There are four main steps in making yeast bread.
Mixing
The first step is to mix the ingredients-flour, water, yeast, and salt-together. During mixing, starch granules absorb water and enzymes digest starch into sugars. The yeast feed on the sugars and produce carbon dioxide and alcohol. Glutenin proteins begin to react into gluten. Mixing can be done by hand, with an electric-mixer, or in a food processor. The later offer the advantage of limited oxygen exposure, which in excess can alter the pigments and flavor of the bread.
Kneading
The second step involves dough development. Through kneading, the dough is stretched, folded over, and compressed over and over. This manipulation strengthens the gluten network. It orients proteins to lay side by side which encourages them to form bonds. Overdevelopment of gluten causes bonds to break and turns the dough sticky and inelastic. Bewrae of overdeveloping the dough when kneading mechanically.
Kneading also aerates the dough. The more air pockets formed, the finer the texture of the final bread. Some breads call for minimal kneading. This results in fewer and larger air cells and their corresponding irregular and coarse texture. The gluten is weak and less developed, but it continues to develop through fermentation and can rise to an airy, tender crumb.
Fermentation (Rising)
The third step is fermentation, where the dough is set aside for yeast cells to produce carbon dioxide. As they do so and the carbon dioxide diffuses into air pockets, the dough rises. Yeast have the highest metabolic activity at 95 degrees F. This produces the greatest amount of carbon dioxide and metabolic by-products, some of which can be sour and unpleasant. For quick rising, it is suggested to keep fermentation at 80 degrees F for a couple of hours. Lower temperatures may extend the fermentation an hour or two, but generally produce more desirable flavors.
The end of fermentation is signaled by the dough's volume and the gluten matrix. Fully fermented dough is about twice its original size and has been stretched to its limit, so a finger impression remains when touched. Fermented doughs feel softer and are easier to handle than freshly kneaded dough.
Fermentation can be retarded by storing dough in the refrigerator. Yeast take 10 times as long to rise bread in cool temperatures. Retarding fermentation not only allows bakers to break up the work of making bread, but it has useful effects too. Long, slow fermentation allows greater flavor development by the yeast and bacteria in the dough. Cold dough handles easier without as much loss of leavening gas. The cycle of cooling and rewarming redistributes gases and promotes the development of a more open and irregular crumb structure.
Baking
The last step is baking. The kind of oven where a bread is baked has an important influence on the qualities of the finished loaf. Traditional bread ovens were made out of clay, stone, or brick. The baker preheated the oven by wood fire to temperatures up to 900 degrees F. The domed roof stored heat and radiated it down onto the loaves. The temperature declines during baking. The dough expands early on. The bread benefits from color and flavor development due to enhanced browning reactions.
Modern metal ovens are not ideal for bread making. The maximum cooking temperature is usually around 500 degrees F. Heat cannot be stored as well within their walls, so modern ovens maintain a heat source of gas or electrical elements. The necessary venting does not allow gas ovens to retain the loaves' steam. Some bakers use ceramic baking stones or ceramic oven inserts that mimic the traditional oven. The oven is preheated to its maximum temperature and provide more intensive even heat during baking.
Steam is important in the early baking stages because it increases the rate of heat transfer from the oven to the dough. As steam condenses onto the dough surface, it forms a thin film of water that temporarily prevents it from drying out into a crust. By doing so, it encourages the initial rapid expansion of the loaf. The hot water film eventually dries into an attractive glossy crust. Professional bakers often inject steam in the first minutes of baking. At home, one can spray water or throw ice cubes into the hot chamber to improve oven spring and crust gloss.
There are three stages of baking plus cooling.
Early baking: Oven Spring refers to the first 6-8 minutes of baking. Heat transfers first from the oven floor to the bottom of the dough, and to the top from the hot air and oven ceiling. Heat moves from the surface through the dough slowly through the gluten matrix; and rapidly through the gas network. Alcohol and water in the dough vaporize. The gas cells expand, and the dough rises. The better leavened the dough, the faster it cooks.
Mid-baking: The dough begins to transform into a sponge when the interior temperature of the dough reaches 155-180 degrees F/68-80 degrees C. At this range, the gluten proteins form strong cross-link bonds, the starch granules gelate, and the amylose molecules leak out. Gas pressure builds and ruptures the walls, turning the closed network of bubbles into an open network of pores similar to a sponge.
