Emerging pulsed electric field (PEF) and static magnetic field (SMF) combination technology for food freezing

The potential of an innovative freezing technique by applying combined pulsed electric field (PEF) and static magnetic field (SMF) was successfully tested and validated. 0.9% sodium chloride (NaCl) solution was frozen under the PEF (1.78 V cm⁻¹, duty ratio: 0.5) at the frequencies of 0–20 kHz and different SMF conditions (attractive and repulsive). At −20 °C, an increase in the working frequencies reduced the phase transition time and the shortest phase transition time (1443 ± 2 s) was obtained at 20 kHz. The patterns of ice crystals became uniformly round (roundness: 0.88–0.90) under the PEF. The effects of attractive and repulsive SMFs showed different patterns in ice crystal formation with the shortest phase transition time under the repulsive SMF. The combined PEF (1.78 V cm⁻¹ at 20 kHz) and repulsive SMF resulted in an effective and synergistic freezing process, forming uniformly round and the smallest mean size of ice crystals in the shortest phase transition (1004 ± 3 s).     

水产品超声辅助浸渍冷冻技术研究进展

目的 为延长水产品销售周期,最大程度保持其营养品质,促进水产品冷冻保藏及加工技术的发展和产业化。方法 归纳总结超声辅助浸渍冷冻的作用机理,并探究其对冰晶形成和生长以及水产品保鲜加工的影响。结果 超声辅助浸渍冷冻能有效提高冷冻效率,减小冰晶晶核规格,缩短冷冻时间,减缓水产品蛋白质变性与脂质氧化速率,改善水产品质构、保持其营养价值,是水产品冷冻处理的有效途径。结论 超声辅助浸渍冷冻技术能在一定程度上解决水产品冷冻过程中存在的问题,为水产品冷冻技术产业化提供支持。超声辅助浸渍冷冻技术在作用机制、适用范围和特定环境下的作用衰减等方面存在着一定缺陷,还需深入阐述其作用机理,为冷冻智能装备开发提供保障。     

Ice-cream storage energy efficiency with model predictive control of a refrigeration system coupled to a PCM tank

A model predictive controller (MPC) was developed in order to optimise energy management of an ice-cream warehouse refrigeration system coupled to a phase-change material (PCM) tank. The controller's internal model was based on a steady-state refrigeration cycle model and energy balances on the PCM tank and warehouse. Considering crystal growth in ice-cream, the quality evolution was based on Ostwald ripening equation. The product temperature field was determined by a 1-D finite difference equation. An original PCM model was developed for better performance of Levenberg–Marquardt's optimisation algorithm which was modified in order to take into account control variables' bounds. Simulation of the ice-cream storage for a period of 90 days lead to good results on the optimised control sequence with efficient energy management thanks to the PCM tank. Ice crystals size remained below the defined target: 26 μm. This study showed the great potential of MPC to reduce energy consumption and guarantee food quality.     

Investigating the ice crystal morphology in frozen cooked rice based on size,fractal dimension and ANN modeling

Cooked rice was frozen in an air blast freezer at freezing temperatures of −5, −10, −15, −30 and −45 °C and stored at −5, −10, −15, −30 and −45 °C for up to 90 days. Cross-sectional images were captured using a fluorescence staining method, and then the equivalent diameters and perimeters of ice crystals were determined. The size and fractal dimensions were affected by the relationship between the freezing and storage temperatures. Artificial neural network analysis revealed that the behavior of response surface curves of both sizes and fractal dimensions were similar. The models demonstrated that the storage temperature of −25 °C for 90 days was required for maintaining the size and morphology of ice crystal immediately after freezing at −25 °C. These models would be effective tools for identifying the optimum operating conditions of frozen cooked rice based on the relationship between the ice crystal size and morphology.     

Study on the supercooling degree and nucleation behavior of water-based graphene oxide nanofluids PCM

This article aimed to study the supercooling degree and nucleation behavior of nanofluids phase change material. The nanofluids were prepared by adding small fraction of graphene oxide nanosheets in deionized water without any dispersants. The supercooling degree of nanofluids with different concentrations was tested experimentally. The results show that the supercooling degree can be reduced by 69.1%, and the nucleation was started in advance, shorting the time by 90.7%. Theoretical analysis based on the heterogeneous nucleation theory indicates that ice crystal nucleus cannot grow on the thickness surface of the graphene oxide nanosheet, while it can grow on the top or bottom surface of the nanosheet only when the supercooling degree ΔT and the nanosheets size D meet the formulaD⋅ΔT≥4.2×10⁻⁸. Our study implies that graphene oxide nanofluids have the potential to be used as PCMs in cold storage applications because of their low supercooling degree and rapid nucleation behavior.     

