Effect of combined microwave‐hot air drying and superheated steam drying on physical and chemical properties of rice

Jasmine rice (Oryza sativa L.) was subjected to two drying operations: combined microwave‐hot air drying (MHA) at initial power intensity of 3, 4 and 6 W g^?1 and superheated steam drying (SHS) at 300 °C and 400 °C. During drying, kinetic rate constants of SHS were significantly higher than those of MHA. Both drying operations could decrease enthalpy of starch gelatinisation from 9.28 J g^?1 to 1.64–6.17 J g^?1, increase gelatinisation extent to 33.51–82.33%, decrease crystallinity from 28.87% to 18.15–21.33%, improve scavenging ability of 1,1‐diphenyl‐2‐picrylhydrazyl, increase ferric reducing antioxidant power and increase hardness of cooked rice from 5.66 N to 5.83–6.55 N, depending on microwave power and drying medium temperature. However, taste profiles and liking scores were comparable to the regular brown rice. Therefore, MHA and SHS operations could be potentially used for reducing drying process and promoting antioxidant activity.     

Effect of microwave vacuum drying and hot air drying on the physicochemical properties of durian flour

Unripe durian cv. Monthong was subjected to microwave vacuum drying (MVD) at 1200, 1600 and 2200 W and hot air drying (HAD) at 40, 50 and 60 °C to produce durian flour. Drying rate of MVD and HAD was 0.34–0.58 kg water kg dry solid^?1 min^?1 and 0.02–0.06 kg water kg dry solid^?1 min^?1, respectively. An increase in drying rate by either increasing hot air temperature or increasing microwave power decreased the degree of crystallinity from 21.95% to 2.31% and from 7.72% to 4.05%, respectively. Moreover, the increased drying rate caused a decrease in endothermic enthalpy (ΔH_gel) and pasting properties. Starch content of the durian flour was 41.40–47.03%. The starch granule morphology of durian flour was disrupted which indicated gelatinisation of flour during drying. Due to a short drying process, the MVD flour had less a*‐value (P ≤ 0.05) than the HAD flour.     

Effects of ultrasound-assisted frying on the physiochemical properties and microstructure of fried meatballs

The aim of this study was to investigate the effects of power ultrasound (0, 200, 400, 600 and 800 W, 20 kHz) on the quality of fried meatballs. The frying time and frying temperature were also considered as fixed factors. The meatballs were fried for 8, 12 and 16 min at 120, 140 and 160 °C. Results showed that the ultrasonic groups saved about 1 to 3 min than the control group to 80 °C. Hardness, springiness, cohesiveness and chewiness of fried meatballs were significantly different among different ultrasonic treatments (P < 0.05). As for colour, the ultrasonic treatments could significantly increase the L*values. Cooking yield was from 82.58% to 85.50% in ultrasonic treatments at 120 °C for 8 min. High moisture retention and cooking yield were shown in fried-assisted ultrasound treatments and were consistent with the microstructure of scanning electron microscopy (SEM). Our data show that ultrasound-assisted frying could improve the quality of meatballs.     

Anti-glycated activity of polysaccharides of longan (Dimocarpus longan Lour.) fruit pericarp treated by ultrasonic wave

Ultrasonic wave was used to extract the polysaccharides of longan fruit pericarp (PLFP) in this work. The anti-glycated activity of PLFP was evaluated. Through artificial neural network toolbox of MATLAB software, a mathematical model between ultrasonic conditions and anti-glycated activity was constructed. The R² and MSE (mean square error) values of the model were calculated to be 0.98 and 0.13, respectively, which suggested a good fitness of the neural network. Response surface plots showed that ultrasonic power, time and temperature had complicated and significant effects on the anti-glycated activity of PLFP. The optimal ultrasonic conditions for obtaining the highest anti-glycated activity were predicted to be 276 W, 24 min and 69 °C. The predicted anti-glycated activity was 60.4%. The experimental determinations under these conditions were not significantly different (P > 0.05) to the predicted value. It indicated the good prediction and optimisation capability of the artificial neural network.     

