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.     

Food protein network formation and gelation induced by conductive or microwave heating: A focus on hen egg white

Heat-induced protein polymerization, gelation and the associated molecular scale changes were investigated during conductive and microwave heating (CH and MH, respectively). Given identical temperature-time profiles, polymerization of bovine serum albumin, ovalbumin, egg white (EW) and wheat gluten protein occurred faster and to a greater extent with CH than with MH. Moreover, lanthionine cross-links were present in some MH but in none of the CH samples. Gelation of EW protein by CH or MH was studied in more detail. Differential scanning calorimetry and fluorescence spectroscopy indicated that EW proteins initially denatured to a larger extent with CH than with MH. Size exclusion high performance liquid chromatography demonstrated that CH resulted in more heavily cross-linked EW protein networks than MH. Homogeneous EW gels were obtained by 15 min CH at 80 °C while 120 min MH at this temperature still resulted in inhomogeneous (with liquid and solid zones) gels. This was affirmed by low resolution ¹H nuclear magnetic resonance measurements which indicated more mobile water in MH than in CH samples. However, MH gels had higher firmness than the corresponding CH gels. Thus, MH here led to less covalent cross-linking than CH, but still resulted in firmer gels most likely because its protein network relied heavily on non-covalent interactions.Industrial relevanceThis study highlighted differences between microwave and conventional, conductive, heating of different protein materials. While literature is divided about the origin of differences noted between these different heating modes, it is clear from our data that heating mode, given identical temperature-time profiles, impacts the changes that proteins undergo during heating. The knowledge that microwave heating favors protein unfolding, aggregation and cross-linking to a lower extent than conductive heating can likely be exploited in industrial applications.     

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.     

Exploring the heating patterns of multiphase foods in a continuous flow,simultaneous microwave and ohmic combination heater

A continuous flow, simultaneous microwave and ohmic combination heater was designed and fabricated to heat treat particulate foods without leaving solids under-processed. Heating uniformity of the combination heater was examined by numerically analyzing the electric field distribution under microwave and ohmic heating. In addition, to minimize the reflection of microwave power, impedance matching of the microwave cavity was conducted with a vector network analyzer. Performance of the heater was studied using food mixtures containing sodium chloride solutions (0.2–0.5%) and carrot particulates. Heating patterns of liquid–particle mixtures were investigated and compared under individual and combination heating modes. Energy efficiencies were also determined for corresponding heating methods. The results showed that maximum solid–liquid temperature differences under microwave and ohmic heating were about 8.1 and 8.0 °C, respectively. However when microwave and ohmic heating techniques were applied simultaneously, there was no significant temperature difference between solid and liquid phases. Energy efficiency of combination heating was higher than microwave heating and a maximum increase in energy conversion of 12.8% was obtained. The findings opened new and very promising opportunities to thermally process particulate foods with improved uniformity, organoleptic, and nutritional quality in addition to reduced food safety problems.     

A simplified approach to assist process development for microwave assisted pasteurization of packaged food products

The purpose of this study was to develop a convenient method to assist the food industry in developing process schedules for production of ready-to-eat meals using microwave assisted pasteurization system (MAPS). An analytical model was applied to estimate the temperature increase in the cold zone in packaged foods during heating in a 915 MHz single-mode microwave system. This model was validated in a pilot-scale four-cavity MAPS using mashed potato-gellan gum model food with different thicknesses (22 to 36 mm) and salt contents (0.0 to 1.0%). Mobile sensors were placed in the packages to measure temperature at the pre-determined cold spots. For 2.48 min of microwave heating with 5, 5, and 8.7 kW 915 MHz microwave powers, the highest temperature increase at the cold spot during microwave heating was 33.2 °C in the 22 mm thick model food with 0.6% salt content, whereas the lowest temperature increase was 10.3 °C in the 36 mm thick model food with 1.0% salt content. There was a deviation of 1.9 ± 1.2 °C between experimental and predicted data with an R² of 0.89. A simplified chart was developed based on the validated analytical results to allow rapid prediction of temperature increases in MAPS as influenced by food dielectric properties and package thickness. Examples were used to illustrate how the chart could assist in process scheduling. The chart can help assess the heating rates of various pre-packaged food products in a specific industrial MAPS or guide product development for desired heating uniformity.     

