Anyone who has been educated as a chemist -- as I have -- knows that benzoic acid is a fairly stable compound, and that there is no obvious mechanism by which it can easily be decarboxylated to form benzene at low temperatures. It has been known for a very long time that AT SUFFICIENTLY HIGH TEMPERATURES (a few hundred Celsius) this will happen, but that hardly seems relevant to what is likely to happen at or near room temperature. In addition, copper (not found in foods) can catalyze the decarboxylation of benzoic acid in refluxing quinoline (i.e. at a temperature around 238°C), again far removed from conditions likely to be encountered in food. It is worth noting that benzoic acid used to be obtained by dry distillation of gum benzoin, and benzoic acid boils at 249°C, so it is fairly stable to thermal decomposition.
There is one well-known type of reaction by which benzoic acid could form benzene, but it was regarded as negligible because the circumstances necessary for it to come about seemed unlikely to be encountered by accident. This is a free-radical decarboxylation. Such reactions have been studied much more intensely in the last couple of decades as chemists have come to realize that radical reactions could be much more subtle and complex than older research had suggested. Understanding of the role of reactive oxygen species in biological processes has also grown, not entirely for the same reasons. It turns out that the hydroxyl radical (OH) which is often implicated in cellular damage is also the culprit in benzene formation. But why would OH be found in soft drinks? The answer is O2 from air. It reacts with various hydrogen-containing (usually organic) compounds to form such species as hydroperoxyl (OOH) radicals, which go on to form traces of hydrogen peroxide (HOOH). Still no benzene, though. Now we need to add some catalyst to convert HOOH to radicals, and certain metal salts, notably iron and copper (but not Al) do just that. That finally gets us the OH radicals, which remove a hydrogen from benzoic acid (IF it's present as the acid, and not a salt) to give a carboxyl radical, which may very well grab a H off of some other molecule, or IF there's nothing else for it to grab, simply fragment into CO2 and phenyl radical. Still no benzene! But wait, IF the phenyl radical can find a hydrogen (which its precursor couldn't) it can pull that off and form benzene. So there's quite a number of IF's in there, and it was thought really improbable that such a sequence of reactions would occur by itself, that is without someone very carefully designing a reaction mixture that got all the conditions just right. Even then, it wouldn't be a very efficient way to make benzene. (Also, it was found that other types of radicals tested would not cause decarboxylation as hydroxyl radical did.)
Of course, if you're worried about trace contaminants, efficiency doesn't matter -- if there are 10,000,000 parts per billion (ppb), i.e. 0.1% of benzoic acid present, only one in every million molecules needs to decompose this way for 10 ppb of benzene to form. 5 ppb is considered the acceptable limit by the FDA. (It may very well be the metal catalyst which is the limiting factor; if this could be reduced to zero conc'n, there would be no benzene formed. But iron is found in trace amounts throughout Nature, so some low conc'n of iron is unavoidable.) So when it was reported that the combination of ascorbate (vit. C), a metal catalyst, and benzoate (at low pH, where it forms the acid) found in soft drinks could in fact form benzene (see second link) in detectable quantities, it came as a surprise to almost everyone, but did lead to action being taken.
Just to make clear -- this is not something that anyone "actually educated as a chemist" would know "from day 1". The formation of benzene by this devious pathway is not at all transparently obvious; on the contrary, its elucidation required a considerable amount of sleuthing. It is not obvious at all that it should actually happen, nor does it appear to happen to more than a few molecules in a million. I do not know if your job is one which requires you to consider a one-in-a-million possibilty as "significant", but if it is, I would think you would have more understanding of what was involved in this case.
Apparently, the soft drink mfgrs themselves had initially approached the FDA with concerns about benzene. FDA investigated, published the results, and worked with mfgrs to reformulate the drinks. Subsequently, benzene levels were found to be very low. This happened back in the early 90's, i.e. the Bush I/Clinton years.
Apparently, the key factor was the realization that ascorbic acid -- a VITAL NUTRIENT -- reacts with O2 with particular ease to form H2O2 and similar species. Ascorbic acid also reacts very easily with OH radical, though, so the use of AA as an anti-oxidant would seem safe and well-grounded, and not likely to serve as a significant source of OH. Apparently, though, the combination of AA, traces of iron, and O2, in just the right combination, still leads to the formation of benzene, at least about one time in a million. So keeping the vital nutrient and anti-oxidant, vitamin C, OUT of soft drinks actually prevents the formation of benzene! Of course, one could always leave out the benzoic acid, but in its absence the growth of mold, yeast, and some bacteria becomes a problem. In a perfect world, no benzoate would be used. Realistically, no one wants to buy foods or drinks with mold growing in them, so it will probably continue to be used in traces. It is interesting to note that YEAST is one organism which appears particularly affected by benzoate -- by inhibition of fermentation -- and it is in yeast where the supposed effect on mitochondrial DNA was observed. There aren't enough details in this article to tell, but this researcher may have jumped to conclusions and raised a false alarm. It will be worth looking for follow-up to this report.
From the reports in 2005 and 2006, it sounds very much like the FDA and mfgrs have quit caring about this problem, and have been reformulating their drinks without regard to benzene formation. Of course, this is happening under Bu**sh**, so that's hardly surprising. With the public at large identifying vit. C as an antioxidant, and believing that antioxidants are A Good Thing, mfgrs have been quick to promote its use in their formulas -- a decision made by marketers, not chemists.
http://en.wikipedia.org/wiki/Benzoic_acidhttp://pubs.acs.org/cgi-bin/abstract.cgi/jafcau/1993/41/i05/f-pdf/f_jf00029a001.pdfhttp://en.wikipedia.org/wiki/Benzene_in_soft_drinkshttp://www.bfr.bund.de/cm/245/indications_of_the_possible_formation_of_benzene_from_benzoic_acid_in_foods.pdfFinally, to provide a little context...
Benzene in soft drinks should be seen in the context of wider environmental exposure. Taking the worst example found to date, of a soft drink containing 87.9ppb benzene, someone drinking a 500ml can would ingest 44μg (micrograms) of benzene. Whilst there is no justification for a soft drink to contain high levels of benzene ("There is a difference here between a small and unavoidable risk, and a small but avoidable risk.” <5>), the casual consumption of such a drink is unlikely to pose a significant health hazard to a particular individual (see, for example, the EPA IRIS document on benzene<6>). However, spread out over billions of people consuming soft drinks each day, there would be a small number of cancers caused by this exposure<7>.
The UK Food Standards Agency has stated that people would need to drink at least 20 litres per day of a drink containing benzene at 10 μg to equal the amount of benzene you would breathe from city air every day.<8> Daily personal exposure to benzene is determined by adding exposure from all sources.
* Air: A European study found that people breathe in 220μg of benzene every day due to general atmospheric pollution. A motorist refilling a fuel tank for three minutes would inhale a further 32μg.<9><10>. The estimated daily exposure from "automobile-related activities" is 49 μg and for driving for one hour is 40 μg.<8>
* Smoking: For smokers, cigarette smoking is the main source of exposure: estimates are 7900μg per day (20-cigarette-per-day smoker)<9>, 1820 μg/day, and 1800 µg/day.<8>
* Passive smoking: Benzene intake from passive smoking is estimated at 63 μg/day (Canada) and 50 µg/day.<8>
* Diet and drinking water: 0.2 – 3.1 μg/day<8>
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