The aging of bread – modifications, and prevention
Fresh bread is a product with a short shelf-life and during its storage, chemical and physical alterations occur, known as staling. As a result of these changes, bread quality deteriorates gradually as it loses its freshness and crispiness while crumb firmness and rigidity increase.
The aging of bread – modifications, and prevention
The molecular basis of staling is examined by reviewing what is known about the components of wheat flour, factors that affect staling rate, and the various mechanisms that have been proposed.
Bread staling is a complex phenomenon in which multiple mechanisms operate – fig. 22 (Katina et all., 2006). Polymer crystallization with the formation of supramolecular structures is instantly involved.
The most plausible hypothesis is that retrogradation of amylopectin occurs, and because water molecules are incorporated into the crystallites, the distribution of water is shifted from gluten to starch/amylopectin, thereby changing the nature of the gluten network.
The role of additives may be to change the nature of starch protein molecules, to function as plasticizers, and/or retard the redistribution of water between components.
The storage temperature has a great influence on the rate of bread staling. For storage temperatures of −18°C water activity decreases and it is maintained at an almost constant level for 23 days.
During storage, the starch molecules are associated and generate a new crystalline order. As observed with the recrystallization kinetics, at −18°C only crystal growth may occur, whereas at 25 °C and 4°C there would be not only growth but also the formation of new crystals. At 4°C the rate of starch retro gradation is the highest for the studied conditions (Russel, 1983).
Some solutions to prevent the aging of bread:
- Maltogenic amylases can help improve the softness of bread over the shelf-life and are used by bakeries to counter the effects of staling, especially in long-life products.
- Lipases produce monoglycerides and fatty acids from triglycerides which complex with starch to inhibit staling.
- The combination of bran sourdough and enzyme mixture significantly improved the volume, texture, and shelf-life of wheat bread supplemented with wheat bran (20 g bran/100 g of flour).
The use of fermented bran improves the structure of the gluten network and may alter water migration between starch, protein, and bran particles during storage.
The antistaling effect of the combined use of bran sourdough and enzyme mixture was due to reduced starch retro graduation rate slowed the increase in rigidity of polymer structure, and due to degradation of cell wall components leading to altered water distribution between starch–protein matrix (Katina et all.,2006).
Fats are known to delay staling of bread. Research has shown that when fat is added to defatted flours where native polar lipids have been removed, the anti-staling is lost. The most prevalent theory is the complex creation with starch constituents (amylase and amylopectin) Anti-staling effect is more prevalent in fat percentages higher than 3 % than the flour weight.
– monoglycerides in the form of hydrate have proven to be the top anti-staling surfactants and surpass the performance of shortenings both on antistaling but also on softening of bread.
– water activity control – water activity affects food chemistry and can be controlled by removal (dehydration or drying) or by chemically binding the water, reducing its activity.
Some examples: propylene glycol, sucrose, and sodium chloride used to achieve water activities of 0.78 or 0.79 in semi-moist food; a combination of sugar (7%), glycerol (2%), propylene glycol (1%), and salt (1.5%) predicted aw of 0.85 in cereal-based products; using glycerol as 15 to 45% of the food product an aw less than 0.85 could be achieved.
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