Bacterial Extracellular Ice Nucleator Effects on Freezing of Foods

Extracellular ice nucleators (ECINs) were incorporated into foods and subjected to subzero freezing. Time-temperature profiles, ice-formation patterns and textures were examined by thermocouple, microscopy and texture analyzer. Onset temperatures (initial freezing), enthalpies and freezing rates were measured by DSC. Addition of ECINs to liquid foods elevated ice nucleation temperatures and promoted freezing. Solid or semisolid products frozen with ECINs resulted in a fiber-like texture. These effects were more apparent at –10°C or higher. Differential scanning calorimetry revealed onset temperatures were increased 11°C by addition of ECINs, but length of time to complete the phase transition was extended at constant cooling rates. Results indicated that ECINs can be used instead of whole bacterial cells for efficient freezing and textural modification.     

不同冻结速率对鸡胸肉品质的影响

为研究冻结速率对鸡胸肉品质的影响,将24只三黄鸡屠宰并预冷8 h后取胸肉作为研究对象,样品分为3个处理组,分别置于-15、-25、-35℃下冻结,测定冻结速率、解冻汁液流失率、加压失液率、蒸煮损失率、色差、蛋白质溶解度、TBARS值、Ca2+-ATPase活力的变化。结果表明:冻结速率对鸡胸肉解冻汁液流失率、加压失液率、蒸煮损失率、色差、TBARS值、Ca2+-ATPase活力有显著影响(P<0.05),而对蛋白质溶解度无显著影响(P>0.05),且冻结速率越大,鸡胸肉品质越好。     

Freezing of strawberries by immersion in CaCl2 solutions

The present investigation studied the freezing of strawberries by immersion in CaCl₂ solutions, analysing drip loss, pectin content and the degree of esterification of the pectins, total and cell-wall bonded calcium contents, the ratio bonded calcium/total calcium, and textural parameters. In addition, the effect of immersion in pectin methylesterase (PME) solutions prior to immersion freezing (IF) was analysed. The firmness of thawed fruit decreased by approximately 74% with respect to fresh strawberries, and neither IF-CA (freezing by immersion in CaCl₂ solution) nor PME–IF-CA (immersion in PME solution + freezing by immersion in CaCl₂ solution) provided significant benefit in maintaining firmness when compared to slow freezing (SF). However, IF provided a significant benefit in reducing drip loss of thawed strawberries when compared to SF, but pre-treatment with PME did not provide any additional benefit.     

冻结速率和冻藏温度对文蛤蛋白质冷冻变性的影响

利用IMP热电偶测温系统，对文蛤分别在-18℃和-30℃冻结温度下的冻结过程进行了测量，对-18℃和-30℃下3种贮藏形态的贝肉冻结样品和冻藏样品肌原纤维蛋白的Ca~(2+)-ATPase活性、Mg~(2+)-ATPase活性、盐溶性蛋白溶解度及添加抗冻剂后防止其冷冻变性的影响进行了研究。结果发现，冻结对贝肉蛋白质变性影响不大，冻藏使蛋白质发生显著的变性，温度低的变性小；在贮藏贝肉的3种形态中，无完整细胞形态的贝糜的变性幅度最大，碎贝肉次之，贝肌肉形态的保持得较好；添加抗冻剂能有效防止贝肉蛋白质冷冻变性，尤其能使贝糜形态贝肉蛋白质的稳定性大大提高。     

复合磷酸盐对粒冻贻贝失水率影响的研究

采用4种不同浓度的复合磷酸盐处理贝肉,研究了复合磷酸盐对粒冻贻贝解冻后失水率的影响。结果表明:经复合磷酸盐处理的粒冻贻贝与对照品的失水率有极显著差异(P<0.01)。通过对贮藏时间与粒冻贻贝失水率关系的研究,可知复合磷酸盐具有明显控制粒冻贻贝的失水率,提高产品贮藏特性。复合磷酸盐的最佳处理浓度范围为,三聚磷酸钠质量分数为1.0%～2.0%,六偏磷酸钠质量分数为0.4%～0.8%。     

超声波介入鲜猪皮的脱脂工艺研究

本论文研究了超声波介入常规皂化工艺对鲜猪皮的脱脂影响,发现超声波可以显著提高猪皮的脱脂率,并确定出了最佳脱脂工艺;超声波脱脂使猪皮表皮明显脱落,有利于表皮角蛋白的去除.     