Late baking: Starch continues to gelate thoroughly. Continued cooking encourages surface browning reactions that improve color and flavor. Though limited to the crust, these reactions affecte the flavor of the whole loaf because their products diffuse downward. Bread is done when its crust has browned and the inner structure has set. Fully cooked bread feels light and hollow.
Cooling
The temperature varies inside a loaf immediately after being removed from the oven. During cooling, the differences even out. Most moisture loss occurs at this stage as moisture in the interior diffuses outward. Small rolls dry out the most, while large loaves the least. As temperature declines, starch granules become firmer, which later allows even slicing. This firming continues over the next day and starts the process of staling.
Showing posts with label fermentation. Show all posts
Showing posts with label fermentation. Show all posts
Tuesday, April 3, 2012
Sunday, April 1, 2012
Yeast metabolism
Baker's and brewer's yeast is Saccharomyces Cerevisiae, also referred to as the "sugar-eating fungus."
Yeasts feast on glucose and fructose from sugar, and on maltose from the broken-down starch granules in the flour.
Yeast metabolize sugars for energy and produce ethyl alcohol and carbon dioxide following this equation:
C6H12O6 -----> 2(CH3CH2OH) + 2(CO2) + ATP
In making beer and wine, the carbon dioxide escapes the liquid and concentrates the alcohol. In making bread, carbon dioxide and alcohol become trapped in the dough. The flexibility of the dough accomodates the expanding gas by inflating or "rising." The ethyl alcohol, along with other by-products of fermentation, give yeast-leavened breads their typical aroma. At the time of baking, the heat expells both carbon dioxide and alcohol from the dough, leaving a flavorful network of empty air pockets.
A small amount of added table sugar increases yeast activity, whereas a large amount decreases it. Too much sugar dehydrates the yeast. To compensate, bakers of sweet breads add more yeast than ordinary, and allow longer times for the bread to rise.
Yeast are also sensitive to salt, and are greatly affected by temperature. Cells grow and produce gas most rapidly at around 95 degrees F/35 degrees C.
Yeasts feast on glucose and fructose from sugar, and on maltose from the broken-down starch granules in the flour.
Yeast metabolize sugars for energy and produce ethyl alcohol and carbon dioxide following this equation:
C6H12O6 -----> 2(CH3CH2OH) + 2(CO2) + ATP
In making beer and wine, the carbon dioxide escapes the liquid and concentrates the alcohol. In making bread, carbon dioxide and alcohol become trapped in the dough. The flexibility of the dough accomodates the expanding gas by inflating or "rising." The ethyl alcohol, along with other by-products of fermentation, give yeast-leavened breads their typical aroma. At the time of baking, the heat expells both carbon dioxide and alcohol from the dough, leaving a flavorful network of empty air pockets.
A small amount of added table sugar increases yeast activity, whereas a large amount decreases it. Too much sugar dehydrates the yeast. To compensate, bakers of sweet breads add more yeast than ordinary, and allow longer times for the bread to rise.
Yeast are also sensitive to salt, and are greatly affected by temperature. Cells grow and produce gas most rapidly at around 95 degrees F/35 degrees C.
Sunday, March 4, 2012
Buttermilk
True buttermilk is composed of the low-fat portion of milk that remains after the cream has been churned into butter. Traditionally, its thickness and flavor develop by mild fermentation. It has remnants of fat globules that make it an excellent emulsifier like lecithin; a characteristic that make it valuable in the preparation of finely-textured foods. True buttermilk is slightly acidic and has a subtle, complex flavor. Regrettably, it is prone to spoilage and off-flavors.
A shortage of true buttermilk after World War II led to the development of imitation “cultured buttermilk.” It is made from ordinary skim milk that is fermented. The process follows that of yogurt, but the fermentation is stopped abruptly by rapid cooling. The gelled milk is then agitated to produce a thick, smooth liquid. Most buttermilk sold in the US is not true buttermilk but rather imitation cultured buttermilk.
Bulgarian buttermilk has yogurt cultures that have replaced the cream cultures. It has increased acidity by fermentation also at higher temperatures; thus it resembles yogurt.
