胶原和明胶的玻璃化转变

本文综述了胶原和明胶的玻璃化转变的研究现状及进展，讨论了影响其玻璃化转变温度（Ｔｇ）的各种因素，以及Ｔｇ在皮胶原化学研究中的作用。     

Thermal Unfolding of Gelatin in Solids as Affected by the Glass Transition

ABSTRACT: The thermal unfolding or denaturation of solid-state proteins during processing can affect their functionality. This project explored the relation between the glass transition temperature (T_g) and thermal unfolding temperature (T_m) using gelatin as a model system. Freeze-dried gelatin was prepared containing various polyol types at several concentrations. Using differential scanning calorimetry, T_m andT_g were determined. Moisture and polyols (that is, glycerol, xylitol, sorbitol, sucrose, and trehalose) promoted a lowering of both T_g andT_m, the extent of which depended upon the plasticizer type and concentration. For all the data, T_m > T_g, and a plot of T_m against T_g indicated a linear relationship ( R²= 0.95). These results suggest that formulations must be in the rubbery state to have the necessary mobility for hydrogen bond disruption that leads to protein unfolding. Glass transition data should be considered when developing processing parameters for proteinaceous systems.     

大豆蛋白的玻璃化转变

在有关玻璃态和玻璃化转变理论的基础上,综述了大豆蛋白玻璃化转变的分析方法、转谷氨酰胺酶处理的大豆蛋白低水分的玻璃化转变及大豆蛋白分子结构与其玻璃化转变关系,并对今后的研究方向作了展望。     

Glass Transition of Tuna Flesh at Low Temperature and Effects of Salt and Moisture

Glass transition of fresh red meat of bigeye tuna (Thunnnas obesus) and its filtrate occurred between —71°C and —68°C, independent of cooling rate from 1 K/min up to 50 K/min. Glass transition at low temperature appeared to occur in the liquid part. Neither dilution nor concentration of the filtrate affected the glass transition temperature (Tg) but the solute concentration of the freeze concentrated phase (Cg) was affected. Addition of salt to the filtrate caused a decrease of ～20 K in Tg. Results suggested the possibility of qualitative improvement in quality of fish by lowering the storage temperature by 15 K below the present storage temperature (—55°C).     

玻璃化转变对食品稳定性的影响

本文首先简要介绍玻璃化转变对黏度、扩散和反应速率的影响,而后从物理稳定性、化学稳定性和生物学稳定性三个方面详细论述了玻璃化转变对食品稳定性的影响。在玻璃化转变温度附近温度降低使黏度增大,但小分子物质的扩散系数不再进一步减小。食品可能因发生玻璃化转变而丧失稳定性,但玻璃化转变并不能成为衡量食品稳定性的唯一标准,发生玻璃化转变也不意味着食品稳定性立即丧失。根据玻璃化转变可以较好地预测食品的物理稳定性,而难以有效预测食品的化学和生物学稳定性。     

水分含量对草莓玻璃化转变温度的影响

在冻干和低温保存中,玻璃化转变温度是一个非常重要的参数。本文用差示扫描量热仪测量得到不同湿基水分含量的草莓汁的玻璃化转变温度。实验发现水分含量较高时(>50%),草莓发生的是部分玻璃化转变,不同湿基水份含量草莓汁的Tg’基本相同,水分对其的影响较小。而水分含量较低时(<45%),草莓降温时往往出现完全玻璃化转变,不同湿基水分含量草莓的Tg有很大不同,完全玻璃化转变温度随着水分含量的减少而升高,拟合得到Tg随水分变化的公式。     

浅述用差示扫描量热法研究食品的玻璃化转变

食品材料所处的状态———玻璃态或橡胶态会对食品体系的粘度、脆度、结晶、塌陷以及冰晶的形成等物理化学及质构等性质产生重要的影响。介绍了差示扫描量热法(DSC)测定玻璃化转变温度在食品中的一些运用。     

Pulse NMR Study of Glass Transition in Maltodextrin

A pulse nuclear magnetic resonance (NMR) technique was used to study the glass transition in maltodextrins. The spin-lattice relaxation time (T₁ and spin-spin relaxation time (T₂) of different molecular weight maltodextrins at different moisture contents and temperatures were measured using the pulse NMR tech nique. T₁, and T₂ were plotted against temperatures. From the resultant curves, the state transitions were observed and the corresponding state transition temperatures were determined. The state transition temperatures were very close to the glass transition temperatures (T_gs) determined with differential scanning calorimetry, suggesting a strong relationship between glass transition and proton relaxation behavior. Results indicate pulse NMR-based instrumentation could be very effective for the study of glass transition in food polymers.     

