The mechanical behavior of bread resembles that of plastic materials.
LEFT OUT IN THE KITCHEN, at room temperature, bread goes stale. Frozen, it seems to change more slowly, but at what temperature must it be kept in order to stay in the same state as when it comes out of the oven? 7°C (45°F)? 0°C (32°F)? –10°C (14°F)? Physical chemists at the École Nationale Supérieure de Biologie Appliquée à la Nutrition et à l’Alimentation (ENSBANA) in Dijon have sought to answer this question using their knowledge of polymers, which are very long molecules formed by the linking of subunits called monomers. This seemed to be a natural approach, for foods contain many polymers: The molecules that constitute the starch granules in flour are linear or ramified chains of glucose molecules known respectively as amylose and amylopectin, proteins are chains of amino acids, and so on.
At high temperatures polymers are in a liquid state because they have sufficient energy to move in a disordered fashion, allowing their mass to flow. When polymers are cooled, they initially form a rubbery solid in which certain polymer chains crystallize while preserving the ability to slide past one another. Then, at temperatures lower than the temperature of vitreous transition, the chains are immobilized and the material solidifies, with their crystalline parts dispersed in an amorphous rigid part, or glass. The structure of the solid phase depends on the cooling. When the cooling is rapid, the viscosity increases too quickly for the molecules to be able to crystallize, and the vitreous part predominates.
Thus many foods are kinds of glass: Sugar cooked with water becomes concentrated with the evaporation of the water and gradually forms a glass; powdered milk, coffee, and fruit juice sometimes also appear in a vitreous state. What about a fresh loaf of bread? Is it initially a rubbery solid that then vitrifies or partially crystallizes as it goes stale? Martine Le Meste, Sylvie Davidou, and Isabelle Fontanet at ENSBANA studied this question by recording the mechanical behavior of various hydrate samples as a function of temperature and comparing the reactions of loaves of bread with those of extruded flat breads, such as crackers.
When one heats bread dough, which is essentially a mixture of flour and water, the starch granules in the flour release their amylose molecules into the water, as we have seen. As the bread cools, the amylose molecules form a gel that traps the water and the amylopectin. In order to prepare variously hydrated breads, the Dijon team first completely dehydrated a series of samples by placing them for a week in desiccators, where the water was absorbed by phosphoric anhydride. The samples were then rehydrated under controlled hygrometric conditions and coated with an impermeable silicone grease. A viscoelastometer was used to measure the force transmitted by the samples when they were deformed in a controlled way, yielding a coefficient of rigidity known as Young’s modulus.
The researchers found that bread remains in a rubbery state as long as the temperature is higher than the vitreous transition temperature, –20°C (–4°F). On the other hand, analysis of the vitreous transition temperature as a function of water content showed that a part of the water does not freeze and that it plays a plasticizing role.
Freezing Bread
These observations have practical implications. The many results obtained by polymer chemists allow us to predict the changes in the mechanical properties of bread and its cousins as a function of their water content, crystallinity, and so on. Among other things, even if the water that freezes is immobilized, freezing will not arrest such changes as long as the temperature is higher than the vitreous transition temperature. At temperatures between –20°C and 0°C (–4°F and 32°F), then, bread continues to undergo structural alteration. To preserve bread without compromising its textural characteristics, the freezing temperature must be lower than the vitreous transition temperature.
The loss of freshness in bread had long been attributed to the phenomenon of starch retrogradation, in which amylose progressively crystallizes, releasing its water. The Dijon team observed instead a co-crystallization of amylose and amylopectin into hydrated crystals. Lipids counteract the loss of freshness that occurs over time because they bind with the amylose, forming crystals that retard the co-crystallization of the amylose and amylopectin.
Nonetheless, the firmness associated with stale bread does not result solely from this co-crystallization. The behavior of the amorphous, or vitreous, regions seems to play a major role. Water is an important parameter in storage, for it works to plasticize these regions, which in turn affects the rate and type of crystallization that occur.