2.13. Off-flavours

A strange, extraneous type of aroma normally not present in a food may arise through loss of "impact compounds" or a shift in aroma concentration or change in composition of the individual components of the aroma. This is designated as an aroma defect, or simply as an off-flavour. Some common off-flavours which can arise during food processing and/or storage are listed in the following table:

Food product	Off-flavour	Cause
Milk	Sunlight flavour	Photooxidation of methionine to methional (with riboflavin as a sensitizer)
Milk powder	Bean-like	The level of O3 in air too high: ozonolysis of 8.15- and 9.15-isolinoleic acid to 6-trans-nonenal
Milk fat	Metallic	Autoxidation of pentaene- and hexaene fatty acids octa-1.cis-5-dien-3-one
Milk products	Malty	Faulty fermentation by Streptococcus lactis, var. multigenes; formation of phenylacetaldehyde and 2-phenylethanol from phenylalanine
Milk powder	Gluey, glutinous	Degradation of tryptophan to o-amino-acetophenone
Mutton-meat	Sweet, acidic	4-Methyloctanoic acid, 4-methylnonanoic acid
Peas, deep frozen	Hay-like	Saturated and unsaturated aldehydes, octa-3,5-dien-2-one, 3-alkyl-2-methoxy-pyrazines, hexanol
Orange juice	Grapefruit note	Metal-catalyzed oxidation or photooxidation of valencene to nootkatone:
Orange juice	Terpene note	d-Limonene oxidation to carvone and carveol:
Passion fruit juice	Aroma flattening during pasteurization	Oxidation of (6-trans-2-trans)-6-(but-2-enyliden)-1,5,5- -trimethylcyclohex-1-ene to 1,1,6-trimetyl-1,2-dihydronaphthalene:
Beer	Sunlight flavour	Photolysis of humulon: reaction of one degradation product with hydrogen sulfide yielding 3-methyl-2-buten-1-thiol
Beer	Phenolic note	Faulty fermentation: hydrocinnamic acid decarboxylation by Hafnia protea

Examples of microbial metabolites which may be involved in pigsty-like and earthy-muddy off-flavours are skatole (I; faecal-like, odour threshold in water: 10 mg/kg), 2-methylisoborneol (II; earthy-muddy, 0.03 mg/kg) and geosmin (III; earthy, 0.01 mg/kg):

I II III IV

2,4,6-trichloroanisole with an extreme low odour threshold (IV; mouldy-like, 3*10^-5 mg/kg) is an example of an off-flavour substance which is produced by fungal degradation and methylation of pentachlorophenol fungicides.

Off-flavours can also be introduced by the absorption of chemicals form the air, from the water used in processing, from packaging materials and from the environment in which the food has been grown, processed and stored.The chemicals responsible are principally volatile organic compounds which can be present at concentrations less than 1 ppb.