A unifying model to estimate thermal tolerance limits in ectotherms across static, dynamic and fluctuating exposures to thermal stress

Temperature tolerance is critical for defining the fundamental niche of ectotherms and researchers classically use either static (exposure to a constant temperature) or dynamic (ramping temperature) assays to assess tolerance. The use of different methods complicates comparison between studies and here we present a mathematical model (and R-scripts) to reconcile thermal tolerance measures obtained from static and dynamic assays. Our model uses input data from several static or dynamic experiments and is based on the well-supported assumption that thermal injury accumulation rate increases exponentially with temperature (known as a thermal death time curve). The model also assumes thermal stress at different temperatures to be additive and using experiments with Drosophila melanogaster, we validate these central assumptions by demonstrating that heat injury attained at different heat stress intensities and durations is additive. In a separate experiment we demonstrate that our model can accurately describe injury accumulation during fluctuating temperature stress and further we validate the model by successfully converting literature data of ectotherm heat tolerance (both static and dynamic assays) to a single, comparable metric (the temperature tolerated for 1 h). The model presented here has many promising applications for the analysis of ectotherm thermal tolerance and we also discuss potential pitfalls that should be considered and avoided using this model.

Shelf-life Prediction Modeling and Physicochemical Changes of Canned African Giant Snail (Achachatina achatina) Based Products during Storage Using Sensory and Kinetic Data

Canned African giant snails (brine, egusi and sauce) of low acidity (pH 4.5) were produced using Thermal Death Time (TDT) techniques. Sixty cans of each product were stored for 6 months at normal temperature (30oC). The physicochemical and sensory properties of these products were evaluated monthly. And also, 28 cans of each product were equally evaluated at accelerated storage temperatures stress (50, 60, 70 & 80oC) for 6 weeks. Changes in pH and overall acceptability were described by first and second order kinetics respectively. The extent of proteolysis, peroxidase activities, and extent of fat hydrolysis i.e peroxide and thiobabituric acid values did not change significantly (P > 0.05) during storage. Regression analysis was used to fit models for Gibbs free energy of activation for physicochemical changes as a function of temperature and to predict models for overall acceptability as a function of pH. Shelf-life prediction models were fitted based on sensory and pH kinetic data. Shelf-lives of 27.6 months, 27-25 months and 18-16.8 months were estimated for snail in brine (SIB) snail in egusi (SES) and snail in sauce (SIS) respectively during storage at 30OC.

The old and the large may suffer disproportionately during episodes of high temperature: evidence from a keystone zooplankton species

Widespread declines in the body size of aquatic ectotherms have been attributed to the poorer ability of older, larger individuals to tolerate high temperature. Here, using the thermal death time curve framework, we investigate the relationship between temperature tolerance and size/age by measuring the change in heat tolerance of the keystone zooplankton species Daphnia magna across a range of temperature intensities (and hence exposures of varying duration) among individuals that differed up to 3-fold in size and thus varied in age also. Across the gradient of exposure temperatures, younger, smaller individuals were more tolerant than older, larger individuals. This suggests that the young and the small may be better equipped to withstand temperature challenges that are both intense/brief and more moderate/prolonged. Our study generalizes results obtained from more acute tolerance assays, providing physiological evidence consistent with the observed reductions in ectotherm body size as a response to warming in aquatic systems.

Determination of Co-value as an indicator of nutritive value of pâté sterilised by regular and optimized regimes

Introduction. Heat treatment of food provides appropriate shelf life and safety, as well as adequate sensory properties and better digestibility, but excessive thermal processing leads to the loss of nutritive value. In practice, sterilised meat products are usually overheated, which leads to a loss of nutritive value. Therefore, it is necessary to find the optimum between the preservative effect of heat treatment and maintenance of the nutritive value of food. Control of the sterilisation efficiency is based on the Fo value, an indicator of the lethal effect of heat treatment, while the cooking (Co) value indicates a reduction of the nutritive value of heat treated products. Materials and Methods. During p?t? sterilisation processes, two working heat treatment regimes were used: the regime used in regular, commercial production and an optimised regime. Heat treatment measurements were carried out using six thermocouple probes placed in the geothermal centres of six chosen cans. For each heat treatment, schematic diagrams were produced. Fo values were determined using Ellab software, and Co values by the graphic method in a semi-logarithmic Thermal Death Time diagram. Results and Conclusions. During p?t? sterilisation, where the effective time was 55 minutes (regular regime) at 114?C and 3.2 bar pressure, an average Fo value of 7.90?0.43 was achieved. By reducing the effective sterilisation time to 45 minutes (optimised regime), an average Fo value of 3.81?0.5 was achieved. Both regimes were sufficient to ensure product safety (Fo >3). The average Co value for the regular sterilisation regime was 109.83?1.33 minutes, while for the optimised regime, it was 88.67?4.27 minutes, i.e., an average reduction of 19.26%. This corresponds to a proportionally higher nutritional value of the product treated using the optimised regime. Data on the Fo and Co values achieved during the sterilisation process can make a significant contribution to obtaining products with a better nutritional value through an optimised sterilisation process, without jeopardising product safety.

Evaluating Pediococcus acidilactici and Enterococcus faecium NRRL B-2354 as Thermal Surrogate Microorganisms for Salmonella for In-Plant Validation Studies of Low-Moisture Pet Food Products

Pediococcus acidilactici ATCC 8042 and Enterococcus faecium NRRL B-2354 were investigated as potential surrogates for Salmonella serovars using thermal death time kinetics in products such as dry pet foods. The D-values of P. acidilactici ATCC 8042, E. faecium NRRL B-2354, and a cocktail of seven Salmonella serovars associated with low-moisture products were determined in a preservative-free dry pet food product at moisture levels of 9.1, 17.9, and 27.0% and heated between 76.7 and 87.8°C. The D-values were calculated by least squares linear regression. The D-values of P. acidilactici ATCC 8042 were higher than those for the Salmonella serovar cocktail but lower than those for E. faecium NRRL 2354. At 9.1% moisture, D-values of 6.54, 11.51, and 11.66 min at 76.7°C, 2.66, 3.22, and 4.08 min at 82.2°C, and 1.07, 1.29, and 1.69 min at 87.8°C were calculated for Salmonella serovars, P. acidilactici ATCC 8042, and E. faecium NRRL B-2354, respectively. The data suggest that the thermal inactivation characteristics of P. acidilactici ATCC 8042 can be utilized as a surrogate to predict the response of Salmonella in dry pet food products that are thermally processed at <90°C.