MATHEMATICAL MODELLING OF THIN-LAYER DRYING OF KIWIFRUIT SLICES

The effect of temperature on the drying kinetics of kiwifruit was investigated. The drying process was carried out at temperatures of 50, 55 and 60C, air velocity of 2.4   m/s and relative humidity between 10–25%. Drying time decreased considerably with increased air temperature. Drying process took place in the falling rate period. Twelve mathematical models available in the literature were fitted to the experimental data. The models were compared by four statistical parameters; i.e., determination of coefficient, mean relative percent error, reduced chi-square and root mean square error, and the best-fit model was selected. The modified Henderson and Pabis and Verma et al. models were given the best results in describing thin-layer drying of kiwifruits. The effective diffusivity of water during air-drying varied from 1.743 to 2.241  ×  10 ^{− 10}   m²/s over the temperature range investigated, with activation energy equal to 22.48   kJ/mol.

PRACTICAL APPLICATIONS

Drying can be described as an industrial preservation method in which water content and activity of agricultural products are decreased by heated air to minimize biochemical, chemical and microbiological deterioration. Kiwifruit has a very short life because of softening and vitamin loss during cold storage. The use of drying prolongs the shelf-life of the kiwifruit, as the water content reduction slows down deterioration reactions. In this study, drying characteristics of kiwifruits were studied in a convectional hot-air dryer. The objectives of the present study were to determine experimentally the thin-layer drying characteristics and rehydration capacity of samples, and to fit the experimental data to 12 mathematical models available from the literature.     

MICROWAVE and CONVECTIVE DRYING of POTATO SLICES

Potato slices were dried using microwave drying, combined microwave and convective drying, and convective drying. Rehydration kinetics were also studied. Drying rates of the different drying methods were determined and microwave drying was compared with convective drying. Microwave drying has a potential for producing better quality dried products while considerably reducing drying duration (i.e., 10 min vs. 10 h). Effective moisture diffusivity profiles were calculated using Fick's diffusion model in one dimension. Statistical analysis showed diffusivity to increase with increasing internal temperature but to decrease (in microwave drying) with increasing moisture content, with high correlation (R²≥ 0.973).     

节能型微波干燥马铃薯片的特性研究

通过对微波干燥马铃薯片的特性研究，得出了在微波干燥条件下马铃薯片的失水及耗电特性。并在此基础上，得出了各指标的回归方程，从而对各参数进行了优化设计，得出了各参数的最佳组合。     

MATHEMATICAL MODELING OF ROUGH RICE DRYING IN A SPOUTED BED

A theoretical model, based on the well-known man and energy transport equations for a continuous media, was rigorously developed to describe the liquid diffusion controlled drying of cereal grain in a batch operated, well-mixed spouted-bed dryer. The equilibrium moisture content equation and the effective diffusion coefficient are required to solve the heat and mass transfer equations. Numerical techniques were used to solve these equations. The model was tested using laboratory-scale experiments where rough rice was dried in a spouted-bed. A good agreement was obtained between the experimental and theoretical model.     

STUDIES ON THE EFFECT OF SURFACTANT, BLANCHING AND OSMOTIC PRETREATMENTS ON THE CONVECTIVE DRYING OF POTATOES

Moisture removal from solids is an integral part of food processing. To approach the drying process of a given product it is necessary to have a knowledge of the drying kinetics. However, foodstuffs are often subjected to certain chemical and physical treatments, such as surfactant impregnation, osmotic dehydration or blanching, prior to convective drying. The drying behavior of potatoes was monitored within an experimental air tunnel dryer, with the mechanism of internal moisture migration being identified as diffusion. This was subsequently evaluated on the basis of the liquid diffusion theory. Experimental data revealed osmotic concentration as a favorable process. A 29±2% reduction in diffusion coefficient was observed following osmotic dehydration with a salt/sugar solution, however, this provided a 63±3% reduction in drying time. Structural modifications were induced by blanching, with this rendering a 10±1 and 27±2% lowering of the moisture transfer rate during drying at 30 and 60C, respectively.     

EFFECT OF SONIC ENERGY ON THE AIR DRYING OF APPLE AND SWEET POTATO CUBES

Cubes (2 cc) of apple and sweet potato were dried with and without sound energy in air at velocities of 5 – 20 fps and temperatures of 125 – 175°F. A stem jet or electronic whistle provided sound energy at 9,800 and 13,000 Hz with an intensity up to 135 dB. Results indicated that sonic energy had no effect on drying rate under all experimental conditions.     

