Food Analysis

Determination of total glucose and xylose in instant coffee by reverse phase HPLC-UV

This article from Issue 10 of the Analytix Reporter, produced by Merck, describes an RP-HPLC method based on the UV label (PMP) tagging of sugars that was used for a sensitive determination of total glucose and xylose sugars in instant coffee using an UV detection.

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CoffeeAbstract
This is a method for the determination of total glucose and xylose in coffee samples by reverse phase HPLC-UV. The limit of detection for glucose and xylose is 53.2 and 33.8 ppm respectively for freeze dried coffee.

Introduction
Coffee is an indispensable beverage for many people. The adulteration of coffee with coffee husks, cereal grains and soy beans to raise the profit margin is well known. Typical markers for such adulteration include glucose and xylose. Instant coffee is considered to be adulterated if it contains more than 2.46% total glucose and 0.45% total xylose.

Although there are established methods for sugar determination in coffee e.g. AOAC Method 995.13 and ISO Method 11292:1995, they all require the HPAEC-PAD instrument.

Here, we demonstrate the determination of total glucose and xylose using a procedure to release the sugars from the coffee followed by an SPE cleanup. The released sugars are next derivatized with a UV tag, 1-phenyl-3-methyl-5-pyrazolone (PMP). A final clean up by liquid-liquid extraction with dichloromethane is done before HPLC injection. The standard addition technique was chosen as it corrects for varying levels of matrix interferences with different coffee samples.

Method
For full details, please download the article.

Results and Discussion
Both glucose and xylose peaks were symmetrical and eluted at ~9.8 and ~11.5 minutes respectively. The freeze dried coffee has a more complex HPLC profile compared to the coffee mixture sample. The freeze dried coffee samples 1 and 2 had a total xylose content >0.42% w/w. The coffee mixture samples 1 and 2 had a high glucose content.

Conclusion
We can determine total glucose and xylose in coffee by reversed phase HPLC-UV. This is a sensitive
isocratic separation that can be completed within fifteen minutes with the Purospher® STAR RP-18e fully porous particle column. The method can be modified using Fused-Core® or Chromolith® columns for even faster separation while still applicable to conventional HPLC and to UHPLC instruments.

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