Mass transfer properties of osmotic solutions. I. Water activity and osmotic pressure

Abstract

In this review paper data on water activity, solute activity and osmotic pressure of”; binary and multi‐component osmotic solutions are provided. The Characteristics of the osmotic solutions are needed for the optimization of mass transfer during osmotic process, and for the improvement of final product quality. The vant Hoff equation and Gibbs Duhem theorem are commonly used to estimate osmotic pressure and solute activity. Water activities can be easily estimated through experimental determination of the freezing point depression. The possibilities of the group contribution models such as the Analytical Solution of Groups (ASOG) approach are also explored. The future needs especially in the case of multicomponent solutions consisting of electrolyte and non‐electrolyte mixtures are pointed out.     

Colour and flavour changes during osmotic dehydration of fruits

Andes berry (Rubus glaucus Benth.) and tamarillo (Solanum betaceum Cav., dark-red strain) fruit were separately submitted to osmotic dehydration with three different osmotic agents: sucrose (70%), sucrose (70%)-glycerol (65%) 1:1, and ethanol. This process decreased the water activity in the fruits and promoted the transfer of main pigments (anthocyanins) and flavour constituents to the osmotic solutions. Tristimulus colorimetry was applied to qualitatively evaluate the process of obtaining colour- and flavour-enriched osmotic solutions. It was found that after successive DIS (Dewatering–Impregnation–Soaking) cycles, it is advisable to obtain high C_ab colour parameter values, as criteria for the evaluation of osmotic solutions. Based on colour parameters, anthocyanin content, and volatile analysis, the ethanol and sucrose osmotic solutions from Andes berry were selected as promising raw materials for developing natural additives.Industrial relevanceOsmotic dehydration is one of the processes used to reduce or avoid detrimental phenomena in fruit and vegetables without a sensorial and nutritional quality loss. Its economic viability depends on the possibility of reusing the osmotic solution in successive DIS cycles. The present work demonstrates that the osmotic solutions, usually considered waste material of osmotic dehydration, could be used as a source of natural pigments and flavours, furthermore exhibiting antioxidant properties. At a large scale, this finding is most significant because it would render this environmentally friendly process more economical.     

An alternative method for the industrial monitoring of osmotic solution during dehydration of fruit and vegetables: A test-case for tomatoes

In this paper, an alternative procedure for in situ and continuous monitoring of the status of the hypertonic solutions used in industrial osmotic dehydration (OD) process of fruit and vegetables is presented and validated. The proposed method, which is based on microwave reflectometry technique, circumvents the limitation of traditional control methods, since it discovers the relation between the characteristic parameters of osmotic solutions and the corresponding measured reflection coefficients. In fact, as the OD proceeds, the dielectric characteristics of the solution, associated to the reflectometry measurements, can be directly related to the variation of the water activity (a_w), which is the parameter that is typically considered to assess the dewatering capacity of the solution. As a result, through the method presented herein, a continuous monitoring of the dewatering capacity of the osmotic solution, associated to its qualitative status, is possible, which, in turn, is useful also for optimizing the OD industrial process. The proposed method was preliminarily validated on reference aqueous solutions of sucrose. Successively, the method was tested also in a typical industrial application, involving the OD process of tomatoes.     

OSMOTIC DEHYDRATION OF STRAWBERRIES IN A BATCH RECIRCULATION SYSTEM

Physical and chemical characteristics of two cultivars of strawberries during osmotic dehydration in sucrose and glucose solutions were investigated. Temperature was found to have a significant effect on the water and sugars (glucose, fructose and sucrose) exchange between strawberry and the osmotic solution. Mass transfer was found not to be significantly different between cultivars. Glucose gain was found to be higher than sucrose for the strawberries osmotically dehydrated in glucose and sucrose solutions at the same mole fraction, respectively. Sugars other than the osmotic sugar were found to decrease in concentration during the osmotic process. The combination of 63% sucrose solution with 25C process temperature for 2 h was able to remove more than 40% of moisture and load less than 0.1% of sucrose in the strawberries.     

