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More on bacteria
10 Answers
Here are more questions on bacteria :
1. What are some roles played by E.coli in daily life?
( I know they digest, but is there anything else?)
2. What are the factors leading to the contamination of milk?
3. What reduces the contamination of milk?
Thanks again in advance!
1. What are some roles played by E.coli in daily life?
( I know they digest, but is there anything else?)
2. What are the factors leading to the contamination of milk?
3. What reduces the contamination of milk?
Thanks again in advance!
Answers
1 to 10 of 10
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I'll leave you to check out Ethel's link regarding your first question although you need to be careful with Wikipedia.
Regarding your second question:
The pH of milk (around 6.6), the temperature of the udder (around 38 degrees C) and the high nutritional value of milk are ideal for bacterial growth. However, as the udder is normally sterile, bacterial growth does not occur under normal conditions � it only arises with an infected udder.
During milking, milk can become contaminated with microorganisms, mainly from the milking equipment, and it will, if maintained at a temperature above 15 degrees C, coagulate due to the production of acid by such organisms as lactic acid bacteria and various coliforms. Therefore, great care must be taken to ensure that the milk is produced hygienically.
In general, improvements in the microbial quality of raw milk have been due to better hygiene during milking, improved design of milking equipment making it easier to clean, cooling of the milk below 5 degrees C within a few hours of production and holding the milk in cleanable stainless steel bulk-storage tanks until it is collected for transportation to the factory.
(continued)
Regarding your second question:
The pH of milk (around 6.6), the temperature of the udder (around 38 degrees C) and the high nutritional value of milk are ideal for bacterial growth. However, as the udder is normally sterile, bacterial growth does not occur under normal conditions � it only arises with an infected udder.
During milking, milk can become contaminated with microorganisms, mainly from the milking equipment, and it will, if maintained at a temperature above 15 degrees C, coagulate due to the production of acid by such organisms as lactic acid bacteria and various coliforms. Therefore, great care must be taken to ensure that the milk is produced hygienically.
In general, improvements in the microbial quality of raw milk have been due to better hygiene during milking, improved design of milking equipment making it easier to clean, cooling of the milk below 5 degrees C within a few hours of production and holding the milk in cleanable stainless steel bulk-storage tanks until it is collected for transportation to the factory.
(continued)
The sources of microorganisms in milk include the udder, the bedding on which the cow lies, food eaten by the cow, the human milker, the air, water used to wash the udder and the milking and storage equipment.
Cows suffering from diseases such as salmonella, tuberculosis and brucellosis may shed the bacteria that cause these diseases into their milk. However, such milk is not normally a major source of bacteria as far as raw milk is concerned. The bacteria responsible for Mastitis is a different story as it can occur in subclinical and clinical forms and a wide range of bacteria can be responsible for the disease. The bacteria invariably occur in vast quantities in the milk of an infected cow.
The major source of contamination of raw milk is improperly cleaned milking equipment and thorough cleaning of such equipment should always take place after each milking. Residues of milk left in the equipment may contain sufficient nutrients to sustain bacterial growth at ambient temperatures. I won�t bore you with technical details of the bacteria involved.
Hope this helps. Let me know if you want me to provide further information.
Cows suffering from diseases such as salmonella, tuberculosis and brucellosis may shed the bacteria that cause these diseases into their milk. However, such milk is not normally a major source of bacteria as far as raw milk is concerned. The bacteria responsible for Mastitis is a different story as it can occur in subclinical and clinical forms and a wide range of bacteria can be responsible for the disease. The bacteria invariably occur in vast quantities in the milk of an infected cow.
The major source of contamination of raw milk is improperly cleaned milking equipment and thorough cleaning of such equipment should always take place after each milking. Residues of milk left in the equipment may contain sufficient nutrients to sustain bacterial growth at ambient temperatures. I won�t bore you with technical details of the bacteria involved.
Hope this helps. Let me know if you want me to provide further information.
Regarding your third question:
The major factor that has reduced bacterial contamination of milk is Pasteurisation. It first took place nationwide around 1940 due to the prevalence of Mycobacterium tuberculosis in much milk production in the UK. The bacterium causes tuberculosis in a wide spectrum of mammals.
Batch Pasteurisation has been replaced by a method known as high temperature, short time or HTST where the milk is heated to 72 degrees C for around 15 seconds. Other temperatures and durations are sometimes used but the HTST method is now universal throughout the world. Over 99.9% of the raw bacteria found in milk are heat labile and are killed when subjected to 72 degrees C for 15 seconds. Spores of Clostridium and Bacillus are not affected by pasteurisation and organisms such as Micrococcus, Microbacterium and various Enterococcus species can withstand the treatment. The last three organisms generally originate from improperly cleaned equipment and are known as thermoduric bacteria. Thermoduric bacteria are fascinating as some, such as Streptococcus thermophilus can grow as a biofilm in the pasteuriser during long processing runs. The organism grows rapidly in milk and is removed and used as a starter culture in yogurt production.
(continued)
The major factor that has reduced bacterial contamination of milk is Pasteurisation. It first took place nationwide around 1940 due to the prevalence of Mycobacterium tuberculosis in much milk production in the UK. The bacterium causes tuberculosis in a wide spectrum of mammals.
