Effect of Drying on Microstructure of Plant Tissue

Drying causes many chemical and physical changes in plant tissue. Physical changes are mostly due to stresses developed in the tissue and are pronounced by macro- and microalterations of size, shape and internal structure. The extent and direction of these changes depend on the mode of drying. Convective drying of apple tissue results in numerous breaks of cell walls and formation of many microcavities. In consequence, the dominating cross-sectional area of cells moves toward values smaller than those observed in raw apple. Cavities formed during convective drying can be pictured in 2D plane as pentagons joined together or ellipses with large ratio between axes. Puff-drying forms porous structure with cavities twice as large as in raw apple. The process homogenizes the structure, and spread of cell sizes is even smaller than that observed in raw apple. Broken cell walls are clearly visible in microphotographs of sliced samples. Many large cavities with cross-sectional area exceeding 40,000 µm² are characteristic of puff-dried apple while such areas are hardly found in raw apple. The structure of freeze-dried apple is similar to that formed during puff-drying. Large irregular cavities and broken cell walls are clearly seen in microphotographs. However, larger cavities are present in freeze-dried apple than in puff-dried one. Cross-sectional area of cavities in freeze-dried apple can be simulated by pentagons joined together or elongated ellipses.     

SNIPPETS ON DRYING

Abstract:

Potato v. Irga was subjected to blanching, and thereafter was dried by convection, puff-drying, and freeze-drying. Microstructure of raw, blanched, and dried tissue was analyzed under the light microscope using computer image analysis. It was found that the tissue of the investigated variety is built up from cells much smaller than those described in literature for other cultivars. Blanching caused starch gelatinization and increase of cross-sectional area of cells. There was no evidence of broken cell walls. Convective drying resulted in cell shrinkage and some breaking of cell walls. It was estimated that some 12% of cells lost integrity during drying. Limited disintegration of the tissue caused by convective drying is attributed to small size of cells, large contact area, and high cohesive forces between cells. Puff-drying damaged the tissue much more than the convective drying. The most devastating to tissue microstructure was freeze-drying, probably freezing per se.     

Effect of Mode of Drying on Microstructure of Potato

Potato v. Irga was subjected to blanching, and thereafter was dried by convection, puff-drying, and freeze-drying. Microstructure of raw, blanched, and dried tissue was analyzed under the light microscope using computer image analysis. It was found that the tissue of the investigated variety is built up from cells much smaller than those described in literature for other cultivars. Blanching caused starch gelatinization and increase of cross-sectional area of cells. There was no evidence of broken cell walls. Convective drying resulted in cell shrinkage and some breaking of cell walls. It was estimated that some 12% of cells lost integrity during drying. Limited disintegration of the tissue caused by convective drying is attributed to small size of cells, large contact area, and high cohesive forces between cells. Puff-drying damaged the tissue much more than the convective drying. The most devastating to tissue microstructure was freeze-drying, probably freezing per se.     

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.     

EFFECT OF PRETREATMENT AND DRYING ON THE QUALITY OF OYSTER MUSHROOMS (PLEUROTUS OSTREATUS)

Fruiting bodies of oyster mushroom were dried by hot-air drying, vacuum-drying and freeze-drying in order to compare food qualities after drying. Prior to drying, mushrooms were subjected to blanching or dipping in sodium metabisulphite solution (1 or 5 g/L), or dipping in citric acid solution (1 or 5 g/L). Drying of raw mushrooms was taken as a control. Blanching reduced the attractiveness of dry mushrooms; sodium metabisulphite improved it. It has been found that pretreatment and drying method affect the course and rate of drying. Samples subjected to hot-air drying and vacuum-drying were darker than those freeze-dried, which were clearly more attractive. The hot-air and vacuum-dried mushrooms on rehydration were inferior in quality to the freeze-dried samples. Flavor of the freeze-dried mushroom was not significantly different from that of the hot-air dried mushrooms. Food quality of dried mushrooms depends significantly on the type of drier used.     

Evaluation of the usefulness of serial combination processes for drying of apples

Abstract

Serial combination drying processes are currently studied as alternatives to conventional drying processes. Compared to freeze-drying (FD), serial combination drying appears to be faster and less expensive, while providing better product quality than hot-air drying (HAD) and microwave-vacuum drying (MVD) as previously demonstrated for carrots. Using the example of carrots, it has also been shown that the drying front moves radially outwards over the course of FD. This unexpected behavior was suggested to originate from the carrots’ heterogeneous structure. It was hypothesized that apple pieces behave differently. Here, this hypothesis was proven by using micro-computed tomography measurements of partly freeze-dried apple pieces. In order to improve the drying process of apple pieces, several single and combination drying processes were carried out. Processes were evaluated by using drying time and sample quality as relevant parameters. Sample quality was determined by analyzing the 3D-structure, rehydration behavior, color, and ingredient retention. Results showed that single MVD is a well suitable drying technique for apple pieces, producing dried products of equal quality to FD. Different from carrots, serial combinations are not necessary to improve the quality of dried apple pieces. Nonetheless, especially a combination of HAD and MVD was useful to obtain specific structures such as puffed pores that did not result from single MVD.     