Effect of Air-Drying Conditions on Physico-chemical Properties of Osmotically Pre-treated Pomegranate Seeds

The drying of pomegranate seeds was investigated at 40 °C, 50 °C and 60 °C with air velocity of 2 m/s. Prior to drying, seeds were osmodehydrated in 55 °Brix sucrose solution for 20 min at 50 °C. The drying kinetics and the effects of osmotic dehydration (OD) and air-drying temperature on antioxidant capacity, total phenolics, colour and texture were determined. Analysis of variance revealed that OD and air-drying temperature have a significant influence on the quality of seeds. Both anthocyanin and total phenolic contents decreased when air-drying temperature increased. The radical diphenylpicril-hydrazyl activity showed the lowest antioxidant activity at 60 °C. Both chromatic parameters (L*, C* and h°) and browning index were affected by drying temperatures, which contributed to the discolouring of seeds. The final product has 22%, 20% and 16% of moisture; 0.630, 0.478 and 0.414 of a _w; 151, 141 and 134 mg gallic acid equivalent/100 g fresh matter (FM) of total phenolics; 40, 24, 20 mg/100 g FM of anthocyanins and 46%, 39% and 31% of antioxidant activity, for drying temperatures of 40 °C, 50 °C and 60 °C, respectively. In view of these results, the temperature of 40 °C is recommended as it has the lowest impact on the quality parameters of the seeds. Differential scanning calorimetry data provided complementary information on the mobility changes of water during drying. Glass transition temperature (Tg′) depends on moisture content and as consequence, on drying conditions. In fact, Tg′ of seeds dried at 60 °C (Tg′ = −21 °C) was higher than those dried at 50 °C (Tg′ = −28 °C) or 40 °C (Tg′ = −31 °C) and osmodehydrated seeds (Tg′ = −34 °C). During OD and drying process, the texture of seeds changed. The thickness of seeds shrank by 55% at 60 °C.     

Effect of Drying on the Color of Tarragon (Artemisia dracunculus L.) Leaves

The effect of drying conditions on the color of tarragon (Artemisia dracunculus L.) leaves was studied. Tarragon leaves were dried at temperatures of 40 to 90 °C with a constant airflow of 0.6 m/s. The samples were collected at 7%, 10%, 20%, and 30% moisture content wet basis for evaluation of the color change. The color parameters of fresh and dried leaves were measured by a colorimeter. The individual parameters of L*a*b* and L*C*h° color systems were evaluated and h° proved to be the best parameter to monitor color change. The smallest change of the color parameters was observed at 40 °C, in which temperature was low, and also at 90 °C, when drying time was short. The biggest change occurred at the temperatures of 50 to 70 °C. Most of the color change happened before the material reaches 35% moisture content. The combination of drying time and temperature defines the change of color.     

The effects of middle infrared radiation intensity on the quality of dried tomato products

The effects of air temperature (T) (unheated and 35 °C), infrared radiation intensity (IP) (894, 1004, 1190, 1314 and 1410 Wm^?2) and air velocity (Av) (1.0, 1.5 and 2.0 ms^?1) on drying characteristics of tomatoes, drying time (DT), specific energy consumption (SEC) and quality parameters were investigated. DT and SEC varied between 114 and 249 min, 10.04 and 18.22 MJ kg^?1 water, respectively, and were significantly affected by process variables. Shrinkage ratio (S) and rehydration ratio (R_r) were varied between 0.129 and 0.214, 2.11 and 2.86 for all the drying conditions, respectively. Total colour change (ΔE) varied from 5.30 to 13.19. Vitamin C content decreased, while lycopene content increased with infrared drying. For optimum points, desirability of functions of all targets had satisfactory, very good and excellent desirability values and ranged between 0.54 and 0.99.     

Moisture transfer and microstructure change of banana slices during contact ultrasound strengthened far-infrared radiation drying

To explore water transfer and structure change during contact ultrasound strengthened far-infrared radiation drying (CU-FIRD) on banana slices, the effects of far-infrared radiation (FIR) temperature and contact ultrasound (CU) power on drying process, water transfer change, microstructure and pore evolution were studied. The results showed that the drying time could be shortened obviously by increasing FIR temperature and CU power. The effective moisture diffusivity values of banana slices were between 2.28 × 10⁻¹⁰–6.12 × 10⁻¹⁰ m²·s⁻¹, which increased with the rises of FIR temperature and CU power. The moisture changes in banana slices during CU-FIRD were analyzed by low-field nuclear magnetic resonance (LF-NMR). There were three kinds of water status in banana slices including bound water, immobilized water and free water. Free water was the main water in banana, and the increases of both FIR temperature and CU power could obviously shorten the removal time of free water. The immobilized water in banana slices increased first and then decreased during the drying process, and the improvements of FIR temperature and CU power could accelerate its mobility and migration. There was no significant change for bound water, which was the main water left in banana slices after drying. The SEM and shrinkage ratio results showed that higher FIR temperature and CU power could reduce the structure shrinkage of banana samples, increase the porosity, and generate more diffusion micro-channels with larger size, so as to speed up the migration process of water in banana. CU-FIRD could improve the porous structure of banana samples, enhance the heat and mass transfer process, and accelerate the migration and diffusion of water.     

