Effects of different drying methods on the physical characteristics and flavor of dried hawthorns (Crataegus spp.)

ABSTRACT

The objective of this study was to compare the physical characteristics and flavor of dried hawthorns obtained by freeze-drying (FD), microwave freeze-drying (MFD), atmospheric freeze-drying (AFD), and heat pump drying (HPD). The parameters including moisture content, product temperature, rehydration ratio, chroma, hue angle, yellowness index, total color difference, and energy consumption were investigated. The use of fuzzy reasoning for the sensory evaluation of hawthorn quality was also performed. As expected, AFD, FD, and MFD produced better accepted dried hawthorn products than did HPD. Nevertheless, FD consumed the highest energy and had a long drying time, but its product was the best; AFD had a similar energy cost as HPD, but its drying time was the longest; and MFD had a higher energy cost and longer drying time than HPD, but its product quality was similar to that of FD products. As a result, MFD and AFD had potential to replace FD to yield dried hawthorns with high product quality and relatively low cost.     

Browning behavior of button mushrooms during microwave freeze-drying

In order to achieve faster drying along with high product quality, microwave freeze-drying (MFD) was applied to dry button mushrooms. Although MFD can lead to similar product quality compared with freeze-drying (FD), the color deterioration of MFD mushroom is higher than FD ones. Therefore, two drying methods were used to investigate the browning behaviors of button mushrooms during MFD in terms of parameters including polyphenol oxidase activity, total phenolic content, moisture content, reducing sugar content, and vitamin C content. It was found that both whiteness and browning degree can be used to describe the browning kinetics of button mushrooms during the MFD process. Both nonenzymatic browning and enzymatic browning took place during MFD of button mushrooms, but the effect of enzymatic browning was more significant, which should be controlled during MFD. In order to avoid enzymatic browning during MFD of button mushrooms, a relatively low microwave power at initial stage of MFD should be adopted. Meanwhile, a low microwave power also should be used at the end stage of MFD so as to reduce nonenzymatic browning.     

Trends in Microwave-Assisted Freeze Drying of Foods

Microwave-assisted freeze drying (MFD) can be accomplished in two distinct ways: freeze drying assisted concurrently with microwave application (MFD-1) and freeze drying and assisted microwave/vacuum microwave drying in two consecutive separate drying stages (MFD-2). MFD is a rapid dehydration technique that can be applied to certain foods, particularly to seafoods, solid soup, and fruits and vegetables. MFD involves much less drying time and energy consumption than conventional freeze-drying methods. Currently, this technology has been successfully used to dry many food materials and has potential in the food industry. Increasing concerns over product quality, energy savings, and production costs have motivated researchers and the industry to adopt MFD technologies. The advantages of MFD include shorter drying time, energy savings, improved product quality, and flexibility in producing a wide variety of dried products. However, current applications are limited to small categories of foods due to high startup costs and relatively complex technology compared to conventional freeze drying. This article presents a concise review of recent progress in MFD R&D and makes recommendations for future research to bridge the gap between laboratory research and industrial applications.     

冷冻干燥过程强化中冷冻阶段优化的研究进展

冷冻干燥产品质量高,但时间长、能耗高。本文综述了冷冻干燥过程强化中冷冻阶段的优化方法,控制冷冻速率、调节冰晶成核和退火处理可以获得大而均匀的冰晶从而提高升华干燥阶段速率,但物料内部比表面积的减小会降低解吸干燥阶段速率,这类常规的冷冻阶段优化方法对弱吸湿性的物料有一定的强化效果。有机溶剂具有较高的蒸气压,作为共溶剂时可以增加传质推动力,但较低的有机溶剂残留量要求阻碍了其进一步应用。“初始非饱和多孔介质冷冻干燥”的技术思想是将液体物料首先制备成具有一定初始孔隙的冷冻物料,然后再进行冷冻干燥。物料具有的初始孔隙为水蒸气的迁移提供了便捷的通道,而且纤薄的固体基质也有利于结合水的解吸,可以同时强化升华干燥阶段和解吸干燥阶段。该技术思想是过程低消耗和产品高质量的完美结合,为解决冷冻干燥过程速率低的问题提供了新的方案。     

Studies on the Microwave Freeze Drying Technique and Sterilization Characteristics of Cabbage

The vacuum freeze-drying (FD) technique used in the food industry can yield a high-quality product, but it is very expensive and requires a long processing time. Besides, the quantity of microorganisms in FD products can often exceed the required standard. As a result, it will be important to develop a new freeze-drying technique. In this article, cabbage was used as a model material, and the microwave field was used as a heat source to supply sublimation heat so that the drying time was shortened greatly. The effect of the microwave sterilization during the drying process was evaluated. Effects of the pressure, thickness of material being dried, and the input microwave power on such indices as drying time and the microorganism number were studied. Compared with the method of ordinary freeze drying, microwave freeze drying (MFD) can greatly reduce the drying time and has a notable sterilization effect.     

