20 May / June 2016
Are you Made of Sugar?
by Stephanie Kappes and Dr Gabriele Zierfels Metrohm AG, Ionenstrasse, 9000 Herisau, Switzerland
ska@metrohm.com
Carbohydrates constitute the biggest part of the biomass on Earth. They are produced by photosynthesis and are present in all plants and plant- based materials. The amount and composition of carbohydrates in a sample can reveal a wide range of different information, depending on the context. As a result, they are subject to analysis in various industries.
Carbohydrates are Everywhere
In the food industry, carbohydrate and sugar content are key factors in determining the nutritional value of food and beverages. In environmental analysis – to mention but one example – the anhydrosugar levoglucosan, which is produced by the pyrolysis of cellulose and acts as a tracer for biomass combustion, is determined in aerosols. These are just two of the many applications of carbohydrate analysis.
Carbohydrates are composed of one or more monosaccharide units, each of which has a carbonyl group (aldehyde or ketone group) and several hydroxyl groups [1]. Because mono-, di-, and oligosaccharides are water-soluble, ion chromatography, which is performed in the aqueous phase, is particularly suitable for analysing them [2,3]. Determination can thus be performed directly – without prior extraction to the organic phase. However, a high-capacity column is necessary because sugars are relatively large molecules which are in many cases similar in structure (e.g., glucose and galactose; see Figure 1). If the column capacity is too low, similar sugars elute together and cannot be detected separately.
Sugars in Foods
From December 2016, the European Union (EU) requires that nutritional values are indicated on all foodstuffs, with the exception of unprocessed products and products sold loose (regulation no. 1924/2006). What is already an established practice, i.e., indicating the calorific value and certain nutrients including sugar and carbohydrates, is now set to become mandatory.
Figure 1: Structural formulae of glucose and galactose. The molecules differ only in the position of the OH group at the C4 atom (highlighted with an asterisk).
Along with starch, which is a polymer of glucose, the usable carbohydrates found in foodstuffs are largely in the form of sugars. According to the EU definition, this includes all mono- and disaccharides with the exception of polyvalent alcohols. The majority of sugars in foodstuffs is made up of the monosaccharides glucose, fructose, galactose, and the disaccharides sucrose, lactose, and maltose. Table 1 shows a multitude of sugars and related compounds that can be analysed using ion chromatography. The table indicates the matrices, i.e., foodstuffs and beverages, in which the compounds can be determined, as well as the necessary sample preparation steps.
Table 1: Sugars and related compounds that can be analysed by ion chromatography. The plus sign indicates the matrices in which each analyte can be determined.
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