强各向异性冰晶生长对速冻食品品质的影响

为降低食品速冻保鲜工艺中冰晶形态对细胞组织结构的破坏程度,以细胞溶液为研究对象,采用六重对称强界面能各向异性相场模型,探讨了冰晶生长演化过程及过冷度对冰晶生长行为的影响。结果表明:冰晶生长模拟结果与冰晶沿着固体表面生长的实际形态一致,两者初期形貌轮廓具有6个顶点,顶点处快速生长发展成主枝,主枝根部向内凹陷形成"缩颈",主枝和侧枝尖端界面出现棱角,部分侧枝具有熔断趋势,冰晶形态经过6主枝小平面结构向六重对称多分支小平面结构转变。随着过冷度增加,固液界面前沿无量纲温度曲线斜率依次增加其温度变化加剧。在过冷度经过0.45时,冰晶水平尖端无量纲曲率半径最小,有效降低冰晶对食品细胞壁结构的破坏,冰晶生长由扩散控制转变为动力学控制。     

界面各向异性对速冻保鲜过程中冰晶生长行为的影响

在速冻保鲜过程中,界面各向异性显著影响冰晶生长行为。采用相场模型再现细胞溶液中水分凝结形成冰晶的过程,从界面能和界面动力学两个方面探讨各向异性对冰晶生长行为的影响规律。结果表明:仅有界面能各向异性作用时晶粒中心沿着[100]向外延伸形成五重对称冰晶主枝,仅有界面动力学各向异性作用时晶粒中心沿着■向外延伸形成五重对称冰晶主枝,界面动力学对冰晶形态的影响比界面能更显著。界面能与界面动力学各向异性共存时模拟结果与冰晶实际形貌一致,晶粒中心存在紧实的固相区且边缘向外延伸出冰晶主枝,主枝上出现侧向分支,侧向分支彼此竞争生长;随着冷冻时间推移,晶粒形貌由冰球体经五边形冰盘向五重对称树枝状冰晶转变。     

速冻保鲜过程中冰晶生长形态研究

为了研究速冻保鲜过程中冰晶形成的微观机理,降低冰晶形态对细胞组织结构的破坏程度,以细胞溶液为研究对象,采用五重对称弱各向异性条件相场模型,再现过冷溶体中的冰晶形态演化过程,分析了不同各向异性条件下的冰晶形态演化规律。结果表明:界面动力学各向异性或界面能各向异性单独存在时,优先生长方向[100]和[■]快速发展成冰晶主枝,主枝根部受溶质和热量富集影响形成"缩颈",冰晶整体为五角星状。在界面能各向异性ε_c为0.02时,随着界面动力学各向异性强度的增加,界面能和界面动力学各向异性交互作用,优先生长方向由[100]逐渐向[■]转变,冰晶整体为薄圆冰盘状。     

冰晶生长对冷冻水产品品质影响及新型冷冻方式研究进展

冷冻技术在水产品保鲜方面应用广泛,但水产品在冻结过程中受冰晶形成、晶体生长与冰晶重结晶的影响,易造成细胞结构的破损,导致水产品品质下降。为深入分析冰晶生长与水产品品质变化间的关系,该文总结了冰晶生长对水产品的蛋白质变性、脂肪氧化、持水力、质构及感官特性影响,综述了物理场辅助冻结及添加抗冻剂、重结晶抑制剂等减小冰晶尺寸的新方法,为控制冰晶生长及提高冷冻水产品品质提供了理论参考     

果汁冷冻浓缩过程中冰晶形成规律的研究

本研究测定了橙汁和菠萝汁的冰点曲线,并以此探讨在果汁冷冻浓缩时水分冻结率的变化规律。结果表明,果汁在开始冻结的温度附近结冰率变化最大;以后逐渐减慢。橙汁和菠萝汁的最大冰晶生成区分别是-0.8～-7.9℃和-0.7～-6.7℃。冷却介质温度越低,冰晶的生长速度越快,但最终冰晶尺寸越小。当介质温度为-10℃左右时,可获得较大的冰晶尺寸。本文也研究了搅拌作用及果汁预处理因素对冰晶生长的影响。     

冻肉有学问 解冻有讲究

正吃肉要吃新鲜的,但因为各种原因,寻常百姓家总会储存些冻肉。冻肉储存时间不宜过长,否则容易失水,导致口感很差。对于大家容易忽略的冷冻和解冻过程,其实也有讲究和窍门。速冻有优势在冷冻过程中,肉先降温到零下1℃至零下5℃,这个温度区间叫"冰晶生成带",肉中大多数水分会在这个温度结成冰,但如果长时间停留在此温度下,冰晶会不断生长。由于冰的体积比水大,     

冷冻干燥法制备纤维素基多孔材料的研究

以植物纤维为原料,研究了利用冷冻干燥法制备纤维素基多孔材料过程中纤维悬浮液浓度和冷冻温度对多孔材料微观结构和性能的影响,并探讨了冷冻过程中冰晶对纤维的作用方式和多孔材料微观结构的形成机制。结果表明,随着纤维悬浮液浓度的升高,冰晶的结构从平面状演变为层状,导致多孔材料的Z向微观形貌从各向同性转变为各向异性层状孔隙结构,有助于提高其抗欧拉失稳能力,使应力-应变曲线平压区缩短,密实化区向低应变点偏移。随着冷冻温度降低,冰晶凝固前沿处纤维受到的黏滞阻力增大,从而使其被冰晶吞没而均匀分散,材料两面差减少;另外,降低冷冻温度可降低层状冰晶的厚度,使多孔材料Z向孔隙尺寸减小,有助于提高其抵抗应力变形的能力,使应力-应变曲线中的密实化区向低应变点偏移。     

静电场对冷冻食品冰晶生长影响的相场法模拟

目的:探究外加静电场对食品冷冻过程中冰晶生长的影响规律.方法:基于相场法中经典的Kobayashi模型,将电场自由能密度引入相场模型,采用有限差分法五点差分格式离散偏微分方程,编程实现了冰晶生长的模拟及可视化.结果:模拟结果和其他模拟结果以及试验结果呈现的冰晶组织特征一致,冰晶形态具有典型的六重对称性;随着电场强度的增...     

