Freeze Drying of Apple Slices with and without Application of Microwaves

In this study, Fuji apple slices were dehydrated using freeze drying (FD) combined with microwave assisted with vacuum drying (VMD). The optimal parameter for the diversion point of moisture content from FD to VMD process was at the moisture level of 21%, and for VMD the optimal parameter for vacuum pressure was at 9.15 kPa and microwave power density was at a level of 3.18 w/g. The results show that the two-step technique can significantly reduce total FD time required by up to 40%, while the nutritional value of the dried apple chips remained unchanged compared to FD used alone.     

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.     

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.     

Effect of Calcium Ion and Microwave Power on Structural and Quality Changes in Drying of Apple Slices

Effects of impregnated calcium ion and microwave (MW) power on texture, rehydration, shrinkage, color, and other selected indexes of freeze-dried apple slices were investigated. Apple slices were dried by freeze drying and microwave freeze drying (MFD) separately. Vacuum impregnated (VI) and non-vacuum-impregnated apple slices were dried at various microwave power levels. Non-vacuum-impregnated apple slices were found to preserve their mechanical structure better than the VI-treated ones. Microwave application resulted in decreased protopectin fraction as well as total pectin content. As microwave power varied from 1.2 to 2.0 W/g, the total pectin content decreased from 0.810 to 0.521 (expressed as g galacturonic acid/100 g fresh sample). The effect of microwave power in decreasing the drying time was significant. The color of MFD apple slices varied to a minor extent with MW power level over the range of parameters studied.     

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.     

Study of the drying process of ginger (Zingiber officinale Roscoe) slices in microwave fluidized bed dryer

ABSTRACT

To research about the temperature and moisture characteristics of ginger slices in microwave fluidized bed drying (MFD), some microwave drying experiments in different powers and hot-air drying experiment were used, and the drying quality and moisture drying curves were studied in contrast, finding MFD with advantages such as good quality and high efficiency. In order to understand the water flow characteristics of ginger slices in MFD, low nuclear magnetic resonance was used to detect the materials in different drying stages, and it was found that free water (FW), immobilized water (IW), and bond water (BW) could be converted to the other forms, especially the form that combined with the dry matter is much firmer. FW is the main water state in fresh ginger slices and converts to the IW quickly in the beginning stage, and then both FW and IW decreased until the final stage is reached. Finally, the content of BW increased, and the material can be stored safely. Gingerol is the main flavor component of ginger, and it was detected through LC-20A high-performance liquid chromatography; it was found that the content of 6-gingerol, 8-gingerol, and 10-gingerol increased in various drying processes, especially 10-gingerol that has a positive role to protect the taste of the ginger.     

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.     

Effect of microwave energy dissipation on drying process of berry puree under microwave foam drying conditions

ABSTRACT

Microwave foam drying (MFD) is a novel drying technology particularly for the drying of agricultural products with high viscosity and thermal-sensitive components. The effect of dissipation of microwave energy on the drying process of berry puree was investigated in MFD conditions. The results show that the dissipation of microwave energy induces the vapor pressure inside the berry puree to improve the formation of bubbles. The changes of pressure inside bubbles of berry puree follow Sine equations. The favorable conditions of bubbles formation inside berry puree are the temperature of 70–80°C with the corresponding viscosity of 70–80?pa?s and moisture content of 80–85% (wb). The dissipation of microwave energy in berry puree tends to decaying trend due to the changes of dielectric properties. Moisture content has the most significant positive effect on both dielectric constant and dielectric loss factor of berry puree, followed by the density, and the drying temperature has the least but significant effect on both indicators. The focusing phenomenon of microwave irradiation depended on the dielectric properties determines the distribution of temperature of berry puree. The findings in current work contribute to a better understanding the MFD behavior of berry puree and will help to optimize the drying conditions of MFD for enhancing energy efficiency and drying quality.     

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.     

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.     

Optimization of the microwave drying process for potato chips based on the measurement of dielectric properties

A microwave drying system with functions of automatic power and temperature control was developed to dry potato chips. The mass and moisture content of the sample are available online. The dielectric properties of the samples were measured during microwave drying, during which the microwave absorption capacity of the samples was analyzed. Afterward, a reasonable power and temperature control scheme was formulated. The enhanced microwave drying technology of potato chips is found by comparing product quality under different schemes. Results show that the change in the dielectric properties of materials during microwave drying is closely related to moisture and temperature variations. A three-stage different temperature control with predefined variable power profiles resulted in the best 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.     

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.     

Comparison of four pretreatments on the drying behavior and quality of taro (Colocasia esculenta L. Schott) slices during intermittent microwave vacuum-assisted drying

ABSTRACT

Effect of different pretreatments on drying behavior and quality of taro slices during intermittent microwave vacuum-assisted drying were investigated. Taro slices were subjected to the following pretreatments: blanching (B), blanching and freezing (B?+?F), blanching and immersion in maltodextrin solution (B?+?M), and blanching and immersion in maltodextrin solution and freezing (B?+?M?+?F). The reduction in amylose content, the increase in amylose to amylopectin ratio, the change of mechanical properties, the increase in sample bulk density and reduction in sample porosity and structure collapse during pretreatment processes facilitated heat and moisture transfer and quality changes during drying of taro slices. During drying process, all the pretreatment reduced the drying time of samples. The drying rate included a warming-up period and a falling rate period. The values of effective moisture diffusivity increased gradually as the moisture content decreased. The B?+?M?+?F pretreatment had the shortest drying time, the highest drying rate and the biggest effective moisture diffusivity. Moreover, for B?+?M?+?F pretreatment, the parameters including hardness, crispness, color, and microstructure had remarkable changes as compared with the other pretreatments.     

Continuous real-time monitoring and neural network modeling of apple slices color changes during hot air drying

A hot air drying system equipped with real-time computer vision system was used to investigate the effects of drying variables on apple slices color changes. Drying experiments were conducted at drying air temperatures of 50–70 °C, drying air velocities of 1–2 m/s, and samples thicknesses of 2–6 mm. A multilayer perceptron (MLP) artificial neural network (ANN) was also used to correlate color parameters and moisture content of apple slices with drying variables and drying time. The effects of drying air temperature and sample thickness on color changes were dominated over the effect of drying air velocity. However, non-linear and somewhat complex trends were obtained for all color parameters as function of moisture content. The MLP ANN satisfactorily approximated the color and moisture variations of apple slices with correlation coefficient higher than 0.92. Therefore, the computer vision system supplemented with ANN can be used as a non-invasive, low cost, and easy method for fast and in-line assessing and controlling of foodstuffs color and moisture changes during drying.     

Studies on Decreasing Energy Consumption for a Freeze-Drying Process of Apple Slices

The shape, color, flavor, and rehydration capacity of freeze-dried (FD) products are all better than other dried products. However, the energy consumption during FD is very high, which limits the application of this drying method for common materials. In this article, microwave vacuum drying (MWVD) was applied before or after FD to decrease the energy consumption during FD. Moreover, energy consumption was divided into two parts: valid and invalid energy consumption. Apple slices were used as an example to calculate the saving percentage of invalid energy consumption by comparing combination drying with FD. Apple slices freeze dried for 8.28 h first and then dried by MWVD have the best appearance, with a savings of 39.20% in invalid energy consumption. But apple slices freeze dried for 6 h first followed by MWVD have the highest savings of invalid energy consumption, 54.02%, while still maintaining an acceptable appearance.     

Moisture flow characteristics and variation patterns of dielectric constants in bitter gourd slices during microwave hot-airflow vibrating drying

Abstract

The uniformity of microwave-dried bitter gourd slices was poor, and a microwave hot-airflow vibrating drying (MHAVD) apparatus was used to carry out an in-depth elucidation of moisture flow characteristics underlying the variation in dielectric characteristics. Low-field nuclear magnetic resonance and imaging (LF-NMR/MRI) was used to measure the moisture state and distribution during different drying stages, and the relationship model between moisture semaphore and moisture ratio were analyzed. A vector network analyzer was used to measure the patterns in the variation of dielectric characteristics in bitter gourd slices at different drying stages. As a result, the temperature and moisture distribution of bitter gourd slices during MHAVD was uniform caused by hot air and vibration, and the water semaphore in NMR gradually decreased, but the bound water semaphore increased during certain drying stages. There is a significant linear relationship between the total semaphore A₂ and the moisture ratio M_R in bitter gourd slices per unit dry basis mass in the NMR spectrum (R² = 0.99). As moisture decreases, the ability of the material to store microwaves and convert electromagnetic energy to heat energy decreases, the ability of the material to absorb microwaves decreases, and the microwave penetration significantly increases. The moisture flow characteristics in bitter gourd slices is related to the variation of dielectric characteristics during MHAVD, and drying uniformity was improved.     

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.     

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.     

Heat and mass transfer in apple cubes in a microwave-assisted fluidized bed drier

In the present study, a pilot scale microwave assisted fluidized-bed dryer was designed and used to dry apple cubes. A model was developed to describe heat and mass transfer in apple cubes during drying in a combined microwave-assisted fluidized-bed dryer. A numerical solution based on the finite difference method was used to develop the model for moisture distribution and temperature variation of samples. The model was validated using experimental data, including average moisture content, center and surface temperature at various air temperatures and microwave power densities.