鸭梨的冻结特征及影响因素分析

运用自制热镀锡膜铜-康铜热电偶测温方法测定了鸭梨果实不同部位的冻结曲线,对鸭梨果实与汁液的冻结特点作了比较,并对冰点温度与部分品质参数的关系进行了探讨.试验结果表明,鸭梨果实的冻结曲线有3种形式,典型的形式具有过冷点、初始冰点、平衡冰点、终止冰点等关键特征点;果实不同部位的过冷点和冰点温度及其出现的时间顺序存在差异,越靠近果心,冻结速率越小,冰点温度越高;果实与果汁冻结特征的不同点在于果实有“特征小段”而果汁没有,且果汁较果实容易冻结;在一定范围内,鸭梨果实冰点温度与可溶性固形物含量呈极显著负相关,而与单果重、硬度相关性不显著.     

Dynamics of Solidification in 2% Corn Starch-Water Mixtures: Effect of Variations in Freezing Rate on Product Homogeneity

Directional solidification studies of gelatinized corn starch-water mixtures were undertaken in order to examine, in situ, the freezing behavior of this food model. The solidification rate was controlled by varying sample cell velocity as it moved from a 25°C hot stage to a -25°C cold stage. While freezing at cell velocities ≤7.5 μm/sec, starch granules were alternately pushed or entrapped by the advancing solid-liquid interface producing a segregated structure consisting of alternating high-starch and low-starch bands. At a cell velocity of 10 μm/sec, the frozen product was homogeneous. The relationship between the solid-liquid interface velocity and segregation behavior was quantified and compared to an existing model of particle-interface interactions.     

The Effect of Freezing Conditions on the Quality of Freeze-Chilled Reconstituted Mashed Potato

Freeze-chilling involves freezing and frozen storage of food products followed by thawing and chilled retail display. The objective of this trial was to examine the effect of different freezing conditions (−30 °C, −60 °C, −90 °C) on selected quality parameters of freeze-chilled reconstituted potato flakes. Lowering the freezing temperature reduced the time required for freezing, gave a softer product and led to a reduction in drip loss. Freeze-chilling resulted in a firmer product and a higher drip loss compared to chilling alone. There was no difference in vitamin C content or in taste panel acceptability between the chilled and the freeze-chilled products.     

冷冻处理对丝织物结构和性能的影响

将蚕丝织物在-28℃条件下进行冷冻处理,利用扫描电镜、红外光谱分析仪、X射线衍射仪对处理前后丝织物的形态结构进行表征,利用织物拉伸仪和织物透气性测定仪等对织物的力学性能、吸湿性能、透气性等进行测试和分析。结果表明,对丝绸织物湿冷处理后,织物的表面形态发生变化,力学性能与冷冻时间有明显的相关性,透气性能明显增加。     

Freezing Rate and Frozen Storage Effects on Color and Sensory Characteristics of Pineapple Fruit Slices

This study was carried out to evaluate the influence of freezing process (cold room –18°C and air-blast freezer –50°C) and frozen storage (–18°C for 0 to 12 mo) on color and sensory quality of pineapple fruit slices (Smooth Cayenne and Red Spanish cultivars). L, –a and +b color parameters showed differences (P ≦ 0.05) with cultivar, but not with freezing rate. No differences (P ≥ 0.05) were found in sensory analysis (color and appearance) between the cultivars, frozen at different rates, compared to fresh product, or after 1 yr frozen storage. The two cultivars are suitable for freezing.     

