Collagen and fats-enriched smoked sausages: effects on Listeria monocytogenes thermal resistance and textural changes

The principal aim of this work was to investigate the effects of both composition and processing conditions on thermal resistance of Listeria monocytogenes deliberately inoculated in poultry meat sausages. In particular, the survival of Listeria monocytogenes as well as of mesophiles and Enterobacteriaceae through the processing phases (i.e. smoking, cooking and post-packaging pasteurization) of sausages with different collagen and fat contents was evaluated. Moreover, the deactivation curves during the cooking phase of two strains of Listeria monocytogenes in relation to the percentages of collagen (0-4.5% w/w) and lipids (0-8%) added to the meat batter were obtained. The comparison of the experimental data relative to the addition of different fat contents outlined that the inactivation dynamics of the two strains were similar: in particular, they were characterised by a first order kinetics with lower slopes when the collagen percentage added to the meat was reduced. However, the addition of the highest collagen content resulted in a biphasic behaviour, commonly referred as \u201ctailing\u201d. Such phenomena could be the consequence of changes in the heat conductivity of the system associated with protein gelation and microstructure changes induced by the heat treatment. The comparison of microbiological and rheological (dynanometric index and shearing stress) data evidenced a relation between the increase of the dynamometric index and microbial heat inactivation. In particular, the highest deactivation rates were observed in the samples characterised by a rapid microstructural change dynamics, i.e. samples with added 8% of fats. The composition of meat batter influenced also the evolution of water activity (Aw) during the heat treatment: in fact, all the curves relative to sausages with added collagen and a low fat percentage (4%) showed a sharper Aw reduction if compared with that of the control sample (without collagen and fats added). However, in the presence of 8% of fats, the Aw values were scattered. This effect could be attributed to an interference with the Aw analytical measurement (i.e. water transition to vapour phase) or to the presence of barriers to water diffusion which resulted in a reduced protein gelation degree and hence in a lower protein-water interaction. On the other hand, it has been proposed that that microbial heat resistance of Bacillus subtilis and Bacillus pumilus inoculated in model system increases as the water activity of the system, or better the water vapour pressure, is reduced (Lerici et al., unpublished dat