The artful transformation of milk into cheese occurs in three stages: In the first stage, lactic acid bacteria convert milk sugar into lactic acid. The second stage involves the addition of rennet and the subsequent curdling of the casein proteins and drainage of the watery whey from the concentrated curds. The third stage is ripening. Protein and fat-digesting enzymes present in the milk, from the bacteria and molds, and from the rennet work together to create the unique texture and flavor of the cheese.
Nearly all cheeses are curdled with a combination of starter bacteria acid and rennet. Acid and rennet give different curdle structures. Acid yields a fine, fragile gel; whereas rennet produces a course but robust, rubbery one. Fresh cheeses and small, surface-ripened goat cheeses begin with predominantly acid coagulation. Large semihard and hard cheeses curdle in rennet-dominant coagulation. Cheeses of moderate size and moisture have moderate content of both.
After curdling, the excess water is drained from the curds. For soft cheeses, whole curd is allowed to drain by gravity alone for many hours. The curd of future firmer cheeses is precut to increase surface area and is actively pressed to expel more moisture. Cut curd may also be cooked in its whey to 130° F/55° C to further expel whey and encourage flavor production by bacteria and enzymes. All cheese is later placed into molds and pressed to its final shape and moisture.
Salt is added to new cheese either by mixing it with the curds or applying dry salt or brine to the whole cheese. In addition to taste, salt inhibits the growth of spoilage microbes and acts as a regulator of cheese structure and the ripening process. Salt draws moisture out of the curds and firms the protein structure. Most cheeses contain 1.5 to 2% salt by weight.
Ripening, or affinage, refers to the process of bringing cheese to the point at which flavor and texture are at their best. Cheeses are said to be alive. They begin young and bland, mature into fullness of character, and eventually decay into harshness and coarseness. The length of vitality depends on the type of cheese. The cheesemaker manipulates the maturation process by controlling the temperature and humidity. Specialist cheese merchants in France are also affineurs; they buy freshly made cheese and carefully mature it in their own premises to sell at their best. Industrial producers ripen their cheeses only partly, then refrigerate them to suspend their development before shipping. This technique maximizes shelf life and stability though quality suffers greatly.
Showing posts with label lactic acid bacteria. Show all posts
Showing posts with label lactic acid bacteria. Show all posts
Sunday, May 27, 2012
The ingredients of cheese
Milk
Cheese is concentrated milk five- to tenfold by the removal of water. The basic character of the milk defines the basic character of the cheese. Milk from cows is more neutral than others. Sheep and buffalo milk have relatively high fat and protein contents therefore make richer cheeses. Goat's milk has less casein protein, so it produces a more crumbly curd.
The cow breed also produces distinct flavors. Today most dairy comes from black and white Holstein or Friesian cows. These cows have been bred to maximize milk yields on a standarized feed. Traditional breeds, such as the Brown Swiss and others, produce a lower volume of milk but one that is richer in protein, fat, and other favorable cheese constituents.
The animal's diet also affects flavor. Today most cows are fed on a standarized diet year round. The diet is composed of silage and hay from few fodder corps such as alfalfa or maize. This feed produces neutral milk that can be made into very good cheese. However, herds allowed to graze on pastures give milk with aromatic complexities that make extraordinary cheese. Pasture cheese has traces of local climate and seasonal flavors. It is also deeper yellow color due to greater carotenoid pigments in fresh vegetation. Beware of bright orange cheeses as these have been artificially dyed.
Flavor is also affected by whether the milk used is raw or pasteurized. Pasteurization, to eliminate disease and spoilage bacteria, has been a practical necessity in industrial cheese making, which requires milk to be pooled and stored. Since 1940s, the FDA requires that any cheese made from unpasteurized milk be aged at least 60 days at a temperature above 35 degrees F/2 degrees C. In the 1950s the US also banned imports of raw-milk cheeses aged less than 60 days. This essentially means that soft cheeses made with raw milk are contraband. The French, Swiss, and Italian regulations actually forbid the use of pasteurized milk for the traditional production of cheeses such as Brie, Camembert, Comté, Emmental, Gruyère, and Parmesan. Pasteurization kills useful milk bacteria, and inactivates enzymes in the milk that work on flavor development. However, pasteurization is no guarantee of safety as milk and cheese can be contaminated in later processing. Most outbreaks in recent years have involved pasteurized products.
Rennet
At least 2,500 years ago, shepherds began using pieces of the first stomach of a young calf, lamb, or goat to curdle milk for cheese. Later, people made a brine extract from the stomach. This was, conceivably, the first semipurified enzyme. Modern methods allow for the production of that same enzyme, chymosin, to be produced in a bacterium, a mold, and a yeast. Most cheese today uses this engineered "vegetable rennets." In Europe, rennet from a calf stomach is required for traditional cheese making.
Traditional rennet is made from the fourth stomach or abomasum of a milk-fed calf less than 30 days old, before chymosin is replaced by other protein-digesting enzymes. Chymosin is unlike other enzymes because it attacks only one milk protein at just one point. It targets the negatively charged kappa-casein that repels individual casein particles from each other. Thus, chymosin allows the casein particles to bond and form a continuous solid gel which is better known as the curd.
Acidity alone reduces the zeta potential and causes milk to curdle, so why rennet? Acid disperses casein proteins and their calcium glue before it allows the proteins to come together. Some casein and most of the calcium are lost in the whey. In addition, the acidity required to curdle milk is so high that some of the flavor-producing enzymes work very slowly or not at all. The curd produced is weak and brittle. By contrast, rennet leaves the casein micelle proteins intact and causes them to bond into a firm, elastic curd.