Ultrasound assisted nucleation of some liquid and solid model foods during freezing

Power ultrasound has been proven to be useful in promoting the nucleation of ice in water-based solutions, and different mechanisms have been proposed to describe this phenomenon. In the present work, the use of ultrasound waves to induce dynamic nucleation in deionised water, sucrose solution, and agar gel samples was studied, and the mechanism of ultrasound assisted nucleation was discussed. The samples were frozen in an ethylene glycol–water mixture (− 20 °C) in an ultrasonic bath system after putting them into tubing vials. Ultrasound irradiation (25 kHz, 0.25 W cm⁻¹) was applied continuously for 1, 3, 5, 10 or 15 s at different sample's temperatures in the range of 0 °C to − 5 °C. The nucleation temperatures of the water, sucrose solution and agar gel samples without ultrasound irradiation, occurred stochastically at − 7.4 ± 2.4 °C, −10.6 ± 1.7 °C and − 7.5 ± 0.92 °C, respectively and followed normal distributions. Unlike agar gel samples, the nucleation temperatures of water and sucrose were induced by applying ultrasound for 5 s at different temperatures after a short delay, and linear relationships between the ultrasound irradiation temperatures and the nucleation temperatures were observed. Evaluation of the effect of different durations of ultrasound application on agar gels indicated that 1 s was not long enough to induce nucleation, 3 s was optimal, 5 s and 10 s produced heat and inhibited nucleation, and 15 s did not exhibit significant differences from 3 s and 10 s. It was concluded that longer irradiation durations (especially 5 s and 10 s) caused the sample to heat up, which interrupted or delayed the nucleation. Ultrasound irradiation for 3 s induced nucleation in agar gel samples at different temperatures resulting in a linear relationship between irradiation and nucleation temperatures. The observations indicated that the Hickling's theory, according to which vigorous collapses of bubbles are the only driving mechanism of nucleation, is not adequate to describe the ultrasound assisted nucleation. The results, however, were in agreement with results of some other researchers suggesting that secondary phenomena such as flow streams are also important for the initiation of nucleation. In conclusion, the use of ultrasound as a means to control the crystallisation process offers promising application in food freezing, though further investigations are needed for understanding the mechanisms, especially in solid foods.     

硫酸钠型矿盐卤水冷冻过程中芒硝结晶动力学的研究

采用连续稳态法对高浓度氯化钠水溶液中十水硫酸钠在MSMPR结晶器中的结晶过程进行了研究,测定了相关动力学参数,成核速率方程为:B0=1.245 E-63exp((-94 396.5)/(RT))ΔC2.907M0.111 5t,生长速率方程为:G=0.206 4exp((-15 825.46)/(RT))ΔC0.549 3.通过实验验证了动力学参数的可靠性,并对冷却结晶的影响因素进行了分析讨论.     

Effect of vacuum impregnation on the phenolic content of Granny Smith and Stark Delicious frozen apple <Emphasis Type="Italic">cvv</Emphasis>

ABTS•+ test, o-diphenols (spectrophotometric) and HPLC-DAD phenolic content in vacuum impregnated apple slices from Stark Delicious and Granny Smith cvv. were studied. Vacuum impregnation (VI) was carried out in a pilot plant using an aqueous solution containing 37.9% dextrose, 15.2% sucrose, 1.0% ascorbic acid, 0.25% calcium chloride and 0.25% sodium chloride. The impregnation time was 30 min and the vacuum pressure was 100 mbar; the temperature was 25 °C during all processing steps and the solution/fruit ratio was 11:1. At the end of VI, samples were left 5 min in syrup. They were then rinsed with water, drained in a vibrating screen and quickly frozen. A significant decrease (p < 0.05) of the o-diphenol content was seen (17.84 and 12.32% of the initial content in Stark and Granny varieties, respectively). The same trend was confirmed by HPLC-DAD where reductions in total phenols were 21.57 and 26.86% in Stark and Granny, respectively. Individual phenolic compounds showed different rates of depletion, although in some cases there was no reduction. The ABTS•+ test was strongly affected by the presence of ascorbic acid (AA), which was much higher in treated samples. Sensory evaluation showed higher values of hardness, crispness, juiciness and sourness in VI Granny Smith than VI Stark delicious, with a higher retention of texture parameters in the former. Finally, sweetness was slightly higher in VI Stark. An erratum to this article can be found at     