Dielectric properties of water relevant to microwave assisted thermal pasteurization and sterilization of packaged foods

Microwave assisted thermal sterilization and pasteurization systems, as emerging thermal processing technologies, use circulation water to reduce edge heating of the food packages in the microwave heating sections. Yet, the influence of the circulation water on microwave heating efficiency for food is unknown. This research studied the dielectric properties of purified water from a reverse osmosis unit and tap water over microwave frequencies of 150–2500 MHz at 23–120 °C and evaluated microwave loses in the circulation water. The values of the dielectric constant of purified and tap water decreased (P < 0.05) with increasing temperature and slightly decreased with increasing frequency. The value of the loss factor of purified water also decreased with temperature (P < 0.05) but increased with frequency. The trend was more complicated for the loss factor of tap water. The penetration depth of 915 MHz microwave power in purified water was 399 and 457 mm at 90 and 120 °C, respectively, it was reduced to 161 and 136 mm in tap water. The microwave power loss in the 18–30 mm deep purified water, depending on the thickness of the food packages, was 4–7%, compared to 11–20% in tap water. Thus, using purified circulation water in microwave assisted thermal sterilization (MATS) and pasteurization systems (MAPS) can significantly improve the efficiency of microwave heating of packaged foods.Industrial relevanceThe information from this research are highly relevant to industrial operations of microwave-assisted thermal sterilization and pasteurization systems of pre-packaged foods. The dielectric property data will contribute to the literature related to the physical properties of water in microwave heating.     

Dielectric properties of salmon (Oncorhynchus keta) and sturgeon (Acipenser transmontanus) caviar at radio frequency (RF) and microwave (MW) pasteurization frequencies

Radio frequency (RF) and microwave (MW) heating provide an important advantage of more rapid heat penetration in pasteurization processes for heat labile high value foods, which to date, have only been pasteurized by conductive heating. The objectives of this work were to determine the dielectric constant, loss factor and power penetration depth for salmon (0.8% and 2.3% total salt) and sturgeon (0.20 and 3.3% salt) caviars at RF frequency of 27 MHz and MW frequency of 915 MHz (20–80 °C). The dielectric constant (ε′) and dielectric loss factor (ε″) for salmon and sturgeon caviar increased markedly with increasing temperature at 27 MHz but not at 915 MHz. Power penetration depth was higher at 27 MHz compared to 915 MHz, and in unsalted compared to salted roe. Power penetration depth tended to decrease as temperature increased.     

Temperature-dependent dielectric properties of honey associated with dielectric heating

Dielectric properties of pure yellow-locust and jujube honey and their water solutions at seven final water content levels from 17.4% to 42.6% were measured with an open-ended coaxial-line probe from 10 to 4500 MHz at 20-80 °C. The results showed that the dielectric constants of pure honey increased with increasing temperature when frequency was above 40 MHz. For honey solutions, the dielectric constant decreased with increasing temperature at the lower frequencies, but increased at the higher frequencies. At any given water content, the relaxation frequency shifted to a higher frequency with increasing temperature, while the loss factor peaks showed little change. There was a strong negative linear correlation between penetration depth and frequency in log-log plot for pure honey at 20-80 °C. The research will be helpful in developing effective honey pasteurization processes with radio-frequency and microwave dielectric heating.     

