Dielectric properties of sea cucumbers (Stichopus japonicus) and model foods at 915 MHz

Short-time microwave (MW) sterilization is a feasible technology to produce high-quality shelf-stable sea cucumbers (SCs) (Stichopus japonicus). Selection of a model food matching the sea cucumbers in dielectric properties (DPs) is one of the most important steps for developing the MW processing. The test results revealed that rehydrated sea cucumber has much lower relative dielectric loss factor (9.73-5.62) than muscle foods, including salmon fillets and sliced beef, which were reported in the literature. The whey protein gel formulations that had been developed in our laboratory as a tool in heating pattern studies for those products are, therefore, not appropriate for sea cucumber. Adding 1.0% gellan powder sharply reduced the amount of whey protein concentrates needed to form firm gels and significantly lowered the dielectric loss factor. The dielectric properties of the sea cucumbers and model food samples with different formulations were measured using a custom-built temperature controlled test cell and an Agilent 4291B impedance analyzer in the temperature range 20-120 °C. Based on comparison of the measured dielectric properties and the calculated microwave power penetration depths among the sea cucumbers and model foods, appropriate formulation with whey protein concentration 5%, whey protein isolation 3%, gellan gum 1%, d-ribose 0.5% and water 90.5% was chosen as the model food for the sea cucumbers for the purpose of MW processing development.     

Using whey protein gel as a model food to study dielectric heating properties of salmon (Oncorhynchus gorbuscha) fillets

It is desirable to develop rapid commercial microwave and radio frequency sterilization processes to produce high quality shelf stable muscle foods, particularly aquatic foods. Whey protein gels containing d-ribose and salt were studied as a model food to determine heating patterns in salmon fillets during high temperature microwave sterilization processes. Dielectric constant (?′) and loss factor (?″) of whey protein gels with d-ribose (0.5 g/100 g, 1 g/100 g, and 1.5 g/100 g) at different salt contents (0, 0.1 g/100 g, 0.2 g/100 g, 0.3 g/100 g, 0.4 g/100 g, and 0.5 g/100 g) and frozen and thawed pink salmon (Oncorhynchus gorbuscha) fillets were determined over the frequency range of 27-1800 MHz at temperatures ranging from 20 to 120 °C. The dielectric properties of whey protein gels containing 1 g/100 g d-ribose and 0.2 g/100 g or 0.3 g/100 g salt closely matched the dielectric behavior of salmon fillets in both radio frequency (RF, 27 MHz) and microwave (MW, 915-1800 MHz) ranges. Altering the salt content had a greater impact on dielectric constant and loss factors at lower frequencies. These results suggest that whey protein gel may be a good model food for microwave sterilization process development, particularly for determining the locations of cold and hot spots in complex muscle foods.     

Characterization of sol-gel transitions of food hydrocolloids with near infra-red spectroscopy

Near infra-red spectroscopy (NIR, 600-1100 nm) was used to characterize sol-gel transitions of food hydrocolloids including whey protein (WP), high acyl (HA) and low acyl (LA) gellan gums. Whey protein (20% w/w), LA and HA gellan (1% w/v) aqueous solutions with selected calcium concentrations (0-40 mM) were used. Principal components analysis (PCA) and principal component regression (PCR) were performed to analyze spectral data of gel dispersions at a series of controlled temperatures (20-95 °C). Gelling temperatures of gellan dispersions were determined directly based upon the PCA results without requiring for reference values. The gelling temperatures determined with the NIR method were similar to those obtained by dynamic rheological tests and dielectric property test. The PCR models showed a good predictability for temperatures of gellan dispersions with selected calcium concentrations during cooling or heating process. This study indicates that the NIR method has a great potential to detect structural change, and to monitor time and temperature dependant behaviour of food hydrocolloids during sol-gel transition period.     

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.     

Physical properties of egg whites and whole eggs relevant to microwave pasteurization

Microwave pasteurization is a novel thermal processing technology in which non-uniform heating may be a major challenge. In this study, the suitability of using egg whites (EWs) and whole eggs (WEs) as model foods to evaluate the heating uniformity and to determine the cold and hot spots during microwave pasteurization was investigated. The samples were prepared from mixtures of water with commercial EW or WE powders at different solid concentrations (20%, 25%, 27.5%, and 30%) and salt contents (0, 50, 100, and 200 mM). Critical physical properties for desirable model food systems include appropriate dielectric properties, gelation temperatures, gel strengths, and water holding capacities (WHCs). The gelation temperature of liquid EW and WE were 70 and 80 °C; both fell in the pasteurization temperature range. At 915 MHz, the dielectric constants of liquid EW and WE samples and their heat induced gels decreased with solid concentration while the loss factor was not affected. Loss factors of liquid EW and WE samples increased linearly with salt addition, which could be explained by the linear increase of electrical conductivities by adding salt. The strength and WHC of heat induced EW and WE gels increased linearly with solid concentration, while salt addition had no significant effect. The results demonstrated the suitability of using EW and WE as model foods to determine the heating uniformity during microwave pasteurization process.     