CHANGES OF RHEOLOGICAL PROPERTIES OF APPLE TISSUE UNDERGOING CONVECTIVE DRYING

The aim of this work was to investigate rheological properties of apple cubes undergoing convective drying. Compression-relaxation test was used to follow changes of properties under investigation.

Raw apple appeared to be very heterogeneous material from the rheological point of view. The relationship between stress and strain was concave downwards and the concavity increased until water content of 2·5 g/g d.m. was reached. Then the relationship begun to straighten and for dry material developed stress was linearly dependent on strain. The resistance of deformation decreased with decreasing water content. Analysis of relaxation showed that the rate of relaxation increased with decreasing water content and the unrelaxed stress was smaller the lower was the water content.

Apple cubes undergoing drying are pictured as material composed of three compartments with different rheological properties. The outer layer is dry, inelastic and difficult to deform. Underneath is moist material with no turgor and relatively easy to compress, and the core with properties of raw apples. During drying the second layer grows in expense of the core and porous structure with many voids filled with air is formed. Share of each compartment in the volume of apple cube undergoing drying affects average rheological properties of the material.     

EFFECT OF PRETREATMENT AND DRYING ON THE QUALITY OF OYSTER MUSHROOMS (PLEUROTUS OSTREATUS)

Fruiting bodies of oyster mushroom were dried by hot-air drying, vacuum-drying and freeze-drying in order to compare food qualities after drying. Prior to drying, mushrooms were subjected to blanching or dipping in sodium metabisulphite solution (1 or 5 g/L), or dipping in citric acid solution (1 or 5 g/L). Drying of raw mushrooms was taken as a control. Blanching reduced the attractiveness of dry mushrooms; sodium metabisulphite improved it. It has been found that pretreatment and drying method affect the course and rate of drying. Samples subjected to hot-air drying and vacuum-drying were darker than those freeze-dried, which were clearly more attractive. The hot-air and vacuum-dried mushrooms on rehydration were inferior in quality to the freeze-dried samples. Flavor of the freeze-dried mushroom was not significantly different from that of the hot-air dried mushrooms. Food quality of dried mushrooms depends significantly on the type of drier used.

     

Drying of Tomato Pretreated with Calcium

Abstract

Pieces of tomato pericarp were pretreated by soaking in CaCl₂ solution before convective drying. Drying was interrupted at prescribed water contents and volume of the piece, its area, perimeter and shape factor were measured. It was found that pretreatment cause shrinkage of tomato pieces by some 15%. Drying causes further shrinkage, which in both nontreated and pretreated tomato is larger than the volume of evaporated water. Microscopic examination showed that pretreatment of tomato with calcium ion causes larger tissue structure alterations in comparison to those observed in nontreated tomato. Calcium concentration gradients cause variable resistance of the tissue to deformation. During dehydration tissue is torn apart and large cavities are formed. It results in faster drying of pretreated tomato in comparison to drying of nontreated tomato.     

Effect of Calcium Ion and Microwave Power on Structural and Quality Changes in Drying of Apple Slices

Effects of impregnated calcium ion and microwave (MW) power on texture, rehydration, shrinkage, color, and other selected indexes of freeze-dried apple slices were investigated. Apple slices were dried by freeze drying and microwave freeze drying (MFD) separately. Vacuum impregnated (VI) and non-vacuum-impregnated apple slices were dried at various microwave power levels. Non-vacuum-impregnated apple slices were found to preserve their mechanical structure better than the VI-treated ones. Microwave application resulted in decreased protopectin fraction as well as total pectin content. As microwave power varied from 1.2 to 2.0 W/g, the total pectin content decreased from 0.810 to 0.521 (expressed as g galacturonic acid/100 g fresh sample). The effect of microwave power in decreasing the drying time was significant. The color of MFD apple slices varied to a minor extent with MW power level over the range of parameters studied.     