Sour cream
Sour cream is a leaner, firmer version of crème fraîche. It contains 20% milk fat and enough protein to curdle when cooked. It appears to have originated from Central and Eastern Europe and brought to the US in the 19th century. Since then, Americans have added a small amount of rennet, which contains enzymes that cause protein coagulation. The result is a heavier, firmer sour cream than its European counterpart.
Acidified sour cream is made by coagulating milk with acid instead of through fermentation. It is therefore also known as non-fermented sour cream. Manufacturer versions of low fat and non-fat sour cream replace butterfat with starch, plant gum, and dried milk protein.
Crème fraîche
Crème fraîche is 30% milk fat pasteurized at moderate temperatures. It is not made from UHT (Ultra High Temperature) pasteurized or sterilized milk. Two versions are available: liquid and thick. The liquid crème fraîche is unfermented. It has a shelf life of 15 days. The thick version is fermented with a typical cream culture. It has a shelf life of 30 days.
A home-made version of crème fraîche is made by adding cultured buttermilk or sour cream to heavy cream (1 tbsp per cup) and letting it stand at cool room temperature for 12 to 18 hrs or until thick.
Cream cultures
Cream cultures such as sour cream, crème fraiche, and buttermilk are indigenous to Western and Northern Europe. These products result from the slow fermentation produced by the mesophilic bacteria Lactococci and leuconostoc species. These bacteria have three important characteristics that make them ideal for production of creams and buttermilk: 1. They grow best at moderate temperatures. The process of fermentation can be kept at lower temperatures than those that produce yogurt. 2. They are moderate acid producers. Again, it prevents the formation of yogurt from too acidic a condition. 3. They complement flavor by turning citrate into diacetyl; a compound which gives the fermented milk product a characteristic buttery flavor.
Yogurt
Yogurt originated in the warm climates of Southwest Asia and the Middle East. Though it has been produced for thousands of years, it only gained popularity in Europe in the early 20th century. By the 1920s, yogurt attained factory-scaled production. Broader popularity for yogurt came after the French developed a means to give it a creamy texture and added fruit flavors.
Yogurt production follows the common path of fermented milks: heat and fermentation. First, the milk is prepared by heating it to concentrate proteins and denature the whey protein lactoglobulin. This treatment improves the consistency of the yogurt. A denatured lactoglobulin allows casein proteins to bond and form a fine matrix that retains liquid in its small interstices instead of coagulating into semi-solid curds. The milk is then cooled to a warm temperature optimal for bacterial production of lactic acid.
The bacteria used for the fermentation of yogurt are very thermophilic. Industrially, these bacteria include Lactobacillus delbrueckii, subspecies bulgaricus, and Streptococcus salivarius, subspecies thermophilus. These bacteria stimulate each other and in combination acidify milk rapidly. They are also notable for their production of flavor compounds dominated by acetaldehyde, which gives yogurt its fresh, tart flavor.
Fermentation of yogurt at high temperatures of 40 to 45 degrees Celsius (104-113 degrees F) grow bacteria that multiply quickly and produce large amounts of lactic acid. Milk proteins set in 2-3 hrs, but produce a coarse protein network. The proteins assemble in thick strands which give it firmness but leak whey protein readily. In comparison, fermentation of yogurt carried out at 30 degrees Celsius (86 degrees F) grows bacteria more slowly and produces a finer, more intricate network that better retains whey. Yogurt prepared at this lower temperature takes 18 hours to set.
Reduced fat milk yogurt is firmer than the regular kind due to the addition of milk proteins used to mask the lack of fat. The extra milk proteins add density to the coagulated protein network. Manufacturers also add gelatin, starch and other stabilizers.
Frozen yogurt is somewhat of a misnomer. Commercial frozen yogurt is made from iced milk with small dose of yogurt in it, usually in a ratio of 4:1.
Saturday, March 3, 2012
Health benefits of lactic acid bacteria in fermented milk
Lactic acid bacteria do more than just pre-digest lactose and produce yogurt. Back in the early 1900s, the Russian immunologist Ilya Metchnikov proposed that lactic acid bacteria in fermented milks help eliminate toxic microbes in our digestive system. To support Mr. Metchnikov’s prescient claim, research in recent decades suggests that Bifidobacteria, fostered in breast milk, colonizes the infant intestine and keep it healthy through acidification and production of antibacterial substances. Once the infant is weaned, the Bifidobacteria recede in favor of a mixed population of Streptococcus, Staphylococcus, E. coli, and yeasts.
