Tomato (Solanum Iycopersicum) puree is created by homogenising the flesh of tomato fruits. The viscosity of this material and the fibrous content is of commercial interest. Tomato puree consists of suspended particles (consisting of whole cells, broken cells and cellular fragments) in an aqueous serum. The contribution of the non-soluble and soluble material to tomato puree rheology was studied with reference to the varying composition of solids and firmness of tomato fruit at four stages of ripeness; mature green, breaker, pink and red ripe. When purees from the red ripe and the breaker fruit with initial total solids contents of approximately 5.5% were diluted to a range of total solids content (between 5 and 1%), both samples decreased in viscosity with total solids. To observe the effect of the serum on tomato puree viscosity, the pellet fraction containing the particles was spun out of the tomato purees by centrifugation so that the particle fraction and the serum could be assessed separately. On further investigation, the viscosity of puree was shown to be affected only to a small degree by the viscosity of the serum. The fact that stirred viscosity was independent of whether the suspension medium constituted of sucrose solution or serum phase showed that little contribution to the viscosity is coming from the soluble hydrocolloids in the tomato sera. Most dominant for viscosity was the particle fraction. The particle fraction was affected by the maturity of the fruit which in turn was associated with changes in fruit texture, particle size and size distribution of the puree. The particles did not behave as rigid spheres, as shown by the large departure from the corresponding Krieger Dougherty fit (Krieger and Dougherty 1959), ripened fruit deviating the furthest. This suggested the more ripened fruit has softer (less rigid) particles, which was shown to have softer tissue before the pureeing process. This may also have made the tissue more susceptible to rupture into smaller particles during pureeing. Quantitative data for size distribution measured by laser light scattering show that as the tomato ripened from green to red, particles were smaller; the mean diameters for the particles in the four puree samples were 893J.lm (mature green), 542J.lm (breaker), 377J.lm (pink), and 262J.lm (red ripe) and of a broader size distribution. Pureed mature green (unripe) fruit were shown to offer more potential as an effective natural viscosifier for foods such as soups and sauces, but they may not meet requirements of taste or colour. The last part of this research (chapter 7) therefore focused on tomatoes with variations in their genes rather than ripeness, harvested at breaker + 7 days. Tomatoes at the same stage of ripeness from the Solanum pennel/ii tomato introgression lines (ILs) were grown and harvested from the glass houses at the University of Nottingham. A screening process was developed to compare the viscosity of small samples of tomato puree from 55 of the ILs, 164 fruits in total; the remaining 21 lines did not produce quantities of fruit sufficient for analysis. The samples were diluted with water to the same total solids of 3.5%. The mean stirred viscosity value for all the lines measured was 132 ± 45 cPo The viscosity was calculated from typically three replicate measurements on three individual fruits per line. Although the particle volume and weight fraction of the puree and the initial total solids content had a strong influence on stirred viscosity, it was shown that the variation between individual fruit was greater than that between the different ILs. Previous texture studies of the IL lines showed marked variations yet there was no clear relationship between the puree viscosities and the expected texture of the whole fruits.

Cette étude porte sur l'amélioration de la qualité de la tomate séchée par microondes assistés par air chaud (MOAC). Le principal but recherché est la préservation de la qualité de la tomate séchée en évitant notamment l'apparition de tâches noires qui souvent surviennent lors d'un chauffage microondes suite à une surchauffe locale du produit quand il devient sec. Pour ce faire, un système de séchage microondes assistés par air chaud avec la capacité de contrôler automatiquement la puissance spécifique a été développé sous labView®. Après une première étude où la puissance microondes appliquée est maintenue constante, une seconde stratégie de séchage MOAC, avec deux modes différents de régulation de la puissance spécifique, a été mise en oeuvre. Ces modes consistent à ajuster la puissance microondes appliquée à la masse du produit, représentative de la quantité d'eau restante. Dans le premier mode, l'adaptation est faite toutes les secondes (ajustement "continu") alors que dans le second mode, la puissance a été ajustée seulement chaque cinq minute (ajustement intermittent). L'ajustement permet de travailler avec un rapport puissance appliquée sur masse résiduelle du produit (puissance spécifique) constant. Tous les paramètres du procédé incluant la puissance appliquée on été enregistrés chaque seconde. De même, la température et la masse du produit sont mesurées chaque seconde. Le produit final a été caractérisé en réalisant des mesures de couleur, de texture, de structure ainsi que de taux résiduel de lycopène. La première étude sur le séchage MOAC réalisée à puissance constante (puissance spécifique variable), a produit des tomates avec des points chauds par endroit et plus de la moitié de la teneur en lycopène de la tomate est dégradée. En revanche, les tomates séchées par MOAC utilisant les deux modes de régulation du séchage, présentent une meilleure qualité en termes de couleur et de structures comparées à deux tomates séchées commerciales de référence (d'origine italienne et ivoirienne). Par ailleurs, l'observation de la structure des tomates par Microscopie Electronique à Balayage (MEB), a permis d'expliquer les changements observés au niveau de la texture de la tomate séchée. D'un point de vue chimique, le taux résiduel de lycopène de la tomate séchée par les modes de régulation (1 et 2) est situé respectivement entre 64,78 et 55,63% pour les puissances et températures d'air basses (3W/g-30°C) et entre 30,91 et 28,75% pour les puissances et températures d'air élevées (7W/g-50°C), contre 30,56 et 22,30% pour les tomates commerciales d'origine italienne et ivoirienne. / The aim of this study was to improve the quality of dried tomato by the use of microwaves combined to hot air (MOAC). The main point was to preserve the quality of dried tomato avoiding the appearance of black spots which often occur during microwaves heating which result from local overheating of the product when it becomes dry. Therefore, an MOAC drying system with ability of automatic control of the power density was developed under LabView®. After an initial study where the applied microwave power remained constant during the drying, a second MOAC drying strategy, with two different control modes, was implemented. This modes consisted to adjust the microwave power density to the remaining amount of water (actually the weight of the product). In the fist one, it is adjusted every second, while in the second, the power was adjusted only every 5 min. Therefore the ratio between the power applied and the weight of the product (i.e. the specific power) remained constant. All the process parameters, including the power applied and the product temperature and weight were recorded every second. The final product was characterized by performing measurements on color, texture, structure and residual lycopene content. In our initial study on MOAC drying performed at constant power (i.e. variable specific power), we have obtained dried tomatoes with hot spots and with a residual lycopene content lower than 50%. The tomatoes dried at constant specific power showed a better physical aspect and a higher nutritional quality, even compared to two commercial products from Italy and Ivory Coast. In addition, the structure of dried tomatoes observed by Scanning Electron Microscopy (SEM) allowed to explain changes in dried tomatoes texture. Chemically, the residual rate of lycopene in tomatoes dried at constant specific power ranged respectively between 64.78 and 55.63% for low power and air temperatures (3W/g-30 °C) and between 31.91 and 28.75% for high power and air temperatures (7W/g-50° C), while only 30.56 and 22.30% for commercial tomatoes from Italy and Ivory Coast.