Effect of gellan,alone and in combination with high-methoxy pectin,on the structure and stability of doogh,a yogurt-based Iranian drink

Doogh is a traditional Iranian drink prepared by fragmentation and dilution of yogurt, with addition of salt and flavouring. In the present work, we have used viscometry, microscopy, particle-size analysis and measurements of serum separation to explore the effect of very low concentrations of gellan gum (0.01, 0.03 and 0.05 wt %), alone or in combination with 0.25 wt % high-methoxy pectin (HMP), on its structure and stability. HMP is known to prevent association of casein particles in acidic milk drinks by steric stabilisation, forming a protective layer bound electrostatically to the surface of the particles. Doogh incorporating 0.25 wt % HMP alone showed satisfactory stability on storage for ∼10 days at 5 °C, but after 15 days there was obvious separation into a dense sediment and a much clearer upper layer that occupied more than 80% of the total volume, which we attribute to progressive sedimentation of individual sterically-stabilised particles. Samples incorporating gellan (with or without HMP), by contrast, showed rapid development of a clear serum phase, with little further separation at longer times (up to ∼1 month), suggesting expression of fluid by contraction of a gel network (i.e. syneresis rather than sedimentation). Particle size increased dramatically (more than 10-fold) with increasing concentration of gellan, and at the highest concentration studied (0.05 wt %) a continuous network of casein-rich strands was observed by phase-contrast microscopy. The concentration of NaCl used in the doogh samples (0.5 wt % ≈ 85 mM) is known to be sufficient to maintain gellan in its ordered (double-helix) conformation, which has higher charge density than individual molecules of HMP. We suggest that network structure is formed by electrostatic attachment of gellan to fragments of acid-casein gel, thus increasing particle size and inhibiting surface-coverage by HMP, with weaker associations between gellan helices allowing the samples to flow. Observed decrease in serum volume with increasing concentration of gellan is attributed to formation of progressively stronger coupled networks with greater resistance to syneresis. Stabilisation of doogh with 0.05 wt % gellan in combination with 0.25 wt % HMP had no adverse effect on organoleptic acceptability, and reduced serum volume on protracted storage to ∼10% (in comparison with over 80% for the same concentration of HMP alone), suggesting that gellan could be of practical value in extending the shelf-life of doogh (and related acidic milk drinks).     

An investigation of the dielectric and thermal properties of frozen foods over a temperature from –18 to 80°C

The dielectric properties (dielectric constant [ε′] and dielectric loss factor [ε″]) for three frozen meals (basil fried chicken, green curry with chicken, and congee with minced pork) were measured at frequency 2.45 GHz from –18 to 80°C. Thermal properties (thermal conductivity [k] and specific heat capacity [c]) of each food item were characterized using their composition for improving the modeling of microwave thawing process of frozen products. For all frozen meals, similar trends in the dielectric and thermal properties values were observed as a function of temperature. In all samples of frozen foods, the dielectric properties (ε′, ε″) rapidly increased with temperature for the range from –10 to 0°C. Thereafter, dielectric properties linearly increased with temperature for basil fried chicken and congee with minced pork but linearly decreased with temperature for green curry with chicken from 0 to 80°C. The dielectric properties data bases were used to calculate power reflected (P_r), power transmitted (P_t), dielectric loss tangent (tanδ), and penetration depth (d_p). These parameters were related to the dielectric properties. The thermal conductivity values of all samples decreased with increasing temperature in the frozen stage and little changed after thawing. While the specific heat capacity values increased first and then do not change considerably. The temperature-dependent material properties can give insight into how the food product interacts with the incident electromagnetic radiation.     

