Open sesame
[November 3, 2008]


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Food irradiation is one of the traditional ways to ensure the safety of many types of foods and protect the consumer. It reduces or eliminates food-borne bacteria such as E. coli, Salmonella and Campylobacter, controls pests and parasites, and extends shelf life, while having virtually no effect on the nutritional value of the food. The process has been endorsed by major health organisations, including the WHO and the US Centers for Disease Control and Prevention (CDC).

Initial fears that irradiation would produce dangerous or radioactive substances in foods have been proven groundless. However, irradiated foods still need to be stored and handled in the same way as non-irradiated foods, using basic food safety rules to prevent subsequent contamination by bacteria.

There are three main irradiation types, with gamma-irradiation being the most common. Gamma rays from radioactive cobalt or caesium are directed at the food and can penetrate several feet, so large quantities can be processed at once, like fruit in a hopper. The newest technology, X-ray irradiation, can also penetrate longer distances but electron beams only reach depths of about 3 cm in food.

Irradiation does have some positive side effects, such as killing any living cells present. For instance, treatment prevents potatoes from sprouting during storage. There are also some minor chemical changes to the food, which have been put to good use. Minute amounts of hydrocarbons and a group of compounds known as 2-alkylcyclobutanones (ACBs) are produced by the gamma-irradiation of fatty-containing foods and they have become established marker compounds for irradiation.

Two main types of hydrocarbons are formed by the radiolytic breakdown of triglycerides (TGs), the most common form of fat in foods. Their compositions depend on that of the fatty acid groups in the TGs. Saturated alkanes with one carbon atom less than the parent fatty acid, and alkenes with two carbon atoms less than the parent and a new double bond at the C1 position are produced.

The ACBs are formed from TGs by a cyclisation reaction. The number of carbon atoms is the same as that of the parent fatty acid, so determines the length of the alkyl chain as the ring size is fixed. They are always detected in irradiated foods but never in non-irradiated foods treated by other processes (freezing, microwaving, UV irradiation) so are ideal irradiation markers.

So, the presence of hydrocarbons and ACBs show that food has been irradiated, facilitating compliance testing. But what happens to these compounds if the foods are treated further after irradiation, say by roasting? Do they remain in the food, are they removed, or are their levels changed?

For sesame seeds, this problem has been tackled by Joong-Ho Kwon and Jeongeun Lee from the Kyungpook National University in Korea and Tusneem Kausar from the University of Sargodha, Pakistan. The characteristic aroma, texture and colour od sesame seeds are developed by roasting, which is carried out before the oil is extracted by pressing. Korea is a major importer of sesame seed oil so needs to be assured of its safety, regardless of the processing steps.

The researchers bought sesame seeds from local markets and gamma-irradiated a portion at doses up to 4 kGy. Then treated and untreated seeds were steamed or roasted and some of the roasted seeds were subjected to a commercial pressing method to extract the oil. Following two different extraction procedures, hydrocarbons and ACBs were removed from the seeds and oils for analysis by GC/MS with electron ionisation to measure their concentrations.

Four major ACBS were detected, corresponding to the various fatty acid components of the TGs: 2-dodecyl- (from palmitic acid), 2-tetradecyl- (stearic), 2-(5'-tetradecenyl)- (oleic) and 2-(5',8'-tetradecadienyl)- (linoleic). The levels of all four compounds increased linearly with irradiation dose, with 2-(5'-tetradecenyl)- being the most abundant, probably because of the high oleic acid content (38.7%) in sesame seeds. Steaming, roasting and oil extraction had no marked effect on their concentrations.

The levels of the hydrocarbons also increased linearly with irradiation dose and three of them were found only in irradiated samples. One of them, 1-hexadecene, was absent from untreated (roasting, etc.) samples so is no good as an indicator of irradiation. However, 1,7-hexadecadiene and 8-heptadecene were present only after irradiation. The team found that their levels increased after roasting, steaming and oil extraction, in some case by more than 100%. This does not affect their suitability as irradiation markers, since it is their presence, not their content, which is significant.

So, 2 particular hydrocarbons and the ACBs as a group have been confirmed as irradiation markers in sesame seeds that have been processed further after gamma-irradiation.

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Article by Steve Down

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.


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