Sub-Micron Grinding of a Food Product

Abstract

This thesis describes how the activity of a preservative product used in food coatings can be optimized. This project is partly sponsored by the European Marie Curie Framework projects as part of the BioPowders research training network. DSM Food Specialties hosts and co finances this project. The research project focuses on food coating such as cheese coatings. A preservative additive is often incorporated to prevent microbial growth on the food products. The shelf life of the food product is defined as the time without any microbial growth. The preservative being poorly water soluble is mainly present in the form of particles and slowly dissolves and diffuses in the coating. The approach in this research is to reduce the particle size of the preservative particles in the coating. At a given concentration, the number of particles and the total surface area of product available increases with decreasing particle size. This enables to reduce the distances between the particles. The increase in specific surface area improves the product dissolution rate. At a given concentration in the food coating, the product diffuses along shorter distances when decreasing product particle size. This is shown to improve the shelf life of the food product. The smaller particles are obtained by milling and specific efforts are made to understand and characterize the milling process. The preservative product initially of a mean size of 15?m (d4,3), is ground in a stirred media mill down to sub micron sizes. Part I of the thesis presents in the first chapter how the process parameters influence the time of grinding and the level of the contamination by heavy metals. Particles in a stirred media mill can undergo different breakage mechanisms depending on the strength of the product and the forces applied by the grinding media on the particles. In the second chapter of Part I the grinding mechanisms are determined. The particles mainly break under long stresses of high intensities and shear stresses. The particles are broken by cleavage and abrasion. In the third chapter of Part I is attempted to characterize the milling of the preservative products by using a grinding time prediction by a model based on population balances. The grinding profile of the preservative product is defined as the mean particle size as function of the number of stress events or “stress number”, SN. SN represents the number of stresses that an initial particle needs to undergo during the grinding process to reach the desired fine particle size. That representation of the grinding profile is product specific. As presented in chapter 4 of Part I, no significant differences are observed between the grinding profiles measured in different tested mills. Any similar stirred media mill is therefore expected to present a similar grinding profile. A correlation is found between the mill parameters (mill rotation speed, surface area of grinding media in contact with the impellors and the grinding media size) and the breakage mechanism and the grinding profile. In order to fully characterize the ground particles, different techniques commercially available for particle sizing are tested. The results are presented and discussed in the first chapter of the second part of this thesis. The minimal product particle size is ca 0.2?m. Different techniques point out as well the presence of larger particles. It is seen by imaging techniques such as SEM and Cryo-TEM that those larger particles are most likely aggregates of the ground particles. The stability of the ground particles is further investigated in the second chapter of Part II. The stability of the particle suspension is determined by measuring the initial sedimentation rate. This measurement technique shows differences in the aggregate size for different pH values. Changing the pH values influences the zeta-potential of the particle surface. Using the DLVO theory, the stability of ground particles shows a good correlation with the energy barrier between the particles. The higher the energy barrier the less aggregation occurs and the smaller the aggregates are. The energy barrier increases with an increase of electrostatic repulsion, i.e. the zeta-potential. The particle suspension becomes more stable when driving away the pH from the isoelectric point. The ground particles are tested in cheese coating. The use of preservative particles from different sizes is presented and discussed in Part III. Using an accelerated shelf life test, it is shown experimentally that protection against fungal growth increases when higher product concentrations are used. Also micronized product particles provide the longest protection against mould growth. A decrease in product particle size increases protection against fungal growth. A model is designed and the different aspects regulating the concentration of the product in the coating are included: the product diffusion, the product dilution, the particle distribution, the product degradation and the limit of sensitivity of the microbe to the product. The weakest point in a coating is the point where the concentration first reaches the limit of sensitivity of the microbe to the product. That point is the position in the middle of two particles. It is seen from the model that the distribution of the particles in the coating is of great importance. Reducing the particle size or increasing the concentration enables the increase of the specific number of particles. Thus the distribution of particles in the matrix changes and the distances between the particles reduce. The diffusing molecules in the coating reach the position in middle of two particles faster; this increases the shelf life of the food product.