Ultrasound-assisted Immersion Freezing Research Articles

Effect of infused CO2 in a model solid food on the ice nucleation during ultrasound-assisted immersion freezing

To understand the mechanism of ice nucleation induced by ultrasound in solid food matrix, the effect of infusion of CO2 (ICO) in a model solid food system (gelatin gel) on the dynamic nucleation of ice during ultrasound-assisted immersion freezing (UIF) was investigated. The results showed that the content of absorbed CO2 in a gelatin gel samples increased with ICO-treated time increase and a great number of micro bubbles were observed in the samples by light microscope. The nucleation temperatures of the samples with/without ICO treatment increased by the application of ultrasound. In addition, there was a better correlation between the nucleation temperature and the ultrasound onset temperature for the samples infused with CO2. Moreover, compared to the samples without ICO treatment, the nucleation delay was shorter and nucleation temperature was closer to ultrasound irradiation temperature. It can be concluded that cavitation and ultrasonic irradiation parameters are factors affecting the nucleation of model solid food and CO2 infusion can be used to initiate the nucleation of solid samples during UIF.

Ultrasound-assisted immersion freezing accelerates the freezing process and improves the quality of common carp (Cyprinus carpio) at different power levels

This study evaluated the effect of ultrasound-assisted immersion freezing (UIF) on the freezing speed and quality of common carp (Cyprinus carpio) at different ultrasonic power levels. The results revealed that UIF increased the speed of freezing, with UIF at 175 W (UIF-175) being optimal. Compared to refrigerator freezing and immersion freezing, UIF samples had less thawing and cooking loss. Microstructural observation showed that UIF-175 reduced pore diameter and ice crystal size. As ultrasonic power increased, the L*-value first decreased and then increased (P < 0.05), and there were no significant differences in a*- or b*-values (P > 0.05). UIF-175 samples exhibited the shortest T21 and T22 relaxation time, which indicated that this appropriate ultrasonic power level reduced the mobility of free and immobilized water. Therefore, UIF at appropriate ultrasonic power significantly decreased the freezing time of fish samples and improved the fish meat quality.

Effects of ultrasound-assisted freezing at different power levels on the structure and thermal stability of common carp (Cyprinus carpio) proteins

This study investigated the effect of ultrasound-assisted immersion freezing (UIF) at different ultrasonic power levels on the myofibrillar protein primary, secondary and tertiary structures of common carp (Cyprinus carpio). Furthermore, protein thermal stability, electrophoresis pattern, and microstructure of the muscle tissue were also studied. Compared with a control, an ultrasonic power of less than 175 W had no significant negative effect on protein primary structure (P > 0.05), including total sulfhydryl, reactive sulfhydryl, carbonyl groups, free amino groups, dityrosine content, and surface hydrophobicity. UIF at 175 W (UIF-175) minimized the changes in protein secondary and tertiary structures. There were no obvious changes in the SDS-PAGE patterns of the control and frozen sample proteins. Microstructure analysis showed that an appropriate ultrasonic power (UIF-175) promoted the formation of smaller and more uniform ice crystals, reduced the damage of muscle tissue by ice crystals, and maintained the sarcomere integrity. In addition, UIF-175 samples had higher protein thermal stability. Overall, ultrasound treatment at a proper power (UIF-175) effectively minimized the changes in protein structure and protected the protein thermal stability during freezing process.

Changes in microstructure, quality and water distribution of porcine longissimus muscles subjected to ultrasound-assisted immersion freezing during frozen storage

The effect of ultrasound-assisted immersion freezing at 180 W (UIF-180) on the microstructure, quality and water distribution of porcine longissimus muscles during frozen storage was evaluated. The size of the ice crystals increased with extended storage time, and UIF-180 samples had a smaller size and uniform distribution of ice crystals. The thawing and cooking losses in the UIF-180 sample were significantly lower than that in air freezing (AF) and immersion freezing (IF) samples (P < 0.05). AF samples had a higher cutting force at 0 days. During the 60–180 days of the storage period, the cutting force of UIF-180 samples was significantly higher than that of AF and IF samples (P < 0.05). Low field-nuclear magnetic resonance results showed that UIF-180 decreased water migration during frozen storage. UIF-180 samples had significantly lower lipid oxidation and higher redness than that of the AF and IF samples (P > 0.05). Overall, UIF at particular powers is an efficient method in reducing quality deterioration of muscles during long-term frozen storage.

