Browsing by Author "Assis, Fernanda Rosa"
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- Mathematical modelling of the osmotic dehydration of physalisPublication . Assis, Fernanda Rosa; Morais, Rui Manuel Santos Costa de; Morais, Alcina Maria Miranda Bernardo dePhysalis was osmotically dehydrated with 60 °Bx sucrose or sorbitol solutions at 60 °C and with a mass ratio of sample to solution of 1:4, at atmospheric pressure or under vacuum at 150 mbar. The Crank’s, Peleg’s and Page’s models were tested to describe the mass transfer kinetics for water loss (WL) and solids gain (SG). The effective diffusivities of both water and solute were around 10-11 m2 s-1 under all conditions. Peleg’s model presented the best fit. The use of sorbitol as the osmotic agent resulted in an increase in the WL rate. In experiments with sucrose solutions, a higher WL was obtained under vacuum than at atmospheric pressure. The SG was particularly low during osmotic dehydration. Thus, the use of sorbitol as the osmotic agent was shown to be a promising alternative to sucrose.
- Modelling and optimization of osmotic dehidration combined with other methods for drying fruitsPublication . Assis, Fernanda Rosa; Bernardo, Alcina Maria Miranda; Morais, Rui Manuel Santos Costa deThe general objective of this Ph.D. programme was to study and to optimize the development, in terms of process efficiencyand the product quality,of fruit-based dry products with intermediate water content through osmotic dehydration,or reduced water content by subsequent combination of this technique with other drying methods.The first part of the work focused on the osmotic dehydration (OD) of apple cubes and physalis. The adequacyof thefit of some mathematical models to describethe mass transfer kinetics inthe products was also studied.The objectives were to perform the OD of apple cubes and to study the effects of the solute (sucrose orsorbitol) and the concentration (40 and 60 °Bx) in the osmotic solution, the mass ratio ofsample to solution (1:4 or1:10), the temperature (25, 40 and 60 ºC), and the pressure (1 bar and 150 mbar) on the water loss (WL) and the solid gain (SG).Regarding physalis, the OD was performed with a mass ratio of sample to solution of 1:4 at 60 °C and the pressure was1 bar or150 mbar. Another objective was to evaluate the changes in the microstructure of the apple tissue after the osmotic treatment with sucrose orsorbitol. Thus, in the apple cubes, the sorbitol, the increase of the temperature and of the soluteconcentration in the osmotic solution resulted in an increase ofthe ODrate, but the mass ratio of the sample to the solution and the pressure did not affect the process. In the physalis, the initial WLrate increased when the vacuum was applied during the OD with sucrose and tended to increase with the use of sorbitol. At the end of the DO of apple cubes, it was observed,by microstructure analysis,that this process affected the size and shape parameters of plant cells and the changes were more pronounced in samples osmotically dehydrated with sorbitol. The OD process caused the shrinkage of the cells and,consequently,a reduction of volume, the plasmolysis and the folding of the cell walls.In the second part of the work, some drying methods were studied to obtain cut apple with reduced water content, namely hot air drying, microwave drying and freeze-drying.The effect of the osmotic pre-treatment with sucrose orsorbitol solutionswas also studied. For the best conditions of each method, the drying kinetics were compared amongthe different methods and the quality —water activity(aw), the colour, the total phenolic content(TPC), the antioxidant activity(AA)and the rehydration ability —of the dried apple cubes were evaluated.Theadequacyof thefit of some mathematical models to describe the water content during drying was also evaluated. Thus, the effect of the temperature (25, 55, 70 and 80 ºC) was studied on the hot air drying of osmotically dehydrated apple cubes (60 ºC, 60 ºBx sucrose orsorbitol solutions, mass ratio of sample to solution 1:4).In the microwave vidrying,the effect of the osmotic pre-treatment andthepower (160, 350, 500, 650, 750 and 850 W) on the WLkinetics of apple cubes was also evaluated. Finally, the WLkinetics of the freeze-drying of apple cubes were studied. Concerning thedried cut apple with reduced water content, the osmotic dehydration as pre-treatment before the hot air drying increased the drying rate and reduced the drying time. The osmotic agent did not affect the drying rate, but the use of sorbitol reduced more the drying time and the awof the final product.In the microwave drying, the increase of the power level reduced the drying time and this reduction was higherfor samples osmotically dehydrated with sorbitol. OD prior to freeze-drying did not present a relevant advantage and the awwas lower in non-treated samples. The type of drying and the pre-treatment did not affect the rehydration rate of the dried apple cubes, but the control samples presentedhigher equilibrium water content than the osmotically dehydrated ones.The OD and subsequent drying by hot air, microwave and freeze-drying, significantly decreased the TPC and the AA. These quality parameters of the osmotically dehydrated samples were not affected by the subsequent drying, but only by the OD.In order to compare the effect of the osmotic agent used in the OD on the consumer’sacceptance, the sensory analysis of hot air dried apple cubes pre-treated osmotically with sucrose orsorbitol was carried out. There was no difference in theresults and the final product was well accepted by consumers.Among the drying methods studied, microwave drying without osmotic pre-treatment produced dried apple cubes with good quality and in a short time. Sorbitol is a good alternative to sucrose as theagent in the osmotic pre-treatment of apple cubes and physalis. Furthermore, sorbitol isa prebioticwith health benefits.