Effect of inulin on texture and clarity of gellan gels

The effect of inulin on the gelation of 0.5 wt% low acyl gellan in the presence of increasing concentrations of potassium chloride was investigated by large deformation compression experiments and visible light absorbance. The sugar and salt concentrations varied from 0 to 15 wt% and 40-100 mM, respectively. Stress and strain at failure along with Young’s modulus were calculated from each compression curve. Samples prior to compression were refrigerated at 5 °C for 24 h. Reduced gel strength and firmness were observed for all inulin and salt concentrations. Increasing amounts of inulin resulted in an increase in gel strength which was greater for higher potassium concentrations. Elasticity values did not exhibit great diversion. Inulin at concentrations of 5 and 10 wt% led to less turbid gels than those with only gellan. Samples containing 15 wt% inulin gave absorbance readings greater than 1.     

A New Chemical Marker-Model Food System for Heating Pattern Determination of Microwave-Assisted Pasteurization Processes

New chemical marker-model food systems with d-ribose and NaOH precursors as color indicators and gellan gels as chemical marker carrier were explored for the assessment of the heating pattern of in packaged foods processed in microwave-assisted pasteurization system (MAPS). In determining appropriate precursor concentrations, a solution of 2% (w/w) d-ribose and 60 mM NaOH was heated at 60–90 °C for 0–20 min. The solution absorbance at 420 nm increased linearly, while the color parameters L* decreased linearly with heating time at all processing temperatures. In storage, the produced brown color was stable at 4 and 22 °C within 7 days. The new chemical marker-model foods were prepared by mixing 2% (w/w) d-ribose and 60 mM NaOH with 1% (w/v) low-acyl gellan gum and 20 mM CaCl₂·2H₂O solution. The dielectric constant of the model food samples decreased with the addition of sucrose, and the loss factors increased with the addition of salt. After processing in the pilot MAPS, the heating pattern and cold and hot spots in the new chemical marker-model food system could be clearly recognized and precisely located through a computer vision method. This is the first time that the caramelization reaction was used as a time-temperature indicator in gellan gel model food. This study shows the possibility of using the new chemical marker-model food system for heating pattern determination of the MAPS.     

Sugar and water contents of honey with dielectric property sensing

The dielectric properties of pure yellow locust, jujube and rape flower honey and their water-adulterated products with water content from 18% to 42.6% were measured with open-ended coaxial-line probe technology and a network analyzer from 10 to 4500 MHz at 25 °C. Dielectric constants of pure honeys and water-added honey samples decreased monotonically with increasing frequency, and increased with increasing water content. Dielectric relaxation was evident in the dielectric loss factors. The critical frequency and the maximum loss factor increased with increasing water content. There were strong linear correlations between the dielectric constant and the total soluble solids and water contents. The linear coefficients of determination were higher than 0.995 from 650 to 960 MHz. The good linear correlations and the sufficient penetration depth >20 mm below 960 MHz, suggest that microwave dielectric properties could be used in developing sensors to determine sugar and water contents.     

Improvement of heating uniformity in packaged acidified vegetables pasteurized with a 915 MHz continuous microwave system

Continuous microwave processing to produce shelf-stable acidified vegetables with moderate to high salt contents poses challenges in pasteurization due to reduced microwave penetration depths and non-uniform heating. Cups of sweetpotato, red bell pepper, and broccoli acidified to pH 3.8 with citric acid solution containing 0-1% NaCl were placed on a conveyor belt and passed through a microwave tunnel operating at 915 MHz and 4 kW with a 4 min residence time. The time-temperature profiles of vegetable pieces at 5 locations in the package were measured using fiber optic temperature sensors. Addition of 1% NaCl to the cover solution lowered microwave penetration into vegetable pieces and decreased the mean temperature in cups of acidified vegetables from 84 to 73 °C. Soaking blanched vegetables for 24 h in a solution with NaCl and citric acid prior to processing improved microwave heating. Heating was non-uniform in all packages with a cold spot of approximately 60 °C at a point in the container farthest from the incident microwaves. More uniform heating was achieved by implementation of a two-stage rotation apparatus to rotate vegetable cups 180° during processing. Rotating the cups resulted in more uniform heating and a temperature of 77 °C at the cold spot. This is above the industrial standard of 74 °C for in-pack pasteurization of acidified vegetables. The effective treatment involved blanching, soaking for 24 h in a NaCl and citric acid solution, and 180° rotation. This work has contributed to a better understanding of the influence of salt addition and distribution during dielectric heating of acidified vegetables using a 915 MHz continuous microwave system.     

