Microwave-assisted Vacuum Drying Research Articles

Statistical electromagnetics for industrial pharmaceutical lyophilization.

Lyophilization is a common unit operation in pharmaceutical manufacturing but is a prolonged vacuum drying process with poor energy utilization. Microwave-assisted vacuum drying has been investigated to accelerate the lyophilization process. However, the literature lacks methodical approaches that consider the lyophilizer, the lyophilizate, the microwave power uniformity, the resulting heat uniformity, and the scalability. We present a microwave-vacuum drying method based on the statistical electromagnetics theory. The method offers an optimum frequency selection procedure that accounts for the lyophilizer and the lyophilizate. The 2.45GHz frequency conventionally utilized is proven to be far from optimum. The method is applied in a microwave-assisted heating configuration to pharmaceutical excipients (sucrose and mannitol) and different myoglobin formulations in a lab-scale lyophilizer. At 18GHz frequency and 60W microwave power, the method shows nearly three times speed-up in the primary drying stage of sucrose relative to the conventional lyophilization cycle for typical laboratory batches. The uniformity of the microwave power inside the chamber is controlled within ± 1 dB. The resulting heating uniformity measured through residual moisture analysis shows 12.7% of normalized SD of moisture level across the batch in a microwave-assisted cycle as opposed to 15.3% in the conventional cycle. Conventional and microwave lyophilized formulations are characterized using solid-state hydrogen-deuterium exchange-mass spectrometry (ssHDX-MS), solid-state Fourier transform infrared spectroscopy (ssFTIR), circular dichroism (CD), and accelerated stability testing (AST). Characterization shows comparable protein structure and stability. Heat and mass transfer simulations quantify further effects of optimal volumetric heating via the high-frequency statistical microwave heating.

Effect of microwave‐assisted vacuum drying on the drying kinetics and quality parameters of Moringa oleifera leaves

AbstractMicrowave‐assisted vacuum drying (MAVD) has been studied to improve drying kinetics and physical and antioxidant properties of dried moringa leaves. Microwave power and vacuum level produced a significant reduction (p < .05) in drying time and improvement of drying rate. It was further confirmed by an increase in effective diffusivity value (Deff) by an increase in power and vacuum levels. The reliance of Deff on microwave power was further confirmed by an increase in activation energy from 13.564 to 17.31 W/g with an increased power level. Fitting experimental moisture ratio to the thin layer drying models revealed that the Midilli et al. model could better describe the MAVD of moringa leaves with the highest R2 values. Physical characteristics such as color and rehydration ratio were better for MAVD samples than hot air‐dried samples. Increased drying rate and rehydration ratio were due to the open porous structure of the moringa leaves dried by MAVD, as revealed from the SEM imaging. The vacuum level and power level were significant (p < .05) to the total color difference but the change in color at power levels of 270 and 450 W were insignificant (p > .05). Also, the vacuum level was significant (p < .05), and the microwave power level was insignificant (p > .05) to the rehydration ratio of the MAVD moringa leaves. Phenolic composition and antioxidant properties of MAVD moringa leaves showed increased phenolic content and improved antioxidant activity. Microwave power showed a significant effect (p < .05) on antioxidative properties, whereas vacuum level (p > .5) showed an insignificant effect. The highest power level of 450 W and vacuum level of 500 mmHg produced better quality moringa leaves with good physical and antioxidative properties.Practical ApplicationsDrying is the most common and oldest preservation method for storing food for a longer time. It is very much important to dry moringa leaves that have high moisture content and very less shelf‐life. But conventional drying methods cause great nutritional and textural changes. Microwave‐assisted vacuum dryer combines the advantage of quick and shorter drying of microwave system and lower temperature process as in vacuum system. This eventually reduces the drying time and temperature stress on the food product. This study aids in scale‐up and design a MAVD system for moringa leaves based on its drying kinetics. MAVD of moringa leaves was best modeled by Midilli et al. model. Evaluation of physical characteristics of leaves after drying and its comparison with hot air‐dried one revealed better color and rehydration property that is considered as the key character of dried samples. MAVD also produced increased antioxidant properties with good retention of phenolic components. So, the method of Microwave Assisted Vacuum Drying (MAVD) has got tremendous importance that could be industrially exploited in Moringa leaf processing plants.

Foam Structure Preservation during Microwave-Assisted Vacuum Drying: Significance of Interfacial and Dielectric Properties of the Bulk Phase of Foams from Polysorbate 80-Maltodextrin Dispersions.

