Phase Transitions in Foods: Basic Science for the Modern Scientist

How can I control the size, diameter, and shape of a baking cookie while reducing the processing time? How can I control the network de-swelling of co-gels, such as maltodextrins and milk proteins? How can I reduce the effect of small amounts of minor constituents (e.g., fats) in cereal starches? Can “fat bloom” in chocolate manufacturing be eliminated?

All these questions can be answered through the study of phase transitions in foods.

In the preface to their book, Phase/State Transitions in Foods . . . Chemical, Structural, and Rheological Changes—the proceedings of IFT’s 1997 Basic Symposium—Rao and Hartel (1998) stated that “An important goal of food scientists, technologists, and engineers is to develop desirable food structure and texture. Structure and texture in a food are the result of interactions among the food’s chemical components and the phase/state transitions induced in them during processing and storage.”

I have been a food scientist for almost 50 years, but my introductory chemistry courses in food science didn’t mention these interesting scientific phenomena. Thinking that there was still time to understand “phase transitions” in food product development, I attended a presentation at the Chicago Section IFT meeting this past May by pioneer food polymer scientists Harry Levine and Louise Slade, Kraft Foods Fellows, East Hanover, N.J. I came away with a new respect for how these basic scientists had developed improved foods, with a more reliable prediction of results and production savings through reduced processing, solely by the application of polymer science.

On the ride home, my two fellow longtime practical application-oriented food scientists and I still did not understand Levine and Slade’s attempt to explain exactly how they could measure several sources of changing ingredient data and apply them to consistently control the quality of the size and texture of baked cookies, among many other food products . . . but we believed that it works!

Acquiring Some Basic Knowledge
After considerable literature research and without trying to make every reader an expert polymer scientist, I realized that one needs to start with some basics of polymer science and then apply them to food characteristics. So here goes:

From a physics standpoint, “phase transition” is defined as a transformation of a thermodynamic system from one phase to another. The characteristic of a phase transition is an abrupt sudden change in one or more physical properties.

Referring to IFT’s Scientific Status Status Summary, “Glass Transitions in Low Moisture and Frozen Foods: Effects on Shelf Life and Quality” (Roos et al., 1996), Reid (1996) stated, “One of the most significant steps during the last half century was the recognition of relative water vapor pressure, often referred to as ‘water activity,’ as an important predictive factor for product stability. . . . Beginning some 15 years ago, a group of food scientists, encouraged by the pioneering work of Harry Levine and Louise Slade, . . . have come to realize that the physical state of the food system has important implications to both preservation and properties. Levine and Slade successfully applied polymer science principles to food systems, in particular to carbohydrate-containing aqueous systems, and focused upon the temperature dependence of molecular mobilities. They determined that glass transition, where the state of a material changes from a solid glassy state to a supercooled, viscous (or rubbery) liquid, played a key role. The temperature range over which the transformation from rubbery to glassy state occurs is an important transition range in stability properties of the food. . . . A range of techniques has been employed to identify the characteristic temperature range for such transitions in a variety of systems.”

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The Glass-Transition Phase
Polymer chemistry describes semi-crystalline solids as having both amorphous and crystalline regions. Depending on the temperature, the amorphous regions can be either in the glassy or rubbery state. The temperature at which the transition in the amorphous regions between the glassy and rubbery state occurs is called the glass-transition temperature T_g.” The glass transition is a property of only the amorphous portion of a semi-crystalline solid. The crystalline portion remains crystalline during the glass transition.

Thermodynamic transitions are also classified as first-order or second-order. First-order transitions show a transfer of heat between the system and the surroundings, and undergo an abrupt volume change; this is the condition of “melting.” Second-order transitions have no transfer of heat, but the heat capacity does change. As stated above, even crystalline polymers will have some amorphous portion (as high as 40–70%) and therefore can have both a glass-transition temperature and a melting temperature. Glass transition (or glass-liquid transition, GLT) is the name given to the phenomena observed when a glass is changed into a supercooled melt during heating, or to the reverse transformations during cooling.

