Effect Of Ice-Nucleation Bacteria On Model Food Freezing

Energy consumption and ice growth which directly affects the quality of frozen food haveimportant effect on the economic feasibility of the freezing process. Ice nucleation active bacteriacan initialize ice nucleation at higher subzero temperature, which leads to reduction of freezing timeand formation of smaller ice crystals. Ice nucleation active bacteria may have profound potential infood freezing industry, however, the relevant research are limited, which may be explained by themulti-disciplinary characteristic of the study. It concerns bacteriology, food engineering andthermophysics. In order to have a better understanding of ice nucleation active bacteria in foodfreezing, further study is needed. The aim of this thesis is to quantitatively study how theconcentration of ice nucleation active bacteria will influence food freezing process. Theoretical andexperimental works were carried out as follows:1. Pseudomonas syringae pv. panici was chosen as ice-nucleation active bacteria and its activitywas measured with the droplet method from Vali. For the first and the second batches ofcultured bacteria, the ice nucleation activity units per unit volume were measured to be 3.29×10~4 INA/mL and 6.9×10~5 INA/mL, respectively.2. The effects of cooling temperature, sample size and concentration of ice-nucleation bacteria onfood analogue freezing were studied by the cooling curve method. It was found that: 1) lowcooling temperature and large sample size led to the reduction of supercooling degrees; 2)bacteria exhibited better activity in the case of high cooling temperature and/or small samplesize; 3) initial freezing temperatures were independent of the concentrations of ice-nucleationbacteria. Further more, the relationship between the supercooling degree and the concentrationof ice-nucleation bacteria for 10% sucrose solution, 0.9% NaCl solution and 77% Tylose wereinvestigated. It could be expressed as△T=a-bexp (-kC), where C was the concentration ofice-nucleation bacteria, a, b and k were empirical parameters. From the experimental results weconcluded that the aggregation states of the ice nucleator protein led to variety of heterogeneousice nucleator size in food samples. The degree of supercooling may be dependent on theice-nucleator surface area and volume.3. The glass transition temperatures and enthalpies of fusion of 77% Tylose with and without theice-nucleation bacteria were measured by differential scanning calorimeter (DSC). Afteraddition of ice-nucleation bacteria with concentration of 7.60×10~3 INA/g, the glass transitiontemperature increased from -52.5℃to -47.0℃, while the enthalpy of fusion increased from203 J/g to 209 J/g. 4. The effect of ice-nucleation bacteria on ice crystal formation was investigated. Frozen Tylosegels were freeze-dried, then sliced, photographed by scanning electronic microscope (SEM) andanalyzed with image analysis software. Mean ice crystal sizes were determined inside the gel as25.7, 15.8 and 15.4μm for the samples when the concentrations of the ice-nucleation bacteriawere 0, 2.53×10~3 and 7.60×10~3 INA/g, respectively. It could be seen that the mean ice crystalsize was reduced with the addition of ice-nucleation bacteria. The results indicated thatice-nucleation bacteria might be used for the quality improvement of frozen food.5. A mathematical-physical model was established to describe the freezing process in food stuffmixed with ice-nucleation bacteria. The model included the relationship between thesupercooling degree and the concentration of ice-nucleation bacteria measured fromexperiments. Succar's semi-empirical equations and Pham's empirical equations wereintroduced to calculate the thermophysical properties. Freezing times predicted by this modelwere compared with experimental data for 77% Tylose samples (a food analogue). The averagediscrepancy was less than 10%, which shows the model has a good accuracy.6. Application of ice-nucleation bacteria in partially freezing, quick freezing and cold storage wereestimated through examples, where the concentration of ice-nucleation bacteria in cooled meatballs was changed in the range of 0 to 6.94×10~3 INA/g. The results demonstrated that outputimprovement and energy saving could be expected if the temperature of the cooling mediumwas relatively high, however, if the temperature of the cooling medium was relatively low, theaddition of ice-nucleation bacteria almost did not affect the freezing time.On the whole, the effects of ice-nucleation bacteria on food freezing process were influencedby many factors. Mathematical-physical analysis could be utilized to predict the relationshipbetween concentration of ice-nucleation bacteria and freezing time. With the application of bacterialice nucleation, some current food freezing processes may be modified to operate at higher subzerotemperatures which leads to energy savings, and smaller ice crystals are formed which lead toimprovement of product quality.