Vacuum-microwave Finish Drying Research Articles

Volatile and polyphenol composition, anti-oxidant, anti-diabetic and anti-aging properties, and drying kinetics as affected by convective and hybrid vacuum microwave drying of Rosmarinus officinalis L

The preservation of herbs during post-harvest operations is crucial in maintaining their potency. The preservation of Malaysian rosemary (Rosmarinus officinalis L.) was investigated by determining the effect of various drying processes on active volatile components and their respective bioactivities. The drying processes included convective drying at 50, 60, and 70 °C; freeze-drying; vacuum-microwave drying (VMD) at 240, 360, and 480 W; and combination of convective pre-drying (CPD) for 30, 60, and 120 min followed by vacuum-microwave finish drying (VMFD) at 360 W. Complete moisture removal was achieved the fastest by VMD at the highest wattage. Semi-theoretical models, namely, modified Henderson–Pabis, Page, and modified Page had the best prediction accuracy of drying kinetics with the highest R2 (>0.9704) and the lowest RMSE (<0.0565). Sixty-seven compounds were identified from fresh and dried rosemary leaves with camphor (1.45 g kg−1 DW), α-pinene (1.64 g kg−1 DW), and α-terpineol (1.79 g kg−1 DW) as the dominant volatiles. The highest volatile percentage, phenolic content, and antioxidant activities were determined from convective-dried leaves at 60–70 °C, whereas the best anti-diabetic and anti-aging effects were detected from freeze-dried leaves. Non-identified compounds probably have a substantial contribution to the bioactivities of freeze-dried leaves. VMD at 480 W was the most efficient method with the least energy consumption. The sequential method of CPD-VMFD performed moderately among the investigated processes in all aspects. Future studies should further improve the drying processes and investigate the nonvolatile fraction of dried rosemary.

Influence of different drying methods on the quality of Japanese quince fruit

The aim of this study was to determine the effect of different drying methods – convective drying (at 50, 60, 70 °C), vacuum-microwave drying (at 120, 240, 480 W and 480 W with reduction to 120 W), a combination (of convective pre-drying (at 50, 60, 70 °C) and by vacuum-microwave finish drying (at 120 W)) and freeze-drying – on the quality factors of Japanese quince fruit, including phenolic compounds, antioxidant activity, and color. Drying kinetics were determined. The highest content of bioactive compounds was in samples after freeze-drying (total phenolic content – 57 g/kg dw; polymeric proanthocyanidins – 41 g/kg dw). The antioxidant activity measured by oxygen radical absorbance capacity (ORAC) assay was the highest for samples after vacuum-microwave drying at 480 W (0.8 mol Trolox/kg dw). Unfavorable changes in color, formation of hydroxymethylfurfural (HMF) and degradation of l-ascorbic acid as well as phenolic compounds were noted along with the increasing drying temperature and increasing magnetron power. The method that will ensure the proper appearance of dried fruits, storage stability and at the same time guarantee the retention of bioactive compounds seems to be freeze-drying.

Characterisation of the Convective Hot-Air Drying and Vacuum Microwave Drying of Cassia alata: Antioxidant Activity, Essential Oil Volatile Composition and Quality Studies

The preservation of active constituents in Cassia alata through the removal of moisture is crucial in producing a final product with high antioxidant activity. This study aims to determine the influences of various drying methods and drying conditions on the antioxidant activity, volatiles and phytosterols content of C. alata. The drying methods used were convective drying (CD) at 40 °C, 50 °C and 60 °C; freeze drying; vacuum microwave drying (VMD) at 6, 9 and 12 W/g; and two-stage convective pre-drying followed by vacuum microwave finish drying (CPD-VMFD) at 50 °C and 9 W/g. The drying kinetics of C. alata are best described by the thin-layer model (modified Page model). The highest antioxidant activity, TPC and volatile concentration were achieved with CD at 40 °C. GC–MS analysis identified the presence of 51 volatiles, which were mostly present in all samples but with quantitative variation. The dominant volatiles in fresh C. alata are 2-hexenal (60.28 mg 100 g−1 db), 1-hexanol (18.70 mg 100 g−1 db) and salicylic acid (15.05 mg 100 g−1 db). The concentration of phytosterols in fresh sample was 3647.48 mg 100 g−1 db, and the major phytosterols present in fresh and dried samples were β-sitosterol (1162.24 mg 100 g−1 db). CPD-VMFD was effective in ensuring the preservation of higher phytosterol content in comparison with CD at 50 °C. The final recommendation of a suitable drying method to dehydrate C. alata leaves is CD at 40 °C.

