Osmotic Dehydration of Tropical Fruits and Vegetables

Because of the microstructural complexity of plant tissue, osmotic dehydration cannot simply be explained as a pure osmotic process in which cell membranes act as a semipermeable barrier allowing water to pass through. Instead, osmotic dehydration is considered a process in which many simultaneous mechanisms, acting at different levels, are responsible for mass transport. Different compositional and structural profiles are induced in fruits and vegetables, depending on process variables and the tissue microstructure. Compositional-structural profiles that are developed with gas-liquid exchanges in the tissue during osmotic process have a significant impact on physical (optical), textural and chemical properties (e.g., flavour profile) of the final product, which is in part influenced by the differences in the number of cells that are altered and unaltered during the treatment. This review focuses on changes in the physical, chemical, and cellular structure of fruits and vegetables, some technologies commonly applied to increase mass transfer during osmotic dehydration (OD), potentials and industrial applications of OD, and the challenges of osmo-drying technology.     

Impact of Thermal Treatment and Storage on Color of Yellow-Orange Cactus Pear (Opuntia ficus-indica[L.] Mill. cv. 'Gialla') Juices

ABSTRACT:  The impact of heating at 75, 85, and 95 °C during 60 min on overall color and betalain retention of yellow-orange cactus pear ( Opuntia ficus-indica [L.] Mill. cv. 'Gialla') juice after a 24-h color regeneration period was investigated. The addition of 0.1% isoascorbic acid prior to heating minimized color alteration as well as betalain degradation to a significant level. A correlation between the total betaxanthin retention and the indicaxanthin/isoindicaxanthin ratio was detected in heated juice samples, thus proving the indicaxanthin/isoindicaxanthin ratio as a valuable indicator of thermally treated cactus pear juices. Notably, the same ratio was instrumental for retrospectively calculating the initial betaxanthin content. Additionally, storage experiments over a period of 6 mo were conducted. Samples kept in the dark exhibited notably better color and pigment retentions as compared to illuminated samples. A significantly stabilizing effect was also observed in samples with isoascorbic acid fortification. While indicaxanthin and betanin were predominantly degraded through hydrolytic cleavage upon both heat exposure and storage, isomerization was only marginal for betanin. In contrast, 2-decarboxy-betanin formation was exclusively found for nonfortified heated samples while decarboxylated indicaxanthin could not be detected.     

Modification of Transverse NMR Relaxation Times and Water Diffusion Coefficients of Kiwifruit Pericarp Tissue Subjected to Osmotic Dehydration

The objective of the present study was to evaluate cellular compartment modifications of kiwifruit (Actinidia deliciosa) outer pericarp tissue caused by osmotic treatment in a 61.5 % sucrose solution, through the quantification of transverse relaxation time (T ₂) and water self-diffusion coefficient (D _w) obtained by low field nuclear magnetic resonance means. Raw material ripening stage was taken into account as an osmotic dehydration (OD) process variable by analyzing two different kiwifruit groups, low (LB) and high (HB) °Brix. Three T ₂ values were obtained of about 20, 310, and 1,250 ms, which could be ascribed to the proton populations, located in the cell walls, in the cytoplasm/extracellular space, and in the vacuoles, respectively. According to T ₂ intensity values, vacuole protons represented between 47 and 60 % of the total kiwifruit protons, for LB and HB kiwifruits, respectively. The leakage of water leading to vacuole shrinkage seemed to cause concentration of solutes, retained by the tonoplast, making the vacuole T ₂ value decrease along the OD. As expected, the D _w values of raw kiwifruits were lower than the value of the free pure water. The water mobility (and hence D _w), depending on the kiwifruit distinctive cellular structures and solutes, decreased even more during OD due to water loss and sugar gain phenomena. D _w represents an average value of the diffusion coefficient of the whole kiwifruit tissue protons. In order to obtain D _w values specific for each cellular compartment, a multiple component model fitting was also used. According to these results, the vacuole water self-diffusion coefficient (D _w,v) was much higher than D _w.     

