One of the most gregarious of fruit trees, guava, Psidium guajava L., which belong to the myrtle family (Myrtaceae), is almost universally known by its English common name or its equivalent in other languages. In Malaya, it is generally known either as guava or jambu batu. The guava is native to the Caribbean and common throughout all warm regions of tropical America and in the West Indies. The fruits are oblong to pear shape but because of out-crossing, some elongate to round variants exist. It has a light yellow, pink blushed skin with white, red or salmon-colored flesh. Guavas emit a strong, sweet, pungent fragrance with flavor ranges from strawberry to lemon. Guava fruit has many seeds in the centre though seedless varieties are also available. The fruit weighs 400-600 g on the average, but can reach a weight of 1 kg. Freshly harvested mature or ripe guava fruit is very popular in Malaysia. Guavas like other tropical fruits continue to ripen after harvest and should not be refrigerated unless overripe. The guava bears fruit all year round in Malaysia. The commercial varieties grown in Malaysia usually have light-green skin when ripe, though there are varieties with yellow skins and are smooth with very faint grooves radiating from the stalk end. Guavas are generally sweet which the flesh may be white, light-yellow, pink or salmon; with textures ranging from crunchy to pulpy. Table 2.1 presents the summary of guava fruit characteristics. Guava consists of 90.91% water and 10% solids. Their density is 1050 kg/m3, specific heat is 3.97 kJ/kg°C and thermal conductivity is 0.56 W/m°C. Table 2.2 presents physico-chemical properties of seedless guava fruit. Guava is a great fruit because it has great amount of nutrients such as vitamin C (more than 3 times as much Vitamin C as an orange), vitamins A and B. Guavas are useful sources of nicotinic acid, phosphorous, and soluble fiber. Calcium is typically not found in high amounts in many fruits though it is available in guava fruit. Guavas are sodium free and low in fat (0.2 g/100 ged.p) and calories (42 cal/100 ged.p). They are very good for the immune system and are beneficial in reducing low-density lipoprotein and protecting the heart. Some studies find a lower risk of cancer among people who eat more fruits and vegetables rich in dietary fiber; carotenoids and vitamin C. Table 2.3 shows nutritional values of guava fruit.
According to Krokida and Marinos-Kouris (2003) the main objective of dehydration and drying is to remove water until the water activity is low enough to prevent growth of microorganisms and slow down the rate of biological reactions. The mechanisms of drying involve vaporization of surface water and water movement under capillary forces, diffusion of liquid, and water vapor by using heat that affects physical and chemical properties, which will change the shape, crispness, hardness, aroma, flavor and nutritive value of the fresh produce. Although these terms are often interchangeable, it may be necessary to distinguish as "drying" refers to the process of moisture removal due to simultaneous heat and mass transfer (i.e. thermal drying). Drying as such, refers primarily to the removal of moisture in the vapor phase, whereas dehydration is a more encompassing term and includes methods of moisture removal can be done without heat (e.g. compression, reverse osmosis, filtration, etc.). Jayaraman and Das Gupta (1992) classified drying process of fruits and vegetables in three basic technologies: Solar drying, air drying in batch or continuous mode and sub atmospheric drying such as vacuum and freeze dryer. Low temperature/low energy processes such as osmotic dehydration have recently received more attention. The main drawback in fruit drying is damage to the sensory characteristics and loss of nutritional components due to long periods of drying at relatively high temperatures. These include the loss of aroma volatiles, oxidation of pigments and vitamins and case-hardening of certain products. Case-hardening is a common defect of dried fruits and is caused by drying is too fast compared to the rate of diffusion of moisture in the product. Under these conditions, the outer layers overdry and inhibit moisture diffusion, leaving the interior wet. A variety of novel methods have recently been investigated for drying fruits and vegetables. Among them are vacuum, microwave drying and osmotic dehydration. The application of osmotic dehydration of fruits, and to a lesser extent in vegetables, has gained more attention in recent years as a technique for production of intermediate moisture (IMF) and shelf-stable foods, or as a pretreatment prior to drying in order to reduce energy consumption and heat damage. Osmosis is the movement of water through a semi-permeable membrane of fruit or vegetables. This process occurs when fruit or vegetable immersed in a hypertonic solutions (solution with high concentration of sugar or salt). The driving force for water diffusion is set up because of difference in osmotic pressure of cells and surrounding hypertonic solution (Talens, Escriche, Martinez-Navarrete and Chiralt 2003; El-Aouar, Azoubel, Barbosa and Murr 2006; Rizzolo, Gerli, Prinzivalli, Buratti and Torreggiani 2006; Rodrigues and Fernandes 2007). Osmotic dehydration involves three simultaneous counter-current fluxes of mass transfer (Figure 2.1). The first two are a major flux of water from fruit or vegetable cells to hypertonic solution and in reverse, some solutes are transferred from the solution into the fruit or vegetable