Drying of Phyla nodiflora Leaves: Antioxidant Activity, Volatile and Phytosterol Content, Energy Consumption, and Quality Studies

Drying is an important process in the preservation of antioxidants in medicinal plants. In this study, leaves of Phyla nodiflora, or commonly known as frog fruit, were dried using convective drying (CD) at 40, 50, and 60 °C; vacuum-microwave drying (VMD) at 6, 9, and 12 W/g; and convective pre-drying followed by vacuum-microwave finish drying (CPD–VMFD) at 50 °C and 9 W/g. Drying kinetics of P. nodiflora leaves was modelled, and the influences of drying methods on the antioxidant activity, total phenolic content, volatile and phytosterol contents, energy consumption, water activity, and color properties were determined. Results showed that drying kinetics was best described by modified Page model. VMD achieved highest drying rate, whereas VMFD considerably reduced the drying time of CD from 240 min to 105 min. CPD–VMFD was the best option to dry P. nodiflora in terms of retaining volatiles and phytosterols, with lower energy consumption than CD. Meanwhile, VMD at 6 W/g produced samples with the highest antioxidant activity with 2,2′-Azinobis (3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) and ferric reducing antioxidant power (FRAP) value of 11.00 and 15.99 µM Trolox/100 g dw, respectively.

Antioxidant Activity, and Volatile and Phytosterol Contents of Strobilanthes crispus Dehydrated Using Conventional and Vacuum Microwave Drying Methods.

The preservation of active constituents in fresh herbs is affected by drying methods. An effective drying method for Strobilanthes crispus which is increasingly marketed as an important herbal tea remains to be reported. This study evaluated the effects of conventional and new drying technologies, namely vacuum microwave drying methods, on the antioxidant activity and yield of essential oil volatiles and phytosterols. These drying methods included convective drying (CD) at 40 °C, 50 °C, and 60 °C; vacuum microwave drying (VMD) at 6, 9, and 12 W/g; convective pre-drying and vacuum microwave finish drying (CPD-VMFD) at 50 °C and 9 W/g; and freeze-drying (FD). GC–MS revealed 33 volatiles, and 2-hexen-1-ol, 2-hexenal, 1-octen-3-ol, linalool, and benzaldehyde were major constituents. The compounds β-sitosterol and α-linolenic acid were the most abundant phytosterol and fatty acid, respectively, in fresh S. crispus. The highest phenolic content was achieved with CD at 60 °C. The highest antioxidant activity was obtained with CD at 40 °C and VMD at 9 W/g. On the contrary, the highest total volatiles and phytosterols were detected with CD at 50 °C and VMD at 9 W/g, respectively. This study showed that CD and VMD were effective in producing highly bioactive S. crispus. A suitable drying parameter level, irrespective of the drying method used, was an important influencing factor.

The Influence of Drying Method on Volatile Composition and Sensory Profile of Boletus edulis

The objective of this study was to evaluate the influence of different drying methods on aroma and sensory profile of Boletus edulis (cepe). The drying methods tested were convective drying (CD), freeze-drying (FD), vacuum microwave drying (VMD), and a combination of convective predrying and vacuum microwave finish-drying (CPD-VMFD). Fresh and dried cepe volatiles, analyzed by SPME and GC-MS, showed the presence of 53 volatile compounds, most of them present in all dried samples but with quantitative variation. The major volatile compounds in fresh and dried cepe were 1-octen-3-ol (3405 µg 100 g−1·db), 3-octanone (429 µg 100 g−1·db), and hexanal (355 µg 100 g−1·db). The results showed that drying of cepe mushrooms caused major losses of aroma compounds; however, the highest content of volatile compounds and the highest intensity of most of the key positive sensory attributes were found in samples after (i) CD at 80°C (3763 µg 100 g−1·db), (ii) CD at 70°C (3478 µg 100 g−1·db), and (iii) CPD at 60°C and VMFD at 480/240 W (2897 µg 100 g−1·db).

Quality of pomegranate pomace as affected by drying method.

During the industrial manufacturing of pomegranate juice, large amounts of pomace are produced. The aim of this work was to find the effective method to dry pomegranate pomace to open new commercial applications for this co-product. The effects of three drying methods: (i) convective drying (CD) at 50, 60, and 70°C; (ii) vacuum microwave drying (VMD) at 240, 360, and 480W, and (iii) a combined method (CPD-VMFD); convective pre-drying (60°C) followed by vacuum microwave finish drying (360W), on drying kinetics and quality of PomP (pomegranate pomace obtained after preparing pomegranate juice by squeezing only arils) were evaluated. The shortest treatments were VMD at 240 and 360W (52 and 33min, respectively); besides, these treatments led to interesting values of the green-red coordinate, a*, (12.2 and 4.1, respectively), total phenolic content (4.0 and 4.1mgeq gallic acidg-1dry weight, respectively), and antioxidant activity (30.8 and 29.0µmolg-1dry weight, respectively). On the other hand, this study demonstrated that this co-product is a rich source of punicic acid (average value=66.4%), being a good opportunity for the pharmaceutical and nutraceutical industries. Moreover, no significant changes in the fatty acid profile was observed as affected by the drying treatments, and no off-flavors were generated by any of the drying methods.

