Standard methods for the enumeration of fungi in cereals and cereal products

Abstract

Three separate investigations involving the methodology of counting yeasts and moulds in cereals and cereal products are covered in this report. These are summarised separately below.

Counts in flour: 24 laboratories in different parts of the world were each sent subsamples from two samples of flour and asked to carry out counts of moulds and yeasts using the method normally adopted. The need for standardisation was emphasised by the wide diversity of media and methods used. Despite this, mould counts in the samples were reasonably uniform. Yeast counts varied widely and it is clear that some workers have difficulty in separating these organisms and that some media are better than others for this purpose. Problems with bacterial contamination occurred with some of the selective media used. Samples of domestic flours tested at the same time generally contained low counts of fungi (i.e. <1000 per g).

Effects of presoaking and mixing: In a collaborative exercise involving 17 laboratories from around the world, six samples of cereal and cereal product were examined for counts of fungi after soaking in saline/peptone diluent for 0, 30 and 60 min and after mixing for 2 min and again after 10 min at each soaking time. Despite using the same method and materials, both the within and between laboratory variability with some samples was unacceptably high. Mixing for 10 min or soaking for 30 min saw a clear improvement in recovery over mixing for 2 min without soaking in most laboratories. Of these treatments the latter is preferred on grounds of convenience. Soaking for 60 min gave no additional benefit. Agreement between laboratories on the composition of the mould flora in the samples was remarkably good.

Monitoring mycological media: Evidence is presented to indicate that tests to monitor the performance of each batch of mycological medium is not just desirable but is necessary if accurate estimations of the numbers and types of fungi in cereals and cereal products are to be obtained. Not only is there a tendency for variation in performance of batches of medium from the same suppliers but also results can vary with the same medium from different suppliers. The effects of this variability are particularly pronounced with selective mycological media. A method whereby plates of medium are stab inoculated with a range of test moulds and yeasts and the colony diameter measured with incubation time is suggested for monitoring purposes.