Relaxations, glass transition and engineering properties of food solids Yrjö H. Roosa a
School of Food and Nutritional Sciences, University College Cork, Cork, Ireland (
[email protected])
ABSTRACT The glass transition of food solids has received considerable attention and its relationships with food solids behaviour in various processes and food storage have been well established. The glass transition properties for food components have been obtained primarily from calorimetric measurements and their limit has been in identifying a transition temperature range with no particular information on the kinetics of changes associated with the transition. On the other hand, theories on fragility of glass forming materials have advanced with some reference to food and pharmaceutical applications. Information on enthalpy relaxations and their use to derive fragility of glass formers in food is also available. Understanding glass transitionrelated relaxations and their coupling with engineering properties of food materials is a challenging and developing area of food materials science. The glass formation of complex food solids systems and their stability is of utmost importance in the development of advanced nutrient delivery systems. Our studies have shown that knowledge of the macroscopic glass transition behaviour of food systems may often be misleading in the prediction of characteristics of food components and their storage stability. For example, the glass transition and relaxation times determined for mixtures of carbohydrates and proteins vary and need to be interpreted carefully when coupled with measurements of the flow properties of powders or reaction kinetics. We have found that the contact time of particles for liquid bridging in stickiness measurements may be governed by the mobility of selected molecular species forming food solids. This showed varying relaxation times of reactive components which may affect physicochemical properties and kinetics in food processing and storage. The new information can advance innovations in food formulation by mapping engineering properties of food components and their mixes and the engineering of novel nutrient delivery systems. Keywords: alpha-relaxation; glass transition; food processing; kinetics; water activity.
INTRODUCTION Engineering properties of food solids are highly dependent on their physical state, i.e., amorphous, crystalline, or liquid. Variations of food material properties and state may occur as a result of changes in external thermodynamic conditions, such as pressure and temperature, and internally in materials because of changes in plasticiser or solvent contents. The Food Polymer Science approach introduced by Levine and Slade [1] has been successful in understanding time-dependent characteristics of amorphous food components and cryostabilization in the manufacturing of frozen foods. Unfortunately, real food systems are composed of numerous miscible, partially miscible, immissible as well as partially crystalline, and partially amorphous components. This makes the understanding of the properties of individual food systems in various processing and storage conditions greatly challenging and different from the behaviour of synthetic polymers. Glass transition data have been published for numerous food components, primarily carbohydrates [2] and proteins [3], as well as food solids, such as milk [4] and apples [5], which typically include carbohydrates, proteins, lipids and minor components in complex and often cellular, localised structures. Mechanical properties of food systems have been measured by dynamic mechanical analysis and also related to dielectric properties [6]. It has also been recognised that the flow properties and stickiness of powders are affected by the glass transition [7]. The glass transition measurement of a material gives information on the temperature range over which dramatic changes in material properties may occur, but no information of the extent of changes of material characteristics at specific conditions, e,g., during food processing and storage. Furthermore, it is well-known that the glass transition and material properties related to the glass transition are time-dependent [8]. The changes in relaxation times above the glass transition according to the WilliamsLandel-Ferry relationship was emphasised by Levine and Slade [1] and the ‘fragility’ concept developed by
Angell [9] has aimed at showing relaxation properties of ‘strong’ and ‘fragile’ glass formers. Our approach has been to develop knowledge on the dielectric and mechanical properties of food systems at temperatures around and above the calorimetric glass transition. The present review will address the glass transition, relaxations and engineering properties of food components and real food systems. It will also discuss the complexity of food systems and the effects of various components and their miscibility to food processing and storage stability. GLASS TRANSITION AND RELAXATION TIMES The glass transition is an important property of food solids in such processes as dehydration, extrusion and freezing as well as to understanding of properties of confectionary, edible films and frozen foods as a few examples. The glass transition is a universal concept similarly to other changes in states of materials. However, the glass transition is a property of materials existing in a supercooled, nonequilibrium state below their respective equilibrium melting temperature with no well-defined structure. Therefore, glassy materials may also show an indefinite number of glass structures with varying levels of molecular packing and order. These nonequilibrium properties are underpinning, for example, enthalpy and volume relaxations occurring around the glass transition. The glassy state and properties of supercooled liquids above the glass transition have been of great interest in the polymer science. The Williams-Landel-Ferry, WLF -relationship (1) of Williams et al. [10] was developed on the basis that most inorganic and organic glass formers showed similar decreases in relaxation times and viscosity over the temperature range of Tg to Tg+100K (Figure 1). The Vogel-Tammann-Fulcher, VTF -relationship (2) may also fit to relaxation times and viscosity data above the Tg. The use of these relationships often assumes that the viscosity of the supercooled liquid approaches 1012 Pa s at the glass transition and the dielectric relaxation time becomes approximately 100 s at the onset of the calorimetric glass transition [9]. The WLF constants, C1 and C2, have often been assigned ‘universal’ values of -17.44 and 51.6, respectively, when Ts = Tg has been taken as the reference temperature. Although the WLF – relationship may fit to viscosity and relaxation times data above the glass transition, it is obvious that changes around the glass transition occur gradually showing an upward concavity when plotted against T-Tg as shown by Peleg [11].
