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.     

Two-parameter Lorentzian distribution for monitoring physical parameters of golden colored fruits during drying by application of laser light in the Vis/NIR spectrum

Inadequate drying conditions of tropical fruits can alter the cellular structure and chemistry, which can lead to undesirable characteristics of the final product. The purpose of this research is to test the feasibility of light scattering of three laser sources operating in the Vis/NIR range by calculating a two-parameter Lorentzian distribution (LD) for quality assessment of two golden-colored fruits, namely mango and litchi, during drying. Linear mixed models showed that blue light at 473 nm was the most adequate to monitor changes in browning (R² = 0.81) and moisture content (R² = 0.80) of litchi. For mango, NIR light at 785 nm was affected by the hardness (p < 0.001), whereas moisture content showed a strong influence on the calculated LD functions at both 532 (p < 0.001) and 785 (p < 0.05) nm with a good fitting of the prediction model (R² = 0.91). Laser light can provide an economically-effective solution to obtain information from the fruit tissue non-destructively during industrial drying processes.Industrial relevanceConvective drying is still the most common dehydration technique in industrial applications, while many agricultural products remain susceptible to thermal damage. Quality parameters of these products are not only affected by drying conditions, but are also crop-specific. Meanwhile, the use of laser light backscattering is an innovative and rapid technique to estimate a range of quality parameters during drying of agricultural products. The results of this research provide a basis for robust, in-line quality control during industrial drying processes. In this respect, the application of laser backscattering systems presents many advantages: rapid, real-time product information, eradication of laborious and costly destructive analytical methods, continuous monitoring to avoid over-drying (improved energy efficiency) and quality losses, easy integration into existing drying systems and high resistance to harsh conditions typically found in dryers such as high temperatures and dust.     

Rehydration kinetics and soluble solids lixiviation of candied mango fruit as affected by sucrose concentration

Rehydration behaviour of candied mango samples was studied. The influence of candy process conditions and rehydrating medium on the rehydration kinetics was studied. In this sense, water gain, solute loss and compositional changes in the fruit liquid phase were fitted using Peleg’s model. Mango cubes were candied using two osmotic dehydration steps (applying vacuum impregnation in the first) plus air drying at 35 °C till 80% or 90% soluble solids was obtained. Four combinations of sucrose solutions (^°Bx) was used in the osmotic steps: 25–65, 45–45, 45–65 and 65–65. Rehydration process were carried out at 10 °C for 8 h on sucrose solutions (0, 10, 20 and 30^°Bx). Candied samples using 45^°Bx sucrose in the two osmotic steps showed the highest solute retention at equilibrium. A higher drying level of candied fruit also implied the greatest solute retention.     

Prediction of moisture content uniformity of microwave-vacuum dried mangoes as affected by different shapes using NIR hyperspectral imaging

The prediction of moisture content uniformity on mango slices as affected by four different shapes (square, rectangle, regular triangle, and round shape) during microwave-vacuum drying (MVD) was investigated using near-infrared hyperspectral imaging in combination with multivariate chemometric analysis. Applying spectral pretreatment of a 2nd derivative followed by mean-center to raw spectra was found to be greatly beneficial for the reduction of noise and scattering levels. Seven wavelengths (951, 977, 1138, 1362, 1386, 1420, and 1440 nm) with larger absolute values of regression coefficients derived from a partial least square regression model were identified as feature variables for moisture prediction. An optimized model based on the selected wavelengths was developed using multivariate linear regression, achieving a high prediction accuracy with R_p² = 0.993 and RMSEP = 1.282%. From the moisture distribution map, a similar non-uniform drying pattern was found on square, rectangle and regular triangle-shaped samples, while round-shaped mango slices achieved better drying results.Industrial relevanceThe current study suggested that NIR hyperspectral imaging was a promising technique in predicting the moisture content of mango slices during MVD, and non-uniformity of moisture distribution and the effect of sample geometry should be taken into account when the microwave-vacuum method is implemented in drying.     

Effects of Osmotic Dewatering and Oven Drying on β-Carotene Content of Sliced Light Yellow-Fleshed Sweet Potato (Ipomea batatas L.)

