THIN LAYER METHOD ANALYSIS OF SPOUTED BED DRIED MALAYSIAN PADDY – CHARACTERISTIC DRYING CURVES

A lab‐scale spouted bed dryer was used for the drying of paddy. The Malaysian grown paddy is a sample of Geldart's Group D particle. Drying was carried out in the dryer at two different internal configurations, namely with or without the installation of a draft tube, and at several drying temperatures (60–90C), air flow rates (1.14–1.45 m/s) and bed heights (30–45 cm). Drying kinetics of paddy in spouted bed showed only induction and falling rate periods, without constant drying rate period. Thin layer method analysis shows characteristic drying rate curves of linear and exponential functions for the first and second falling rate period, respectively, in conventional spouted bed drying. A single power function was observed in spouted bed drying with a draft tube, mainly as a result of a better tempering effect that gave a lower drying rate gradient.     

Stability of probiotic Lactobacillus plantarum in dry microcapsules under accelerated storage conditions

This study investigated the stability of freeze dried and fluid bed dried alginate microcapsules coated with chitosan containing model probiotic bacteria, Lactobacillus plantarum, during storage for up to 45 days at different water activities (0.11, 0.23, 0.40 and 0.70) and temperatures (4, 30 and 37 °C). The loss in cell viability was around 0.8 log in the case of fluid bed drying and around 1.3 in the case of freeze drying, with the former method resulting in dried capsules of smaller size (~ 1 mm vs 1.3 mm), more irregular shape, and with a rougher surface. In both cases, the water activity and water content were less than 0.25 and 10% w/w, respectively, which favours high storage stability. The storage stability studies demonstrated that as the water activity and temperature decreased the survival of the dried encapsulated cells increased. Considerably better survival was observed for fluid bed dried encapsulated cells compared to freeze dried encapsulated cells and freeze dried free cells with 10% sucrose (control), and in some cases, e.g. at 4 and 30 °C at water activities of 0.11, 0.23 and 0.40, there was more than 1 log difference after 45 days, with concentrations higher than 10⁸ CFU/g after 45 days of storage. The results indicate that fluid bed drying is an effective and efficient manufacturing method to produce probiotic containing capsules with enhanced storage stability.     

Mitigation of acrylamide and hydroxymethyl furfural in instant coffee by yeast fermentation

Acrylamide being known as carcinogenic and hydroxymethyl furfural (HMF) being known as cytotoxic compounds are heat induced process contaminants found in instant coffee. Today's instant coffee production method involves roasting of coffee beans, grinding, flavor and aroma separation, extraction, concentration, and drying steps. During roasting, acrylamide and HMF are formed in varying amounts depending upon the degree of heat treatment as a result of the Maillard reaction. This study was conducted in order to reduce the concentrations of acrylamide and HMF in instant coffee. Instant coffee (20%, w/v) was mixed with sucrose (0–10, w/v) and baker's yeast (Saccharomyces cerevisiae, 1–2%, w/v) in a tightly closed glass vessel. The mixture was fermented at 30 °C for 48 h. The kinetics of acrylamide and HMF degradation was investigated. HMF and acrylamide contents were reduced exponentially at varying rates, depending upon fermentation medium and time. After 24 h, HMF concentration was decreased by 61.2%, 75.7%, 93.6% and 99.2% in the fermentation media containing none, 1%, 5%, and 10% of sucrose, respectively. After 48 h, acrylamide concentration was decreased by about 70%. These results revealed that yeast fermentation is promising for the mitigation of HMF and acrylamide in instant coffee.     

Anthocyanin stability and antioxidant activity of spray-dried açai (Euterpe oleracea Mart.) juice produced with different carrier agents

Anthocyanin stability and antioxidant activity of powdered açai juice was evaluated throughout 120 days. Powders were produced by spray drying using four types of carrier agents: maltodextrin 10DE, maltodextrin 20DE, gum Arabic and tapioca starch. Samples were stored at different temperatures (25 and 35 °C) and water activities (0.328 and 0.529), in order to verify the effect of these conditions on anthocyanin degradation and antioxidant activity reduction. Anthocyanin degradation exhibited two first-order kinetics: the first one, with higher reaction rate constant, up to 45–60 days of storage, and the second one, after this period, with lower degradation rate. Both temperature and water activity negatively affected anthocyanin stability. Antioxidant activity also decreased with increasing water activity, but was higher for the powders stored at 35 °C. Maltodextrin 10DE was the carrier agent that showed the best pigment protection and the highest antioxidant activity, for all the conditions studied.     