Microbes
A handful of modern cheese is made with purified cultures, but mostly it is made using a portion of the previous batch's starter.
Starter bacteria consists of lactic acid bacteria which initially acidify the milk, persist in the drained curd, and generate much of the flavor during the ripening process of semihard and hard cheeses such as Cheddar, Gouda, and Parmesan. The numbers of starter bacteria drop dramatically during cheesemaking, but their enzymes continue to work for months. There are two broad groups of starters: Lactococci (mesophilic) and Lactobacilli and Streptococci (thermophilic). Most cheeses are acidified by the mesophilic group, while the few that undergo a cooking step, such as mozzarella and the Italian hard cheeses, are acidified by the thermophilic group.
In addition, there are other microbes that give some cheeses their characteristic looks and flavors:
Cheese is concentrated milk five- to tenfold by the removal of water. The basic character of the milk defines the basic character of the cheese. Milk from cows is more neutral than others. Sheep and buffalo milk have relatively high fat and protein contents therefore make richer cheeses. Goat's milk has less casein protein, so it produces a more crumbly curd.
The cow breed also produces distinct flavors. Today most dairy comes from black and white Holstein or Friesian cows. These cows have been bred to maximize milk yields on a standarized feed. Traditional breeds, such as the Brown Swiss and others, produce a lower volume of milk but one that is richer in protein, fat, and other favorable cheese constituents.
The animal's diet also affects flavor. Today most cows are fed on a standarized diet year round. The diet is composed of silage and hay from few fodder corps such as alfalfa or maize. This feed produces neutral milk that can be made into very good cheese. However, herds allowed to graze on pastures give milk with aromatic complexities that make extraordinary cheese. Pasture cheese has traces of local climate and seasonal flavors. It is also deeper yellow color due to greater carotenoid pigments in fresh vegetation. Beware of bright orange cheeses as these have been artificially dyed.
Flavor is also affected by whether the milk used is raw or pasteurized. Pasteurization, to eliminate disease and spoilage bacteria, has been a practical necessity in industrial cheese making, which requires milk to be pooled and stored. Since 1940s, the FDA requires that any cheese made from unpasteurized milk be aged at least 60 days at a temperature above 35 degrees F/2 degrees C. In the 1950s the US also banned imports of raw-milk cheeses aged less than 60 days. This essentially means that soft cheeses made with raw milk are contraband. The French, Swiss, and Italian regulations actually forbid the use of pasteurized milk for the traditional production of cheeses such as Brie, Camembert, Comté, Emmental, Gruyère, and Parmesan. Pasteurization kills useful milk bacteria, and inactivates enzymes in the milk that work on flavor development. However, pasteurization is no guarantee of safety as milk and cheese can be contaminated in later processing. Most outbreaks in recent years have involved pasteurized products.
Rennet
At least 2,500 years ago, shepherds began using pieces of the first stomach of a young calf, lamb, or goat to curdle milk for cheese. Later, people made a brine extract from the stomach. This was, conceivably, the first semipurified enzyme. Modern methods allow for the production of that same enzyme, chymosin, to be produced in a bacterium, a mold, and a yeast. Most cheese today uses this engineered "vegetable rennets." In Europe, rennet from a calf stomach is required for traditional cheese making.
Traditional rennet is made from the fourth stomach or abomasum of a milk-fed calf less than 30 days old, before chymosin is replaced by other protein-digesting enzymes. Chymosin is unlike other enzymes because it attacks only one milk protein at just one point. It targets the negatively charged kappa-casein that repels individual casein particles from each other. Thus, chymosin allows the casein particles to bond and form a continuous solid gel which is better known as the curd.
Acidity alone reduces the zeta potential and causes milk to curdle, so why rennet? Acid disperses casein proteins and their calcium glue before it allows the proteins to come together. Some casein and most of the calcium are lost in the whey. In addition, the acidity required to curdle milk is so high that some of the flavor-producing enzymes work very slowly or not at all. The curd produced is weak and brittle. By contrast, rennet leaves the casein micelle proteins intact and causes them to bond into a firm, elastic curd.
Microbes
A handful of modern cheese is made with purified cultures, but mostly it is made using a portion of the previous batch's starter.
Starter bacteria consists of lactic acid bacteria which initially acidify the milk, persist in the drained curd, and generate much of the flavor during the ripening process of semihard and hard cheeses such as Cheddar, Gouda, and Parmesan. The numbers of starter bacteria drop dramatically during cheesemaking, but their enzymes continue to work for months. There are two broad groups of starters: Lactococci (mesophilic) and Lactobacilli and Streptococci (thermophilic). Most cheeses are acidified by the mesophilic group, while the few that undergo a cooking step, such as mozzarella and the Italian hard cheeses, are acidified by the thermophilic group.
In addition, there are other microbes that give some cheeses their characteristic looks and flavors:
- The Propionibacteria- Propionibacter shermanii is the hole maker, important in Swiss starters. It produces carbon dioxide gas that makes up the holes in cheese.
- The Smear Bacteria-Brevibacterium linens gives some strong cheeses their characteristic stink, such as Münster, Epoisses, Limburger.
- Molds, especially Penicillium, require oxygen to grow, can tolerate drier conditions than bacteria, and produce powerful protein- and fat-digesting enzymes that improve the texture and flavor of certain cheeses.
- Blue molds include Penicillium roqueforti, which gives Roquefort cheese its veins of blue; and P. glaucum, which colors the interior of Stilton and Gorgonzola.
- White molds include P. camemberti, which contributes to the creamy texture of Camembert, Brie, and Neufchâtel.
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.
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