Osmotic Dehydration of Pineapple with Impregnation of Sucrose, Calcium, and Ascorbic Acid

Mass transfer was evaluated during osmotic dehydration of pineapple in solutions with until four components aiming to investigate the solutes concentration influence on impregnation. In the first step, the experimental trials for optimization of solution concentration were based on 2³ factorial design. In the second step, effective diffusion coefficients were determined. Equations representing the influence of the concentration of sucrose, calcium lactate, and ascorbic acid in osmotic solutions on water loss and gains of sucrose, calcium, and vitamin C were found. Results showed that both calcium lactate and sucrose concentration affected calcium and sucrose gain. On the other hand, only vitamin C gain was significantly affected by the ascorbic acid concentration in the studied concentration range. However, when comparing diffusivities in pineapple immersed in sucrose solutions, with and without calcium lactate, with and without ascorbic acid, it was possible to verify that diffusivities of water, sugar, and calcium increased in presence of ascorbic acid in solution. Calcium in solution diminished the water and sucrose diffusivities. High calcium and vitamin C contents were obtained in 1 h immersion in the solutions studied.     

Protein–polysaccharide interactions: The determination of the osmotic second virial coefficients in aqueous solutions of β-lactoglobulin and dextran

Solutions containing dextran and solutions containing mixtures of dextran +β-lactoglobulin are studied by membrane osmometry. The low concentration range of these solutions is considered. From the measured osmotic pressures the virial coefficients are obtained. These are analyzed using the osmotic virial coefficient of β-lactoglobulin solutions published earlier by us [Schaink, H.M., & Smit, J.A.M. (2000). Determination of the osmotic second virial coefficient and the dimerization of beta-lactoglobulin in aqueous solutions with added salt at the isoelectric point. PCCP, 2, 1537–1541]. The second cross-virial coefficient A₁₂ is found to be positive indicating a repulsive and probably mainly steric interaction between neutral in nature dextran and and practically uncharged β-lactoglobulin (pH=5.18). The measurements show that the β-lactoglobulin has only a small tendency to form multimers in the presence of dextran. The phase diagram of solutions of dextran+Whey Protein Isolate (appr. 60% β-lactoglobulin) is also presented. The McMillan–Mayer equation of state that considers only the second virial coefficients is found to be unreliable for the extrapolation up to the concentrations at which phase separation is expected.     

MASS TRANSFER IN PLANT MATERIALS IN CONTACT WITH AQUEOUS SOLUTIONS of ETHANOL and SODIUM CHLORIDE: EQUILIBRIUM DATA

A process for osmotic dehydration of plant materials using aqueous solutions of ethanol and sodium chloride is described. Equilibrium data for blanched and unblanched carrots in contact with binary aqueous solutions of ethanol and sodium chloride are presented. Based on the equality of activities of the solutes in carrots and the external solution a successful representation of equilibrium has been obtained in terms of the measurable practical variables such as the total solids in the material at full turgor, solute concentration in the bath and the temperature of contacting.     

Some methods for isolation and assays of enzymes occurring in cereals and legumes

Abstract

This review deals with isolation techniques and enzymatic assays which are applicable to the study of enzymes occurring in cereals and legumes. In the section of the paper concerned with separation and purification methods, various types of Chromatographic and electrophoretic techniques are described and specific examples are presented. The discussion of assay methods cites selected assay methods suitable for monitoring enzyme activities in plants using natural, synthetic, dye‐labeled or radiolabeled substrates in different technical combinations. Immunochemical assays of enzymes, characterization of antibodies, and practical examples illustrating how these techniques can be used are also presented.     

Kinematic viscosity and water activity of aqueous solutions of glycerol and sodium chloride

During osmotic dehydration, a removal of water from foodstuffs is achieved with a decrease in the water activity of the food sample. This dehydration process involves the utilization of highly concentrated solutions with one or several solutes that increase considerably the viscosity of the liquid phase. This property is fundamental in the studies of mass and momentum transfer processes. Glycerol and sodium chloride are studied as systems employed in this type of process due to the advantages for the final dehydrated product. Kinematic viscosities of binary and ternary aqueous solutions of these solutes were measured at various concentrations (from 0 to up 5.0 mol kg^–1) and temperatures (from 20 up to 50 °C). Water activities for each indicated solution at 25 °C are also reported. Experimental data for both physical properties were simultaneously correlated with concentration and temperature for binary and ternary solutions with a suitable accuracy. Additionally, relationships between kinematic viscosities and water activities were established.     

