The BACIS Archives

[ POM - 98011 ]

• Sensory Properties and Molecular Structures of Aromatic Compounds

INTRODUCTION

Benzene derivatives are called aromatic compounds, because within this group of compounds a lot of substances exist, which posses aromatic, spicy and tasteful odours. Below the structural features and the sensory properties, e.g. odour qualities and threshold values, of aromatic compounds will be discussed.

DISCUSSION

Aromatic compounds concern mono- and poly-substituted benzene compounds as, for instance, benzaldehyde, anethole and anisaldehyde, eugenol and vanillin, safrole and heliotropin. As substituent one finds in these compounds the following functional groups: aldehyde (-CHO), hydroxyl (-OH), alkoxyl (RO-), allyl (-CH2-CH=CH2), propenyl (-CH=CHCH3) and dioxymethylene (-O-CH2-O-). The group of compounds concerning, for example, anethole, eugenol, safrole, myristicin, elemicin and apiole is also called the phenylpropanoid group, due to the fact that their molecules consist of a benzene nucleus (phenyl-group) and a propane-substituted sidechain.

The sensory qualities of the allyl- and propenyl-substituted benzene derivatives are very characteristic, that is to say these compounds are character-impact compounds for certain natural isolates as, for instance, in:

Compound	Allyl/Propenyl	Other Substituents	Sensory Quality
methylchavicol	allyl	methoxyl	basil-like
anethole	propenyl	methoxyl	anise-like
eugenol	allyl	hydroxyl & methoxyl	clove-like
safrole	allyl	dioxymethylene	sassafras-like

The allyl-substituted benzene derivatives can be oxidised to the corresponding aldehydes, which in turn also possess very characteristic odour qualities as, for example, in:

Compound	Sensory Quality
benzaldehyde	bitter almond-like
2-methoxybenzaldehyde	hawthorn-like (aubepine)
3-methoxy-4-hydroxybenzaldehyde	vanilla-like
3,4-dioxymethylenebenzaldehyde	heliotrope-like

Various studies about the sensory properties and structural features of aromatic compounds have been published. Ranson and Belitz (Z Lebensm Unters Forsch (1992)) reported on investigations in structure-activity relationships of alicyclic and aromatic compounds. They studied threshold values and odour qualities of a series of alicyclic and aromatic aldehydes. They found that aldehydes bearing terminal phenyl or cycloalkyl substituents exhibited threshold minima at eight carbon atoms, as did saturated chain aliphatic aldehydes. Straight alkyl chains resulted in lower threshold values than cyclic structures.

According to published data the threshold values of substituted benzaldehydes in air (mg/m³) are:

Compound	Substituent	Threshold Value
benzaldehyde	none	0.014 - 0.6
o-tolualdehyde	2-methyl	0.001
salicylaldehyde	2-hydroxy	0.014 - 0.4
anisaldehyde	2-methoxy	0.0015 - 0.034
vanillin	4-hydroxy-3-methoxy	0.000001 - 0.002
heliotropin	3,4-dioxymethylene	0.0005 - 0.075