cells (Torregiani and Bertolo 2001; Osorio, Franco, Castaño, González-Miret, Heredia and Morales 2007). Since the membrane responsible for osmotic transport is not perfectly selective, natural solutes present in the cells such as sugars, organic acids, minerals, salts, etc. can be leached into the osmotic solution (Lazarides, Gekas and Mavroudis 1997; Matusek, Czukor and Merész 2008). It should be mentioned that solute transfer is usually limited due to differential permeability of cellular membranes. Therefore, the transfer of more water than solute characterizes this process (Bildweel 1979; Garcia, Mauro and Kimura 2007). The cell is the smallest biological unit with the characteristics of living matter, connected to each other by the middle lamella (Figure 2.2). Tonoplast or vacuolar membrane which separates the vacuolar contents from the cytoplasm and, the plasmalemma or plasma membrane which separates the protoplasm from the primary cell wall are two major membranes of the plant cells. In addition, protoplasmic connections, or plasmodesmata, connect the protoplasm of adjacent cells which this continuous network of protoplasm is referred to as symplasm. The fraction of the cell outside the protoplasm forms another continuous network which known as apoplasm. The nature of the protoplasm is generally studied to determine how different substances can penetrate into the living cells. From these studies, the fact that the protoplast of a plant cell is highly permeable to certain substances, but only slightly permeable to others was established. Sugars, such as glucose or sucrose could slowly pass through the protoplasmic membranes. These hypertonic solutions cause plasmolysis due to the greater total concentration it has than that in the cell sap when the cell is in zero turgor. According to Nobel (1983), the cell wall and the plasma membrane are permeable to water and certain solutes. Water will be contained in the cell, under pressure as long as there is a high internal concentration of solutes. This capacity of the cell is measured by its cell potential. The difference between the magnitudes of cell turgor pressure and osmotic potential is the potential of water. Net osmosis occurs if the potential of water inside the cell is not equal to that in the extracellular fluid outside: the process occurs from a region of high water potential to that of a lower water potential at a rate proportional to their difference. The lipid layer of the plasma membrane provides the greatest resistance to osmosis. Thus, its permeability determines the rate of osmosis into or out of the cell as a response to a change in potential of water. Ferrier and Dainty (1977) reported that the main resistance to water flow into the cell is the cell membrane rather than the apoplast; but in some cases, the resistance of the apoplast and its water capacity may contribute significantly to the water potential equilibrium time constant of the tissue. The apoplast is that area in the cell wall and intercellular spaces where water and solutes could move around. Heat and mass transfer gradients during dehydration process produce changes in the chemical, physical and structural characteristics of the plant tissue. The knowledge of these changes are important because they are related to quality factors (Perera 2005) and some aspects of food processing, such as food classification (Rahman 2005), and design of equipment. It is becoming increasingly difficult to ignore the physical changes of fruits and vegetables because of their relationship with quality aspects such as product color and texture. Transparency and color of the fruits and vegetables may change considerably during osmotic dehydration process due to the following facts: Degradation or loss of fruit pigments and development of browning during the process ;Water loss implies not only an increase in the effective pigment concentration, which could enhance selective light absorption, but also an increase in the refractive index in the tissue liquid phase that promotes surface reflection. These two simultaneous effects could lead to divergent effects on colour attributes (especially clarity and chrome), depending on the product; Exchange of a gas phase for external liquid near the sample surface due to the action of hydrodynamic mechanisms (HDM). This exchange induces more homogeneous refractive indexes in the tissues, which promote light absorption against scattering, giving the product transparency. Of these above-mentioned changes, the alteration of fruit pigments or browning is the most drastic for product acceptance, since it implies changes in the characteristic of product hue. Several studies have revealed that these effects can be minimized using by low temperatures and a protective role of sugars on some plant pigments such us chlorophylls and anthocyanins. Different factors that contribute to mechanical properties of cellular tissue are cell turgor, cell bonding force through middle lamella, cell wall resistance to compression or tensile forces, density of cell packaging that defines the free spaces with gas or liquid, and some factors such as sample size and shape, temperature and strain rate. Chiralt et al. (2001) published a paper in which they introduced loss of cell turgor, alteration of middle lamella, alteration of cell wall resistance, establishment of water solute concentration profiles, changes in air and liquid volume fractions in the samples, and changes in sample size and shape as the main changes that are induced by osmotic pretreatment