Three-dimensional measurement of ice crystals in frozen beef with a micro-slicer image processing system

A novel technique has been developed for measuring the three-dimensional (3-D) structure and distribution of ice crystals formed in frozen beef by using a micro-slicer image processing system (MSIPS). The system has functions to reconstruct the 3-D image based on the image data of exposed cross-sections obtained by multi-slicing of a frozen sample with the minimum thickness of 1 μm and to display the internal structure as well as an arbitrary cross-section of the sample choosing observation angles. The size and distribution of ice crystals can be determined from the 2-D quantitative information, such as the periphery and area of the crystals. The effects of freezing conditions on the morphology and distribution of the ice crystals were demonstrated quantitatively from the observations of raw beef stained by fluorescent indicator. For the samples frozen at −15 °C, the network structure of ice crystals were observed mainly at intercellular space, having approximately 100 μm in cross-sectional size, while that prepared at −120 °C showed the spherical crystals of 10–20 μm in diameter within the cells. The 3-D image of the sample demonstrated that the growth of ice columns was restricted by the intrinsic structure of muscle fibers. The proposed method provided a new tool to investigate the effects of freezing conditions on the size, morphology and distribution of ice crystals.     

Effects of vacuum cooling on the enzymatic antioxidant system of cherry and inhibition of surface-borne pathogens

The study aimed to determine the effects of vacuum cooling on the enzymatic antioxidant system of cherry and inhibition of surface-borne bacteria during storage. Cherries were vacuum-cooled and then stored at 1 °C and 95% relative humidity for 1 week. Changes in the enzymatic antioxidant system of cherry were measured throughout the storage period. The catalase activity and peroxidase activity of vacuum-cooled cherries were higher than those of the control during storage. In addition, the malondialdehyde content of cherry with vacuum cooling was lower than that of the control. Both groups showed intermediate levels of polyphenol oxidase activity. Scanning electron microscopy studies demonstrated the inhibitory effect of vacuum cooling on the morphology of E. coli on the cherry surfaces. This study offers new insights into the physiological mechanics of cherry fruit after vacuum cooling and storage and provides experimental evidence for better control of cherry quality in practice.     

The effect of supercooling on ice structure in tuna meat observed by using X-ray computed tomography

Several studies have reported that freezing a homogeneous food such as soy bean curd with deep supercooling (supercooled freezing) results in the formation of many particle ice and homogenous ice structure. However, ice crystal morphology may be affected by the cellular structure of the food. In this study, the ice crystal structure in tuna meat, a cellular food, frozen by the supercooled freezing method was investigated by X-ray computed tomography and compared with ice structures in tuna meat frozen by conventional freezing methods. The results showed that rod-like ice crystals grew parallel with the myofibers, and inhomogeneous ice structures formed in tuna meat frozen by the supercooled freezing method regardless of the degree of supercooling, in contrast to the ice structure in frozen soy bean curds. These ice crystals linked with each other to form rod-like ice structures due to mobility limitations imposed by the cellular structure.     

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.     

Phase change material for the thermal protection of ice cream during storage and transportation

Ice cream is a very temperature sensitive product and temperature variations during the storage and distribution steps may result in a reduction of quality. It is possible to improve the ice cream storage and transportation conditions by using an additional packaging with a low thermal diffusivity. This paper studies a phase change material (PCM) packaging and compares its performance to a polystyrene packaging configuration. The impact on temperature fluctuations and ice crystal size distribution was characterized experimentally during long term storage and temperature abuse. The results show that the use of an additional PCM packaging has a significant impact on the final quality of the product by keeping ice cream temperature stable and close to the phase change temperature of the PCM material. These results were compared with the insulation material results and discussed, showing that a material with a buffering heat capacity can be more efficient to reduce temperature fluctuations than a low conductivity material, and that the same results can be usually obtained with a much thinner layer of material.     

