Microbiology of food taints

Fresh and processed foods are often spoilt by the presence of undesirable flavours and odours caused by microbial action. The aim of this paper is to review the current knowledge of microbiologically induced taints that occur in a wide range of foodstuffs, including meats, poultry, fish, crustaceans, milk, dairy products, fruits, vegetables, cereals and cereal products. Examples have been chosen where the compounds responsible for the taint have been identified and sufficient data obtained to demonstrate the involvement of microorganisms. However, in some cases the full identity of the causative organism may not have been elucidated. The types of microorganisms covered by this review include bacteria, fungi, yeasts, actinomycetes and cyanobacteria. Although cyanobacteria do not in general infect foods, their presence in aqueous systems and water supplies can lead to off-flavours in aquatic organisms and processed foodstuffs. Several examples of each of these processes are discussed. Wherever possible, the likely biosynthetic pathway used by the microorganism to produce the offending compound in a foodstuff is indicated.     

Volatile off-flavour compounds in desiccated coconut

Aliphatic methyl ketones and secondary alcohols have been isolated from desiccated coconut and shown to be responsible for the rancid off-flavour. Two series of compounds have been isolated. The series with an odd number of carbon atoms (C_5,7,9,11) comprising pentan-2-one, pentan-2-ol, heptan-2-one, heptan-2-ol, nonan-2-one, nonan-2-ol and undecan-2-one is derived from even chain fatty acids one carbon atom longer, present in the coconut oil by a modified β-oxidation due to growth of moulds. Heptan-2-one and nonan-2-one were present in the greatest concentration in the rancid coconut. These compounds gave an odour reminiscent of rancid almonds and turpentine respectively while heptan-2-ol and nonan-2-ol gave an odour similar to rancid coconut as well as a musty, stale odour. This type of off-flavour has been called ketonic rancidity and is an oxidative variation of the hydrolytic type of rancidity. The presence and origin of the even numbered series (C_6,8), hexan-2-one, hexan-2-ol and octan-2-one is discussed. The increase in C_5,6,7,8,9,11 oxidation products in the rancid samples occurred at the expense of C_8,10,12 short chain fatty acids.