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When is honey honey, and when is it not? This question has recently exercised a team of French analytical scientists from the University of Lyon, who turned to anion exchange chromatography to help provide an answer. Like many revered and comparatively expensive food stuffs, honey is open to adulteration, where unscrupulous producers and traders mix pure honey with cheaper syrups, especially corn syrup. Unfortunately, this adulteration is increasingly difficult to detect, because the adulterers are becoming adept at modifying the sugar profiles of the syrups to match closely the sugar profile of honey. But the profiles are obviously not identical and so differences can still be found if looked for closely enough. For instance, a number of research groups have shown that the concentration of small oligosaccharides - sugar polymers made up of less than 12 linked sugar molecules - can be used to distinguish pure honey from adulterated honey. This is because honey tends to contain higher concentrations of monosaccharides such as glucose and fructose and small oligosacchraides such as trehalose. The converse of this is that syrups tend to contain higher concentrations of polysaccharides comprising more than 12 linked sugar molecules, mainly glucose molecules. So Bernard Herbreteau and his team from the University of Lyon decided to develop a technique for detecting these larger polysaccharides in honey and syrups, as a novel way of distinguishing between them. The problem with doing this is that even though polysaccharides are present at higher concentrations in syrups than in honey, they are still present at much lower concentrations than the monosaccharides and polysaccharides. Honey and syrups both contain much more glucose and fructose than anything else. This meant that Herbreteau and his team first needed to remove the monosaccharides and small oligosaccharides from the honey and syrups. Fortunately, they had just developed a solid-phase extraction (SPE) process for doing just that, which essentially involves trapping the polysaccharides on a liquid chromatography column. Because monosaccharides and small oligosaccharides make up the vast majority of honey and syrups (around 85%), the SPE process greatly concentrates the polysaccharides, with the resultant fraction enriched by a factor of 67. This has the fortunate effect of helping to increase the sensitivity of the subsequent analysis. This involves separating the enriched polysaccharide fraction by anion exchange chromatography (AEC) followed by pulsed amperometric detection. Herbreteau and his team used an AEC column specially designed for separating carbohydrates, which is able to separate polysaccharides based on a wide range of factors, including charge, size and degree of branching. Testing this technique on samples of pure honey and corn syrup, Herbreteau and his team found that they could separate and identify polysaccharides with between 12 and 18 linked sugar molecules. Each of these polysaccharides formed clear peaks in the subsequent chromatogram. Furthermore, the corn syrup samples contained higher concentrations of polysaccharides than the honey samples. Herbreteau and his team then created adulterated honey samples, by adding corn syrup to pure honey at concentrations of either 1% or 0.1%. Using their technique, they found that they could unambiguously distinguish pure honey from honey adulterated with 1% corn syrup, based purely on the polysaccharide concentrations. It proved more difficult to distinguish pure honey from honey adulterated with 0.1% corn syrup, although adding 0.1% corn syrup did increase the size of some of the polysaccharide peaks. But again this shouldn't cause a problem, because syrup is always added to honey at concentrations greater than 0.1%: it's not worth doing otherwise. Herbreteau says that this technique could also be used to detect adulteration of other revered sugar-containing foodstuffs, such as fruit juices and maple syrup. Related links:
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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