Ultrasound-assisted Freezing Research Articles

Effects of tissue pre-degassing followed by ultrasound-assisted freezing on freezing efficiency and quality attributes of radishes.

The rapid freezing technique for porous foods using tissue pre-degassing followed by ultrasound-assisted freezing (UF) was developed, and its effects on quality attributes of radishes including tissue air volume, hardness, total calcium contents, bonded calcium contents, retention rates of bonded calcium and microstructures were investigated. To evaluate the freezing efficiency, parameters including total freezing time, phase transition time, and the increases of freezing rate and phase transition rate were determined. Besides, multivariate statistical analyses including principal component analysis (PCA) and hierarchical cluster analysis (HCA) were performed to visualize and further analyze the quality differences of radishes under different treatments. Results suggested that decreasing tissue air volumes can significantly shorten the phase transition time of UF. Samples treated by pre-degassing for 5min at -0.09MPa followed by UF (D-0.09MPa5min-UF) showed the freezing rate and phase transition rate increased by 28.8% and 29.8%, respectively, as compared with the same pre-degassed samples frozen by immersion freezing (D-0.09MPa5min-IF). Retention rates of bonded calcium positively correlated with the sample hardness, announcing the importance of bonded calcium maintenance during radish freezing. Both PCA and HCA indicated that D-0.09MPa5min-UF endowed radishes with quality attributes more similar to the fresh ones, which was further verified by microstructure analysis, showing remarkably alleviated plasma membrane puncture, cell separation and deformation in D-0.09MPa5min-UF samples. The current study proved that the technique of tissue pre-degassing followed by UF could effectively improve the freezing efficiency and quality attributes of frozen radishes, and thus have great potentials in rapid freezing of porous foods.

Development of a single/dual-frequency orthogonal ultrasound-assisted rapid freezing technique and its effects on quality attributes of frozen potatoes

A novel ultrasound-assisted freezing system that can generate orthogonal ultrasound was developed, and the effects of single/dual-frequency orthogonal ultrasound-assisted freezing as compared with single direction ultrasound-assisted freezing on the quality of potatoes were investigated. Inner links between ultrasonic intensity, cavitation yield, and potato quality attributes including drip loss, hardness, total calcium content, l-ascorbic acid content, total phenolic content and microstructure were revealed. Besides, principal component analysis (PCA) was used to analyze and visualize the quality variation of potatoes before and after freezing. Results showed that orthogonal ultrasound could predominately decrease drip loss and enhance hardness by increasing the freezing rate and shortening the total freezing time. Microstructure observation indicated that dual-frequency orthogonal ultrasound-assisted freezing endowed potatoes with the finest pores and the plumpest tissue, which was in agreement with the PCA result. This study announces the great potential of the dual-frequency orthogonal ultrasound-assisted freezing technique in promoting the freezing quality.

Effects of ultrasound-assisted freezing on the water migration of dough and the structural characteristics of gluten components

The effects of ultrasound-assisted freezing on the freezing time and water migration of dough, and the structural characteristics of gluten components were investigated. The effects of ultrasound-assisted freezing in the whole immersion freezing process (UWF) on the freezing time were better than those of ultrasound-assisted freezing in the maximum ice crystal generation zone. The shortest freezing time was obtained at 80 W/L, and was 577 s shorter than that with traditional immersion freezing. The UWF treatment at 80 W/L significantly (p < 0.05) affected the absorption enthalpy, freezable water content and water migration of frozen dough. In UWF compared with traditional immersion freezing, the SH content of gluten, glutenin and gliadin was significantly (p < 0.05) higher, by 12.06%, 27.55% and 21.65%, respectively. The surface hydrophobicity of gluten, glutenin and gliadin in UWF treated samples significantly (p < 0.05) decreased, by 19.67%, 13.21% and 9.17%, respectively. The secondary structure of gluten components was also significantly changed by UWF. The network of gluten, the chain structure of glutenin and the gliadin particles were all changed by UWF treatment. These findings indicated that UWF is an effective method to improve the moisture distribution in dough and reduce the damage to protein molecular structure caused by freezing.

Effects of multi-frequency ultrasound on freezing rates and quality attributes of potatoes.