Preparation of stable microcapsules from disrupted cell of Haematococcus pluvialis by spray drying

Haematococcus pluvialis, including astaxanthin, disrupted by high‐pressure homogenisation was microencapsulated with Maillard reaction products as wall materials by spray drying. The microcapsules were characterised by scanning electron microscope, size analysis and also the storage stability. The optimised cell disruption process for H. pluvialis based on response surface optimisation was 70 MPa of pressure, 7.38% of H. pluvialis concentration and homogenisation once with a disruption rate of 98.96 ± 0.12%. The optimised spray drying process consisted of a wall‐to‐core material ratio of 2.4:1, inlet temperature of 180 °C and outlet temperature of 80 °C with a microencapsulation rate and microcapsule production rate of (92.1 ± 0.1)% and (97.7 ± 0.2)%, respectively. Characterisation and stability test showed that this microencapsulation process ensured the stability of astaxanthin.     

Microwave drying of mango ginger (Curcuma amada Roxb): prediction of drying kinetics by mathematical modelling and artificial neural network

Curcuma amada (Mango ginger) was dried at four different power levels ranging 315–800 W to determine the effect of microwave power on moisture content, moisture ratio, drying rate, drying time and effective diffusivity. Among the fifteen thin layer drying models considered for evaluating the drying behaviour, the semi‐empirical Midilli et al., model described the drying kinetics very well with R² > 0.999. Drying rate and effective diffusivity increased as the microwave power output increased. Activation energy was estimated by a modified Arrhenius type equation and found to be 21.6 kW kg^?1. A feed‐forward artificial neural network using back‐propagation algorithm was also employed to predict the moisture content during MW drying and found adequate to predict the drying kinetics with R² of 0.985.     

Effect of process variables on the drying rate of mango pulp by Refractance Window

Refractance Window (RW) process is considered a novel and promising drying method, which uses hot water in contact with a polyester film (Mylar) at its bottom face to heat up and dry out a solution spread on the film surface. The aim of this study was to investigate the effect of process variables on the RW drying of mango pulp: water temperature (75, 85 and 95 °C), product thickness (2, 3 and 5 mm) and radiant source (transparent and painted Mylar film). Films' transmissivities to infrared radiation were determined and the drying kinetics of mango pulp with the nine possible pairs of layer thickness and water temperature were assessed. Mylar film was partially transparent to infrared radiation, while the black film (Mylar painted) blocked all infrared radiation emitted from the hot water. RW evaporation capacity was up to 10 kg m^− 2 h^− 1 (pulp with 2 mm, water at 95 °C), indicating a very efficient drying process. RW is more efficient than black film process for pulp thickness up to 3 mm. In fact, this study clearly established that radiative heat transfer contributes to less than 5% of the total amount of energy delivery to food during the RW drying process.     

A Feasibility Study on the Drying of Lime Juice: The Relationship between the Key Operating Parameters of a Small Laboratory Spray Dryer and Product Quality

Concentrated lime juice with 20% total soluble solids was dried using three levels of maltodextrin (40%, 50%, and 60% solid base) and a BüCHI B-190 spray dryer. The drying air temperature and flow rate were 140 °C, 155 °C, and 170 °C and 47.1, 53.5, and 57.8 m³/h, respectively. A total of 27 experiments were conducted (in triplicate) with the feed rate, temperature, and compressed air flow for the atomizer kept constant. Analysis of the data revealed that product recovery increased when the maltodextrin in the feed and drying air flow rate were increased, but the inlet air temperature had no significant (P < 0.05) effect on powder recovery. The moisture content of the powder varied with the maltodextrin level, inlet air temperature, and air flow rate. Using scanning electron microscope images, larger particle size was observed with the increase of maltodextrin concentration. Powder bulk density also increased from 0.41 to 0.68 g/cm³, depending on the level of maltodextrin, whereas the drying air temperature and flow rate were not as effective. The results of this study are beneficial for scaling up the spray drying of lime juice to the food industry.     

Changes of microwave structure/dielectric properties during microwave freeze‐drying process banana chips

This experimental study investigated the trend of structure and dielectric properties during microwave freeze‐drying process banana chips. The mass of banana samples was 160 g, the microwave power set as 2 W g^?1 and the highest drying temperature set as 55 °C. The whole drying process can be finished within 6 h. A network analyser and light microscope were used to determine the dielectric properties and structure. The dielectric properties, ε′ (from 20.80 to 1.20) and ε′′ (from 7.74 to 0.15), and the size of cell get smaller as the drying process continues, especially during the 3–4 h drying, which is the end of primary drying stage and the beginning of secondary drying stage. The trend of dielectric properties and microstructure of samples during drying can be an exact indication of drying stage of MFD.     