which affecting mechanical behavior of plant tissues. So, on the basis of the above-mentioned changes, the expected changes in mechanical response provoked by osmotic processes will be a decrease in the stress-strain relationship, an increase in the ratio viscous-elastic character and changes in the failure mode. Different process conditions can induce different mechanical behavior to a differing degree, thus affecting sample mechanical response in a different way. In this sense, mechanical damage in cell arrangement such as cell debonding, which is associated with sample deformation when vacuum pulse was applied have reported by Chiralt and Talens (2005). They also reported that long immersion times for kiwifruit caused negative textural effects. In contrast, strawberry samples were observed to behave differently. Mutanda et al. (1998) observed different cellular alteration reached in osmotic dehydration treatments carried out with 65 and 35 °Brix which cell wall alteration and cell debonding is much more evident in samples treated with 65 °Brix sucrose solutions. In addition to the compositional changes initiated by mass transport during osmotic dehydration, different chemical changes have been reported. Many of these changes can be attributed to alterations that were induced in the enzyme system of cells by osmotic stress and the proportion of some enzyme activities, changes in pectic fractions and volatile constituents, and micronutrient flow from the tissue to the osmotic solution. Tovar et al. (2001) reported that osmotic treatment provoked changes in the physiology of a mango slice which resulted in activity of enzymes involved in the metabolic paths. Activities of peroxidase and polygalacturonase increased in osmotically treated cucumber using polyethylene glycol solution, which caused changes in pectin methylation degree and tissue texture. Synthesis of anthocyanins was promoted in cultures of grape cells and strawberry cells when osmotic stress increased due to mannitol or sucrose concentration, depending on the pH. Torreggiani et al. (1998) pointed out that osmotic treatments provoked changes in pectic fractions in strawberry and kiwi tissues which these changes were correlated with induced textural modifications. Several chemical alterations such as changes in the volatile profile, or the development of chemicals (i.e., ethanol or acetaldehyde) associated with changes in respiration paths initiated by application of osmotic treatment. Pfannhauser (1988) carried out an investigation on the behavior of the volatile profile during the osmotic dehydration of a kiwi showed an increase in some esters and a decrease in aldehydes. In another study, Torres et al. (2006) concluded that osmotic treatments promoted changes in the volatile profile of a mango. They observed a decrease in the volatile concentration of 3-carene and nonanal when mango samples were treated using 30 °Brix, especially in PVOD-processed samples. Treatment with 65 °Brix sucrose solution promoted the greatest losses of volatile compounds in all cases. Rizzolo et al. (2006) pointed out that osmotic concentration, the type of osmotic agent and immersion time influenced the volatile profiles of strawberry slices. Influence of the osmotic agent on total volatiles during osmotic dehydration was evaluated based on esters, since esters significantly contribute to the perception of aroma of various strawberry cultivars. As for esters, osmosis in sorbitol caused significant decreases in ethyl and octyl acetates, methyl and ethyl butanoates, and methyl hexanoate and maximum concentration changes after 2h of osmosis for ethyl 2-methylpropanoate for (E)-2-hexenyl acetate. On the other hand, maximum concentration change of ethyl propanoate, coupled with a rapid decrease in ethyl butanoate after 2h of osmosis using sucrose solution was observed. Most of the observed changes at a macroscopic level, are caused by changes occurred at microstructural/cellular level. Therefore, the study of the microstructural changes is an important task in order to understand the changes occurred in the physic-ochemical properties of plant tissue during dehydration. A fresh plant tissue is composed by cells connected one to each other by the middle lamella (Figure 2.4). These cells are in turgor pressure which it gives elastic mechanical characteristics to the plant tissue. The cellulose of the cell wall gives rigidity and strength to the tissue, whereas pectin and hemicellulose of the middle lamella give plasticity and dictate the degree which the cells can be pulled apart during deformations (Lewicki and Pawlak 2003). When plant tissue is submerged in hypertonic solution water will leave the cell by osmosis. As a result different phenomena can be observed during dehydration such as plasmolysis (Figure 2.4b), detachment of the middle lamella, or cell debonding (Figure 2.4c) and cell rupture (Figure 2.4d). Lewicki and Porzecka-Pawlak (2005) draws our attention to loss in the turgor pressure, shrinkage and deformation of cells which is accompanied with plasmolysis during osmotic dehydration of apple. They also mentioned that the detachment of the middle lamella or cell debonding is due to the degradation or denaturation of the components of the middle lamella, as well as to the micro-stresses produced in the cellular tissue because of water removal. This phenomenon has an influence on the mechanical properties of the product. Lewicki and Pawlak (2003) pointed out that cell rupture is related to cell membrane and cell wall degradation and