沙棘原浆冷冻浓缩工艺的响应面优化

为了避免在热浓缩过程中沙棘原浆里V_C、含硫氨基酸等热敏性物质的损耗,缓解褐变现象。以沙棘原浆为试材,利用单因素实验研究冷冻浓缩加工过程中冷冻温度、沙棘浆浓度、冷冻时间对取冰质量、取冰纯度的影响。并采用Box-behnken响应面设计,进一步对冰晶质量和冰晶纯度进行联合优化。结果表明,经过响应面联合优化并验证后得到的最佳的工艺条件为:冷冻温度-7℃、沙棘浆浓度12.4 Brix、冷冻时间13 min,此条件下实际结晶强度为33.67 g,结晶纯度为8.1 Brix。此时能够在最经济的浓缩条件下得到最大的冰晶生成量,且冰晶纯度最高。通过对冷冻浓缩前后以及复原汁的营养成分对比得知,浓缩后沙棘汁的总酸、总可溶性固形物和总黄酮的含量显著上升,浓缩过程对各营养成分的损失均低于10%。     

单颗粒花生挤压过程中的力学-出油特性

以单颗粒半粒花生为研究对象，利用原位观测结合质构仪测试的方式对花生进行挤压实验，研究花生在不同的压距、压速、保压时间及温度条件下的结构-力学-出油特性，并且建立相关的曲线拟合模型。结果表明：压距逐渐增大，花生所受峰值力也逐渐增大，花生表面被挤压开始变形，逐渐产生裂纹，压力较大时，花生变成饼状，裂纹布满整个表面；峰值力随着温度的升高先缓慢升高后急剧降低，再趋于稳定；压速及保压时间对峰值力无明显影响。递增的压距和保压时间皆能促进花生出油，而压速增加则会抑制花生出油；花生出油量随着温度的升高先缓慢增大后急剧减小，然后趋于稳定。花生颗粒挤压出油的最优条件为压距5 mm、压速0.5 mm/s、保压时间360 s和温度75 ℃。     

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.     

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.     

Control of ice crystal nucleation and growth during the food freezing process

Freezing can maintain a low-temperature environment inside food, reducing water activity and preventing microorganism growth. However, when ice crystals are large, present in high amounts, and/or irregularly distributed, irreversible damage to food can occur. Therefore, ice growth is a vital parameter that needs to be controlled during frozen food processing and storage. In this review, ice growth theory and control are described. Macroscopic heat and mass transfer processes, the relationship between the growth of ice crystals and macroscopic heat transfer factors, and nucleation theory are reviewed based on the reported theoretical and experimental approaches. The issues addressed include how heat transfer occurs inside samples, variations in thermal properties with temperature, boundary conditions, and the functional relationship between ice crystal growth and freezing parameters. Quick freezing (e.g., cryogenic freezing) and unavoidable temperature fluctuations (e.g., multiple freeze–thaw cycles) are both taken into consideration. The approaches for controlling ice crystal growth based on the ice morphology and content are discussed. The characteristics and initial mechanisms of ice growth inhibitors (e.g., antifreeze proteins (AFPs), polysaccharides, and phenols) and ice nucleation agents (INAs) are complex, especially when considering their molecular structures, the ice-binding interface, and the dose. Although the market share for nonthermal processing technology is low, there will be more work on freezing technologies and their theoretical basis. Superchilling technology (partial freezing) is also mentioned here.     

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。     

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.     

新技术在食品冷冻过程中的应用

文中对超声冷冻技术、高压冷冻技术、冰核活性细菌和冰核活性蛋白及抗冻蛋白在食品冷冻过程中的应用作一介绍。     

A Novel Technique for Ice Crystal Visualization in Frozen Solids Using X-Ray Micro-Computed Tomography

A novel technique, using an X‐ray micro‐computed tomography system (X‐ray micro‐CT system) has been developed for visualization of the two‐ (2‐D) and three‐dimensional (3‐D) ice crystal structures formed during freezing. The system reconstructs all 3‐D images based on a set of 2‐D images obtained by multiple slicing of an X‐ray shadow image. This study demonstrates the capability of the technique to characterize the ice crystal microstructure of mycoprotein products after freezing. Results are presented for the 2‐D ice crystals formed within mycoprotein frozen at different rates. The method requires freeze‐drying of the sample to remove frozen water before scanning to indicate ice crystal and internal structure of the material at a depth of 1 cm. The dendrite spacing of ice crystals has been related to the freezing conditions of the material.     

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,从而达到鱼糜冷冻保护最大化。本文首先探讨了鱼糜在冷冻过程中的品质变化规律,重点概述冷冻稳定剂的种类(糖类、蛋白水解物类、胶体类及多酚类)及其冷冻稳定机理,展望鱼糜冷冻保护策略的发展趋势。本文旨在从增强鱼糜主要组分的冻融稳定性角度出发,可作为现有冷冻保护剂的有效补充,以最大程度地提高鱼糜制品的冷冻品质。