Yeast as Natural Control for Aflatoxin

Yeast can now be used for natural control of deadly aflatoxins produced by Aspergillus flavus.

The yeast can control growth of A flavus by competing for nutrients that might otherwise be used by the toxin producing mold. Field testing in pistachios prevented aflatoxin incidence up to 97 percent. The natural control would be more effective if accompanied by good agricultural practices.aflatoxin poison symbol

The yeast are applied to crops before harvest. It can also be applied on stored grains. It controls growth of Aspergillus flavus and other crop damaging microorganisms.

The eco-friendly technology is patented to Agricultural Research Service (ARS) plant physiologist Sui-Sheng (Sylvia) Hua. Not yet available for use here in Philippines.

Optimal Conditions for Yeast Fermentation

water jug ferment

The optimum condition for yeast fermentation are as follows:
1) Absence of oxygen.
2) Temperature range of 20° to 30° C
3) pH range of 4 to 4.5
4) Minimum water activity of 0.85
5) Sugar content of less than 40%

The conditions are explained by Fermented Fruit and Vegetables, A Global Perspective, by Mike Battcock and Sue Azam-Ali:

Most yeasts require an abundance of oxygen for growth, therefore by controlling the supply of oxygen, their growth can be checked. In addition to oxygen, they require a basic substrate such as sugar. Some yeasts can ferment sugars to alcohol and carbon dioxide in the absence of air but require oxygen for growth. They produce ethyl alcohol and carbon dioxide from simple sugars such as glucose and fructose.

C6H12O6 = 2C2H5OH + 2CO2

Glucose      =yeast=    ethyl alcohol + carbon dioxide

In conditions of excess oxygen (and in the presence of acetobacter) the alcohol can be oxidised to form acetic acid. This is undesirable if the end product is a fruit alcohol, but is a technique employed for the production of fruit vinegars.

Yeasts are active in a very broad temperature range – from 0 to 50° C, with an optimum temperature range of 20° to 30° C.

The optimum pH for most micro-organisms is near the neutral point (pH 7.0). Moulds and yeasts are usually acid tolerant and are therefore associated with the spoilage of acidic foods. Yeasts can grow in a pH range of 4 to 4.5 and moulds can grow from pH 2 to 8.5, but favour an acid pH (Mountney and Gould, 1988).

water jug ferment

In terms of water requirements, yeasts are intermediate between bacteria and moulds. Bacteria have the highest demands for water, while moulds have the least need. Normal yeasts require a minimum water activity of 0.85 or a relative humidity of 88%.

Yeasts are fairly tolerant of high concentrations of sugar and grow well in solutions containing 40% sugar. At concentrations higher than this, only a certain group of yeasts – the osmophilic type – can survive. There are only a few yeasts that can tolerate sugar concentrations of 65-70% and these grow very slowly in these conditions (Board, 1983). Some yeasts – for example the Debaromyces – can tolerate high salt concentrations. Another group which can tolerate high salt concentrations and low water activity is Zygosaccharomyces rouxii, which is associated with fermentations in which salting is an integral part of the process.