Microstructural Changes during Drying of Apple Slices

Abstract

In recent years some effort has been done trying to relate microstructural changes of dehydrated foods with macroscopical properties and quality factors of the processed product. This work assesses the microstructural changes of apple slices submitted to convective drying. Apple slices (25.7 mm diameter, 2.6 mm thickness) were dried in an oven at 70°C on a screen sample holder in order to allow mass transfer in any side of the sample. Microphotographs of the same sample were taken at determined time intervals using a stereomicroscope. The objects in these photographs were identified and classified in three size groups: (a) the cells group, containing objects with an area less than 0.03 mm²; (b) the mixed cells-intercellular spaces group, with an area between 0.03 and 0.06 mm², and (c) the intercellular spaces group, including objects with an area higher than 0.06 mm². Geometrical parameters of such objects, namely size (area, perimeter, equivalent diameter, and major and minor axis length) and shape parameters (compactness, elongation, and roundness) were evaluated. Shrinkage of cells and intercellular spaces was very clear during drying, observing a decrease of size with moisture content. Concerning shape factors, compactness remained constant, roundness slightly decreased during drying, and elongation increased in the final stage of the process. Macroscopical changes of the entire disc followed the same behavior as microscopical ones, suggesting that changes at the two scale levels are strongly related.     

Scanning Electron Microscopic Study of Microstructure of Gala Apples During Hot Air Drying

Microscopic changes that occur in plant food materials during drying significantly influence the macroscopic properties and quality factors of the dried food materials. It is critical to study the microstructure to understand the underlying cellular mechanisms to improve the performance of food drying techniques. However, there is limited research on such microstructural changes of plant food material during drying. In this work, Gala apple parenchyma tissue samples were studied using a scanning electron microscope for gradual microstructural changes as affected by temperature, time, and moisture content during hot air drying at two drying temperatures: 57 and 70°C. For fresh samples, the average cellular parameter values were as follows: cell area, 20,000 µm²; ferret diameter, 160 µm; perimeter, 600 µm; roundness, 0.76; elongation, 1.45; and compactness, 0.84. During drying, a higher degree of cell shrinkage was observed with cell wall warping and an increase in intercellular space. However, no significant cell wall breakage was observed. The overall reductions in cell area, ferret diameter, and perimeter were about 60, 40, and 30%. The cell roundness and elongation showed overall increments of about 5% and the compactness remained unchanged. Throughout the drying cycle, cellular deformations were mainly influenced by the moisture content. During the initial and intermediate stages of drying, cellular deformations were also positively influenced by the drying temperature and the effect was reversed at the final stages of drying, which provides clues regarding case hardening of the material.     

In Situ ESEM Examination of Microstructural Changes of an Apple Tissue Sample Undergoing Low-Pressure Air-Drying Followed by Wetting

Low-pressure drying of apple tissue has been visualized in situ using an environmental scanning electron microscope (ESEM). Both freshly cut and pre-boiled sample tissues were tested for drying and followed by wetting processes and the differences between the two types of samples noted. Bubble formation was observed when drying the freshly cut sample and were found to form only around the intact cells (intracellular spaces or channels). Boiling produced a more uniform surface and no bubble formation could be observed. Air-drying of the two tissue samples was conducted as a complementary investigation so that the effect of boiling can be better ascertained.     

In Situ ESEM Examination of Microstructural Changes of an Apple Tissue Sample Undergoing Low-Pressure Air-Drying Followed by Wetting

Low-pressure drying of apple tissue has been visualized in situ using an environmental scanning electron microscope (ESEM). Both freshly cut and pre-boiled sample tissues were tested for drying and followed by wetting processes and the differences between the two types of samples noted. Bubble formation was observed when drying the freshly cut sample and were found to form only around the intact cells (intracellular spaces or channels). Boiling produced a more uniform surface and no bubble formation could be observed. Air-drying of the two tissue samples was conducted as a complementary investigation so that the effect of boiling can be better ascertained.     

Numerical Investigation of Case Hardening of Plant Tissue During Drying and Its Influence on the Cellular-Level Shrinkage

Dried plant food materials are one of the major contributors to the global food industry. Widening the fundamental understanding of different mechanisms of food material alterations during drying assists the development of novel dried food products and processing techniques. In this regard, case hardening is an important phenomenon, commonly observed during the drying processes of plant food materials, which significantly influences the product quality and process performance. In this work, a mesh-free-based 2D numerical model developed by the authors is further improved and used to simulate the influence of case hardening on shrinkage characteristics of plant tissues during drying. In order to model the fluid mechanisms of plant cells, smoothed particle hydrodynamics (SPH), which is a popular mesh-free technique used to solve hydrodynamics problems, is used. The cell wall mechanisms are modeled using the discrete element method (DEM). The model is fundamentally more capable of simulating large deformations of multiphase materials compared to conventional grid-based modeling techniques such as finite element methods (FEM) or finite difference methods (FDM). Case hardening is implemented by maintaining distinct moisture levels in the different cell layers of a given tissue. In order to compare and investigate different factors influencing tissue deformation under case hardening, four different plant tissue varieties (apple, potato, carrot, and grape) are studied. The simulation results indicate that the inner cells of any given tissue undergo limited shrinkage and cell wall wrinkling, compared to the case-hardened outer cell layers of the tissues. For a given dried tissue condition, the case-hardened cellular deformations are highly influenced by the unique characteristics of the different tissues, such as cell size, cell fluid turgor pressure, and cell wall properties.     

