A fundamental study on physical properties and stability in food systems : the relationship with molecular dynamics

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.O controlo das propriedades físicas e da estabilidade dos alimentos é requisito essencial para o fornecimento de produtos seguros e saudáveis aos consumidores. Por este motivo, desde há muito tempo que o tema tem despertado a atenção e a curiosidade dos cientistas que trabalham na área alimentar. A literatura tem vindo a sugerir que as propriedades físicas e a estabilidade só podem ser plenamente compreendidas se a dinâmica molecular e a estrutura dos alimentos for tida em consideração; ou seja, é necessário um conhecimento da composição, da estrutura e da dinâmica molecular dos sistemas alimentares, entendendo o arranjo tridimensional dos vários elementos estruturais e das suas interacções. No caso particular dos alimentos, a dinâmica molecular da água desempenha um papel fundamental no seu comportamento. A “disponibilidade” da água influencia profundamente a qualidade química, física e microbiológica dos sistemas alimentares. A compreensão das alterações na localização e mobilidade da água do sistema representa um passo significativo no conhecimento dos mecanismos que estão associados às reacções de degradação dos alimentos. A ressonância magnética nuclear (RMN), através da análise dos tempos de relaxação da magnetização nuclear, tem sido considerada uma poderosa técnica para investigar a dinâmica da água e avaliar estruturas físicas em sistemas complexos como os alimentos. A aplicação desta técnica pode ser muito útil na previsão de alterações físico-químicas como a textura, a viscosidade ou a migração da água na matriz. Esta tese considerou dois sistemas alimentares distintos: (i) filmes de origem biológica, pelo seu interesse como matrizes modelo e potencial para a indústria alimentar; e (ii) fruta minimamente processada, pela sua complexidade e reconhecida importância económica nos mercados de alimentos. Os filmes com origem biológica, neste caso específico provenientes do quitosano, possuem várias aplicações industriais como é o caso das embalagens ou revestimentos comestíveis, que têm como objectivo prolongar a vida útil dos produtos. Como modelo para sistemas alimentares mais complexos têm as vantagens de: serem facilmente reprodutíveis; e tal como os alimentos, de um ponto de vista fundamental podem ser considerados biopolímeros comestíveis parcialmente cristalinos, e parcialmente amorfos. Apesar da vasta informação que existe na literatura sobre as propriedades físicas dos filmes de quitosano, uma abordagem sistemática para a identificação da contribuição das soluções formadoras do filme, assim como a influência da dinâmica molecular nessas propriedades, revela-se ainda necessária. Nesta tese, a importância da composição das soluções formadoras nas propriedades dos filmes é realçada através da monitorização de soluções formadoras com diferentes proporções polímero/plasticizante. A importância destas soluções foi avaliada também nas alterações das propriedades termomecânicas dos filmes assim como a sua influência na dinâmica molecular dos mesmos (através de técnicas de RMN). Os resultados demonstram que a composição das soluções formadoras influenciou as propriedades mecânicas e térmicas dos filmes, bem como as propriedades relacionadas com a água (atividade, solubilidade, permeabilidade e a dinâmica molecular). A concentração de quitosano afetou o coeficiente de consistência das soluções formadoras, o que pode ser relacionado com diferenças na estrutura e na retenção de água dos filmes. Por outro lado, a quantidade de plasticizante usado na preparação das soluções formadoras é responsável pela composição dos filmes, enquanto a razão polímero/plasticizante determinou a espessura, logo a estrutura dos filmes. Através dos estudos de RMN foi possível compreender o rearranjo molecular dos filmes, demonstrando o papel importante que a água, como componente, desempenha neste tipo de matrizes, revelando diferenças de comportamento entre esta e o plasticizante. Estes resultados revelaram ainda que existe uma relação entre a dinâmica molecular quer da água quer do plasticizante nos filmes com as propriedades macroscópicas dos mesmos. As frutas são alimentos com uma estrutura celular muito complexa, ricos em água que pode estar presente quer nos espaços intracelulares, quer nos extracelulares. As frutas, minimamente processadas, devido ao ferimento a que são sujeitas, tem altas taxas metabólicas que provocam rápidas alterações fisiológicas, bioquímicas e de degradação microbianas, resultando, por exemplo, em perda de cor e textura. A segunda parte desta tese dedica-se ao estudo de pêra e melão minimamente processados. Estas frutas são muito diferentes no que diz respeito à estrutura. As amostras foram estudadas durante 7 dias de armazenamento em condições de refrigeração. Foram avaliados alguns dos parâmetros de qualidade mais relevantes, como é o caso da cor e da textura. A atividade da água (aw) e a dinâmica molecular da água (T2), analisada através de uma técnica de RMN, foram também monitorizadas durante o tempo de armazenamento. Os resultados mostram que quer o processamento quer o tempo de armazenamento afetaram os parâmetros de qualidade, bem como a dinâmica da água nos sistemas. Observou-se ainda uma relação entre os parâmetros de qualidade e os valores da dinâmica da água. Esta relação foi diferente para os dois frutos estudados, realçando o papel da estrutura na estabilidade dos alimentos. Nesta tese evidencia-se o interesse e a utilidade dos estudos de dinâmica molecular, utilizando a técnica de RMN como ferramenta na avaliação das propriedades físicas e da estabilidade de sistemas alimentares e complexos

