Refractance Window Drying: An Emerging Technology for Heat-Sensitive Food Products
Ultrasound and infrared assisted conductive hydro-drying is a new patented drying method that can dry food materials with high quality and high energy efficiency. In this study, okra slices were dried using UIACHD with different settings and also by freeze-drying and oven drying. The UIACHD drying curves, moisture content, color, vitamin C content, hydrocolloids rheology and microstructure of the dried okra sample as affected by drying method were investigated.
The average value of the radiation penetration depth in 2% CMC was 1.20 ± 0.19 mm. Although the penetration depth is known to depend on the nature of the material and temperature of the emitting source , the value obtained depended only on the material (CMC 2%) in the range of temperatures evaluated (70, 80, and 90 °C). It was, as expected, independent of the type and thickness of the films Revolutionizing Food Drying: The Advantages of Refractance Window Dryer evaluated. The radiative heat flux emitted by the water at 70, 80, and 90 °C and the radiative heat flux transmitted through the film (water–film system) determined during the first 50 s were measured . It was observed that the heat flux was significantly higher for times longer than 50 s . The heating of the film after this time increased its emissivity, causing an increase in heat flux.
The drying behavior and characteristics of date paste were studied at temperatures of 60, 70, and 80ºC as thin layer with sample thicknesses of 1, 1.5, and 2 cm in a laboratory scale vacuum chamber. Modeling of drying kinetics of date paste was investigated based on the specific temperatures and sample thicknesses. The experimental moisture loss data were fitted to eight thin layer drying models available in the literature. The modified Henderson-Pabis, Verma, and Jena-Das models showed better fitness to the experimental drying data compared to the other models. The effective moisture diffusivity ranged between 6.0854×10-8 and 4.868×10-7 m 2 s-1. Effective diffusivity increased with the increase in temperature and sample thickness.
Accordingly, resonant absorption occurs in carbohydrates, which heat the water in the CMC. Subsequently, a thermal heating effect of the water occurs due to the dipole orientation in the electromagnetic field of radiation. Figure 1 shows the drying kinetics of CMC with thicknesses of 1.5, 2.5, and 3.5 mm for the three films at 90 and 70 °C.
These compounds interact with amino groups from proteins and undergo Maillard reaction during the exposition to air at high temperatures, long drying times and the amount of water. In addition, color changes appear due the degradation of some thermosensitive compounds such as anthocyanins and carotenes, which results in a loss of functionality and color. A quick dehydration of the product to 15–20% moisture content can reduce Maillard reaction time. For this reason, the drying methods are designed to accelerate the drying time in order to achieve the desired moisture content while reducing the browning time . Microwave drying is based on the volumetric heating occurring when electromagnetic waves pass through the material causing a molecules oscillation. This oscillation generates thermal energy that is then used to remove water from the wet material.
Refractance window drying is one such technique that gained a lot of attention in recent years, owing to the numerous benefits it claims. The technique involves drying purees and liquids placed over a thin infrared transparent film that essentially forms a ‘window’ through which drying occurs. Importantly, product temperatures are kept low and rapid drying occurs as all three modes of heat transfer are involved. Refractance window drying has found several applications not only in food industries but also in pharmaceutical, nutraceutical, cosmetic and pigment handling industries. The objective of this work is to present the recent trends in refractance window drying of foods with emphasis on the underlying mechanism and effect on product quality.
The dryer 100 includes a cover 101 that provides a cover and headspace above a drying belt 110 for the dryer 100, an air supply manifold 120 that introduces conditioned air 102 into the dryer 100 and an air outlet exhaust manifold 130. In one embodiment, low pressure air is distributed through adjustable slots, or air knives, to effectively distribute the air across the entire width of the drying belt. The present drying system dries a liquid or slurry product placed on a continuous drying belt by properly directing conditioned air across the surface of the product, according to one embodiment.