Porosity,Bulk Density,and Volume Reduction During Drying: Review of Measurement Methods and Coefficient Determinations

Several experimental methods for measuring porosity, bulk density, and volume reduction during drying of foodstuffs are available. These methods include, among others, geometric dimension, volume displacement, mercury porosimeter, micro-CT, and NMR. However, data on their accuracy, sensitivity, and appropriateness are scarce. This article reviews these experimental methods, areas of applications, and limits. In addition, the concept of porosity, bulk density, and volume reduction and their evolution as a function of moisture content during drying are presented. In this study, values of initial porosity (?₀) and density ratio (β) of some food products are summarized. It has been found that ?₀ is highly dependent on the type of food products, while β ranges from 1.1 to 1.6. The possibility of calculating solid density based on food compositions has also been validated. The inter-predictions between porosity, bulk density, and volume density have been made mathematically evident.     

Frying of Potatoes: Physical,Chemical, and Microstructural Changes

Frying is one of the oldest unit operations and is used not only in industry but also at home. The most commonly fried vegetable is potato, for important commercial products such as potato chips, par-fried potatoes, and french fries. Quality parameters of interest for fried potatoes include physical and chemical properties such as color, mechanical properties (e.g., crispness, hardness, etc.), structural properties (e.g., porosity and roughness), oil content, and water content, among others. Some chemical contaminants such as acrylamide and furan are heat-generated during the frying of potato slices or strips, leading to final fried pieces with considerable amounts of these contaminants. The controllable variables in industrial frying processes are generally potato variety, oil type, frying time, and frying temperature. Therefore, the study of the quality changes during frying is critical because knowledge regarding kinetics parameters will enable prediction of the final quality in fried potatoes and improvements in the final product value by selecting properly the processing conditions. Finally, modern techniques such as computer vision provide valuable tools to quantify and predict physical and chemical properties of potato pieces during frying in a fast and noninvasive way. In addition, computer vision can allow us to classify fried potatoes in different quality classes previously determined by sensorial panels.     

Combined Electrohydrodynamic (EHD) and Vacuum Freeze Drying of Sea Cucumber

A combination of electrohydrodynamic drying (EHD) and vacuum freeze drying (FD) is examined as an improved method for dehydrating sea cucumbers. The energy consumption, shrinkage and rehydration ratio, protein content, and sensory properties, such as the color and trimness, of the dried product in the EHD–FD method are measured. Compared with FD, the combined process consumes less drying time and has lower energy consumption than EHD drying alone. Also, the product processed by combined drying displays lower shrinkage, higher rehydration rate and higher protein content, along with better sensory qualities.     

Changes in the size of pores during shrinkage (or expansion) of cement paste and concrete

The often-argued and seldom-resolved issue of relating the bulk shrinkage of concrete to changes in the size of pores is analyzed using a simple model. It is shown that the pores in a composite containing both shrinking and nonshrinking solid phases can themselves either shrink or expand when the matrix shrinks, depending on the amount of restraint in the system. The analysis also applies to expansive deformations, which occur with some types of chemical attack of cement paste. A mathematical relationship between the degree of restraint in a composite and the relationship between bulk volume changes and pore volume changes is given. These observations provide guidelines for interpreting the meaning of gaps that often form between aggregate and paste in concrete.     

Computer Vision System (CVS) for In-Line Monitoring of Visual Texture Kinetics During Shrimp (Penaeus Spp.) Drying

In this study, the effects of drying medium temperature and velocity were surveyed on the image texture features of shrimp (Penaeus spp.) batches in a dryer equipped with a perpendicular dual-view computer vision system (CVS). This was carried out by applying an innovative rotation- and scale-invariant image texture processing approach with the capability of eliminating the effects of sample shrinkage on the visual textural features. Moreover, the variations in image texture parameters were investigated with moisture ratio, color, and geometrical characteristics of the shrimp samples. Drying experiments were conducted at hot air drying (HAD) temperatures of 50–90°C and superheated steam drying (SSD) temperatures of 110–120°C with drying medium velocities of 1–2 m/s. Several configurations of a multilayer perceptron artificial neural network (MLP-ANN) were also used to predict the moisture ratio and the geometrical characteristics of the shrimp batch using the image texture parameters. Generally, the image texture features were significantly affected by drying medium temperatures (p < 0.01), and the effects of drying medium velocities on the textural properties were nonsignificant (p > 0.05). Additionally, the higher drying temperatures generated products with uniform and regular texture patterns. The SSD produced samples with somewhat nonuniform and irregular texture patterns compared with HAD at 90°C. Finally, selected MLP-ANN topologies successfully predicted the moisture ratio and the geometrical characteristics of the shrimp batch using the textural properties with correlation coefficients higher than 0.99.     