Physico-Chemical and Mechanical Properties of Apple Disks Subjected to Osmotic Dehydration and Different Drying Methods

The effect of sucrose infusion (SI) pretreatment and dehydration methods (freeze and air drying) on physical and textural properties of apple disks were analyzed. Dried samples were humidified between 11% and 43% relative humidity (RH) at 20 °C. Control samples (air- and freeze-dried) behaved similarly regarding water sorption and glass transition temperature. SI process caused important changes in the water sorption behavior of air-dried samples. Nuclear magnetic resonance relaxation times values (T ₂) for freeze-dried apples were higher than those for air-dried samples. Samples subjected to previous SI always presented lower T ₂ values because they had lower water contents. The dehydration method also affected the mechanic behavior. Air-dried samples exhibited higher F _max values during puncture assay than those obtained for freeze-dried samples. SI samples showed higher F _max values for both drying methods. The crust formed during air drying generated crispier materials along the whole RH range, while freeze-dried matrices were more deformable with the increase in RH. SI pretreatment also allowed diminishing browning development. The results obtained are useful in the choice of processing technologies of organoleptically acceptable dehydrated fruits for direct consumption or for their incorporation into compound foods.     

Mass Transfer Modeling and Shrinkage Consideration during Osmotic Dehydration of Fruits and Vegetables

During osmotic dehydration of fruits and vegetables, as water and/or other substances are removed from the material, shrinkage follows depending on the extent of net mass loss. Mass transfer is usually predicted through modeling. However, common models developed for osmotic dehydration of fruits and vegetables make assumptions that often deviate far from reality, including large heterogeneity, variability and complexity in properties of fruits and vegetables. This generates some skepticism about such models and minimizes their potential industrial reliability. This paper reviews osmotic dehydration of fruits and vegetables through a basic approach, provides a critical view on modeling and points out the factors that affect shrinkage and mass transfer based on an extensive evaluation of pertinent literature.     

Study of the Pseudo-equilibrium During Osmotic Dehydration of Apples and Its Effect on the Estimation of Water and Sucrose Effective Diffusivity Coefficients

The study of the equilibrium is needed not only for modeling of the osmotic process as a unit operation but also for a better understanding of the mass transfer mechanisms involved in this kind of systems. A true equilibrium state usually takes very long time to achieve; therefore, a pseudo-equilibrium state is often employed. Experimental pseudo-equilibrium states for water loss and solid gain during the osmotic dehydration of apple slices (5?×?50?×?50?mm³) at different osmotic syrup concentrations (30%, 40%, 50%, and 60% (w/w) of sucrose) were evaluated. Four empirical mathematical methods (Slopes, Azuara, Zugarramurdi and Lupín, and Equal concentration) were used in order to calculate the pseudo-equilibrium values obtained and then to compare them against to the experimental ones. Additionally, the effective diffusion coefficients for water and sucrose were calculated by using those pseudo-equilibrium values. Experimental pseudo-equilibrium values increased with concentration of the osmotic syrup, ranged between 24% and 56% for water loss and 11% and 28% for solid gain; the predicted pseudo-equilibrium values followed the same trend. The decreasing order of accuracy for pseudo-equilibrium values and effective diffusion coefficients, among the methodologies evaluated, was Equal concentration method > Azuara method > Slopes method > Zugarramurdi and Lupín method. Although the Equal concentration method has no theoretical accuracy, it is independent of the kinetic data presenting a higher advantage over the other three mathematical methods evaluated.     

国内外果蔬渗透脱水的研究进展

阐述了果蔬渗透脱水的定义和技术特点,介绍了渗透脱水在国内外的研究和应用现状,探讨了该技术在生产应用中存在的问题和应用前景。     

Heat Treatment at 38 °C and 75–80 % Relative Humidity Ameliorates Storability of Cactus Pear Fruit (Opuntia ficus-indica cv “Gialla”)

Cactus pear fruit cv. Gialla were exposed to 38 °C and 75–80 (low relative humidity (LRH)) or 100 % relative humidity (saturated relative humidity (SRH)) for 36 h to evaluate their effect on decay incidence, peel disorders, and overall quality during 4 weeks’ storage at 5 °C plus 1 week at 20 °C or 5 weeks at 20 °C. Both treatments were equally effective in reducing peel disorders in cold-stored fruit, whereas decay incidence was significantly lower in fruit exposed to LRH than to SRH. The better LRH decay control was attributed to faster desiccation and fall-off the cladode piece left at harvest on the stem, which is a major onset site of infection. Both treatments reduced weight loss, had only a transient effect on respiration activity, ethylene production rate, and did not affect chemical composition, eating quality, or visual appearance of fruit.     

