Thermal kinetics of enzymatic time-temperature integrators (TTIs) were experimentally evaluated under both conventional and microwave heating systems in the pasteurization temperature range (50 to 90°C). Recent developments of process evaluation methodologies have Shown that standardized enzymatic time-temperature integrators (TTIs) could be successfully used for fast and correct quantification of thermal processes. Promising results have been reported for the alpha-amylase based TTI from Bacillus subtilis (BAA), which was chosen in this study as the TTI to compare the effectiveness of continuous-flow heating systems with microwave and conventional heating modes. Thermal inactivation kinetics of alpha-amylase was studied by measuring the residual activity of heat treated samples in isothermal conditions in a temperature range of 50 to 95°C and pH range, 5.0 to 6.9. Based on a first order rate of inactivation kinetics, kinetic parameters, decimal reduction time, D, and temperature sensitivity indicator, z, were calculated. (Abstract shortened by UMI.)

Continuous thermal pasteurisation of various bulk liquid media is an important step in the food and allied industries. The design of a continuous flow pasteuriser is typically predicated on mathematical models developed from experimental data - usually batch, bench - scale, ethods. Of particular interest is the effect of combined pasteuriser temperature ( T ) and liquid pH on inactivation and survivor of contaminants. However, bench - scale thermal survivor data may not adequately mirror those in a continuous flow pasteuriser. This research presents the development and experimental validation of rigorous models for thermal pasteurisation of bacteria as affected by combined process T - pH in both batch, bench - scale capillary studies ( static ) and in a pilot continuous flow pasteuriser ( dynamic ), within a defined liquid and range of exposure time, temperature and pH ( t - T - pH ). Five integrated stages in synthesis and model analysis were undertaken using stringent criteria for goodness of fit of an adequate model established. First, four published predictive models were assessed against published static data ( n [subscript T] = 248 ) for the thermal inactivation of Escherichia coli ( ATCC 25922 ) in a Carbopol ® 941 liquid food simulant in batch capillaries over a range of t - T - pH. The models tested were the Classical Arrhenius, Davey Linear - Arrhenius ( D - LA ), Square - Root ( Belehradek ) and a third - order Polynomial model ( nOP ). Analysis showed the D - LA model best satisfied the criteria for model selection and explained 96.0 % V in the thermal inactivation rate coefficient. Second, the D - LA model was assessed against limited, published dynamic data ( n [subscript T] = 109 ) for the same E. coli strain in identical food simulant. The model explained 60 % V in the thermal inactivation rate coefficient. On average, model predictions of survivor numbers from the dynamic data were less than that predicted from the static data, i.e. for a given ( t - T - pH ) more bacterial cells were apparently inactivated in the continuous flow pasteuriser than in bench - scale, batch capillary studies. Overall, however it was not clear from extensive analyses of available data whether there is a statistically significant difference in survivor numbers of viable E. coli between batch static and continuous flow dynamic data. Third, although the D - LA model best satisfied the criteria for goodness of fit of a model, it failed to accurately predict the observed tails in the static survivor data. New models ( KDT and a modified KDT ) were synthesised to predict tails and shoulders in survivor data. The modified KDT ( MKDT ) form gave improved predictive capability over the KDT model when assessed against published static survivor data for E. coli and L. monocytogenes ( n [subscript T] = 355 ) in the Carbopol food simulant. This model, however, could not be readily integrated with equations describing the performance of a continuous flow pasteuriser. Analyses indicated that a greater density of dynamic survivor data for E. coli was needed. Fourth, a pilot continuous flow pasteuriser was constructed and used to generate a greater density of dynamic survivor data of E. coli ( ATCC 25922 ) in a Carbopol ® 941 carrier liquid for rigorous comparison with predictions from the Lin ( 1976 ) isothermal continuous laminar flow process model. Direct steam injection heating was used. Extensive dye and digital - video studies, in a section of glass holding tube confirmed the practical implementation of the assumptions of laminar flow and rapid condensation of steam. Extensive practical experiments highlighted a non - isothermal condition along the holding tube. A highly linear dependence ( R ² > 0.90 ) of exposure temperature with holding tube length, i.e. exposure time, was demonstrated. This was accounted for using mathematical approaches and quantitatively incorporated into a D - LA model for the rate coefficient in an extended Lin process model. A block experimental design of 4 T ( 54, 56, 58, 60 ° C ) x 4 pH ( 4.5, 5.5, 6.5, 7.5 ) x 3 replicates with a total of ( n [subscript T] = 834 ) exposure times ( 16 - 198 s ) was carried out in the pilot continuous flow pasteuriser. Findings highlighted that greater numbers of E. coli were thermally inactivated in the flow pasteuriser than predicted. From a practical operating view, the predictions from the extended Lin model were therefore conservative - with reduced risk to public health. Highly significant differences in the rates of heat - up of bacteria in the pilot pasteuriser ( dynamic ) ( 0.0104 s ) compared with that in the batch ( static ) capillary tubes ( 1.6 s ) and, mode of heat transfer, together with partial effects of dispersion with increasing length of pasteuriser holding tube, are postulated to be the controlling process influences for the difference between the experimental survivor data and the extended Lin model predictions. The lack of agreement between the continuous pasteuriser data and predictions from the extended Lin model indicated that this model cannot be practically applied. A direct comparison of the experimentally derived dynamic survivor data from the pilot pasteuriser ( as ln N / N [subscript 0] ) was also made with both the published static and dynamic data at a number of defined t - T - pH. This comparison revealed that overall, more E. coli were inactivated in the pilot continuous flow pasteuriser than described by published batch static capillary and dynamic data. Importantly, these comparisons showed that batch thermal survivor data for E. coli do not adequately mirror those obtained in continuous flow systems. Fifth, in a search for an improved model for the inactivation data, the newly derived MKDT model was assessed against the experimental pilot pasteuriser data. This model was rejected, however, because it could not account satisfactorily for all tails in survivor curves. A Weibull form model with two coefficients ( a scale factor ( α ) and a shape factor ( β ) ) also did not adequately predict tailing and could not be reliably extrapolated with holding time. However, a modified Weibull form, also with two model coefficients ( β [subscript 0], β [subscript 1] ), did give an improved fit to available experimental data. This research highlighted statistically significant differences between the dynamic thermal survivor data for E. coli and standard bench - scale static capillary data for a defined liquid and range of t - T - pH. It is likely that findings from this study can be generalised. However, validation should be carried out for a range of common indicator micro - organisms in a range of liquid foods. / Thesis (Ph.D.)--School of Chemical Engineering, 2006.