Structural analysis of thermokarst lake ice in Beiluhe Basin,Qinghai–Tibet Plateau

The high altitude and cold climate of the Qinghai–Tibet Plateau (QTP) means that rivers and lakes are covered by ice for 6–7 months of the year. The ice cover is a major influence on environmental and ecological conditions, since it alters the heat and energy exchange with local atmosphere, as well as determines the design and construction of civil buildings. However, research on these ice covers has not been conducted previously. For the first time, the present work focuses on in-situ observations of ice growth and decay, and the inner structural analysis of ice samples from a thermokarst lake, Beiluhe basin, in the central plateau. Ice was sampled on two occasions during fast growth and slow growth periods. Ice crystals, densities and gas bubbles were investigated and classified into ice types. Vertical profiles of the ice crystal type, crystal size, fabrics, gas bubble shape and size, gas content, as well as ice density were determined. The formation mechanism was discussed based on QTP meteorology and stratigraphy.     

超声辅助冷冻技术在食品浸渍式冷冻中的研究进展

目的 概述超声辅助冷冻技术在食品浸渍式冷冻中的研究进展,为今后新型技术辅助浸渍式冷冻提供研究方向.方法 研究超声辅助冷冻的成核机理和对冰晶生长的影响,以及超声对溶液性质的影响,并对超声辅助冷冻技术在食品浸渍式冷冻中的应用进行综述.结论 超声辅助浸渍式冷冻技术可以广泛应用在各类食品加工中,有很大的研究和发展空间.超声辅助冷冻技术能够进一步加快食品浸渍式冷冻的冻结速率,减小食品内部冰晶尺寸,较好地改善了冷冻食品的品质和风味,在食品加工中具有广阔的应用前景,可以高效应用于食品工业.     

Three-dimensional measurement of ice crystals in frozen dilute solution

A Micro-Slicer Image Processing System (MSIPS) has been applied to observe the ice crystal structures formed in frozen dilute solutions. Several characteristic parameters were also proposed to investigate the three-dimensional (3-D) morphology and distribution of ice crystals, based on their reconstructed images obtained by multi-slicing a frozen sample with the thickness of 5 μm. The values of characteristic parameters were determined for the sample images with the dimension of 530×700×1000 μm. The 3-D morphology of ice crystals was found to be a bundle of continuous or dendrite columns at any freezing condition. The equivalent diameter of ice crystals were in the range of 73–169 μm, and decreased exponentially with increasing freezing rate at the copper cooling plate temperature of −20 to −80 °C. At the T_cp −40 °C, the volumes of ice crystals were in the range of 4.6×10⁴ μm³ to 3.3×10⁷ μm³, and 36 ice columns were counted in the 3-D image.     

Freeze–thaw stability of emulsions with soy protein isolate through interfacial engineering

In this paper, we examined to the influence of interfacial composition on freeze–thaw stability of oil in water emulsions. An electrostatic layer-by-layer deposition method was used to create the multilayered interfacial membranes with different compositions of primary emulsion (Soy protein Isolate); secondary emulsion (Soy protein Isolate – octenyl-succinate starch); tertiary emulsion (Soy protein Isolate – octenyl-succinate starch – Chitosan). The primary, secondary and tertiary emulsions were subjected to from one to two freeze–thaw cycles (−20 °C for 24 h, +25 °C for 18 h) and then their stability was assessed by ζ-potential, particle size, microstructure and creaming stability measurements. The crystallization behaviour of emulsions was studied by differential scanning calorimetry (DSC). Primary and secondary emulsions were unstable to droplet flocculation when the water phase crystallized, whereas tertiary emulsions were stable, which was attributed to the relatively thick biopolymer layer surrounding the oil droplets. These results showed the interfacial engineering technology used in the study could therefore lead to the creation of food emulsions with improved stability to freezing and thawing.     

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.     

Measurement of solubility and density of water + lithium bromide + lithium chloride and water + lithium bromide + sodium formate systems

Solubility of aqueous solutions containing lithium bromide + lithium chloride and lithium bromide + sodium formate were measured (LiBr/NaHCO₂ = 2 and LiBr/LiCl = 2 by mass ratio) at different temperatures. Visual polythermal method was used in the temperature range of (283.15–340.15) K and mass fraction range of (0.4–0.8). Also density of mentioned systems was reported in the temperature range of (288.15–333.15) K. Each set of experimental measurements were correlated using least-square regression as a function of temperature. Our results indicate that solubility of LiBr + LiCl is higher than LiBr and its density is lower than density of aqueous solution of LiBr.     

