Species of Drechslera producing metabolites

Fungi which grow in stored commodities are xerophilic, that is, capable of growing at reduced water activity (aw). Some xerophilic fungi either will not grow or grow only very slowly at the high aw values of media traditionally used to enumerate yeasts and moulds in foods and feedstuffs. Populations of xerophiles may therefore be underestimated or go undetected if the media and methods normally used for detecting fungi in foods are not modified. Because ofthe wide range of aw values over which various food spoilage fungi will grow, the choice of a plating medium is very important, as it will determine the types of fungi which will be isolated or enumerated. The characteristics of the commodity being examined should be critically assessed, and a medium chosen which reflects those characteristics. High aw media are suitable for high aw foods such as meat, seafood, fruits, vegetables, salads etc., but for stored commodities like grains, nuts, and spices, and dried and intermediate moisture foods such as confectionery, dried fruits, and dried meat and seafood products, high aw media will enumerate only the surface mycoflora, and will not detect the significant fungi that are capable of growing in the product. If a medium of reduced aw is used, then type of solute is the next consideration. For foods which are high in sugar, a glucose or glycerol based medium may be most suitable, while for salty foods, the use of a reduced aw medium containing some salt (but not necessarily based entirely on salt) may be more appropriate. For general purpose enumeration of xerophilic fungi in stored commodities, a glycerol based medium, Dichloran 18% glycerol agar (00 18) is recommended. For enumeration of spoilage fungi in dried. salted seafood or meat products, a medium containing some salt is prererable. For this purpose, Malt extract Yeast extract 5% NaCl12% glucose agar (MY5-12, aw 0.953) has been developed. For foods which contain high sugar concentrations, and are subject to spoilage by extreme xerophiles, Malt extract Yeast extract 50% Glucose agar (MY5OG, aw 0.89) is recommended. FUNGAL spoilage of stored commodities is due almost entirely to growth of xerophilic fungi; that is, fungi which are capable of growth at water activity (aw) 0.85 or below. The growth of microorganisms in foods is controlled by many factors, one of the most important being aw, but interactions with pH also play a part (Fig. 1). If the aw of foods or eommodities is below 0.90 or the pH is less than 45, then few bacteria will grow. If the aw of a food or commodity is below 0.85, then spoilage will be caused almost exlusively by moulds and yeasts. Foods of reduced aw are often referred to as intermediate moisture foods, and their aw is generally between 0.85 and 0.60. Commodities with aw below 0.60 are microbiologically stable, * CSm.O Division of Food Processing, PO Box 52, North Ryde, NSW 2113, Australia. 65 as there are no microorganisms capable of growth at aw values so low. Intermediate moisture foods can be separated into several groups on the basis of the types of fungi that are able 10 grow in them and cause spoilage (Hocking 1991). These are: 1. stored commodities cereals, oilseeds, nuts, and spices; 2. high sugar foods confectionery, dried fruits, and jams; 3. dried meats and meat products; and 4. dried seafood products. The mycoflora of stored commodities and of dried meats is fairly similar. Usually, Eurotium species are dominant, with the Aspergillus restrictus series also very 1.0 ,-----"""1,,:---0:-, ........ ---------, " ' .... ',