Aeration of model gels: Rheological characteristics of gellan and agar gels

Model food sols containing gellan and agar at different solid concentrations were aerated. The rheological characteristics of the setted gels were determined. These were the compression characteristics (fracture strain/stress/energy, firmness and the total energy for compression) and the stress relaxation characteristics (extent of relaxation and relaxation time). Fracture strain of gels increased due to aeration indicating that the aerated samples exhibited a delayed fracture due to the presence of incorporated air. The extent of increase in fracture strain for aerated agar gels over the corresponding non-aerated samples is 1.7-23.1% while it is 4.3-15.1% for gellan gels. The extent of relaxation increased due to aeration while relaxation time (λ) decreased. Relaxation time for aerated agar and gellan gels is 9.6-40.7% and 19.6-37.6% lower than the non-aerated samples. At a low concentration of hydrocolloid, the effect of aeration was usually non-significant (at p ? 0.05) while prominent changes in rheological behaviour occurred at a high concentration. Aeration of food gels offered the possibility of rheological modifications to suit the consumer requirements in developing specialty fabricated gels.     

Gellan gum for reducing oil uptake in sev,a legume based product during deep-fat frying

Gellan gum, a high molecular weight anionic linear polysaccharide produced by pure culture fermentation from Sphingomonas paucimobilis ATCC 31461 is used in a variety of food applications that are based on its unique gelling profile. The present work reports on the effective use of gellan gum on the oil uptake of a traditional Indian deep-fat fried product, sev that is based on chickpea flour. The effect of addition of gellan gum at 0.25–0.75% (w/w) (based on chickpea flour) on the dough texture, and that of the sev prepared was also evaluated using TA.XT2i Texture Analyzer. Addition of gellan gum at 0.25% (w/w) markedly reduced the oil content in the sev from 37.02% in the control to 27.91%. The reduction in oil content beyond 0.25% gellan gum addition was not significant (P = 0.05). Furthermore, while addition of gellan gum significantly altered the texture of dough, it did not significantly affect the texture of sev (P = 0.05). Addition of 0.25% gellan gum in combination with sodium alginate (0.25–1.00%), carboxymethylcellulose (0.25–1.00%) or soy protein isolate (2.5–10.0%) did not affect oil uptake significantly (P = 0.05) as compared to that prepared by the addition of 0.25% gellan gum alone.     

Development of a dielectric spectroscopy technique for the determination of key biochemical markers of meat quality

Dielectric measurements of different standard solutions (nucleotides, nucleosides, lactic acid and myoglobin) were assayed simulating the concentrations of these substances in early postmortem meat. These assays were performed for considering the potential use of dielectric spectra for the quality control of meat. Good correlations among solutions of Adenosine Triphosphate (ATP), Inosine Monophosphate (IMP) and lactic acid with loss factor at punctual frequencies (0.5, 0.915 and 1 GHz) were found. The other assayed substances did not present a marked effect over the electromagnetic spectra. Good correlations of dielectric properties with IMP and lactic acid content of real pork meat samples were also found at 0.5 GHz. This work is presenting prospective data of dielectric spectra for certain key biochemical markers in order to consider its potential application as a non-destructive control sensor for the prediction of pork meat quality.     

Prediction of the degree of starch gelatinization in wheat flour dough during microwave heating

In order to produce quick-boiling noodles, we made partially pre-gelatinized wheat flour dough by the microwave heating method. A cylindrical-shaped piece of wheat flour dough containing 3% NaCl was heated by 117 W microwave oven operating at 2450 MHz. The sample dough was intermittently heated to allow sufficient gelatinization of the starch granules of the dough. The resultant changes in the internal temperature profiles of the sample dough were measured and compared with numerical prediction, which is the two-dimensional heat conductive equation with a term for internal heat generation based on Lambert’s law. In order to calculate the internal heat generation during microwave heating, we measured the dielectric properties of wheat flour dough at 2450 MHz from 10 to 70 °C by the open-ended coaxial probe method. The calculated temperature history could describe the feature of the experimental one during intermittently microwave heating. Furthermore, the progress of starch gelatinization according to the heat transfer in the sample dough was predicted by the Runge–Kutta gel method. A tendency that the total gelatinization degree increased slowly during the microwave intermittent heating was obtained in the calculated result.     

