Coffee is an extremely popular beverage with a significant economic importance. Quality assurance, including tracing adulterants in coffee, is therefore an established process and a requirement for consumers.

Carbohydrates which make up to 50% of raw coffee beans function as flavor, viscosity, and aroma agents [1]. They also serve as authenticity tracers, because unadulterated soluble coffee is exclusively made from pure roasted coffee [2,3]. There are clear specification criteria for the quality assessment by ISO 24114 and AFCASOLE (e.g., a limit of <2.46% total glucose and <0.45% total xylose expressed as mass fractions of total carbohydrates) [3]. AOAC 996.04 and ISO 11292 give analytical requirements for instant coffee quality tests regarding analysis of free and total carbohydrates. Ion chromatography (IC) allows precise quantification of the mandatory analytes arabinose, fructose, galactose, glucose, mannose, sucrose, mannitol, and xylose according to AOAC and ISO. The presented IC method is extremely sensitive and overcomes a very common challenge of possible analyte coelutions, such as with rhamnose.

Instant coffee powders (≈ 300 mg per 100 mL) of two instant coffee brands (Jacobs coffee GOLD and a customer sample) were prepared as described in AOAC and ISO to determine the amount of free carbohydrates (arabinose, fructose, galactose, glucose, mannose, sucrose, and mannitol) and total carbohydrates (arabinose, galactose, glucose, mannose, xylose, and mannitol) after acid hydrolyzation.

For the determination of free carbohydrates, coffee powders were dissolved in 100 mL ultrapure water (UPW) and then filtered (0.25 μm). For total carbohydrate analysis, coffee powders were hydrolyzed in HCl (0.1 mol/L) at 100 °C (150 minutes), diluted to 100 mL with UPW, and filtered with an Ag+-H+-cartridge combination. A final dilution (10 to 50-fold) is recommended with UPW.

The carbohydrates specified above for dissolved (free) and total carbohydrate analysis were baseline separated on a Metrosep Carb 2 column with a binary high-pressure gradient combined with a flow gradient (940 Professional IC Vario ONE/HPG configuration) (Figure 1). Amperometric detection was performed after PCR with 300 mmol/L NaOH to improve the detection sensitivity of the method.

Figure 1. Schematic showing the sample flow path from sample introduction with an 858 Professional Sample Processor, to the 940 Professional IC with column (Metrosep Carb 2 - 250/4.0), amperometric detector (Wall-Jet cell with Au and Pd electrodes), and the high pressure gradient pumps for eluent A (UPW) and B (200 mmol/L NaOH and 1 mmol/L NaAc). To increase sensitivity, 300 mmol/L NaOH is added as a PCR (Post Column Reaction) solution. Chromatography for anions is often referred as HPAEC (high performance anion exchange chromatography) but is simplified here to the generic term of IC.

For the two analyzed instant coffee samples, the free carbohydrate content (results not shown) after dissolution in UPW ranged from 0.2 to 27 g/kg. The mass fractions show unique patterns for both samples. In the Jacobs brand, arabinose and mannose dominate (≈ 35 mass%), while the largest peaks for the customer-provided instant coffee brand corresponded to glucose (≈ 20 mass%) and fructose (almost 40 mass%).

Total carbohydrate content after acid hydrolysis is especially crucial for quality control and purity assessment (Table 1). ISO 24114 set limitations for total glucose and xylose of 2.32 and 0.42%, respectively. The total carbohydrate content of both tested samples show a distinct distribution (Table 1 and Figure 2). Both brands contain similar fractions of galactose and arabinose. Glucose, mannose, and xylose contents differ in a broader range.

Looking more closely at the quality criteria, the purity of Jacobs coffee GOLD can be approved as unadulterated product. The customer-provided brand indicates adulteration and would fail a respective control.

Table 1. Carbohydrate concentrations (g/kg) determined by IC-PCR- PAD after acid hydrolysis in two instant coffee samples (Jacobs coffee GOLD and a customer sample). The total carbohydrate content is expressed as the individual mass fractions (M%) of mannitol, arabinose, galactose, glucose, mannose, and xylose (ISO 11292). Additionally, quantification of rhamnose, fructose, ribose and sucrose is possible (Figure 2). Purity indicators are given by the limits for total glucose (<2.32%) and total xylose (<0.42%) (ISO 24114:2011).
  Jacobs (g/kg) [M%] Customer (g/kg) [M%]
Mannitol ND 9 [2.6%]
Arabinose 28.3 [6.5%] 36 [10.2%]
Galactose 190.0 [43.9%] 197.8 [56.2%]
Glucose 6.3 [1.5%] 34.2 [9.7%]
Mannose 207.1 [47.8%] 68.5 [19.4%]
Xylose 1.2 [0.3%] 6.7 [1.9%]
Total carbohydrate content 436.9 [100%] 352.2 [100%]
Figure 2. Chromatogram overlay of (A) a 5 mg/kg mixed carbohydrate standard and the diluted (1:10) samples of (B) Jacobs coffee and (C) customer instant coffee after acid hydrolysis. The separation and detection were performed according to the setup listed in Figure 1. For better performance, quantification of fructose and ribose should be performed using peak height.

With the presented method the requirements of AOAC 996.04 and ISO 11292 for the determination of dissolved and total carbohydrates in instant coffee are fulfilled. An excellent separation of the required carbohydrates can be achieved by combination of a binary high-pressure gradient and a flow gradient on a Metrosep Carb 2 column. An additional benefit of this method eliminates peak overlap between rhamnose and arabinose, an overall constraint of the ISO-method. Overall, the precise quantification of all required carbohydrates plus fucose and ribose can be performed.

Automation and Inline Sample Preparation are additional improvements to increase the sample throughput and save laboratory time and money.

IC with amperometric detection is a robust, highly specific and precise valuable addition for analytical laboratories performing carbohydrate analysis.

  1. Araya and Rao (2007), Crit Rev Food Sci Nutr. 47(1), 51–67.
  2. Girard et al. (2006), J AOAC Int. 89(4), 99–1003.
  3. AFCASOLE (Association of European producers of soluble coffee) statement on the authenticity of soluble coffees of 6 July 1995; as confirmed by the ECF (European Coffee Federation, legal successor of AFCASOLE) in January 2007.
Contact

Metrohm USA

9250 Camden Field Pkwy
33578 Riverview, FL

Contact