The effects of pre-treatment in osmotic salt solution followed by oven-drying on the moisture removal, and β-carotene content of yellow-fleshed sweet potato were investigated. Potato rings of 3 cm diameter and thicknesses of 2 mm, 3 mm and 4 mm were soaked in NaCI solutions of 10, 15, 20, 25 and 30% w/v, for varying time of 20, 40, 60, 80 and 100 minutes. The pre-treated slices were then oven-dried in a convective oven at 60 °C. Moisture diffusivity was determined by the method of slopes, while β-carotene was measured by UV/VIS- spectrophotometer. Moisture diffusivity values during drying were between 3.648 × 10^− 7 m²/s and 1.459 × 10^− 6 m²/s, while losses in β- carotene ranged from 47 to 86%. The amount of water removed by pre-treatment with NaCl solution decreased significantly as the concentration and the time soaking increased, however, the water out flow was found to increase significantly with increase in thickness of the samples, where the 4 mm size had the highest (1.535 g H₂O/g sample) and 2 mm the least (1.434 g H₂O/g sample) Osmotic pre-treatment in salt solution had no effect on β-carotene, but subsequent oven-drying however degraded the β-carotene.     

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.     

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.     

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.     

Free-standing thin film interactions with small particles

In order to investigate foam stability and coating ability via interactions with particles a free-standing film of initial thickness 150 μm to 350 μm was created within a metallic ring of 16 mm diameter. The weight change of the film is monitored and attributed to water loss with time. The effects of film drying are investigated with respect to the film′s interaction with a solid, spherical, 3 mm diameter glass bead. This particle was dropped onto the film and high-speed camera imaging was used to examine the moment it interacts with the film. In addition to the water content of the film, the relative humidity of the environment, the initial film thickness of the film and the moment the bead interacts with the film are all factors that influence the quality of the film and hence how it interacts with the glass bead. The bead would cause defect points in the film to turn into tears. Films with a higher final water content presented with less tears and gave the particle a more homogeneous coating.Industrial relevanceThin films are often used in experimental and theoretical studies as a basis to investigate foam behavior (Bergeron & Radke, 1992; Myers, 1998, Stubenrauch and Von Klitzing, 2003, Weaire and Kermode, 1983).The importance of foam interaction with small solids spans a wide range of industrial and scientific fields such as agriculture, food science, pharmaceuticals, drilling fluids, household & personal care products (Dickinson, 2010, Eisner et al., 2007, Hunter et al., 2008, Mason et al., 2006, Murray, 2007, Quebaud et al., 1998). Liquid coatings are often used within the food industry, most notably for snack foods and animal feeds (Baldwin et al., 2011, Beckett, 2008, Rockey et al., 2010). What is under investigation here is how a foam, represented via a liquid thin film, can be used to coat a small spherical particle. In addition the drying of the thin, surfactant-based films is taken under consideration with respect to the interaction of such films with solid particles.     

Edible films produced with gelatin and casein cross-linked with transglutaminase

In this study, transglutaminase was used to produce cross-linked casein, gelatin and casein–gelatin blend (100:0, 75:25, 50:50, 25:75 and 0:100) edible films. Cross-linking was investigated by SDS–PAGE. Mechanical and water vapor barrier properties of the films were characterized using ASTM procedures, and the film morphology was evaluated using scanning electron microscopy. The casein–gelatin film showed significant greater elongation values (P < 0.05) with or without transglutaminase treatment, as compared to films made from gelatin or casein alone. Mixtures of casein and gelatin produced a synergistic effect only observed in the film elongation, while no improvement was detected for tensile strength and water vapor barrier properties, except for the casein:gelatin (75:25) formulation with transglutaminase, which showed the lowest water vapor permeability value (5.06 ± 0.31 g mm/m² d kPa). Enzymatic cross-linking also induced a substantial increase in the high molecular weight protein components in the film forming solutions.     

Mastication effects on carotenoid bioaccessibility from mango fruit tissue

The release of carotenoids from fresh fruits or vegetables is determined by the encapsulating plant tissue matrix, intracellular carotenoid location within the cell, and the mastication process. The objectives of this study were to assess the particle sizes obtained after mastication of mango fruit tissue, and how the resulting degree of plant tissue rupture affects carotenoid bioaccessibility. A fine and a coarse chewer were selected after screening 20 healthy volunteers for in vivo human mastication, and the collected chewed boluses were subjected to wet sieving fractionation, followed by an in vitro gastric and small intestinal digestion model. Confocal micrographs show that the smallest particle size fraction (0.075 mm) consists mostly of fragmented cells and the largest size fraction (2.8 mm) contains bulky clusters of whole cells and vascular fibers. Higher amounts of total carotenoids (211–320 μg/100 g) were observed in the larger particle size fraction (2.8 mm) relative to the 1 mm (192–249 μg/100 g) and 0.075 mm fractions (136–199 μg/100 g). Smaller particles showed a greater % release of total carotenoids after in vitro digestion. Xanthophyll derivatives are more bioaccessible than β-carotene for all particle sizes. The effects of particle size or degree of fine vs coarse chewing are unexpectedly small (p > 0.05), but the process of chewing substantially reduced the release of β-carotene and xanthophylls by 34% and 18%, respectively. While there is a (small) particle size effect, this appears to not be the primary factor controlling bioaccessibility for soft tissues such as mango, in contrast to previous reports that a single cell wall appears to be enough to prevent bioaccessibility of carotenoids in more robust carrot tissues.     