Rheological analysis of emulsion-filled gels based on high acyl gellan gum

Emulsion-filled gels are widely used in cosmetic, food, and pharmaceutical industry. As rheological properties of these systems are strongly dependent on the properties of the gelled polymer network, rheological characteristics of gels containing high and low acyl gellan gum were analyzed. Under the processing conditions low acyl emulsions were unstable, thus in the present work the influence of oil and hydrocolloid concentrations on the viscoelastic behavior of emulsion-filled gels containing high acyl gellan gum was studied. Increasing gellan concentration (from 0.1 g/100 g to 0.5 g/100 g) produced stronger gels, while oil fraction (10 g/100 g–30 g/100 g) slightly affected the elastic behavior of the emulsions reinforcing the structure and the elastic characteristics of the gellan matrix. Sauter diameter (d₃₂) was measured for all emulsions and an average value of 12 μm was obtained. Rheological data (oscillatory and creep–recovery tests) were successfully modeled to interpret the structural characteristics of the gelled emulsions. The broadened Baumgaertel–Schausberger–Winter spectrum was used to represent the linear viscoelastic behavior of the continuous phase and the emulsified system, showing that the rheological behavior of the systems was controlled by the highly structured continuous phase rather than the contribution of filler lipid droplet in the emulsion. Relaxation spectra were validated using creep–recovery experiments. Regardless of hydrocolloid concentration, creep compliance of the gel emulsions decreased compared with their respective gels, showing that the inclusion of oil droplets produced a reinforcement of the structure and the gel strength of the matrix.     

Dielectric properties of egg whites and whole eggs as influenced by thermal treatments

Effects of cooking on dielectric properties of liquid whole eggs and liquid egg whites were studied in connection with radio frequency and microwave heating processes to preserve shelf-stable products. Dielectric measurements were made using an open-ended coaxial probe method over a temperature range of 20 and 120 °C at radio frequencies 27 and 40 MHz, and microwave frequencies 915 and 1800 MHz. Thermal denaturation of liquid egg whites and whole eggs influenced the dielectric constants and dipole loss component of eggs, as reflected by changes in loss factors above 60 °C. In addition, loss factor of liquid whole eggs was generally smaller than that of egg whites and larger than the loss factor of egg yolk. Ionic conductivity was a dominant factor determining the dielectric loss behavior of egg products at radio frequencies, whereas dipole water molecules played an increasing role with an increase in microwave frequencies.     

Microwave assisted solid state reaction synthesis of methionine complexes of iron (II)

Methionine complexes of iron (II) were synthesized by employing a simple and novel microwave assisted solid state route in a domestic microwave (MW) oven (2.45 GHz frequency, 700 W power) and six crucial reaction parameters were investigated on the basis of their individual effects on the yield of the synthesized compounds. Results suggest that optimal synthesis conditions are microwave irradiation time, 260 s; the airflow temperature, 100 °C; and the amount of reactant, 1.5 g. The synthesized compounds were characterized by infrared spectrometry. From the results of chemical analysis and infrared spectrometric analyses, the minimum molecular formula of the synthesized compounds could be determined as Fe(Met)₂ · H₂O, in which iron, nitrogen element and water account for 15.93%, 7.97% and 0.07%, respectively. The microwave method was found to be simple, fast and efficient, with high selectivity for the preparation of these compounds.     

Non-additive response of starch systems in different hydration states: A study of microwave-absorbing properties

In the food industry, microwave-based curing and modification represent green, low-energy-consumption techniques for food processing, offering significant advantages. Water, a representative high-dielectric material, can react with many food components; however, few studies on microwave absorption to these components have been reported. In this study, commercially available potato starch and rice starch (the latter a representative cereal starch) were selected as the research materials. Based on vector network analysis of these materials, the bow coaxial probe and improved arch methods were used to study the dielectric properties and microwave reflection loss of the potato starch and rice starch systems, with different hydration levels, at 2.45 GHz. Their absorption behaviour was compared with the classical model to study the absorption properties of the starch systems under different hydration conditions. The results showed that at ambient temperature, potato starch and rice starch had similar dielectric responses, which both showed a steady rise with increasing water content. Therefore, because of their similar dielectric properties, the two systems behaved similarly in the microwave field. However, further experiments found that the microwave absorption properties of both starches differed significantly from that of water: starch–water systems with different compositions varied nonlinearly in their microwave absorption performance, consistently with the “true single layer of water” theory.     

Dielectric spectroscopy measurements for moisture prediction in Vidalia onions

Microwave sensing offers an opportunity to determine nondestructively the amount of moisture in materials by sensing the dielectric properties of the material. Dielectric properties of Vidalia onions grown in southeastern Georgia were measured with an open-ended coaxial-line probe and network analyzer in the range from 200 MHz to 20 GHz. Frequency dependence and moisture dependence of dielectric properties were analyzed for moisture contents between 8% and 91%. Moisture content was linearly correlated with the dielectric constant at higher frequencies for the entire moisture range. A density-independent function that incorporates both the dielectric constant and loss factor was tested across multiple frequencies and moisture ranges. Use of this function enabled prediction of moisture content with high accuracy (R² = 0.99) up to 40% moisture content.