Aroma perception of individual volatile compounds in fresh tomatoes (Lycopersicon esculentum, Mill.) as affected by the medium of evaluation

Abstract

Odor thresholds of volatile compounds in fresh tomato (Lycopersicon esculentum Mill.) were estimated and odor units calculated to determine whether the medium of evaluation affects aroma perception. The ‘ascending method of limits’ was used to determine odor thresholds of cis-3-hexenal, hexanal, trans-2-hexenal, hexanol, cis-3-hexenol, 2-isobutylthiazole, 6-methyl-5-hepten-2-one, geranylacetone, 2-pentenal, β-ionone, 1-penten-3-one, 3-methylbutanol, 3-methylbutanal, acetone and 2-phenylethanol in deionized water, an ethanol (100 ppm)/methanol (500 ppm)/water mixture and a deodorized tomato homogenate. Cis-3-hexenal exhibited the highest level of odor units in all three media. Odor thresholds were lower in deionized water for all compounds than in the ethanol/methanol/water mixture (simulating levels found in homogenized tomato) and the thresholds were even higher in the deodorized tomato homogenate for most compounds. Distinct differences were noted in aroma descriptors for the compounds in different media. The results suggest that both qualitative and quantitative changes are occurring in the perception of volatile compounds in the different media and that ethanol and methanol alter perception of tomato aroma.

Introduction

Characteristic tomato (Lycopersicon esculentum) flavor represents a complex interaction of volatile and nonvolatile compounds (Petro-Turza, 1987). Some investigators have emphasized the importance of volatile compounds on the aromatic component of flavor (Buttery et al., 1987; McGlasson et al., 1987; Langlois et al., 1996; Baldwin et al., 1998; Brauss et al., 1998; Krumbein and Auerswald, 1998), while others have emphasized the contribution of sugars and acids (Stevens et al., 1979; Hobson and Bedford, 1989; Malundo et al., 1995).

Tomato taste is attributed to a combination of sweetness (fructose and glucose) and sourness (citric and malic acids) (Malundo et al., 1995). While over 400 volatile components have been identified in fresh tomatoes, the most important compounds are thought to be cis-3-hexenal, hexanal, trans-2-hexenal, hexanol, cis-3-hexenol, 2-isobutylthiazole, 6-methyl-5-hepten-2-one, geranylacetone, 2-phenylethanol, β-ionone, 1-penten-3-one, 3-methylbutanol, 3-methylbutanal (Buttery and Ling, 1993), acetone and 2-pentenal. The main precursors of these volatile compounds in tomato are the free amino acids, fatty acids and carotenoids (Table 1).

The C₆ aldehydes (hexanal, cis-3-hexenal, trans-2-hexenal) are known as ‘green’ compounds, as they impart a fresh, green character to tomato aroma, while ketones (acetone, geranylacetone and β-ionone) are contributors to a fruity aroma. These compounds have also been isolated from the nosespace of individuals consuming fresh tomatoes (Brauss et al., 1998)

It is not clear how much volatile compounds in tomatoes interact with other components in the fruit. The objective of this study was to determine the odor thresholds of the primary volatile compounds in fresh tomato aroma in three different media — deionized water, an ethanol/methanol/water mixture (simulating levels normally found in homogenized ripe tomatoes) and a deodorized tomato homogenate — to determine if the medium affects aroma perception.