Determining the critical relative humidity at which the glassy to rubbery transition occurs in polydextrose using an automatic water vapor sorption instrument

Similar to an increase in temperature at constant moisture content, water vapor sorption by an amorphous glassy material at constant temperature causes the material to transition into the rubbery state. However, comparatively little research has investigated the measurement of the critical relative humidity (RHc) at which the glass transition occurs at constant temperature. Thus, the central objective of this study was to investigate the relationship between the glass transition temperature (Tg), determined using thermal methods, and the RHc obtained using an automatic water vapor sorption instrument. Dynamic dewpoint isotherms were obtained for amorphous polydextrose from 15 to 40 °C. RHc was determined using an optimized 2nd-derivative method; however, 2 simpler RHc determination methods were also tested as a secondary objective. No statistical difference was found between the 3 RHc methods. Differential scanning calorimetry (DSC) Tg values were determined using polydextrose equilibrated from 11.3% to 57.6% RH. Both standard DSC and modulated DSC (MDSC) methods were employed, since some of the polydextrose thermograms exhibited a physical aging peak. Thus, a tertiary objective was to compare Tg values obtained using 3 different methods (DSC first scan, DSC rescan, and MDSC), to determine which method(s) yielded the most accurate Tg values. In general, onset and midpoint DSC first scan and MDSC Tg values were similar, whereas onset and midpoint DSC rescan values were different. State diagrams of RHc and experimental temperature and Tg and %RH were compared. These state diagrams, though obtained via very different methods, showed relatively good agreement, confirming our hypothesis that water vapor sorption isotherms can be used to directly detect the glassy to rubbery transition. Practical Application: The food polymer science (FPS) approach, pioneered by Slade and Levine, is being successfully applied in the food industry for understanding, improving, and developing food processes and products. However, despite its extreme usefulness, the Tg, a key element of the FPS approach, remains a challenging parameter to routinely measure in amorphous food materials, especially complex materials. This research demonstrates that RHc values, obtained at constant temperature using an automatic water vapor sorption instrument, can be used to detect the glassy to rubbery transition and are similar to the Tg values obtained at constant %RH, especially considering the very different approaches of these 2 methods--a transition from surface adsorption to bulk absorption (water vapor sorption) versus a step change in the heat capacity (DSC thermal method).     

Use of ramping and equilibrium water vapor sorption methods to determine the critical relative humidity at which the glassy to rubbery transition occurs in polydextrose

Recent research has demonstrated that the critical relative humidity (RHc) values, obtained using automatic water vapor sorption instruments, can be used to detect the glassy to rubbery transition. However, reported time dependency of these RHc values suggests that additional research be carried out using equilibrium water vapor sorption methods. Thus, the objectives of this study were to: (1) determine the RHc for amorphous polydextrose at various temperatures using both instrumental (Dynamic Vapor Sorption [DVS] ramping and equilibrium) and saturated salt slurry methods, and (2) compare the RHc values obtained via sorption methods to the glass transition temperature (Tg) values obtained via differential scanning calorimetry (DSC). When plotted as a "glass curve" on a state diagram, the RHc values (plotted as a function of temperature) were found to be similar to the Tg values (plotted as a function of relative humidity). Of the 3 sorption methods employed, at 25 °C, the saturated salt slurry exhibited the lowest RHc value (34.3%), followed by the DVS equilibrium method (41.7%), and the DVS ramping method (49.9%). The RHc DVS equilibrium method was closest to the calculated DSC Tg onset RHc value (41.6% at 25 °C). These water sorption methods show promise as practical tools for predicting the quality and stability attributes of amorphous materials by being able to routinely determine the location of the glassy to rubbery transition. Future research applying these sorption methods to more complex amorphous food systems is suggested. Practical Application: Despite its extreme usefulness, the Tg, a key element of the Food Polymer Science approach, remains a challenging parameter to routinely measure in amorphous food materials. Recent research has demonstrated that the RHc values can be used to detect the glassy to rubbery transition. However, reported time dependency of these RHc values suggests that additional research be carried out using equilibrium water vapor sorption methods. Therefore, in this research 2 instrumental (DVS ramping and equilibrium) methods and the traditional saturated salt slurry method were used to obtain RHc values, comparing them to DSC obtained Tg values. The water sorption methods show promise as practical tools for predicting the quality and stability attributes of amorphous materials by being able to routinely determine the location of the glassy to rubbery transition. Future research applying these sorption methods to more complex amorphous food systems is suggested.     