At a low temperature, the amorphous regions of a polymer are in the “glassy state,” where the molecules are frozen in place and the polymer will be hard, rigid, and brittle. If the polymer is heated, it eventually reaches its Tg, the molecules will move around, and the polymer will reach its “rubbery state.” The rubbery state lends softness and flexibility to a polymer. An example is the glass transition of chewing gum. At body temperature, the gum is soft and chewable. This is characteristic of an amorphous solid in the rubbery state. If you drink a cold drink while chewing gum or hold an ice cube on the gum, it becomes hard and rigid. Therefore, the Tg of the gum is somewhere between 0 and 37ºC.

A polymer may also have a higher Tg than we would like for various ingredients in a food formulation or process. Scientists have a solution for this in the introduction of plasticizers. These are small molecules which get between the polymer chains and space them out from each other. This allows them more free movement and lowers the Tg of a polymer, making it more pliable and easier to work with in food manufacturing. Water is probably the best and most common example of a plasticizer for foods.

Applying What We Have Learned
Although food polymer science may have had an early beginning in the application of intermediate-moisture technology (water activity, a_W), phase transition technology is far beyond a_W. Food polymer science involves time, molecular weight, and composition of food factors. This same science can be used for ingredient selection by knowing the fiber structure, function, network, film, gel, and glassy matrix properties of ingredients. T_g can have good predictive value regarding the effects of temperature and water content in varied operations like drying, extrusion, and flaking. Processes such as baking, air- and freeze-drying, extrusion, and flaking may operate through the glass transition range.

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Le Meste et al. (2002) presented an excellent summary of potential food applications. Encapsulation continues to have a high interest for allowing controlled release of volatile aromas and flavors, without associated losses due to evaporation, oxidation, or ingredient interactions. Spray drying and extrusion are by far the most common flavor encapsulation techniques. The release of volatiles is promoted by temperature and water content conditions that bring the dry glassy material to the rubbery state (To and Flink, 1978; Whorton and Reineccius, 1995). The relatively high mobility of water and oxygen in glassy matrices is also responsible for the limited shelf life of encapsulated materials in dried food products.

Edible films and barriers can be expected to respond to GLT, as both their mechanical and barrier properties are strongly affected by temperature, ambient humidity, and plasticizer content. Crispness, a popular texture characteristic of various low-moisture foods, is lost when water content is raised above a threshold of 6–9% in crackers, popcorn, and potato chips (Katz and Labuza, 1981). Hardening effects in glassy cereal products, such as dried and extruded breads, are followed by softening at higher hydration (Fondanet et al., 1997; Roudaut et al., 1998). In the processing operations of air- or freeze-drying, or in the storage of dried products, GLT can be responsible for reduction in volume and porosity. Water content, temperature, and time dependence are controlling factors in structure collapse, stickiness, and caking of powder products (To and Flink, 1978). The formation and thickness of a crust is controlled by the relative rates of drying and collapse (Achanta and Okos, 1995).

Crystallization is an important quality factor in the texture of confectionery and ice cream. Numerous studies continue to determine the impact of Tg phenomena on the chemical stability of foods, with a focus primarily on non-enzymatic browning reactions (Karmas et al., 1992; Roos and Himberg, 1994; Lievonen et al., 1998; Craig et al., 2001).

Problems to Solve
The inability to determine unique T_g for specific ingredients makes the application of glass transition concepts to food technology a continuing problem. Neither molecular mobility nor microbial stability can yet be predicted by phase technology with complete confidence in products such as intermediate-moisture foods. Data on other parameters, such as width of the transition, fragility, non-linearity, and non-exponentiality have not been collected for food materials as they relate to chemical or structural attributes. However, cost-saving methodology for several food processes has resulted from recent applications of phase technology concepts to food products.

It will only be a matter of time, additional studies, and better understanding by food scientists before measurement and control the many parameters of this basic scientific approach is applicable to improvement of food products.

The most popular methods being used to determine the temperature range of GLTs are differential scanning calorimetry (DSC) and mechanical spectroscopy and dynamic mechanical thermal analysis (DMTA). Both techniques may provide significantly different values (Kalichevsky et al., 1992; Biliaderis et al., 1999). These methods give an indication of a transition at the macroscopic level. The application of nuclear magnetic resonance, electron spin resonance, and solute translational diffusion may give other information on the molecular mobility in various phases and microstructural locations.

by DEAN DUXBURY
Contributing Editor
Consultant, Oak Brook, Ill.
[email protected]

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