| 1. |
- Elustondo, Diego, et al.
(författare)
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Mathematical modeling of moisture evaporation from foodstuffs exposed to subatmospheric pressure superheated steam
- 2001
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Ingår i: Journal of Food Engineering. - 0260-8774 .- 1873-5770. ; 49:1, s. 15-24
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Tidskriftsartikel (refereegranskat)abstract
- In this work, a reduced expression to calculate the drying rate of foodstuffs being dried with low pressure superheated steam is developed. It is based on a theoretical drying mechanism which assumes that water removal is carried out by evaporation in a moving boundary making the vapor to flow through the dry layer built as drying proceeds. The theoretical model can be applied to any form and features dimensionless parameters to allow for the influence of form, shrinking effect and boiling point rise. To make the application easier, a simplified expression was derived, which has only two experimentally determined parameters and allows the calculation of the drying rate of a given product. Different foodstuffs were dried with this technique and the empirical parameters were calculated from the experimental data fitting equation. Interestingly, those parameters can be considered constant within the 10000-20000 Pa pressure range and the 60-90°C temperature range
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| 2. |
- Elustondo, Diego, et al.
(författare)
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New thermal conductivity probe design based on the analysis of error sources
- 2001
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Ingår i: Journal of Food Engineering. - 0260-8774 .- 1873-5770. ; 48:4, s. 325-333
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Tidskriftsartikel (refereegranskat)abstract
- The thermal conductivity determination by means of the thermal probe is a classical non-stationary method, suitable for foodstuffs because of the relatively short period of time and the small temperature rise required during experimental measurements. Even though its simplicity, this method has theoretical and practical implicit errors, their main sources being evaluated in this paper. The theoretical analysis shows that experimental errors can be kept within acceptable values, if the testing time period is within certain limits, out of which the errors grow quite rapidly. The error arising from the calculation of the thermal conductivity by means of an approximate equation is studied and a new way to express it is presented. Also, testing times were calculated for different probe dimensions. Finally, on the basis of two former probe which exhibits some advantages upon each other, a new probe design is proposed which recoveries those features.
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| 3. |
- Funebo, Tomas, et al.
(författare)
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Microwave heat treatment of apple before air dehydration : Effects on physical properties and microstructure
- 2000
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Ingår i: Journal of Food Engineering. - 0260-8774 .- 1873-5770. ; 46:3, s. 173-182
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Tidskriftsartikel (refereegranskat)abstract
- Golden delicious apple cubes were heated with microwave energy of high intensity (20 W/g), as a pre-treatment before air-dehydration at 40 °C, 60 °C and 80 °C. After the microwave treatment extending for 0.75 up to 5 min, the cubes were finish-dried with only forced air at 2 m/s. Dehydrated and rehydrated samples were analyzed with a puncture test using a texture analyzer. The microstructure of the samples was studied with confocal laser scanning microscopy (CLSM). Dried apple pieces were harder and more shrunk when pre-treated with microwaves, compared to only air dehydration. Despite the shrinkage and increased firmness, the rehydration capacity of microwave `blanched' apple cubes was higher than when they were only air-dehydrated. Image analysis of the rehydrated apples showed that large voids (up to 0.5 mm in diameter) had developed, and that these increased with air temperature. Higher magnification of the images revealed cell separation and disruption of cell walls, caused by the microwave heating.
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| 4. |
- Prothon, F., et al.
(författare)
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Application of the Guggenheim, Anderson and De Boer model to correlate water activity and moisture content during osmotic dehydration of apples
- 2004
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Ingår i: Journal of Food Engineering. - 0260-8774 .- 1873-5770. ; 61:3, s. 467-470
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Tidskriftsartikel (refereegranskat)abstract
- Water activity and moisture content are the most valuable characteristics for assessing the stability of dried foods. The aim of this work was to evaluate if a desorption isotherm model could be used to describe the relationship between the water activity and moisture content in the product during osmotic dehydration. This model can lead to a better understanding of how water activity of a product can be reduced during osmotic dehydration. The Guggenheim, Anderson and De Boer (GAB) model was chosen because it has been proven to best fit the desorption and adsorption of foods over a large range of water activities. The studied material was apple pieces, and the osmotic medium with a water activity of 0.939 was prepared with sucrose. The osmotic dehydration process was studied by collecting moisture content and water activity data at different temperatures and times. The model was shown to fit well the experimental points at temperatures 25, 45 and 55 °C. At 65 °C, however, the model is satisfactory if aw is below 0.970 but not above. The effect of temperature can be expressed on the C-parameter of the GAB model. © 2003 Elsevier Ltd. All rights reserved.
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| 5. |
- Skjöldebrand, Christina, et al.
(författare)
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A computer simulation program for evaluation of the continuous heat treatment of particulate food products. : Part 1: Design
- 1993
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Ingår i: Journal of Food Engineering. - : Elsevier BV. - 0260-8774 .- 1873-5770. ; 20:2, s. 149-165
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Tidskriftsartikel (refereegranskat)abstract
- Computer simulation programs used by the food industry today are not very well developed, partly because few food processes ar easily described by mathematical equations. The heat treatment of food, however, is one group of processes for which mathematical modelling exists. Programs used for prediction of the time-temperature relationship for various food products during heat treatment have been developed at SIK. A specially designed program calculates the temperature in viscous liquid foods with discrete solid particles. This program facilitates study of the influence of processing conditions on the product quality in an aseptic processing system. The F value is a measure of the safety and a so-called C value (cook value) is a measure of the quality of the product. The simulation program has been further developed to take into account product flow rate, residence time distribution, particle size and geometry, equipment specifications, particle load and the thermal properties of the product. The simulation program makes it possible to analyse the entire sterilisation system, including cooling, and determine the required holding time to reach a desired lethality value in the centre of the fastest-moving particle. The design of the program is described in this paper, and some applications are treated in a following paper. © 1993.
