High hydrostatic pressure (HHP) could be assessed as an unconventional method to ensure the stability of food quality. Pressure not only extends the shelf life of food by partly eliminating microorganisms, but also changes its properties, providing the opportunity to create new food products with innovated structure. Technological advances in fruit processing create the possibility of combining traditional drying with unconventional methods of its preservation. The attractiveness of processed by HHP dried fruit is high quality. The advantages of HHP and the benefits of its application in fruit drying explain the need for an interest in using this method by technologists and manufacturers. The most important is to extend the application in various sectors of the food drying to create new, attractive food products.
The purpose of the food preservation is to eliminate or prevent the growth and development of microorganisms, protect against infection with microbial pathogens and inhibit physical and chemical changes [1,2,3,4,5,6]. For more than two centuries, the most common methods used in the preservation of plant materials have included heat-based physical methods of processing. The continual development of food preservation methods is driven by the reluctance of consumers to the presence of extrinsic chemicals and the concerns related to the use of broadly defined biotechnology [3, 4, 7, 8].
The heterogeneity of the internal structure of fruit and vegetables within single variety, or even single fruit is causing the plant tissue to be a material susceptible to various types of thermal, mechanical or enzymatic processes. Technological treatment changes the structure of the raw fruit by modifying not only enzymatic reactions occurring in the tissue, but most of all, by affecting the conditions of the heat and mass exchange that take place in the plant material [6, 9,10,11,12,13,14,15,16]. Interference in the structure of tissue plant material, using various kinds of pre-treatment, reduces the resistance of mass exchange during processing, leading to a product of high quality. Quality of fresh and processed fruit is correlated with a number of factors, which mainly include the structure of material, type of the processing, pre-treatment and storage of raw materials and products [4, 14, 17,18,19,20,21,22,23,24].
Internal structure of fruit
Food structure is the organization of food constituents at multiple spatial scales and their interactions. In simple terms, different types of food structure are formed when food ingredients are mixed together to make a food product. Insights into food structure and how it changes during processing are essential for producing high-quality food. Structure that is relevant to food properties can be considered at different length scales. At the smallest scale from ∼1 Å to ∼10 nm is the molecular structure which is composed of the constituents such as water, minerals, vitamins, flavor components, fatty acids, lipids and protein monomers. The next level up is the microscopic level: aggregation of molecules and their assembly into components, colloids and networks which are typically measured between nanometers and micrometers (from ∼10 nm to ∼100 µm). A range of classic supermolecular structures in food are casein micelles, milk fat globules, starch granules, biopolymer network/gels such as the gluten structure in dough, pectin gel and other. This type of structure also includes cellular structure and whole cells that forms fruit tissue, fibers that build muscle tissue. The macroscopic level of structure (from ∼100 µm) considers those features that are perceived by human senses, such as texture and taste. They are often associated with specific types of food, e.g., fruit, vegetables, fruit or vegetable purees, meat dishes such as hamburgers and sausages, porous structures such as cakes and biscuits .
Fruit are composed mainly of crumb tissue called parenchyma, occurring in almost all organs of plants where it fills the compartments between other tissues. Parenchyma consists of living, occasionally elongated parenchymatous cells. These lumpy formations of the crumb tissue are usually quite large. Studies show that their diameter ranges from 0.01 to 0.5 mm, and in some cases reaches even several millimeters, such as in the citrus fruit. The crumb tissues play in the fruit essential physiological functions of metabolism and energy conversion; they are also involved in carbon assimilation and transpiration [26, 27].
Parenchyma cell walls are generally thin and made of cellulose. The interior of parenchyma cells is occupied by a large vacuole surrounded by cytoplasm, which lines the cell wall with a thin layer. A characteristic feature of parenchyma tissue is the intercellular spaces providing all the cells with oxygen and allowing gas exchange with the atmosphere. The formation of intercellular spaces is schizogenic, i.e., associated with separation of the cell walls, especially in areas where several cells contact each other. Lysogenic spaces, formed as a result of enzymes action, may also appear, resulting in dissolution of the cell walls and loss of some cells. Intercellular spaces are usually combined in a single system of channels, which play a very important role as means of maintaining the connection between the atmosphere and the cells located in the deeper parts of the plant. They constitute from 1 to even 50% of tissue volume, depending on the type of fruit, the species and variety as well as the degree of ripeness [28,29,30]. Walls, which shape the fruit cells, play a very important role and have many important functions. First of all, provide a mechanical support for cell protoplast and controls protoplasm serving as a barrier that reduced pathogens penetration into the cell and allows the transport of other compounds. The cells walls are permeable to water and small molecules, and do not constitute any obstacle to the transport of these components, in and out of the cell. The task is also protecting the cells from disruption in the event of a significant difference in the concentrations of osmotically active substances between the cell and the external solution [9, 10, 20, 24, 31].
