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- Recovery of heat-injured listeria innocuaPublication . Miller, Fátima A.; Brandão, Teresa R. S.; Teixeira, Paula; Silva, Cristina L.M.Listeria innocua was subjected to thermal inactivation and the extent of heat-injured cells was quantified. Cultures were heated in liquid medium for different times, using temperatures in the range of 52.5 to 65.0 °C, and plated on Tryptic Soy Agar with 0.6% yeast extract (TSAYE) used as non-selective medium and on TSAYE plus 5% NaCl (TSAYE+NaCl) and Palcam agar with selective supplement (Palcam agar) as selective media. The difference observed in counts in non-selective and in selective media gave an indication of cell injury during the heat treatment. D- and z- values were calculated for all conditions considered. For each temperature, D-values obtained using non-selective recovery procedures were higher than the ones obtained using the two selective media. When comparing the selective media, it can be concluded that Palcam agar allowed recovery and growth of thermally injured cells and so it was less inhibitor than TSAYE+NaCl. Another important result was the influence of temperature on the degree of cellular injury. As temperature increases, the degree of heat-injured cells also increases, and consequently concern has to be taken with the temperature and the counting medium used in food processing studies. The results of this work clearly demonstrated that selective media used for Listeria monocytogenes enumeration/detection might not be suitable for the recovery of heatinjured cells, which can dangerously underestimate the presence of this foodborne pathogen.
- Heat inactivation of Listeria innocua in broth and parsley under non-isothermal conditionsPublication . Miller, Fátima A.; Ramos, Bárbara F.; Brandão, Teresa R. S.; Teixeira, Paula; Silva, Cristina L. M.
- Effect of temperature, pH and type of acid on the inactivation of Listeria innocuaPublication . Miller, Fátima A.; Brandão, Teresa R. S.; Teixeira, Paula; Silva, Cristina L. M.
- Heat inactivation of Listeria innocua in broth and parsley under non-isothermal conditionsPublication . Miller, Fátima A.; Ramos, Bárbara F.; Brandão, Teresa R. S.; Teixeira, Paula; Silva, Cristina L. M.
- Influences of physicochemical stresses on injury and inactivation behaviour of Listeria innocuaPublication . Miller, Fátima A.; Ramos, Bárbara; Gil, Maria M.; Brandão, Teresa R. S.; Teixeira, Paula; Silva, Cristina L. M.Many minimally processed foods depend on a combination of inhibitory factors to reduce the hazard of foodborne illness. Therefore, inactivation of Listeria innocua was studied according to a 24 factorial experiment designed to draw conclusions about temperature (52.5 °C and 65.0 °C), pH (4.5 and 7.5), water activity (aw=0.95 and 0.99) and solute type (NaCl and glycerol) effects. Three different recovery media were used to assess injured cells. Survival data were fitted with a Gompertz-based model and kinetic parameters (shoulder, maximum inactivation rate – kmax, and tail) were estimated. Results showed that shoulder was affected by temperature, pH and combined effects; kmax was influenced by all factors and their combinations; and tail was affected by aw, temperature and aw/pH combination. Results demonstrated the potential occurrence of microbial cross-protection survival techniques between the various stresses, e.g. heat and osmolarity. Indeed, this work clearly established that, to avoid hazards, Listeria inactivation must be evaluated with a maximum of environmental factors that undergo alterations. Only thus, appropriate food preservation treatments can be developed and consequently, the safety of food products can be assured.
- Comparison of recovery methods for the enumeration of injured Listeria innocua cells under isothermal and non-isothermal treatmentsPublication . Miller, Fátima A.; Ramos, Bárbara; Brandão, Teresa R.S.; Teixeira, Paula; Silva, Cristina L.M.This study compares the feature of different media with the combination of selective with non-selective media in a TAL method for recovery of Listeria innocua cells exposed to thermal treatments. Experiments were conducted in broth at constant temperature (52.5 and 65.0 ºC) and pH (4.5 and 7.5) conditions, using NaCl or glycerol to adjust water activity to 0.95. Four different media were used in bacterial cell enumeration: (i) a non-selective medium e TSAYE, (ii) two selective media e TSAYE þ 5%NaCl and Palcam Agar and (iii) TAL medium (consisting of a layer of Palcam Agar overlaid with one of TSAYE). Two food products were used as case studies aiming at comparison of results obtained on selective and TAL media enumeration. Parsley samples were inoculated with L. innocua and subjected to posterior thermal treatments both under isothermal (52.5, 60.0 and 65.0 ºC) and non-isothermal (heating rate of 1.8 ºC/min from 20.0 to 65.0 ºC) conditions. The recovery capability of TAL method was also studied when a pre-cooked frozen food (i.e. meat pockets) was fried (oil temperature of w180 ºC). TAL method proved to be better than Palcam Agar in terms of capability to recover injured cells and was effective in L. innocua enumeration when non-sterile samples were analysed.
- On the assessment of an agar overlay method for the recovery of heat-injured Listeria innocua in parsleyPublication . Miller, Fátima A.; Ramos, Bárbara F.; Brandão, Teresa R. S.; Teixeira, Paula; Silva, Cristina L. M.
- Sigmoidal thermal inactivation kinetics of listeria innocua in broth: Influence of strain and growth phasePublication . Miller, Fátima A.; Gil, Maria M.; Brandão, Teresa R. S.; Teixeira, Paula; Silva, Cristina L.M.Listeria innocua inactivation was studied within the temperature range 52.5–65.0 ºC, comparing two different strains (10528 and 2030c) and two growth phases (exponential and stationary). Survival curves may present a sigmoidal behaviour, with an initial shoulder (L), followed by a maximum inactivation rate (kmax) period and a final tailing tendency. A Gompertz-inspired model was used to fit experimental data, and kinetic parameters (L, kmax and tail) were estimated by non-linear regression analysis. The influence of temperature, growth phase and strain on kinetic parameters was studied using a 23 factorial experimental design. Results showed that temperature and growth phase were the most significant variables affecting the kinetic parameters. Listeria thermal inactivation varied from a log-linear tendency till a pronounced sigmoidal behaviour, depending on the studied factors.
- Recovery of heat-injured Listeria innocuaPublication . Miller, Fátima A.; Brandão, Teresa R. S.; Teixeira, Paula; Silva, Cristina L. M.
- Heat inactivation of Listeria innocua in broth and food products under non-isothermal conditionsPublication . Miller, Fátima A.; Ramos, Bárbara F.; Gil, Maria M.; Brandão, Teresa R. S.; Teixeira, Paula; Silva, C. L. M.The objective of this work was to study the effect of three linear temperature profiles (heating rates of 1.5, 1.8 and 2.6 °C/min, from 20 to 65 °C) on Listeria innocua inactivation in liquid medium. The inactivation was also analyzed in artificially contaminated parsley (heating rate of 1.8 °C/min) and throughout a frying process, using a pre-cooked frozen food as case study. Inactivation showed a sigmoidal behaviour and all data was fitted with a Gompertz-inspired model. Results demonstrated that, in liquid media, Listeria inactivation is influenced by the temperature profile used. As heating rate increases, the shoulder decreases and the tail effect disappears. If Listeria was in parsley, its heat resistance increased (for identical experimental conditions in broth). Besides model adequacy was proven in all studied situations, the heating rate affected parameters’ precision.