micro-stresses during water removal. Cell rupture leads to the formation of cavities of different size and shape which increases the porosity of the product. Barat and Fito (2001) observed the suction of hypertonic solution into intercellular spaces of apple tissue subjected to osmotic dehydration which cause shrinkage of cells, deformation of the solid matrix and increase in size of the intercellular spaces. In the study on osmotic dehydration under atmospheric and sub-atmospheric pressure, Barat et al. (1998) identified different changes in both intercellular spaces and cells. In osmotic dehydration under atmospheric pressure intercellular spaces become larger and more cylindrical in shape. Cells were shrunken with irregular shape whereas the detachment of plasma membrane from the cell wall was not observed. On the other hand, application of sub-atmospheric pressure caused the separation of plasma membrane from the cell wall and filling the empty space with hypertonic solution. Over the past 30 years, the factors that influence the process of osmotic dehydration were studied extensively. Several factors affect the mass transfer during osmotic dehydration process including raw material characteristics, type and concentration of hypertonic solution, fruit: solution ratio, shape and geometry of sample, temperature, immersion time and possible enhancing method such as blanching, use of vacuum pressure, microwave power, centrifugal force, ultrasonic waves, high electric field pulse and high pressure treatment. The factors are briefly discussed below: Among factors that influence the osmotic dehydration process, the raw material characteristics play a fundamental rule. The great variability between different fruits is mainly due to the compactness of tissue, initial soluble and insoluble solid content, presence of pectin and other cellulosic components, intercellular spaces and enzyme activity . Chilat and Talens (2005) pointed out that mass transport in the bulk of the material is determined by the characteristics of the tissue such as cell size, porosity, tortuosity and permeability of cell membrane. Yamaki and Ino (1992) conducted a survey on mature and immature apples showed that plasma membrane and tonoplast are permeable to sugars. They noted that the permeability to sugars increase during maturation and the increase is much greater in tonoplast than plasma membrane. However, the permeability of plasma membrane is higher than that of tonoplast regardless of the stage of maturation. The selection of a particular osmotic agent and its concentration depends on several factors. The organoleptic characteristics, solubility, cost, molecular mass and lowering capacity of the compound on water activity are the most important factors. The two most common solute types used for osmotic treatments are sugars (mainly with fruits) and salts (with vegetables, fish, and meat), with relevance for sucrose and sodium chloride, which show advantages already described by several authors. Several studies have revealed that the type of osmotic agent and solution concentration have a significant effect on weight reduction, water loss, solids gain and water activity during the osmotic dehydration. In this regard, Matusek et al. (2008) compared the effect of Fructo-oligosaccharides (FOS) and sucrose as osmotic agents in osmotic dehydration of apple cubes. They reported that osmotic behavior of fructo-oligosaccharides differs from sucrose due to the higher molecular size which could decrease the rate of solute diffusion. Glucose as osmotic agent is utilized by many authors for several food products in orderto obtain higher amounts of water loss and solid gain than sucrose. Torreggiani (1993) and Mandala (2005) reported that the low molecular weight sugar such as glucose due to the high rate of penetration can increase the absorption of sugar during dehydration process. It is obvious since low mass sugar used as osmotic agent the main effect of process instead of dehydration is a solid enrichment. In agreement with Torreggiani (1993), Argaiz, López-Malo, Palou and Welti (1994) found that glucose has a more intensive effect on water activity depression than polysaccharides such as sucrose and maltodextrines at same moisture content. In summary, sucrose can be recommended as one of the best osmotic agents due to its low cost, molecular weight and size, effectiveness, convenience and desired flavor, especially when the osmotic dehydration is used before drying. The presence of this sugar on the surface of the dehydrated sample is a barrier to contact with oxygen, thereby reducing the oxidative reactions. Temperature is one of the most important factors influencing osmotic dehydration process. The temperature has two different effects during dehydration: (1) it enhances diffusion rates; (2) it can disrupt the integrity of the cellular material when it exceeds a critical value. Kinetics of mass transfer during osmotic dehydration process is a temperature-dependent phenomenon. Higher process temperatures generally conducive to rapid loss of water through swelling and plasticizing of cell membranes and in highly concentrated solution, the diffusion rate of both water and solute increase through decreasing viscosity of the osmotic solution. The effect of temperature on kinetics of water loss, sugar gain and weight loss during osmotic dehydration of fruits and vegetables has been widely investigated by several authors. They concluded that the kinetics of mass transfer increases with the temperature during osmotic