16 May / June 2016
Figure 5: Carbohydrate profiles of honey samples. Conditions are described in the Experimental section. Peaks [concentration in standard (mg/L)]: 1) fucitol [1], 2) trehalose [2], 3) fucose [2], 4) rhamnose [2], 5) glucose [2], 6) fructose [3], 7) sucrose [5], 8) kojibiose [1], 9) melezitose [7], 10) gentibiose [8], 11) 1-kestose [2], 12) nigerose [3], 13) maltose [15]
Results and Discussion:
Coffee is enjoyed worldwide and its popularity has been growing. The green coffee bean is approximately 50% carbohydrate by weight, and the carbohydrates play a major role in the fl avor profi le of the brewed coffee. Measuring the carbohydrate content of the green coffee bean, roasted coffee, and instant coffee is one way to determine coffee quality, and for instant coffee, whether it has been adulterated. In 2014, Pauli et al. showed how HPAE-PAD could be used to detect adulteration of coffee with roasted soybean or wheat [6]. The AOAC International has approved an HPAE-PAD method using
the Dionex CarboPac PA1 column for determining free and total carbohydrates in instant coffee [4]. The method requires over one hour per sample and a post- column addition of sodium hydroxide for analyte detection. After the introduction of the Dionex CarboPac SA10 column, a method using that column was reported that required less than 10 min per sample and no post-column sodium hydroxide addition [7]. Figure 1 shows the separation of coffee carbohydrates on the Dionex CarboPac SA10-4µm column. Mannitol, fucose, sucrose, arabinose, galactose, glucose, xylose, mannose, and fructose are resolved in less than six minutes. This separation delivers better resolution of the eight
Table 1: Carbohydrate determination of an instant coffee sample. There were three sample preparations and each sample was injected in triplicate.
carbohydrates than published in reference seven, and was applied to the determination of free and total carbohydrates in instant coffee (Figure 2). Arabinose is the most prominent monosaccharide in the free carbohydrate analysis while galactose and mannose are the most prominent in the total carbohydrate analysis of the instant coffee. The major carbohydrates in both the free and total carbohydrate analyses of the instant coffee sample were quantifi ed (Table 1). These determinations were validated by the good spike recovery achieved for each analyte in both types of analyses. Note the extremely low concentrations of glucose and fructose in Figure 2. Elevated concentrations of glucose and fructose are markers for roasted wheat and roasted soybean adulteration [6]. A second instant coffee sample (not shown) had a similar profi le of free and total carbohydrates. This method will not resolve galactose and rhamnose as well as fructose and ribose. While rhamnose and ribose are two of the carbohydrates separated in AOAC Method 995.13, earlier work showed very low concentrations of rhamnose in the free and total carbohydrate analysis of instant coffee and no detectable ribose [7] and therefore this method is suitable for instant coffee determinations.
The last decade has witnessed the emergence of functional beverages, which are beverages formulated to provide a specifi c benefi t. Electrolyte replacement beverages, a.k.a. sports drinks, and energy drinks are two such beverages. These are typically sweetened with glucose, fructose, sucrose, or some combination of these three carbohydrates. Figure 3 shows an analysis of the glucose, fructose, and sucrose contents of four functional beverages. The samples were diluted, fi ltered, and injected. The three carbohydrates are well resolved and therefore easy to quantify. Note the variety of the ratios of the three analytes in these products as well as confi rmation that the sugar free drink does not have any detectable glucose, fructose, and sucrose. The same method can be used to determine the carbohydrate content of two fruit juices (Figure 4). The peak at about 2 min in the apple juice sample is probably sorbitol, which is known to be a major component of apple juice’s sweetness and will elute early from this column.
HPAE-PAD has been used for analysing the carbohydrate content of honey since 1990 [8]. The method has the required sensitivity to detect honey’s small quantities of disaccharides, trisaccharides, and small
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