The effects of multi-frequency ultrasound assisted freezing on the freezing rate, microstructure, quality properties (drip loss, firmness, total calcium content, l-ascorbic acid content and total phenol content) of potatoes were studied. The results indicated that the freezing effects of multi-frequency ultrasound was better than those of single-frequency ultrasound. Multi-frequency ultrasound could significantly increase the freezing rate and preserve the quality of frozen samples better. With increase in the number of ultrasonic frequencies, the freezing effect was more obvious. In addition, scan electron microscopy (SEM) images showed that the ice crystals formed by the multi-frequency ultrasonic treatment were fine and uniformly distributed, which caused less damage to the frozen potato samples. From the analysis of the quality attributes, the nutritional values of the samples after multi-frequency ultrasonic treatment was higher, but attention should be paid to the negative influence of the hydroxyl radical generated by the multi-frequency ultrasound.

Sono-physical and sono-chemical effects of ultrasound: Primary applications in extraction and freezing operations and influence on food components.

Ultrasound is an advanced non-thermal food-processing technology that has received increasing amounts of interest as an alternative to, or an adjuvant method for, conventional processing techniques. This review explores the sono-physical and sono-chemical effects of ultrasound on food processing as it reviews two typical food-processing applications that are predominantly driven by sono-physical effects, namely ultrasound-assisted extraction (UAE) and ultrasound-assisted freezing (UAF), and the components modifications to food matrices that can be triggered by sono-chemical effects. Efficiency enhancements and quality improvements in products (and extracts) using ultrasound are discussed in terms of mechanism and principles for a range of food-matrix categories, while efforts to improve existing ultrasound-assist patterns was also seen. Furthermore, the progress of experimental ultrasonic equipments for UAE and UAF as food-processing technologies, the core of the development in food-processing techniques is considered. Moreover, sono-chemical reactions that are usually overlooked, such as degradation, oxidation and other particular chemical modifications that occur in common food components under specific conditions, and the influence on bioactivity, which was also affected by food processing to varying degrees, are also summarised. Further trends as well as some challenges for, and limitations of, ultrasound technology for food processing, with UAE and UAF used as examples herein, are also taken into consideration and possible future recommendations were made.

Effect of ultrasound-assisted freezing on the textural characteristics of dough and the structural characterization of wheat gluten.

To better understand the effects of ultrasonic treatment in the whole freezing process (UWF) and the maximum ice crystal formation zone (UMF) on the quality of frozen dough, the textural properties of dough and the structure of gluten were investigated. The results showed that the UWF and UMF treatments improved the textural properties of frozen dough and obtain the best effect at the 60W/L power densities. Ultrasound-assisted freezing reduced the destructive effect of disulfide bonds on dough, and led to a state of dynamic equilibrium of hydrophobic groups. UWF treatment at 80W/L and UMF treatment at 40W/L had positive effects prevented the secondary structure from destruction by freezing. The network of gluten treated by ultrasound-assisted freezing was more uniform and smaller than that of traditional freezing samples, which was similar to the network structure of fresh protein. According to Pearson's correlation analysis, there was a high correlation between SH, α-helix content and springiness. There was a significant positive correlation between β-turn and G', G″, and there was a significant negative correlation between β-turn and hardness. These results suggest that ultrasound-assisted freezing improved the process quality of dough though reducing the damage to gluten structure caused by freezing.

Effects of micro-nano bubbles on the nucleation and crystal growth of sucrose and maltodextrin solutions during ultrasound-assisted freezing process