Properties of crumb tissue depend on the kind of fruit, its species and variety. Differences depend also on the degree of maturity of fruit tissue. Strawberries (Fragaria X ananassa) and apples (Malus domestica) are good examples of structure diversity. Strawberries belong to the group of soft fruit which has a delicate structure. Currently, berries are gaining much interest as a functional food and an important part of a healthy diet. Strawberries are one of the most popularly consumed and commercially valuable berries of the Rosaceae family. They are rich sources of vitamins and many health-promoting bioactive compounds. The antioxidant potential of strawberries has been reported as high among various fruit because of the high content of vitamin C and polyphenolic compounds including anthocyanins and ellagitannins. Owing to this fruit’s content of bioactive phytochemicals, increased attention has been given to identifying the potential health benefits associated with its consumption . Studies on the internal structure of the different varieties of strawberries revealed that the most regular in shape and arrangement spaces, in the whole structure of the fruit, identified as cells, are found in a strawberry cultivar Pandora (Fig. 1A). Although cultivars Bounty (Fig. 1B) and Senga Sengana (Fig. 1C) have a much more disorganized internal structure, and shapes of individual cells are more random than those in the cultivar Pandora (Fig. 1B, C). It was observed during visual evaluation of fruit that both cultivars Senga Sengana and Bounty had a more compact structure and fleshy parenchyma .
One of the basic geometric parameters of microscopic images analysis that can highlight the specific differences in the plant tissue of strawberry cultivars is the projection area identified as cells (Fig. 1A, B, C ). Based on the analysis of the distributions of projection area sizes, it was found that the tested cultivars Pandora, Bounty and Sengana Senga had sizes in the range of 0.01–0.07 mm2 (Fig. 1D). In addition, it was observed that more than 88% of the cells in the internal structure of cultivar Senga Sengana had projection area at about 0.01 mm2. However, in the case of the cultivar Pandora and Bounty, distribution of area sizes identified as cells shifted toward higher value, and most of the spaces had the areas in the range of 0.01–0.04 mm2. Single spaces of a size of up to 0.07 mm2 were also found in the internal structure of Pandora cultivar (Fig. 1B, D ) . Projection area sizes and the microstructure presented in the photos from the electron microscope identify the structure of the strawberries, which depends on the variety of fruit. Therefore, geometric parameters can be a good feature to distinguish the varieties of examined fruit, which makes it easier to optimize their use in technological processes to obtain products with repetitive properties, stable quality and acceptable sensory characteristics. However, chromatographic analysis of pectin fractions (at the molecular level) investigated during the ripening of strawberries showed that the structural complexity of pectins decreased during fruit development. Modification of the chemical, biochemical and nanostructured pectin matrix during maturation may contribute to the process of modifying the physical properties of the cell wall, possibly reducing its mechanical strength and ultimately leading to a reduction in the hardness of the fruit .
Apples, unlike strawberries, are fruit available throughout the whole year and are treated as a model, researched plant material due to tissue composition and the ingredients content, which amount is determined by fruit ripeness and their degree of processing. Apples are some of the most ancient and popular fruits in the world. While apples are mostly consumed fresh, they are processed into beverages, jam, jelly, and other forms of foods. Polyphenols are the bioactive compounds, and their stability, bioaccessibility, bioavailability, and the antioxidant and anti-inflammatory effects are affected by many factors. The consumption of apples and its processed products or extracts rich in polyphenols have been linked to reduced risk in cancer, cardiovascular disease, diabetes, and many other diseases although some of them require further confirmation . Storage could change the internal structure of the fruit tissue, which could largely determine the changes occurring during the processing of raw materials, in terms of their structure and physical properties of final product.
The shape of the cells in apples tissue dependents on the function they perform, and their diameter may be range in the 161–265 µm, depending on where they are located. During storage and processing, alterations in the structure can cause physical and chemical changes, which is mainly consequence of shrinkage of the cell walls. The result is a softening of the fruit tissue, due to the enzymatic transformation of apples protopectins, insoluble in water, to soluble pectins [8, 18, 36,37,38,39].
