It was not until the early 1980s that food scientists, led by Drs. Harry Levine and Louise Slade, realized that synthetic polymer principles are applicable to food systems (1–8). Various thermal analysis techniques have demonstrated the similarity between them (9–11).Figure 1shows the five regions of viscoelasticity of a synthetic polystyrene: AB glassy region, BC glass transition region, CD
Figure 1 Five regions of viscoelasticity, illustrated by using a polystyrene sample. Also shown are the strain (ε)–time curves for stress applied atxand removed aty: (a) glassy region, (b) leathery state; (c) rubbery state; and (d) viscous state. (From Ref. 12.)
rubbery plateau region, DE rubbery flow curves for stress applied atxand re- moved aty. In the glassy state (AB), the material behaves like an elastic solid.
In the viscous state (EF), it is a liquid, while it behaves like a viscoelastic material in the temperature ranges from B to E. In The modulus–temperature curve is very sensitive to many structural factors, such as molecular weight, degree of cross-linking, percentage crystallinity, copolymerization, plasticization, and phase separation (13).
The glass transition temperature (Tg) is a function of product composition, molecular weight of the continuous structural matrix, degree of branching, degree of cross-linking, crystallinity, and degree of plasticization. For un-cross-linked molecules, the drop in modulus is about three decades nearTg. The magnitude of drop in modulus in the glass transition region decreases as the degree of cross- linking or molecular entanglement increases, which is the case for low-moisture gluten samples. As shown inFigure 2,for a gluten sample equilibrated in 65%
relative humidity (RH), the drop in modulus is less than two decades (13). The degree of viscosity drop at a constant (T ⫺ Tg) has been used as an index of
‘‘fragility.’’ Unfortunately, organic glass usually is more fragile than inorganic glass. Thus, there is minimal opportunity for applying this concept in stabilizing food systems. A high-crystallinity sample has lower modulus drop atTgdue to
Figure 2 Typical DMTA plot for gluten (RH⫽65%), showing tanδ, log loss modulus (E″), and log elastic modulus (E′) as a function of temperature. (From Ref. 13.)
the reduced amount of amorphous region, and is directionally higher inTg. Plasti- cizers decrease bothTgand the rubbery modulus of a PVC-diethylhexyl succinate system (Fig. 3), whileFigure 4 shows that water is a powerful plasticizer for gluten (13). Crispy breakfast cereal has modulus of around 109Pa. The modulus for glucose glass is 8⫻109Pa, while it is 8⫻108Pa for glucose/sucrose glass at 2–3% moisture, regardless of their ratios (15).
As shown inFigure 1,the modulus in the rubbery plateau region is rela- tively constant, and its temperature range (CD) is a function of molecular weight and the number of entanglements per molecule. Thus, as shown inFigures 2and 5, a gluten sample at 65% relative humidity, due to its high molecular weight and degree of entanglement, has very long rubbery plateau, while sorbitol has almost no visible one (15). A slice of microwave-heated bread has the modulus of 106–107Pa at room temperature, which is in between the glass transition and rubbery plateau regions as typified by its leathery-rubbery texture.
Figure 3 Dynamic shear modulus of polyvinyl chloride plasticized with various amounts of diethylhexyl succinate plasticizer. (From Ref. 14.)
In contrast to sorbitol, which goes into rubbery and viscous flow right after the glass transition region, the gluten DMTA curve lacks the rubbery flow and viscous flow regions. This is probably due to the relatively high degree of cross- linking and/or entanglement. Molasses, honey, and batters are examples of food systems in the viscous flow region at room temperature.
A blend of incompatible polymers will phase-separate and show more than oneTg, as indicated inFigure 6for sodium caseinate–water and fructose–water systems (16). From all these observations, we can conclude that food systems can indeed be viewed as synthetic polymer systems.
Air cells or pockets help soften some products, such as breakfast cereals, that would otherwise be too hard. Gas cells inside some products can be from many sources: entrapped air during mixing, yeast leavening, chemical leavening, injected inert gases, carbon dioxide, ethanol. Those gas cells will eventually have the same gaseous composition as that of the packaged environment. One area that differs from the synthetic polymer system is the presence of fat in food matrices. In bakery products, fats and emulsifiers do not affectTg, but they do decrease the rubbery modulus (17). That is why they are sometimes called tender- izers instead of plasticizers. The tenderizing effect at serving temperature is a
Figure 4 DMTA plot for gluten samples stored under different RH values. (From Ref. 13.)
function of the solid fat index, fat content, and fat crystalline form. Due to their lubricating effect, fats and emulsifiers also enhance the perceived moistness of bakery products. Moistness also results partly because the true moisture content of the nonfat portion is higher than the apparent moisture content, which is based upon the total system. No wonder that fat-free bakery products usually taste dry and not as tender.