Developments in osmotic dehydration technique for the preservation of fruits and vegetables

In recent years much attention has been focused on maintaining the freshness of fruits and vegetables by immersion of cellular materials containing water in an osmotic solution. It results in the development of intermediate moisture products having lower water activity, which is imparted by solute gain and water loss. During the process, chemical, physical and biological activities, which deteriorate the foods, are lowered considerably; hence extends the shelf life of food products. In this process moisture is withdrawn from the product at ambient temperature by diffusion, so phase change has been avoided. Besides, it helps to improve the nutritional and sensory attributes of food products and is less energy intensive process as compared to other drying techniques. Osmotic dehydration is influenced by various factors such as osmotic agent, time and temperature, solute concentration, solution to sample ratio, agitation and geometry of the materials. Recently, osmotic dehydration has been combined with several other methods namely, pulsed high electric field, high hydrostatic pressure, ultrasound, centrifugal force, vacuum and gamma irradiation. These techniques have been employed during or after osmotic treatment to enhance osmotic dehydration performance by increasing the cell membrane permeability and mass transfer rate. These combined operations reduce the drying time, minimizing further energy costs. In this study, various segments of osmotic dehydration techniques and its application in food processing as well as recent advances in osmotic dehydration have been reviewed.Industrial relevanceThe osmotic dehydration technique is gaining popularity as a mean of obtaining minimally processed food. This review paper deals with the kinetics and mechanisms of osmotic dehydration technique for the preservation of fruits and vegetables. The various factors effecting osmotic mass transfer rate in food have been reviewed. In this paper, the combined effect of osmotic dehydration and several other innovative techniques (pulsed high electric field, high hydrostatic pressure, ultrasound, centrifugal force, vacuum and gamma irradiation) on the quality and shelf life of fruits and vegetables have been reviewed. These techniques have been employed during or after osmotic treatment to enhance osmotic dehydration performance by increasing the cell membrane permeability. These combined operations reduce the drying time, minimizing further energy costs as well as improving the quality of fruits and vegetables during storage.     

The Compound Characteristics Comparison (CCC) approach: a tool for improving confidence in natural compound identification

ABSTRACT

Compound identification is the main hurdle in LC-HRMS-based metabolomics, given the high number of ‘unknown’ metabolites. In recent years, numerous in silico fragmentation simulators have been developed to simplify and improve mass spectral interpretation and compound annotation. Nevertheless, expert mass spectrometry users and chemists are still needed to select the correct entry from the numerous candidates proposed by automatic tools, especially in the plant kingdom due to the huge structural diversity of natural compounds occurring in plants. In this work, we propose the use of a supervised machine learning approach to predict molecular substructures from isotopic patterns, training the model on a large database of grape metabolites. This approach, called ‘Compounds Characteristics Comparison’ (CCC) emulates the experience of a plant chemist who ‘gains experience’ from a (proof-of-principle) dataset of grape compounds. The results show that the CCC approach is able to predict with good accuracy most of the sub-structures proposed. In addition, after querying MS/MS spectra in Metfrag 2.2 and applying CCC predictions as scoring terms with real data, the CCC approach helped to give a better ranking to the correct candidates, improving users’ confidence in candidate selection. Our results demonstrated that the proposed approach can complement current identification strategies based on fragmentation simulators and formula calculators, assisting compound identification. The CCC algorithm is freely available as R package (https://github.com/lucanard/CCC) which includes a seamless integration with Metfrag. The CCC package also permits uploading additional training data, which can be used to extend the proposed approach to other systems biological matrices.