The effects of micro-nano bubbles (MNBs) on the ultrasound-assisted freezing process of sucrose and maltodextrin solutions were investigated and different freezing characteristics of both nucleation period and crystal growth period were analyzed. Results showed that the introduction of MNBs was effective for the acceleration of freezing process, including the ice nucleation and crystal growth of both sucrose and maltodextrin solutions. Under optimal combination conditions, both sucrose and maltodextrin solutions with MNBs achieved a remarkable reduction in the supercooling degree and freezing time (Tn = −1.24 ± 0.09 °C, tp = 92 ± 3.88 s and t = 230 ± 4.82 s for sucrose solution, and Tn = −1.97 ± 0.13 °C, tp = 102.9 ± 8.62 s and t = 240.2 ± 3.72 s for maltodextrin solution) as compared with conventional immersion freezing (Tn = −7.87 ± 4.65 °C, tp = 126 ± 17.86 s and t = 303 ± 28.12 s for sucrose solution without MNBs, and Tn = −8.24 ± 4.73 °C, tp = 127 ± 18.12 s, t = 303.83 ± 17.86 s for maltodextrin solution without MNBs). This study suggested that the introduction of MNBs was a feasible method for enhancing ultrasound-assisted freezing process.

Numerical simulation of heat transfer and phase change during freezing of potatoes with different shapes at the presence or absence of ultrasound irradiation

As novel processes such as ultrasound assisted heat transfer are emerged, new models and simulations are needed to describe these processes. In this paper, a numerical model was developed to study the freezing process of potatoes. Different thermal conductivity models were investigated, and the effect of sonication was evaluated on the convective heat transfer in a fluid to the particle heat transfer system. Potato spheres and sticks were the geometries researched, and the effect of different processing parameters on the results were studied. The numerical model successfully predicted the ultrasound assisted freezing of various shapes in comparison with experimental data of the process. The model was sensitive to processing parameters variation (sound intensity, duty cycle, shape, etc.) and could accurately simulate the freezing process. Among the thermal conductivity correlations studied, de Vries and Maxwell models gave closer estimations. The maximum temperature difference was obtained for the series equation that underestimated the thermal conductivity. Both numerical and experimental data confirmed that an optimum condition of intensity and duty cycle is needed for reducing the freezing time, as increasing the intensity, increased the heat transfer rate and sonically heating rate, simultaneously, that acted against each other.

Recent developments in novel freezing and thawing technologies applied to foods

ABSTRACTThis article reviews the recent developments in novel freezing and thawing technologies applied to foods. These novel technologies improve the quality of frozen and thawed foods and are energy efficient. The novel technologies applied to freezing include pulsed electric field pre-treatment, ultra-low temperature, ultra-rapid freezing, ultra-high pressure and ultrasound. The novel technologies applied to thawing include ultra-high pressure, ultrasound, high voltage electrostatic field (HVEF), and radio frequency. Ultra-low temperature and ultra-rapid freezing promote the formation and uniform distribution of small ice crystals throughout frozen foods. Ultra-high pressure and ultrasound assisted freezing are non-thermal methods and shorten the freezing time and improve product quality. Ultra-high pressure and HVEF thawing generate high heat transfer rates and accelerate the thawing process. Ultrasound and radio frequency thawing can facilitate thawing process by volumetrically generating heat within frozen foods. It is anticipated that these novel technologies will be increasingly used in food industries in the future.

Infusion of CO2 in a solid food: A novel method to enhance the low-frequency ultrasound effect on immersion freezing process

In this study, influence of infusion of CO (ICO) in a model solid food system (gelatin gel) was investigated during low-frequency ultrasound (LFU) assisted immersion freezing process. Low-frequency ultrasound will cause a rapid gas bubble formation in the CO infused food matrix enhancing the ice crystal nucleation. The freezing time, ice crystal size distribution and physico-chemical properties in the treated sample were investigated by Confocal Laser Scanning microscopy (CLSM), low-field NMR (LF-NMR), SEM, DSC, FTIR and TPA. The results indicated that infused CO can significantly (p < 0.05) decrease the freezing time and ice crystal size. The freeze-thawed samples subjected to combined ICO and LFU treatment had significantly (p < 0.05) lower water loss and higher gel strength and texture properties than the sample with LFU treatment only. LF-NMR study demonstrated that the combined treatment also influenced the water mobility and distribution of freeze-thawed samples. Compared to control sample, secondary structure of gelatin proteins of all the frozen samples changed significantly, α-helix decreased, while β-sheet and random coil increased. Industrial relevance The existence of CO in the samples promoted the crystallization of ice in solid food model during ultrasound assisted freezing process. Thus, the infusion of CO will have promising application in freezing of real foods (such as fruits and vegetables, dough, and fish).