The Effect of Selected Fruit Juice Concentrates Used as Osmotic Agents on the Drying Kinetics and Chemical Properties of Vacuum-Microwave Drying of Pumpkin

The study examined the osmotic dehydration of pumpkin slices in chokeberry, flowering quince, and raspberry concentrated juices. Products obtained were subjected to vacuum-microwave finish drying (VMD). The objective of the study was to evaluate the drying kinetics and the chemical properties, that is, total polyphenolics content and antioxidant capacity of the vacuum-microwave-dried pumpkin products. The concentration and temperature of the juices were 40°Brix and 45°C, respectively. The pumpkin slices were pretreated in concentrated juices for 0.5, 1, 2, 3, and 6 hours. Vacuum-microwave finish drying was carried out at the power of magnetrons that ensured the maintenance of the safe temperature (below 90°C) of the slices measured with the use of infrared camera. The results of the study showed that the moisture content of samples during the pretreatment in concentrated juices was decreasing until the equilibrium stage. The logarithmic model was used to describe the drying kinetics of pumpkin during VMD. Osmotic pretreatment resulted in a decrease in colour coordinates, improved the antioxidant activity of dried product, and prolonged the duration of VMD.

Influence of osmotic dehydration pre-treatment and combined drying method on physico-chemical and sensory properties of pomegranate arils, cultivar Mollar de Elche

“Mollar de Elche” is the most popular Spanish pomegranate cultivar (intense sweetness and easy-to-chew arils); however, arils have pale pink colour and flat sensory profile. “Mollar the Elche” arils first underwent an osmotic dehydration pre-treatment (OD) with concentrated juices: (i) chokeberry, (ii) apple, and/or (iii) pomegranate cultivar “Wonderful”, to improve their antioxidant capacity, colour, and sensory profile complexity, and later the arils were dried by a combined method (convective pre-drying+vacuum microwave finish drying). The use of OD provided dried arils with characteristic sweetness, and improved colour and aromatic complexity. The recommended OD methods were those using (i) pomegranate, and (ii) pomegranate with chokeberry juices; they improved the total anthocyanin content (mean of 368mgkg−1), red colour (a∗ coordinate 15.6), and antioxidant capacity (e.g. ABTS mean of 5.7mmolTrolox100g−1). However, further research is still needed because freeze-dried arils had the highest anthocyanin content.

Chemical Composition, Antioxidant Capacity, and Sensory Quality of Dried Sour Cherry Fruits pre-Dehydrated in Fruit Concentrates

The aim of this study was to determine the effect of combined drying methods by osmotic dehydration (OD) using several fruit concentrates (apple, blackcurrant, chokeberry, sour cherry, raspberry, quince, flowering quince) or sucrose solution, before combined drying involving convective drying (CD) and vacuum-microwave finish drying (VMFD), on the quality parameters (phenolic compounds, antioxidant activity, color, and other chemical parameters (titratable acidity, pH, dry matter, and water activity)) of dried sour cherry (DSC). OD in some concentrates resulted in enriching the DSC in polyphenols (from 6.4 to 22.4 % OD in sour cherry and chokeberry, respectively), especially in polymeric procyanidins (up to 1.5 times OD in the chokeberry concentrate), but it also reduced the content of anthocyanins (from 10.14 to 65.07 % OD in blackcurrant and sucrose solution, respectively). OD pre-treatment stimulated also the antioxidant activity and other chemical parameters of the DSC. The study revealed that DSC osmo-dehydrated prior to combined drying approach allowed for obtaining attractive final products.

Dying methods affect the aroma of Origanum majorana L. analyzed by GC–MS and descriptive sensory analysis

Drying is a useful option to extend the shelf life of marjoram (Origanum majorana L.), which is a widely used as herb. The influence of drying method on the quality of marjoram aroma was evaluated. The drying methods tested were: convective drying (CD), vacuum–microwave (VM) drying, and a combination of convective pre-drying and VM finish-drying (VMFD). Marjoram’s CD kinetics was described by a logarithmic model, while vacuum–microwave drying kinetics was described by Page model. VMFD drying kinetics followed the model of Henderson and Pabis. Volatile compounds of marjoram samples were extracted by hydrodistillation and analyzed by gas chromatography. Thirty three compounds were tentatively identified and quantified, with cis-sabinene hydrate (229mg100g−1 dry basis, db) and terpinen-4-ol (169mg100g−1db) being the major components. The total concentration of volatiles of fresh marjoram (825mg100g−1db) was reduced by most of the drying treatments, except for the treatments VMD at 240 and 360W. The VMD method at 240W was the best option for drying marjoram followed by the combined method with 50°C and 240 or 360W; the drying times were relatively short (≈162 and 158min, respectively), and aroma quality was good according to instrumental data (total concentration of volatiles) and sensory evaluation results (high scores of fresh and vegetable odor).