The survivability of Bacillus anthracis (Sterne strain) in processed liquid eggs

In this study, we investigated the survival and inactivation kinetics of a surrogate strain of Bacillus anthracis (Sterne strain) in whole egg (WE), egg white (EW), sugared egg yolk (YSU), and salted egg yolk (YSA) at low (−20, 0, and 5 °C), moderate (15, 20, 25, 30, 35, and 40 °C), and high storage temperatures (45, 50, 55, and 60 °C). Outgrowth of the spores was measured as lag phase duration (LPD). Replication of vegetative cells was measured in terms of growth rate (GR) and maximum population density (MPD). Spore inactivation was recorded as inactivation rate and percent reduction in viable count. In general, spore viability decreased at low and high temperatures and increased at moderate temperatures. At 0 and 5 °C, a 60–100% reduction in spore viability was seen within 2–3 weeks in WE and YSU, 0–30% in YSA, and 50–100% in EW. At −20 °C, however, no drop in spore titer was observed in YSU and EW but a 20% drop in titer was seen in YSA and 50% in WE within 2–3 weeks. At high temperatures, WE, EW, and YSA produced a 20–50% drop in the spore titer within 1–4 h whereas YSU showed 100% inactivation within 0.75 h. At moderate storage temperatures, as the temperature increased from 15 to 40 °C, LPD decreased from 13.5 to 0.75 h and MPD reached 0.27–2.2 × 10⁹ CFU/ml in YSU and WE, respectively. Markedly lower growth was observed in YSA (LPD = 24–270 h, MPD = 9 × 10⁵ CFU/ml) and spores were inactivated completely within 1–6 h in EW. The survivability and inactivation data of B. anthracis in liquid egg products reported in this investigation will be helpful in developing risk assessment models on food biosecurity.     

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.     

Temperature and moisture dependent dielectric properties of legume flour associated with dielectric heating

Dielectric properties data are important in developing thermal treatments using radio frequency (RF) and microwave (MW) energy and are essential in estimating heating uniformity in electromagnetic fields. Dielectric properties of flour samples from four legumes (chickpea, green pea, lentil, and soybean) at four different moisture contents were measured with an open-ended coaxial probe and impedance analyzer at frequencies of 10-1800 MHz and temperatures of 20-90 °C. The dielectric constant and loss factor of the legume samples decreased with increasing frequency but increased with increasing temperature and moisture content. At low frequencies and high temperatures and moisture contents, negative linear correlations were observed between the loss factor and frequency on a log-log plot, which was mainly caused by the ionic conductance. At 1800 MHz, the dielectric properties data could be used to estimate the legume sample density judging from high linear correlations. Loss factors for the four legume samples were similar at 27 MHz, 20 °C and low moisture contents (e.g. <15 g/100 g). At the highest moisture content (e.g. 20 g/100 g) soybean had the highest loss factor at 27 MHz and 20 °C, followed by lentil, green pea, and chickpea. The difference in loss factor among the four legumes did not show clear patterns at 915 MHz. Deep penetration depths at 27 MHz could help in developing large-scale industrial RF treatments for postharvest insect control or other applications that require bulk heating in legumes with acceptable heating uniformity and throughputs.     

A microwave and ohmic combination heater for uniform heating of liquid-particle food mixtures

Microwave and ohmic combination heating was proposed to improve the uniformity of thermal processing of particulate foods. Thermal patterns of a liquid-particle mixture in a small test cell were studied using both experimental and simulation approaches. Carrot cubes (10 mm × 10 mm × 10 mm) and 0.1% NaCl salt solution were used as model foods. The temperature distribution of solid and liquid phases was examined using individual and combination heating methods. Under ohmic heating, the liquid was heated faster by 18.9 °C after 250 s. The heating rate of a carrot cube was faster than liquid under microwave heating and temperature rise of carrot was approximately 11.2 °C higher than that of solution after heating of 70 s. Samples experienced different heating patterns over time during the combination heating. Carrot samples showed a thermal lead initially when heated under microwave and the trend reversed during the second stage when ohmic heating was applied. Liquid-particle temperature difference was reduced as the combination heating proceeded, and came to be less than 2 °C at the end. Results obtained from simulation showed similar patterns and all prediction data agreed well with the experimental data. The prediction errors for sample temperatures ranged from 5.7% to 11.6%. The results provided better understanding for designing a continuous flow combination heater that can produce uniform temperature of solid-liquid mixtures. PRACTICAL APPLICATION: If successful, this combination heating technique will find its way to effective aseptic or sterile processing of low acid multiphase foods containing large particulates (such as soups with meatballs or vegetables) that has not been a commercial reality in the United States.     

