Technical aspects in freeze-drying of foods

Drying is a widespread concept in the food industry, typically used to convert a surplus crop into a shelf-stable commodity. With advancement of technology, however, there is interest in moving forward from the traditional convective air drying that is most widely used today for foods, to maintain at a very high level the nutritional and organoleptical properties of the initial fresh product. Freeze-drying (FD) produces the highest quality food product obtainable by any drying method, but it is considered the most expensive operation for manufacturing a dehydrated product owing to high energy consumption and high costs of both operation and maintenance. Microwave freeze-drying (MFD) and atmospheric freeze-drying (AFD) have been developed to reduce the FD energy consumption. The product quality of these two drying methods is similar to FD, due to removal of water content in materials by sublimation in both MFD and AFD. Although a significant amount of scientific research has been carried out in the field of sublimation drying, there are only a few comprehensive summarizations about the various sublimation drying methods. As a result, this review aims to highlight some of the latest and most notable advancements in sublimation-drying of foods, with main emphasis given to recent developments of reducing energy consumption of FD process and suggests future research areas on sublimation-related drying.     

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.     

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.     

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.     

Optimization of microwave freeze drying strategy of mushrooms (Agaricus bisporus) based on porosity change behavior

ABSTRACT

For dehydrated foods, porosity is a crucial parameter which affects mass and heat transfers, and is related to the product quality. It is important to monitor porosity change behavior to optimize the drying process. To achieve faster drying along with high product quality, microwave freeze drying (MFD) was applied to mushrooms dehydration processing. The relationship between porosity (include total, open-, and closed-pore porosity) and dried mushroom qualities was studied, and a suitable microwave loading strategy was obtained. It was found that lots of open pores could transform to closed pores while the moisture content (MC) was below 0.25?±?0.05?g/g (db), and the closed-pore porosity was arrived at a relatively stable level at moisture content below 0.17?±?0.03?g/g (db). Both total porosity and closed-pore porosity had a significant influence (P?<?0.01) on texture of MFD mushroom, and open-pore porosity had a significant effect (P?<?0.01) on rehydration ratio of MFD mushroom. According to the porosity change behavior of mushrooms, a dynamic microwave loading strategy can be used to reduce drying time and keep product quality during MFD process.     

Drying kinetics and product quality of green soybean under different microwave drying methods

The objective of this study was to investigate the effect of different microwave drying techniques on the drying kinetics and product quality of dried green soybean. Experiments were conducted using microwave vacuum drying (MVD), pulse-spouted vacuum microwave drying (PSMVD), pulse-spouted microwave drying (PSMD), and microwave freeze drying (MFD). Parameters of apparent density, color, processing temperature, expansion ratio, rehydration ratio, texture, and microstructure of the dried products were determined. The results showed that the MFD green soybean had a small change on bright color, compared with the fresh samples, but the drying time was the longest among the four methods. The bright color value of PSMVD/PSMD/MVD, respectively, was 79.77, 71.43, and 55.45, and drying time of them was slightly different. The PSMVD/PSMD showed advantages over MVD by improving the product quality.     

Drying Characteristics and Quality of Restructured Wild Cabbage Chips Processed Using Different Drying Methods

The drying characteristics of restructured wild cabbage chips dried using microwave vacuum (MVD), hot air (AD), and microwave freeze drying (MFD) were compared. Some of the key quality parameters of restructured wild cabbage chips such as fracturability and color and sensory characteristics were measured. Results showed that the drying time was reduced with the increase of microwave power (MVD/MFD) and in the case of air drying by the temperature (AD). Drying time was the shortest in the MVD process. Optimal quality of dried chips was obtained with the MFD process at a microwave power level 2.0 W/g.     

Atmospheric Freeze Drying—Modeling and Simulation of a Tunnel Dryer

Drying is an important unit operation in processing of foods and other biotechnological products. Vacuum freeze drying is said to be the best drying technology regarding product quality of the end product, but the disadvantages are, among others, expensive operational costs and batch drying. Atmospheric freeze drying was introduced to lower the production costs of high-quality dried foods, and the need of simulation tools became important in estimations of the industrial drying processes.