List of abbreviations: Acidic: acidic moiety; aliph: aliphatic chain; AUC: area under the ROC curve; bs: best glycosidic structure; CCC: Compounds’ Characteristics Comparison; Cees: Carbons estimation errors; CO: Carbon to Oxygen ratio; Het: Heterocyclic moiety; IMD: Isotopic Mass Defect (and Pattern); LC-HRMS: Liquid Chromatography – High Resolution Mass Spectrometry; md: mass defect; MM: Monoisotopic Mass; MS: Mass Spectrometry; MSE: Mean Squared Error; nC: number of Carbons; NN: Nitrogen; pC: percentage of Carbon mass on the total mass; Pho: Phosphate; PLSr: Partial Least Square regression; ppm: parts per million; QSRR: Quantitative structure-retention relationship; RMD: Relative Mass Defect; ROC: Receiver Operating Characteristics; rRMD: residual Relative Mass Defect; RT: retention time; Sul: Sulphur; UPLC-ESI-Q-TOF-MS: Ultra Performance Liquid Chromatography – ElectroSpray Ionization –Quadropole – Time of Flight – Mass Spectrometry; VAT: Vitis arizonica Texas     

The mathematical modelling of the osmotic dehydration of shark fillets at different brine temperatures

The effect of brine temperature (20, 30, 40 and 50 °C) on the osmotic drying behaviour of shark slabs (10 × 5 × 1 cm) in saturated (100°) brine was investigated. The parameters investigated were weight reduction, water loss, salt gain and water activity. Salt uptake and moisture data were analysed using various mathematical solutions based on Fick's Law of Diffusion and the effective diffusion coefficients were predicted after considering the process variables. The expressions presented by Azuara et al. (1992) , based on the model presented by Crank (1975) , were successfully used to predict the equilibrium point and to calculate diffusion coefficients at not only the initial stages of dehydration, but also at different times during the osmotic process.     

Time domain nuclear magnetic resonance to monitor mass transfer mechanisms in apple tissue promoted by osmotic dehydration combined with pulsed electric fields

Pulsed electric field (PEF) technology is gaining momentum as a pre-treatment to enhance mass transfer of vegetable tissues obtained by further processing. In this study PEF pre-treatment increased osmotic dehydration (OD) effectiveness, in terms of water loss and solid gain in apples, as a function of electric field strength and number of pulses. Mass transfer was particularly high when average electric fields of 250 and 400 V cm^-1 were applied. Time domain nuclear magnetic resonance (TD-NMR), with the use of a contrast agent, clarified structural changes that drive mass transfer. Treatments at 100 V cm^-1 redistributed water between vacuole, cytoplasm and extracellular space, while at 250 and 400 V cm^-1 the membrane breakages caused the loss of cellular compartmentalization. Two non-destructive and fast acquirable parameters, the longest measured relaxation time (T₂) and water self diffusion coefficient (D_w), allowed the separate and accurate observation of PEF treatment and osmotic dehydration effects.Industrial relevanceThe developed non-destructive method, here described, allows the measure of the effects of PEF treatment on apple tissue which can be exploited to have reliable control of the process within minutes. Since mass transfer parameters depend on subcellular water redistribution, the present work provides a tool to boost the development and optimization of agri-food processes on fresh vegetable tissues.     

Antioxidant activity of an anatolian herbal tea—Origanum minutiflorum: isolation and characterization of its secondary metabolites

Origanum species are significant aromatic and medicinal plants used in food and pharmaceutical industries. Isolation of bioactive compounds was executed on n-butanol extract to yield the compounds responsible for the activities. Tricosan-1-ol (1), (8E,16E)-tetracosa-8,16-diene-1,24-diol (2), azepan-2-one (3), 3,4-dihydroxybenzoic acid (4), apigenin (5), eriodictyol (6), globoidnan-A (7), luteolin (8), rosmarinic acid (9), apigenin-7-O-glucuronide (10), and vicenin-2 (11) were isolated by chromatographic methods (column chromatography and semi-preparative High Performance Liquid Chromatography (HPLC) and structures were elucidated on the basis of spectroscopic techniques including 1D/2D nuclear magnetic resonance (NMR) and Liquid chromatography/Time-of-flight/Mass spectrometry (LC-TOF/MS). The isolated compounds and extracts were applied for antioxidant assays including 1,1-diphenyl-2-picrylhydrazyl (DPPH^?) scavenging, 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS^?+) scavenging, reducing power, and cuprac techniques. 3,4-Dihydroxy benzoic acid (4), eriodictyol (6), luteolin (8), and rosmarinic acid (9) revealed the considerable antioxidant activities.     