Optimization of tyrosinase inhibition activity of ultrasonic-extracted polysaccharides from longan fruit pericarp

Various ultrasonic conditions were employed to prepare polysaccharides from longan fruit pericarp (PLFP) and the Lineweaver–Burk equation was then used to determine the effect of PLFP on inhibition of tyrosinase activity. This result showed that PLFP acted as a non-competitive inhibitor of tyrosinase. The highest slope was observed for ultrasonic extraction, followed by the hot-water extraction, suggesting that the ultrasonic treatment of PLFP increased the inhibition of tyrosinase activity. Furthermore, a multilayer feed-forward neural network trained with an error back-propagation algorithm was used to evaluate the effects of ultrasonic power, time and temperature on the slope value. The trained network gave a regression coefficient (R²) of 0.98 and a mean squared error (MSE) of 0.58, implying a good agreement between the predicted value and the actual value of the slope, and confirmed a good generalization of the network. Based on the artificial neural network-genetic algorithm, the optimal ultrasonic extraction conditions to obtain the highest slope value (154.1) were determined to be 120 W, 12 min and 57 °C. Application of response surface plots showed the slope value as a function of every two factors under various ultrasonic extraction conditions, which can be observed directly. Therefore, the artificial neural network provided a model with high performance and indicated the non-linear nature of the relation between ultrasonic conditions and slope value.     

Mathematical Modeling and Experimental Study on Thin-Layer Vacuum Drying of Ginger (Zingiber Officinale R.) Slices

The vacuum-drying characteristics of ginger (Zingiber officinale R.) slices were investigated. Drying experiments were carried out at a constant chamber pressure of 8 kPa, and at four different drying temperatures (40 °C, 50 °C, 60 °C, and 65 °C).The effects of drying temperature on the drying rate and moisture ratio of the ginger samples were evaluated. Efficient model for describing the vacuum-drying process was chosen by fitting five commonly used drying models and a suggested polynomial was fitted to the experimental data. The effective moisture diffusivity and activation energy were calculated using an infinite series solution of Fick’s diffusion equation. The results showed that increasing drying temperature accelerated the vacuum-drying process. All drying experiments had only falling rate period. The goodness of fit tests indicated that the proposed two-term exponential model gave the best fit to experimental results among the five tested drying models. The average effective diffusivity values varied from 1.859 × 10⁻⁸ to 4.777 × 10⁻⁸ m²/s over the temperature range. The temperature dependence of the effective moisture diffusivity for the vacuum drying of the ginger samples was satisfactorily described by an Arrhenius-type relationship with activation energy value of 35.675 kJ/mol within 40–65 °C temperature range.     

Extraction of polyphenols and antioxidants from pomegranate peel using ultrasound: influence of temperature,frequency and operation mode

The objective of this research was to investigate the effects of temperature (40–70 °C), frequency (37 and 80 kHz) and ultrasonic operation mode (normal, pulse and sweep), and their interactions on the recovery of phenolic compounds and antioxidants present in pomegranate peel using ultrasound-assisted extraction. The content of individual phenolic compounds, total phenolic compounds (TPC), antioxidant capacity (AC) and total extract yield (X₀) were analysed. Significant effects were observed between some sources of variation in recovery of individual compounds at TPC and X₀, and no significant effect was observed for AC. The process and conditions studied shown to be interesting to extract antioxidants, since a high antioxidant capacity was verified in the extracts. Temperatures around 50–60 °C, using 37 kHz frequency, and normal or pulse modes were the conditions that provided better yields of phenolic compounds and X₀. Regarding AC, the best condition observed was at 70 °C, 80 kHz frequency and continuous mode.     

Effects of microwave and hot‐air drying methods on colour, β‐carotene and radical scavenging activity of apricots

The effects of drying by microwave and convective heating at 60 and 70 °C on colour change, degradation of β‐carotene and the 2,2‐diphenyl‐1‐picrylhydrazyl radical (DPPH) scavenging activity of apricots were evaluated. Microwave heating reduced significantly the drying time (up to 25%), if compared with convective one, also owing to the higher temperature reached during the last phase of the process, as monitored by infrared thermography. Colour changes of apricot surface, described with lightness and hue angle, in both drying methods followed a first‐order reaction (0.927 ≤ R² ≤ 0.996). The apricots dried by microwave were less affected by the darkening phenomena. The evolution of β‐carotene in fresh apricots (61.2 ± 5.6 mg kg^?1 d.w.) during the drying highlighted a wider decrease (about 50%) when microwave heating was employed for both the temperatures used. Radical scavenging activity increased (P < 0.05) in all dried samples except for hot‐air dried apricots at 60 °C.     