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.

Emerging techniques for assisting and accelerating food freezing processes: A review of recent research progresses

ABSTRACTFreezing plays an important role in food preservation and the emergence of rapid freezing technologies can be highly beneficial to the food industry. This paper reviews some novel food freezing technologies, including high-pressure freezing (HPF), ultrasound-assisted freezing (UAF), electrically disturbed freezing (EF) and magnetically disturbed freezing (MF), microwave-assisted freezing (MWF), and osmo-dehydro-freezing (ODF). HPF and UAF can initiate ice nucleation rapidly, leading to uniform distribution of ice crystals and the control of their size and shape. Specifically, the former is focused on increasing the degree of supercooling, whereas the latter aims to decrease it. Direct current electric freezing (DC-EF) and alternating current electric freezing (AC-EF) exhibit different effects on ice nucleation. DC-EF can promote ice nucleation and AC-EF has the opposite effect. Furthermore, ODF has been successfully used for freezing various vegetables and fruit. MWF cannot control the nucleation temperature, but can decrease supercooling degree, thus decreasing the size of ice crystals. The heat and mass transfer processes during ODF have been investigated experimentally and modeled mathematically. More studies should be carried out to understand the effects of these technologies on food freezing process.

Effect of ultrasound-assisted freezing on the physico-chemical properties and volatile compounds of red radish

Power ultrasound, which can enhance nucleation rate and crystal growth rate, can also affect the physico-chemical properties of immersion frozen products. In this study, the influence of slow freezing (SF), immersion freezing (IF) and ultrasound-assisted freezing (UAF) on physico-chemical properties and volatile compounds of red radish was investigated. Results showed that ultrasound application significantly improved the freezing rate; the freezing time of ultrasound application at 0.26W/cm2 was shorten by 14% and 90%, compared to IF and SF, respectively. UAF products showed significant (p<0.05) reduction in drip loss and phytonutrients (anthocyanins, vitamin C and phenolics) loss. Compared to SF products, IF and UAF products showed better textural preservation and higher calcium content. The radish tissues exhibited better cellular structures under ultrasonic power intensities of 0.17 and 0.26W/cm2 with less cell separation and disruption. Volatile compound data revealed that radish aromatic profile was also affected in the freezing process.

Direct contact ultrasound assisted freezing of mushroom (Agaricus bisporus): Growth and size distribution of ice crystals

To reduce the size of ice crystals in mushroom (Agaricus bisporus) contact ultrasound (300 W, 20 kHz) was applied during freezing and frozen storage. Stereomicroscopy was used to observe the ice crystal morphology, and DSC and NMR spectroscopy were performed to evaluate the water states in the samples. Results indicated that ultrasound irradiation initiated the nucleation of ice and reduced the mean size of ice crystals during freezing and frozen storage, and therefore improved the frozen product quality compared to the control samples. Most of the ice crystals in the ultrasound assisted frozen (UAF) samples were in the size range of 0–80 microns while that for the control samples were in the size range of 50–180 microns. SEM photos also proved that due to the application of ultrasound, the sizes of the ice crystals was reduced. This micro-scale information on the documentation of ice crystals will assist in understanding the ice crystal growth phenomena in an ultrasound assisted freezing process.

Hydrocolloids and Cryoprotectant used in Frozen Dough and Effect of Freezing on Yeast Survival and Dough Structure: A Review

Advantages of freezing of dough are reduction of losses caused by aging of products. But freezing deteriorates the baking quality of frozen bread dough. These include gradual loss of the dough strength, reduced yeast activity and deterioration in the texture of the final product. The negative effects of freezing and defrosting of dough can be reduced by incorporating hydrocolloids in dough. Furthermore Biogenic ice nucleators, such as extracellular ice nucleators (ECINs) isolated from Erwinia herbicola, are a group of lipoglyco proteinsthat has been demonstrated to minimize the supercooling. Ice nucleation materials function as heterogeneous ice nucleators to minimize the supercooling of water. Mechanism of cryoprotective effects from extracellular ice nucleators was possibly that ECINs helped in preserving the viability of yeast cells during freeze/thaw cycles. At the end of this study we reviewed the ultrasound-assisted freezing; this method can improve the quality of frozen food without the need for additional additives. DOI: http://dx.doi.org/10.3126/ijls.v9i3.12439 International Journal of Life Sciences 9 (3): 2015; 1-7

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.