This study aimed at examining the cause of differences in the structure preservation of polysorbate 80–maltodextrin foams during microwave-assisted vacuum drying (MWVD) versus conventional vacuum drying (CVD). Aqueous dispersions of 3% polysorbate 80 and 0–40% maltodextrin were characterized for their dielectric and interfacial properties, and results were related to their drying performance in a foamed state. Surface tension and surface dilatational properties as well as dielectric properties clearly responded to the variation in the maltodextrin content. Likewise, the foam structure preservation during CVD was linked to the maltodextrin concentration. Regarding MWVD, however, foams collapsed at all conditions tested. Nevertheless, if the structure during MWVD remained stable, the drying time was significantly reduced. Eventually, this finding could be linked to the dielectric properties of polysorbate 80 rather than its adsorption kinetics and surface film viscoelasticity as its resonant frequency fell within the working frequency of the microwave drying plant.

Influence of interfacial characteristics and dielectric properties on foam structure preservation during microwave-assisted vacuum drying of whey protein isolate-maltodextrin dispersions

This study aimed at assessing the correlation between interfacial and dielectric properties of whey protein isolate (WPI)-maltodextrin (MD) dispersions and their foam structure stability during conventional vacuum drying (CVD) and microwave-assisted vacuum drying (MWVD). Interestingly, the presence of MD clearly increased the surface activity of WPI, whereas the surface dilatational properties were rather unaffected. Besides, loss factor and resonant frequency linearly increased with MD concentration, while the dielectric constant decreased. Regarding drying, foam structure preservation during CVD was mainly correlated with the dilatational elasticity. Thereby, all samples remained stable throughout the drying process. Compared to CVD, MWVD required ≥20% MD to avoid foam collapse. In addition, MWVD-dried samples exhibited larger bubble sizes, which was linked to the volumetric heating and higher mechanical stress as compared to CVD. Overall, high surface dilatational elasticity and bulk viscosity were identified as key parameters to ensure foam structure preservation during vacuum drying. • Maltodextrin content ≥30% affects interfacial properties of WPI. • Dielectric properties of WPI depend on maltodextrin content and temperature. • Foam decay is related to interfacial properties. • Foam structure stability during MWVD is linked to surface elasticity.

Effect of Intermittent Microwave Volumetric Heating on Dehydration, Energy Consumption, Antioxidant Substances, and Sensory Qualities of Litchi Fruit during Vacuum Drying

To examine the processing characteristics and high quality of an improved microwave vacuum drying system, litchi fruits were dried using intermittent microwave volumetric heating while microwave vacuum drying at 2 W/g was carried out for comparison; the intermittent microwave heating profiles were set as (1) 5 min drying-on, 5 min drying-off; (2) 5 min drying-on, 10 min drying-off; and (3) 5 min drying-on, 15 min drying-off. Energy consumption during drying was determined, and physicochemical properties such as moisture content, vitamin C, total phenolics, color, and sensory evaluation of dried products were assessed. In microwave vacuum drying, intermittent microwave volumetric heating was found to be energy-efficient (about 32 KJ/g to 45 KJ/g) and saved at least 31% of energy consumption compared with microwave vacuum drying as well as decreasing product browning. In addition, microwave volumetric heating had no substantial effects on sugar and protein contents, while antioxidants were affected significantly (p ≤ 0.05). Moreover, sensory evaluation showed that intermittent microwave-assisted vacuum drying (IMVD) increased the acceptance of the dried product compared with microwave vacuum drying (MVD).

Effect of different drying methods on antioxidant characteristics of bee-pollen

In this study, microwave drying (MWD) and microwave-assisted vacuum drying (MW-VD) (500, 675 mbar) at power levels of 300, 450, 600 and 900 W, hot-air drying (HAD) and vacuum drying (VD) (300, 500 mbar) at temperatures of 35, 50, 65 °C and freeze drying (FD) of bee-pollen were investigated in relation to its total phenolic content (TPC), antioxidant vitamin composition (C and E) and antioxidant capacity. HAD at 35 °C provided similar vitamin C content as compared to fresh pollen while all other treatments led to significant loss ranging between 14.3 and 61.3%. Retention of vitamin E level in dried pollen ranged between 71 and 87%. Except vitamin C, MWD at lower power levels provided better or similar values of above mentioned properties as compared to HAD and FD treatments. The data indicated that pollen dried by MW-VD had higher antioxidant activity compared to samples dried by HAD irrespective of the applied pressure or power level.