Blanching, Freezing and Frozen Storage Influence Texture of White Asparagus

We studied effects of three methods of blanching in conjunction with freezing, on texture of white asparagus as defined by three measures: maximum shear force, cutting energy, and total fiber content. We also assessed shelf life of asparagus kept in frozen storage at -22°C. Methods of blanching were total immersion in hot water, progressive immersion in hot water and steam. An increase in total fiber content was found throughout frozen storage. This increase correlated with lignification of vascular bundles in the basal segment of spears, even during frozen storage. This was reflected in an increase in maximum shear force and cutting energy required. The shelf life of frozen asparagus was 12 mo using total fiber content as a criterion.     

Effects of sodium bicarbonate containing traces of citric acid in combination with sodium chloride on yield and some properties of white shrimp (Penaeus vannamei) frozen by shelf freezing, air-blast and cryogenic freezing

Effects of sodium bicarbonate with traces of citric acid in combination with sodium chloride on yield, freezing time, freezing rate, freezing loss and cutting force of white shrimp frozen by shelf, air-blast and cryogenic freezing with/without precooking were investigated. Shelf freezing was done at −40 °C ± 2 °C while air-blast freezing was carried out at −35 °C ± 2 °C, and cryogenic freezing was done at −35 °C, −40 °C and −60 °C. The freezing loss in the non-treated samples was 8.25, 4.6-5.84 and 1.92-3.48 g/100 g fresh shrimp for peeled samples frozen without precooking and increased to 21.85, 17.54-26.97, 17.92-20.31 g/100 g fresh shrimp in the precooked samples frozen by shelf, air-blast and cryogenic freezing, respectively. The treatment of sodium bicarbonate containing traces of citric acid at 4 g/100 ml with sodium chloride at 3 g/100 ml lead to the increase of yield thus reduced the freezing loss by about 6.83-10.28 and 6.41-12.4 g/100 g fresh shrimp for the frozen-thawed samples frozen as uncooked and cooked products, respectively. The toughening of shrimp was observed while sodium bicarbonate containing traces of citric acid treatment with sodium chloride could reduce the texture change occurred during the freezing.     

冷冻对马铃薯品质的影响

新鲜马铃薯在-20℃冷冻24h,然后在20℃解冻1h,接着对其组织状态、Vc流失、酶促变色和矿物质含量等指标进行测试分析,并与未经冷冻处理的对照组进行比较,以研究冷冻对马铃薯品质的影响。     

低温冷冻处理对桑蚕丝纤维结构的影响

研究不同冷冻温度处理后桑蚕丝纤维的表观形态及内部结构的变化情况。采用低温冷冻设备对普通脱胶桑蚕丝进行-10℃~-86℃冷冻处理,测试了不同冷冻温度下桑蚕丝纤维的外观形态、红外光谱图、X射线衍射图谱、热学性能、直径和体积质量。研究结果表明:冷冻处理后,在桑蚕丝纤维表面会出现局部孔洞、裂隙结构现象,且冷冻温度越低,这种结构变化越明显;丝蛋白中的无规线团结构及α-螺旋结构有向更稳定结构转变的趋势,且结晶结构向更规整的SilkⅡ型结构转化,蚕丝的热稳定性改善,最大吸热分解温度提高2℃以上,对应的分解焓值提高5 J/g以上,而冷冻后纤维在直径及体积质量方面的变化并不明显。认为:冷冻处理可用于桑蚕丝纤维的改性。     

Physico-chemical studies on a wide composition range of low-moisture glucose–fructose mixtures: rates of crystallisation

The crystallisation rates of low-moisture (from 2% to 10% w/w) glucose–fructose mixtures were investigated at a variety of storage temperatures (from 0 to 60 °C). It was found that d -fructose considerably retards the rate of crystallisation. High storage temperatures induced a decrease in the 'threshold' moisture content, which is necessary for the initial nucleation and further development of the glucose and/or fructose crystals. This knowledge of crystallisation rates can be exploited in terms of storage of confectionery products containing d -glucose, d -fructose or both of them, and honey.