The parenchyma of apples has different content of gases in the intercellular spaces, protoplasm and vacuoles. These values are ranging from 18 to 28% of the volume [31, 40]. The gases enclosed within the plant tissue have a major influence on the fruit processing technology . Heat exchange process is significantly hindered in the material of high porosity due to the low heat conductivity coefficient for gases. They cause significantly prolonged thermal treatment that, in turn, results in increase energy consumption [41, 42]. Apples parenchyma is built from loosely packed cells of the projection area from 0.002 to about 0.071 mm2 (Fig. 2 A, B) , which mainly consists of water and carbohydrates but also starch, lipids, proteins, minerals, and other components [8, 31, 43].
Thermal reactions cause an arrest of vital functions of the protoplasm in living cells, reducing at the same time semi-permeable properties of the cell walls, which results in partial leakage of vacuole content and a decrease turgor. However, it is still a barrier for some substances dissolved in water. The mechanism of this type of changes, related in particular to diffusion processes, is not entirely explored [44,45,46]. Drying evokes changes in cell wall flexibility and the effect of these changes is a significant shrinkage and changes fruit structure . Physical changes in the structure of apples tissue modify its properties from visco-elastic, responsible for the deformations in the final product, to the formation of a rigid structure, resulting from the crystallization of the cellulose present in the cell walls. Stiffening of the structure, as a result of formation of crystalline domains in the amorphous polymer forms of the plant tissue inhibits deformation of the material and significantly affects the structure and mechanical properties of the dried plant material [8, 14, 47,48,49,50].
The structure is a very important trait for various types of fruit, thus the preservation of its proper form, or the possibility of its modification, is one of the aim of all the methods of food processing. The structure is also a major factor influencing the assessment of the quality of fruit. The cell wall polysaccharides consist of mainly of pectins, hemicellulose and cellulose. Pectins are a major building component of the fruit tissue cell wall, providing turgidity and elasticity of the tissue [8, 27, 43, 45, 51].
Physical properties of fruit products largely depend on cell morphology and turgor. Fluid-filled thin-walled parenchyma cells and substances in the extracellular space, containing interstitial fluids or air, determine the response of the structure during technological processing [8, 10, 21, 22, 50, 52,53,54]. Significant loss of elasticity and structural changes could be observed during fruit processing. It occurs not only during the traditional methods such convective drying, but also those classified as new or unconventional, such as high hydrostatic pressure (HHP) [4, 5, 8, 24, 38, 55].
The processability, texture, flavor, and shelf-life qualities of food are controlled not just by chemical composition, but also by how the various ingredients are distributed and interact at the nano and microscopic length scales. Food structures vary enormously from relatively homogenous liquids to complex, multiphase colloidal systems containing fats, proteins, polysaccharides, salts, and water in the form of fibers, droplets, crystals, networks or even air bubbles. The size, shape and distribution of these structures greatly influence product stability as well as sensory properties and bioavailability of nutrients for absorption when food is consumed and digested .
High hydrostatic pressure
HHP treatment significantly improves heat and mass exchange in diffusion processes. This happens regardless of the diversity of plant tissue being processed, and independently of the combination of parameters applied [4, 15, 38, 39, 53, 57]. There are many reasons for the high pressure treatment application in the fruit processing. One of the main obstacle for its widespread use are economic concerns [2, 7, 55, 58, 59].
Safety and quality of fruit products result from their preservation with thermal methods. At the same time, the use of these methods deteriorates the quality of the raw material, which has a further impact on the quality of the final product. Changes caused by high temperature, usually considered as adverse, can be reduced by lowering the heat dose. However, this increases the risk of the product becoming microbiologically unstable. This is the reason for an increased use of food production techniques in the processing of fruit ensuring effective protection against microflora, and to a large extent preserving the structure of processed materials. HHP is one of the methods fulfilling these criteria [41, 55, 60, 61].
Application of HHP in the fruit processing is considered by as one of the alternatives for thermal preservation [4, 7, 24, 62,63,64,65]. Moreover, combination of HHP with other technological processes, e.g., drying, may facilitate desired changes in the raw material. Improving of the sensory and nutritional quality, and prolonging shelf life could be expected [50, 60, 61, 66].
