Imaging food freezing using X-ray microtomography

X‐ray micro‐computed tomography (X‐ray micro‐CT) has been applied to visualise ice crystal structures formed during freezing of a number of foods. Materials were frozen unidirectionally at ?5 °C and then freeze‐dried to remove the ice crystal structure and leave voids that can be measured by the X‐ray. The system reconstructs the 3‐D image based on a set of 2‐D images, and is capable of micrometre‐scale visualisation. This study demonstrates the capability of the technique to characterise the internal ice crystal microstructure of a range of frozen materials; meat, fish, chicken, potato, cheese and carrot. Results show the voids corresponding to the ice crystals formed within these materials at different directions to the heat flux and various axial positions. Electron microscopy of the same materials, both fresh and frozen at ?5 °C, indicates the same shape of voids seen by the tomographic technique. Ice crystal parameters such as size, area and width can be quantified by the technique. Ice crystals in carrot were larger than in the other materials, while cheese and potato had the lowest values. The ice crystal distribution of all the experimental materials varied with axial distance from cooling surface; the closer the measurement was to the cooling surface, the smaller the crystal size. The results demonstrate that X‐ray micro‐CT might be useful in the analysis of frozen foods.     

Size and location of ice crystals in pork frozen by high-pressure-assisted freezing as compared to classical methods

In high-pressure-assisted freezing, samples are cooled under pressure (200 MPa) to - 20 °C without ice formation then pressure is released (0.1 MPa) and the high super-cooling reached (approx. 20 °C), promotes uniform and rapid ice nucleation. The size and location of ice crystals in large meat pieces (Longissimus dorsi pork muscle) as a result of high-pressure-assisted freezing were compared to those obtained by air-blast and liquid N(2). Samples from the surface and centre of the frozen muscle were histologically analysed using an indirect technique (isothermal-freeze fixation). Air-blast and cryogenic fluid freezing, having thermal gradients, showed non-uniform ice crystal distributions. High-pressure-assisted frozen samples, both at the surface and at the central zones, showed similar, small-sized ice crystals. This technique is particularly useful for freezing large pieces of food when uniform ice crystal sizes are required.     

速冻食品的冰晶形态及辅助冻结方法研究进展

文章简述了食品速冻技术以及不同冻结速度下形成的冰晶形态及观测冰晶的方法,综述了压力辅助冻结、电场辅助冻结以及磁场辅助冻结等技术在近几年的研究进展。直接法能直接观察到冻结过程形成的冰晶,而间接法则是通过观察冻结后冰晶在食品内部留下的间隙来分析冰晶特征。压力辅助冻结能提高过冷度,在压力释放时水分瞬间冻结,使形成的冰晶细小且分布均匀;电场辅助冻结能降低成核温度促使形成更小尺寸的冰晶;磁场辅助冻结能增强氢键抑制冰晶的生长,3种辅助冻结方式有利于提高冷冻食品的品质,具有良好的应用前景。     

不同方式冻结后养殖大黄鱼肌肉理化特性在冻藏期间的变化

采用高压转换冷冻法（PSF法）和常规空气冷冻法（CAF法）冷冻大黄鱼,并在-20℃下储藏3个月。结果表明,CAF法生成的冰晶大而少,肌纤维严重收缩和变形,相反PSF法生成的冰晶少而多,分布均匀,对肌纤维的破坏程度小,且205MPa下冰晶更小。3个月后PSF冷冻的样品组织显微结构均没有明显改变;PSF法冷冻的样品在冻藏期间盐溶性蛋白含量均没有显著变化（p>0.05）,而CAF法冷冻的样品在冻藏期间盐溶性蛋白含量显著减少（p<0.05）;较之CAF,PSF法冷冻的样品弹性和咀嚼性均显著提高（p<0.05）。因此,PSF法在大黄鱼保鲜方面具有潜在的应用前景。      

Ice Recrystallization Inhibition in Ice Cream by Propylene Glycol Monostearate

ABSTRACT: The effectiveness of propylene glycol monostearate (PGMS) to inhibit ice recrystallization was evaluated in ice cream and frozen sucrose solutions. PGMS (0.3%) dramatically reduced ice crystal sizes in ice cream and in sucrose solutions frozen in a scraped‐surface freezer before and after heat shock, but had no effect in quiescently frozen solutions. PGMS showed limited emulsifier properties by promoting smaller fat globule size distributions and enhanced partial coalescence in the mix and ice cream, respectively, but at a much lower level compared to conventional ice cream emulsifier. Low temperature scanning electron microscopy revealed highly irregular crystal morphology in both ice cream and sucrose solutions frozen in a scraped‐surface freezer. There was strong evidence to suggest that PGMS directly interacts with ice crystals and interferes with normal surface propagation. Shear during freezing may be required for its distribution around the ice and sufficient surface coverage.     

