Different molecular types of Pseudomonas fragi have the same overall behaviour as meat spoilers

The functional diversity of a population of sixty-five different strains of P. fragi isolated from fresh and spoiled meat was studied in order to evaluate the population heterogeneity related to meat spoilage potential. The strains were characterized for the proteolytic activity at 4°C on beef sarcoplasmic proteins and only 9 strains were found to be proteolytic. An iron-dependent growth behaviour was shown when each strain was grown in citrate medium containing either myoglobin, haemoglobin or iron chloride as iron sources. Increase of maximum population and µmax in presence of different iron sources was registered. The release of volatile organic compounds (VOC) by each strain in beef during aerobic storage at 4°C was evaluated by GC-MS. A considerable variability of occurrence of each molecule in the GC-MS profiles obtained by the different strains was observed ranging from 3% to 79% although the strains showed a high degree of similarity. In particular, ethylhexanoate, ethyloctanoate, ethylnonenoate, ethyldecanoate, 1-octen-3-ol, 3-octanone, 4-methylthiophenol, 2-pentylfurane were produced by more than 50% of the strains. Representative strains were used to spoil meat in the same conditions used for the VOC analysis and the samples were evaluated by a sensory panel. The results of the sensory analysis indicated that the different strains could significantly affect the odour of meat and strains characterized by production of esters gave fruity odours to the spoiled meat. However, the similarity of strains based on the sensory profiles does not necessarily match the similarity shown in VOC profiles. P. fragi has a significant role in the microbial ecology of meat and the influence of meat-related sources of iron on the growth behaviour of many different strains suggests that meat can be an ecological niche for P. fragi. Regardless of the proteolytic and lipolytic capacities shown in vitro, different molecular types of P. fragi can release odour active volatile molecules and play a similar overall role as spoilage agents of meat.