The drying kinetics of kale (Brassica oleracea) in a convective hot air dryer

The effect of air temperature and sample thickness on the drying kinetics of kale was investigated using a convective air dryer at a fixed airflow rate of 1 m/s and drying air temperatures of 30, 40, 50 and 60 °C. The sliced kale leaves were dried in wire trays in 10, 20, 40 and 50 mm thick layers. The drying rate increased with drying air temperature but decreased with layer thickness. The effective diffusivity for 10 mm thick layers was found to increase with the drying air temperature and ranged between 14.9 and 55.9 × 10⁻¹⁰ m²/s. The effect of temperature on diffusivity could be expressed by an Arrhenius type relationship with a high R² of 0.9989. The activation energy of kale was found to be 36.115 kJ/mol. When four drying models were developed using the experimental data the Modified Page model was found to be marginally better than the other models in estimating the drying curve over the experimental temperature range.     

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.     

Improving ambient temperature stability of probiotics with stress adaptation and fluidized bed drying

Stabilization efficiency in terms of long term ambient temperature storage viability of the probiotic strain Lactobacillus casei CRL 431 was compared using freeze and fluidized bed drying techniques. Fluidized bed drying was able to retain 2.5 log cfu/g higher viability after 52 weeks of storage at 25 °C. A combination of fluidized bed drying and osmotic stress adaptation to the probiotic cells yielded further improvement of 0.83 log cfu/g higher viability compared to the unstressed cells. The findings were validated with other two lactobacilli and two bifidobacterium strains with probiotic characteristics and significant improvements in storage stability over freeze dried samples were observed. Fortification of vitamin E in the stabilization matrix as an antioxidant improved the stability by 0.18 log cfu/g during 20 weeks storage period at 25 °C, whereas any similar benefit of fortifying inulin as a prebiotic was not observed.     

Mathematical modeling on hot air drying of thin layer apple pomace

The hot air convective drying characteristics of thin layer apple pomace were evaluated in a laboratory scale dryer. The drying experiments were carried out at 75, 85, 95 and 105 °C and at the air velocity of 1.20 ± 0.03 m/s. Different mathematical models were tested with the drying behavior of apple pomace in the dryer. The results indicated that the Logarithmic model can present better predictions for the moisture transfer than others; the drying process took place in two falling rate periods, the effective diffusivities in the second period were about six times greater than that in the first period; the general relationship of moisture ratio against drying duration in the thin-layer convective drying of apple pomace were found and can be used in design and operation of the convective drying, suggested by 0.9923 of square correlation coefficient and 0.00001 of chi square.     

Moisture diffusivity and shrinkage of broad beans during bulk drying in an inert medium fluidized bed dryer assisted by dielectric heating

Drying behavior of broad beans (Vicia faba) was studied in a pilot scaled fluidized bed dryer with inert particles assisted by dielectric heating. The effective diffusion coefficient of moisture transfer was determined by Fickian method at four different air drying temperatures of 35, 45, 55 and 65 °C. Correlations for moisture diffusivity as a function of moisture content and temperature of the drying medium were developed. The values of moisture diffusivity were obtained within the range of 1.27 × 10^?9–6.48 × 10^?9 m²/s and the activation energies for FBD and FBD + DE were found to be 27.71 and 17.10 kJ/mol, respectively. The shrinkage behavior of the broad beans was also investigated by considering the volume ratio of (V/V_O) to be function of moisture content alone and fitting a polynomial of the third order. The dielectric heating power was also found to be effective on the rate of drying.     

Microbial cells as colloidal particles: Pickering oil-in-water emulsions stabilized by bacteria and yeast

Thermally-inactivated baker's yeast (Saccharomyces cerevisiae) and lactic acid bacteria (Lactobacillus acidophilus and Streptococcus thermophilus) were used to generate and stabilize model oil-in-water (O/W) emulsions containing up to 80 wt.% dispersed oil. With optimized compositions, cell-covered dispersed oil droplets were stable against droplet coalescence and bulk phase separation for over four months. From a textural perspective, these emulsions were self-supporting and exhibited a mayonnaise-like consistency. The microbial cells acted as Pickering-type stabilizers by residing at the oil–water interface. The three-phase contact angle of the yeast at the oil–water interface measured using confocal microscopy was 30 ± 9°, demonstrating its ability to stabilize O/W emulsions. These microbial cells may be used in the design of processed food emulsions with an ‘all-natural’ designation as well as for the replacement of common synthetic surfactants to permit clean label declarations.     