Definition of the optimum freezing rate-1. Investigation of structure and ultrastructure of beef M. longissimus dorsi frozen at different freezing rates

The influence of freezing rate on location, shape and size of ice crystals formed during freezing of beef M. longissimus dorsi, as well as its influence on ultrastructure, were investigated. Muscle samples were frozen at different rates: 0·22 cm/h and 0·39 cm/h (cooling agent was chilled air), and 3·33 cm/h, 3.95 cm/h, 4·92 cm/h and 5·66 cm/h (cooling agent was liquid carbon dioxide which expanded in the sucking-pipe of the tunnel freezer).

It was found that by slow freezing (freezing rates 0·22 cm/h and 0·39 cm/h) 30·00 μm). An increase in the freezing rate was followed by a change in ice crystal location. In this case they had also been formed intracellularly. The number of crystals increased while their size decreased.

The most intensive fibre damage was found in samples frozen at a rate of 0·22 cm/h, and the least in samples frozen at a rate of 3·95 cm/h with a freezing temperature of −50°C.     

Microstructure of high-pressure vs. atmospheric frozen starch gels

The aim of this study was to find out whether the ice crystal size of a starch gel, a model food system, could be reduced by high-pressure freezing compared with freezing at atmospheric pressure. The size and number of pores in thawed gels was determined by light microscopy and image analysis, and was taken as an indirect measure of ice crystals formed during the different freezing processes studied.The pore size and the total area occupied by the pores were clearly reduced by high-pressure freezing at 150–240 MPa compared with freezing at atmospheric pressure at the same cooling rate. The pore size in the high-pressure (nor in the atmospheric) frozen gels did not increase during a storage time of 3 months at − 24 °C (still air) at atmospheric pressure.Industrial relevanceHigh-pressure processing at subzero temperatures is not yet industrially applied. More evidence on the benefits of high-pressure freezing or thawing on the quality of real food materials as well as development of processing equipment is needed for commercialization of the processes. This study demonstrates that the pore size of frozen and thawed starch gels can be reduced by high-pressure freezing compared with freezing at atmospheric pressure. The reduced pore size was assumed to be a result of smaller ice crystals formed in the high-pressure freezing process. Based on this study, no conclusions can be drawn on the possibility of high-pressure freezing to improve the quality of real foods of a more complex composition and structure.     

Effect of freezing temperature on the color of frozen salmon

New freezing methods developed with the purpose of improved product quality after thawing can sometimes be difficult to get accepted in the market. The reason for this is the formation of ice crystals that can give the product a temporary color loss and make it less appealing. We have here used microscopy to study ice crystal size as a function of freezing temperature by investigating the voids in the cell tissue left by the ice crystals. We have also investigated how freezing temperature affects the color and the visible absorption spectra of frozen salmon. Freezing temperatures previously determined to be the best for quality after thawing (-40 to -60 °C) were found to cause a substantial loss in perceived color intensity during frozen state. This illustrated the conflict between optimal freezing temperatures with respect to quality after thawing against visual appearance during frozen state. Low freezing temperatures gave many small ice crystals, increased light scattering and an increased absorption level for all wavelengths in the visible region. Increased astaxanthin concentration on the other hand would give higher absorption at 490 nm. The results showed a clear potential of using visible interactance spectroscopy to differentiate between poor product coloration due to lack of pigmentation and temporary color loss due to light scattering by ice crystal. This type of measurements could be a useful tool in the development of new freezing methods and to monitor ice crystal growth during frozen storage. It could also potentially be used by the industry to prove good product quality. PRACTICAL APPLICATION: In this article we have shown that freezing food products at intermediate to low temperatures (-40 to -80 °C) can result in paler color during frozen state, which could affect consumer acceptance. We have also presented a spectroscopic method that can separate between poor product color and temporary color loss due to freezing.     