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| 6. |
- Skjöldebrand, Christina, et al.
(författare)
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A computer simulation program for evaluation of the continuous heat treatment of particulate food products. : Part 2: Utilisation
- 1993
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Ingår i: Journal of Food Engineering. - 0260-8774 .- 1873-5770. ; 20:2, s. 167-181
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Tidskriftsartikel (refereegranskat)abstract
- A computer program for calculation of temperatures in a liquid containing particles during continuous heat treatment has been developed at the SIK. The program facilitates the study of the influence of processing on product quality in an aseptic processing system. The calculated F value is related to the safety, and the cook value (C value) is related to the quality of the product. In this paper, results of some calculations using the program are presented. They concentrate on the influence of the processing conditions on the process design. These results indicate that factors such as particle heat transfer coefficient, both in the holding tube and in the scraped surface heat exchangers (SSHEs), flow conditions, particle diameter and sterilisation temperature have consequences for the holding tube length, i.e. the processing time, for a given lethality. For example, for small particle heat transfer coefficients (< 100 W m2 K), a small change will have a large effect on the required processing time (i.e. the holding tube length). However, at particle heat transfer coefficients > 150-200 W m2 K a small change does not greatly affect the processing time (i.e. holding tube length). The program can thus be used to examine whether the surface heat transfer coefficient is the rate-limiting factor, or whether the heat conduction inside the particles is more important in the design of the process. © 1993.
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| 7. |
- Skjöldebrand, Christina, et al.
(författare)
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Optical properties of bread in the near-infrared range
- 1988
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Ingår i: Journal of Food Engineering. - : Elsevier BV. - 0260-8774 .- 1873-5770. ; 8:2, s. 129-139
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Tidskriftsartikel (refereegranskat)abstract
- This paper reports on a method for determination of the optical properties of bread. The optical properties of white bread have been studied. The measurements were carried out for dough, crumb and crust. The transmission and reflection measurements were made with a Beckman spectrophotometer with an integrating sphere. Transmission functions and penetration depths were calculated and compared with previous investigations made at this institute (SIK). The influence of peak power wavelength and spectral range (800-1250 nm and 1250-2500 nm) on the penetration depth for crust and crumb have been determined. This study has shown that it is possible to calculate penetration depth for any radiator using transmission data in the range 800-2500 nm. © 1989.
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| 8. |
- Thorvaldsson, K., et al.
(författare)
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A model for simultaneous heat, water and vapour diffusion.
- 1999
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Ingår i: Journal of Food Engineering. - 0260-8774 .- 1873-5770. ; 40:3, s. 167-172
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Tidskriftsartikel (refereegranskat)abstract
- A model for simultaneous heat, water and vapour diffusion was developed to use for prediction of the diffusion of water inside foods during heat processing. The model is based on Fourier's and Fick's Laws. The diffusion of liquid water is separated from the diffusion of water vapour. The model was evaluated in a drying process. Slabs of bread crumbs, 12×12×2 cm3 in size, were dried in a conventional oven at 210 °C and the local water content and temperatures were measured during the drying in the centre, halfway to the centre and at the surface. From the measurements, the diffusion coefficient as a function of temperature and concentration and several other material parameters were estimated. The results show that the measured water content slowly increases in the centre of the sample. During the increase, the temperature remains on a plateau. When the centre starts to dry the temperature increases. Halfway to the centre the water content also increases slightly before it starts to dry out, while the surface starts drying immediately. The simulated water content levels and temperatures conform well to the experimental values and show that the evaporation and condensation model describes well the diffusion mechanisms in a porous food.
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| 9. |
- Thorvaldsson, K., et al.
(författare)
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Method and instrument for measuring local water content inside food
- 1996
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Ingår i: Journal of Food Engineering. - : Elsevier BV. - 0260-8774 .- 1873-5770. ; 29:1, s. 1-11
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Tidskriftsartikel (refereegranskat)abstract
- A new method of measuring local water content inside food during heat processing has been evaluated. The method uses a fibre optic NIR-Instrument. The evaluation was mainly based on investigations of the influence of structure and temperature. The instrument was found to be very sensitive to both the structure and the temperature of the sample. The dependence on the temperature of the sample can be incorporated into the calibration and is thereby not an impediment to the measurements. The structural dependence makes quantitative measurements at different places in the sample difficult for inhomogeneous materials. However, measurements of dynamic changes in the water content in one spot work well for both homogenous and inhomogeneous materials and are probably the best application of the instrument. By using two or more optic fibres, the diffusion of water can also be measured. Copyright © 1996 Elsevier Science Limited.
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| 10. |
- Thorvaldsson, K., et al.
(författare)
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Water transport in meat during reheating
- 1996
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Ingår i: Journal of Food Engineering. - : Elsevier BV. - 0260-8774 .- 1873-5770. ; 29:1, s. 13-21
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Tidskriftsartikel (refereegranskat)abstract
- A study of heat and water transport in meat during a heating process was conducted. The meat samples were heated from 10 to 100°C in a conventional oven at 175°C and local water content and temperatures were measured during the heating process. Using whole meat from bovine muscles, the mechanism behind water transport inside the sample during heating was investigated. The anisotropy of meat in terms of heat and water transport was also studied. The study yielded two main results: When heating commences, water moves towards the centre of the samples. At a centre temperature of ?70°C, water transport stops and changes direction. A theory for transport phenomena has been developed and is described in this paper. The results also indicate that, as expected, both water transport and heat transport are faster parallel to the meat fibres than perpendicular to them. Copyright © 1996 Elsevier Science Limited.
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