The comparison of ultrasound-assisted immersion freezing, air freezing and immersion freezing on the muscle quality and physicochemical properties of common carp (Cyprinus carpio) during freezing storage

This study investigated the impact of ultrasound-assisted immersion freezing (UIF), air freezing (AF), and immersion freezing (IF) on the ice crystal size, protein thermal stability, and physicochemical properties of common carp (Cyprinus carpio) muscle during frozen storage. UIF samples had smaller ice crystals throughout the storage period than AF and IF samples did, which led to less damage to the muscle tissue. Low-field nuclear magnetic resonance analysis revealed that UIF reduced the mobility and loss of immobilized and free water. The thawing and cooking losses in the UIF samples were significantly lower than those in the IF and AF samples (P < 0.05). The AF samples had a higher shear force (P < 0.05) than UIF and IF samples did at the beginning of storage, and then the shear force reduced rapidly. During the 90–180 days, the shear force of the UIF samples was higher than that of the AF and IF samples (P < 0.05). Decreases in the Tmax and enthalpies were observed for all of the treatments during storage, and the UIF samples had a higher protein thermal stability than AF and IF samples did. The UIF samples showed lower thiobarbituric acid reactive substance and total volatile basic nitrogen values during storage than the AF and IF samples did (P < 0.05). Principal component analysis showed that there were significant correlations between the freezing methods and the ice crystal size, protein thermal stability and physicochemical characteristics of frozen muscles. Overall, UIF was an effective way to inhibit the deterioration of frozen fish during frozen storage.

Synergistic effects of ultrasound and soluble soybean polysaccharide on frozen surimi from grass carp

Ultrasound and water soluble soybean polysaccharide (SSPS) were applied during the freezing of grass carp surimi. Ultrasound-assisted immersion freezing (UF) process was observed from 0 °C to −15 °C. Based on characteristic freezing time, tempering-stage freezing rate and the quality change of surimi after a 14-day storage period, the optimal sonication was performed 5 times at 300 W, for 10 s, with 40 s intervals. The cryoprotective effects of SSPS content on surimi myofibillar protein were subsequently investigated during 28-day frozen storage at −18 °C. The Ca2+-ATPase activity, total sulphydryl content, active sulphydryl content, salt extractable protein content, whiteness and water-holding capacity of frozen surimi were determined after adding SSPS (0%, 1%, 3% and 5%). The results showed that synergism of SSPS and UF occurred. Consequently 3% SSPS was best for mitigating the protein denaturation during processing.

Freezing Efficiency and Quality Attributes as Affected by Voids in Plant Tissues During Ultrasound-Assisted Immersion Freezing

Voids, filled with air, in plant tissues, can attenuate ultrasound, resulting in weakening the effectiveness of ultrasound during immersion freezing. The effect of voids on ultrasound-assisted immersion freezing (UF) in selected plant tissues, apple, radish, and potato was investigated in the present study. The freezing time and quality attributes of firmness, drip loss, total calcium content, and total phenolic content were investigated in apple, radish, and potato samples treated by normal immersion freezing (IF) and UF. The results showed that the more the percentage voids in the plant tissues, the lower the effectiveness of the ultrasound treatment. The total freezing time reduction (%) due to UF was a power function of the volume of voids (%): y = 0.018x−1.057 (R2 = 0.994). Ultrasound at 0.62 W/cm2 (28 kHz) resulted in the best firmness and lowest drip loss in potato, while no significant (p > 0.05) differences in quality attributes were observed between IF and UF in apple samples. These findings indicated that UF was more effective in freezing fruit or vegetables with a highly dense structure.

Influence of ultrasound-assisted immersion freezing on the freezing rate and quality of porcine longissimus muscles

The objective of this study was to evaluate the effect of ultrasound-assisted immersion freezing (UIF) on the freezing rate and quality of porcine longissimus muscles under different ultrasound powers. UIF with a certain level of ultrasound power significantly (P>0.05) accelerated the freezing rate. The phase transition times of samples treated with UIF at 180W (UIF-180) were the shortest. There were no significant differences (P>0.05) in a* (redness), b* (yellowness), pH values or cooking loss among UIF, IF, and control (fresh muscle) samples. Investigation of the microstructure of frozen muscles demonstrated that UIF-180 remarkably reduced the size of ice crystals and made their distribution more uniform. UIF-180 samples showed a significant (P<0.05) reduction in thawing loss and T21 and T22 relaxation times compared with other treatments, which meant that UIF at certain powers could reduce the mobility and loss of immobilized and free water. The results showed that UIF at certain powers significantly increased the freezing rate of muscle samples and improved meat quality.