Effect of different drying methods on the quality of restructured rose flower (Rosa rugosa) chips

Rose flower chips were produced by freeze drying (FD), pulse spouted vacuum-microwave freeze drying (PSVMFD), vacuum-microwave freeze drying (VMFD), and microwave-assisted vacuum drying (MVD). Drying time, color attributes (L*, a*, b* and h), flavor, texture, Vitamin C, flavonoids content, shrinkage value, DPPH radical scavenging capacity, and microstructure were compared. Results showed that MVD was the fastest (120 min) method of drying and FD was the slowest (560 min) method to achieve the target moisture content (under 5%). Considering product quality, FD samples exhibited the highest vitamin C (2.40 ± 0.14 mg/100 g) and flavonoids content (88.33 ± 0.22 mg/100 g), with the strongest DPPH radical scavenging capacity, followed by PSVMFD samples. Additionally, FD samples showed the lowest shrinkage (3.89 ± 0.01%) followed by PSVMFD samples (15.02 ± 0.02%) whereas the MVD chips had the highest shrinkage (50.12 ± 0.12%). FD and PSVMFD samples exhibited loose porous structure, while VMFD and MVD samples showed more compact structure. Considering the drying time and product quality comprehensively, PSVMFD was a novel and promising drying method to produce high quality rose flower chips.

HMF formation, diastase activity and proline content changes in bee pollen dried by different drying methods

Bee pollen, as a protein rich and energy enhancing food source, can be used as a valuable complement to our daily diet. In this study, multifloral bee-pollen was dehydrated to a moisture content less than 8% by hot-air (HAD) and vacuum (VD) (300, 500 mbar) drying at 35, 50 and 65 °C, microwave (MWD) and microwave assisted vacuum drying (MW-VD) (500, 675 mbar) at 300, 450, 600 and 900W power levels and freeze drying. The water activity, diastase number (DN), proline and 5-hydroxymethyl-2-furfural (HMF) contents in fresh and dried pollen samples were determined and compared. A greater reduction was observed in DN of samples subjected to MWD and MW-VD at 600 and 900W as compared to other drying treatments. Drying by microwave irradiation at 900W power level caused the highest, almost 72–76%, reduction in DN values. A higher retention of enzyme activity, ranging between 87 and 93%, was observed at 50 °C during HAD and VD. Proline and HMF content were affected by the power level during MWD and MW-VD. A significant reduction in proline content was observed only in microwave treated samples at the highest power level. Except for the highest power level, MW-VD leads to a significant increase in HMF levels ranging between 4.9 and 9.2 mg/kg while HAD and VD provided similar HMF content in samples as compared to fresh pollen.

Quality of restructured cookies made from old stalks of Asparagus officinalis using various drying methods

ABSTRACTThe restructured crisp cookies were made from the old stalks of Asparagus officinalis. Temperature distribution, expansion ratio, moisture content, and color were measured to evaluate the drying uniformity of the cookies by applying pulse-spouted microwave-assisted vacuum drying (PSMVD) and microwave-assisted vacuum drying (MVD). Besides, bulk density, expansion ratio, microstructure, and textural properties were used to compare the products’ quality obtained by three different drying methods including vacuum drying, MVD, and PSMVD. Results showed that the products dried by PSMVD were more uniform compared to those samples dried by MVD. The pulse-spouted mode in drying chamber substantively improved the microwave-vacuum-dried products’ quality, drying uniformity, and drying characteristics.

Drying of restructured chips made from the old stalks of Asparagus officinalis: impact of different drying methods.

Old stalks of Asparagus officinalis, which account for one third of the total length of each spear, are always discarded as waste. To make full use of the resource, a kind of restructured Asparagus officinalis chip was made. The effects of pulse-spouted microwave-assisted vacuum drying (PSMVD), microwave-assisted vacuum drying (MVD) and vacuum drying (VD) on texture, color and other quality parameters of restructured chips were then studied to obtain high-quality dried chips. Results indicated that the drying time was significantly affected by drying methods, and PSMVD had much better drying uniformity than MVD. The expansion ratio and crispness of chips increased with increasing microwave power and vacuum degree. The browning reaction of samples in VD was more serious, which was confirmed by the results of color test and electronic nose. The PSMVD drying method showed much better drying uniformity than MVD. The dried chips obtained by PSMVD showed optimal quality and were more readily accepted by consumers. © 2015 Society of Chemical Industry.

Effects of Four Different Drying Methods on the Quality Characteristics of Peeled Litchis (Litchi chinensis Sonn.)

Four different drying methods, namely, air drying (AD), microwave-assisted vacuum drying (MWVD), coupled microwave–hot air drying (MWAD), and freeze drying (FD), were studied, in terms of drying time, color, reducing sugar content, shrinkage, microstructure, and sensory evaluation. The drying rate of MWVD and MWAD were notably faster than that of AD and FD. The highest reducing sugar content was observed in MWAD, followed by MWVD. No significant differences were observed in reducing sugar between air-dried and freeze-dried products. FD had the best color and shrinkage among the other drying methods. The color and shrinkage of MWVD drying is close to the fresh litchis and much better than hot air and MWAD. Scanning electron microscopy (SEM) images showed that the MWVD and FD litchis have a porous structure, whereas AD and MWAD litchis have a compact structure. The sensory evaluation suggests that peeled litchis dried by the four studied drying methods are of a quality that is accepted by consumers.