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⎛ τ ⎞ ⎛ η ⎞ −C1 (T − Ts ) log aT = log⎜ ⎟ = log⎜ ⎟ = ⎝ τ s ⎠ ⎝ ηs ⎠ C2 + (T − Ts )
(1)
η = ηse B /(T −Ts )
(2)
where aT is the ratio of the relaxation times τ and τs or viscosities η and ηs at temperature T and a reference temperature Ts, respectively, and C1, C2 and B are constants.
Figure 1. Viscosity (A) and relaxation time, τ, (e.g., dielectric relaxation) (B) above the glass transition temperature, Tg, predicted by the Williams-Landel-Ferry (WLF) –relationship with the universial constants, C1 = -17.44 and C2 = 51.6 K.
Figure 2. Viscosity, η, (A) and relaxation time, τ, as predicted by the Williams-Landel-Ferry (WLF) –relationship (1) (B) above the glass transition temperature, Tg, shown in ‘Tg –scaled’ Arrhenius plots according to Angell [9] with experimental viscosity data for glucose [12], glycerol [13] and water [14]. According to Angell [9], fragility is shown as the F½=(1-Tg/T) fragility or ‘steepness index’, m=[mmin(1+F½)/(1-F½)], where Fmin is 16 for τmin and 17 for ηmin. FWLF refers to the WLF predicted τ at T-Tg = 100K.
FRAGILITY The ‘Fragility’ concept developed by Angell [9] aims at using a fragility parameter, m, to classify glassforming liquids according to their deviation from the Arrhenius temperature-dependence above their respective glass transitions to ‘Strong’ and ‘Fragile’ materials. Strong liquids are those following the Arrhenius relationship while fragility increases with increasing deviation from linearity of relaxation times against reciprocal temperature (Figure 2). Fragility may also be derived from the parameter D of the modified VTF relationship (3) as FVTF=1/D which varies from 0 to 1. Fragility has also been defined as F½ fragility [15] as shown in Figure 2 for viscosity.
τ = τ 0e DT0 (T −T0 )
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(3)
The fragility approach with experimental data shows that SiO2 is a strong glass former with Arrhenius behaviour above the Tg. Organic glass formers and water appear highly fragile glass formers. Although the fragility indexes in Figure 2 for glycerol, glucose and trehalose are different, they show similar changes in viscosity and relaxation times above the Tg and should be noted as equally fragile. The difference in the apparent fragility of these materials, as an example, is a serious problem and limitation of the fragility approach which results from the differences in the individual Tg values and the Tg/T -scaling. Water shows the highest fragility and water-plasticised food components may be assumed to increase in fragility with increasing water plasticisation. Despite these limitations of the fragility concept, it appears quite obvious that the ‘fragility’ of water plasticised food materials increases with increasing water content. Dielectric (DEA) and dynamic mechanical (DMA) studies of food materials have shown a significant decrease in relaxation times around the glass transition when derived from the respective frequencydependent dielectric loss or loss modulus α-relaxation temperatures. Knowledge of the relaxation times and effects of food composition on the relaxation properties of amorphous components in complex foods is fundamental for understanding food properties in processes and storage at high solids contents or low temperatures. Our studies have shown that the Tg value of a glass former and the presence of other components and water are more significant factors affecting food characteristics than fragility as defined by Angell [9,15].
