Application of microwaves dielectric spectroscopy for controlling osmotic dehydration of kiwifruit (Actinidia deliciosa cv Hayward)

Dielectric spectroscopy studies have been performed on fresh and osmotically dehydrated kiwifruits (Actinidia deliciosa cv Hayward). The osmotic treatment consisted on the immersion the samples into 65% (w/w) sucrose aqueous solution at 30 °C during different treatment times from 5 to 1440 min. Some physical–chemical parameters were measured in fresh, treated and reposed (24 h at 30 °C) samples. Dielectric spectra were measured in the frequency range from 500 MHz to 20 GHz by an Agilent 85070E Open-ended Coaxial Probe connected to an Agilent E8362B Vector Network Analyzer in the fresh, treated and reposed samples. It has been demonstrated that the dielectric technique is a good method to control the osmotic treatment in kiwifruit.Industrial relevance: The results of this research article are demonstrated to be useful for controlling candying of kiwifruits in bakery industries. Thus, the industrial relevance is clear in order to optimize the osmotic dehydration times and the final quantity of sugars added by using a non destructive technique that can be implemented in process line. Dielectric spectroscopy, which can be considered an emerging technology, has the advantage of being an objective and a rapid technique. For all these reasons we are sending to this journal “Innovative Food Science and Emerging Technologies” our results.     

Nonlinear thermodynamic approach to analyze long time osmotic dehydration of parenchymatic apple tissue

Osmotic dehydration experiments of Granny Smith apple were carried out in order to model the operation by using a nonlinear irreversible thermodynamic approach. Samples were immersed into 65% (w/w) sucrose aqueous solution at 30 °C during 180, 360, 480, 720, 1463, 1577, 1722, 3375, 4320, 7200, 8540, 10,270 min. Some physical-chemical parameters were measured in fresh, treated and reposed (24 h at 30 °C) samples. The results allow identifying three differentiated steps in the osmotic operation process. At the beginning of the treatment, the cells were turgid and the wall-membrane system stored high level of mechanical energy. In this step, the highest velocity of shrinkage occurred. In the second step, the main driving force was diffusional, with important concentration profiles in the tissue; in this step, the deformation of the tissue reached the maximum shrinkage level of approximately 60%. It produces an important structural change in the tissue removing the concentration profiles and changing the mechanism driving forces. Last step shows internal relaxation of the tissue, increasing the mechanical transport behaviours.     

Application of microwaves dielectric spectroscopy for controlling pork meat (Longissimus dorsi) salting process

Salting treatment is a common operation in the production of high quality meat products, one of its main problems is the difficulty to control the simultaneous water and salt fluxes into the product and control the protein fibers transformation throughout process. Dielectric properties of salted porcine meat are strongly related to its structure and composition. Thus, dielectric properties measurement appears as a promising method for controlling on-line the salting process in meat industry. Dielectric spectroscopy studies have been performed on raw and salted Longissimus dorsi pork samples. Dielectric spectra were measured in the frequency range from 500 MHz to 20 GHz by an Agilent 85070E open-ended coaxial probe connected to an Agilent E8362B Vector Network Analyzer immediately after salting process and also 16 h. Salt content, moisture, water activity and volume were also measured. It has been demonstrated that the dielectric properties are a good tool to analyze the share of the added sodium chloride molecules in the muscle tissue during the salting treatment (bounded and solved ions).     

Dielectric spectroscopy of osmotic solutions and osmotically dehydrated tomato products

Complex permittivity is the physical property that describes the interaction between matter and an electromagnetic field. This property is related to the structural and physico-chemical properties such as water and soluble solids content or water activity of the material.

The complex permittivity of different aqueous solutions, fresh and osmotically dehydrated cherry tomatoes was measured and related to the products’ composition. Results showed that fruits osmotically dehydrated with sucrose solutions reached the lowest loss factors but the maximum penetration depth (D_p). On the other hand, fruits immersed in binary salt solutions showed the inverse tendency. For products impregnated with ternary solutions an intermediate behaviour was observed. Calcium lactate addition had no significant effects on complex permittivity at 2.45 GHz, but its presence increased the effective loss factor at low frequency.

The dielectric properties of fruits and vegetables can be modified by controlling osmotic process variables (time, temperature, pressure and osmotic solution composition) in order to improve their further drying with microwaves.     