Spray drying of red (Hylocereus polyrhizus) and white (Hylocereus undatus) dragon fruit juices: physicochemical and antioxidant properties of the powder

This study aims to investigate physical stability and antioxidant properties of spray‐dried red (Hylocereus polyrhizus) and white (Hylocereus undatus) dragon fruit powder upon storage at various relative humidity (RH). Inlet air temperatures of 120 °C (red dragon fruit) and 110 °C (white dragon fruit) as well as maltodextrin concentration of 30% (w/v) were selected as the spray drying conditions as powder was obtained at these minimum conditions. The powder was ranging from 3 to 7 μm in particle size with spherical morphology. The powder had lower antioxidant content and antioxidant activities compared with the control before spray drying. Storage of powder at 43%, 54% or 75% RH at 25 °C for 25 days resulted in structural changes correlating to the depression of glass transition temperatures (T_g) to below storage temperature. At 33% RH, no visible structural changes were observed. Antioxidant properties of the powder remained unchanged after 25 days' storage at the studied RHs.     

Microwave assisted fluidized bed drying of nutmeg mace for essential oil enriched extracts: An assessment of drying kinetics,process optimization and quality

The drying mechanism of the myristicin enriched nutmeg mace has been optimized in a microwave assisted fluidization bed dryer (MWFBD) through multiple linear regression (MLR) and artificial neural network (ANN) modeling. The developed drying technique overcomes the non-uniformity heating problems in microwave heating and prolonged drying in fluidized bed drying. During the novel method of drying selected input variables were drying air temperature (DT) (40–50 °C) and microwave power (MP) (480–800 W) and output variables involving colour, oil yield, and myristicin quantity have been investigated by a continuous air velocity of 5.1 m/s. Six mathematical models about one to four conditions have fitted with an experimental design. Suitable enforcement of such models was evaluated through statistical measures. The coefficient of determination (R²) of MLR varied from 0.89 to 0.98, and the sum of squared error (SSE) varied within 6 × 10⁻³ to 158.18, while R² of ANN fluctuated from 0.82 to 0.95, and the mean squared error (MSE) varied between 0.006 and 0.1450, which shows MLR design performance superior than ANN design. The processing conditions of 48.24 °C DT and 637.431 W MP with a drying time of 1.3 h were identified as optimum conditions with a desirability value of 0.98 to obtain maximum oil yield (13.38%) and good colour (L* (20.83), a* (17.34), b* (8.62)) of nutmug mace. Moreover, no myristicin (5.92%) degradation was observed compared with the sun and convective drying. Among the tested models, page and logarithmic models gave a better prediction of moisture ratio.     

Quantity and chemical composition of essential oil of peppermint (Mentha × piperita L.) leaves under different drying methods

This work was aimed at quantitative and qualitative analyses of the essential oil of peppermint leaves under different drying methods. Thin layer drying experiments of the leaves were performed in shade, hot air dryer (at temperatures of 50°C, 60°C, and 70°C), and microwave oven (at power levels of 200, 400, and 800 W). Essential oils of the fresh and dried samples were extracted by hydrodistillation and analyzed using gas chromatography-mass spectrometry (GC/MS). The highest (22.24 g/kg _{dry matter}) and the lowest (1.33 g/kg _{dry matter}) oil yields were obtained from the hot air-dried leaves at temperature of 50°C and microwave-dried leaves at power of 800 W, respectively. In general, increasing drying temperature decreased the essential oil content. The GC/MS analysis of essential oils showed that the chemical compounds belonged mostly to oxygenated monoterpenes class (72.34–86.41%). The chemical compounds group was significantly (p < 0.01) decreased by microwave drying at power levels of 200 and 400 W. The assessed drying methods caused significant (p < 0.05 and/or p < 0.01) variations in the main constituents of the peppermint leaves essential oil including menthol, menthone, menthofuran, 1,8-cineole, and menthyl acetate. The minimum (35.01%) and maximum (47.50%) concentrations of menthol, as the major compound of the oil, were found in hot air-dried leaves at temperature of 50°C and microwave-dried leaves at power of 400 W, respectively. The percentage of menthone, as the second constituent in the essential oil, was significantly lost (p < 0.01) under microwave drying.