Studies on the Microwave Freeze Drying Technique and Sterilization Characteristics of Cabbage

The vacuum freeze-drying (FD) technique used in the food industry can yield a high-quality product, but it is very expensive and requires a long processing time. Besides, the quantity of microorganisms in FD products can often exceed the required standard. As a result, it will be important to develop a new freeze-drying technique. In this article, cabbage was used as a model material, and the microwave field was used as a heat source to supply sublimation heat so that the drying time was shortened greatly. The effect of the microwave sterilization during the drying process was evaluated. Effects of the pressure, thickness of material being dried, and the input microwave power on such indices as drying time and the microorganism number were studied. Compared with the method of ordinary freeze drying, microwave freeze drying (MFD) can greatly reduce the drying time and has a notable sterilization effect.     

Comparative study on the flavonoids extraction rate and antioxidant activity of onions treated by three different drying methods

Onions, rich in flavonoids, with antioxidant, antibacterial, and other biological activities, are often used as extraction of raw materials to obtain health products with high flavonoids content. However, the amount of raw material consumption is large due to low yield of extracts from fresh onions. As a result, dried onions often are used as extraction materials. The effects of air drying (AD), freeze-drying (FD), and microwave freeze-drying (MFD) on the flavonoids extraction yield of onions were investigated. The most effective and economical method was hot AD. On the other hand, the flavonoids extraction rate of onions dried by MFD was the fastest. FD could also obtain high extraction rate, but it cost the highest energy consumption. In order to obtain relatively rapid, economic, and effective extraction of onion flavonoids, onions can be dried by MFD or AD method.     

A Novel Atmospheric Freeze-Drying System Using a Vibro-Fluidized Bed with Adsorbent

Atmospheric freeze drying (AFD) in a vibro-fluidized bed dryer coupled with an adsorbent and multimode heat input is proposed for dehydration of food products at lower cost than the traditional freeze-drying process under vacuum. The aim of this project is to study the proposed AFD system using a vortex tube to produce low-temperature dry air, an alternative for producing dried food products of high quality. An experimental setup was designed and built to permit simultaneous application of convection, conduction, and radiation heat input to the drying material above its freezing point to ensure sublimation. A parametric evaluation over a broad range of possible parameter values was carried out using cubic-shaped potato and carrot as model heat-sensitive products. The influence of various system parameters on drying kinetics, quality, and functional properties of the dried products (color, rehydration properties, and morphology) were investigated. Comparison between physical quality and drying characteristics of the AFD system with AFD using fixed bed, fluidized bed dryer, and also with traditional vacuum freeze drying were carried out to investigate the viability of this new system. Results indicate that proposed system is an alternative to reduce the process time as well as to maintain the product quality at lower cost.     

A Novel Atmospheric Freeze-Drying System Using a Vibro-Fluidized Bed with Adsorbent

Atmospheric freeze drying (AFD) in a vibro-fluidized bed dryer coupled with an adsorbent and multimode heat input is proposed for dehydration of food products at lower cost than the traditional freeze-drying process under vacuum. The aim of this project is to study the proposed AFD system using a vortex tube to produce low-temperature dry air, an alternative for producing dried food products of high quality. An experimental setup was designed and built to permit simultaneous application of convection, conduction, and radiation heat input to the drying material above its freezing point to ensure sublimation. A parametric evaluation over a broad range of possible parameter values was carried out using cubic-shaped potato and carrot as model heat-sensitive products. The influence of various system parameters on drying kinetics, quality, and functional properties of the dried products (color, rehydration properties, and morphology) were investigated. Comparison between physical quality and drying characteristics of the AFD system with AFD using fixed bed, fluidized bed dryer, and also with traditional vacuum freeze drying were carried out to investigate the viability of this new system. Results indicate that proposed system is an alternative to reduce the process time as well as to maintain the product quality at lower cost.     

A novel atmospheric freeze dryer using simultaneous application of subzero and hot air streams using a vortex chiller

The objective of this research was to develop an atmospheric freeze-drying (AFD) system for heat-sensitive materials. A laboratory scale AFD system consisting of a vortex chiller was designed and fabricated. A vortex tube was used to convert compressed air into a subzero and a hot air stream. The system used the simultaneous application of subzero air stream and hot air carrier gas coupled with radiant heat input for drying heat-sensitive materials. A parametric evaluation of this drying system was performed. Comparison of drying characteristics and the physical quality (e.g., color, shape, size) with vacuum freeze and traditional AFD was also performed. Results revealed that the proposed system is a suitable and viable alternative without compromising the quality compared to traditional freeze-drying system.     