The analytical solution is given by , which is valid only for the falling rate drying period. Where Xt is the moisture content at a given time, Xe is the equilibrium moisture content and X0 is the initial moisture content (g water/g dry solid). However, MR was simplified to , taking into account that the RW™ equipment used for the experiments does not control the relative humidity of air in contact with the material [21-23]. The drying kinetics were fitted using Newton's and Midilli's models, widely used for modeling moisture ratio in biological materials [19-21].
During impingement processing, the boundary layer around the product is broken through high velocity air impinging upon the surface. This process increases the heat and mass transfer rate, and allows efficient removal of moisture, baking, curing, toasting and roasting. Nozzle Dry Impingement Ovens reduce processing time significantly when compared to lower velocity convection or through-air systems. According to one embodiment, an apparatus includes a drying belt configured to receive a product to be dried on a first surface of the drying belt, and a heat medium in contact with a second surface of the drying belt. The heat medium is configured to heat the product and is maintained at a pre-determined temperature.
Singh D.B., Kingsly A.R.P. Effect of convective drying on qualilty of Anardana. Zou K., Teng J., Huang L., Dai X., Wei B. Effect of osmotic pretreatment on quality of mango chips by explosion puffing drying. Pydi Setty Y., Ramana Murthy J.V. Development of a model for drying of solids in a continuous fluidized bed dryer. The effect of dehydration on essential oils and/or volatile compounds has been reported mostly in aromatic herbs. The two basic forms of consuming culinary herbs are fresh and dried.
Papaya is a tropical fruit which originated from Mexico and Central America, and it is a good source of vitamins . Papaya is highly perishable, mainly due to its high moisture content (between 80 and 85% wb), so it is important to find alternatives for its preservation and/ or processing [3-5]. The cost of Refractance Window drying equipment is approximately one third to one half that of a freeze dryer to dry a similar amount of product, while the energy costs to operate RW dryers are less than half of freeze dryers. The RWD method has become attractive for applications in the food industry, especially because the dried products are of high quality and the equipment is relatively inexpensive. Besides its ability to handle a diverse liquid product, it can be used effectively to transform fruits, vegetables, herbs and other related products into value added powders and concentrates. Both capital investment and operating cost of refractance window drying is economical than other techniques.
The average value of the radiation penetration depth in 2% CMC was 1.20 ± 0.19 mm. Although the penetration depth is known to depend on the nature of the material and temperature of the emitting source , the value obtained depended only on the material (CMC 2%) in the range of temperatures evaluated (70, 80, and 90 °C). It was, as expected, independent of the type and thickness of the films Revolutionizing Food Drying: The Advantages of Refractance Window Dryer evaluated. The radiative heat flux emitted by the water at 70, 80, and 90 °C and the radiative heat flux transmitted through the film (water–film system) determined during the first 50 s were measured . It was observed that the heat flux was significantly higher for times longer than 50 s . The heating of the film after this time increased its emissivity, causing an increase in heat flux.
The drying behavior and characteristics of date paste were studied at temperatures of 60, 70, and 80ºC as thin layer with sample thicknesses of 1, 1.5, and 2 cm in a laboratory scale vacuum chamber. Modeling of drying kinetics of date paste was investigated based on the specific temperatures and sample thicknesses. The experimental moisture loss data were fitted to eight thin layer drying models available in the literature. The modified Henderson-Pabis, Verma, and Jena-Das models showed better fitness to the experimental drying data compared to the other models. The effective moisture diffusivity ranged between 6.0854×10-8 and 4.868×10-7 m 2 s-1. Effective diffusivity increased with the increase in temperature and sample thickness.
Accordingly, resonant absorption occurs in carbohydrates, which heat the water in the CMC. Subsequently, a thermal heating effect of the water occurs due to the dipole orientation in the electromagnetic field of radiation. Figure 1 shows the drying kinetics of CMC with thicknesses of 1.5, 2.5, and 3.5 mm for the three films at 90 and 70 °C.
