Microstructure and rheological properties of plant particle suspensions prepared using different physical treatments

Doktorsavhandling, 2011

Suspensions of plant particles are very common products or by-products in many industries such as food, paper and biofuel. An improved understanding of the complex relationship between microstructure and rheology of these plant systems might help design, control and predict their structural and material properties. This thesis focuses on plant suspensions for application in food products structured mainly with comminuted plant material such as soups, drinks and sauces. Carrot, broccoli and tomato were selected as model plant materials due to the fact that they form a significant part of the Western diet and to their high content in specific micronutrients. The main objective of this work was to generate plant particle suspensions by physical treatments and to evaluate the relationship between the microstructure and the rheology of such systems, always considering the possible implications for nutrients bioaccessibility. It was shown that plant tissues can be disrupted into different particle morphologies, depending on the plant origin, by using mechanical and thermal processes including high pressure homogenisation as an intense shear treatment. While carrot and broccoli break in a stepwise fashion from clusters of cells to single cells and cell fragments with compact cell walls; tomato was more easily disrupted into fragments with an open wall structure. Suspensions, with potential to structure food products, were generated and characterised by techniques such as confocal scanning laser microscopy, cryo-scanning electron microscopy and rheology. The dependency of the yield stress and elastic modulus G’ on particle dry mass fraction was found to be approximately a power-law, independently of the plant origin and particle morphology. Furthermore our results gave evidence that the viscoelasticity is dominated by both attractive forces and elastic interactions acting together. As an example of commercially available food products, 5% olive oil was added to the suspensions generating stable plant based emulsions which displayed a range of rheologies and microstructures with specific in vitro bioaccessibility of carotenes. The findings in this thesis might help in the selection of plant raw materials for specific textural applications and to design food products structured mainly using plant material, while reducing the amount of other texturisers and stabilisers. Furthermore optimisation of the textural and nutritional quality of these types of products could help to achieve the recommended levels of fruit and vegetables consumption, which may reduce the risk for different types of cancer and cardiovascular diseases.