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.     

Effects of drying methods and conditions on antimicrobial activity of edible chitosan films enriched with galangal extract

The aim of this work was to study the effects of drying methods and conditions (i.e., ambient drying, hot air drying at 40 °C, vacuum drying and low-pressure superheated steam drying within the temperature range of 70–90 °C at an absolute pressure of 10 kPa) as well as the concentration of galangal extract on the antimicrobial activity of edible chitosan films against Staphylococcus aureus. Galangal extract was added to the film forming solution as a natural antimicrobial agent in the concentration range of 0.3–0.9 g/100 g. Fourier transform infrared (FTIR) spectra and swelling of the films were also evaluated to investigate interaction between chitosan and the galangal extract. The antimicrobial activity of the films was evaluated by the disc diffusion and viable cell count method, while the morphology of bacteria treated with the antimicrobial films was observed via transmission electron microscopy (TEM). The antimicrobial activity, swelling and functional group interaction of the antimicrobial films were found to be affected by the drying methods and conditions as well as the concentration of the galangal extract. The electron microscopic observations revealed that cell wall and cell membrane of S. aureus treated by the antimicrobial films were significantly damaged.     

Properties of edible sodium caseinate films and their application as food wrapping

Sodium caseinate solutions (2.5 g protein/100 g solution) containing glycerol (gly) at gly:protein ratios of 0, 0.08, 0.16, 0.24 or 0.32 w/w were dried over 48 h at 50±5% RH and 23±2 °C to form transparent flexible films of approximately 35 μm thickness. Maximum load, tensile strength and elastic modulus of films decreased and the moisture content increased significantly with increasing glycerol content. Elongation and water vapour permeability of films increased significantly only at gly:protein ratios of 0.24 and 0.32. Wrapping bread samples in the caseinate films reduced hardness during 6 h storage at ambient temperatures relative to unwrapped controls. However, the caseinate films were not as effective in this regard as synthetic polyvinyl chloride film.     

Molecular and structural characteristics of cod gelatin films modified with EDC and TGase

Cod gelatin films before and after cross-linking of gelatin with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) or transglutaminase (TGase) have been characterized by Fourier transform infrared (FT-IR) spectroscopy and differential scanning calorimetry (DSC) analysis. For comparison, a film prepared from unmodified pig gelatin has been also analysed. The difference spectra showed that cod gelatin during the film formation involved first of all water-to-amide hydrogen bonds, and the film from pig gelatin contained water-to-amide, amide-to-amide and water-to-water hydrogen bonds. A higher number of hydrogen bonds in the structure of the film from pig gelatin contributed to much better recovering of the helical structure in this film than in the film from cod gelatin, as the peaks at about 1663 cm^?1 in the amide I band and at about 1537 cm^?1 in the amide II band in the second-derivative spectra revealed. The recovered helical structure, in turn, resulted in a significantly higher melting enthalpy value in the case of the film from the pig gelatin. After modification of cod gelatin with EDC or TGase, the inter-chain cross-linkages formed in the films led to the conformation of gelatin with no indications of helical ordering. An increase of melting temperature of gelatin films by 7 °C on EDC and by 10 °C on the TGase modifications was related to the formation of covalent cross-links, and a decrease of glass temperature by 28 °C and 7 °C on EDC and TGase cross-linking, respectively, demonstrated the plasticizing effect of water.     

Estimation of the diffusivities of sodium chloride,potassium sorbate and sodium bisulphite in mango slices processed by hurdle technology

The fractional amount of sodium chloride, potassium sorbate and sodium bisulphite were evaluated in mango slices immersed in limited volumes of syrup at 25, 50 and 70 °C. The syrup contained 250 g sucrose, 1.5 g sodium chloride, 0.5 g potassium sorbate and 0.25 g sodium bisulphate per kilogram of solution. The sodium chloride concentration in the syrup was confirmed with a flame photometer, and the concentrations of potassium sorbate and sodium bisulphite were determined using high-performance liquid chromatography (HPLC). Fick’s second law was used to calculate effective diffusion coefficients and to predict solute content in the mango slices. Diffusion coefficients were affected by temperature and were correlated by the Arrhenius equation. The experimental data fit the proposed mathematical model well, allowing prediction of the system’s behavior at different temperatures. The resultant diffusivities ranges were 2.63–3.54 × 10^?9 m²/s for sodium chloride, 3.88 × 10^?9–8.3 × 10^?10 m²/s for potassium sorbate and 1.83 × 10^?7–5.98 × 10^-8 m²/s for sodium bisulphite.     