Beyond Tg: optical luminescence measurements of molecular mobility in amorphous solid foods

Current research recognizes the importance of the glassy state for stabilizing amorphous solid foods and the importance of the glass transition (Tg) as an index temperature for food stability. This reflects a realization that molecular mobility modulates both physical properties such as texture and physical and chemical processes such as reaction rate, crystallization rate, solute diffusion, and collapse; the Tg is the onset temperature for cooperative translational motion in amorphous solids. The existence of complex structural features in foods with lengths ranging from nanometers to centimeters and the recognition that complex vibrational motions are activated in the glassy state, brings about the need to go beyond macroscopic measurements of product Tgs. It is now important to determine how molecular structure and mobility as well as microscopic organization modulate the macroscopic physical properties of foods.Luminescence spectroscopy provides a powerful arsenal of biophysical tools to investigate the structure and molecular mobility of amorphous solids; measurements of emission intensity, energy (wavelength), and polarization can provide direct information about molecular structure and mobility. Luminescence measurements of the effect of hydration and temperature on amorphous solid proteins and sugars reviewed here indicate that the molecular mobility due to local vibrational and rotational motions (as opposed to global translational mobility) in glassy foods is actually quite extensive with motions occurring on time scales ranging from nanoseconds to seconds. There is thus a need to identify how the rates of specific chemical and physical processes involved in food degradation are influenced by specific modes of molecular mobility in amorphous solid foods.     

Amorphous state and delayed ice formation in sucrose solutions

Phase transitions of amorphous sucrose and sucrose solutions (20–100% sucrose) were studied using differential scanning calorimetry, and related to viscosity and delayed ice formation. Glass transition temperature (Tg) was decreased by increasing water content. Ice formation and concurrent freeze concentration of the unfrozen solution increased apparent Tg. Tg could be predicted weight fractions and Tg values of components. Williams–Landel-Ferry (WLF) relation could be used to characterize temperature dependence of viscosity above Tg. Crystallization of water above Tg was time dependent, and annealing of solutions with less than 80% sucrose at –35°C led to a maximally freeze-concentrated state with onset of glass transition at –46°C, and onset of ice melting at -34°C. The state diagram established with experimental and predicted Tg values is useful for characterization of thermal phenomena and physical state at various water contents.     

Glass Transition and Food Technology: A Critical Appraisal

ABSTRACT: Most low water content or frozen food products are partly or fully amorphous. This review will discuss the extent to which it is possible to understand and predict their behavior during processing and storage, on the basis of glass transition temperature values (Tg) and phenomena related to glass transition. Two main conclusions are provisionally proposed. Firstly, glass transition cannot be considered as an absolute threshold for molecular mobility. Transport of water and other small molecules takes place even in the glassy state at a significant rate, resulting in effective exchange of water in multi-domains foods or sensitivity to oxidation of encapsulated materials. Texture properties (crispness) also appear to be greatly affected by sub-Tg relaxations and aging below Tg. Secondly, glass transition is only one among the various factors controlling the kinetics of evolution of products during storage and processing. For processes such as collapse, caking, crystallization, and operations like drying, extrusion, flaking, Tg data and WLF kinetics have good predictive value as regards the effects of temperature and water content. On the contrary, chemical/biochemical reactions are frequently observed at temperature below Tg, albeit at a reduced rate, and WLF kinetics may be obscured by other factors.     

Water Plasticization and Crystallization of Lactose in Spray-dried Lactose/Protein Mixtures

ABSTRACT: Water plasticization led to depression of the glass transition causing significant changes in the physico-chemical and crystallization properties in storage of lactose and lactose/protein (3:1) mixtures. Glass transition (Tg) and crystallization temperatures (Tcr) were determined using differential scanning calorimetry. Whey protein isolate (WPI), albumin, and gelatin increased the Tg of dry powders; when Na-caseinate was used, a decrease was observed. In the presence of proteins and water, a decrease of Tg at aw ≤ 0.23 was observed. At aw ≤ 0.33, proteins increased the Tg In the anhydrous state, Tcr decreased in the presence of proteins possibly because of browning. WPI, Na-caseinate, albumin, and gelatin delayed lactose crystallization in humidified samples, with albumin and gelatin delaying it more than WPI at all storage humidities. Temperature difference between an observed instant crystallization and glass transition (Tcr to Tg) was larger for humidified samples containing proteins than for lactose. Various proteins and water affect crystallization behavior of amorphous lactose differently in spray-dried powders. This should be considered in evaluating sugar crystallization properties in food products including dairy powders.     