Bacteria such as L. fermentum, L. casei, L. brevis, and L. acidophilus adhere to the human intestinal wall. Their presence shields it from other microbes by the secretion of antibacterial compounds, and by boosting the immune response to infection. Furthermore, research suggests that these bacteria also dismantle cholesterol and reduce the production of carcinogens.
The consumption of fermented milks for health purposes, however, is debatable. At the time of Mr. Metchnikov’s claim, milk fermentation involved a dozen or more microbes. Industrial versions nowadays usually limit it to two or three. This biological narrowing does not only affect flavor and consistency, but its health value as well. In addition, live cultures in industrial buttermilk and yogurt grow well in milk but cannot survive in the human body. Some manufacturers are adding "probiotic" Lactobacilli and Bifidobacteria to their cultured milk products in an attempt to mimic the original fermented milks. Such products are advertised for the health benefits afore noted.
Bacteria such as L. fermentum, L. casei, L. brevis, and L. acidophilus adhere to the human intestinal wall. Their presence shields it from other microbes by the secretion of antibacterial compounds, and by boosting the immune response to infection. Furthermore, research suggests that these bacteria also dismantle cholesterol and reduce the production of carcinogens.
The consumption of fermented milks for health purposes, however, is debatable. At the time of Mr. Metchnikov’s claim, milk fermentation involved a dozen or more microbes. Industrial versions nowadays usually limit it to two or three. This biological narrowing does not only affect flavor and consistency, but its health value as well. In addition, live cultures in industrial buttermilk and yogurt grow well in milk but cannot survive in the human body. Some manufacturers are adding "probiotic" Lactobacilli and Bifidobacteria to their cultured milk products in an attempt to mimic the original fermented milks. Such products are advertised for the health benefits afore noted.
Lactic acid bacteria and fermented milk
Lactose is almost uniquely found in milk. Few bacteria can break down lactose into usable forms of energy. Lactic acid bacteria, also known as Probiotic bacteria, use the enzyme lactase to break down the disaccharide lactose into the more usable monosaccharides glucose and galactose. The bacteria produce lactic acid as a by-product of lactose digestion. The low pH preserves the milk as other microbes cannot survive in acidic conditions. In addition, casein proteins come together in semi-solid curds. The flavor and texture favorably change in a process referred to as milk fermentation.
The history of milk fermentation dates back at least 2,000 years. In the early years, people believed that the fermentation was a spontaneous process. Later, the process was managed by the inoculation of fresh milk with fermented milk. By the late 19th century, bacteria had been identified as a causative agent, though the process was not entirely understood. By the 1900s, starter cultures of unknown mixed bacteria became commercially available. By the 1930s, pure single-strain cultures had evolved for the specific production of sour creams, yogurts, and cheese.
Fermenting lactic acid bacteria include species in the genera Lactobacillus, Lactococcus, Streptococcus, Leuconostoc, and Pediococcus. Some of these species also colonize the mouth, intestine, and vagina of mammals as normal flora.
The history of milk fermentation dates back at least 2,000 years. In the early years, people believed that the fermentation was a spontaneous process. Later, the process was managed by the inoculation of fresh milk with fermented milk. By the late 19th century, bacteria had been identified as a causative agent, though the process was not entirely understood. By the 1900s, starter cultures of unknown mixed bacteria became commercially available. By the 1930s, pure single-strain cultures had evolved for the specific production of sour creams, yogurts, and cheese.
Fermenting lactic acid bacteria include species in the genera Lactobacillus, Lactococcus, Streptococcus, Leuconostoc, and Pediococcus. Some of these species also colonize the mouth, intestine, and vagina of mammals as normal flora.
Thursday, February 9, 2012
Chocolate Production
There are three groups of cacao trees: the Criollos, Forasteros, and Trinitarios. The Criollos produce some of the finest flavors, but are disease prone and low-yielding trees. They account for less than 5% of the world crop. The Forasteros provide full-flavored beans and are high-yielding. They account for most of the world crop. Trinitarios are hybrids of the other two.