Effect of hydrophobic plasticizers on functional properties of gelatin-based films

The aim of this study was the production and characterization of gelatin-based films using hydrophobic plasticizers derived from citric acid and soy lecithin as emulsifier. The films were characterized as to their mechanic properties, permeability to water vapor, opacity, morphology and possible interactions using Fourier transform infrared spectroscopy. Tensile strength values (TS) varied from 36 to 103 MPa, however, the increase in the concentration of plasticizers (acetyltributyl citrate and tributyl citrate) reduced TS by 57% and no relation was observed between plasticizer quantities and the elongation in the quantities tested. Permeability to water vapor varied between 0.17 and 0.34 (g mm/m² h kPa), slightly increasing with the increase in concentration of plasticizers. The effectiveness in the use of soy lecithin emulsifier in the homogenization between the compounds could be proven by microscopic observation using confocal laser microscopy.     

Effect of PLA lamination on performance characteristics of agar/κ-carrageenan/clay bio-nanocomposite film

Multilayer films composed of PLA and agar/κ-carrageenan/clay (Cloisite® Na⁺) nanocomposite films were prepared, and the effect of lamination of PLA layers on the performance properties such as optical, mechanical, gas barrier, water resistance, and thermal stability properties was determined. The tensile strength (TS) of the agar/κ-carrageenan/clay nanocomposite films (67.8 ± 2.1 MPa) was greater than that of PLA films (43.3 ± 3.6 MPa), and the water vapor permeability (WVP), water uptake ratio (WUR), and water solubility (WS) of the nanocomposite films were higher than those of PLA films. The film properties of the multilayer films exhibited better properties of the component film layers. Especially, the WVP and water resistance of the bionanocomposite film were improved significantly, while the OTR of the PLA film decreased profoundly after lamination with PLA layers. Thermal stability of the bionanocomposite also increased after lamination with PLA layers.     

Numerical modelling of the food freezing process in a quasi-hydrofluidisation system

The paper presents a study of food freezing using hydrofluidisation method, characterised by very high heat transfer coefficients exceeding 2500 W · m⁻² K⁻¹, which constitutes a novel and promising technology. A numerical analysis of a system with a stationary group of food products was performed. This study aimed to investigate the geometrical parameters governing the freezing process, i.e., the position of food products above the orifices (from 50 mm to 70 mm), their mutual position, the diameter of orifices (3 mm or 5 mm), and the spacing of the orifices in an array (from 8 mm to 12 mm). In this method, the freezing time was from 3 min for 10 mm spherical food sample in moderate refrigerating medium temperatures up to 12 min for 30 mm spherical sample. Moreover, the freezing times at different liquid temperatures were compared in the range of −20 °C to −5 °C. Reducing the temperature by 5 K may lead to shortening the process by up to 50%. The hydrofluidisation method was assessed versus the immersion freezing for spherical products of different sizes showing the reduction of the process time from about 35% to over 60%.     

Modeling the inactivation kinetics of fruit bromelain in pineapple during high-pressure and thermal treatments

The stability of fruit bromelain (FBM) in pineapple pulp was studied within a high-pressure domain of 0.1–600 MPa/30–70 °C/1 s–30 min. The pulse effect was quantified as a function of pressure, temperature, pressure build-up and decompression times. A maximum of 60% reduction in FBM activity was obtained after a single pulse of 600 MPa/70 °C. Upon applying nth order model, the obtained reaction order (n) for thermal (0.1 MPa/30–70 °C) and high-pressure (100–600 MPa/30–70 °C) inactivation was 1.1 and 1.2, respectively. The inactivation rate constant (k) ranged from 1.2 to 45.0 × 10^− 3 U^n − 1 min^− 1. The activation energy was nonlinearly dependent on pressure (P); whereas, the activation volume was linearly related to temperature (T). The nonlinear dependence of k on P and T was modeled by an empirical equation. The D-values obtained from the empirical model appeared to be more realistic than those from the log-linear kinetics.Industrial relevancePineapple fruit bromelain (FBM) has numerous health benefits and therapeutic effects. It is a protease enzyme that helps in digestion. Processing of pineapple pulp needs attention towards retaining the maximum FBM activity in it. A detailed kinetic study of FBM within a broad range of pressure–temperature–time domain will help in designing a high-pressure process for the pineapple pulp with respect to its bromelain stability.