摘要:

Moisture diffusivity is an important parameter needed in the analysis, design and optimization of drying processes for food and other materials. Published data on moisture diffusivities of food materials are scarce and, sometimes, inconsistent due to a lack of a precise and repeatable experimental technique. Most experimental data are limited to low and moderate drying temperature (<70°C), whereas in the food industry hot air of up to 100°C is usually used in the falling rate period to speed up the drying processes. In this study, the effective moisture diffusivities of Red Delicious apple tissues were determined from drying curves produced with a Perkin Elmer thermogravimetric analyzer, using the slope method. The experiments were conducted at lour temperatures 60, 80, 100 and 120°C. Two well defined falling rate periods were observed. The effective moisture diffusivity, for the four temperature levels ranged from 3.2 × 10−7to 7.9 × 10−8m2/s for the first falling rate period and 3.8 × 10−8to 4.7 × 10−8m2/s for the second falling rate period. The temperature dependence of the effective diffusivity can be described with an Arrhenius-type equation.

ISSN:0737-3937    

DOI:10.1080/07373930008917771

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Shrinkage of potato cubes, parallelepipeds and cylinders was addressed during drying. A basic methodology by using image analysis was set-up in order to measure and monitor geometrical changes along the drying process. Both measurements by image analysis and directly with a calliper were carried out. Results from both methods showed a good agreement with no bias. Particle shape was found to influence shrinkage in a different way according to the axis. In fact the larger dimension shrinks less than the shorter one. This could be linked to the influence of the core drying in shrinkage. Experiments were also carried out for cauliflower stems. The difference in shrinkage between directions was greater for cauliflowers, due probably to the effect of alignment of fibers along the axis. Good agreement with existing data in the literature for specific volume was found.

ISSN:0737-3937    

DOI:10.1080/07373930008917772

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Small specimens ofPinus radiatahave been tested to determine the creep strain that occurs during the kiln drying of boards. The samples have been tested over a range of temperatures from 20°C to 140°C. The samples, measuring 150 × 50 × 5 mm, were conditioned at various relative humidities in a pilot-plant kiln, in which the experiments at constant moisture content (MC) in the range of 5-20% MC were undertaken to eliminate mechano-sorptive strains. To determine the creep strain, the samples were brought to their equilibrium moisture content (EMC), then mechanically loaded under tension in the direction perpendicular to the grain. The strain was measured using small linear position sensors (LPS) which detect any elongation or shrinkage in the sample. The instantaneous compliance was measured within 60 sec of the application of the load (stress). The subsequent creep was monitored by the continued logging of strain data from the LPS units.

ISSN:0737-3937    

DOI:10.1080/07373930008917773

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The effect of pretreatment on color of convective dried products (namely apple, banana, potato and carrot) was investigated. Five different types of pretreatment were taken into consideration; microwave, osmotic, sulfite, water blanching and steam blanching. Color characteristics were identified by measuring the color parameters (namely, Lightness (L), Redness (a) and Yellowness (b)) using a Hunter Lab chromatometer. The type of pretreatment was found to significantly affect the three color parameters. In addition, Redness (a) and Yellowness (b) were found to follow a first order kinetic model. Untreated dried materials showed an extensive browning, indicated by a significant drop of the L parameter and a corresponding increase of a, b parameters. Osmotically and microwave pretreated samples supressed browning compared to the untreated samples. In this case, lightness decreased slightly, while a, b increased slightly. Sulfite pretreatment prevented significantly color deterioration, while water and steam blanching also prevented enzymatic browning during convective drying.

