COMBINED DEHYDRATION METHODS - From Fresh Fruit to High-quality Ingredients
Abstract: Numerous products from the food industry contain fruit and/or berries, in the form of fillings or ingredients, e. g. in yoghurt, some bread products/cereal products, ice cream, etc. In order to produce these ingredients and improve their characteristics, many processing techniques are available. This thesis focuses on dehydration methods that take into account the fragile and complex cellular tissues of fruit and facilitate the incorporation of fruit pieces into numerous industrial recipes. The target product should have an acceptable ability to rehydrate and to regain a texture that is as near to fresh as possible. The structural changes and the stabilisation in terms of water activity were particularly studied. Water and its functionality at macro- and micro-levels of the cellular tissue were reviewed as well as its transfer during dehydration. In the context of dehydration, fruit and vegetables collapse as a result of water loss when water is transported from the cells to the surroundings via the cell walls and the intercellular spaces. Collapse can be partially prevented by reinforcing the cell walls and filling the intercellular spaces with carbohydrate solutions and other biopolymers before dehydration with methods such as osmotic treatment or vacuum impregnation. Microwave-assisted air-drying with controlled temperatures at 50, 60 and 70°C was applied to apple cubes after different pre-treatments. The use of microwaves shortened the time of drying compared with air-drying alone. The combination of osmotic treatment and microwave-assisted air-drying produced dehydrated samples of improved appearance and higher volume compared with the samples obtained without osmotic pre-treatment. Other pretreatments were also studied in combination with microwave-assisted air-drying, and the best results were obtained with calcium infusion at ambient temperature, particularly with the rehydrated texture. Pre-freezing was the most damaging process for the apple tissue. Important parameters of osmotic treatment of apple were also studied. When trehalose was used as the osmotic agent it was shown to be at least as efficient as sucrose in removing water except when applied in crystalline form. Vacuum application in combination with osmotic treatment had an immediate and significant effect on the composition of the samples. It enhanced the solid gain by filling most of the extracellular spaces. Modelling of the relation between water activity and moisture content during osmotic treatment was also successfully accomplished. In conclusion, this thesis showed that a combination of different techniques is a good way to achieve high-quality ingredients from fresh fruit, but more research is needed to better understand the complexity of the plant tissue and its behaviour during processing.
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