Batch Pasteurisation has been replaced by a method known as high temperature, short time or HTST where the milk is heated to 72 degrees C for around 15 seconds. Other temperatures and durations are sometimes used but the HTST method is now universal throughout the world. Over 99.9% of the raw bacteria found in milk are heat labile and are killed when subjected to 72 degrees C for 15 seconds. Spores of Clostridium and Bacillus are not affected by pasteurisation and organisms such as Micrococcus, Microbacterium and various Enterococcus species can withstand the treatment. The last three organisms generally originate from improperly cleaned equipment and are known as thermoduric bacteria. Thermoduric bacteria are fascinating as some, such as Streptococcus thermophilus can grow as a biofilm in the pasteuriser during long processing runs. The organism grows rapidly in milk and is removed and used as a starter culture in yogurt production.
(continued)
Pasteurisation also inactivates several enzymes in milk, including lipase and alkaline phosphatase. The lack of this alkaline phosphatase is used as a test to confirm that the milk has been correctly pasteurised.
Cheese is sometimes manufactured in some countries by a method using milk heat-treated to a temperature lower than pasteurisation. This treatment is called Thermisation and generally involves heating the milk to 63 degrees C for 10-15 seconds. The enzymes in the milk are not inactivated during such treatment and greater cheese flavours are said to result. Thermisation is also used in Europe to reduce the number of organisms in raw milk as it is taken to the factory and therby to prolong the keeping qualities of the raw milk.. Nevertheless, the Thermised milk is always pasteurised before being used for cheesemaking.
Cheese is sometimes manufactured in some countries by a method using milk heat-treated to a temperature lower than pasteurisation. This treatment is called Thermisation and generally involves heating the milk to 63 degrees C for 10-15 seconds. The enzymes in the milk are not inactivated during such treatment and greater cheese flavours are said to result. Thermisation is also used in Europe to reduce the number of organisms in raw milk as it is taken to the factory and therby to prolong the keeping qualities of the raw milk.. Nevertheless, the Thermised milk is always pasteurised before being used for cheesemaking.
I can think of at least four alternatives to heat treatment for reducing the numbers of bacteria in milk.
1. Treatment with Hydrogen Peroxide. This is a very effective bactericidal agent, which is used in the USA and other countries. Adding an enzyme known as Catalase destroys the excess H2O2.
2. Lactoperoxidase-H2O2-Thiocyanate. Lactoperoxidase is an indigenous enzyme in milk and reduces H2O2 in the presence of a suitable reducing agent. One such reducing agent is the thiocyanate anion, which is oxidised to various species that are strongly bactericidal. Now milk contains a low concentration of indigenous thiocyanates that arises from catabolism of glucosinolates (from members of the Cruciferae family) by bacteria in the rumen of the cow. Milk contains no H2O2, which must be added or produced in situ via oxidation of glucose by glucose oxidase, from xanthine by xanthine oxidase or from a starter culture grown in the prescence of oxygen. The LPO system is very effective for the �cold pasteurisation� of milk. It has not been adopted worldwide nevertheless for various technical reasons and problems with patents and economics.
(continued)
1. Treatment with Hydrogen Peroxide. This is a very effective bactericidal agent, which is used in the USA and other countries. Adding an enzyme known as Catalase destroys the excess H2O2.
2. Lactoperoxidase-H2O2-Thiocyanate. Lactoperoxidase is an indigenous enzyme in milk and reduces H2O2 in the presence of a suitable reducing agent. One such reducing agent is the thiocyanate anion, which is oxidised to various species that are strongly bactericidal. Now milk contains a low concentration of indigenous thiocyanates that arises from catabolism of glucosinolates (from members of the Cruciferae family) by bacteria in the rumen of the cow. Milk contains no H2O2, which must be added or produced in situ via oxidation of glucose by glucose oxidase, from xanthine by xanthine oxidase or from a starter culture grown in the prescence of oxygen. The LPO system is very effective for the �cold pasteurisation� of milk. It has not been adopted worldwide nevertheless for various technical reasons and problems with patents and economics.
(continued)
3. Bactofugation. A high percentage (up to 99%) of somatic and bacterial cells and spores can be removed by centrifugation at high gravitational forces using a special centrifuge called a bactofuge. The cells and spores are denser than milk serum and are concentrated in the resulting sludge. The sludge is removed in milk sterilisation but when the serum is required for cheesemaking, the spores are sterilised and the bacteria are returned to the milk.
4. Microfiltration. This is a membrane seperation process, similar to reverse osmosis, nanofiltration and ultrafiltration, except that large pore-sized membranes are used. The semipermeable membranes used retain the bacteria but allow milk constituents, including most of the casein micelles to pass through in the permeate. The process can only be applied to skimmed milk, as the fat globules in whole milk block the pores of the membrane and reduce its efficiency. Normally what happens is that the cream is separated out, pasteurised and then added back to the filtered skimmed milk
Hope all this helps and apologies if I've got too technical! Let me know if there's anything else.
4. Microfiltration. This is a membrane seperation process, similar to reverse osmosis, nanofiltration and ultrafiltration, except that large pore-sized membranes are used. The semipermeable membranes used retain the bacteria but allow milk constituents, including most of the casein micelles to pass through in the permeate. The process can only be applied to skimmed milk, as the fat globules in whole milk block the pores of the membrane and reduce its efficiency. Normally what happens is that the cream is separated out, pasteurised and then added back to the filtered skimmed milk
Hope all this helps and apologies if I've got too technical! Let me know if there's anything else.
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