The Effect of Glass Transition Temperature on the Procedure of Microwave–Freeze Drying of Mushrooms (Agaricus bisporus)

Freeze drying (FD) yields the best quality of dried mushroom but at the cost of a long drying time and high overall cost. Air drying (AD) gives an unacceptably poor quality product. To achieve faster drying along with a high-quality product, a microwave–freeze drying (MFD) technique was developed to dry mushrooms. The relationship between dried mushroom quality and the glass transition temperature during the MFD process was studied to optimize the MFD process. According to the change tendency of the glass transition temperature of mushroom during MFD, a step-down microwave loading scheme for the MFD process was developed to obtain good product quality.     

The drying strategy of atmospheric freeze drying apple cubes based on glass transition

Freeze drying (FD) yields the best quality of dried apple cubes but at the cost of long drying time and also the overall cost. To achieve economical freeze drying along with a high quality product an atmospheric freeze drying (AFD) technique was developed to dry apple cubes. The effect of different air temperature loading scheme on product quality and drying process was studied during AFD. According to the glass transition temperature of apple, a step-up temperature loading strategy for AFD process was developed to reduce the drying time by almost half and provided a similar good product quality.     

Microwave-Assisted Atmospheric Freeze Drying of Green Peas: A Case Study

Atmospheric freeze drying (AFD) is based on the sublimation of ice due to a pressure gradient (convective drying), and is a dehydration process for temperature-sensitive products. Since the process is slow in general, microwave radiation (MW) was applied in order to increase the sublimation in fluid and fixed bed conditions at drying temperatures of ?6°C, ?3°C, and 0°C. The modified Weibull model was used to describe the drying behavior for all investigations. With 280 Watt power supplied to the magnetrons, it was possible to reduce drying time by approximately 50%. The drying efficiency was approximately 30%, while the SMER was increased by 0.1 to 0.3 kg_waterkWh^?1, which gives better energy efficiency for the microwave drying system used in this investigation. The product quality (color reduction and particle size/porosity) was well preserved in fixed bed drying at ?6°C and ?3°C, while the product quality was reduced significantly in microwave AFD experiments at 0°C and in a fluid bed. The drying rates of AFD in a fluid bed condition were not as high as those in a fixed bed. MW-AFD in a fixed bed condition at temperatures of ?6°C and ?3°C performed best regarding product quality, drying time, and process control.     

Serial combination drying processes: A measure to improve quality of dried carrot disks and to reduce drying time

Conventional drying processes like hot air-drying (HAD), freeze-drying (FD) and microwave vacuum drying (MVD) have specific advantages and disadvantages concerning product quality and drying time. Recent studies have shown that serial combinations of these processes can lead to better product quality (water content, color, hardness, rehydration, volume, and ingredient retention) and favorable process parameters (shorter time, less energy consumption). However, little is known about the timing of the changeover point between the different processes. In this study, we investigated the development of quality parameters (volume retention, rehydration properties) during FD, HAD, and MVD either as a single or as a serial combination process with varying changeover point. Therefore, carrot disks were processed in a modular drying processor to different final relative moisture contents. On this basis, recommendations were derived for certain combinations of drying processes to achieve desired product quality parameters, partially with shorter drying times.     

Diffuse interface model of the freeze-drying process of individually frozen products

Abstract

Vacuum freeze-drying (VFD) is a dehydration method based on the sublimation of the liquid phase contained in a certain product, previously frozen, at low pressure and temperature. Since it is a time and energy consuming process, it is crucial to select the best processing conditions to minimize drying duration, thus reducing the energy requirement. Additionally, product temperature must be monitored since it plays an important role in preserving product quality. The aim of this study was to develop a Diffuse Interface Model (DIM) for in-silico simulation of the freeze-drying process of individually frozen products. Due to the geometrical features of the samples, and to the role of radiation in the heat transfer to the product, the usual one-dimensional approach is inappropriate. Using a DIM, each cell of the computational domain can be described as a porous solid matrix filled by ice and vapor with a time-varying composition, thus allowing the use of a fixed computational grid and making the computation effort less demanding in comparison to moving interface-based models. Drying of eggplant cubic samples was considered as case study: model parameters were estimated by fitting the experimentally measured product temperature and drying time to the calculated ones. The model was proven to be reliable in providing an accurate estimate of both the drying time and the product temperature. Therefore, it can be used for off-line process design and optimization, minimizing the experimental effort required to design and optimize the process.     