ISSN:0737-3937    

DOI:10.1080/07373930008917774

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Drying related properties of apple are evaluated for various different drying methods (namely, convective, vacuum, microwave, osmotic and freeze drying), and their corresponding process conditions. The examined properties are drying kinetics, equilibrium material moisture content, density, porosity, color and viscoelastic characteristics. The effect of various process factors on these properties is described through particular mathematical models. The model parameters are estimated by fitting the corresponding model equations on a wide range of experimental data. Drying kinetics is greatly affected by the characteristic particle size and drying air temperature for convective drying, while for the case of microwave drying they are affected by the vacuum pressure and the emitted radiation power. Equilibrium material moisture content is affected by the temperature and the humidity of the surrounding air, while the osmotic pretreatment shifts the sorption isotherms to higher water activity levels. The quality properties examined, are significantly affected by the drying method. More specifically, osmotic dehydration decreases the porosity of the final product, while it prevents color deterioration and enchances the viscous nature of dehydrated apple. Freeze-dried apples develop the highest porosity, have the most elastic structure and the lowest rate of color deterioration.

ISSN:0737-3937    

DOI:10.1080/07373930008917775

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Immersion frying was studied as a high temperature drying process and analogies drawn between periods found in drying and similar periods in immersion frying. Analysis of external heat transfer during immersion frying showed a highly complex system of free and forced convection augmented by boiling conditions. Oil flow was found to be driven downward by buoyancy forces due to cooling at the sample surface then upward due to entrainment in vapor bubbles during boiling conditions. Experimental work showed the convective heat transfer coefficient to be a dynamic property ranging from 300 to 1100 W/m2°C and to be strongly coupled with bulk movement of the oil. Heat flux measurements found a peak flux of nearly 30,000 W/m3. Based on analysis of bubble dynamics it is hypothesized that heat flux increases with increased oil degradation through a reduction in vapor bubble size and increase in bubble frequency due to changes in interfacial properties of the oil.

ISSN:0737-3937    

DOI:10.1080/07373930008917776

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Our measurements proved that the convective heat transfer coefficient (h) has a larger valuehwetat the constant drying rate period and after that it falls down to a minimum one:hdryin the equilibrium dried state (see Fig.1). Our measurements showed also that the heat of vaporisation in the last phase of the falling drying rate period is far greater than it was in the constant drying rate period (see Fig.10).

ISSN:0737-3937    

DOI:10.1080/07373930008917777

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Non-linear partial differential equations are presented for two dimensional heat and mass transfer within a single grain kernel during drying. In this model, the moisture evaporation inside the kernel is considered. The moisture is assumed to diffuse to the outer boundary of the kernel in liquid form and evaporate on the surface of the kernel. The influence of temperature and moisture content on grain properties is also considered in the simulation. The Non-linear partial differential equations are solved using the finite element method and simulation data is verified on a thin layer dryer for wheat kernels. The comparison shows that the simulated results have a high accuracy with average relative error of about 5%. The results of the finite element analysis can be used for grain quality evaluation, drying simulation studies and stress analysis of grain kernel.

ISSN:0737-3937    

DOI:10.1080/07373930008917778

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Various treatments were used to determine polyphenol oxidase activity changes during osmotic dehydration of banana slices. A temperature of 35 °C and 5.0 pH were used as the most adequate conditions for polyphenol oxidase activity determination when 4-methyl catechol was used as a substrate. Enzyme activity change was determined in the central and edge region of fruit. The central part of the banana slice showed 2.4 times higher activity than in the edge region. A difference in the kinetics of enzyme activity decrease also occurred during four hours of drying. When the drying was carried out at 70 °C, the enzyme was completely inactivated after 60-90 minutes of specific osmotic treatments. After four hours of osmotic drying at 50 and 60 °C, polyphenol oxidase activity was still present; the residual activity varied from 1.3 to 15 %.

ISSN:0737-3937    

DOI:10.1080/07373930008917779

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Picnometry and image analysis were used to measure and analyze the dimensional shrinkage ofBrachiaria brizanthaseeds due to reduction in the moisture content. The data were statistically treated and an equation to relate the shrinkage phenomenon with the moisture content was obtained.

ISSN:0737-3937    

DOI:10.1080/07373930008917780

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor