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Summary: Three freeze–thaw (FT) pre‐treatments using different freezing methods that freezing at −80 °C (FT‐80), freezing at −25 °C (FT‐25) and vacuum freezing (VFT) were used for carrot slices, the effects and mechanisms of FT pre‐treatment on the drying characteristics and product quality of carrots during microwave freeze drying (MFD) were studied. The results showed that FT pre‐treatment under different freezing methods affected the water state of carrots, resulting in varying degrees of moisture loss and shortening drying time by 14.3% to 42.8%. VFT treatment caused the greatest water loss (35.33%) during FT and significantly increased the effective moisture diffusivity in the MFD process. FT treatments did not cause any significant degradation in the colour of the carrots. The VFT and FT‐80 treatments resulted in an elevated rehydration ratio, increased crispiness of MFD carrots and products with a porous microstructure and higher porosity. In contrast, the FT‐25 treatment led to a decrease in porosity due to destructive damage to the carrot cell structure. However, this damage favoured the extractability of carotenoids, resulting in a higher carotenoid content than the other treatments. This study provides a theoretical foundation for the practical implementation of FT pre‐treatment using various freezing methods on fresh foods prior to drying. It is recommended to employ VFT pre‐treatment before MFD due to its efficiency and effectiveness. [ABSTRACT FROM AUTHOR]

Heating uniformity is always a concern in microwave drying for foodstuff, where unsatisfied product quality could be caused by over‐ or inadequate‐heating. To deal with this concern, multi‐physics field rotation simulation models and intermittent microwave drying modes were designed and evaluated in this study. Four drying modes with different intermittent time ratios (R1/2, R1/3, R1/4, and R1/5) were first applied in drying experiments. Their effects on temperature distribution, drying characteristics, and quality aspects (chromatic aberration, rehydration ratio, vitamin C content, and sensory evaluation) for carrot samples were compared and analyzed. The results revealed that the carrot quality in R1/4 mode drying was increased by up to 64% in comparison with the other three modes. The root‐mean‐square error of the moisture ratio was maintained at a lower value, and a better consistency was also achieved in R1/4 mode. To further improve the drying performance and efficiency, a novel stage intermittent‐time control strategy was attempted, that is, an appropriate strategy was used in each drying stage. With this method, the best drying effect could be obtained. The above work can promote a deeper understanding of the drying kinetic and provide novel operation strategies in the microwave drying of food products. Practical applications: A multi‐physics field rotational simulation model (MFRSM) coupled with an intermittent microwave drying process, including electromagnetic heating, heat, and mass transfer, was developed to characterize the drying process of rotating samples in an intermittent microwave dryer. The MFRSM was helpful to analyze the continuous microwave drying process of the sample and deepen the understanding of the internal heat and mass transfer process of the sample. Four modes with different intermittent schedules were designed, and simulation models were also established. The drying characteristics in different modes were analyzed; a long intermittent time could affect the drying efficiency on account of the driving force, while a short intermittent time would reduce the product quality because of the continuous high temperature. R1/4 mode was founded in terms of drying efficiency and drying quality. An optimized stage intermittent‐time control strategy (R‐IO mode) was designed. The drying time in the three stages was shortened by 25% in total compared with R1/4 mode. Meanwhile, the vitamin C retention, the taste quality of the dried products were improved. These works are expected to provide benefits for the understanding of the intermittent microwave drying modes on drying kinetic using the MFRSM. It can also be useful in the development of an IMD for food materials. [ABSTRACT FROM AUTHOR]

In order to optimize the microwave vacuum drying of raspberries, the temperature variation of different constant power microwave vacuum conditions was investigated. The conventional microwave vacuum drying (MVD) and the three different steady temperature control scheme was selected. A microwave intermittent vacuum drying scheme (MIVD), microwave vacuum drying power curves control scheme (MVD-T), and microwave vacuum drying power curves with intermittent control scheme (MIVD-T) were developed and conducted. As shown, temperatures under control schemes were all kept at more moderate ranges compared to constant power, where MIVD-T showed less fluctuation by adopting power curve changing with time as input variable. It was found that temperature control schemes could improve the appearance, as MVD-T and MIVD-T both reduced carbonization on the exterior and interior of raspberries. For antioxidant properties, there were differences between different schemes, in which MIVD-T could maintain more total monomeric anthocyanin (TMA) compounds and keep other antioxidant compounds similar to other drying methods. Control schemes such as MIVD, MVD-T, and MIVD-T were effective in obtaining more steady temperature materials than conventional microwave vacuum drying and were beneficial for drying quality. [ABSTRACT FROM AUTHOR]

Four drying methods, hot air drying (HAD), microwave constant temperature drying (MWD), microwave vacuum temperature‐controlled drying (MIVD‐T50), and freeze drying (FD), were investigated for their effects on raspberries in terms of drying time, color, microstructure, rehydration, water activity, phenolic compounds, flavonoids, anthocyanins, and DPPH scavenging ability. It was found that microwaves combined with vacuum took the shortest time. The color of MIVD‐T50 is similar to that of FD and much better than that of HAD and MWD. A smooth surface and fluffy and porous structure inside were observed in the FD and MIVD by scanning electron microscopy (SEM) images. FD has the highest energy consumption, followed by HAD, MIVD‐T50 and MWD. The retention rates of total phenols and flavonoids in HAD, MWD and FD were lower than those in MIVD‐T50, and FD had the advantage of retaining anthocyanin content. In general, MIVD‐T50 has significance for the commercial production of raspberry. Practical applications: The use of natural antioxidants in food processing not only enhances the shelf life of products but also improves their nutritional value. Raspberry is a rich source of antioxidants and is widely used in food processing. However, the drying process is crucial in preserving its antioxidant quality, and different drying methods affect the antioxidant content differently. According to the results of the experiments, the drying method has a considerable impact on the energy usage and quality of dried raspberries. Compared with the HAD and MWD treatments, both FD and MIVD‐T50 showed better nutrient content preservation and antioxidant activity, which may be caused by the lower temperature drying process of FD or the stronger efficiency of MIVD‐T50. However, the drying time of FD is longer, and the energy consumption is higher. Therefore, MIVD‐T50 is a promising raspberry drying technology with low energy consumption, a short drying time, and high quality. It has huge development potential and broad commercial scale production market prospects. [ABSTRACT FROM AUTHOR]

Based on the anti-resonance characteristics of multi-degree-of-freedom system, negative stiffness structures containing nonlinear inclined springs (NIS) are introduced to form a two degree-of-freedom (DOF) quasi-zero stiffness (QZS) vibration isolator. The remarkable feature of this system is the use of nonlinear springs to achieve the nonlinear stiffness. The restoring force generated by the NIS includes both linear stiffness term and cubic stiffness term. First, through static characteristic analysis, the relation between parameters is derived when the system meets QZS conditions. Influences of mechanical and structural parameters on stiffness characteristics are studied. Then, nonlinear dynamic equations of the two DOF vibration isolation system with QZS are established. Using the harmonic balance method, the frequency-domain analytical solutions and expression of force transmissibility are deduced. Effects of damping ratio, vertical stiffness ratio, mass ratio and excitation amplitude on the dynamic response and force transmissibility are numerically discussed. The isolation performance of the system is in comparison with single DOF QZS system and two DOF QZS system with linear inclined springs (LIS). The results indicate that smaller structural parameter E and nonlinear inclined springs with softening stiffness are conducive to obtain smaller system stiffness and larger low-stiffness interval near equilibrium position. Furthermore, by selecting suitable parameters, the system has advantages of achieving the lower initial isolation frequency and wider isolation frequency band. In addition, the attenuation rate of force transmissibility in a certain frequency region is able to be obviously accelerated and the low-frequency isolation performance of the system can be dramatically improved. [ABSTRACT FROM AUTHOR]

The effect of ultrasonic soaking assisted microwave drying on the optimum cooking time and physicochemical properties of brown rice were investigated. After the ultrasonic soaking, the optimum cooking time of brown rice was significantly reduced, and the physical properties improved. The rice bran treated with high‐temperature soaking and high‐power ultrasound showed a porous structure with dense striped gaps. Accordingly, faster heat and mass transfer brought about the acceleration of drying rate and the reduction of drying time. The pretreatment and microwave drying reduced the gelatinization enthalpy (ΔH), which was beneficial to enhance the degree of gelatinization of starch. Microwave drying obviously increase the breakdown viscosity of brown rice flour, bringing about better tasting qualities. The content of free fatty acids, color parameters, and crystalline structure remained stable after pretreatment and microwave drying. Novelty impact statement: Appropriate high‐power ultrasound at the soaking temperature of 60°C assisted microwave drying significantly altered the structure of brown rice bran, reduced the optimum cooking time, and improved the physical properties of brown rice. Ultimately, our research proved that soaking and ultrasound‐assisted microwave was a promising way to produce instant brown rice with high quality. [ABSTRACT FROM AUTHOR]

