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Bright greenish-yellow fluorescence test for aflatoxin estimation
by Prisnar Siriacha
There are three approaches to aflatoxin estimation in corn: (1) a presumptive test to identify corn lots that may contain the toxin and to determine whether a corn lot should be analysed for aflatoxin or not (2) rapid screening methods to establish the presence or absence of the toxin and to know the range of aflatoxin level; and (3) quantitative methods to determine types and, aflatoxin contents.
Since most samples do not contain a detectable amount of aflatoxin, there is a need for a method which correctly identifies the many negative samples with minimum expenditure of time and money. Such a method is known as the BGYF test or black light test. Corn is inspected under the UV lamp (365 µm.) for a characteristic bright greenish yellow fluorescence in broken and damaged kernels. The test takes 5 minutes or less.
The basis of the presumptive test for aflatoxin in corn known as the bright greenish-yellow fluorescence (BGYF) or the 'black light' test is the characteristic fluorescence under long-wave ultraviolet light (365 µm.) associated with the presence of kojic acid produced from Aspergillus flavus or A. parasiticus, aflatoxin producing fungi, or possibly mycotoxin itself.
Marsh (1) postulated that BGYF resulted from the action of heat labile peroxidase in the living plant tissues (cotton fibers, corn, etc.) and from some microorganisms on kojic acid produced by many fungal species including A. flavus and A. parasiticus. Thus the BGYF test indicates the growth of the fungi that may have resulted in the production of aflatoxin.
Because the BGYF test is so easily done, attempts have been made to establish aflatoxin levels by numbers of BGYF particles or by the weight and by area of BGYF particles in whole kernel samples. Although a relationship existed between numbers of BGYF particles and kernels and aflatoxin levels, the correlation was not high enough to encourage use of the numbers as an indication of aflatoxin content (2). So BGYF test can not provide a satisfactory quantitative estimate for aflatoxin but should only be used to identify lots for further chemical analyses.
Previous studies indicate that the aflatoxin content of samples with BGYF kernels may range from none to very high concentrations (4). When there are no BGYF particles in 4.5 kg samples of cracked maize, the probability is very low that the sample contains aflatoxin (5). Therefore, an effective aflatoxin screening programme may consist of accepting lots with no BGYF in the samples, and only analyzing those with BGYF kernels.
PROCEDURES FOR BGYF TEST
1. Usually, at least fresh 4.5 kg samples should be collected, but even larger ones would be better.
2. Sample should be cracked or coarsely ground in a straub disc mill or coffee grinder before BGYF inspection because the fluorescence can occur under the seed coat (3). Sometimes it can be detected as a dull gold color under the seed coat, usually in the germ area, and becomes fully visible when cracked. The grinding process creates dust and if the maize dust contains aflatoxin, it presents a potential hazard to workers inhaling it, thus efficient exhaust fume hoods are needed.
3. Ground maize is discharged in a monolayer onto a black tray.
4. Inspect under long-wave ultraviolet light (365 µm.) or black light in a darkened chamber or room. A high-intensity light is recommended, but lower intensity lights may be used in complete darkness. Goggles that screen out UV light must be used to lessen eye strain and prevent possible eye damage from continuous exposure.
BGYF has a bright glow, sometimes called a firely glow, that differentiates it from other fluorescence materials in corn. False results may be reduced by use of the color standards Tinopal BHF (Ciba-Geigy Corporation, Greensboro, NC 27409) or a Black-Ray green fluorescing crayon (Ultra-Violet Products, Inc., 5100 Walnut Grave Avenue, San Gabriel, CA 91778).
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