Key odorants in various cheese types as determined by gas chromatography-olfactometry

Abstract

Cheese odour-active compounds identified by gas chromatography-olfactometry (GC-O) are compiled in nine tables dealing with alcohols, aldehydes, ketones, esters, lactones, furans, nitrogen-containing compounds together with pyrazines and sulphur-containing compounds, terpenes and miscellaneous compounds, and, finally, aromatic compounds and free fatty acids. For each component, the odour descriptors, the cheese variety in which it was evaluated, the concentration (or an indication of the odour intensity), the extraction method, the GC-O type of analysis, and, if available, the perception threshold values are also provided. The sensory properties and metabolic origin of major cheese aroma compounds are discussed in the text, explaining to a certain extent the discrepancies between different authors.

Introduction

Volatile (aroma) components of various dairy products have received a great deal of attention. So far, more than 600 volatile compounds have been identified in cheese (Maarse & Visscher, 1989). However, there is a general agreement that only a small fraction of these compounds are really responsible for cheese flavour. In many cases, the most abundant volatiles may have little, if any, odour significance in dairy products. This leads to great confusion in the literature between odour-active compounds and those which are insignificant. Such distinction should become a challenge to flavour chemists in the future.

Many studies have used gas chromatography, coupled with mass spectroscopy (GC-MS), to analyse the aroma of dairy products and several reviews have been published on this topic (Badings & Neeter, 1980; Gallois & Langlois, 1990; Engels, Dekker, de Jong, Neeter, & Visser, 1997; Sablé & Cottenceau, 1999; Lawlor, Delahunty, Wilkinson, & Sheehan, 2001). However, GC-MS is a useful method for identifying and quantifying flavour substances, but it does not allow to establish whether such compounds are odour-active or not. In complement, gas chromatography-olfactometry (GC-O) provides a valuable tool for investigating the pattern of odorants in terms of both their odour descriptors and activity. GC-O can be defined as a collection of techniques that combine olfactometry, or the use of the human nose as a detector, to access odour activity in defined air streams, with the very popular gas chromatographic separation of volatiles (Friedich & Acree, 1998). Theoretically, the human nose can have a detection limit down to 10⁻¹⁹ moles for certain odours (Reineccius, 1994). Therefore, it can be successfully used for the analysis of chromatographic effluents and in many cases is much more sensitive than an instrumental detector such as a flame ionisation detector.

Several techniques have been proposed for the extraction, isolation and gas chromatographic injection of volatile components of dairy products (Bosset & Gauch, 1993; Mariaca & Bosset, 1997; Deibler, Acree, & Lavin, 1999). These procedures can be divided into five main groups: (i) direct extraction methods by which an organic extract is obtained by liquid–liquid or liquid–solid partitioning; (ii) supercritical fluid extraction (SFE) methods using CO₂; (iii) steam distillation or “stripping” methods that produce large quantities of aqueous extracts which then have to be concentrated in a further step by liquid–liquid partitioning or by cryoconcentration; (iv) high-vacuum (or molecular) distillation methods, which are, in principle, variants of the previous group but work under higher vacuum and lower temperature; (v) static, as well as dynamic, headspace techniques in which the analysed sample is an aliquot of the gas-phase containing the volatile components released by the food.

The data that are generated by subsequent GC-O analysis can be evaluated using the following types of analysis: (i) Aroma Extract Dilution Analysis (AEDA) (Ullrich & Grosch, 1987; Grosch, 1994); (ii) Aroma Extract Concentration Analysis (AECA) (Kerscher & Grosch, 1997); (iii) CharmAnalysis^TM (Combined Hedonic Aroma Response Measurement) (Acree & Barnard, 1984; Acree, Barnard, & Cunningham, 1984); (iv) Osme (from the Greek word meaning “odour”) (McDaniel, Miranda-Lopez, Watson, Michaels, & Libbey, 1990; Miranda-Lopez, Libbey, Watson, & McDaniel, 1992); (v) Nasal Impact Frequency (NIF) and Surface of Nasal Impact Frequency (SNIF) analyses by using a panel of trained persons to do the sniffing (Pollien et al., 1997).

So far, a comprehensive list of cheese key odorants, determined by GC-O, has not been established. Thus, the aim of the present paper is to summarise the current knowledge on this topic and compile key odour compounds determined in various cheese varieties using different extraction procedures and GC-O methods.