Theoretical analyze and modeling of coda wave propagation in multilayer 

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Fouling in heat exchanger of dairy industries

Since the beginning of human civilization, fire, as a method of heat treatment, is used to improve the quality of food. With correct processing parameters, heat treatment is able to eliminate contamination without destroying the nutrients. The principle of this basic idea is still applied presently [6]. Heat treatment techniques are widely applied in the dairy industry, for example Pasteurization (Figure 1.2). The advantages of heat treatments are the guarantee of elimination of the majority of pathogenic microbial, without secondary Heat treatment techniques mainly used in the food industry is pasteurization and sterilization [7]. Most of the enzymes found in nature will be deactivated at a temperature higher than 60 C. Microorganism can be destroyed by deactivating the enzymes. In the pasteurization, the temperature is around 70 – 80 C and the process last for 15 – 20 s. The number of microbials could be effectively decreased with pasteurization. Sterilization is a strict heat treatment technique. The sterilization temperature is usually higher than 110 C and it last for 2 – 5 s. Food products can be stored for long periods with the sterilization treatment.
Deposits is formed very frequently during heat treatment. Fouling in the equipment are always formed in an undesirable way [8, 9].
Other authors have proposed that in agriculture and food industry, the fouling could be classified into 3 types by their composition [10] :
— Organic fouling : Organic molecule is one of the essential component in dairy pro-ducts, for example the protein. And it is common to have organic fouling in dairy production.
— Mineral fouling : Mineral component existed in water and products may adhere on surfaces, especially during heat treatment.
— Microbiology fouling : This kind of fouling is caused by the contamination of mi-croorganisms.
In fact, in the real production, the industry have to face the combination of several fouling types [11, 12]. For example in dairy industry, both the protein (organic) and the calcium (mineral) in milk will take part in the fouling formation.

Importance of the French dairy sector and requirement to control fouling

As a motor of French economy, the dairy industry’s exports in 2018 are 7 billion euros, while the total volume of business is about 30 billion euros. This sector provide more than 250,000 jobs in France, which contribute a lot to the social stability and prosperity. In fact, the agriculture and food sector (more than 11.5 billion euros) is the second largest contributor after the aeronautic industry (more than 22 billion euros) (Source : douanes 2011). In the agriculture sector, 13% product come from dairy industry. The value of dairy products in 2018 is about 8.7 billion euros (Source : La filiére laitière française). As the second producer in Europe and the 8th producer in the world (Eurostat, 2011 ; Agreste, 2011), 23.8 billion litres of milk is used to produce dairy product in France every year.

Importance of heat treatment for texturing and stabilization of dairy product

Heat exchanger is widely used in dairy industry. This kind of equipment is used in many production step, especially for the heat treatments, in order to guarantee the quality of product. Different kinds of heat exchangers, in form of plate or tube, are used to achieve heat treatments (Figure 1.3). Nevertheless, heat treatments are not the perfect solution. I many food production, for example the dairy industry, heat treatments will cause the formation of deposit on the inner surfaces of equipment [2, 13, 14].

Economic, environmental and health impact of biofilm formation on processing surfaces

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Biofilm is able to help the bacteria surviving in unsuitable environment. This charac-teristic leads to intractable problems in many sectors. Recently, studies in the medical field have shown that bacterial biofilms are the reason for some chronic and refractory diseases [45]. Antibiotics are widely used in the medical field. Unfortunately, biofilms have the inherent resistance for the antibiotic and host immunity [46, 47]. Biofilm infections are difficult to be cured by antibiotic or immunity [48]. Problems caused by biofilm exist in many industries (water system industry, medical industry, etc.). In addition, biofilms can reduce the quality of drinking water [49]. In food industry, some studies have mentioned that the meat product, seafood product and dairy food product were found to be contaminated by bacteria [50, 51, 52]. Even the retail raw chicken sample was found to be contaminated by S. aureus (a kind of common pathogenic bacteria) in China [53].
The contamination source in food industry is variety. It could be water, dust, equip-ment, crude materials, food handlers, etc. Sharma and Anand have stated the persistence of biofilms on pasteurization lines of commercial plants [54]. Since the adhesion of biofilm is rather persistent, a completely formed biofilm on food contact surfaces could play a role of continuous contamination source. The biofilm is now known as the major contami-nation source of food product contaminations. It has been enumerated that most of the food contact surfaces (including dairy, meat and seafood sectors) were contaminated by biofilms [55, 56].
All these studies lead to a fact that the food sector provide a suitable environment for the development of biofilm. The food safety will then be challenged, and the public hygienic risks will be increased.

Table of contents :

Chapitre 1 Context 4
1.1 Fouling : a major issue for different industrial sectors
1.1.1 Fouling in heat exchanger of dairy industries
1.1.2 Importance of the French dairy sector and requirement to control fouling
1.1.3 Importance of heat treatment for texturing and stabilization of dairy product
1.1.4 Economic and environmental impact of dairy fouling
1.1.5 Dairy composition and its fouling
1.1.6 Fouling mechanisms
1.1.7 Factors affecting fouling formation
1.2 Biofilm formation in dairy industries
1.2.1 Economic, environmental and health impact of biofilm formation on processing surfaces
1.2.2 Mechanisms of biofilm formation
1.3 Cleaning of deposit
1.3.1 Cleaning mechanisms of dairy deposits and biofilms
1.3.2 Factors affecting cleaning
1.3.3 Environmental footprint of cleaning
1.4 Monitoring of surface fouling factor and cleaning endpoint
1.4.1 Direct methods – off-line detection techniques
1.4.2 Indirect methods
1.4.3 Monitoring fouling : still a challenge
1.5 Generality on coda wave and CWI (Coda Wave Interferometry)
1.5.1 Acoustic wave propagation
1.5.2 Principle of coda wave interferometry
1.5.3 Current applications of CWI
1.6 Conclusion
Chapitre 2 Theoretical analyze and modeling of coda wave propagation in multilayer 
2.1 Introduction of the studied structure and modeling
2.2 Hypotheses and limited conditions
2.3 Propagation of acoustic waves
2.4 Mathematical modeling
2.5 The simulation results by impedance matrix method
2.6 Useful indicators of coda signals
2.6.1 Decorrelation coefficient
2.6.2 Decorrelation coefficient result with impedance matrix method
2.6.3 Decorrelation coefficient result with theoretical analyze result
2.7 Conclusion
Chapitre 3 Monitoring of cleaning procedure 
3.1 Wax cleaning procedure
3.1.1 Materials and method
3.1.2 Experimental results
3.1.3 Discussion
3.2 Cleaning of protein deposit
3.2.1 Materials and method
3.2.2 Experimental results
3.2.3 Conclusion
Chapitre 4 Detection of biofilm formation 
4.1 Materials and method
4.1.1 The biofilm formation assay
4.1.2 Ultrasound acquisition and description of coda processing
4.1.3 Environmental temperature
4.2 Experimental results and interpretation
4.3 Conclusion
Conclusion and perspectives


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