealand Food Safety Authority
68-86 Jervois Quay
PO Box 2835
Wellington
NEW ZEALAND
Phone: +64 4 894 2500
Fax: +64 4 894 2501
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