RELAXATION TIMES IN FOOD SYSTEMS According to Angell [9,15] relaxation times, such as dielectric relaxation times and calorimetric relaxations at the onset Tg, are typically 100 s and assumed to approach 10-14 s at high temperatures (Figure 2). Relaxation times of food systems decrease to 10-3 s at 20-30°C above the Tg. This corresponds to a decrease in viscosity from 1012 Pa s to 105 Pa s, which agrees with critical viscosities for collapse in freeze-drying [16] and stickiness in spray drying [17], as well-known examples. The WLF –relationship has been useful in relating viscosities of supercooled liquids to their stickiness [17] and times to crystallisation of amorphous sugars [18]. The assumption made is that the universal WLF constants apply and that the viscosity of the glass forming liquids above the Tg follows the WLF – relationship, as shown in Figure 1. The data in Figure 1 show stickiness at a surface viscosity of 106 to 108 Pa s corresponding to a dielectric relaxation time of 10-4 to 10-2 s and a surface contact time of 1 to 10 s at the ‘sticky point’ [17]. These values were shown to apply to a noncrystalline 7:1 mixture of sucrose and fructose [17]. The sticky points were found approximately at 20°C above the onset temperature of the glass transition, Tg, measured by differential scanning calorimetry (DSC) [18]. Several other studies have confirmed the relationships of glass transition and stickiness of amorphous solids [19]. The dielectric and mechanical analyses of food systems allow determination of relaxation times at the αrelaxations which can be related to stickiness characteristics of food powders as well as other mechanical changes of food solids, such as collapse of structure or viscosity and diffusion. Dielectric analysis of fructose and glucose show that the α-relaxation time decreases from 102 s at the onset of the calorimetric Tg to 10-2 to 10-3 s at the sticky point at T-Tg of 20°C. Crystallization of amorphous sugars occurs time-dependently above the glass transition. The dielectric αrelaxations suggest that an α-relaxation time of 1-2 s at 10°C above the onset Tg corresponds to time to crystallization of amorphous lactose [20] in 10 days and α-relaxation times of approximately 10-2 s correspond to crystallization within 30 hours. The relaxation times data show that there is a significant increase in mobility of carbohydrate components in food materials within the glass transition temperature range measured by DSC. The most important range to be considered in using the Tg data of sugar-containing food materials in relating glass transition to processing characteristics and storage stability is within the T-Tg of 20°C, but this may become different for complex food matrices. DIELECTRIC AND MECHANICAL RELAXATIONS OF FOOD SOLIDS Amorphous food components have been studied for glass transitions, dielectric relaxations, mechanical relaxations, spectroscopic properties and various other characteristics showing changes in molecular mobility at and around the glass transition. There are, however, very few studies on the properties of food solids with varying carbohydrate and protein compositions (Figure 3). Our recent approach in studies of stickiness properties of food solids has been to use materials, particularly dairy-based systems, with various carbohydrate and protein compositions [21-23]. These materials have been studied for their sticky points and dielectric and mechanical relaxations at various water activities, aw. The results have revealed significant differences in the glass transition and relaxation behaviour of systems containing low molecular weight sugars with maltodextrins or proteins. We have shown that carbohydrates and proteins may form phaseseparated regions in food systems, e.g., food powders. In a carbohydrate-protein system, the carbohydrate phase typically showed an almost protein content-independent glass transition in a DSC study, and the glass transition approached that of the carbohydrate at increasing water contents [21]. The sticky point was found for skim milk-milk protein solids systems to occur at increasing temperatures above the Tg the higher was the protein content [21]. The results also showed that although the Tg was at an almost constant, aw-dependent temperature, stickiness developed at the higher temperature the higher was the protein content leading to a larger temperature difference between the sticky point and glass transition [22]. The α-relaxation time corresponding to the sticky point decreased with increasing aw, but there were increases in the temperature difference of the sticky point to Tg with increasing aw. This could be related to aw-dependent interactions of the components in water plasticized systems and availability of the carbohydrate phase for the formation of liquid bridges at particle surfaces [22].
Figure 3. Food components at high solids contents, e.g., in food powders. Miscible components, such as sugars, maltodextrins and water, form amorphous structures showing glass formation according to the component properties (A); carbohydrate and protein systems are plasticised by water, but show phase separation of protein and the glass-forming properties are primarily determined by the carbohydrate phase (B); high molecular weight components, such as starch, may show partial crystallinity and phase separation from a continuous, low molecular weight, glass-forming carbohydrate matrix (C); a lipid phase is phase separated from a continuous, glass-forming carbohydrate phase (D).
Skim milk-maltodextrin solids systems showed quite different properties from those of skim milk-protein systems [23]. Skim milk-maltodextrin solids systems formed a carbohydrate rich phase with a maltodextrin content-dependent Tg described in Figure 3 (A and B). The sticky point occurred at approximately 20°C above the maltodextrin content- and molecular size-dependent Tg. These studies have shown that stickiness in carbohydrate-protein powders is affected by the carbohydrate, and the sticky point is the higher the higher is the protein content. In skim milk-maltodextrin solids systems, a higher maltodextrin content gave a higher Tg and the sticky point was at a higher temperature but at an approximately constant T-Tg. Similar type differences in glass-forming properties and behaviour affecting characteristics of various other food systems, as shown in Figure 3, may be expected. CONCLUSION Understanding glass transition-related relaxations and their coupling with engineering properties of food materials is essential for the design of complex food and nutrient delivery systems. The macroscopic glass transition behaviour of food systems may often be misleading in the prediction of characteristics of food components and their storage stability, as relaxation times determined for mixtures of carbohydrates and proteins vary and need to be interpreted carefully. The fragility concept, because of its limitations, cannot explain glass forming-properties of food systems, but studies of relaxations around and above the glass transition give new information that can advance innovations in food formulation by mapping engineering properties of food components and their mixes and the engineering of novel nutrient delivery systems.
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