Ultrasound-Assisted Osmotic Treatment of Model Food Impregnated with Pomegranate Peel Phenolic Compounds: Mass Transfer,Texture, and Phenolic Evaluations

Food impregnation with nutraceutical components due to the health beneficial property is of great importance for food processing industry. In this study, osmotic dehydration was used to impregnate model food with phenolics extracted from pomegranate peel. Intermittent acoustic treatment was applied to enhance mass transfer. This process was carried out at three sucrose concentrations of 40, 50, and 60% and two levels of power ultrasound, 50 and 100% in an experimental setup, which was equipped with a pump circulating osmotic solution frequently. Results showed that increase in sucrose concentration resulted in an increase in the amounts of water loss and solutes gain. Additionally, application of higher power ultrasound led to higher values of water loss and solid gain. Mass transfer modeling using Azuara model predicts water loss and solid gain values at equilibrium. Results revealed the good correlation of experimental values with the model (due to the R² values greater than 0.94). The microstructure of samples was investigated using scanning electron microscopy (SEM). Images revealed pores and cavities made by ultrasound waves as the result of spongy effects. Texture profile analysis (TPA) was applied for the determination of hardness, springiness, and gumminess of the samples. Results also showed significant effects of the sucrose concentration and ultrasound power on textural properties. Measurements of total phenolic content and antiradical activity, which were carried out by a colorimetric method and antiradical scavenging assay, EC50, respectively indicated that osmotic dehydration is a possible way for uptaking phenolic compounds of pomegranate peel presented in osmotic solution into food matrices.     

Evaluation of the mechanical properties and diffusion coefficients of osmodehydrated melon cubes

The objective of this work was to evaluate the influence of osmotic agents, their concentration and temperature, on the mass transfer kinetics and mechanical properties of osmodehydrated melon cubes. Samples were immersed in a hypertonic solution of sucrose or maltose up to 8 h under controlled temperature and agitation. Water loss, sugar gain and mechanical properties were analysed throughout the process. Mass transfer kinetics was modelled according to Peleg and Fick equations. Higher water loss, lower sugar uptake and stress at rupture values more similar to fresh fruit were observed in samples treated with maltose solutions. Peleg’s model showed the best adjustment to all the experimental data, however the k₁ and k₂ parameters did not show a clear trend with the solution concentration or temperature increase. The effective diffusivity obtained using Fick’s equation varied from 3.93 × 10^?9 to 6.45 × 10^?9m² s^?1 for water loss and from 7.57 × 10^?10 to 3.14 × 10^?9m² s^?1 for sugar gain.     

ARTIFICIAL CELL STUDIES IN SIMULATED APPLE AND POTATO STARCH CELL COMPLEX DURING OSMOTIC DEHYDRATION

ABSTRACT

Visking tubing was employed as an artificial cell system to investigate the behavior of water in simulated apple and potato starch cell complex during osmotic dehydration. The study involved a factorial experimental design at three levels of temperature (32.2, 40 and 55C) and five levels of solute concentration solutions (four sucrose concentrations: 40, 50, 60 and 70%; one sodium chloride concentration: 0.5%). A time period of 0–3 h was implemented with 30‐min sampling period for every treatment. Three phases of a sharp decrease in the initial weight of the visking tubing in the first 30 min were observed for all temperatures and solute concentrations. In all the treatments, there was an incremental increase in water loss with increased temperature and sucrose and salt concentrations. Water loss was higher for simulated apple and 1% potato starch solutions compared with 15% potato starch solution. Starch in the visking tubing competed for the removal of water during osmotic dehydration. The amount of starch contributed to the differences in the rate of water loss in the visking tubing.