Potential Life Cycle Carbon Savings for Immersion Freezing of Water by Power Ultrasound

Since the food cold chain produces large amounts of greenhouse gases (GHGs), its carbon footprint (CF) has caused widespread concern. The process of freezing, one of the most common operations, is used in numerous food processes, and new freezing methods appear constantly. Ultrasound-assisted immersion freezing is an emerging technique that can significantly improve product quality compared to the conventional immersion freezing. In the current study, the CFs of conventional immersion freezing and ultrasound-assisted immersion freezing were investigated according to the principle of life cycle assessment (LCA). The freezing time of 1 mL of deionized water in a flask was 264 ± 15 s for conventional freezing, while the time of 1 mL of deionized water in a flask for the ultrasonic-assisted freezing could be lowered to 188 ± 5, 182 ± 5, 193 ± 6, and 201 ± 8 s under the four ultrasonic frequencies of 28, 40, 50, and 80 kHz, respectively. By improving heat transfer and enhancing ice nucleation, the freezing time of ultrasonic-assisted freezing was reduced significantly (P < 0.05) compared to conventional immersion freezing. In addition, electricity use and carbon emissions from electricity use at four frequency levels were reduced significantly (P < 0.05) compared to conventional immersion freezing. The final CFs of the freezing process for 1 mL of frozen water were 11.65 ± 0.52, 9.16 ± 0.24, 8.95 ± 0.17, 9.33 ± 0.22, and 9.60 ± 0.27E−5 kgCO2eq (equivalent) for the control, 28, 40, 50, and 80 kHz, respectively. Most of the final product CF was related to electricity and the flask used, contributing 78.1 and 19.7 % respectively for the conventional freezing, and about 70 and 25 % respectively for the ultrasound-assisted freezing. This research demonstrated that ultrasonic-assisted freezing as an emerging technology could not only improve the food quality but can also reduce the CF of a product. In addition, sensitivity analysis showed that the use of flask and improving the electricity use efficiency could decrease the carbon emissions of the product significantly.

Experimental analysis and modeling of ultrasound assisted freezing of potato spheres

In recent years, innovative methods such as ultrasound assisted freezing have been developed in order to improve the freezing process. During freezing of foods, accurate prediction of the temperature distribution, phase ratios, and process time is very important. In the present study, ultrasound assisted immersion freezing process (in 1:1 ethylene glycol–water solution at 253.15K) of potato spheres (0.02m diameter) was evaluated using experimental, numerical and analytical approaches. Ultrasound (25kHz, 890Wm−2) was irradiated for different duty cycles (DCs=0–100%). A finite volume based enthalpy method was used in the numerical model, based on which temperature and liquid fraction profiles were simulated by a program developed using OpenFOAM® CFD software. An analytical technique was also employed to calculate freezing times. The results showed that ultrasound irradiation could decrease the characteristic freezing time of potatoes. Since ultrasound irradiation increased the heat transfer coefficient but simultaneously generated heat at the surface of the samples, an optimum DC was needed for the shortest freezing time which occurred in the range of 30–70% DC. DCs higher than 70% increased the freezing time. DCs lower than 30% did not provide significant effects on the freezing time compared to the control sample. The numerical model predicted the characteristic freezing time in accordance with the experimental results. In addition, analytical calculation of characteristic freezing time exhibited qualitative agreement with the experimental results. As the numerical simulations provided profiles of temperature and water fraction within potatoes frozen with or without ultrasound, the models can be used to study and control different operation situations, and to improve the understanding of the freezing process.

Effects of different freezing methods on the quality and microstructure of lotus (Nelumbo nucifera) root

The effects of three freezing methods, air blast freezing (ABF), immersion freezing (IF) and ultrasound-assisted immersion freezing (UIF), on quality and microstructure of lotus roots were investigated. The parameters used to evaluate the freezing methods effect were the freezing time, color, firmness, drip loss, vitamin C and microstructure of the final frozen products. The results showed that the UIF products had several advantages in terms of the freezing time, color, firmness and drip loss over ABF and IF. No significant difference (p > 0.05) of vitamin C content was observed between the ABF and IF products, while significant difference (p < 0.05) of vitamin C was observed between UIF and ABF/IF products. ABF caused the largest destruction to the tissue, while the microstructure of the UIF products was the best preserved. It is concluded that UIF processing was a better freezing method for lotus root with improved quality and less damaged microstructure than the two other methods.

Effect of physicochemical properties on freezing suitability of Lotus (Nelumbo nucifera) root

The initial physicochemical properties of different lotus roots were studied with the aim to evaluate their influence on freezing suitability. Six physicochemical properties indicators (starch content, whiteness index, maximum diameter, protein amount, titratable acidity and vitamin C) of lotus root were determined by principal component analysis. It was observed from the experimental results that the initial physicochemical properties of lotus root can directly affect the freezing suitability in terms of initial freezing temperature, transition phase time, total freezing time, relative firmness and drip loss. Correlation analysis suggested that relative firmness increases in proportion to starch content, and initial freezing temperature and drip loss are inversely proportional to the starch content. Compared to immersion freezing, power ultrasound can significantly improve the freezing rate and relative firmness but decrease drip loss. These results suggested that high quality frozen lotus root can be achieved via appropriately selecting high starch content lotus root and using ultrasound-assisted immersion freezing.

