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.     

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。     

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.     

Glass Transition Explanation for the Effect of Polyhydroxy Compounds on Protein Denaturation in Dehydrated Solids

Dehydrated proteins are frequently subjected to thermal stress in the presence of other components. The effect such substances may have upon protein structure, and therefore function, has not been fully investigated. Thus, the effect of added polyhydroxy components on the denaturation of lyophilized β-lactoglobulin, ovalbumin, and ribonuclease, as determined by differential scanning calorimetry, was evaluated. The denaturation temperature, T_m, of the globular proteins decreased in the dry state after the addition of sucrose, sorbitol, or glycerol. The thermal stability (based on T_m) of the dehydrated proteins appeared to correlate with the glass transition temperature (T_g) of the polyhydroxy component, which was assumed to be related to the T_g of the mixture. The lower the T_g of the component, the greater was the degree of protein destabilization. Thus, glass transition data may be used to predict the effect that a component would have on denaturation of dehydrated proteins at elevated temperatures.     

Glass Transition Values of Muscle Tissue

Reports of glass transition (Tg') values for frozen muscle tissue are not common and reported values are mostly much lower than would be expected. Tg’ values for muscle tissue and isolated proteins were studied using differential scanning calorimetry. Apparent Tg's of mackerel, cod and beef were similar (ca ?11 to ?13°C) and substantially higher than most published values (?15 to ?77°C for tuna and beef), but in accord with expectations for substances of high molecular weight. Dialyzed insoluble and soluble protein fractions from mackerel yielded apparent Tg’ values (ca ?7°C) that were similar, with both being higher than those for whole muscle. Apparent Tg’ values of ca ?7°C were determined for aqueous samples of gelatin and collagen, but none was detected for zein.     

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。     

Glucose Loss and Maillard Browning in Solids as Affected by Porosity and Collapse

The effects of porosity and collapse on the kinetics of glucose loss and brown pigment formation via the Maillard reaction were analyzed in solid low-moisture model food systems. Equimolar glucose and glycine were incorporated into amorphous polyvinylpyrrolidones having different porosities, ranging from high porosity to totally collapsed. Samples were equilibrated and stored in desiccators at water activity 0.33 or 0.44 and 25 °C. Glucose loss and brown pigment development were monitored for 3 months. Glucose loss rate constants were lower in collapsed systems compared to non-collapsed systems. The extent of porosity did not affect glucose loss rate constants. Collapse and porosity had minimal effects on the browning rate constants. Porosity and collapse affect chemical reactivity in low-moisture solids and should be considered during product development and shelf-life testing.     

Starch Conversion During Extrusion as Affected by Added Gelatin

Waxy maize starch was modified by twin-screw extrusion cooking in the presence and absence of gelatin. X-ray diffraction, microscopy, rheology, and differential scanning calorimetry studies showed that, at constant specific mechanical energy and extruder heater temperatures, the degree of starch conversion decreased with increasing levels of gelatin. It was hypothesized that during extrusion cooking, the gelatin acts as a lubricant, protecting the starch from being converted since more mechanical energy would be dissipated in the gelatin phase than the starch phase.     

Roles of Water and Solids Composition in the Control of Glass Transition and Stickiness of Milk Powders

Abstract: Plasticization and glass transition of amorphous components in food powders often result in stickiness and caking. The glass transition temperature (T_g) of milk powders was measured by differential scanning calorimetry (DSC) and a viscometer method was used to determine sticky-point temperatures. Water sorption isotherms were established for varying solids compositions. Lactose contents were analyzed by high-performance anion exchange chromatography with pulsed amperometric detection (HPAE-PAD) and proteins were identified using SDS-PAGE gel electrophoresis. Solids composition and water affected both the T_g and stickiness behavior. Stickiness was governed by carbohydrates and water plasticization. At low protein contents, precrystallization of lactose decreased the sticky point temperature, but increasing protein content in all milk powders decreased stickiness at all water activities. The results showed that glass transition can be used to describe time-dependent stickiness and crystallization phenomena, and it can be used as a parameter to control and reduce stickiness of dairy solids with various compositions. Practical Application: Glass transition of component sugars in milk powders with various water contents was responsible for a solid-liquid transformation which resulted in their viscous flow at particle surfaces and stickiness of the powders. Stickiness leads to wall deposition in dehydration and caking of powders in storage when the amorphous carbohydrate-rich components gain liquid characteristics. High protein contents in milk powders decreased stickiness, but precrystallization of lactose prior to spray drying increased stickiness at low protein content. Milk powders in storage gained higher water contents with increasing protein contents, but stickiness was reduced and lactose crystallization was delayed which improved storage stability.     

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).     

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。     

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。     

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.     

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.     

Quality Deterioration of Freeze-dried Foods as Explained by their Glass Transition Temperature and Internal Structure

ABSTRACT Several quality parameters, such as volumetric shrinkage, internal structure, color changes, and glass transition temperature (Tg) of dry solids, were investigated during freeze-drying of strawberry, apple, and pear, at various combinations of freezing and freeze-drying temperatures. Process conditions had different impacts on the quality parameters. Both mercury intrusion and electron microscopy methods revealed a smaller pore radius and a denser internal structure for pear as compared to the other fruits. The shrinkage phenomena observed in freeze-dried products could be interpreted from the Tg and the internal structure of the materials. Therefore, the determination of Tg of dry products could be used as an indicator for the best choice of products to be freeze-dried.     

Glycine Loss and Maillard Browning as Related to the Glass Transition in a Model Food System

The effects of water activity and the glass transition on the rate constants for glycine loss and brown pigment formation due to the Maillard reaction were evaluated in a model food system. Equimolar glucose and glycine were incorporated into amorphous polyvinylpyrrolidones of various molecular weights and moisture contents. Glycine loss and brown pigment formation were quantified during storage at 25°C. At constant water activity, rate constants were higher in systems with lower glass transition temperatures. Glycine rate constants decreased upon matrix collapse, but browning rate constants were not affected by collapse. Changes associated with the glass transition influence bimolecular reactions and should be considered during product formulation and shelf-life testing.     

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。     

Measurement of Glass-Rubber Transition Temperature of Rice by Thermal Mechanical Compression Test (TMCT)

TMCT (Thermal Mechanical Compression Test) was applied to measure the T_g-r (glass-rubber transition temperature) of rice flour and individual rice kernels at low moisture content (2.4–19.5% wet basis). As expected, T_g-r decreased with increasing moisture content. The T_g-r temperatures measured by TMCT were comparable with those measured by DSC (Differential Scanning Calorimetry), TMA (Thermo-mechanical Analysis), and DMTA (Dynamic Mechanical Thermal Analysis) as reported in the literature. These results indicated that the TMCT technique could be applied to measure the glass-rubber transition of a single grain of rice or the rice in powder form. The sensitivity of the test was observed higher for the rice kernel than rice flour. TMCT technique has the advantage of usefulness and cost-effectiveness compared with the other techniques.