Physical Changes in Bamboo (Bambusa phyllostachys) Shoot During Hot Air Drying: Shrinkage, Density, and Porosity

The apparent density of bamboo (Bambusa phyllostachys) shoot was investigated at a moisture content range of 10-92% wet basis by weighing the product in air and determining the buoyancy force in toluene. An analysis of variance (ANOVA) unveiled that moisture content significantly affected apparent density at 95% confidence level. The true density of the bamboo shoot was determined by grinding the dried sample to exclude all internal pores and using density bottles. The experimental data fitted well to a general density equation for fruits and vegetables proposed by Lozano et al. (1983) and to a second order polynomial (SOP) model. The internal porosity generated during drying varied in a nonlinear (quadratic) fashion. Shrinkage at different moisture content levels was measured by evaluating the dimensional changes in bamboo shoot slabs (5.0 × 3.2 × 1.8 cm) by drying in a convection oven and a tray dryer operating at 70°C and 7.2% relative humidity. Shrinkage was affected statistically by decreasing moisture content at 95% confidence level. Shrinkage was compared with available models in literature and found to be oriented based on fiber direction and distinctly different from the isotropic volume change in fruits and vegetables.     

Drying with Internal Vaporisation : Introducing the Concept of Identity Drying Card (IDC)

ABSTRACT

Different drying configurations (convective drying with moist air and superheated steam, microwave drying and vacuum drying) on different materials (isotropic and anisotropic) were experimentally studied in order to model and visualise the evolution of internal pressure and temperature. To be able to do so, in addition to measuring the average moisture, a method which can determine internal-local pressure and temperature simultanously by using specially designed sensors was developed. In combination with the experiments, the numerical code TRANSPORE has been used to simulate drying processes. A less comprehensive but more comprehensible analytical model was also provided to facilitate the better understanding of internal phenomena. Based on the results gained from measurement and numerical analysis, the dynamic distribution and development of local temperature and pressure inside seasoned medium are coupled together by a temperature-pressure graph, which is herewith called “Identity Drying Card” (IDC), a new concept initialid in the paper. By using IDC, the internal profile of temperature and pressure, the dominant transport properties (penncability and difisivity), the mechanism of transport (diffusion, convection or both) and the phase transitions during drying can be visualised. More specifically, the amount of dry air, the moisture content in the hygroscopic rcgion or the danger due to internal mechanical loads of handled materials can be figured out with the aid of IDC.     

Extraction and Biological Evaluation of Matrix-Bound Nanovesicles (MBVs) from High-Hydrostatic Pressure-Decellularized Tissues

Decellularized tissues are widely used as promising materials in tissue engineering and regenerative medicine. Research on the microstructure and components of the extracellular matrix (ECM) was conducted to improve the current understanding of decellularized tissue functionality. The presence of matrix-bound nanovesicles (MBVs) embedded within the ECM was recently reported. Results of a previous experimental investigation revealed that decellularized tissues prepared using high hydrostatic pressure (HHP) exhibited good in vivo performance. In the current study, according to the hypothesis that MBVs are one of the functional components in HHP-decellularized tissue, we investigated the extraction of MBVs and the associated effects on vascular endothelial cells. Using nanoparticle tracking assay (NTA), transmission electron microscopy (TEM), and RNA analysis, nanosized (100–300 nm) and membranous particles containing small RNA were detected in MBVs derived from HHP-decellularized small intestinal submucosa (SIS), urinary bladder matrix (UBM), and liver. To evaluate the effect on the growth of vascular endothelial cells, which are important in the tissue regeneration process, isolated SIS-derived MBVs were exposed to vascular endothelial cells to induce cell proliferation. These results indicate that MBVs can be extracted from HHP-decellularized tissues and may play a significant role in tissue remodeling.     