Influence of Osmodehydration Pretreatment and Combined Drying Method on the Bioactive Potential of Sour Cherry Fruits

The aim of this study was to determine the effect of osmotic dehydration (OD) time in apple concentrate on dehy- dration kinetics and quality parameters (polyphenol content and antioxidant activity) of sour cherries before further processing. Additionally, the study was focused on the effects of combined drying (convective drying+vacuum-microwave finish drying (CD-VMFD)) applied to fresh and osmodehydrated sour cherries in respect to drying kinetics, including the temperature evolution of dried material with time, as well as on quality factors of the finished product. Analyses were conducted for four kinds of sour cherries: frozen with stone (FS), frozen without stone (FW), thawed with stone (TS), and thawed without stone (TW), which were osmodehydrated for 180 min in the same operating conditions. The study revealed that the decrease of water loss to solid gain (WL/SG) ratio during OD of frozen sour cherries in apple concentrate associated with in- creased values of WL/SG was considerably enhanced by stone removal and the process of thawing. A logarithmic model was proposed for overall description of drying kinetics of combined drying. While OD time was increasing, the drying constant was decreasing for CD and obtained the maximum value for VMFD at the optimal time of OD, which was associated with achieving the peak temperature of VMFD material. It was found that the best variant of pretreatment of sour cherries, to be used for OD in apple concentrate, was freezing the fruits without stones. These fruits were characterized by a high content of polyphenols, high antioxidant activity, and the largest WL/SG ratio during OD process. It was also stated that the optimal method for sour cherry drying was the combined method consisting of OD (120 min) followed by CD (90 min) and VMFD, while maintaining the initial parameters.

Chemical Composition, Antioxidant Capacity, and Sensory Quality of Pomegranate (Punica granatum L.) Arils and Rind as Affected by Drying Method

The objective of this study was to evaluate the application of: (1) freeze drying, (2) convective drying (50, 60, or 70 °C), (3) vacuum–microwave drying (240, 360, or 480 W), and (4) a combined method of convective pre-drying and vacuum–microwave finish drying in the processing of pomegranate arils and rind. The quality parameters under study included sugars and organic acids, punicalagins and ellagic acid, total polyphenols, total antioxidant activity, and sensory quality. In general, drying led to a reduction in all studied parameters; however, the behavior of arils and rind was different. Vacuum–microwave drying at 240 or 360 W was the best drying treatment for arils, while rind required freeze drying or soft conditions of convective drying (50 °C). Further research is needed to obtain proper results with convective pre-drying and vacuum–microwave finish drying of arils and rind. With proper selection of the drying protocol, high-quality dried arils will be available for consumers; these arils will be characterized by high contents of fructose (25 g 100 g−1), phytic acid (2.2 g 100 g−1), punicalagins (0.57 mg g−1), total polyphenols (1.6 mg eq gallic acid g−1), high antioxidant capacity (0.6 mg eq Trolox g−1), and high intensities of garnet color, sweetness, sourness, and fresh pomegranate aroma. Besides, dried rind with very high contents of active compounds (123 mg g−1 of punicalagins and 108 mg eq gallic acid g−1) and high antioxidant capacity (26 mg eq Trolox g−1) will be also available as functional material.

Drying kinetics and quality of beetroots dehydrated by combination of convective and vacuum-microwave methods

Beetroot cubes were dehydrated by convective drying in hot air at 60 °C and by the combination of convective pre-drying (CPD) until moisture content 1.6, 0.6 or 0.27 kg/kg db and vacuum-microwave finish drying (VMFD) at 240, 360 or 480 W. The control samples were obtained by freeze-drying (FD). The drying kinetics of beetroot cubes was described with an exponential function. VMFD significantly reduced the total time of drying and decreased drying shrinkage in comparison with convective method. A critical moisture content divided the temperature profile of samples during VMFD into increasing and falling periods. At the falling temperature period a significant increase in the colour parameters L ∗, a ∗ and b ∗ was found. VM treated samples as well as FD ones exhibited lower compressive strength, better rehydration potential and higher antioxidant activity than those dehydrated in convection. Increasing the microwave wattage and decreasing the time of CPD improved the quality of beetroot cubes dried by the combined method.