Osmotic dehydration of the Indian fig (Opuntia ficus indica) with binary and ternary solutions

The objective of this work was to study osmotic dehydration (OD) of the Indian fig with two binary solutions (sucrose/water and glucose/water) and a ternary solution (sucrose/NaCl/water) according to a 2³ factorial design with independent variables: temperature (30–50 °C), immersion time (90–240 min) and concentration (40–60 °Brix). The dependent variables were water loss (WL), solid gain (SG) and dehydration efficiency index. The temperature had greater influence on the WL in the three hypertonic solutions studied; the concentration had greater influence on the SG in the three hypertonic solutions investigated and the best conditions for the OD of the Indian fig were in glucose solution at 40 °Brix, 40 °C and 165 min.     

Osmotic Dehydration and Vacuum Impregnation on Physicochemical Properties of Chilean Papaya (Carica candamarcensis)

ABSTRACT: The effect of osmotic dehydration (OD) at atmospheric pressure and vacuum impregnation (VI) treatments on some physiochemical parameters of papaya (aw, pH, color, firmness, and microstructure) was analyzed. Osmotic treatments were carried out on papaya with 55°Brix and 65°Brix sucrose solutions at 30 °C. VI with 65°Brix osmotic solution was the most effective in reducing aw due to the highest sucrose gain during osmotic treatment. Color differences were associated to loss of clarity in line with transparency gain. Scanning electron microscopy observations show that osmotic dehydration caused shape changes and size reduction of papaya cells; also differences in microstructural features were observed between OD-treated and VI-treated samples. Moreover, the largest firmness observed in VI samples compared with OD treatments was associated with the thickness of the middle lamella between cells, which was greater in VI than OD treatments. Improvement in texture and palatability of papaya was obtained with VI rather than OD treatment compared with fresh papaya.     

The Influence of Selected Osmotic Dehydration and Pretreatment Parameters on Dry Matter and Polyphenol Content in Highbush Blueberry (Vaccinium corymbosum L.) Fruits

The paper presents an assessment of the influence of selected highbush blueberry pretreatment methods and parameters on the process of osmotic dehydration conducted in 65 °Brix sucrose solution for 5 to 240 min at 30–70 °C. The pretreatment methods used included: fruit immersion in boiling water (15 s) and in 0.5 % NaOH solution (15 s at 95 °C), exposure to ultrasound at atmospheric pressure (vibration frequency of 35?±?5 kHz, 500 W, for 15 min.) and at low pressure (0.92 kg?cm^?1), and enzymatic processing; pectinase (enzyme activity of 46,000 PGU/mL; 0.6 mL/90 g of fruits; 30 min at approx. 22 °C) and lipase (enzyme activity of 750 PGU/mL; 0.7 mL/90 g of fruits; 30 min at approx. 22 °C) were used. Dehydration was also conducted in the presence of pectinolytic enzymes. The dehydrated material was analyzed in terms of the content of dry matter, total polyphenols, and particular polyphenols using high performance liquid chromatography. It was observed that dehydration was much more intensive at 60 and 70 °C, but such temperatures led to substantial losses of phenolic compounds (by 15–30 % after 2-h dehydration) and unfavorable changes in the texture of the final product. A promising method of pretreatment is fruit immersion in solutions containing pectinolytic and lipolytic enzymes, which increase dry matter content by 26 % (after 1 h of dehydration at 30 °C) with a low loss of phenolic compounds (4 %). Among the identified anthocyanins, the greatest retention during dehydration at various temperatures was displayed by petunidin-3-galactoside (over 80 % after 1 h of dehydration) and petunidin-3-glucoside (over 78 %).