The high-pressure treatment is a preservation method that makes use of an interaction between three basic physical parameters: pressure, temperature and time. However, when food is preserved by means of this technique, the pressure is responsible for the transfer of energy to the entire system. Pressure range considered as high is broad and ranges from 100 to 1000 MPa depending on the application, and the time of material exposure to pressure ranges from fractions of seconds to several minutes [7, 15, 38, 55, 57, 61, 67, 68]. The selection of pressure value time and temperature of the process is dependent on the type of product being preserved. The processing temperature usually does not exceed 50 °C. Pressure propagates in the product at the same time and in all possible directions, which gives an advantage in comparison to thermal methods [2, 15, 53, 55, 61].
An important advantage of the high pressure fruit preservation, as compared with thermal methods, is the absence of temperature gradient between the surface layer and the interior of the product being preserved. Therefore, the thermal effect which may influence the quality are eliminated, while adequate safety is maintained [4, 5, 24, 38, 41, 50, 69]. As a result of HHPs, the temperature of the product increases by approximately 3–7 °C per 100 MPa, which has significant consequences for the process control. To develop the pressurization conditions, the following parameters of the process must be taken into account: temperature, time needed to reach the desired pressure, height and duration of pressure, decompression time, pH, and water activity. Low molecular weight compounds, including flavor compounds, pigments or biologically active molecules, such as vitamins, remain intact during the HHP treatment. Changes in the structure of other components, for example, in proteins, including enzymes, in some cases may limit the suitability of this method as a process of gentle food processing, while in others may be beneficial in shaping the desired properties of fruit products .
HHP of fruit is often accompanied by a reduction in volume, causing in many cases a shift in the equilibrium of some reactions towards more compact or concentrated systems. As a result of these interactions, macromolecules undergo conformational alterations, temperature of phase transition also changes, whereas the chemical reaction rate is increased or decreased, depending on the direction of volume change. The influence of high pressure on hydrogen, ionic and hydrophobic (non-covalent) bonds is ambiguous. They can be disrupted or formed under the action of HHP, depending on volume changes of the system. Therefore, proteins, nucleic acids, starches and pectines, which quaternary structures are formed with these bonds, denature, coagulate or gelatinize. Hence, it follows that the most frequently mentioned consequences of the use of non-thermal preservation using high pressure is mainly the destruction of microorganisms ensuring the proper quality of the food. However, it should be noted that HHP can also cause protein denaturation or can be responsible for their structural modification. HHP processing can either inactivate or activate enzymes, depending both on the level of pressure applied and the type of enzyme. The effect of the pressure is also a change in the fruit properties [53, 55, 58, 61, 67, 68].
HHP acts on the non-covalent binding, thus it has little effect on the chemical components associated with very desirable properties of food. In contrast to the heat treatment, HHP does not lead to a loss of nutritional value, taste, color and vitamin content [7, 53, 67, 68]. However, it was found that HHP of high-porosity food, due to the instability of the air in the fruit tissue, may cause irreversible structural changes. When raw materials with very delicate cellular structure, e.g., strawberry, were processed using the high-pressure, analyses showed a substantial loss of their elasticity and changes in their cell structure that were probably caused by alterations in the conformation of the cells due to the damage of their walls under pressure. Loss of the ability of fruit cells to keep the cell juice in the tissue structure can be a reason for the loss of turgor and ultimately collapse of the structure of the parenchyma, leading to changes in the physicochemical properties of the fruit tissue [4, 8, 24, 50, 53, 70, 71].
The application of HHP on persimmon cubes seems to be positive since it enhances the diffusion of soluble compounds. However, treatment conditions should be optimized to obtain HHP treated persimmon with suitable textural properties. The intercellular spaces of persimmon flesh, occupied by air in the original fruit, were progressively replaced with intracellular liquid due to the tissue degradation caused by the HHP treatments. In this way, HHP favored the diffusion of tannins and other soluble components to the intercellular spaces .
However, the loss of structure stability and change in the properties of the fruit tissue upon high pressure are considerably lower than upon heat treatment. Therefore, the preservation of fruit with a high-pressure technique is considered to be a method that does not significantly affect the desired characteristics of the tissue structure. Cellular damage is primarily caused by the separation of the cell wall, and the benefit is a significant reduction of biochemical changes. Furthermore, Basak and Ramaswamy  determined the textural changes in plant tissue as a temporary loss of structure that allows the tissue to regain turgor when pressure is decreased.