The work investigated the influences of blanching (B) and ultrasonic‐assisted blanching (UB) as pretreatment on mass transfer, drying kinetics, color, contents of nutrient, and aroma compounds of hot‐air‐dried perilla leaves. The samples were pretreated at two temperatures (70℃ and 80℃) and two durations of pretreatment (1 min and 3 min). Principal component analysis and hierarchical cluster analysis were used to distinguish sample characteristics of different pretreatments. The results showed that the condition of UB pretreatment had a higher drying rate compared to B pretreatment. The effective moisture diffusivity (Deff) of pretreatment for UB is higher than B. The UB pretreatment method yielded the lower ΔE value and higher preserved content of chlorophyll, flavonoid, and rosemary acid at the pretreatment temperature of 70℃. Moreover, GC‐MS results showed a lower perilla ketone relative content under the same conditions. According to the results, the quality of the dried perilla leaves after performing low‐temperature UB pretreatment (70℃) was improved relatively. Practical applications: Perilla leaves are rich in antioxidant substances and have extensive culinary usage given their characteristic aroma and flavor. Convective drying is known to have some adverse effects on agricultural products. However, Ultrasonic pretreatment, as a modern food processing technology, can shorten the drying time and preserve the color, aroma, and nutrient content of food. The purpose of this study was to investigate the quality of dried perilla leaves with B and UB pretreatments, and to compare the differences in drying characteristics, color, nutrient content, and aroma compounds by different pretreatments. The pretreatment and dehydration parameters should be considered with respect to further utilization of dried perilla leaves. Nonetheless, sonication is worth to be taken into account due to the significant effects of energy expenditures, inhibiting browning of perilla leaves without damaging the quality of final product. [ABSTRACT FROM AUTHOR]

Instantly sensing the food quality attributes is a prerequisite to regulate the drying process for final product quality improvement. Novel sensing technologies have been "pushed" to food drying engineering from other research disciplines. Electronic nose, computer vision, hyperspectral imaging, near-infrared, nuclear magnetic resonance, and dielectric properties have been "transplanted" for this purpose. Various physicochemical attributes of food quality, including flavors, moisture contents, colors, shapes, texture, and chemical contents, have been sensed during the food drying process. Sun and solar drying, hot air convective drying, freeze drying, and microwave drying have been involved in these attempts. Numerous data processing and analysis techniques have been employed to make clear the meaning of multivariate variables for the above technologies. With the assistance of these technologies, a great diversity of drying processes have been modulated and food product quality had been improved. To promote their industrial application, more research is required to be conducted and more practical sensing method development is suggested along this direction. [ABSTRACT FROM AUTHOR]

Fenjiu spirits are famous for their multifarious flavors in China. Nevertheless, the uniformity and homogeneity of the products are always challenges for the producers. Three flavor measurement methods, that is, zNose™, GC‐MS, and spirit tasters, were employed and correlated for evaluation of Fenjiu flavors' difference. An automatic blending system was designed to improve the uniformity, where base liquors of Dazongjiu, Dajiu, and Daijiu, as well as pure water, were used for blending, and BPNN was employed for regression and error minimization. Results showed that zNose™ results could be correlated with GC‐MS in 95%; hence, zNose™ can replace GC‐MS in Fenjiu flavor measurement. With the zNose™ assistance, an experimental scale, fully automatic blending system could be optimized with different modern mathematical algorithms to improve the Fenjiu products uniformity. We believe that this detailed work will advance the scientific knowledge in the field and help to facilitate the uniformity and homogeneity of Fenjiu products. [ABSTRACT FROM AUTHOR]

Fresh ginkgo biloba (GB) seeds are seasonal and susceptible to microbial spoilage, especially the growth of mold owing to GB seeds' high-water content, which greatly limits their shelf life. As an efficient and special heating method, radio-frequency (RF) heating can be used to dehydrate ginkgo to preserve its nutritional value and reduce postharvest losses. However, the non-uniformity of RF heating restricts the development in industrialized application. In this study, the RF drying of GB seeds was performed to investigate the effect of RF heating on temperature. The distribution law of the entire temperature field was also observed. Using numerical simulation method, the coupling model of electromagnetic and heat transfer was established. The model was validated by the 6-min heating profile of GB seeds in a 12 kw and 27.12 MHz RF system. The model was also qualitatively validated by comparing the simulated temperature profiles on the three planes in the GB seeds with the corresponding thermal images. Quantitative validation was performed by comparing the simulated temperature of GB seeds on the three planes with experimental temperature acquired at places using thermocouples. Furthermore, the model can be effectively used to identify the distribution of electric fields in different positions and to achieve satisfactory heating uniformity. [ABSTRACT FROM AUTHOR]

