Interactions between Volatile and Nonvolatile Coffee Components. 1. Screening of Nonvolatile Components MARIELLE CHARLES-BERNARD, ² KARIN KRAEHENBUEHL,ANDREAS RYTZ, AND DEBORAH D. ROBERTS* ,‡ Nestle ´ Research Center, P.O. Box 44, Vers-Chez-les-Blanc, CH-1000 Lausanne 26, Switzerland This study is the first of two publications that investigate the phenomena of coffee nonvolatiles interacting with coffee volatile compounds. The purpose was to identify which coffee nonvolatile(s) are responsible for the interactions observed between nonvolatile coffee brew constituents and thiols, sulfides, pyrroles, and diketones. The overall interaction of these compounds with coffee brews prepared with green coffee beans roasted at three different roasting levels (light, medium, and dark), purified nonvolatiles, and medium roasted coffee brew fractions (1% solids after 1 or 24 h) was measured using a headspace solid-phase microextraction technique. The dark roasted coffee brew was slightly more reactive toward the selected compounds than the light roasted coffee brew. Selected pure coffee constituents, such as caffeine, trigonelline, arabinogalactans, chlorogenic acid, and caffeic acid, showed few interactions with the coffee volatiles. Upon fractionation of medium roasted coffee brew by solid-phase extraction, dialysis, size exclusion chromatography, or anion exchange chromatography, characterization of each fraction, evaluation of the interactions with the aromas, and correlation between the chemical composition of the fractions and the magnitude of the interactions, the following general conclusions were drawn. (1) Low molecular weight and positively charged melanoidins present significant interactions. (2) Strong correlations were shown between the melanoidin and protein/peptide content, on one hand, and the extent of interactions, on the other hand (R ) 0.83-0.98, depending on the volatile compound). (3) Chlorogenic acids and carbohydrates play a secondary role, because only weak correlations with the interactions were found in complex matrixes. KEYWORDS: Coffee; headspace analysis; SPME; aroma stability; polysaccharides; melanoidins; chlorogenic acids; size exclusion chromatography; anion exchange chromatography; dialysis; SPE INTRODUCTION In addition to their physiological effects, coffee beverages are appreciated for their aroma and taste characteristics. Coffee aroma is the result of a complex balance of about 800 volatile compounds mainly formed during the roasting process. After being roasted, these compounds continue to be affected by environmental factors, their intrinsic instability, and interactions with matrix components. Interactions between volatile and non- volatile compounds may be of physical (reversible) or chemical (reversible or irreversible) nature (1). Recently, Hofmann et al. (2, 3) have observed that coffee aroma rapidly changes after preparation of the coffee brew. A strong decrease of the sul- fury-roasty odor note was noticed at the same time as a decrease of 2-furfurylthiol, a key coffee odorant, in the headspace above coffee brew. Similar observations were made upon thermal processing of canned liquid coffee drinks (4). Our screening study performed with 21 coffee volatiles chosen for their chem- ical reactivity and/or their aroma impact indicated that thiols, sulfides, pyrroles, and diketones present significant interactions with coffee brew constituents, while aldehydes, esters, pyrazines, and guaiacols were not affected by 1% coffee brew upon 24 h storage (5). The nonvolatile components of the coffee matrix play a major role in this instability. Indeed, model mixtures of coffee aroma without matrix are far more stable (6). Due to the complexity of coffee matrix, the relative contribution of its different com- ponents to the coffee aroma staling is still very poorly un- derstood. Coffee nonvolatile components comprise carbohy- drates, proteins/protein fragments, low molecular weight acids, chlorogenic acids, minerals, caffeine, trigonelline, lipids, and up to 30% of unknown molecules usually called melanoidins. Many of these classes of compounds have been shown to trap physically or chemically aroma components and could be involved in coffee aroma instability (1). Because only a few nonvolatile coffee compounds are commercially available, preparative fractionation is a necessary tool to screen the families of compounds responsible for coffee aroma instability. In the * Author to whom correspondence should be addressed [phone/fax (636) 728-1608; e-mail flavorscience@charter.net]. ² Present address: 8 rue Pierre Breton, FR-54110 Dombasle-sur-Meurthe, France. ‡ Present address: Food and Flavor Science Consulting LLC, 17632 Ailanthus Dr., Chesterfield, MO 63005. 10.1021/jf048021q CCC: $30.25 © xxxx American Chemical Society PAGE EST: 8.3 Published on Web 00/00/0000