Influence of power ultrasound on ice nucleation of radish cylinders during ultrasound-assisted immersion freezing

The effect of ultrasound-assisted immersion freezing on the dynamic nucleation of ice and the delay from ultrasound application onset to nucleation commencement of radish cylinder samples were studied. The samples were frozen in 30% (w v−1) CaCl2 solution (−20 °C) in an ultrasonic bath system. To evaluate nucleation, ultrasound irradiation (20 kHz) was carried out with different durations (0 s, 3 s, 7 s, 10 s or 15 s), onset temperature (−0.5, −1, −1.5 and −2 °C) and intensities (0.09, 0.17, 0.26 and 0.37 W cm−2). The results showed that ultrasound irradiation was able to induce nucleation and the nucleation temperature of radish cylinder samples exhibited a good fit to linear equation with the ultrasound irradiation temperature under 7 s duration and 0.26 W cm−2 intensity. Ultrasound irradiation temperature at −0.5 °C for 7 s duration with intensity of 0.26 W cm−2, was an optimal ultrasound application conditions for the nucleation inducement of radish cylinder samples. The results of the current study implied that ultrasound offered promising application to control the crystallization process in freezing of solid foods.

Effect of ultrasound irradiation on some freezing parameters of ultrasound-assisted immersion freezing of strawberries

In this study, the effect of ultrasound irradiation temperature and ultrasound intensity on the freezing and nucleation in strawberry samples was studied. The application of ultrasound irradiation at different temperatures was able to induce nucleation at lower degree of supercooling compared to the control samples. The achieved degree of supercooling in the ultrasound irradiated strawberries was linearly correlated to the ultrasound irradiation temperature. At the ultrasound irradiation temperature of −1.6 °C, the characteristic freezing time (CFT) was significantly shorter than that in the control sample (p < 0.05). The application of ultrasound at higher intensities was found to effectively shorten the CFT. The degree of supercooling in ultrasound irradiated samples was not linearly correlated to ultrasound intensity. In conclusion, the combination of ultrasound irradiation temperature and intensity can be effective in controlling nucleation and freezing processes of perishable fruits such as strawberry.

Influence of Ultrasound on Freezing Rate of Immersion-frozen Apples

The use of power ultrasound within the food industry is an innovative subject. Application of sound to monitor a process or product is common, e.g. in quality assurance. However, the use of ultrasound to directly improve processes and products is less popular in food manufacturing. In the present work, ultrasound-assisted immersion freezing was investigated on apple samples. Because the apple parenchyma is mechanically anisotropic, the influence of applying ultrasound on radial or tangential orientated samples was also examined. Apple cylinders were immersed in an ultrasonic bath system, which operates at 40 kHz frequency. Experiments were carried out at a power level of 131.3 W (0.23 W/cm2), and ultrasound was applied intermittently for different times from temperatures below and close to the initial freezing point. Results showed that ultrasound application at 0°C or −1°C for 120 s in total, with 30 s intervals, significantly improved the freezing rate represented by the characteristic freezing time up to 8% (P < 0.05), compared to immersion freezing without ultrasound. Results of the effect of ultrasound waves applied on radial or tangential cut samples sonicated for 120 s from −1°C and/or 0°C indicated that at the power level considered there were no significant differences among the ultrasonic radial or tangential irradiated samples of these treatments, though the freezing rates were enhanced and different (P < 0.05) from the control treatment. Some evidence of the influence of ultrasound to induce primary nucleation was also observed.

Microstructural change of potato tissues frozen by ultrasound-assisted immersion freezing

Power ultrasound has proved to be very useful in controlling crystallisation processes since sonication can enhance both the nucleation rate and crystal growth rate by producing fresh and/or more nucleation sites. Therefore, in this study, power ultrasound was applied to assist the freezing process. The results showed that the freezing rate of potato sample was improved with the application of ultrasound, compared to that without ultrasound. Higher output power and longer exposure time to ultrasound would help to enhance the freezing rate. However, the heat produced when ultrasound passes through the medium limited the power applied and the exposure time. In this work, the freezing rate was fastest with an output power of 15.85 W and a treatment time of 2 min. The analyses conducted on the microstructure of potato tissue using cryo-scanning electron microscopy technique showed that the plant tissue exhibited a better cellular structure under ultrasonic power of 15.85 W. Much less intercellular void and cell disruption was observed. This was attributed to high freezing rate obtained under high ultrasonic level and thus the domination of intracellular small ice crystals.