微波杀菌研究进展及其在食品工业中的应用现状

综述了微波杀菌研究进展及其在食品工业中的应用现状。微波加热对微生物和酶的热力效应,目前已深入到对杀菌对象不同种类微生物的致死和酶的钝化作用机理研究,而非热力效应还难以量化。微波杀菌还必需对内容物以及包装材料进行研究。微波杀菌技术在食品工业中的应用受到技术、经济和商业因素制约,杀菌过程缺乏有效的在线温度测定和控制的手段,目前还难以建立一套可靠的程序和依据来评估微波杀菌的效果和安全性。     

Dielectric properties of ground almond shells in the development of radio frequency and microwave pasteurization

To develop pasteurization treatments based on radio frequency (RF) or microwave energy, dielectric properties of almond shells were determined using an open-ended coaxial-probe with an impedance analyzer over a frequency range of 10–1800 MHz. Both the dielectric constant and loss factor of almond shells decreased with increasing frequency, but increased with increasing temperature and moisture content. The absolute value of the slopes of log–log plots between loss factor and frequency increased with increasing temperature at low frequencies, especially at high temperatures and moisture contents. The effective electrical conductivity of shell samples was close to zero at the lowest moisture content (6% w.b.) and 3–9 times larger at 90 °C than 20 °C for the highest moisture content (36% w.b.). A good linear relationship was observed between permittivity and density at 1800 MHz. The power penetration depths at RF range (27 and 40 MHz) were about 6–24 times as deep as those for microwave frequencies (915 and 1800 MHz) at each corresponding temperature and moisture content. It is likely that RF energy may provide uniform heating and high throughput treatments for controlling Salmonella in in-shell almonds after washing.     

Dynamic high pressure-induced gelation in milk protein model systems

The structure-functional properties of milk proteins are relevant in food formulation. Recently, there has been growing interest in dynamic high-pressure homogenization effects on the rheological-structural properties of food macromolecules and proteins. The aim of this work was to evaluate the effects of different homogenization pressures on rheological properties of milk protein model systems. For this purpose, sodium caseinate (SC) and whey protein concentrate (WPC) were dispersed at different concentrations (1, 2, and 4%), pasteurized, and then homogenized at 0, 18 MPa (conventional pressure, CP), 100 MPa (high pressure, HP), and 150 MPa (HP+). Differences in viscosity were observed between WPC and casein dispersions according to concentration, heat treatment, and homogenization pressure. Mechanical spectra described the characteristic behavior of solutions except for the WPC 4% pasteurized sample, in which a network formed but was broken after homogenization. Dispersions with different ratios of WPC and SC were also made. In these systems, pasteurization alone did not determine network formation, whereas homogenization alone promoted cold gelation. A total concentration of at least 4% was required for homogenization-induced gelation in pasteurized and unpasteurized samples. Gels with higher elastic modulus (G′) were obtained in more concentrated samples, and a bell-shaped behavior with the maximum value at HP was observed. The HP treatment produced stronger gels than the CP treatment. Similar G′ values were obtained when different concentrations, pasteurization conditions, and homogenization pressures were combined. Therefore, by setting appropriate process conditions, systems or gels with tailored characteristics may be obtained from dispersions of milk proteins.     

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.     

