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Interactions between volatile and nonvolatile coffee components. 2. Mechanistic study focused on volatile thiols.

Marielle Charles-Bernard¹, Deborah D Roberts, Karin Kraehenbuehl

¹Nestlé Research Center, P.O. Box 44, Vers-Chez-les-Blanc, CH-1000 Lausanne 26, Switzerland.

Journal of Agricultural and Food Chemistry

|May 26, 2005

Summary

Volatile thiols, crucial for coffee

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Area of Science:

Food Chemistry
Analytical Chemistry
Organic Chemistry

Background:

Volatile thiols contribute significantly to the desirable
roasty
and
burnt
aroma profiles of coffee.
Understanding the degradation pathways of these key aroma compounds is essential for maintaining coffee quality during processing and storage.

Purpose of the Study:

To elucidate the chemical mechanisms behind the loss of volatile thiols in coffee brew.
To investigate the influence of coffee's nonvolatile matrix on thiol stability.
To differentiate between degradation pathways for various types of thiols.

Main Methods:

Utilized Solid-Phase Microextraction Gas Chromatography-Mass Spectrometry (SPME-GC-MS) to monitor volatile thiols over time.
Employed a coffee brew model system with controlled addition of volatile thiols.
Incorporated specific inhibitors and varied oxygen levels to probe reaction mechanisms and kinetics.

Main Results:

Identified nucleophilic addition as the primary degradation pathway for aliphatic thiols, occurring on oxidized species in the presence of air.
Observed evidence suggesting a parallel radical mechanism for benzylic thiols, particularly inhibited by oxygen absence and radical scavengers.
Established a correlation between thiol hydrophobicity and the extent of interaction with the coffee matrix, potentially influencing degradation rates.

Conclusions:

Coffee aroma loss is driven by two main chemical mechanisms affecting volatile thiols.
Nucleophilic addition is the dominant degradation route for aliphatic thiols, while benzylic thiols may degrade via a radical pathway.
Hydrophobic interactions play a role in accelerating thiol degradation, warranting further investigation.