dehydration process. El-Aouar et al. (2006) suggested that as uptaking of low amount of sugar is favored, low values of solids gain (<10%) can be obtained using temperature of the osmotic medium and contact time in their lowest levels, with the concentration in its highest level. Worthy of note, there is a temperature limit, perhaps 50-60°C, which the cell membrane of the plant tissue is damaged. Ponting et al. (1966) and Videv et al. (1990) pointed out that temperatures above 50 °C caused internal browning and loss of fruity flavor in apple slices. Ratio of sample to solution can express the mass of solution required per unit mass of treated fruit or vegetable. On a lab and industrial scale, the ratio must be as low as possible to restrict plant size and the costs of solution regeneration. Numerous studies are carried out using a large excess of solution in the range of 10:1-30:1 to ensure negligible variation in the solution composition, which makes the interpretation and modeling easier. A number of studies have found that the rate of water loss and solid gain in the osmotic dehydration is directly related to the proportion of sample to solution . As expected, higher ratio offered a higher water loss and higher sugar gain. Immersion time seems to affect the rate of mass transfer and the behavior of membranes during osmotic dehydration. It was reported that the most important changes in water and solids in treated material takes place within the first three hours of the process. The high rate of mass transfer in the beginning of the osmotic dehydration process is due to the large osmotic driving force between fresh fruit and surrounding hypertonic solution. On the other hand, the rapid loss of water and uptake of solids near the surface in the beginning of the process might have been led to the formation of high solids basement layer, which interfered with the concentration gradient across the product-solution interface and acted as a barrier against removal of water and absorption of solids. Similar results were also reported by Hawkes and Flink (1978) and Lenart and Flink (1984) for osmotic dehydration of apple and potato. Baroni and Hubinger (1997) concluded that as time passes, the cell membranes loss their effective barrier for the solute migration, which is then free to penetrate into all parts of the cell. A considerable amount of literature has been published on the various application of the osmotic technique to obtain several kinds of fruit products such as minimally processed or intermediate moisture fruits, or into their application as a pre-treatment in air drying, appertization, extraction of juices or freezing. Osmotic dehydration gains more popularity to produce dehydrated fruits, vegetables and candy due to the fact that energy is becoming expensive globally. Dehydrated papaya is produced in small and medium scale industries in Asian countries like India and Thailand which mainly used in products like fruit bread, desserts, cakes and ice-creams. Candy and dried fruits are produced from other fruits like apple, banana, and jack fruit. Candied ginger obtained by osmotic dehydration is also a popular product. Despite its application, it is necessary to review the advantages of osmotic dehydration. Advantages of osmotic dehydration include: Moisture is efficiently removed from food products without the water going through a phase change; Osmotic dehydration can reduce the overall energy requirement for further drying processes of the product such as hot-air drying and freeze drying; Osmotic dehydration can be used to overcome some of the undesirable effects of heat on the semi-permeable characteristics of cell membranes and quality (color and texture) of food products; The osmotic dehydration process as a pretreatment aids the inhibition of enzymatic reactions. Certain draw backs and difficulties are associated with osmotic dehydration, that need to be studied to improve its efficiency are listed below. High viscosity of the osmotic solution increases resistance to mass transfer; The labile nature of foods will not permit increases in agitation levels beyond a point to overcome this viscosity effect; The low density difference makes the product float requiring needing an additional mechanical means to keep it immersed in the solution; The major constraint for the industrial adoption of osmotic dehydration is the cost of osmotic solution that necessitates a proper means of its recycling. During osmotic dehydration, the solution gets diluted and acquires flavour, and colour of the food. Care should be taken to minimize these so that product quality is not affected by recycling the osmotic solution; The osmotic solution has to be concentrated in order to be recycled which can be achieved through concentration by evaporation and /or by the addition of solute. The processing steps involved in recycling of spent osmotic solution still remain proprietary in the form of patents; Soluble solid leaching and extensive solids uptake are other major draw backs of osmotic dehydration process. Solute uptake and leaching of valuable product constituents often lead to substantial modification of the original product composition, with a negative impact on sensory characteristics and nutritional profile; The large solute uptake causes additional resistance to the mass transfer of water and leads to a lower dehydration rate in complementary drying; The osmotic pretreatment is time-consuming ; Further, there are still some constraints in scaling up osmotic dehydration technology.