PRACTICAL APPLICATIONS

In order to better understand the behavior of water in simulated apple and potato starch cell complex during osmotic dehydration of plant materials, the Visking tubing was used as an artificial cell in simulating biological membranes. Simulated apple and potato starch was studied as representatives of cellulosic and starchy plant materials, respectively. Notably, there was antagonistic effect between starch and salt solutions, starch and sugar solutions similar to sugar and salt solutions as a result of differences in their molecular weights. This study is industrial applicable to achieve efficient osmotic dehydration processes for plant materials that are cellulosic or starchy in nature.     

Evaluation of different drying systems as an alternative to sun drying for figs (Ficus carica L)

The aim of this work was to study the effect of different drying systems on the quality characteristics of dried figs as an alternative to sun drying. Fresh figs were dried as follows: drying in a convection oven at different temperatures; pre-treatment by soaking in osmotic solutions at different concentrations; pre-treatment with K₂CO₃ + olive oil, combined or not with osmotic solutions; pre-treatments with ultrasound combined or not with osmotic solutions; sun drying (control). Moisture, water activity, microbial counts, firmness and sensory quality were measured at the beginning, at the mid-way and at the end points of the drying process. The figs dried with all the artificial drying systems tested needed between 1 and 3 days to reach the required moisture content of 24% wet basis (w.b.), while the sun dried figs needed 15 days to reach the optimal moisture. Regarding mold counts, firmness and sensory quality, the best results were observed for the treatments with ultrasound combined with osmotic solutions. Thus, this treatment could be a good alternative to the sun drying process.Industrial relevanceSun drying of figs is a widely-spread practice in tropical and subtropical countries. Nevertheless, the slowness of the process, depending on the climatic conditions, can result in diverse problems such as the loss of quality and dramatically increases the rapid growth and proliferation of microorganisms such as molds which can produce mycotoxins, resulting in high losses of the product and consequently, economic losses. Therefore, the major problem of these fruits is fungal growth and subsequent mycotoxin production, which are considered an important problem throughout the world in terms of public health, agriculture and economics. For this reason, there is an increasing demand by the productive sector for the development of new drying technologies to shorten the drying time and to enhance the safety of these products by reducing or even inhibiting fungal proliferation due to the optimal and controlled drying conditions of these proposed technologies. This study represents a great opportunity to apply new techniques in order to reduce the drying period and to obtain a product with suitable sensory, safety and quality characteristics. Therefore, this study is a useful tool which will help the productive sector, which is in a critical situation, to avoid all problems mentioned above.     

Glass transitions for freeze-dried and air-dried tomato

Glass transition temperatures of freeze-dried tomato conditioned at various water activities at 25 °C were determined by differential scanning calorimetry (DSC). Air-dried tomato with and without osmotic pre-treatment in sucrose/NaCl solutions was also analyzed. Thermograms corresponding to the low water activity domain (0.11 ⩽ a_w ⩽ 0.75) revealed the existence of two glass transitions, which were attributed to separated phases formed by sugars and water and other natural macromolecules present in the vegetable. Both transitions were plasticized by water and experimental data could be well correlated by the Gordon-Taylor equation in the low-temperature domain, and by the Kwei model in the high-temperature domain. For higher water activities, the low-temperature glass transition curve exhibited a discontinuity, with suddenly increased glass transition temperatures approaching a constant value that corresponds to the T_g of the maximally freeze-concentrated amorphous matrix. The unfreezable water content was determined through the melting enthalpy dependence on the moisture content.     

The prediction of water activity of some supersaturated non-electrolyte aqueous binary solutions from ternary data

Binary data are required for prediction of the water activities of multicomponent aqueous solutions by various methods. At high concentrations, these methods would need binary data beyond the solubility limits of individual solutes. Such supersaturated data are usually lacking or very difficult to determine experimentally. The Zdanovskii–Stokes–Robinson equation, which has been shown to be a reliable and accurate method for predicting the water activity of complex solutions, may be applied in reverse to calculate the water activities of supersaturated binary solutions from ternary data. Hitherto unpublished supersaturated binary a _w data of mannitol, sucrose and urea obtained using this method as well as polynomials for their prediction are presented in this paper.