Drying of Tomato Pretreated with Calcium

Pieces of tomato pericarp were pretreated by soaking in CaCl₂ solution before convective drying. Drying was interrupted at prescribed water contents and volume of the piece, its area, perimeter and shape factor were measured. It was found that pretreatment cause shrinkage of tomato pieces by some 15%. Drying causes further shrinkage, which in both nontreated and pretreated tomato is larger than the volume of evaporated water. Microscopic examination showed that pretreatment of tomato with calcium ion causes larger tissue structure alterations in comparison to those observed in nontreated tomato. Calcium concentration gradients cause variable resistance of the tissue to deformation. During dehydration tissue is torn apart and large cavities are formed. It results in faster drying of pretreated tomato in comparison to drying of nontreated tomato.     

Effects of preparation processes on the structure and properties of collagen gel

In this study, the effects of collagen self-assembly, drying methods, and reswelling on collagen gel structure and biological properties were determined. The results showed that the self-assembly of collagen improved the regularity and pore size of the collagen gel and promoted collagen gel thermostability and cell proliferation. After drying and reswelling, the main structure of the self-assembly gel was composed of lamellar gel walls, but the gel structure shrank to some extent and pore size was reduced. The structure of the self-assembled gel after drying and reswelling was closely related to that of the drying method.     

CHANGES OF RHEOLOGICAL PROPERTIES OF APPLE TISSUE UNDERGOING CONVECTIVE DRYING

ABSTRACT

The aim of this work was to investigate rheological properties of apple cubes undergoing convective drying. Compression-relaxation test was used to follow changes of properties under investigation.

Raw apple appeared to be very heterogeneous material from the rheological point of view. The relationship between stress and strain was concave downwards and the concavity increased until water content of 2·5 g/g d.m. was reached. Then the relationship begun to straighten and for dry material developed stress was linearly dependent on strain. The resistance of deformation decreased with decreasing water content. Analysis of relaxation showed that the rate of relaxation increased with decreasing water content and the unrelaxed stress was smaller the lower was the water content.

Apple cubes undergoing drying are pictured as material composed of three compartments with different rheological properties. The outer layer is dry, inelastic and difficult to deform. Underneath is moist material with no turgor and relatively easy to compress, and the core with properties of raw apples. During drying the second layer grows in expense of the core and porous structure with many voids filled with air is formed. Share of each compartment in the volume of apple cube undergoing drying affects average rheological properties of the material.     

Application of 3D imaging and analysis techniques for the study of food plant cellular deformations during drying

In this investigation, novel 3D imaging and image-analysis tools have been used to observe the deformations of food-plant tissues and single cells during convective air drying at 70°C. A comprehensive investigation was performed to qualitatively and quantitatively analyze the shrinkage and surface wrinkling of Royal Gala apple parenchyma cellular structure during drying. To study the cellular morphology, 3D contour maps produced by a novel 3D image and surface analysis tool, “Nanovea Expert 3D” were used. ImageJ software was used to quantify the single cell morphological characteristics. During the study, each tissue was observed continuously for the gradual morphological alterations. It was evident that there is a significant reduction of surface roughness during the drying process. In the case of individual cells, the area reduced approximately by 20% and diameter by 11%. This study provides conclusive proof that 3D contour maps and images combined with the 2D microscopic images could be a highly valuable source of information in producing data for the validation of 3D computational plant tissue drying models and simulations.     

Effect of Immersion Time in Osmosis and Ultrasound on Papaya Cell Structure during Dehydration

The effect of ultrasound-assisted osmotic dehydration applied at atmospheric pressure for different lengths of time on papaya tissue structure was evaluated. Ultrasound induced the loss of cellular adhesion, formation of large cell interspaces, and light rupture of the cell walls. The changes in the tissue structure caused by ultrasound application increased sugar loss, water loss, and effective water diffusivity. Ultrasound-assisted osmotic dehydration induced a gradual distortion in the shape of the cells, loss of cellular adhesion, and the formation of large channels caused by rupture of the cell walls. The changes caused by the application of osmotic dehydration resulted in high water loss and sugar gain.     

Combined Microwave-Vacuum and Freeze Drying of Carrot and Apple Chips

A combination of microwave-vacuum (MWV) drying and freeze drying was investigated as potential means for drying carrot and apple chips. The sample was first dried by microwave-vacuum to dehydrate some amount of internal free water and then by freeze drying to a final moisture content of less than 7% (wet basis). Chemical properties (carotene and vitamin C retention) and physical properties (shrinkage, color, texture, and rehydration ratio) of carrot and apple slices dried by this method were evaluated and compared with those dried by freeze drying alone, MWV drying alone, and conventional hot air drying, respectively. The comparison showed that the carotene retention of carrot slices and the vitamin C retention of apple slices dried by the current method were close to those of freeze-dried carrot and apple slices and much better than those of conventional hot air–dried ones. The samples prepared by the current method exhibited very close rehydration capacity, color retention, and texture with those of the freeze-dried ones but with a little higher shrinkage. However, the samples still showed the attractive external appearance without marked warp.     

