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  • 2 Glass Transitions in FrozenFoods and BiomaterialsStefan KasapisNational University of Singapore, Singapore

    CONTENTS

    I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

    II. Unfreezable Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

    III. The Concept of State Diagram in Food Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

    IV. Measurement of the Glass Transition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

    A. Sample Preparation and Moisture Determination . . . . . . . . . . . . . . . . . . . . . . . 39

    B. Conventional Differential Scanning Calorimetry . . . . . . . . . . . . . . . . . . . . . . . . 39

    C. Modulated Differential Scanning Calorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . 40

    D. Rheological Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

    1. Viscosity (h) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412. Dynamic Mechanical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

    V. Glass Transitions in Frozen Fruits, Fruit Juices, and

    Model Carbohydrate Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

    VI. Glass Transitions in Ice Cream and Other Fabricated Products . . . . . . . . . . . . . . . . . 47

    VII. Tg Perspective of Collapse Phenomena, Chemical Reactions, and

    Enzymic Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

    VIII. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

    Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

    References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

    I. INTRODUCTION

    High solid systems refer mainly to mixtures of biopolymers and co-solutes and as such are increas-

    ingly popular in the industrial world [1]. The mixtures are used as a base to formulate products with

    a variety of textures and sensory stimuli but mechanistic knowledge behind these properties has

    been lacking. In recent times, the importance of the rubber to glass transition and the development

    of the glassy state became widely appreciated in the understanding and controlling the quality of

    materials [2]. The emphasis now is on mapping out the relationship between the kinetics of vitri-

    fication and the metastability of systems to produce innovative methods of processing and product

    formulations [3].

    Popular science dictionaries define glass as a liquid, which is unable to flow during the time-

    scale of practical observation. Molecules in the liquid remain in a random orientation due to the

    viscosity increase that prevents them from arranging into regular patterns. Therefore, the essential

    requirement for glass formation is a high cooling rate to inhibit preliminary nucleation and crystal

    growth. The temperature at which the sample acquires glassy consistency is known as Tg but it is

    not that well defined as, for example, the melting point (Tm), because the process of vitrification may

    take place over a wide range of temperatures. The resulting glassy system is thermodynamically

    unstable, but derives kinetic stability from its high viscosity [4].

    33

    2006 by Taylor & Francis Group, LLC

  • There has been an extensive work in the literature about the vitrification of pure compounds.

    For instance, values of Tg(anh) of some members of the series of glucose carbohydrates are:

    38.58C for glucose, 958C for maltose, 1308C for maltotriose, 1758C for maltohexose, and about1858C for starch [5]. Examples of partial and total glassy behavior include hair, dry cottonshirts, biscuits, coffee granules, pasta, spaghetti, ice cream, as well as inorganic oxide systems,

    organic and inorganic polymers, and carbohydrate or protein matrices in aqueous environment

    or in mixture with high levels of sugars. An important consideration in the discussion of the

    behavior of these foodstuffs is the concept of plasticization and its effect on the glass transition

    temperature [6]. A plasticizer is defined as a substance incorporated in a material to increase the

    materials workability, flexibility, or extensibility. For example, proteins or polysaccharides are

    plasticized by low-molecular-weight diluents. Water is the most effective diluent-plasticizer and

    increasing concentrations dramatically reduce the glass transition temperature.

    Although the glassy consistency is widely observed, a theoretical treatment is far from trivial.

    Various ideas have been put forward to rationalize the discontinuities in molecular processes

    observed in the vicinity of Tg , but a simple unified theory of the phenomenon is yet to be achieved.

    The prevailing theories focus on thermodynamic, kinetic, or free volume aspects and use a single

    property or parameter to characterize the glass [7]. These are described as:

    (1) The process is considered to be a second-order thermodynamic transition in which the

    material undergoes a change in state but not in phase. A first-order transition exhibits a discontinuity

    in the primary thermodynamical variables of volume, enthalpy, and free energy. Instead, the glass

    transition region records marked changes in the first derivative variables of the coefficient of expan-

    sion (ap), heat capacity (Cp), and so on [8]. Furthermore, the spike in ap and Cp observed at the crys-tallization temperature (first-order transition) has no counterpart during vitrification. The theory

    argues that if measurements could be made infinitely slow, the true underlying transition temperature,

    T2 , would be attained, at which the configurational entropy of the system becomes zero. Using the

    quasi-lattice model of Flory [9], the energy barrier to intramolecular rotation was identified as the

    most critical variable and the T2 was calculated to lie approximately 50 K below the experimental

    Tg. The theory was successful in predicting the effects of molecular weight, copolymerization, plas-

    ticization, and crosslinking on Tg but the validity of describing a kinetically determined transition as a

    system at equilibrium is questionable. Furthermore, T2 cannot be measured experimentally and thus

    its existence cannot be proved.

    (2) The experimental measurement of the glass transition temperature is kinetically deter-

    mined because it depends on the applied frequency of oscillation, cooling or heating rate, and

    sample history [10]. Work has been carried out calorimetrically and experiments involved anneal-

    ing the sample to a temperature above the experimental Tg until equilibrium was established and

    then cooling rapidly to the temperature of interest. The temperature jumps demonstrated consider-

    able volume relaxation and hysteresis effects in materials. A measure of the time-dependent relax-

    ation modes in the glassy state could be given by pinpointing a temperature at which the value of a

    property would approximate the equilibrium value [11]. Thus kinetic postulates do not attempt a

    molecular understanding of the glassy state, but rather model the observed rate-dependent behavior

    in terms of two or more relaxation timescales.

    (3) The approach used extensively by material scientists to develop a mechanistic understand-

    ing of the rubber to glass transition is based on the concept of macromolecular free volume. Accord-

    ing to Ferry [12], holes between the packing irregularities of long-chain segments or the space

    required for their string-like movements accounts for free volume (uf). Adding to that the space

    occupied by the van der Waals radii of polymeric contours and the thermal vibrations of individual

    residues, that is, the occupied volume (u0), we come up with the total volume per unit mass (u) of a

    macromolecule. In polymer melts, the proportion of free volume is usually 30% of the total volume

    and the theory predicts that it collapses to about 3% at the glass transition temperature [13]. At this

    point, the thermal expansion coefficient of free volume (af) undergoes a discontinuity, whichreflects a change in slope in the graph of the linear dependence of total volume with temperature.

    34 Glass Transitions in Frozen Foods and Biomaterials

    2006 by Taylor & Francis Group, LLC

    A schematic representation of the concept of free volume is given in Figure 2.1.

  • The free volume concept is popular partly due to it being intuitively appealing. Often (but not

    invariably), it is able to explain the observed trends correctly in synthetic polymers, low-molecular

    weight organic liquids, and inorganic compounds, and is easy for researchers in materials science

    coming from many different backgrounds [14,15]. This has prompted calls for the universality of

    the approach in glass-forming systems where changes in the free volume appear to be independent

    of chemical features. Nevertheless, there is a tendency to apply the approach to a number of pro-

    cesses in frozen foods without a direct mechanistic justification, which shall be critically evaluated

    in this chapter.

    II. U