Use of citric and lactic acids in ice to enhance quality of two fish species during on-board chilled storage

This work focused on the on-board chilled storage of European hake (Merluccius merluccius) and megrim (Lepidorhombus whiffiagonis). To enhance fish quality, an aqueous solution including citric (1.25 g l⁻¹) and lactic (0.50 g l⁻¹) acids was prepared, frozen, ground and employed as icing medium. Its effect on sensory, microbiological and chemical changes was monitored after 9, 12 and 15 days of on-board storage. Lower (p < 0.05) bacterial growth was detected according to microbiological (aerobe, anaerobe, psychrotrophe, proteolytic, and Enterobacteriaceae counts) and chemical (trimethylamine content) assessments. An inhibitory effect (p < 0.05) on autolysis development (K value assessment) in hake was also detected. Finally, an enhancement of sensory scores (eyes, external odour and gills) in both species was obtained. Results described allow to conclude that on-board employment of such acid-mixture icing system can provide a profitable strategy to obtain higher quality and safe products while unloading.     

The modelling of the freezing process in fine-grained porous media: Application to the frost heave estimation

The solution of the moving-boundary problem, related to heat- and mass-transfer processes in freezing, fine-grained, porous media under phase-transition conditions is presented. It is assumed that a freezing zone, characterized by a wide temperature range of phase transitions, is formed. Therefore a three-zone model is developed. The preservation of the term ∂L/∂t(L is the ice content) in the system of equations has made it possible to determine the ice distribution within the frozen and the freezing zones. For loamy soils the dependence of the freezing process on the characteristic parameters, the Stefan and Lewis numbers, was analyzed. It was found that increasing the enthalpy of phase transition, i.e., decreasing the Stefan number Ste, resulted in diminution of the frozen zone but, at the same time, the ice content within this zone increased. Intensification of the migration process, i.e., increasing the Lewis number Le, also led to diminution of the frozen zone, in which the ice content and, consequently, the total moisture (including ice) were increased. For large Lewis numbers the freezing zone was observed to decrease. When the water migration process is absent (Le = 0), the calculations, which were based on the described model show that in the course of freezing the redistribution takes place only between moisture and ice contents. The total moisture remains constant and equal to the initial water content. The theoretical conceptions and results derived from the analytical solution are in agreement with experimental findings. The presented model predicts the freezing process in porous media and satisfactorily reflects observed phenomena. The utilization of the considered problem solution to the prediction of the frost heave phenomenon in soils freezing processes shown that the calculated frost heave curve matches the experimental results very closely indicating that the model can well reproduce the frost heaving process associated with the freezing. Propagation of the freezing front in the test is predicted the experimental results with reasonable accuracy.     

Development of microwave assisted zeolite–water adsorption heat pump

A microwave assisted zeolite–water adsorption heat pump system was designed, manufactured and investigated experimentally. The influence of operation time of microwave oven on performance of the adsorption heat pump was studied. The performance criteria: coefficient of performance, specific cooling power and volumetric cooing power, were calculated for the designed and tested adsorption heat pump system. The regeneration of adsorbent bed was achieved very rapidly (35 min) by using microwave heating system. The poor thermal conductivity of adsorbent did not affect the periods of isosteric heating and isobaric desorption processes.     

A propane water-to-water heat pump booster for sanitary hot water production: Seasonal performance analysis of a new solution optimizing COP

Electrical heat pumps for sanitary hot water production achieve a high performance with a good matching of water and refrigerant temperature profiles during the heat rejection stage, as it happens in CO₂ systems. This work considers the thermodynamic possibility to adapt the condenser pressure of a propane heat pump to maximize the COP, while producing sanitary hot water up to 60 °C from a heat sink equal to 15 or 25 °C. The performance of the heat pump is calculated through specific models which, in combination with a TRNSYS model of the whole system, allowed to assess its seasonal performance for a hotel in Strasbourg, also varying the control logic and the size of the storage tank. Results obtained led to the conclusion that, for achieving a high seasonal performance, the control logic of the tank has the largest influence.