半干型荔枝干玻璃化转变研究

目的：确定冷冻干燥荔枝干的吸湿等温线,研究半干型荔枝干的玻璃化转变温度。方法：采用差示扫描量热法(DSC)。结果：半干型荔枝干的吸湿等温线与GAB 模型拟合程度好,呈现出“J”形。半干型荔枝干的玻璃化转变温度为－ 77.35℃,经过退火处理后,玻璃化转变温度提高约10℃,基本消除反玻璃化峰。经冷冻干燥处理后的半干型荔枝干的玻璃化转变温度比半干型荔枝干产品的高约7℃。     

杨梅玻璃态保藏技术的研究

以杨梅(东魁)果实为研究对象,测定其玻璃化转变温度(Tg),并将该温度应用于实际的冻结保藏中。研究发现,杨梅部分玻璃化转变温度(Tg')的起始点为－ 44℃,中点为－ 42℃,终止点为－ 39.2℃。与－ 18℃冻藏相比较,－ 40℃冻结保藏(模拟玻璃态保藏)的果实解冻后汁液流失率低,可溶性固形物(SSC)含量高,果汁pH 值和可滴定酸(TA)含量无显著差异;仪器分析和感官评定的结果均表明,－ 40℃冻结保藏更有利于保持杨梅果实的质地品质。     

Glass Transition Temperatures Determined using a Temperature-Cycling Differential Scanning Calorimeter

Glass transition temperatures, T_g, of polystyrene, polyvinylpyrrolidone, polydextrose, gelatin, corn flakes, pasta, and aqueous glucose/glycine solutions were determined using a differential scanning calorimeter which cycled the temperature while the net temperature increased at a constant rate. Operating conditions of the modulated differential scanning calorimeter^(tm) (MDSC^(tm)) for optimizing the endothermic baseline shift associated with the glass transition were a scan rate of 5°C/min with an amplitude of ± 1°C over a modulation period of 60 or 100 sec. The MDSC successfully separated the glass transition from other irreversible thermal changes in simple food ingredients. While the MDSC did not distinctly determine T_g of complex food systems at low moisture contents, glass transitions were observed for solutions and food systems at higher moisture contents. T_g values from MDSC were reproducible and similar to those from standard DSC.     

Glass Transition Studies of Baked Tortilla Chips Using Dynamic Mechanical Thermal Analysis

The objectives were to determine the glass transition temperature (T_g) of baked tortilla chips and find a statistical model that represents the change in T_gwith respect to baking conditions. The dynamic mechanical thermal analysis (DMTA) was used to determine the glass transition temperature of samples. TheT_g of samples was in the range of 59 to 76°C with respect to baking temperature and time. The results indicated that the glass transition temperature increased with increasing baking temperature and at longer baking times. TheT_g of samples with longer baking periods increased up to 1.2 times those at smaller baking periods. Statistical analysis showed a linear relationship for T_gvs. baking time and temperature. A statistical model was obtained using a linear regression to describe the change in T_gin terms of baking temperature and time (R²=0.74).     

Glass Transition Temperature of Casein as Affected by Transglutaminase

The objective of this study was to determine glass transition temperature (T_g) of a-casein with differential scanning calorimetry (DSC) and to study the effect of microbial transglutaminase (MTG) treatment on T_g of a-casein. MTG-treated a-casein was fractionated following the MTG treatment. After adjusting water content, T measurements were made by DSC and the dependencies of T_g on the water content determined. Oscillation DSC (ODSC) was also used. The determination of T_g of MTG-treated samples by conventional DSC was unreliable. However, by comparing with ODSC data, their T could be detected. The MTG treatment elevated the T_g of a-casein, which maybe the reason for improved stability of dry daily products during storage.     

