(500b) The Fragility of Glassy Foods

The non-Arrhenius behavior of amorphous materials near the glass transition temperature is well documented and theoretical models such as the Williams-Landel-Ferry (WLF) and Vogel-Fulcher-Tammann (VFT) have been used successfully in both synthetic and bio - polymers to describe the temperature dependent variation of material properties. A recent development in the synthetic polymer literature is the concept of ?fragility' that was broadly popularized by C.A. Angell in explaining the varying tendencies of liquids to vitrify. Fragility is based on the idea that the strong liquids will have stable structures of local to intermediate order, whereas the structure in fragile liquids will be unstable and they will show a broader range of property changes when going from the liquid or melt state to the glassy state. Fragility has been determined by two differing means: 1) as the ratio of thermodynamic heat capacities in the liquid and glassy states and 2) as the steepness factor, m, determined from dynamic relaxation data. Such studies determining the fragility of synthetic polymers and their influence on structure-function relationships have not been made for food bio-polymers.

In this work, fragility of a particular food bio-polymer - soy flour, has been determined from published and experimental relaxation data and the effect of processing and storage parameters, such as cooking temperature and water activity on fragility are assessed.

Angell plots, which represent relaxation data as a reduced Arrhenius plot of log (a_T) vs. T_g/T (where a_T are shift factors from Time-Temperature superposition experiments), have been developed from published data on the dependence of shift factors, a_T, of uncooked defatted soy flour on water activity (Yildiz and Kokini, 2001). Such plots are also constructed for experimental data obtained on defatted soy flour and soy protein concentrate that have been cooked at processing temperatures of 60°C or 95°C and 145°C in a convection oven or a twin-screw extruder respectively. The published WLF constants, C₁ and C₂, of defatted soy flour at various water activities (Yildiz and Kokini, 2001) and the WLF constants deduced from experimental data for defatted soy flour and soy protein concentrate cooked at various temperatures are used to determine the steepness factor, ?m', in the manner of Huang and McKenna (2001).

Angell plots reveal the extent of the non-Arrhenius behavior in the samples studied. A linear plot of log (a_T) vs. T_g/T indicates strong Arrhenius type behavior near the T_g while a non-linear plot indicates non-Arrhenius behavior with the extent determined by the curvature. The fragility obtained as a steepness factor, m, has high values (~200) for a strongly non-Arrhenius and fragile liquids whereas values as low as ~20 have been observed for strong liquids (Huang and McKenna, 2001).

Preliminary observations in this study have obtained ?m' values of 48, 32, 64 and 25 for defatted soy flour at water activities of 0.43, 0.57, 0.68 and 0.75 respectively. While, the results do not indicate a clear trend with respect to water activity, it can be observed that they are within range of values published for synthetic polymers. Also, the Angell plot for this data is linear in the range of temperatures studied and shows behavior typical of a ?strong' or ?less-fragile' material.

The study of the fragility of glassy foods can provide a window into understanding the mechanism of vitrification in foods. It presents an opportunity to understand the role of intrinsic and extrinsic factors that shape the final structure of a glassy material through unique means of understanding the strength of the interactions involved. Preliminary findings for food polymers reported in this study offer a glimpse on the effects of water activity in the vitrification of defatted soy flour and further work is needed to understand the role that water activity and other factors, such as processing temperatures, play in this system.

References:

1. Yildiz, M. E. and Kokini, J. L., 2001. ?Determination of Williams?Landel?Ferry constants for a food polymer system: Effect of water activity and moisture content?, J. Rheology, 45:903-912.

2. Huang, D. and McKenna, G., 2001. ?New insights into the fragility dilemma in liquids?, J. Chem. Phys., 114: 5621-5630.