食品贮藏加工中玻璃态转变理论的初步探讨

本文从物理化学角度初步研究了玻璃态转变的基本理论。玻璃态转变温度Ｔｇ作为一个重要的食品工程物理参数，可以用来解释食品加工（如干燥、挤压、烘烤、冷冻）和贮藏中，食品发生的一系列变化。     

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。     

用热膨胀法测量非晶态粉体的玻璃化转变温度

根据热膨胀原理,建立了测量非晶态粉体的玻璃化转变温度的装置,给出了测量步骤和数据处理方法,考核了用该方法测量非晶态粉体的玻璃化转变温度的重复性和可靠性。结果表明,所建立的测量装置具有操作简单、造价低廉、测量重复性好等特点,可用于非晶态粉体的玻璃化转变温度Tg的测量。     

Developments in glass transition determination in foods using moisture sorption isotherms

The food polymer science (FPS) approach has been effectively used to investigate the physical stability of amorphous food materials. The glass transition, a key FPS parameter, has traditionally been determined using thermal techniques that scan temperature while holding the plasticizer (moisture) content constant. Moisture sorption isotherms provide information about the physical properties of food as the plasticizer level is adjusted and temperature is held constant. New automatic isotherm generators can be used to produce high resolution, dynamic isotherms much faster than traditional static methods. Dynamic isotherms for a small selection of amorphous materials have been investigated and shown to experience distinct inflection points in the water activity region where the glass transition temperature is close to the experimental temperature. Several studies on amorphous spray dried milk powder and amorphous polydextrose indicate very good agreement between glass transitions determined using thermal techniques and dynamic isotherm methods. This agreement suggests that dynamic isotherms are a viable alternative to traditional thermal methods for investigating glass transitions of amorphous foods.     

Use of DSC, DVS-DSC, and DVS-fast GC-FID to Evaluate the Physicochemical Changes that Occur in Artificial Cherry Durarome® upon Humidification

ABSTRACT: Physicochemical changes that occur in artificial cherry Durarome® (benzaldehyde entrapped in an amorphous sucrose glassy matrix) when exposed to humid environments were evaluated using differential scanning calorimetry (DSC), dynamic vapor sorption (DVS) and DSC (DVS-DSC), and DVS-fast gas chroma-tography-flame ionization detection (DVS-fast GC-FID). The average measured midpoint glass transition temperature (Tg) value for "as is" Durarome was 45.9 ± 1.86°C, indicating that Durarome is in the glassy state at the experimental temperature (T) of 25°C. Humidification of Durarome increased the matrix moisture content and subsequently depressed its Tg. An appreciable percentage of the original benzaldehyde released from the Durarome matrix, measured using the dynamic, open DVS system, began when the Tg was depressed to approximately -5°C, or T-Tg = 30°C. Volatile release initiation is influenced by the physicochemical characteristics of both the matrix material and the volatile compound, as well as the humidification system used (that is, static, closed versus dynamic, open). Thus, it is reasonable to expect that each volatile entrapping matrix system will exhibit a different T-Tg release profile.     

New methods to assess water diffusion in amorphous matrices during storage and drying

Water diffusion coefficient is an important parameter for food processing and stability, strongly influenced by the composition and the physical state of the matrix (amorphous glassy or rubbery/crystalline). In this study, two different methods to assess water diffusion in amorphous matrices were compared. First, an automatic sorption device was used to measure the water sorption kinetics of powders of maltodextrin DE21 and DE47 with defined shapes when equilibrating at different air relative humidity (0–60% at 25 °C). Calculation of water diffusion coefficients was done according to Crank’s and Weibull’s models, and it was not possible when powders undergo glass transition. In the second part of the study a thin film dryer was used to assess the drying kinetics of aqueous solutions of maltodextrin (DE 6-12-29-40). The Yamamoto’s model allowed calculating water diffusion as a function of water content and temperature, above and below glass transition temperature.     

食品添加剂对面团玻璃化转变温度的影响

实现食品的玻璃化贮藏是提高食品质量的有效手段。本研究利用差示扫描量热仪探讨了适用于面团玻璃化转变温度测量的扫描程序、研究了水分含量及若干食品添加剂对面团玻璃化转变温度的影响规律。结果表明,多次扫描法是一种可行的面团玻璃化转变温度的测量方法;水分添加量对面团的玻璃化转变温度有明显影响。在本研究的范围内,面团的玻璃化转变温度随面团水分含量的增加而明显下降。食品添加剂对面团的玻璃化转变温度有显著影响。添加抗坏血酸、海藻糖及明胶时,均可不同程度的使面团的玻璃化转变温度有所升高,这对于在现有储存条件下(-18℃)实现面制品的玻璃化保存,提高速冻面制品的稳定性,改善速冻面制品的质量有重要的应用价值。     