A simplified mathematical model (AFDsim) is developed based on uniformly retreating ice front (URIF) considerations. The model is used to calculate theoretical drying curves of atmospheric freeze dried foods in a tunnel dryer. Studies of thermal and mass transfer properties during drying are essential for understanding the changes in product quality and for designing and dimensioning the drying process. The model can be used to simulate industrial atmospheric freeze drying of different foodstuff in a tunnel. The results from AFDsim modeling are in good accordance with the experimental results.     

PULSED MICROWAVE-VACUUM DRYING OF FOOD MATERIALS

ABSTRACT

Microwave drying of food materials has been investigated over several years as a potential means for reducing the total drying time. However, some quality loss almost always accompanied when foods were dried completely using microwaves due to nonuniform temperature and moisture distribution. Some strategies used to improve dried product quality include combination of microwave and conventional hot air drying. pulsed or intermittent drying, and microwave-vacuum drying. Combination of pulsing and vacuum drying is a useful technique to maximize energy use efficiency and product quality especially for temperature sensitive products such as fruits. Some results of pulsed, microwave-vacuum drying of cranberries are presented. Pulsed drying is more energy efficient than continuous drying. In pulsed drying, the longer the pulsing ratio (i.e. longer power-off time in relation to power-on time) was more energy efficient. The quality of pulse-dried product was also generally better than that of continuous-dried product. The cycle power-on time and pulsing ratio should be carefully selected to obtain maximize the benefits of pulsed, microwave vacuum drying     

Atmospheric Freeze Drying—Modeling and Simulation of a Tunnel Dryer

Drying is an important unit operation in processing of foods and other biotechnological products. Vacuum freeze drying is said to be the best drying technology regarding product quality of the end product, but the disadvantages are, among others, expensive operational costs and batch drying. Atmospheric freeze drying was introduced to lower the production costs of high-quality dried foods, and the need of simulation tools became important in estimations of the industrial drying processes.

A simplified mathematical model (AFDsim) is developed based on uniformly retreating ice front (URIF) considerations. The model is used to calculate theoretical drying curves of atmospheric freeze dried foods in a tunnel dryer. Studies of thermal and mass transfer properties during drying are essential for understanding the changes in product quality and for designing and dimensioning the drying process. The model can be used to simulate industrial atmospheric freeze drying of different foodstuff in a tunnel. The results from AFDsim modeling are in good accordance with the experimental results.     

PULSED MICROWAVE-VACUUM DRYING OF FOOD MATERIALS

Microwave drying of food materials has been investigated over several years as a potential means for reducing the total drying time. However, some quality loss almost always accompanied when foods were dried completely using microwaves due to nonuniform temperature and moisture distribution. Some strategies used to improve dried product quality include combination of microwave and conventional hot air drying. pulsed or intermittent drying, and microwave-vacuum drying. Combination of pulsing and vacuum drying is a useful technique to maximize energy use efficiency and product quality especially for temperature sensitive products such as fruits. Some results of pulsed, microwave-vacuum drying of cranberries are presented. Pulsed drying is more energy efficient than continuous drying. In pulsed drying, the longer the pulsing ratio (i.e. longer power-off time in relation to power-on time) was more energy efficient. The quality of pulse-dried product was also generally better than that of continuous-dried product. The cycle power-on time and pulsing ratio should be carefully selected to obtain maximize the benefits of pulsed, microwave vacuum drying     

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.     

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.     

Drying of a Slip Casting for Tableware Product Using Microwave Continuous Belt Dryer

To demonstrate the potential of microwave drying in the ceramic industry, microwave demolding of tableware product by a continuous microwave belt drier has been investigated. This study focuses on the investigation of the effects of the irradiation time, sample size, microwave power and location of magnetron on overall drying kinetics. The results show that microwave drying has several advantages over the conventional method such as shorter processing time, volumetric dissipation of energy throughout a product, high energy efficiency, reduced amount of mold usage, and offering product quality. Further quantitative validation of experimental data could be very useful, especially in providing information for processing high-performance microwave drying for developing the ceramic industry in Thailand.     

