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in a lab. For example a gas sample can be tested with gas chromatography mass spectrometry using thermal desorption and the resulting spectra is compared to a database to determine which gases are present. There are odour panels and professional smellers. These are people who have trained themselves to be able to identify different odours. There are many references in the literature that give more information on methods. There are monitors available that can detect gases and identify them. In some cases these can be programmed to store an odours profile in memory. If there is a limit to VFAs in the final product in most cases these samples are sent to an outside lab for testing. In the mill there are commercially available test kits to test for VFAs in stock and water samples. While the final product testing may be required the in-mill test can be used as a monitoring tool. The in-mill test will give immediate results while the outside lab test will have a time delay. The other area of testing is the microbial enumeration or detection methods. It is important to understand the nature of Improving process conditions is important but in the case of VFA control this will not provide the complete answer of anaerobic bacteria a good control tool is oxygen. Improving mixing in chests and tanks to keep oxygen levels up will help control the anaerobes. This is an economical option. Good cleaning and housekeeping is also important. If a layer of solids is allowed to build up in the bottom of a chest then anaerobic conditions can result. The aerobic bacteria in the layer will use up the oxygen. Once the oxygen level is low then anaerobic bacteria growth will start. In the same vein leaving water and pulp sitting unused in tanks should be avoided. Again this can lead to anaerobic conditions. It is always important to audit the process to identify unfavorable process conditions and as much as possible to design them out or at least minimise them. Improving process conditions as mentioned above is important but in the case of VFA control this will not provide the complete answer. A microbial control program will be required. One aspect of VFA producing anaerobic bacteria control is the endospores that were mentioned earlier. A portion of an endospore producing bacterial population will always produce endospores. One bacteria will produce WORLD PULPPAPER 67 microbial testing. There is no one test that will test for all the different types of microorganisms especially with odour where it is important to identify the different types of microbes that are causing the odour. When in the problem solving stage a number of different tests may be necessary to help identify the microbes present and audit the system. This article will not discuss in detail the wide range of tests available but will focus on the VFA producing anaerobic bacteria. A common test for bacteria is what is commonly known as the standard plate count. This widely used method tests for aerobic bacteria which will grow on nutrient agar at a temperature of 36 C. Therefore if you are looking for anaerobic bacteria this test will not detect them. You need to use a more specific test such as using reinforced clostridial medium. This tests for clostridial species as well as some other anaerobic bacteria. It is more a yesno test in that you do not end up with a count of the population. CONTROL OPTIONS It was mentioned earlier that in most cases a control program will involve process changes as well as the use of a microbial control program. In the case Table 1. Volatile Fatty Acids Common Name Systematic Name Odour Formic acid Methanoic acid Pungent solvent like nail polish Acetic acid Ethanoic acid Vinegar Propionic acid Propionic acid Body odour Butyric acid Butanoic acid Rancid butter vomit Isobutyric 2-Methylpropanoic acid Rancid butter Valeric Pentanoic acid Fruit like Isovaleric 3-Methylbutanoic acid Body odour