Microwave assisted drying of short-cut (ditalini) macaroni: Drying characteristics and effect of drying processes on starch properties

Macaroni samples were dried by conventional hot air, microwave alone and hot air followed by microwave drying methods. The drying of macaroni samples took place in the falling rate period. Higher drying rates were observed with a higher microwave power level. A diffusion model was employed to fit the experimental drying data in order to characterize the drying rate of samples. Drying only with microwave energy (70 and 210 W) or hot air–microwave energy (70 and 210 W) resulted in substantial shortening of the drying time. The reduction was 61.8%, 87.3%, 61% and 78%, respectively, of the time to dry with hot air. All starch granules of semolina and hot air dried macaroni displayed a clear “Maltese cross” under polarized light. Starch granules of macaroni samples dried with microwave (70–210 W) and combined hot air–microwave (70–210 W) had irregular shapes but retained birefringence in about half or whole of each granule. The starch gelatinization enthalpies of macaroni samples dried with hot air, microwave alone (70 and 210 W) and combined hot air and microwave (70 and 210 W) drying were lower than that of semolina sample. This confirms that starch is not completely gelatinized during drying.     

Physical properties of parboiled wheat and bulgur produced using spouted bed and microwave assisted spouted bed drying

The main objective of this study was to compare the spouted bed and microwave assisted spouted bed drying on affecting physical properties of parboiled wheat and bulgur. Drying was performed at different temperatures (50 °C, 70 °C, 90 °C) and microwave powers (288 W, 624 W). Bulk and apparent density, bulk and apparent porosity, sphericity, color, microstructure and pore size distribution of parboiled wheat were investigated after drying. Yield and water absorption capacity were determined in bulgur samples. The effect of air temperature on physical properties of product except color was not significant in spouted bed drying. More porous structure was observed in wheat samples dried in microwave assisted spouted bed compared to air dried ones. Sphericity and bulk density were higher when high temperature was combined with high microwave power. In microwave assisted spouted bed drying, similar yield value but lower water absorption capacity of bulgur were observed as compared to spouted bed drying.     

Combined effects of ultrasound and pulsed-vacuum on air-drying to obtain unripe banana flour

In order to increase the water migration rate during unripe banana drying process, two pre-treatments were applied. Experiments were performed as following: 20 min of ultrasound (US) + air-drying at 50 °C; 20 min of US + 60 min of pulsed-vacuum (PV) + air-drying at 50 °C; 25 min of US + air-drying at 60 °C and 25 min of US + 60 min of PV + air-drying at 60 °C. Experimental data were adjusted to five models and the Midilli model resulted to fit best experimental data with r² > 0.999, RMSE < 0.0119 and χ² < 0.00012. Increasing in water effective diffusivity, at the two falling rate periods, were observed due to the application of US, whereas the combined technique of US + PV did not improve the water migration, at both air-drying temperatures. The results revealed drying time savings of (28 and 18) % at (50 and 60) °C, respectively.Industrial relevanceIn this work, ultrasound and ultrasound-pulsed vacuum pre-treatments prior to air-drying were applied as an alternative to produce unripe banana flour (UBF). This study indicates that the drying kinetics increased due to the application of ultrasound, preserving the resistant starch content of the final product. Hence, this technology diminishes the drying time and consequently reduces the energy costs, in comparison with to the conventional process. Therefore, the UBF rich in resistant starch content can be considered a functional ingredient that promotes dietary intake of unavailable carbohydrates, which may reduce the risks of non-communicable diseases.     

Thin-layer drying characteristics and modelling of mint leaves undergoing microwave treatment

The effect of microwave drying technique on moisture content, moisture ratio, drying rate, drying time and effective moisture diffusivity of mint leaves (Mentha spicata L.) were investigated. By increasing the microwave output powers (180–900 W) and the sample amounts (25–100 g), the drying time decreased from 12.50 to 3.0 min and increased from 6.60 to 16 min, respectively. To determine the kinetic parameters, the drying data were fitted to various models based on the ratios of the differences between the initial and final moisture contents and equilibrium moisture content versus drying time. Among of the models proposed, the semi-empirical Midilli et al. model gave a better fit for all drying conditions applied. By increasing the microwave output powers and decreasing the sample amounts, the effective moisture diffusivity values ranged from 3.982 × 10⁻¹¹ to 2.073 × 10⁻¹⁰ m² s⁻¹ and from 9.253 × 10⁻¹¹ to 3.162 × 10⁻¹¹ m² s⁻¹, respectively. The activation energy was calculated using an exponential expression based on Arrhenius equation. The relationship between the drying rate constant and effective moisture diffusivity was also estimated; and gave a linear relationship.     

Drying of edamames by hot air and vacuum microwave combination

The characteristics of hot air and vacuum microwave drying were compared using edamame as the raw material, and an optimized combination drying process was then established thereof so as to achieve increased drying rate and enhanced product quality. Edamame was subjected to 70 °C hot air drying for 20 min, and then microwave dried at power intensity 9.33 W/g for 15 min under −95 kPa (gauge pressure). The optimized combination drying process exhibited significantly shortened drying time as compared with conventional hot air drying, and greatly decreased mass loads on the vacuum microwave dryer.     