Effect of Ultrasonically Induced Nucleation on the Drying Kinetics and Physical Properties of Freeze-Dried Strawberry

Power ultrasound has proved to be very useful in controlling crystallization 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 control the freezing step of freeze-dried strawberry. The results showed that when power ultrasound was applied at different temperatures, it increased the nucleation temperature and shortened the characteristic freezing time. The application of power ultrasound in the freezing step increased the drying time in subsequently freeze-dried strawberry samples. This longer drying time was found to be due to increased resistance to moisture diffusion due to the formation of a network of small pores caused by sublimation of small ice crystals induced by the power ultrasound. Scanning electron microscopic images revealed that the freeze-dried sample frozen by ultrasound-assisted freezing (UAFD) had finer cellular structure compared to those frozen in other freezing conditions. UAFD samples had better textural hardness, while the rehydration capacity was lower compared to those of NRFD and NIFD samples.

Ultrasound assisted immersion freezing of broccoli (Brassica oleracea L. var. botrytis L.)

Objective: The aim of this study was to research the ultrasound-assisted freezing (UAF) of broccoli.Methods: CaCl2 solution was used as freezing medium. The comparative advantage of using UAF over normal freezing on the freezing time, cell-wall bound calcium to total calcium ratio, textural properties, color, drip loss and l-ascorbic acid contents was evaluated.Results: The application of UAF at selected acoustic intensity with a range of 0.250–0.412 W/cm2 decreased the freezing time and the loss of cell-wall bound calcium content. Compared to normal freezing, the values of textural properties, color, l-ascorbic acid content were better preserved and the drip loss was significantly minimized by the application of UAF. However, when outside that range of acoustic intensity, the quality of the ultrasound-assisted frozen broccoli was inferior compared to that of the normally frozen samples.Conclusions: Selected the appropriate acoustic intensity was very important for the application of UAF.

The effect of ultrasound-assisted immersion freezing on selected physicochemical properties of mushrooms

The application of power ultrasound during immersion freezing significantly (P < 0.05) affected freezing rate of mushrooms. Ultrasound at 0.39 W cm−2 (20 kHz) reduced nucleation time by 24%, 53% and 34% in Lentinula edodes, Agaricus bisporus and Pleurotus eryngii respectively. Drip losses during the thawing process were found to be 10% less compared to control samples. Whiteness index increased when ultrasound intensity was higher than 0.27 W cm−2 (20 kHz) in L. edodes and P. eryngii whereas, ultrasound at 0.39 W cm−2 (20 kHz) showed the highest chroma value with lower yellowness and browning indices. Ultrasound at 0.39 W cm−2 (20 kHz) resulted in the highest textural hardness values in all the three mushroom varieties. Polyphenol oxidase and peroxidase enzyme activities were also significantly (P < 0.05) reduced with the increase in ultrasound power during ultrasound-assisted freezing (UAF). These results indicated that UAF can be a suitable technology for industrial freezing of mushroom.

The effects of ultrasound-assisted freezing on the freezing time and quality of broccoli (Brassica oleracea L. var. botrytis L.) during immersion freezing

The application of ultrasound during immersion freezing of broccoli was studied and particular attention was given to the effects on freezing time, microstructure, firmness and drip loss of broccoli. Broccoli florets were immersion frozen in an ultrasound-assisted freezer at two frequencies and four different power levels. The results showed that the total freezing time and times required for pre-cooling, phase change and sub-cooling stages of broccoli were significantly reduced by the application of ultrasound-assisted freezing (UAF) at 150 (30 kHz) or 175 W (20 kHz) power level and with judicious combination of process parameters (exposure time, ultrasound irradiation temperature and pulse mode). The microstructure and the firmness of broccoli tissue were better preserved and the drip loss was significantly reduced by the application of optimized UAF compared to the normal immersion freezing. These findings indicate that there is a great potential of UAF in immersion freezing of food.