Phase transitions and the mechanical properties of food biopolymers

At low temperatures dry biopolymer, and products of which they are a major component are usually stiff, hard and brittle. They are plasticized by heat ing and/or moisture sorption to a degree that is primarily determined by th e polymer species, and its molecular weight and structure but also by its m echanical and thermal histories. The temperature range at which the mechani cal changes take place can be very large in dry biopolymers. Moisture lower s the temperature level at which these changes occur and makes the transiti on from a brittle to a plasticized state sharper. In many instances the per tinent temperature-moisture conditions at which biopolymers function, in na tural systems and man made products, happen to correspond to the transition region. Hence many of the theories and models developed for either glassy or fully plasticized synthetic polymers are inapplicable to biopolymers. Al so, the magnitude of different measures of biopolymers mechanical propertie s need not rise and fall in unison as these theories predict. In fact, mode rate moisture sorption can increase certain biopolymers toughness at the sa me time as their brittleness is lost. The relationship between mechanical parameters and temperature or moisture can be described mathematically by several versions of a modified Fermi fun ction. The constants of these models can be combined to produce a single un ified model with which three dimensional plots of stiffness-moisture-temper ature relationships for example can be created. Such models can also be use d to describe the plasticizing and antiplasticizing effects of additives ot her than water. Application of these models to published data shows that co mmon hydrophilic food biopolymers, such as starches and proteins, have a si milar type of stiffness-temperature-moisture relationship at the transition region.