ISO/TC 34/SC 9/WG 28 - Enumeration of micro-organisms at 30 degrees C

ISO 4833-1:2013 specifies a horizontal method for enumeration of microorganisms that are able to grow and form colonies in a solid medium after aerobic incubation at 30 °C. The method is applicable to: a) products intended for human consumption and for animal feed; b) environmental samples in the area of food and feed production and handling. ISO 4833-1:2013 is applicable to: 1) products that require a reliable count when a low limit of detection is specified (below 102/g or 102/ml for liquid samples or below 103/g for solid samples); 2) products expected to contain spreading colonies that obscure colonies of other organisms, e.g. milk and milk products likely to contain spreading Bacillus spp. The applicability of ISO 4833-1:2013 to the examination of certain fermented food and animal feeds is limited and other media or incubation conditions can be more appropriate. However, this method can be applied to such products even though it is possible that the predominant microorganisms in those products are not detected effectively. For some matrices, the method specified in ISO 4833-1:2013 can give different results to those obtained using the method specified in ISO 4833‑2.

ISO 4833-2:2013 specifies a horizontal method for enumeration of microorganisms that are able to grow and form colonies on the surface of a solid medium after aerobic incubation at 30 °C. The method is applicable to: a) products intended for human consumption or for animal feed; b) environmental samples in the area of food and feed production and food handling. ISO 4833-2:2013 is applicable to: 1) products containing heat-sensitive organisms that are likely to form a significant proportion of the total flora (e.g. psychrotrophic organisms in chilled and frozen foods, dried foods, other foods that may contain heat-sensitive organisms); 2) products containing obligately aerobic bacteria that are likely to form a significant proportion of the total flora (e.g. Pseudomonas spp.); 3) products that contain small particles that can prove difficult to distinguish from colonies in a pour plate; 4) products whose intense colour prevents the recognition of colonies in a pour plate; 5) products for which distinction between different types of colony is required as part of the assessment of food quality. In addition to the manual spread plating technique, an annex to ISO 4833-2:2013 also specifies the use of a spiral plater, a rapid method of performing surface colony counts. The applicability of ISO 4833-2:2013 to the examination of certain fermented food and animal feeds is limited and other media or incubation conditions can be more appropriate. However, this method can be applied to such products even though it is possible that the predominant microorganisms in these products are not detected effectively. For some matrices, the method described in ISO 4833-2:2013 can give different results to those obtained using the method described in ISO 4833‑1.