An unusual spoilage condition of chill-stored normal pH vacuum-packed beef characterised by gross distension (blowing) of the packs and of-odours perceived on initial opening of the packs as 'sulphurous' and subsequently as yuity', 'solvent-like' and 'strong cheese' has been chemically defined. Hydrogen and carbon dioxide were the major components of the headspace gases. Other components included hydrogen sulphide, methanethiol, dimethyl sulphide, dimethyl di-and trisulphides, methylthioacetate, 1-butanol, acetic and butanoic acids, and 1-butyl esters of a range of C,-C6 fatty acids. Published sensory data are consistent with components of this mixture being responsible for the off-odours detected. When grown in pure culture on beef under identical storage conditions, a Clostridium sp isolated from one of the blown packs produced the same chemical and sensory spoilage characteristics.

Volatile compounds produced by Pseudomonas fragi and mixed, natural floras on beef of normal pH (5.5-5.8; glucose greater than 1500 micrograms/g) and high pH (6.3-6.8; glucose less than 10 micrograms/g) included a range of alkyl esters and a number of sulphur-containing compounds including dimethylsulphide but not hydrogen sulphide. Production of the last was a property common to the other Gram-negative organisms tested viz. Hafnia alvei, Enterobacter agglomerans, Serratia liquefaciens, Alteromonas putrefaciens and Aeromonas hydrophila, all of which produced similar off-odours and, with the exception of E. agglomerans, 'greening' on high pH meat. Serratia liquefaciens also produced greening of normal pH meat. Acetoin and diacetyl were major end products of Brochothrix thermosphacta but the related 2,3-butanediol was formed only on normal pH meat. The Enterobacteriaceae produced the same compounds but only on normal pH meat and together with Br. thermosphacta were probable sources of these compounds and of the free and esterified branched-chain alcohols detected in the naturally contaminated samples.

The microbial flora of naturally contaminated beef stored in air was similar to that frequently recorded for meat stored under gas permeable films. Compounds produced as a result of microbial growth were acetoin, diacetyl, 3-methyl-1-butanol, 2-methyl-1-propanol, ethyl esters of acetic, propionic, butyric, isovaleric and hexanoic acids, methane thiol, dimethylsulphide, dimethyl disulphide, 1-undecene and 1,4-undecadiene. The first four compounds, which are known end-products of Brochothrix thermosphacta metabolism, were consistently detected at earlier stages of storage than the others, all of which have been shown to be produced by Pseudomonas spp. A pattern of odour development consistent with the chemical changes was also observed.

Acetoin and acetic, isobutyric and isovaleric acids are major end-products, and important components of the spoilage odours, of Brochothrix thermosphacta growing aerobically on meat surfaces or in tryptone-based medium containing glucose, ribose or glycerol. Acetoin and acetic acid are probably derived entirely from the carbohydrates and isobutyric and isovaleric acids from valine and leucine respectively. Glucose and pH are both important factors in controlling the relative amounts of end-products, low glucose and near neutral pH favouring fatty acid formation, high glucose and lower pH values favouring acetoin formation. w \o P

Of the meat strains of streptobacteria, leuconostocs, Enterobacteriaceae and Brochothrix thermosphacta tested, only Hafnia alvei and Serratia liquefaciens showed diamine-producing potential during growth in pure culture on beef stored in vacuum packs at 1 degree C. Both organisms produced cadaverine at concentrations similar to those reported previously in naturally contaminated beef stored under the same conditions. Putrescine concentrations produced by the two organisms, however, were an order of magnitude lower. During the growth on beef of either H. alvei or S. liquefaciens in mixed culture with arginine-utilizing strains of streptobacteria, putrescine as well as cadaverine concentrations were similar to those detected in naturally contaminated samples.