速冻保鲜过程中冰晶生长形态研究

冷冻是保藏食品常用且有效的方法之一。但在传统冷冻过程中,生成的大冰晶会破坏食品组织,导致品质劣变,因此如何利用新型冷冻技术改善冷冻食品的品质成为研究的热点。磁场辅助冷冻技术是一种新型的调控冰晶成核技术。本文综述了磁场调控冰晶成核的机制,重点阐述了磁场冷冻在果蔬、畜禽肉、谷物等食品领域的应用。综述结果发现,虽然磁场冷冻技术在多种食品领域有所应用,但目前研究主要集中于磁场对冷冻食品品质及冷冻参数的影响,而磁场辅助冷冻调控冰晶成核的机制目前尚无统一结论,仍需科研者进一步系统地研究揭示磁场冷冻的作用机制,推动磁场辅助冷冻技术在食品领域的应用,促进冷冻食品品质的提升。

     

Microwave assisted freezing part 2: Impact of microwave energy and duty cycle on ice crystal size distribution

Among innovative freezing processes, electromagnetic wave assistance during freezing has received great attention in recent years. The literature provides interesting studies indicating that smaller ice crystals may be obtained during freezing with the assistance of microwaves. In the companion paper, we designed a laboratory scale prototype to perform experiments of this type in well controlled conditions at 2.45 GHz. In the current study, we performed freezing experiments using this prototype and developed a protocol to measure ice crystal sizes using X-ray micro-tomography. The methodology was first validated by measuring ice crystal distribution in unidirectional conventional freezing. Then several microwave assisted freezing modalities were studied. Experiments were conducted with pulsed and continuously applied microwaves. The impact of microwave irradiation time was studied at a constant incident microwave power, and then at constant energy supplied to the system. We observed a significant reduction of up to 25% in ice crystal size when microwave assistance was used during freezing with both pulsed and continuously applied microwaves. These observations tend to disconfirm the hypothesis in the literature according to which ice crystal size reduction is due to temperature oscillations. The only remaining hypothesis in the literature is that ice crystal size reduction may be due to the perturbation of the H-bond network in water, which could be a precursor to crystalline structure. It appeared that at constant incident microwave power, the increase in irradiation time tended to decrease ice crystal size. No significant difference in ice crystal size reduction was observed for a constant energy supplied to the system. The amount of energy supplied by microwaves seems to be a factor influencing ice crystal size.Industrial relevanceMicrowave assisted freezing is an emerging process which permits reducing ice crystal size, and thus improving frozen product quality. The measurement of ice crystals in samples frozen in controlled conditions of both freezing and microwave irradiation allows studying the main parameters that induce the reduction of their size. Better understanding of the relevant factors influencing nucleation and/or crystal growth can help to optimize the process.     

Amount and Size of Ice Crystals in Frozen Samples as Influenced by Hydrocolloids

Measurement of the heat of fusion of frozen hydrocolloid-water solutions and of ice cream mix by differential scanning calorimetry indicated that hydrocolloids, when incorporated at a concentration of 2% (wt/wt) or less, cause only a small reduction (usually less than 3%) in the amount of ice formed. Based on the microscopic appearance of the ice cream mix, with and without gelatin, it was concluded that gelatin at a concentration of .28% (wt/wt) does not have a significant effect on: 1) the amount of ice that forms in ice cream mix, 2) the size and shape of the ice crystals existing soon after freezing, or 3) the rate at which recrystallization of ice occurs during a 2-wk period at −15 ± 2°C. It would appear likely that gelatin exerts a desirable influence on the sensory texture of frozen desserts by some mechanism other than control of ice crystal size or altering the amount of ice formed.     

鱼糜冷冻稳定剂的种类及作用机理

鱼糜在冷冻贮藏过程中冰晶的形成是导致其品质降低的根本原因.鉴于现有抗冻剂的安全性和经济性,单纯通过抗冻剂来抑制冰晶形成,或许无法最大程度地提高鱼糜的冻藏品质.与抗冻剂的作用对象不同(控制冰晶形成),冷冻稳定剂可作为现有冷冻保护策略的重要补充,通过增强鱼糜组分的稳定性,以抑制其在冷冻过程中的各种生化反应,从而达到鱼糜冷冻...     