EDIBLE COATINGS ON FRESH SQUASH SLICES: EFFECT OF FILM DRYING TEMPERATURE ON THE NUTRITIONAL AND MICROBIOLOGICAL QUALITY

The exposure of foods to the temperature and humidity needed to dry the coatings can have deleterious effects on their quality. A critical analysis of the film drying conditions on quality indexes for butternut squash slices is presented. During the coatings drying, the samples lost significant amounts of weight and ascorbic acid. Only during the drying of the chitosan coating for 100 min at 20C and for 30 min at 50C that the ascorbic acid contents were not significantly reduced. The application of casein and carboxy methyl cellulose coatings did not cause significant reductions in the counts of mesophilic aerobic microorganisms (5.75–6.00 Log CFU/g). Chitosan coated samples presented significant log reductions showing the higher reductions the samples dried for 30 min at 50C (5.02 Log CFU/g). For the preservation of the nutritional and microbiological quality indexes of squash slices, the best coating conditions would be chitosan dried for 30 min at 50C.

PRACTICAL APPLICATIONS

The continuous consumer interest in high quality and food safety, combined with environmental concern, has induced the development and study of new edible coatings that avoid the use of synthetic materials. The edible coatings, formed from Generally Recognized as Safe materials, have the potential to reduce weight loss, respiration rate and improve food appearance and integrity. It is one of the most effective methods to maintain food quality.
Antimicrobial films and coatings have innovated the concept of active packaging and have been developed to reduce, inhibit or delay the growth of microorganisms on the surface of food in contact with the packaged products. In addition, the edible coatings allow the incorporation of food additives with antimicrobial effects that might help reduce microbial spoilage during postharvest.     

PREVENTION OF BROWNING OF POTATO SLICES USING POLYPHENOLOXIDASE INHIBITORS AND ORGANIC ACIDS

Browning of minimally processed potato slices stored at low temperatures without active modification of the atmosphere in the storage container was inhibited by use of combinations of polyphenoloxidase inhibitors with organic acids. Slices of two cultivars‘Atlantic’and‘Russet Burbank’were dipped in solutions of various combinations of 4‐hexyl resorcinol, N‐acetyl‐cysteine and citric acid. Treated potato slices were kept in storage at 5C without browning or decrease in firmness for up to 21 days. The limiting factor in storage became visible microbial decay. Browning of slices from potatoes stored 1 month longer at 10C was also inhibited, but storage was limited to 14 days because of increased susceptibility to decay and decreased firmness with most treatments. However, no differences were found in inhibition of browning or retention of firmness of slices of the two cultivars after treatment with the combination of 0.25 M citric acid, 0.25 M N‐acetylcysteine and 0.01 M 4‐hexylresorcinol.     

EFFECTS OF THERMAL AND ELECTROTHERMAL PRETREATMENTS ON HOT AIR DRYING RATE OF VEGETABLE TISSUE

Cylindrical samples of carrot, potato and yam were dried in a hot‐air dehydrator after preheating to 50C or 80C by three different heating methods (conventional, microwave and ohmic). The results showed that enhancement of drying rate increased with pretreatment temperature. Ohmic pretreatment increased the drying rate more than conventional and microwave heating. Desorption isotherms showed that in the low a_w range, desorption data of preheated and raw materials were similar. However, the isotherms of preheated samples shifted when a_w was high, which indicated that thermal pretreatments altered the structure, and apparently, the water distribution within these materials. For all samples, ohmic pretreatment showed stronger influences on isotherms than microwave heating, while the pretreatment effect of conventional heating was only observed for potato tissue.     

IMPINGEMENT DRYING OF POTATO CHIPS

The effect of superheated steam temperature (115, 130, and 145C) and convective heat transfer coefficient (100 and 160 W/m²C) on the drying rate and product quality attributes (shrinkage, density, porosity, color, texture, and nutrition loss) of potato chips was investigated. Furthermore, potato chips dried by impinging superheated steam (130 and 145C, h = 100 W/m²C) were compared to air dried (same conditions), commercial, and fried potato chips. Temperature and convective heat transfer coefficient had a significant effect on the drying rate during superheated steam impingement drying. Potato chips dried at higher drying temperature and convective heat transfer coefficient showed less shrinkage, lower bulk density, higher porosity, and darker color when compared to chips dried at lower temperatures and convective heat transfer coefficients. They were also less hard and had a lower vitamin C content. A higher rate of evaporation during the falling rate period was obtained when superheated steam drying was compared to air impingement drying. Potato chips produced using superheated steam impingement drying showed more shrinkage, higher bulk density, lower porosity, and lighter color than chips dried with air under the same temperature and with the same convective heat transfer coefficient (130, and 145C, h = 100 W/m²C). Moreover, superheated steam‐dried potato chips retained more vitamin C during the drying process.