Texture Analysis to Measure the Effects of Enzyme Addition on the Staling of Artisanal Bread: Part 1 – Why does Staling Occur?

Bread is a common, low cost food that is eaten readily worldwide, associated with traditions in many countries. 

However, there has been a movement over the last decade or two towards products that are marketed as ‘clean label’, ‘plant-based’ or ‘artisan’, so supermarket shelves are seeing a shift from soft crust, cuboid loaves in coloured plastic bags to rustic looking crusty bread. Artisan bread is a term that has no absolute definition but refers to a style of short shelf-life bread that is usually offered unpackaged (in baskets) and consumed immediately after baking for maximum freshness.

The basic process of making bread dough is the addition of water to flour followed by a large amount of mechanical work. This comes initially from the mixer, and then from the expanding CO2 bubbles in the dough. These bubbles form when added yeast ferments sugars that appear in the dough when they are released from hydrated starch granules by the flour’s natural amylase enzymes. Mechanical work stretches the gluten into sheets that trap the CO2. As the dough expands, the gluten molecules uncoil and reveal more bonding sites, and the dough becomes stronger. In traditional breadmaking, dough must be left to prove for at least three hours for the expanding bubbles to develop the gluten. This long fermentation time develops the bread’s flavour, but is costly in time.

In the 1960s, a research effort was put into decreasing this fermentation time, which resulted in the Chorleywood bread process. This involved a large amount of yeast, a more vigorous mixing process to increase bubble rate formation, and the addition of oxidising agents to promote disulphide bond formation.

Any consumer will know that high moisture baked foods like bread, rolls and cakes become stale when stored for several days. Historically, this was of minimal concern because most bread products were consumed soon after baking. However, modern bakers are more concerned with staling because large wholesale bakeries and distribution chains may cause a day or two lapse between production and purchase. Additionally, traditional ‘clean label’ bread that is seeing a rebirth of popularity does not always include the full complement of additives from Chorleywood bread, and shelf lives are reduced due to faster staling. The importance of bakery enzymes is consequently brought back into focus.

Enzymes are flour and dough additives commonly used to improve the quality of finished baked goods by altering the way flour and dough behave in the mixing bowl and oven. They are classified as ‘processing aids’, which do not have to be declared on product labels (unlike additives). Their status as processing aids is based on the assumption that they are used up in the production process and are not really present in the final product. Consequently, enzymes are an important part of mass-produced ‘artisanal’ bread. Not including enzymes on a product label is a contentious issue, as there is some evidence that even if destroyed in the baking process, they can have adverse effects on the health of some groups of people. However, there is no doubt that they are an important part of the artisanal bread market.

Fast staling is costly to the producer, distributor and consumer. Some breads have very short shelf lives – on the extreme end of this is high bran Arabic bread, which becomes undesirably stale in only 6 hours. Bread staling is responsible for significant financial losses. As far back as 1990, an estimated 3-5% of all baked goods produced in the US were discarded every year, representing a value of $1 billion (Hebeda, Developments in enzymes or retarding staling of baked goods).

Why does staling occur?
When a loaf of bread is removed from the oven after baking, a series of changes occurs that eventually leads to the deterioration of quality. These changes are collectively called ‘staling’. It is a complicated process involving changes in all components of bread and is actually an organoleptic response to these changes - older bread seems ‘stale’ by sight, taste, touch and even sound. Some of the effects that are known to cause staling are moisture loss, starch recrystallisation or starch-gluten protein bond formation.

Moisture loss
Moisture loss might seem the most obvious culprit for staling, but it is actually a minor effect. A five-day old stale loaf that has been stored under proper conditions has the same moisture content as when it was fresh, although it gives the impression of a drier mouthfeel. However, it should be listed as a cause of staling nonetheless.

Starch recrystallisation (aka retrogradation)
Flour contains amylose polymers, which make up about a fifth of the total starch content. During the dough stage, these are initially aligned, side by side, in a microcrystalline structure. This structure is destroyed during baking (a process called gelatinisation), but subsequently crystallisation recommences, giving a crystal form which contains a lot of water of hydration. This reduces the amount of free water in the bread, and it loses springiness and appears to the consumer to dry out.

Starch-gluten protein bond formation
Gluten proteins are also involved in staling. Starch granules form crosslinks with gluten protein fibrils. An increasing number of these crosslinks form during storage, leading to an increased crumb firmness characteristic of staling.

What does this mean to sensory panels, instrumental test results and ultimately the consumer?
When a loaf of bread undergoes staling, the taste of the bread might be altered, but the change in mechanical properties is the more significant effect. The key features of a staling loaf are a crust that becomes less crispy and a crumb structure that shows an increase in firmness and elastic modulus but a decrease in elasticity. Stale bread has a drier, crumblier mouthfeel.

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