Comparison of three microwave‐assisted drying methods on the physiochemical, nutritional and sensory qualities of re‐structured purple‐fleshed sweet potato granules

SummaryPurple‐fleshed sweet potato (PFSP) with high anthocyanin level was attractive to health concern. Three drying methods, namely microwave‐assisted freeze‐drying (MWFD), microwave‐assisted vacuum drying (MWVD) and microwave‐assisted spouted bed drying (MWSBD), were compared in the drying of PFSP granules. Product’s texture, colour, anthocyanin level and energy consumption were investigated. Drying periods varied among drying methods. MWSBD needed the least time to finish the drying process. Different drying methods had a significant effect (P < 0.05) on texture and anthocyanin level. Maximum penetration force of MWSBD treated sample was lowest (14.99 N). Anthocyanin level of MWSBD products was not as high as that of MWFD products, but colour and appearance were fine. MWFD products owned good crispness through sensory evaluation. Energy consumption of MWFD was about two times higher than that of MWVD. Final results indicated that MWSBD may be an alternative way to MWFD with measurements to maintain the anthocyanin level.

Studies on different combined microwave drying of carrot pieces

SummaryThree different combined microwave (MW) drying methods were compared, namely microwave‐assisted vacuum drying (MWVD), microwave‐assisted freeze drying (MWFD), microwave‐enhanced spouted bed drying (MWSD), in terms of drying rate, drying uniformity, product colour, rehydration ratio, retention of β‐carotene and vitamin C, and energy consumption. The drying rate of MWVD and MWSD were much faster than that of MWFD. The largest drying rate was obtained in MWSD with 3.5 W g−1. In general, the colour of MWSD products was very uniform. Rehydration ratio of MWFD carrot pieces was almost the same as the freeze‐dried (FD) products and better than MWVD and MWSD products. In addition, the highest retention of carotene and vitamin C was observed in MWFD carrot pieces. No significant differences were observed in carotene and vitamin C between MWVD and MWSD products. However, the energy consumption in MWFD was the highest.

Study of a Microwave Assisted Vacuum Drying Process Applied to the Granulated Pharmaceutical Drug Hydrochlorthiazide (HCT)

Drying processes have evolved considerably over the years, aiming at the best conditions to shorten the processing time, but maintaining the highest final product quality. The application of vacuums to the microwave drying process might be interesting, especially for thermally sensitive products because the reduced boiling point of the solvent allows a lower temperature processing. The objective of the present work is to study and evaluate the process of drying a granulated product that is the basis of the drug hydrochlorthiazide, with an initial moisture content of 21% in dry basis. This study monitored the drying kinetics, product temperature and power absorbed by the sample using a bench scale vacuum microwave dryer. The equipment consisted of a cylindrical pressure vessel crossed by a wave guide, setting up a system whereby the vacuum pressure, the sample weight and the incident, reflected and residual microwave powers could be measured and evaluated, throughout the entire process. The experimental runs were established with an approximate incident microwave power at 20 W for absolute pressure levels of 50 and 75 mbar, working with samples of about 1.4 g. It was observed that the vacuum microwave process kinetics at both pressure levels showed little difference. The processes were carried out almost entirely in a regime of water evaporation, the product’s temperature remaining below the solvent boiling temperature. The drying times were similar for both processes, whereas the absorbed power was slightly higher at the pressure of 75 mbar.

Microwave Assisted Vacuum Drying and Endpoint Determination Using Mass Spectrometry, Part II, On-Line Moisture Content Profiling

Quantitative methods were developed to compute on-line conventional loss-on-drying (LOD) profiles from mass spectrometry (MS) measurements. Good agreement between calculated LOD values and actual LOD measurements were obtained for single and multiple solvent wet cakes. Characteristic drying curve profiles were determined for calcium carbonate, sand, cellulose, starch, and silica gel solids, and the corresponding critical moisture contents were estimated. Although this work was developed for microwave drying, the methods apply to conventional dryer monitoring as well.

Microwave Assisted Vacuum Drying and End Point Determination Using Mass Spectrometry, Part I

On-line mass spectrometry (MS) was investigated as a tool to monitor microwave vacuum drying operations on laboratory scale. Using several different solvents and calcium carbonate solids, MS was used to monitor the drying end point of single and multiple solvent wet cakes. The microwave drying rates for polar solvents were shown to have a linear correlation with solvent heat of vaporization. Mass spectrometry profiles were also shown to correlate with other drying end point parameters, including rate of recovered solvent, product temperature, and microwave reflected power.