Microwave Freeze Drying of Apple Slices Based on the Dielectric Properties

Freeze drying (FD) yields the best quality of dried apple slices but requires a long drying time and is not cost-effective. To mitigate these problems, a microwave freeze drying (MFD) technique was developed to dry apple slices. The relationship between corona discharge and microwave power at various pressures and initial moisture content conditions was studied to avoid the possibility of corona discharge during MFD. It was found that with change of moisture content and temperature of samples during MFD, the dielectric property also changed, which resulted in dynamic microwave dissipation. Based on the dielectric property of samples, a changed microwave loading scheme can lead to perfect product quality and greatly reduce the drying time; thus, MFD can be used to replace the traditional FD technique.     

Emerging food drying technologies with energy-saving characteristics: A review

Abstract

Drying is one of the most vital preservation techniques used in the food industry. It demands different levels of energy to produce commercially high-quality-dried food products. Novel drying technologies minimize deterioration of the food ingredients and produce novel products for consumers. In recent years, there have been many developments in the technology connected with the industrial drying of foods. Recent research has shown that novel food drying technologies could be utilized to improve the efficiency of drying by lowering the energy consumption and also to enhance the product quality. This article reviews selected energy- saving techniques in drying and discusses some novel combined drying technologies. These include solar-assisted, infrared-assisted, microwave-assisted and similar hybrid drying methods for food drying. Recommendations are also made for future research and development.     

A Combination of Freeze Drying and Microwave Vacuum Drying of Duck Egg White Protein Powders

High energy consumption during freeze drying (FD) is a major concern that limits its application on common food product manufacturing. In this research, fresh duck egg white protein (FDEWP) powder and desalted duck egg white protein (DDEWP) powder were obtained by a combined two-stage tandem drying technologies (FD and microwave–vacuum drying [MVD]) in order to reduce energy consumption while maintaining good product quality. The results showed that the drying time for the FDEWP and DDEWP powders was significantly decreased by FD + MVD compared to those obtained by the FD-only process. The FDEWP powders dried by FD + MVD had a better color (higher L* and lower b*), lower apparent density, and lower foaming stability but higher emulsifying index than those dried by FD only. The DDEWP powder dried by FD + MVD had a product quality similar to that of FDEWP powder, suggesting that the DDEWP powder could be widely used as a food ingredient.     

The energy consumption and color analysis of freeze/microwave freeze banana chips

This study investigated the energy consumption of preparing banana chips by freeze drying (FD) and microwave freeze drying (MFD) methods. The results in this study showed that the energy consumption for 400 g fresh banana (about 95 g dried samples) by FD process and MFD process are about 35.73 × 10⁶ J (9.92 kW h) and 21.76 × 10⁶ J (6.57 kW h), respectively. Compared to the traditional FD process, MFD can save up to 35.7% energy and 40% drying time. Increasing the heating power in the secondary drying stage of MFD process had been confirmed to result in decrease in both the energy consumption and drying time. After increasing the microwave power in the secondary drying stage from 1.0 W/g to 1.5 W/g in MFD process, total energy consumption is about 18.12 × 10⁶ J (5.56 kW h) and drying time can be reduced from 360 min to 270 min. The sensory evaluation of produced banana chips at different drying conditions (1.0 W/g, 1.5 W/g and 2.0 W/g) revealed that the sensory properties are acceptable by the customers except the 2 W/g microwave power dried product. Thus, the method that increased the heating powder in the secondary drying stage of the MFD process could potentially be an effective method to reduce the energy consumption without seriously sacrificing the color of the end product.     

Parametric Study of High-Intensity Ultrasound in the Atmospheric Freeze Drying of Peas

Atmospheric freeze drying (AFD) is a dehydration process that can be used to produce high-end products for the food, pharmaceutical, and biological industries. Evaporation or sublimation at the drying temperatures used for these processes is generally low. Airborne ultrasound can be used to increase drying rates. This parametric study investigates the influence of the drying temperature, drying time, and ultrasonic power for atmospheric freeze drying in the presence of an airborne ultrasonic field. Accelerated effective diffusion of up to 14.8% was obtained for atmospheric freeze drying with a fluid bed. The faster drying in ultrasonic-assisted atmospheric freeze drying is assumed to be due to a higher mass transfer rate at the solid-gas interface, caused by a reduced boundary layer due to a higher turbulent interface. Thus high intensity, airborne ultrasound used with modern drying systems has great potential to accelerate drying, reduce investment and production costs, and improve product quality.