Effect of different freezing processes on the microstructure of Atlantic salmon (Salmo salar) fillets

Salmon fillets were frozen either by pressure shift freezing (PSF, 200 MPa, − 18 °C or 100 MPa, − 10 °C) or by air-blast freezing (ABF, − 30 °C, 1 m/s or 4 m/s) or direct-contact freezing, and then stored at − 20 °C for 6 months. The influence of these treatments on the microstructure of Salmon fillets was studied. The equivalent diameter of the intracellular ice crystals were 14.69 ± 4.11, 5.52 ± 2.11, and 30.65 ± 6.31 μm for the samples subjected respectively to PSF at 100, 200 MPa and ABF (− 30 °C, 4 m/s) after 2 days of storage. Smaller and more regular intracellular ice crystals were observed in fillets frozen by PSF (200 MPa) compared with PSF (100 MPa), ABF and direct-contact frozen ones. Significant differences were observed between the size of the ice crystals obtained after conventional freezing process and PSF. Large and extracellular ice crystals were observed in fillets frozen by ABF (1 m/s) and direct-contact frozen. Minimal changes in the size of ice crystals were observed during a 3 months storage.

Industrial relevance

This paper compares different freezing methods and subsequent frozen storage with respect to their effect on microstructures of salmon fillets. Pressure shift freezing at 200 MPa was superior to conventional freezing regarding small and regular ice crystal formation. Interestingly, during frozen storage for up to 3 months the high quality product obtained via pressure freezing at 200 MPa could be retained. For longer storage periods lower pressures (100 MPa) seem sufficient to achieve stable ice crystals.     

Characterization of Ice Crystals in Pork Muscle Formed by Pressure-shift Freezing as Compared with Classical Freezing Methods

ABSTRACT: Cylindrical specimens of fresh pork muscle packed in plastic bags were frozen by air blaster freezing (ABF), liquid immersion freezing (LIF), and pressure-shift freezing (PSF) (100 to 200 MPa). Sample internal temperature and phase transformations were monitored at center, midway, and surface locations. ABF and LIF resulted in large irregular ice crystals, causing serious muscle structure deformation. PSF ice crystals were generally small and regular, but differed along the radial direction. Near the surface, there were many fine and regular intracellular ice crystals with well-preserved muscle tissue. From midway to the center, ice crystals were larger in size and located extracellularly. Ice crystal formation was affected by super-cooling during/after depressurization and subsequent freezing.     

水-冰-水动态变化引起冷冻肉类食品品质变化机理及控制技术研究进展

低温冷冻是最古老、应用最广泛和最经济有效的肉类食品保鲜方法,但肉类食品经过冻结、冻藏和解冻后,其中80%的水分经历水-冰-水两次相变,冰晶的形成以及后续冰晶的融化会破坏肌细胞结构,使肌肉主要成分如蛋白质和脂肪发生氧化,导致冷冻肉类食品保水性降低、颜色变暗、组织软塌和营养价值减少或丧失。因此,本文拟从水-冰-水动态变化角度概述冰晶的形成和生长过程（冰晶的成核、生长和重结晶）;分析不同温度和过饱和度下冰晶的形态、冻融引起肌肉中发生水-冰-水动态变化的过程;重点综述水-冰-水动态变化迫使冷冻肉微观结构、保水性、氧化稳定性（脂肪氧化、蛋白质氧化和构象改变）以及感官品质（质构、色泽和风味）发生劣变的机理;简述目前一些提升冷冻肉类食品品质的新兴冷冻技术（超声波技术、高压技术、电磁场技术以及添加抗冻剂）,为提高冷冻肉类食品品质及工业化生产提供科学理论依据。     

ICE CRYSTAL FORMATION IN PRESSURE SHIFT FREEZING OF ATLANTIC SALMON (SALMO SALAR) AS COMPARED TO CLASSICAL FREEZING METHODS

