Cereals

A. Fungi

The cereal mentioned here are the grains of wheat, maize, oats, rye…The fungi found in the cereals can be classified into Field fungi and Storage fungi.

Field fungi are those grow on the plants and invade the grains or seeds before the harvesting. The water activity for the Field fungi to grow are > 0.90 (= to 20-25% moisture). The damage done by the Field fungi includes blemishing, discolouration, loss of germinability and mycotoxin production, which occurred before harvesting or during the drying. However, the field fungi cannot growth during storage; they might slowly die during the storage period or that remain viable for a long period. Note: there are still some field fungi that are able to survive in low water activity.

Storage Fungi degrade the quality of the grains after the harvesting. Storage fungi are able to strive at low water activity.

There are 2 types of Field fungi which is commonly present in the maize- Fusarium and Aspergillus. Flavus. The common Fusarium fungi that is present in the Fusarium verticillioides. Other species include Fus. graminearum and Fus. subglutinans. The optimum growing temperature is > 30°C. The maize crops in the Asia are more prone to Asp. flavus than the crops in cooler area such as Midwestern US. The maize can also pre-harvest infected by the Penicillium species. These species include Pen. oxalicum, Pen. funiculosum and Pen. citrinum.


B. Bacteria

Bacillus, Lactobacillus, Pseudomonas, Streptococcus, Achromobacter, Flavobacterium, Micrococcus and Alcaligenus are bacteria that can be found in the cereals. Spore forming bacilli are the main bacteria in the cereal as they can survive in heating and strive if the water activity is not well controlled. Below are the ranges of each bacterium that can be present in the cereal:

C. Factors that effect the growth of microorganisms

Water activity: the water activity is critical in preventing the growth of storage fungi as they are capable of growing at low water activity.
Eurotium, which is an ascomycete state of Aspergillus are the major storage fungi found in grains. Below shows the minimum Aw and the approximate moisture contents of the storage fungi:

Temperature: The optimum temperature for most fungi grow is 10-35°C. In tropical region, Aspergillus are the cause of grain spoilage in which the optima temperature is 30-40°C, whereas the Penicillium is major cause of spoilage in the temperate climate whereby the optimum growth temperature is 20-30°C.


D. Presence of Toxin (mainly focus on maize)

Aflatoxins: It is produce by the Asp. Flavus. The Aflatoxins that usually present in the maize is B Aflatoxins.

Fumonisins: It is form the Fusarium verticillioides which produce toxin Fumonisin B. It may result in the human esophageal cancer.




Reference ( All info and tables are obtained from the below references):

ICMSF. 2005. Micro-organisms in Foods 6; 2nd Edition. Klumer Academic/ Plenum Publishers; USA. Accessed on 10 July 2008.