Modelling of mass transfer during osmotic dehydration of apple tissue pre-treated by pulsed electric field

The osmotic dehydration mechanism of apple sample pre-treated by pulsed electric field (PEF) was investigated over a range of 44.5-65 ° Brix sucrose concentrations. Apple disks treated by PEF field of 0.90 kV/cm during a constant time were immersed in sucrose solution at ambient temperature with 3:1 syrup to apple ratio (w/w). Increase of the initial solute concentration and the PEF treatment resulted in acceleration of the osmotic dehydration (OD). The PEF-treated samples had a higher water loss (WL) and higher solid gain (SG) than the untreated samples. The osmotic dehydration kinetics was studied on the basis of two approaches: Fick's unsteady state diffusion equation and a two-exponential kinetic model. The coefficients of effective diffusion of water and solute were calculated. Their values were higher for samples pre-treated electrically. The effect of PEF was more pronounced for the WL comparatively to the SG. The two-exponential kinetic model permits evaluating of both convective and diffusion stages of OD. The PEF pre-treatment accelerates the kinetics of water and solute transfer during convective and diffusion stages of OD.     

Headspace volatile compounds during osmotic dehydration of strawberries (cv Camarosa): Influence of osmotic solution composition and processing time

The influence of the type of sugar in the osmotic solution and of the time of processing on the volatile compounds of strawberries was studied, in order to optimize the technique as a pre-treatment to further processing. Strawberries of cultivar ‘Camarosa’, cut into 1 cm thick slices, were subjected to osmotic dehydration at 30 °C for 1, 2, 4 and 6 h using either 60% (w/w) sucrose or 60% (w/w) sorbitol solutions. Volatile compounds of fresh and processed strawberry slices were analyzed by static headspace—gas chromatography. Osmotic treatments provoked a loss in volatile compounds due to the migration, mainly of esters, into the osmotic media. The variations in the volatile pattern depended on both time of treatment and type of osmotic solution. The greater changes occurred after 2 h in sucrose; with a promotion of fermentative volatiles (acetaldehyde, ethyl acetate), and a decrease in the other volatiles. Using sorbitol as osmotic agent, the major variations in the volatile pattern occurred after 4 h of osmosis, and the fermentative process was less important than in sucrose, as sorbitol could be less effective in forming a peripheral layer at the fruit surface and/in reducing tissue porosity.     

Development of a dielectric spectroscopy technique for the determination of apple (Granny Smith) maturity

Dielectric measurements of apple were done during its maturity in order to find relations with apple physiological compounds (sugar content, malic acid). An Agilent 85070E open-ended coaxial probe connected to an Agilent E8362B vector network analyzer were used in these experiments. All determinations were made at 30 °C from 500 MHz to 20 GHz. These assays were performed in order to consider the potential use of dielectric spectroscopy for determining the state of fruit maturity. Good correlations among apples Thiault Index with a new defined Dielectric Maturity Index were found. The Dielectric Maturity Index was related to loss factor at two punctual frequencies (0.5 GHz and dipolar relaxation frequency). This work is presenting a non-destructive control method for the prediction of climacteric fruits maturity.Industrial relevanceThe results of this research article are demonstrated to be useful for determining apple maturity. Thus, the industrial relevance is clear because the determination of fruit maturity is important to decide the best uses and storage time of the fruit. In the paper, a Maturity Index based on dielectric properties was developed. The results are obtained for the concrete application of apple fruit but they can be extended to other climacteric fruits although more studies are necessary. Dielectric spectroscopy, which can be considered an emerging technology, has the advantage of being an objective and a non destructive technique. For all these reasons we are sending to this journal “Innovative Food Science and Emerging Technologies” our results.     

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.     

Control of the brewing process by using microwaves dielectric spectroscopy

Dielectric spectroscopy in microwave and radiofrequency region is an emerging control technique used to obtain information about the transformation of biological systems. In microwave region, the main interaction is produced with different food constituents and water molecule. In this context, the dielectric properties were analyzed during beer production in order to improve beer quality. There were also analyzed the most important physical and chemical properties of beer through the process. Good correlations were found between loss tangent at 10 GHz and the ethanol and sugars concentration. Therefore, this technique can be used as a fast, accurate and non-destructive control method of beer production.     