Atlantic salmon were frozen either by pressure shift freezing (PSF) at 100 MPa (−8.4C), 150 MPa (−14C) and 200 MPa (−20C) or by conventional air freezing (CAF) at −30C and glycol/water bath freezing (GBF) at −20C. Temperature and phase transformations of fish were monitored during the freezing processes, and microstructures of ice crystals formed were evaluated for size, shape and location. The mean (± standard deviation) cross-section area of the ice crystals were: 11000 ± 7600, 280 ± 340, 260 ± 300, 63 ± 62, and 23 ± 22 μm² for salmon subjected to CAF, GBF and PSF at 100, 150 and 200 MPa, respectively, as compared with that of the muscle fibers (7200 ± 2500 μm²). The roundness of the fish muscle fibers was 0.67 ± 0.07, while the ice-crystal roundness were: 0.38 ± 0.14, 0.55 ± 0.21, 0.57 ± 0.18, 0.63 ± 0.14 and 0.71 ± 0.14 for the salmon, respectively. CAF created larger and irregular ice crystals, and resulted in irreversible damage to muscle tissues. Due to its higher freezing rate, GBF produced smaller ice crystals than CAF, but the cross-section area and roundness values had larger deviations. The PSF process produced large amounts of fine and regular intracellular ice crystals that were homogeneously distributed throughout the salmon. Microscopic images clearly     

冻结方式对凡纳滨对虾贮藏中组织冰晶及品质的影响

新鲜凡纳滨对虾采用液氮、平板及冰柜冻结结合-20 ℃贮藏,研究不同贮藏时间内其组织冰晶形态与品质变化,以评价出最优的冻结方式。结果显示,随着贮藏时间的延长,平板及冰箱冻结的盐溶性蛋白含量、持水性、感官评分显著降低(p<0.05);而液氮冻结形成的冰晶直径比平板速冻小1/3,比冰箱小1/2;冻结后贮藏180 d其挥发性盐基氮(TVB-N值)≤25 mg/100 g,硫代巴比妥酸值(TBA)仅为0.72 mg/100 g,有效抑制了虾肉肌原纤维蛋白变性及脂肪氧化,能较好维持肌肉组织形态和品质,从而使虾肉的货架期延长至180 d以上。     

Characterization of ice cream structure by direct optical microscopy. Influence of freezing parameters

The main objective of this study was to develop and to set up a new optical direct microscopy method, based on the reflected light flux differences, with episcopic axial lighting to characterize the different phases structure of commercial overrun ice creams. Firstly, the results obtained have been validated by two others methods, a destructive method by dispersion and observation by light microscopy and, an indirect method, by scanning electron microscopy after freeze-drying sample. It was observed that the three methods were in agreement and led to the same conclusions concerning the main freezing parameters influence.So, this technique has been principally used to investigate the effects of the freezing conditions on the ice crystal structure. One of the most important parameters is the freezing rate that governs not only the size, but also the ice crystals texture of the ice cream. The air introduction (overrun) tends to limit the ice crystal growth. Besides, by this method, it was also possible to observe and to quantify the air bubbles size distribution. This direct microscopy method could be extended to other frozen food structure analysis.     

Ice morphology in frozen beef

A histological study was made of pieces of meat which had been frozen from one side, in conditions where the direction of the heat flux was approximately unidirectional and perpendicular to the muscle fibres. The morphology of the ice crystals in tissue frozen under various conditions was studied, and the range of freezing velocities over which no intracellular ice existed, was established. The average diameter D , of ice crystals, measured in different experiments and at different levels in the same piece of meat was related to the characteristic freezing time t _c, by the equation:
D = a + b log t _c     

Ice crystal growth of living onion epidermal cells as affected by freezing rates

The entire crystallization process and the effect of freezing rates on the temperature of spontaneous freezing (TSF) of living onion epidermal cells were analysed in this study. The cells, stained with iodine at a concentration of 1%, were frozen at three different freezing rates, i.e. 2, 1.2, and 0.5°C/min, and were thawed at 5°C/min. The higher freezing rate caused a higher TSF. Moreover, this article presents a method for determining whether a cell is inactivated by a plasmolysis experiment. The results of plasmolysis showed that the formation of ice crystals did not cause fatal mechanical damages to the cells, even when the freezing rate was as slow as 0.1°C/min. Finally, this article also demonstrated the grey-scale changes that occurred in the cells, as evidenced by an image processing technique. This further highlighted the ice crystal growth process and showed the ice crystal content at different locations inside the cells.