Drying kinetics of grape seeds

Drying of grape seeds representing waste products from white wine processing (Riesling), red wine processing (Cab Franc), and juice processing (Concord) was studied at 40, 50, and 60 °C and constant air velocity of 1.5 m/s. Equilibrium moisture content had a significant effect on the normalized drying curve and was determined for each grape seed at each drying temperature. Effective moisture diffusivity ranged between 1.57 and 3.96 × 10⁻¹⁰ m²/s for Riesling seeds, 2.93–5.91 × 10⁻¹⁰ m²/s for Concord seeds, and 3.89–8.03 × 10⁻¹⁰ m²/s for Cab Franc seeds. The temperature dependence of the effective diffusivity followed an Arrhenius relationship, and the activation energies were 40.14 kJ/mol for Riesling seeds, 30.45 kJ/mol for Concord seeds, and 31.47 kJ/mol for Cab Franc seeds. Three thin-layer models were used to predict the drying curves: Page model, Lewis model, and the Henderson–Pabis model. All three models were found to produce accurate predictions compared to the mass average moisture loss for each grape seed variety (percent error less than 10%), and the Lewis model was shown to be an excellent model for predicting all three grape seed varieties (percent error less than 5%).     

Effects of drying methods and conditions on drying kinetics and quality of Indian gooseberry flake

Vacuum drying and low-pressure superheated steam drying (LPSSD) of Indian gooseberry flake (used to prepare Indian gooseberry tea) were carried out at various drying conditions to monitor the drying kinetics and quality degradation of the product during drying. In terms of drying kinetics, the vacuum drying took shorter time to dry the product than LPSSD at every drying condition. In terms of quality, it was found that only the product subjected to vacuum drying at 75 °C and absolute pressure of 7 kPa had similar level of ascorbic acid retention compared to those samples of LPSSD at every condition. The total color difference of this sample was, however, slightly higher than that of product dried by LPSSD. Nevertheless, since the color changes were not of much concern to the consumers of Indian gooseberry tea, vacuum drying at 75 °C and 7 kPa was proposed as the most favorable condition for drying of Indian gooseberry flake in terms of energy consumption.     

Experimental and parametric study on drying of green peas in spouted bed

Drying of Green Peas (Pisum sativum) was studied experimentally in a lab‐scale spouted bed. Experiments were carried out at different operating conditions, and the effect of inlet air temperature and its flow rate, bed height (initial mass of wet particles) and average diameter of particles were examined on performance of dryer and rate of drying. The obtained experimental results were fitted by an exponential form equation to model the drying kinetic behaviour of green peas in a spouted bed. The fitting parameters are presented in the form of mathematical correlations as a function of operating parameters. As expected, it was found that by increasing inlet air temperature and air‐flow rate, the rate of drying increases, but by increasing the bed height and diameter of particles, the rate of drying decreases. The results can help the design engineers to choose the optimum drying conditions for industrial applications.     

Modelling of red abalone (Haliotis rufescens) slices drying process: Effect of osmotic dehydration under high pressure as a pretreatment

Simultaneous application of osmotic dehydration and high pressure as a pretreatment to drying process on red abalone (Haliotis rufescens) slices was studied. During drying process the process time was reduced by increasing temperature from 40 to 60 °C along with the application of different pretreatments: high pressure (350 and 550 MPa), pressure time (5 and 10 min), and osmotic solution (10 and 15% NaCl). Effective moisture diffusivity was determined and varied from 4.35 to 9.95 × 10^− 9 m²/s, for both control and pretreated samples (R² ≥ 0.97). The Weibull, Logarithmic and Midilli–Kucuk models were applied to drying experimental data, where Midilli–Kucuk model was found to be the best fitting model. Furthermore, all drying curves were normalized and then modelled by the same three above models showing a R² ≥ 0.96. As to energy consumption and efficiency values for drying processes were found to be in the range of 777–1815 kJ/kg and 8.22–19.20%, respectively. Thus, knowledge on moisture transfer kinetics, energy consumption and data normalization, is needed to manage and control efficiently drying process under different pretreatment conditions.Industrial relevanceThis article deals with the mass transfer modelling and energy consumption during simultaneous high hydrostatic pressure treatment and osmotic dehydration as a pretreatment to drying process of abalone slices. Water and salt transfer during this combined process was satisfactorily simulated with the Midilli–Kucuk model. Results indicated that application of this combined innovative technology improved abalone slices dehydration rates compared to atmospheric pressure operation resulting in a dried abalone with intermediate moisture content ready to be used as input material of further processes. Furthermore, the different energetic features were determined in order to realize the importance of the changes that can influence to alter process time.