A study on the rehydration ability of isolated apple cells after osmotic dehydration treatments

A study on rehydration of isolated apple cells is presented. Isolated cells previously dehydrated in 35% and 25% sucrose solutions were rehydrated in 5% sucrose under the microscope with the aim of analyzing the phenomena that take place during rehydration. Cells response to rehydration was found to be more heterogeneous than their response to hypertonic treatments. Cells showed different degrees of delay in their response, which was related to differences in the formation and preservation of membrane-to-wall connections. Results confirmed that rehydration success is based on the preservation of the structures along both, dehydration and rehydration treatments. During swelling, Hechtian strands are reincorporated to the protoplast as far as they are formed and preserved during dehydration and rehydration; their absence or shortage leading to a loss of rehydration capacity or even membrane lysis. Different stages have been identified during rehydration, mass transfer being coupled with deformation–relaxation phenomena once the protoplast reaches the cell wall. Phenomenological coefficients for water transfer indicated that rehydration kinetics is faster than water transfer during dehydration.     

Equilibrium water and salt contents of sardine sheets during osmotic dehydration

Our objectives were to experimentally determine the equilibrium water and salt contents and compare the performance of prediction according to the models of Zugarramurdi and Lupín, and Azuara et al. for osmotic dehydration of sardine sheets using brine at different concentrations (0.15-0.27 g NaCl/g) and temperatures (32-38 °C). The high regression coefficients (R²>0.92) indicated the acceptability of both models for determining the equilibrium contents. The coefficients of determination and the modulus of mean relative errors for both models indicated that Zugarramurdi and Lupín model could be advantageous in estimating the equilibrium water and salt contents. Multiple comparison of means showed that equilibrium water and salt contents predicted by the model of Zugarramurdi and Lupín were lower than those by the Azuara et al. model, at all concentration and temperature conditions. The values differed by 9.4-46.9% for equilibrium water content and by 3.5-44.1% for equilibrium salt content.     

Comparison of sucrose and fructo-oligosaccharides as osmotic agents in apple

Fructo-oligosaccharides and sucrose were compared as osmotic agents in the osmotic dehydration of apple cv. Idared. Dehydration process of apple cubes (10  10  10 mm) was performed to determine the weight reduction (WR), moisture content (MC), water loss (WL) and solid gain (SG) over a range of osmotic solutions (40–60% w/v), temperature (40–60 °C) and processing time (20–40 min) The effective diffusion coefficient of water and solute was calculated assuming the processes to be governed by Fick's unsteady state diffusion. The effective diffusion coefficients were found to be of the order of 10^− 9 m² s^− 1 and were effected by the type of solute significantly. The WR, MC, WL and SG were predicted as weighted linear combinations of temperature, concentration of solute and time of OD.

Industrial relevance

The use of fructo-oligosaccharides (FOS) in different fruit based products is an efficient way to enrich human diet with functional component, because of the well-known health benefits of FOS. The osmotic behaviour of fructo-oligosaccharides were studied and compared to the conventional used sucrose. In view of the changes of different osmotics regarding to unit parameters of osmotic dehydration the results give possibility to industrial technology planning of products containing FOS, which are available for consumption in every season of the year and are favourable also in processed form e.g. muesli, dairy products.     

Microwave dielectric spectroscopy for the determination of pork meat quality

The use of dielectric spectra to detect quality defects in porcine muscle during postmortem period was evaluated. The changes in dielectric spectra during meat ageing were also studied. For these purposes, dielectric spectra were measured by using an Agilent 85070E Open-ended Coaxial Probe connected to an Agilent E8362B Vector Network Analyzer from 500 MHz to 20 GHz parallel and perpendicular to muscle fiber direction. This study reveals that dielectric properties at two punctual frequencies (0.5 GHz and 10 GHz) are a useful tool to determine meat quality classes (RFN, PSE, and DFD) soon (6 h) after slaughter. Based on these results, it is possible to develop control algorithms in order to discriminate low quality pork meats and to decide their best uses. Moreover, the evolution of dielectric spectra during meat ageing showed important variations only in DFD meats, while in PSE and RFN meats the variations were observed only at some punctual frequencies of the spectrum during the 24 h after slaughter.     

Centrifugal osmotic dehydration and rehydration of carrot tissue pre-treated by pulsed electric field

This paper studies the centrifugal osmotic dehydration (OD), rehydration and texture of carrot tissue treated by pulsed electric field (PEF). The influence of centrifugal acceleration (0-5430g), salt addition (NaCl/sucrose solutions 0/65, 7/44 and 15/50% w/w), and temperature (20-40 °C) on the OD kinetics was investigated in the presence and absence of PEF (E=0.60 kV/cm, treatment duration t_PEF=0.05 s). The centrifugal OD of samples untreated and treated by PEF was compared with OD under stirring (250 tr/min).Centrifugal force, PEF and salt addition increase the amount of water removed during OD. However, the centrifugal force decreases the intake of solids by the product.The application of PEF enhances both water loss (WL) and solid gain (SG) during OD (under stirring or centrifugation), increases the rehydration capacity of carrot tissue, but somewhat decreases the firmness of rehydrated product.The combination of centrifugal field with PEF and salt addition can be advantageous for reducing the OD duration. The textural study shows that the product obtained with OD is less affected by thermal treatment than the directly dried product.     

