A Fundamental Study on Physical Properties and Stability in Food Systems: The Relationship with Molecular Dynamic

Food systems physical properties and stability are critical for delivering safe and healthy food to the consumers, and thus this is a theme that attracts food scientists for a long time. Recently, literature suggests that stability can only be fully grasped if food molecular dynamics and structure are taken into consideration, i.e. an appropriate understanding of the behaviour of food products requires knowledge of its composition, structure and molecular dynamics, through the three-dimensional arrangement of the various structural elements and their interactions. Food systems behaviour is strongly dependent on the water molecular dynamics. Understanding changes in water location and mobility represents a significant step in food stability knowledge, once that water "availability" profoundly affects the chemical, physical and microbiological quality of foods. Nuclear magnetic resonance (NMR), through the analysis of nuclear magnetisation relaxation times, has been presented as a powerful technique to investigate water dynamics and physical structures of foods. It provides information on molecular dynamics of different components in complex systems. The application of this technique may be very useful in predicting food systems physicochemical changes, namely texture, viscosity or water migration. The research leading to this thesis focused on two main food systems: i) films from biological sources, for their interest as model matrices and potential for food industry; and ii) fresh-cut fruit, due to its complexity and significance in food markets. Films from biological sources, particularly chitosan, present several applications including biodegradable packaging and edible coatings for shelf-life extension. As model food systems, films from biological sources are partially crystalline, partially amorphous, and easily reproducible materials. From a fundamental perspective, foods are mainly edible and digestible biopolymers that are also partially crystalline/partially amorphous. Despite of the wealth of information on literature, a systematic approach to understand the contribution of film forming solutions (FFS) on chitosan films physical properties, as well as the knowledge on its molecular dynamics to such properties, are still uncommon. In this thesis, the relevance of FFS composition on films properties is highlighted through the monitoring of solutions with different polymer/plasticiser ratios. Also the molecular dynamics, evaluated through NMR methodology, was analysed and compared with the films physical properties. Results demonstrated the influence of solutions polymer/plasticiser concentrations on both thermomechanical and water related properties. Chitosan concentration in solutions affected consistency coefficient, and this was related with differences in films water retention and structure. Plasticiser quantities used in FFS are responsible for films compositions, while polymer/plasticiser ratio determined the thickness and thus the structure of the films. NMR allows understanding the films molecular rearrangement, demonstrating that water is also an important component in these matrices and performs differently when compared with the plasticiser. A relationship between water and plasticiser dynamics and films macroscopic properties was also observed. Fruits are high water content products with a complex cellular structure, where water can be present in both intra and extra cellular spaces. Fresh-cut fruit, due to processing, has high metabolic rates with faster physiological and biochemical changes and microbial degradation, which results in product's colour and texture alterations. The second part of this thesis focused on fresh-cut fruits, pear and melon, which were chosen for their significantly different composition and structure. Fresh-cut fruit was monitored during 7 days of refrigerated storage conditions. Relevant quality parameters, such as colour and firmness, were analysed. Water activity (aw) and water molecular dynamics (T2), measured by a NMR technique, were also assessed throughout storage. Results demonstrated that processing and storage affected quality parameters, as was expected, but also system's water molecular dynamics. Throughout storage, it was possible to find relationships between the molecular dynamics and the quality parameters. These relationships were different for the two studied fruits, and the role of microstructure on food stability could be observed. These studies highlight the significance and impact of molecular dynamics on physical properties and stability of foods, and also the usefulness of NMR methodology as a tool to evaluate food physical properties and stability. Therefore, NMR could provide a novel instrument to improve the knowledge of food systems, even when complex.