On plantations and farms, the cacao pods are opened and their contents exposed. The sweet pulp is fermented for 2-8 days. Fermentation of the pulp is a key step in making chocolate flavorful. Three phases occur during fermentation: First, yeasts convert sugars to alcohols and metabolize some of the acids in the pulp. Next, as the oxygen supply in the pods diminishes, lactic acid bacteria attack the pods. Some of these lactic acid bacteria are the same species found in fermented dairy. Last, acetic acid bacteria consume the alcohol produced by the yeast and convert it into acetic acid. The acetic acid then penetrates into the beans, making the cacao beans less astringent. Digestive enzymes within the beans break down proteins and sucrose, which will later produce more aromatic molecules during roasting. The beans soak in some of the flavor of the fermented pulp, which makes the beans more flavorful.
The beans are dried to about 7% moisture, at which point they are resistant to further microbial spoilage. The beans are cleaned and shipped to manufacturers.
The next step is roasting. Manufacturers roast beans to develop their flavor. The roasting needs of cacao beans are milder than those of coffee beans because cacao has an abundance of reactive amino acids that participate in Maillard browning to generate flavor. Therefore, roasting helps preserve the rich flavors within the beans acquired during fermentation.
The shells and nibs are separated after roasting. The nibs are ground into cocoa liquor. After that, the process varies according to the ultimate product desired. For cocoa powder, the cocoa liquor is pressed to remove the cocoa butter, then pulverized. For chocolate, other ingredients are added to the liquor (sugar, milk, vanilla, etc.) and then subjected to conching-a process of extended agitation and added heat. The physical friction breaks up particles of the other ingredients so that they coat the cocoa butter evenly. It also mellows the strong flavor of cocoa by means of aeration. Volatile compounds present in the cocoa evaporate, including acids and aldehydes. Favorable volatiles, such as pyrazines, furaneol, and maltol, become concentrated. These compounds make up much of the characteristic aromas in chocolate. At the end of conching, cocoa butter and lecithin are added to create the creamy texture of chocolate.
Following conching, the liquid chocolate needs to be tempered. Tempering is a process that involves heating and cooling the liquid chocolate to ensure that cocoa butter crystals stabilize and become uniform in size.
Lastly, chococolate is molded and cooled off.
On plantations and farms, the cacao pods are opened and their contents exposed. The sweet pulp is fermented for 2-8 days. Fermentation of the pulp is a key step in making chocolate flavorful. Three phases occur during fermentation: First, yeasts convert sugars to alcohols and metabolize some of the acids in the pulp. Next, as the oxygen supply in the pods diminishes, lactic acid bacteria attack the pods. Some of these lactic acid bacteria are the same species found in fermented dairy. Last, acetic acid bacteria consume the alcohol produced by the yeast and convert it into acetic acid. The acetic acid then penetrates into the beans, making the cacao beans less astringent. Digestive enzymes within the beans break down proteins and sucrose, which will later produce more aromatic molecules during roasting. The beans soak in some of the flavor of the fermented pulp, which makes the beans more flavorful.
The beans are dried to about 7% moisture, at which point they are resistant to further microbial spoilage. The beans are cleaned and shipped to manufacturers.
The next step is roasting. Manufacturers roast beans to develop their flavor. The roasting needs of cacao beans are milder than those of coffee beans because cacao has an abundance of reactive amino acids that participate in Maillard browning to generate flavor. Therefore, roasting helps preserve the rich flavors within the beans acquired during fermentation.
The shells and nibs are separated after roasting. The nibs are ground into cocoa liquor. After that, the process varies according to the ultimate product desired. For cocoa powder, the cocoa liquor is pressed to remove the cocoa butter, then pulverized. For chocolate, other ingredients are added to the liquor (sugar, milk, vanilla, etc.) and then subjected to conching-a process of extended agitation and added heat. The physical friction breaks up particles of the other ingredients so that they coat the cocoa butter evenly. It also mellows the strong flavor of cocoa by means of aeration. Volatile compounds present in the cocoa evaporate, including acids and aldehydes. Favorable volatiles, such as pyrazines, furaneol, and maltol, become concentrated. These compounds make up much of the characteristic aromas in chocolate. At the end of conching, cocoa butter and lecithin are added to create the creamy texture of chocolate.
Following conching, the liquid chocolate needs to be tempered. Tempering is a process that involves heating and cooling the liquid chocolate to ensure that cocoa butter crystals stabilize and become uniform in size.
Lastly, chococolate is molded and cooled off.
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