In addition, fruit tissue of different parenchyma structures, resulting from the degree of processing, subjected to pressure in the range of 100–650 MPa changes its technological, biological, chemical and mechanical characteristics. They are highly dependent on the structure as it was shown in the example of many fruit in both fresh [15, 16, 55] and processed [6, 15]. The effect of HHP can vary depending on processing conditions (pressure, hold time, temperature), food form (whole, pieces, juice, puree, mousse or smoothie) and intrinsic factor of food such as pH. Apples pieces (var. Granny Smith and Pink Lady) in pineapple juice subject to 600 MPa resulted in no visible color change during storage. The effect of treatment significantly reduced residual polyphenol oxidase (PPO) activity. High pressure treatment of apples pieces in glucose solution no significantly effect on the structure of the fruit but addition of sodium pyrosulfite helped to prevent browning during the entire storage period. Depending on pH, pressures of 100–700 MPa were needed for the inactivation of apples PPO. There are no only studies on thermal denaturation of PPO, but also on pressure inactivation of PPO for fruit such as grapes, strawberries, apricots and many others [6, 15, 55].
Studies carried out in the world support the theory described by Basak and Ramaswamy  about the instantaneous change in turgidity of parenchyma, but they also suggest that changes in the permeability of intracellular systems are a major cause of degradation of the structure of the fruit subjected to high pressures [38, 39, 50, 53]. Studies on the effect of structural changes resulting from the HHP treatment found that some biochemical changes, especially in pectins, are closely correlated with the structural changes. This is mainly due to the participation of endogenous enzymes such as polygalacturonase or pectin methylesterase, which significantly affect the degradation in the tissue structure of fruit . Molecules of low methylated pectins, produced in the plant parenchyma and polymerized by polygalacturonase, cause significant softening of the fruit [69, 71].
Application of high pressure to fruit and vegetables changes texture due to liquid infiltration and gas displacement. The changes such as collapse of air pockets and shape distortion result in shrinkage. In many cases, pressure-treated vegetables did not soften during subsequent cooking, which was attributed to the action of PME, which was only partially inactivated by pressure. The visual examination of high pressure-processed tomatoes indicated that textural damage increased with an increase in pressure up to 400 MPa. An increase in pressure between 500 and 600 MPa showed less damage, and the tomatoes appeared similar to untreated samples. These visual observations were in good agreement with the instrumental texture (firmness) and cell rupture parameters. The decrease in firmness below 500 MPa was due to the action of the PG enzyme, which hydrolyzed the low methoxy pectin to water-soluble galacturonic acid. This resulted in more free water being seen along with a decrease in firmness .
Drying of plant tissue
In fruit technology, the most popular preservation method is production of the final products due to decreasing water content. Reduction of water content is necessary to receive the products of the appropriate stability, both microbial and chemical. Reduction of water content in the raw material, which is the result of drying, gives the possibility to create quality products and is conditioned by a number of positive and negative effects. The determination of these effects is associated primarily with the sensory evaluation of products in the form of drought, but also the evaluation of the drying process in terms of its duration, which closely affects its energy consumption and cost. At the same time, drying as the technological operation used in fruit processing allows to achieve products with properties that are impossible to obtain in other processes. Particularly noteworthy changes of selected physical properties of the drought, which may include, among others, the reduction of volume, mass, density of the dry matter constituents and changing of structure, which in turn conduce their storage and transport [8, 10, 24, 39, 41].
Modern techniques of drying and the possibility of combining traditional drying with unconventional methods of fruit preservation create new opportunities for the food industry. The main purpose at the present time is to meet the increasingly high demands of the consumer and to provide him safe, comfortable, repeatable products characterized by functional properties and high nutritional value. The attractiveness of the processed products is the combination of freshness with the convenience for the consumer. Using the associated processing methods is necessary to obtain a safe product of high nutritional value and favorable textural features [2, 21, 39, 42, 75].
There are several possible solutions, both scientific and technological, which can meet these requirements. These include, for example, genetic modification and the impact of gamma rays, which are not accepted by demanding consumers. The group of modern and unconventional methods which ensure the quality of food include HHP. The use of HHP pre-treatment allows not only to extend the shelf life of fruit by eliminating microorganisms, but also changes its structure and properties, giving the opportunity to create new products.