- Osmotic dehydration combined with freeze-drying of apple cubes and comparison with microwave drying and hot air dryingPublication . Assis, Fernanda Rosa; Morais, Rui Manuel Santos Costa de; Morais, Alcina Maria Miranda Bernardo deThe effects of the pre-osmotic treatment on the mass transfer kinetics of the freeze-drying (FD) and on the water activity (aw) and the quality – colour, total phenolic content, antioxidant activity – of apple cubes were studied. The fit of the moisture content was carried out using several mathematical models. The comparison among FD, microwave drying (MWD) and hot air drying (HAD) of osmotically treated apple cubes was also performed according to the drying rate and the aw. The apple cubes were osmotically treated with 60ºBrix sucrose or sorbitol solutions at 60ºC, then frozen and freeze-dried. The FD reduced significantly the moisture content from 6.259, 1.086 and 1.031 to 0.138, 0.099 and 0.074 kgwater.kgdry matter-1 for the control and the samples osmotically pre-treated with sucrose and sorbitol, respectively. The modified Page’s model presented the highest precision of parameter estimates. The FD of samples osmotically pre-treated with the sucrose solution resulted in a higher drying rate than the others samples. The highest reduction in aw was observed in the fresh samples. After FD, the total phenolic content and the antioxidant activity of the osmotically pre-treated samples decreased around 80%, in comparison with the fresh apple, this reduction being due to the osmotic pre-treatment. The freeze-dried control samples achieved a lower aw in comparison with HAD and MWD. However, FD presented a lower drying rate.
- Quality of 'Royal Gala' cut apple during osmotic dehydrationPublication . Bernardo, Alcina Maria Miranda; Pieters, Bert; Assis, Fernanda Rosa; Morais, Rui Manuel Santos CostaThe present work aimed to evaluate quality parameters of 'Royal Gala' apple cubes during osmotic dehydration (OD). We investigated the following OD conditions: osmotic agent, pressure, and temperature. The osmotic agent, being lower after OD with sorbitol than sucrose solutions, mainly influenced the water activity of the product. The color changes increased with increased temperature and were higher in vacuum experiments than at atmospheric pressure. In general, we recommend OD at 25 °C and atmospheric pressure for the preservation of the total phenolic content (TPC) and antioxidant activity (AA) of apple cubes during the process. Peleg's model was found to provide the best fit of TPC and AA data.
- Quality of ‘Royal Gala’ cut apple during osmotic dehydrationPublication . Pieters, Bert; Assis, Fernanda Rosa; Morais, Rui Manuel Santos Costa; Morais, Alcina Maria Miranda BernardoThe present work aimed to evaluate quality parameters of ‘Royal Gala’ apple cubes during osmotic dehydration (OD). We investigated the following OD conditions: osmotic agent, pressure, and temperature. The osmotic agent, being lower after OD with sorbitol than sucrose solutions, mainly influenced the water activity of the product. The color changes increased with increased temperature and were higher in vacuum experiments than at atmospheric pressure. In general, we recommend OD at 25 °C and atmospheric pressure for the preservation of the total phenolic content (TPC) and antioxidant activity (AA) of apple cubes during the process. Peleg’s model was found to provide the best fit of TPC and AA data.