Combined ultrasound and infrared assisted conductive hydro-drying of apple slices

Abstract

Conductive hydro-drying also known as Refractance Window drying is a relatively new drying technology, which uses hot water to carry thermal energy to materials to be dehydrated. It has a high retention of heat sensitive quality parameters (vitamins, antioxidants, and color) with better energy efficiency than freeze-drying as well as many other conventional drying methods. A new ultrasound and infrared assisted conductive hydro-dryer (UIACHD) was developed to increase drying rate while reducing required hot water temperature and increasing the drying material thickness. The goal of this study was to evaluate the performance of the new dryer and to compare the performance of a pilot scale continuous UIACHD with a freeze-dryer and a cabinet dryer in drying apple slices. The physiochemical characteristics of the dried apple slices including flavonoid content, total phenolic compounds, antioxidant activity, vitamin C content and color were measured. In addition, the energy consumption and energy efficiency of the dying methods were evaluated. Results showed that combining ultrasound and infrared with conductive hydro-drying can result in higher drying rates and lower product moisture content. Quality of UIACHD dried apple slices was close to the freeze-dried products and it was significantly better than the cabinet dried products. Moreover, the energy efficiency of UIACHD was considerably better than the cabinet dryer and the freeze-dryer. The results of this study showed that combining ultrasound and infrared with conductive hydro-drying can lead to an energy-efficient process with good quality retention ability.     

Mild enzymatic isolation of mannan and glucan from yeast Saccharomyces cerevisiae

An enzymatic procedure to isolate polysaccharides from the yeast saccharomyces cerevisiae was developed which would both be environmentally friendly and preserve the native structure of the polymers. Two proteases and three enzyme cocktails were employed. To isolate the cell wall polysaccharides, the cell walls were first separated from the internal components by mechanical cell disruption. The quality of cell disruption was investigated for four types of glass beads with diameters from 0.25 – 1.5 mm and monitored via the inner cellular proteins released. With large glass beads (Ø = 0.75 – 1.5 mm) both brewer's and baker's yeast cells exhibited more than 90% cell disruption after as little as 12 min, whereas disruption was still incomplete with smaller beads. The isolated cell wall material was purified on microporous membranes, freeze-dried and then incubated with enzymes. Proteases caused protein lysis in the outer cell wall and released soluble mannan. The insoluble glucan was separated by centrifugation and the mannan was purified by dialysis or chromatography. In the digestion with proteases, complete conversion was achieved at concentrations higher than 240 mg g^–1 of cell wall material within 26 h. Cell walls from baker's yeast had to undergo a preliminary extraction with petroleum ether. Complete conversion was only possible with one of the enzyme cocktails investigated, and this was achieved after as little as 6 h for cell walls from both baker's and brewer's yeast. The activity of the two other enzyme cocktails investigated was weaker, with brewer's yeast cell walls generally reacting more readily than those of baker's yeast.     

Effect of Drying Methods on the Quality of the Essential Oil of Spearmint Leaves (Mentha spicata L.)

Drying is one of the primary processes involved in the manufacture of herbs, which themselves come in the form of stems, leaves, and roots. The quality of the final product depends on the procedure used and drying method. This article presents the results pertaining to the drying behavior of spearmint in both hot air drying and freeze drying conditions. Conventionally, herbs are dried at high temperatures (40–45°C), which results in a deterioration of product quality. The characteristics of vacuum freeze–dried herbs were studied, and the quality of the freeze-dried products was assessed. It was found that drying time and essential oil content were strongly influenced by chamber pressure. Higher chamber pressure tended to lengthen the drying time but preserved the major volatile compounds of spearmint in the final product. The quality of the freeze-dried product was assessed as being lower than that of the raw material but higher than that of a convectively dried product. Four different mathematical models were fitted to the drying data. A water absorption test confirmed that the rehydration ratios of vacuum freeze–dried mint leaves were higher than those that were convectively dried.     

FREEZE DRYING CHARACTERISTICS OF MEDICINAL HERBS

The demand on traditional herb medicine shows a tremendous increase. Conventionally, medicinal herbs are dried at high temperatures, causing quality deterioration. The freeze-drying characteristics of medicinal herbs were studied, and the quality of the freeze-dried products assessed. The herbs studied were medicinal ginger and Javanese pepper.

It was found that drying time, was more influenced by chamber pressure and freezing rate than by the surface temperature of the product. Higher chamber pressures and faster freezing rates tended to shorten the initial (primary) drying time but lengthen the secondary drying time. Quality of the freeze-dried product was assessed as slightly lower than the quality of the raw material, but higher than when oven dried at 35-40°C, and met the MMI standard.