The most widely used physical methods of fruit preservation are based on the processes related to temperature changes. Methods consisting in temperature reduction such as cooling and freezing and these based on considerable rise in temperature, such as sterilization and pasteurization are widespread. But the most common methods involve the drying of fruit, when temperature increase removes water, thereby preventing the growth of microorganisms [1, 6, 41]. Although thermal treatments provide a high level of fruit safety, a loss in quality is significant and may diminish product’s attractiveness. The use of high temperature in most fruit results in the loss of fresh material structure, and is causing changes in sensory and nutritional properties [17, 73,74,75]. Taking into account the properties of the raw material, there are no restrictions related to the choice of the types of fruit used as the material for drying. The following characteristics determine the particular choice of the fruit for this type of processing: drying rate, storage stability, the possibility of rehydration and quality, parameters of drying and pre-treatment of the raw material [4, 5, 24, 38, 39, 57, 61].
High temperature and HHP have the same targets. The primary one may be associated with reversible or irreversible losses of membrane integrity yielding a pronounced increase in the permeability of the semipermeable membranes. Nevertheless, action of both parameters may also include membrane bound or membrane spanning proteins, in particular ion pumps or ion channels. Sometimes at any combination of the respective parameters, cell turgor of delicate fruit and vegetables was always significantly declined immediately after the treatments, e.g., red cabbage leaves . This certainly indicates that the initial effects of these treatments yield a pronounced increase in the permeability of the semipermeable membranes. As a consequence, a pronounced efflux of ions out of the symplast occurs, resulting in the breakdown of the osmotic gradient between protoplasts and apoplast. This gradient is, however, indispensable for turgor to exist or to be built up. HHP and thermal processing of strawberries in ethylene vinyl alcohol copolymer cups were evaluated by examining their impacts on texture and nutritional properties (total phenols, total anthocyanins and antioxidant capacity) and color during 45 days of storage at 4 and 25 °C. Samples treated by HHP and stored at 4 °C showed higher hardness, total phenols, total anthocyanins and antioxidant capacity than samples treated by thermal processing and stored at 25 °C. It suggests that HHP treatment did not lead to cell wall depolymerization and loss of flesh hardness of cupped strawberries. As compared to thermal processing treatment, HHP treatment is possible to cause pectin leaching, but the effect was limited, which was probably due to tighter cell structure and inhibition of pectin depolymerisation  .
The appropriate selection of the drying method, its parameters and adequately selected pre-treatment of fruit can significantly improve the quality of dried products [1, 5, 24, 39, 78,79,80]. The processes that allow to achieve high quality of structure include a combination of traditional methods of water removal, i.e., drying, preservation with sugar and salt, together with unconventional techniques such as the microwave treatment [22, 49, 80,81,82], oscillating magnetic fields, HHP [24, 39, 55, 57, 63], pulsed electric field (PEF) [7, 23, 83, 84], ultrasound [21, 85, 86], ultraviolet, heating with radio waves, ohmic heating, pulsed X-ray, infrared and packing in modified atmosphere and aseptic conditions [55, 87, 88]. A common feature of both unconventional and associated techniques is the ability to achieve expected structure without significantly raising the temperature [17, 89]. Furthermore, the processed fruit have a longer shelf life and improved quality, enabling storage [2, 10, 21, 54, 61, 86, 89,90,91].
Analysis of the results indicated that the HHP pre-treatment improved mass exchange parameters and caused a significant increase in water diffusivity during drying, which in turn, increased drying rate (Fig. 3). Due to the use of HHP as a pre-treatment in apples drying, the final products have lower water content, they retain a high degree of nutritional and organoleptic value of the raw material, and their structure is damaged only to a little extent in compare to drying . The use of the HHP pre-treatment reduced the time of convective drying as demonstrated by Yucel et al.  on the example of apples, carrot and green bean.
Distribution analysis of the size of the projection areas of spaces identified as cells, in a tissue of fresh and convectively dried apples, showed that the range of area sizes of these spaces in the tissue of raw apples was from 0.01 to 0.035 mm2, while in the convectively dried apples it ranged from 0.001 to 0.020 mm2 (Fig. 2A, B)  and (Fig. 5) . Although the spaces identified as cells were smaller than the corresponding spaces in fresh apples, projection area in dried apples shifted towards the space of greater cross-sectional area, which indicated mechanical damage of the cell walls caused by significant internal stresses generated during drying. This action caused the disruption of the cell walls and formation of larger spaces limited by the cell wall due to the fusion of several or more cells, which was also evident in the structure (Figs. 2, 5). Statistical analysis confirmed a significant difference in the sizes of the projection area of apple tissue cells [38, 39]. Similar results were obtained by Lewicki and Pawlak  in dried apple tissue.