淀粉的玻璃化转变温度与含水量的关系

采用热膨胀法测定不同含水量下四种淀粉的玻璃化转变温度,得到了不同含水量下淀粉的玻璃化转变温度Tg实验值,探讨了水的增塑作用及影响机理,得到了含水量与Tg的定量表达式,为预测淀粉体系的玻璃化转变温度及对淀粉的储藏加工提供了依据。     

玻璃化转变与食品的加工、储存和品质

论述了食品的玻璃化转变与其加工、储存工艺及产品品质间的关系。重点介绍了玻璃化转变对食品的冷冻及干燥工艺的影响,淀粉玻璃化转变对食品的品质的影响,冰淇淋的冷冻保藏和玻璃化法超低温保存等。     

Thiamin Stability in Solids as Affected by the Glass Transition

The stability of thiamin was evaluated in solid polyvinylpyrrolidone model systems as a function of water activity (a_w) and the glass transition. In the glassy state, thiamin degradation rate constants correlated with the glass transition temperature (T_g), decreasing as T_g increased. Above a_w 0.4, rate constants correlated better with a_w rather than T_g. However, rate constants decreased in the rubbery state above a_w 0.4, which could be attributed to glass transition-induced structural collapse. Glass transition effects (mobility considerations, collapse) have a larger effect on thiamin stability than a_w, which should be recognized during the development of fortified food products.     

Mathematical Model for Prediction of Glass Transition Temperature of Fruit Powders

Glass transition temperature (Tg) has been identified as a critical factor to predict quality of foodstuffs during processing or storage. Differential Scanning Calorimetry (DSC) was used to determine the Tg of 4 types of berry powders as a function of water content, and equilibrium moisture content was measured over different saturated solutions uusing the gravimetric method. The Guggenheim‐Anderson‐deBoer model was used to predict water activity (a_w). Four models were tested for their ability to predict Tg as a function of the solid fraction. The combined effects of Tg and a_w were incorporated into a new mathematical expression. The expression requires the determination of Tg (dry solids) of the product and data on equilibrium moisture sorption. The mean percent error of the model predictions is less than 3.6% when compared to experimental data.     

基于玻璃化转变理论的蓝莓粉吸湿机制

以‘珠宝’和‘海岸’这两个品种蓝莓为原料制作蓝莓粉,研究吸湿前后蓝莓粉的品质差异;采用静态称量法测定两种蓝莓粉在不同温度（5、15、25、35、45℃）下的吸湿等温线,并构建状态图;探讨了水分吸附过程净等量吸附热、微分熵、熵焓互补等热力学特性。结果表明,两种蓝莓粉吸湿后品质下降,粉体颗粒黏聚,流动性降低;水分吸湿等温线均呈典型的“J”型曲线,GAB模型为描述蓝莓粉水分吸附特性的最适模型。从状态图可知,‘珠宝’和‘海岸’分别在水分含量小于等于0.104 5、0.107 7 g/g（干基）,贮藏温度小于等于-30.30、-32.66℃时,有较好的稳定性。熵焓互补理论表明,两品种蓝莓粉水分吸附过程均为熵驱动的非自发过程。研究结果可为蓝莓粉加工和贮藏条件的选择提供理论依据。     

冻干圣女果粉的水分吸附性质及玻璃化转变温度

根据吸附理论,在水分活度为0.11～0.90范围内,环境温度为25 ℃条件下,采用静态称质量法研究冻干圣女果粉的吸附和解吸等温线;通过差示扫描量热仪（differential scanning calorimeter,DSC）测量不同水分含量下的玻璃态转变温度（glass transition temperature,Tg）,并采用Gordon-Taylor方程对其进行了非线性拟合。结果表明：冻干圣女果粉在25 ℃条件下的吸附等温线和解吸等温线的类型都为J型,属于Ⅲ型等温线;在水分活度为0.23～0.76范围内存在明显的解吸-吸附滞后现象,属于H3型等温线。GAB和Peleg模型都描述冻干圣女果粉的吸附特性。随着水分含量的增加,冻干圣女果粉的Tg显著降低;Gordon-Taylor方程能够较好地拟合其玻璃化转变曲线。对比水分活度贮藏理论和玻璃化转变理论,发现二者在预测冻干圣女果粉贮藏稳定性上存在一定的差异。     

Caking and Stickiness of Dairy-Based Food Powders as Related to Glass Transition

The effects of heating rate, storage temperature and water activity on surface caking (T_sc) and advanced caking (T_ac) of several dairy-based infant formula powders were determined by the modified ampule and sealed glass test tube methods, respectively. The glass transition temperature, T_g, was determined by differential scanning calorimetry. Observed T_sc and T_ac values were higher at faster scan rates (5 and 10°C/min) compared to a slower heating rate (1°C/3 min). In addition, because of the sample size and the difference in viscous flow time constants, T_ac≥ T_sc≥ T_g. As expected, stability towards collapse and sticking decreased with Increasing amounts of low-molecular-weight carbohydrate. The predicted stable storage water activities at room temperature based on T_sc were higher than those observed during storage; however, the water activity at which T_starage= T_g gave a good prediction of the %RH when collapse begins.