These compounds interact with amino groups from proteins and undergo Maillard reaction during the exposition to air at high temperatures, long drying times and the amount of water. In addition, color changes appear due the degradation of some thermosensitive compounds such as anthocyanins and carotenes, which results in a loss of functionality and color. A quick dehydration of the product to 15–20% moisture content can reduce Maillard reaction time. For this reason, the drying methods are designed to accelerate the drying time in order to achieve the desired moisture content while reducing the browning time . Microwave drying is based on the volumetric heating occurring when electromagnetic waves pass through the material causing a molecules oscillation. This oscillation generates thermal energy that is then used to remove water from the wet material.
Refractance window drying is one such technique that gained a lot of attention in recent years, owing to the numerous benefits it claims. The technique involves drying purees and liquids placed over a thin infrared transparent film that essentially forms a ‘window’ through which drying occurs. Importantly, product temperatures are kept low and rapid drying occurs as all three modes of heat transfer are involved. Refractance window drying has found several applications not only in food industries but also in pharmaceutical, nutraceutical, cosmetic and pigment handling industries. The objective of this work is to present the recent trends in refractance window drying of foods with emphasis on the underlying mechanism and effect on product quality.
The dryer 100 includes a cover 101 that provides a cover and headspace above a drying belt 110 for the dryer 100, an air supply manifold 120 that introduces conditioned air 102 into the dryer 100 and an air outlet exhaust manifold 130. In one embodiment, low pressure air is distributed through adjustable slots, or air knives, to effectively distribute the air across the entire width of the drying belt. The present drying system dries a liquid or slurry product placed on a continuous drying belt by properly directing conditioned air across the surface of the product, according to one embodiment.
The analytical solution is given by , which is valid only for the falling rate drying period. Where Xt is the moisture content at a given time, Xe is the equilibrium moisture content and X0 is the initial moisture content (g water/g dry solid). However, MR was simplified to , taking into account that the RW™ equipment used for the experiments does not control the relative humidity of air in contact with the material [21-23]. The drying kinetics were fitted using Newton's and Midilli's models, widely used for modeling moisture ratio in biological materials [19-21].
During impingement processing, the boundary layer around the product is broken through high velocity air impinging upon the surface. This process increases the heat and mass transfer rate, and allows efficient removal of moisture, baking, curing, toasting and roasting. Nozzle Dry Impingement Ovens reduce processing time significantly when compared to lower velocity convection or through-air systems. According to one embodiment, an apparatus includes a drying belt configured to receive a product to be dried on a first surface of the drying belt, and a heat medium in contact with a second surface of the drying belt. The heat medium is configured to heat the product and is maintained at a pre-determined temperature.
Singh D.B., Kingsly A.R.P. Effect of convective drying on qualilty of Anardana. Zou K., Teng J., Huang L., Dai X., Wei B. Effect of osmotic pretreatment on quality of mango chips by explosion puffing drying. Pydi Setty Y., Ramana Murthy J.V. Development of a model for drying of solids in a continuous fluidized bed dryer. The effect of dehydration on essential oils and/or volatile compounds has been reported mostly in aromatic herbs. The two basic forms of consuming culinary herbs are fresh and dried.
Papaya is a tropical fruit which originated from Mexico and Central America, and it is a good source of vitamins . Papaya is highly perishable, mainly due to its high moisture content (between 80 and 85% wb), so it is important to find alternatives for its preservation and/ or processing [3-5]. The cost of Refractance Window drying equipment is approximately one third to one half that of a freeze dryer to dry a similar amount of product, while the energy costs to operate RW dryers are less than half of freeze dryers. The RWD method has become attractive for applications in the food industry, especially because the dried products are of high quality and the equipment is relatively inexpensive. Besides its ability to handle a diverse liquid product, it can be used effectively to transform fruits, vegetables, herbs and other related products into value added powders and concentrates. Both capital investment and operating cost of refractance window drying is economical than other techniques.
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