Abstract:
Milk fouling has been regarded as a major problem in the heat exchangers used in the dairy industry. If the thermophilic bacteria are involved in the milk fouling, problem become much severe. The major issue in the microorganism associated fouling deposits (biofouling) is the lack of knowledge regarding the existence and growth of bacteria on the process equipment surfaces and in the fouling deposits. Therefore, in the present work, milk bio-fouling was investigated through the fundamental approaches. The studies concluded in this investigation are summarised as follows: 1. The nature and extent of milk fouling with and without the addition of Bacillus stearothermophilus have been studied. A fouling detection system using electric conductivity was used. The corresponding thermal resistance of the deposit layer has also measured. The fouling deposits have been analysed visually as well as with a scanning electron microscope (SEM). The results show significant variations in the fouling patterns with and without the presence of large number of bacteria in the flowing medium. 2. Bacillus stearothermophilus was used as a representative thermophilic bacterium and the main milk components were used as substrates to study the growth behaviour of this species. The growth of Bacillus stearothermophilus has been found to promote acidic conditions in both milk and lactose solutions. It has been found that milk proteins sustain Bacillus stearothermophilus in stationary phase longer than the other milk components during the batch growth. Milk protein solution which contained Bacillus stearothermophilus formed thick, dense porous lumps, more rapidly than that without the bacteria. The presence of biological material on heat exchanger surfaces may promote other fouling mechanisms. Therefore, implications of the present study to the interactions between the bacteria and milk fouling are an important information. 3. A simple mathematical model describing the whole process of batch growth of bacteria in a single nutrient component (solution) has been proposed. The essence of this model is a proposal of the use of the growth-history-dependent 'carrying capacity' or 'stagnation' live cell population in the conventional logistic growth model. The rationale of this model has been summarised and the model has been tested using the new experimental data on Bacillus stearothermophilus growing in three different liquids with dairy ingredients. Reasonable agreements between the predicted and the experimental results have been obtained. 4. The phenomena of the bacteria emission from a porous layer has been investigated. Various process fluids were flushed over the top of a model milk foulant layer, which contains high percentages of milk proteins, fat and some bacteria cells to investigate the behavior of the 'resident' microorganisms and how they are 'released' into the flushing liquids. Definitive results were obtained, which have created sufficient interest for a different approach taken later, where the fabric layers were used as the support for the bacteria cells to explore the 'generic' behavior of the porous layer-bacteria system. This study has shown that Bacillus stearothermophilus could multiply on or within a porous layer and 'migrate' from the layer into the fluid during processing. This "migration' is somewhat peculiar in terms of its time-responses but these are reproducible in all the tests performed. The phenomena observed may have an impact on future microbial safety practice in food factories. 5. In this study, the cleaning of milk deposits formed in a heat exchanger in the presence of Bacillus stearothermophilus was investigated using 0.5 wt % sodium hydroxide solution. The efficiency of the cleaning process was evaluated using the electrical resistance method. Three different types of fouling deposits were examined: freshly generated deposits, aged deposits and Bacillus stearothermophilus associated aged deposits. The freshly generated deposits, characterized by porous hollow matrix, were cleaned most easily. The aged deposits were harder and took longer to clean. The bacteria associated deposits were the hardest to clean.