M.Sc. (Chemistry) / Please refer to full text to view abstract

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The technical feasibility of ohmic heating was evaluated for viscous liquids in static ohmic heating cells in three stages. First, electrical conductivities and time/temperature profiles were measured and compared for selected hydrocolloids (carrageenan, gelatin, pectin, starch and xanthan) in water at various concentrations (1--6%). Of the thickening agents examined, carrageenan gave the highest value for electrical conductivity and the shortest time to raise the temperature from 20 to 100°C. It was followed by xanthan. Pectin and gelatin samples were found to exhibit lower, but similar electrical conductivities and heating profiles. Starch samples had the lowest electrical conductivity and the temperature of starch solutions never exceeded 62°C within the specified time limit of 10000 s. / In the second stage, the effect of salt and acid concentration was evaluated on electrical conductivities and time/temperature profiles of the above selected hydrocolloids in water solutions at a concentration necessary to achieve a similar apparent viscosity of 0.2 Pa.s at 300 s-1 and 20°C (carrageenan 1.7%, pectin 2.5%, starch 4.3% and xanthan 2%). / In the third stage, measurements of electrical conductivities, time/temperature and heating rate profiles were performed applying six voltage gradients (5.26, 7.14, 13.16, 17.86, 21.05 and 25 V/cm), with two electrode cross-sectional surface areas (19.84 and 37.95 cm2) and the electrodes separated at three specific distances (10.05, 14.33 and 20.01 cm) at 150 V. Applied voltage gradients had a major effect on temperature and heating rate profiles but no effect on electrical conductivities. / Rheological properties of carrageenan, pectin, starch and xanthan solutions were investigated at various temperatures (20--80°C) and concentrations (carrageenan 1.5, 1.7 and 1.9%; pectin 2.3, 2.5 and 2.7%; starch 3.8, 4 and 4.2%; xanthan 1.6, 1.8 and 2%) in the presence of 1% salt. Both temperature and concentration influenced the rheological properties of solutions. / Finally, electrical conductivity measurements and rheological properties of starch at 4% and 1% salt were used as input to evaluate a theoretical model for the electrical, thermal and flow behaviour in a continuous ohmic heating unit. (Abstract shortened by UMI.)

Graduation date: 1961

Graduation date: 1961

Chicken breast muscles (Pectoralis major and Pectoralis minor) were subjected to low doses of gamma radiation (≤ 0.50 Mrad) within 20-30 min. post-slaughter. pH decline extractability of different muscle protein fractions, shear force, isometric tension pattern and sarcomere length were evaluated at definite intervals of post mortem storage period. pH measurements from 1/2 hour to 72 hours post mortem revealed a stimulation of pH decline immediately after irradiation in 0.25 and 0.50 Mrad treated muscle samples but this effect was subsequently found to be nullified and an ultimate pH of about 5.55 was reached between 6-12 hours in all the samples. Extractability of total soluble and total myofibrillar proteins of muscle increased with post mortem time. A clear effect of irradiation treatments of increased extractability of actomyosin especially in the post rigor period was observed and this effect was more noticeable in 0.50 Mrad than in 0.25 Mrad samples. Comparatively less extractable actomyosin was found during the post rigor period of storage particularly in control samples. There was no significant effect of radiation doses, time of storage or their interactions on the solubility of sarcoplasmic proteins or of four minor fractions of protein and non-protein substances. The level of extractable actomyosin was found to be significantly related (r² = 0.12; df=46) with the corresponding shear press measurements. Irradiated samples had significantly higher mean shear values and more extractable actomyosin than the control samples especially during the post rigor period. Irradiation (0.125 and 0. 250 Mrad) was found to stimulate post mortem development of isometric tension in muscle strips. In irradiated samples maximum tension was developed about 2 hours ahead of the corresponding controls. A 50% decline of isometric tension was achieved in 6-8 hours post mortem, while a 50% decline of tension in control samples required more than 12 hours. The general relationship of isometric tension pattern and extractability of actomyosin was found to be low. Sarcomere length was related (r² = .38; df = 6) with shear values such that a small change in sarcomere length was found to result in a relatively large change in shear press values. Irradiated samples with relatively shorter sarcomeres in the post rigor period were found to have significantly lower mean shear values when compared with their respective control samples. / Land and Food Systems, Faculty of / Graduate