HHP pre-treatment before drying changes apple tissue structure as a result of the applied pressure and environmental conditions (in the presence of isotonic and hypertonic solutions or without solutions). Also affects the physical properties of the dried products, mainly by changing their structure, density, porosity, and causing greater drying shrinkage (Table 1) [38, 39, 57].
The image of apples tissue HHP pre-treated with 300 MPa pressure was significantly different compared to the images of fresh apples, and the distribution of the projection areas ranged from 0.030 to 0.095 mm2 (Figs. 4, 5). Spaces identified as cells had larger size, and distribution of their surface area was similar to that of dried apples without pre-treatment [38, 39].
HHP pre-treatment during apples preservation reduced convective drying time when the process was carried out without the use of liquid medium in the form of isotonic and hypertonic solutions . The HHP pre-treatment under these conditions improved the mass exchange conditions and has resulted in a significant increase in the drying rate. At the same time, it was shown that the prepared material was characterized by much higher water diffusion during convective drying. The presence of liquid medium during the HHP pre-treatment of apples significantly prolonged the convective drying time of apples, which should be recognized as a technological disadvantage of the applied pre-treatment. At the same time, the pressure changes in the range of 100–500 MPa in isotonic solution (apple juice) and hypertonic (sucrose) had a positive effect on the drying process—the higher the pressure, the shorter drying time.
HHP processing (HPP) is used on industrial scale in many countries, most often to extend the shelf life of the fruit due to the reduction of microbial activity or enzyme activity . It can also be used to modify the functional properties of individual components of a raw material and food product, thereby creating new physical characteristics, mainly structure. The importance of HHP as a pre-treatment prior to other preservation methods, such as fruit drying, has increased [39, 57, 61].
Fruit is a diverse group of raw plant materials with respect to the water content, structure, properties and potential use in processing. Therefore, studies are carried out on an appropriate choice of processing methods, leading to preserve structure of fruit in the least modified form.
Water content and the cellular structure in the plant parenchyma are responsible for the processes occurring in the raw material during processing and storage. The technological processes of food production from raw fruit greatly extend their shelf life and in many cases improve their digestibility and absorption capabilities. Moreover, the volume of water present in the final product determines its structure and ensures the product safety to the potential consumer.
Based on the studies conducted on the internal structure of strawberries: Senga Sengana, Bounty and Pandora, it was found that geometric parameters can be a good feature to distinguish the varieties of examined fruit, which favors the qualification of suitable fruit structure for consumption and technological purposes.
The thermal treatment during processing of plant tissue structure not only causes irreversible changes in their structure, but also alters the properties, and the irreversible loss of nutrients results in a significant loss of the quality of processed fruit. Combining modern (high hydrostatic pressure—HHP) and traditional (drying) means of preservation provides many potential applications in fruit processing. However, for various reasons, it is important to identify the cause and direction of changes of the drying process and the structure of the obtained dried fruit products.
The mechanical disruption of the structure of fresh fruit as a result of the application of HHP treatment accelerates changes in the material. It could be a disadvantage when using this technique before drying. It was also found that treatment with HHP changes the diffusion processes occurring in the plant tissue. The type of fruit used in processing directs its use and optimize the applied parameters.
HHP pre-treatment prior to drying changes the conditions of heat and mass exchange in this process. Damage of apples tissue caused by HHP pre-treatment affects the availability of water in the parenchyma tissue, which in turn, changes the course of the water diffusion during drying. Modification of tissue structure resulting from the applied pressure in the presence of an isotonic and hypertonic solutions, affect the drying process and physical properties of the dried product, mainly by changing their structure, density, porosity, and causing shrinkage.
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The authors declare no conflict of interest. No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the authors or any organization with the authors are associated.
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Janowicz, M., Lenart, A. The impact of high pressure and drying processing on internal structure and quality of fruit. Eur Food Res Technol 244, 1329–1340 (2018). https://doi.org/10.1007/s00217-018-3047-y
- High hydrostatic pressure (HHP)