Thermodynamic incompatibility between denatured whey protein and konjac glucomannan

The current study investigated the effect of a neutral polysaccharide, konjac glucomannan, on the heat-induced gelation of whey protein isolate (WPI) at pH 7. Oscillatory rheology (1 rad/s; 0.5% strain), differential scanning calorimetry and confocal laser scanning microscopy were used to investigate the effect of addition of konjac in the range 0-0.5% w/w, on the thermal gelation properties of WPI. The minimum gelling concentration for WPI samples was 11% w/w; the concentration was therefore held constant at this value. Gelation of WPI was induced by heating the samples from 20 to 80 °C, holding at 80 °C for 30 min, cooling to 20 °C, and holding at 20 °C for a further 30 min. On incorporation of increasing concentrations of konjac the gelation time decreased, while the storage modulus (G′) of the mixed gel systems increased to ∼1450 Pa for 11% w/w WPI containing 0.5% w/w konjac gels, compared to 15 Pa for 11% w/w WPI gels (no konjac). This increase in gel strength was attributed to segregative interactions between denatured whey proteins and konjac glucomannan.     

SMALL AMPLITUDE OSCILLATORY TESTING (SAOT). INSTRUMENTATION DEVELOPMENT AND APPLICATION TO COAGULATION OF EGG ALBUMEN,WHEY PROTEIN CONCENTRATE AND BEEF WIENER EMULSION3

An instrument to measure the development of protein gels during thermal coagulation was developed. Very small oscillatory movements, sufficiently small to avoid damaging forming structures, were imposed on the sample trapped within a specially constructed cell, and the torques transferred through the sample sensed with strain gauges. Temperatures were controlled with one heating and one refrigerated (20°C) bath and the sample properties determined through both heating and cooling cycles. Egg white (EW) whey protein concentrate (WPC) and beef wiener emulsion (BWE) were tested. EW and WPC were characterized by delayed onset of gelation followed by high temperature thickening. Cooling further stiffened the gel in both cases. B WE was characterized by an initial decrease in transmitted torque as fat melting was detected. This was followed by a rapid rise in transmitted torque as the protein coagulated, followed by a further increase or stiffening on cooling. Detailed parameters describing the thermal gelation of the three materials are given.     

Characterisation and thermo-reversible gelation of cod muscle protein isolates

Cod (Gadus Morhua) muscle proteins were solubilized using alkaline treatment of the muscle. Solutions of similar protein composition were obtained between pH 10.5 and 12.0, however, pH > 11 was required for optimal yield. Addition of salt (up to 0.25 M NaCl) did not affect protein yield or composition. Light scattering showed that a significant fraction of the proteins was present as large self similar and flexible aggregates. When the pH was decreased below 10, gelation was observed below a critical temperature of about 25 °C, which could be reversed by heating. Slow irreversible aggregation was also observed leading to coarsening and syneresis of the gels or precipitation at higher temperatures. The rate of irreversible aggregation increased with decreasing pH and was fast below pH 8. Homogeneous thermo-reversible self supporting gels that were stable for a period of days could be prepared without heating at a narrow pH range between 8.5 and 9.5.     

Ohmic cooking of whole beef muscle – Optimisation of meat preparation

Uniform ohmic heating of solid foods primarily depends on the uniformity of electrolyte distribution within the product. Different preparation techniques were tested in an attempt to ensure an even salt dispersion within a full beef muscle (biceps femoris). Meat pieces were soaked, injected and tumbled using a range of procedures before ohmic cooking at pasteurization temperatures. A final preparation method (multi-injection (five points) with a 3% salt solution followed by 16 h tumbling) was validated. Selected quality parameters of the ohmically cooked products were compared to steam cooked products. Ohmically heated meat had a significantly (P < 0.05) uniform lighter and less red colour. Cook loss was significantly lower (P < 0.05) in ohmic samples and in relation to tenderness ohmic heated samples were tougher (P < 0.05) though the difference was only 5.08 N. Comparable cook values were attained in the ohmic and conventionally cooked products.