190 Chilled foods procedures allow organisms to be distinguished from each other by the reactions that their colonies cause in the medium. An example would be the inclusion of a pH indicator in a medium to differentiate acid-producing organisms. In most cases, media will utilise a multiple approach system, containing elective selective and differential components in order to ensure that the user can identify and count the target organism The number of types of agar currently available are far too numerous to list For details of these, the manuals of media manufacturing companies(e.g Oxoid LabM, difco, Merck) should be consulted MPN method The second enumerative procedure mentioned earlier was the MPN method. This procedure allows the estimation of the number of viable organisms in a sample based on probability statistics. The estimate is obtained by preparing decimal tenfold) dilutions of a sample, and transferring sub-samples of each dilution to (usually) three tubes of a broth medium. These tubes are incubated, and those that how any growth(turbidity) are recorded and compared to a standard table of esults(Anon. 1986)that indicate the contamination level of the product. As indicated earlier, this method is used only for particular types of test tends to be more labour and materials intensive than plate count methods ddition, the confidence limits are large even if many replicates are studied at each dilution level. Thus the method tends to be less accurate than plate counting methods 8.3.2 Conventional qualitative procedures Qualitative procedures are used when a count of the number of organisms in a sample is not required and only their presence or absence needs to be determined. Generally such methods are used to test for potentially pathogenic microorganisms such as Salmonella spp, Listeria spp, Yersinia spp. and campylobacter spp. The technique requires an accurately weighed sample (usually 25g)to be homogenised in a primary enrichment broth and incubated for a stated time at a known temperature. In some cases, a sample of the primary enrichment may require transfer to a secondary enrichment broth and further incubation. The final enrichment is usually then streaked out onto a selective agar plate that allows the growth of the organisms under test. The long enrichment procedure is used because the sample may contain very low levels of the test organism in the presence of high numbers of background microorgan- isms. Also, in processed foods the target organisms themselves may be in an injured state. Thus the enrichment methods allow the resuscitation of injured cells followed by their selective growth in the presence of high numbers of competing organisms The organism under test is usually indistinguishable in a broth culture, so broth must be streaked onto a selective/differential agar plate. The microorg isms can then be identified by their colonial appearance. The formationprocedures allow organisms to be distinguished from each other by the reactions that their colonies cause in the medium. An example would be the inclusion of a pH indicator in a medium to differentiate acid-producing organisms. In most cases, media will utilise a multiple approach system, containing elective, selective and differential components in order to ensure that the user can identify and count the target organism. The number of types of agar currently available are far too numerous to list. For details of these, the manuals of media manufacturing companies (e.g. Oxoid, LabM, Difco, Merck) should be consulted. MPN method The second enumerative procedure mentioned earlier was the MPN method. This procedure allows the estimation of the number of viable organisms in a sample based on probability statistics. The estimate is obtained by preparing decimal (tenfold) dilutions of a sample, and transferring sub-samples of each dilution to (usually) three tubes of a broth medium. These tubes are incubated, and those that show any growth (turbidity) are recorded and compared to a standard table of results (Anon. 1986) that indicate the contamination level of the product. As indicated earlier, this method is used only for particular types of test and tends to be more labour and materials intensive than plate count methods. In addition, the confidence limits are large even if many replicates are studied at each dilution level. Thus the method tends to be less accurate than plate counting methods. 8.3.2 Conventional qualitative procedures Qualitative procedures are used when a count of the number of organisms in a sample is not required and only their presence or absence needs to be determined. Generally such methods are used to test for potentially pathogenic microorganisms such as Salmonella spp., Listeria spp., Yersinia spp. and Campylobacter spp. The technique requires an accurately weighed sample (usually 25g) to be homogenised in a primary enrichment broth and incubated for a stated time at a known temperature. In some cases, a sample of the primary enrichment may require transfer to a secondary enrichment broth and further incubation. The final enrichment is usually then streaked out onto a selective agar plate that allows the growth of the organisms under test. The long enrichment procedure is used because the sample may contain very low levels of the test organism in the presence of high numbers of background microorgan￾isms. Also, in processed foods the target organisms themselves may be in an injured state. Thus the enrichment methods allow the resuscitation of injured cells followed by their selective growth in the presence of high numbers of competing organisms. The organism under test is usually indistinguishable in a broth culture, so the broth must be streaked onto a selective/differential agar plate. The microorgan￾isms can then be identified by their colonial appearance. The formation of 190 Chilled foods