Bread
Bread dough is prepared by hydrating and solubilizing flour particles and yeast. When water is initially added to the flour a sticky mass is formed. The dough is manipulated until it forms into viscous extensible yet resistant smooth dough.
The hydration of bread dough may be carried out by either water or milk, with milk increasing the food value of the dough. Milk also increases the water binding capacity of the dough resulting in a juicer crumb. One function of water is to dissolve salts and sugars and disperse yeast. Water is also an essential component for the hydrolysis of starch and sucrose, the components of which are food for yeast cells. The water then allows for transportation of food to yeast cells through cell membranes. The gelatinisation of starch during baking and hydration of protein strands for gluten development are also carried out by water. Free water within the dough influences its extensibility too little water and the dough will be stiff and resist stretching and the dough will be slow to rise, too much water and the dough will be too sticky and soft to handle.
Salt added to bread is primarily for flavour, in small amounts it enhances the action of amylases which break down starch. It also inhibits the action of proteases preventing a sticky and hard to handle dough as a result of weakened gluten. A negative effect of salt is the slowing of the fermentation and carbon dioxide production.
The primary function of sugar is food for the yeast. An increase in the rate of addition of sugar increases rate of fermentation up to a ten percent. Adding sugar at greater than ten percent retards the fermentation. This is possibly due to sugar limiting the flow of nutrients into the yeast cells by osmotic pressure. When no sugar is added the fermentation is slower, sugars must be formed from amylolytic activity
Addition of fat results in a more tender and brown crust. The addition of fat to the bread dough has also been associated with greater loaf volume as fat forms an assistive barrier in the walls around gas cells, enabling greater expansion before rupture.
Flour is the source of protein (11-13%) for gluten formation, starch (71%) as a source of fermentable sugars and structural elements, and lipids (1%) which also participate in dough development. The flour from wheat, rye and barley are unique in their functionality to produce bread dough.
Manipulation or kneading of the dough is carried out to develop strong elastic strands of gluten. During kneading the hydrated and swollen particles of the protein fractions glutenin and gliadin adhere to each other and align to form long elastic strands of gluten. Glutenin subunits are joined by disulphide bonds to form long strings. The gluten in developed dough is seen as a continuous network of fibrils covered with a protein membrane in which is embedded starch granules. Kneading is also carried out to remove excessive carbon dioxide in order to prevent over stretching of the gluten strands and to distribute yeast. A well kneaded dough will have evidence of small air pockets. Excessive kneading will tear or over stretch the gluten and reduce its elasticity and the volume of the bread is reduced as gas cells leak and coalesce. The polar galactoylglycerides from added fats are bound to gliadins and glutenins simultaneously, with the hydrophilic portion binding to gliadin and the hydrophobic portion binding to glutenin. Starch grains from the flour become embedded in the protein matrix of gluten that surrounds the gas cells. Starch reduces the amount of gluten formed preventing an excessively cohesive dough.
Bread made from whole wheat flour does not require as extensive kneading as the bran particles cut/ interfere with gluten development, as a result producing a smaller loaf volume. The inclusion of the bran and germ in the weight of flour also reduces the percentage of gluten proteins, resulting in less gluten production. If the whole wheat flour is finely ground and less interference of gluten development occurs production of a loaf with similar volume to white bread is possible.
Rye flour does not form an elastic network like gluten as while it contains sufficient gliadin it is low in glutenin. Rye flour contains pentosans which along with starch form a cohesive structure. Pentosans (gums) interfere with the little gluten production which does occur. Due to the high pentosan content rye flour can absorb up to eight times its weight in water. Unlike starch pentosans do not retrograde and harden after cooking and result in a soft moist texture. Rye flour may also contain a high proportion of alpha-amylase activity due to the fact the grains tend to sprout before harvest. The enzyme breaks down the starch component of the dough structure. Rye flour has poor gas retaining properties and breads made form rye flour tend to be dense, gummy and have a strong flavour.
Soya beans are legumes and not cereal grains. In comparison the wheat flour soy flour has 35% protein, 20% fat, and 15-20% starch. Soy flour contains no gluten-forming proteins, but does increase the nutritional value of the bread. Soy flour may also contain amylase breaking down starch content. The addition of soy flour affects the absorption, mixing and fermentation of the dough. If soy flour is used in the proportion 1/3 cup to 5 cups wheat flour satisfactory dough is produced.
Fermentation occurs mostly during the resting/rising periods. It is when the yeast cells use the available sugars to produce carbon dioxide and alcohol. The available (fermentable) sugars include glucose, fructose, sucrose and maltose). Yeast cells contains an enzyme invertase which catalyses the hydrolysis of sucrose to glucose and fructose and maltose to glucose. Starch also contains enzymes alpha and beta amylase. Alpha amylase hydrolyses the reducing end of the starch polymer to produce limit dextrin. The beta-amylase then acts on the amylopectin left to produce maltose. The volume of the dough increases due to carbon dioxide production. Also during fermentation the pH of the dough decreases from 6.0 to 5.5-5.0 which is the result of lactic and acetic acid which are produced as a result of fermentation by bacteria which are naturally present in flour. The increase in acidity (decrease in pH) increases the capacity of the gluten to absorb moisture and allows a more complete hydration of the dough. Dough becomes easier to handle during fermentation due to the break down of flour proteins to amino acids by protease enzymes.
During the initial temperature increase during baking what is known as oven spring occurs (a quick increase in dough volume) due to expansion of carbon dioxide and other gases as they are heated, softening of the gluten, increased fermentation of the yeast and, increased enzyme activity. Increased fermentation occurs until internal temperatures destroy the yeast. The enzyme alpha amylase continues to hydrolyse starch until the internal temperature reaches 75ºC. Gluten undergoes a gradual change in properties from 50ºC – 80ºC and finally becomes firm. As gluten looses water during baking the starch swells and only partial gelatinisation occurs as the percentage of water is not high enough. As moisture is removed form the crust Mallard browning occurs due to the sugar and protein content.
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