Effective Diffusivities and Energy Consumption of Whole Fruit Chinese Jujube (Zizyphus jujuba Miller) in Microwave Drying

Microwave drying of whole fruit Chinese jujube was performed at 45, 90, and 135 W. Ten commonly used mathematical models of thin-layer drying were compared. The Midilli model was best in describing drying time dependency of product moisture ratios. The initial drying rate and drying rate constant are linearly proportional to microwave power level. An effective diffusivity model was presented and validated with the Renka-Cline algorithm. The model has very high predictive precision, suggested by the relative percentage error of 3.734% on average between the model and the Renka-Cline algorithm. The effective diffusivity was proved to be a linear function of microwave power level and a quadratic function of moisture content. Energy consumption in microwave drying of Chinese jujube decreased as microwave power increased from 45 to 135 W, but 90 W was adequate for high-quality products with less energy consumption.     

Microstructural analysis and structure-property relationship of poly(glycolide-co-1,3-trimethylene carbonate)

A series of linear copolymers of glycolide and 1,3-trimethylene carbonate were synthesized by bulk ring-opening polymerization. The copolymers were characterized by ¹H NMR, ¹³C NMR, viscometry, and differential scanning calorimetry (DSC). The dependency of reaction temperature, reaction time, and the feed composition on the microstructure of the copolymers was examined by ¹³C NMR analysis. The microstructural analysis using ¹³C NMR was useful to calculate the average block length of the glycolyl (L_G) and trimethylene carbonyl (L_T) sequence. The structural change such as transesterification, which was assigned by TGT sequence, was reflected in the average block length and the sequence of each monomeric unit in the copolymer. The average length of glycolyl sequence (L_G) was much longer than that of trimethylene carbonyl sequence (L_T) in polymerization temperature of 100-150 °C. Upon further increasing the polymerization temperature, the L_G decreased, but the change of L_T was insignificant. During the polymerization, transesterification did not occur at 100 °C, but it was observed at a polymerization temperature range of 130-200 °C resulting in the decrease in L_G. As the composition of trimethylene carbonate increased, L_G decreased, but L_T do not show remarkable change. DSC results showed a close relationship between crystallinity and nature of microstructural sequence. The crystallinity of block copolymers was mainly decided by the average length of the glycolyl block.     

Effect of Osmotic Pretreatment on Hot Air Drying Kinetics and Quality of Chilean Papaya (Carica pubescens)

The effect of osmotic pretreatment on the mass transfer kinetics and quality of dried rehydrated Chilean papaya was researched. Osmotic treatments were sucrose solutions of 40, 50, and 60% w/w and dried at 60°C; non-pretreated samples were dried at different temperatures (40, 60, and 80°C). Quality parameters were analyzed: proximal composition, rehydration ratio, water-holding capacity, color, vitamin C content, firmness, and microstructure. Non-pretreated samples showed a clear turgor loss, color loss. and low ascorbic acid retention when rehydrated. Osmotic pretreatment improved the quality of rehydrated papayas, showing higher ascorbic acid retention and best firmness and color. Microstructure is better preserved in the pretreated papayas than in the non-pretreated samples, although this difference is minimal.     

First principles calculations and synthesis of multi-phase (HfTiWZr)B2 high entropy diboride ceramics: Microstructural,mechanical and thermal characterization

First principles calculations were conducted on (HfTiWZr)B₂ high entropy diboride (HEB) composition, which indicated a low formation energy and promising mechanical properties. The (HfTiWZr)B₂ HEBs were synthesized from the constituent borides and elemental boron powders via high energy ball milling and spark plasma sintering. X-ray diffraction analyses revealed two main phases for the sintered samples: AlB₂ structured HEB phase and W-rich secondary phase. To investigate the performance of multi-phase microstructures containing a significant percentage of the HEB phase was focused in this study. The highest microhardness, nanohardness, and lowest wear volume loss were obtained for the 10 h milled and 2050 °C sintered sample as 24.34 ± 1.99 GPa, 32.8 ± 1.9 GPa and 1.41 ± 0.07 × 10^?4 mm³, respectively. Thermal conductivity measurements revealed that these multi-phase HEBs have low values varied between 15 and 23 W/mK. Thermal gravimetry measurements showed their mass gains below 2% at 1200 °C.     

Combined Convective and Microwave Drying of Agglomerated Sand: Internal Transfer Modeling with the Gas Pressure Effect

In order to improve the industrial drying (hot air and microwave) of inserts made of agglomerated sand, a comprehensive internal heat and water transfer model has been proposed. In this model, the internal gas phase pressure effect was made perfectly explicit, especially the phenomena of liquid and vapour transfer by filtration and of liquid expulsion at the surface. This model was validated on the basis of the experimental mean water content and core temperature curves for drying trials at different microwave powers. Then, it was used for comparing the drying time and the internal pressure level calculated for four particular processes: a standard process with high temperature air applied all over the time, a strong process with high power microwaves applied all over, and two processes which alternate the two heating modes. It was demonstrated that the combined and alternative processes provide a real possibility for faster drying with less internal pressure and thus with less cracking risk. The microwaves should be applied only in the first hour of the process and with decreasing power. The decrease of the drying time was around 30% with regard to the hot air standard process.