Drying of a Slip Casting for Tableware Product Using Microwave Continuous Belt Dryer

To demonstrate the potential of microwave drying in the ceramic industry, microwave demolding of tableware product by a continuous microwave belt drier has been investigated. This study focuses on the investigation of the effects of the irradiation time, sample size, microwave power and location of magnetron on overall drying kinetics. The results show that microwave drying has several advantages over the conventional method such as shorter processing time, volumetric dissipation of energy throughout a product, high energy efficiency, reduced amount of mold usage, and offering product quality. Further quantitative validation of experimental data could be very useful, especially in providing information for processing high-performance microwave drying for developing the ceramic industry in Thailand.     

Modularer Trocknungsprozessor zur Anwendung kombinierter Trocknungsverfahren

A modular drying processor for the development of energy and time efficient combined drying processes was constructed. It allows the serial and/or parallel application of different drying methods such as freeze‐, hot‐air‐, infrared‐, contact‐, microwave‐ or vacuum‐drying without interruption between the processes. Depending on the drying method precise product structures could be achieved by using the example of carrot cubes. The serial combination of freeze‐ and microwave vacuum drying shortened the process time about 40 % while highest product quality was obtained.     

Microwave Freeze-Drying Characteristics of Banana Crisps

This experimental study investigated the drying characteristics of microwave freeze drying (MFD) of banana slices of varying maturity. Results show that the drying time was shortened with increase of both microwave power and degree of maturity of the banana. In terms of the quality parameters such as hardness, fracturability, color, and rehydration ratio, high maturity samples with high sugar content displayed better performance. The best sensory scores were obtained for medium maturity bananas processed using 2.0 W/g microwave power.     

Effect of infrared pretreatment on low-humidity air drying of apple slices

The dried apple is used in the preparation of weaning foods and bakery products. The current drying processes, especially hot air (HA) drying, still face the problem of longer processing time and product quality degradation. The low-humidity air (LHA) drying can be an option to retain product quality in heat-sensitive food such as apple. The present work focuses on the effect of pretreatment of apple slices with potassium metabisulfite and infrared (IR) waves on drying characteristics when subjected to LHA drying and comparing the product quality with conventional hot air and freeze drying (FD). Pretreatment with IR waves reduced the drying time by nearly 23 and 17% in LHA and HA drying, respectively. The results indicated that IR-treated and LHA-dried slices retained nearly 82–90 and 72–74% of ascorbic acid and total phenolic content, respectively and was comparable to FD slices. The drying time for LHA was nearly 37% lesser than that for HA drying. LHA-dried apple slices had better color and rehydration ratio compared to FD- and HA-dried slices.     

Microwave Drying of Multi-component Powder Systems

The rate of uptake of microwave technology within the pharmaceutical sector has been slow due to stringent regulatory procedures and unsuccessful microwave trials. However, microwave systems offer significant energy and time savings compared with conventional dryers. Common pharmaceutical excipients, lactose, stearic acid, and active ingredients, Aspirin, Paracetamol, were wetted with water and dried within an experimental atmospheric microwave drying system (2.45 GHz, 90 W). The drying curves obtained showed a constant drying rate, followed by two falling rate periods. Two- and three-component mixtures were dried under the same conditions. The drying characteristics were found to lie between those of the pure components. Drying kinetics of samples wetted with a range of solvents (methanol, ethanol, acetone), and two-component solvent mixtures, were examined. The relative evaporation rates of the solvents showed some dependence on their dielectric properties; latent heat of vaporisation and boiling point having greater influence on drying characteristics. Clearly this research provides information vital to achieve increased acceptance of this technology within the pharmaceutical industry.