Microstructure, composition and their relationship with molecular mobility, food quality and stability

Food stability is a critical parameter for both consumers and producers, since it assures safety, nutritional, and sensorial quality of foodstuffs, and at the same time maximizes shelf-life. For a long time, water activity, aw, was considered a determinant parameter in food stability and physical properties. This concept was challenged with the revolutionary approach to the study of food systems using the glass transition concept. Recently, scientific research suggests that molecular mobility is a fundamental approach to fully attain food physical properties and stability. Current literature suggests that stability can only be fully grasped if molecular mobility and structure are taken into consideration; that is, an appropriate understanding of the behavior 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 are complex mixtures of water, biopolymers, low-molecular weight ingredients, and colloid particles, and the molecular mobility between these different components reflects on the stability of such systems, determining the physical state, microstructure, and composition, which impacts food characteristics. Particularly, food water content, location, and interactions with other components are critical in microbial growth, degradation reactions, and sensorial aspects. Understanding changes in water location and mobility represents a significant step in food stability knowledge, once that water availability profoundly affects chemical, physical, and microbiological quality of foods.info:eu-repo/semantics/acceptedVersio

The impact of metabolic engineering of yeast, designed to produce valuable biomolecules through fermentation, on resulting spent yeast composition

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Antimicrobial potential of formulations, incorporating spent yeast derived from synthetic biotechnology, against Pseudomonas spp.

Yeasts are currently used as cell factories for the sustainable production of high value biomolecules for applications in the pharmaceutical and cosmetic sectors. However, the production of such molecules through fermentation, conducted in bioreactors and making use of genetically engineered yeast strains, generates a number of waste-streams, with spent yeast as the second main by-product of fermentation processes, representing more than 20% of the total waste produced. Since this by-product is rich in several molecules including proteins, vitamins and several minerals, being also a natural source of glucans and mannoproteins with known bioactivities, it is of interest to develop valorization strategies for these residues. In this work, spent yeast was autolysed and a set of formulations, incorporating the resulting supernatant and pellet at different concentrations, were tested as potential antimicrobial solutions to prevent diseases in plants and fungi caused by Pseudomonas spp. The formulations were initially tested against P. aeruginosa and then against strains affecting cultivated mushrooms (Agaricus bisporus), P. tolaasi, and P. agarici; and a number of P. syringae strains responsible for plant diseases including the pathovars actinidifoliorium, tomato, pisi, syringae and atrofaciens. It has been recently shown that spent yeast hydrolysates possess antimicrobial activity against Salmonella enterica, Aeromonas salmonicida, Bacillus cereus and Bacillus subtilis (Martin et al. 2021), suggesting that spent yeast extracts may have potential antimicrobial effect against a range of microorganisms. To date, there are no studies showing the potential of spent yeast extracts against Pseudomonas strains. The results showed that the supernatant of the autolyzed yeast at 0.1% inhibited the growth of P. aeruginosa by about 20% and P. tolaasii by 10% but increased the growth of P. agarici by up to 34%. The combination of supernatant with lactose also slightly increased P. tolaasii growth inhibition (12%) but it reduced the inhibitory effect of supernatant against P. aeruginosa. With regard to plant pathogens, the formulations containing 0.1% and 0.3% of pellet inhibited by 25% the growth P. syringae pv. actinidifoliorium, the bacterium responsible for kiwi canker, but for all the other pathovars, the inclusion of spent yeast extracts in the formulation induced bacterial growth. Among all the tested formulations, the ones including supernatant and pellet at low concentrations were the most promising leading to slight growth inhibition of some environmental Pseudomonas spp.info:eu-repo/semantics/publishedVersio

Thermosonication applied to kiwi juice processing: anti-listerial effect and quality retention

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Thermal and non-thermal Cantaloupe melon juice pasteurization: Assessment of the impact of ozone exposure on microbiological, physicochemical and bioactive characteristics

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