How to deal with visco-elastic properties of cellular tissues during osmotic dehydration

In this work, vacuum impregnated apple discs with different isotonic solutions (sucrose and trehalose) were equilibrated during osmotic dehydration (55°Brix glucose at 40 °C). Changes in sample composition (water and soluble solid contents), weight and volume are analysed. A mathematical model is proposed to describe and quantify the outflow of water from the protoplast as well as the visco-elastic behaviour of the cell. Good correspondence between simulated and measured data of non impregnated samples and samples impregnated with isotonic solutions of sucrose or trehalose during long term osmotic dehydration is obtained. Fitted values of the cell permeability correspond well with tabulated values. Furthermore, also the obtained values of the parameters describing the mechanical properties of the cell wall and Hectian strands seem to reflect the observed structural development of these structures for the different treated samples well.     

Comparison of diffusion of fructo-oligosaccharide components during vacuum impregnation and osmotic dehydration

The osmotic behaviour of fructo-oligosaccharides was investigated during osmotic dehydration of apple cubes. The effect of vacuum impregnation on discrete diffusion coefficient (D _eff) of oligomers was scrutinised. D _eff were determined using the solution of Fick’s unsteady-state diffusion equation developed by Rastogi and Raghavarao. The moisture content decreases logarithmically irrespective of the type of treatment. The applied vacuum does not influence the rate of decrease of moisture content, but it reduces the water content. There are not significant differences among the D _eff of oligomers. Vacuum impregnation has no effect on D _eff values, which are in narrow range of the order of 10⁻⁹ m² s⁻¹ in the observed set of unit operation parameters. The prediction by the used model is acceptable in all cases especially in traditional osmotic dehydration, where the relative standard deviation (RSD) is less than 10%, and it also demonstrates the altered mechanism of vacuum impregnation by the increased RSD values.     

Mass transfer during osmotic dehydration of pineapple: considering Fickian diffusion in cubical configuration

Osmotic dehydration kinetics of pineapple cubes (15×15×15 mm³) was studied over a range of concentration (40-70°B) and temperature (30-50°C) of osmotic solution. The effective diffusion coefficients for water and solute diffusion were determined, considering pineapple as cubical configuration, assuming osmotic dehydration to be governed by Fickian diffusion. The effective diffusion coefficients for water as well as solute were empirically correlated with concentration and temperature of osmotic solution. A high degree of correlation was observed between predicted and experimental values of the effective diffusion coefficients of water (R²=0.99) as well as solute (R²=0.96).     

Effect of osmotic dehydration and vacuum impregnation on respiration rate of cut strawberries

Respiration rate in terms of O₂ consumption and CO₂ production was determined in strawberry halves, both fresh-cut and vacuum impregnated with an isotonic solution. The experimental measurements were also carried out in osmodehydrated samples for different concentration levels (to 30 °Brix) at atmospheric pressure and by applying a vacuum pulse. Changes throughout time of O₂ and CO₂ concentration in the headspace of chambers containing the samples were analysed to determine respiration rates. The effect of temperature on respiration rate in fresh-cut and impregnated samples showed sigmoid behaviour where a sharp increase in respiration levels occurred between 5 and 10 °C. Osmodehydration treatments resulted in a great decrease in O₂ consumption but no notable changes in CO₂ generation, which suggests that anaerobic biochemical pathway became dominant respiration mode due to the treatments. Production of ethanol and acetaldehyde was detected in these cases in agreement with the anaerobic process.     

Optimization of osmotic dehydration of diced green peppers by response surface methodology

Osmotic dehydration of diced green peppers was optimized with respect to temperature (20-40 °C), time (15-600 min), salt (0-10 g/100 g) and sorbitol (0-10 g/100 g) concentrations through response surface methodology. Water loss (WL), solids gain (SG), salt uptake (SA) and sorbitol uptake (SO) were the responses in a 2⁴ central composite rotatable design. Models developed for all responses were significant (p ≤ 0.01) without significant lack of fit. Results suggested that optimum processing conditions of 5.5 g salt/100 g and 6 g sorbitol/100 g at 30 °C after 240 min would result in WL = 23.3%, SG = 4.1%, SA = 8 g/100 g dry pepper and SO = 2.4 g/100 ml extract.