快速测量食品玻璃化转变温度新方法的研究

食品的玻璃化转变是影响食品品质和稳定性的关键因素。准确测量食品的玻璃化转变温度对改进食品的加工和贮藏条件非常重要。核磁共振技术(NMR／MRI)在研究食品玻璃化转变过程和测最玻璃化转变温度方面，具有一定的优势。它作为一种先进的分析测量工具，在食品科学领域具有广阔的应用前景。     

Physical Aging of Maltose Glasses

Physical aging is the alteration in the physical properties of a glass, as a function of time, at temperatures below its glass transition temperature (Tg). The physical aging of maltose glasses, prepared from maltose monohydrate sugar, was studied using differential scanning calorimetry (DSC). The maltose was aged at 30°C for times ranging from 5 min to ～ 10 days. The Tg and change in heat capacity of the unaged and aged maltose glasses were measured. The change in enthalpy relaxation with aging time was determined by integrating the difference between the aged and the unaged DSC thermograms. The enthalpic relaxation data were adequately described by the Williams-Watts equation.     

Crystallization of Amorphous Lactose

Crystallization of amorphous lactose was studied at constant water content and at constant relative humidity above glass transition temperature (Tg). The rate of crystallization was followed using differential scanning calorimetry. Crystallization at constant water content increased the amount of water in the remaining amorphous matrix, which decreased T_g and accelerated crystallization. At constant relative humidity crystallization proceeded at a rate determined by T -T_g which increased with crystallinity. Temperature dependence of time to complete crystallization was confirmed to follow Williams-Landel-Ferry (WLF) equation. The results can be used to evaluate diffusion and crystallization kinetics of amorphous carbohydrates.     

浅述玻璃化转变温度与食品成分的关系

食品体系的玻璃化转变温度会对食品的加工和贮藏过程及食品的品质产生重要影响。重视食品体系的玻璃化转变温度并分析其影响因素,可帮助人们更好的了解食品加工和贮藏特性,提高产品品质。该文就水分含量、碳水化合物、蛋白质、平均分子量及食品添加剂等因素与一定的食品体系的玻璃化转变温度的关系进行了综述。     

论玻璃化转变温度与食品成分的关系

食品体系的玻璃化转变温度,会对食品的加工和贮藏过程及食品的品质产生重要影响.重视食品体系的玻璃化转变温度并分析其影响因素,可帮助人们更好地了解食品加工和贮藏特性,提高产品品质.该文就水分含量、碳水化合物、蛋白质、平均分子量及食品添加剂等因素与一定的食品体系的玻璃化转变温度的关系进行了论述.     

Corn Syrup Solids and Their Saccharide Fractions Affect Crystallization of Amorphous Sucrose

The effects of corn syrup saccharides on the glass transition (Tg) and crystallization of freeze-dried sucrose were studied using differential scanning calorimetry. Corn syrup (CS) solids were fractionated into samples with differing ranges and types of glucose polymers. Tg of the sucrose/CS additive mixtures depended on number-average molecular weight, rather than weight-average molecular weight, of CS material. Additive levels of 10% and 20% (w/w) of CS solids and their fractions interfered with crystallization of amorphous sucrose, whereas levels 50% prevented crystallization. The mechanism by which CS saccharides interfered with crystallization of amorphous sucrose depended not only on Tg but was also influenced by specific interactions between molecules.     

Effect of relative humidity and storage time on the bioactive compounds and functional properties of grapefruit powder

The modified state diagram of freeze-dried grapefruit powder was obtained in order to determine the critical water content and critical water activity that cause the glass transition of the amorphous matrix at storage temperature. At 20 °C these values were in the ranges of 0.031–0.057 g water/g product and 0.089–0.210, respectively. Below those critical values, the glassy state of the amorphous matrix is guaranteed, thus avoiding an increase in the rate of the deteriorative reactions related to the loss of the bioactive compounds in the fruit (organic acids, vitamin C, main flavonoids, and total phenols) which contribute to the antioxidant capacity (AAO) of grapefruit. In the rubbery state, a certain time is needed for these degradative reactions to start. This time depends on the water content of the sample, the greater the water content the lower the time needed. In this study, the powder was stable for a relatively long storage time (3 months) regardless the relative humidity, due to the limited mobility of the molecular system. Between 3 and 6 months had to pass before a significant loss of bioactive compounds was observed; the higher the relative humidity, the greater the loss.