Softness is a key parameter used to judge the freshness and, consequently, the quality of bread. Bakers are therefore interested in maintaining the softness of their bread for as long as possible. Any loss of bread crumb softness is often referred to simply as staling. Staling is defined as any change other than microbial spoilage that occurs after baking, making bread less acceptable to the consumer.
Physical or sensory changes associated with staling include: loss of crumb softness, flexibility, and strength; increase in crumb resilience; tendency to become crumbly; loss of flavor; and change in mouthfeel.
Starch, which makes up approximately 70 percent of flour, is regarded as the main flour component involved in staling. After baking, the gelatinized starch in bread tends to re-associate or, to use another term, retrograde.
After cooling and during the first hours after baking, the initial crumb structure is set by amylose gelatinization, creating a network in which the gelatinized starch granules are embedded. Re-crystallization of amylopectin side chains leads to the increasing rigidity of the starch granules and an overall strengthening of the crumb structure, measured as an increase in crumb firmness. Other factors, however, also have an impact on bread firming, particularly the distribution of water between protein and starch, which undoubtedly, plays an important role.
Starch retrogradation, though, is the main factor with regard to time determined changes in crumb softness. Functional ingredients that limit retrogradation are instrumental in improving crumb softness.
Starch consists of two fractions: amylose and amylopectin in a ratio of approximately 1:3. Both macromolecules comprise glucose units, although with structural differences. Amylose is a relatively small (molecular weight is approximately 250,000), linear and water-soluble macromolecule, while amylopectin is a very large (molecular weight is approximately 205,000), bulky, branched, and water-insoluble molecule.
Figure 1 on page 38 shows the changes that occur from the dough stage to fresh bread and, finally, to old or stale bread. The restoration of bread freshness by heating (toasting) is also indicated. In the dough stage, unswollen starch granules contain crystalline amylopectin, amorphous amylose, and polar lipids. The granules are embedded in gluten, which forms the continuous phase. During baking, the starch granules absorb water and swell. The amylopectin crystals are gradually disrupted at temperatures above 140 degrees Fahrenheit, and gelatinization takes place. Some of the amylopectin molecules expand into the inter-granular space and, at a somewhat higher temperature—around 176 degrees Fahrenheit, some of the
amylose that has not formed complexes with polar lipids leaks from the swollen granules. Within hours after baking, the amylose molecules develop a network, and a sliceable crumb structure is formed, giving the fresh bread its initial firmness. During aging, reformation of the amylopectin’s double helical structure and reorganization into crystalline regions takes place. While the re-association of amylose occurs within hours, the retrogradation of amylopectin takes days.
How Enzymes Work
Enzymes have been applied in bread making for decades. Bakery enzymes such as amylases help modify starch during the baking process. Slowing starch retrogradation, they ensure bread stays soft for longer than bread made without enzymes.
The varying action patterns of the most important amylases are shown in Figure 2. One effect of the enzymes is to reduce starch retrogradation by modifying the starch.
There are two main types of amylase enzymes: endo-amylases, such as classic fungal and bacterial α-amylases, that primarily hydrolyze starch at random within the amylose and amylopectin molecules; and exo-amylases that primarily hydrolyze starch from the non-reducing ends of starch molecules, cutting off two or four glucose units.