BACKGROUND: The combination of sugar osmotic dehydration and microwave vacuum drying is an effective method for the dehydration of blackberries, the retention of their antioxidant properties, and the extension of their shelf life. Mass transfer during the osmotic dehydration of blackberries in sugar solution was investigated together with its influence on microwave vacuum drying characteristics, and the retention rate of anthocyanins in dried frozen blackberries. RESULTS: The concentrations of the osmotic solutions that were tested contained 40%, 50%, and 60% sugar, and the osmotic solution temperatures were 30 °C, 40 °C, and 50 °C. The solution‐to‐blackberry mass ratio was 10:1 (w/w) and the process duration varied from 0 to 5 h. A two‐parameter mathematical model was used to describe mass transfer in the osmotic dehydration of blackberry samples and estimate moisture loss and solid gain in the final equilibrium. The results showed that the dehydration rate and solid gain rate of the blackberries increased with an increase in osmotic concentration, osmotic time, and the temperature of the solution under certain experimental conditions. The effective diffusivity of moisture and solute were estimated using the analytical solution of Fick's second law of diffusion. The moisture and effective diffusivities of sugar in the above osmotic dehydration conditions were in the range of 1.77 × 10−9–2.10 × 10−9 and 1.36 × 10−9–1.60 × 10−9 m2.s−1, respectively. CONCLUSION: The pretreatment of sugar osmosis greatly reduced the microwave vacuum drying time in the latter part of the dehydration period and increased anthocyanin retention. © 2019 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Due to the limitations of single index analysis, the merits and demerits of microwave-assisted permeation process cannot be comprehensively evaluated. However, the analytical hierarchy process (AHP) was applied to optimize the conditions of microwave-assisted diffusion of sugar in blackberry. In this paper, the results of a single-factor experiment were compared with blackberry preserved after the treatment of microwave. An orthogonal rotation test procedure was designed to calculate the composite score of preserved blackberry fruit using AHP, and to determine the optimum processing conditions based on sugar osmosis characteristics. The results showed that the sugar content, vitamin C (VC) content, and color difference of treated blackberry increased with the increase of microwave power, the residence time, and the resulting sugar concentration. The composite grade points are listed to show that the optimum results were as follows: power level, the residence time, and the sugar concentration were, respectively, 210 W, 12 min, and 35%. [ABSTRACT FROM AUTHOR]

Chinese steamed bread with chicken, which is a typical Chinese fast food, is popular among consumers. The model simulated a frozen combination consisting of six chicken nuggets and two Chinese steamed breads with the dynamic change of dielectric properties on the turntable. The temperature-dependent dielectric and thermo-physical properties of chicken meat were measured as a function of temperature from −5°C to 95°C. A mathematical model of the dielectric properties, specific heat and thermal conductivity with temperature dependence was inputted in the model as the material parameters. 7.5 rpm was observed to be the optimal rotation speed. Simulated temperature profiles were compared with experimental temperature profiles after 90 s of microwave (MW) heating at 700 W. The simulated results were consistent with the corresponding experimental results. The developed model can be applied in food products to identify the cold and hot spots to ensure quality in terms of temperature uniformity. [ABSTRACT FROM AUTHOR]

In-package pasteurization treatments based on radio frequency or microwave energy can greatly reduce the danger of microbial spoilage for Chinese steamed bread. The mixture equations methods were used to obtain dielectric properties at the frequency from 1 to 2450 MHz for the temperature range of 25–100ºC and the moisture content range of 40.1–48.5%. The effects of frequency, moisture content, and temperature on Chinese steamed bread dielectric properties were analyzed. The Bottcher Equation was adapted for calculating dielectric constant, the Landau and Lifshitz, Looyenga Equation for dielectric loss factor. Both dielectric constants and dielectric loss factors increased with increasing moisture content and temperature. Radio frequency energies had several times deeper in Chinese steamed bread than microwave energies. [ABSTRACT FROM PUBLISHER]

The effect of food particle mass on the instantaneous fluid dynamic characteristics of a gas–particle spout fluidized bed in a pulsed spouted microwave–vacuum drying (PSMVD) system was investigated by a numerical method. The spout fluidization process in a pseudo-2D spout fluidized bed was simulated by computational fluid dynamics (CFD) using the inviscid two-fluid theory method (TFM) based on the kinetic theory of granular flow. The effect of particle mass on the particle flow pattern, jet penetration depth, and pressure drop was evaluated. Under a specific particle mass, the spout velocity was an important factor controlling particle status in the spout fluidized bed and a critical velocity was identified for effective transition of the flow pattern. With an increase in particle mass, the spout velocity tended to increase to obtain the expected gas–particle flow pattern and the correlation between the particle mass and particle flow pattern was revealed. This research on the fluid dynamic characteristics of a gas–particle spout fluidized bed could be used to improve the uniformity of particle mixing and microwave heating. [ABSTRACT FROM PUBLISHER]