Bread: phases of development

Dough

  • Fermentation

    • The flour used to make dough initially contains small amounts of complex and simple sugars. The complex sugar sucrose is hydrolysed to glucose and fructose in reactions involving the enzyme sucrase originating from the yeast. The complex sugar maltose is hydrolysed to glucose in reactions involving the enzyme maltase, also originating from the yeast. The simple sugars glucose and fructose enter the yeast cells and are broken down into carbon dioxide and alcohol in reactions involving the zymase group of yeast enzymes.

  • Diastasis
    Concurrently with the fermentation process described above, diastasis (Gk. 'separation') of starch takes place. Those starch particles (from the endosperm) which were damaged during the milling of the grain have an increased capacity to absorb water which is necessary for the action of enzymes. Under suitable conditions of moisture, temperature and acidity, starch is hydrolysed to the sugar maltose in a multi-stage process involving the diastatic flour enzymes α-amylase and β-amylase. This maltose and other fermentable sugars present in the flour sustain the yeast so that production of carbon dioxide and alcohol may continue after the supply of sugars initially present in the flour has been exhausted.

    As fermentation proceeds, the dough becomes slightly more acid, owing in part to carbon dioxide dissolving in the water and in part to the presence in the flour of bacteria which produce acetic and lactic acids. This enhances the aroma and flavour of the bread and promotes diastasis, which ripens the dough. Butyric acid is also produced but this is not a flavour improver. The optimum temperature range for the fermentation process is 27 - 38°C.

  • Science of kneading
    Kneading creates shearing forces which break the cross links or bonds between initially randomly aligned gluten protein molecules (glutenin and gliadin) and cause them to stretch and become aligned in parallel. New cross links then form but these are of greater strength, causing the gluten to become stronger and more elastic. The cross-linking also increases the molecular weight and viscosity of the gluten proteins. These effects modify the rheological (flow) properties of the dough, making it yield to gas pressure (rise) more easily yet still retain the gas. Glutenin is the main contributor to the strength and elasticity of the gluten, whereas gliadin acts as a plasticiser. Glutenin and gliadin have therefore to be in correct balance to produce an ideally formed dough. The presence of gluten can be confirmed by a simple and interesting experiment (No 1).

    The shearing action of the kneading process requires that the dough be firm enough to form an integral mass, yet yield under moderate pressure. Wholemeal flour is less responsive to this treatment than white flour because the bran particles that it contains, being non-functional protein, do nothing to sustain the yeast yet cause a more open crumb structure which impairs gas retention. The water absorption of wholemeal flours is greater than that of white flours. This is because a proportion of the water is absorbed by the (chemically inactive) bran particles. The amount of water given in a recipe may need to be varied in order to render dough of optimum consistency. If the dough is too dry it will lack malleability, if too fluid (wet) it will not respond to kneading at all (see also Troubleshooting chart).

  • Maturation or leavening
    (Leaven: L. levare 'to lift'). The structure of the gluten develops further with time, and the dough becomes more gas tight. Carbon dioxide initially dissolves in the water present in the mixture. After the oxygen has been depleted by the yeast, the carbon dioxide comes out of solution and the gas diffuses into the nitrogen bubbles present in the mixture. Continued production of carbon dioxide then causes the dough to expand. Some of the starch becomes gelatinised by absorbing water; this also contributes to increased dough viscosity and gas retention.

  • Characteristic variations
    Each variety of wheat will respond in a characteristic way during dough development because of differences in constituents such as carbohydrates, lipids (non water-soluble fatty acids) and proteins.

    Baking

    Baking kills the yeast, fixes the structure of the loaf, and the heat drives off most of the alcohol. Starch gelatinises at 66°C, and coagulation of the hydrated proteins causes the dough to set. Enzymic action ceases at 77°C. The high temperatures and moist conditions near the surface of bread during baking cause starch to hydrolyse to dextrin. This glazes the crust, which darkens when the dextrin caramelizes. This reaction does not occur when baking Quick bread, but a dull crust will form as a result of moisture loss.

    Staling

    The starch granules in the flour, which are initially crystalline, absorb water during the development of the dough and become partially gelatinised, making them soft. After baking, bread begins to age. The starch recrystalises, becoming brittle again, and it shrinks owing to loss of water. These changes, accompanied by loss of flavour owing to other chemical changes, produce the characteristics of staleness. Staling can be reversed to an extent by briefly warming the bread to 70°C.

    Mould development

    Many kinds of mould spores will develop on bread in warm moist conditions. They come in a variety of colours including black, white, green, pink, and brown. Airborne mould spores include Mucor mucedo, which produces white mould, Penicilium glaucum, which produces green or blue mould, and Aspergillus niger, which produces black mould. Moulds produce mycotoxins which can permeate the substrate, making mouldy food dangerous to eat.

    Rope disease
    This is the commonest disease that effects bread. Some hours after cooling, the bread may manifest a smell like pineapple and a pronounced stickiness. This is caused by Bacillus subtilis and similar species of bacteria which originate in the soil. (It was attributed to Mesentericus vulgatus in early research.) The bacilli produce enzymes which break down the bread, polysaccharides (causing the stickiness), toxins, and chemicals having the characteristic smell. If the bread is eaten it may cause a burning sensation in the mouth and vomiting.

    Rope only develops rapidly in warm, moist, alkaline environments. Acidic flour additives help to arrest the development of rope but calcium carbonate added as a dietary supplement, favours its development. Baking temperatures are not high enough to kill the bacilli. Flour should be stored in cool, dry conditions to reduce the risk of rope development.

    The Bread Pages: Contents

    Homepage