Post by Farmer Nestor on Mar 17, 2011 15:27:25 GMT -8
Bacteria
Bacteria are the most common, and probably the most numerous of microorganisms with which the dairy processing industry is concerned.
They belong to four main groups: (1) cocci, usually gram positive, (2) gram positive non-sporeforming rods, (3) gram positive sporeforming rods, and (4) gram negative non-spore forming rods.
Yeasts
The yeasts most frequently encountered in milk and milk products act upon the lactose to produce acid and carbon dioxide.
Yeasts are more commonly found in raw, cream during hot weather, but are potential contaminants throughout the year.
Moulds
Moulds often grow in large concentrations and are visible as a fuzzy or fluffy growth. They are sometimes observed on the surface, of butter, old cream, khoa, or cheese.
They are black, grey, green, blue or white. They discolour milk products and often produce undesirĀ¬able, at times repulsive, odours and flavours. Moulds are essential in production of the certain kinds of cheese.
Bacteriophages
Bacteriophages are particularly obnoxious in starter cultures used for making cultured milk, butter and cheese.
Phages will, kill the bacterial culture and the whole process of fermentation will be lost.
Microorganisms found in milk can also be described on the basis of the following characteristics;
1. Biochemical activities.
2. Temperature response.
3. Ability to cause infection and disease.
Biochemical activities
If allowed to stand under condition that permit bacterial growth, raw milk of a good sanitary quality will rapidly undergo a series of chemical changes. The principal change is lactose fermentation to lactic acid.
This change is brought about by acid uric lactic organisms, especially Strepotococcus lactis and certain lactobacilli. These include two distinct biochemical types, homo-and heterofermentative. In homofermentation lactic acid is the major product of lactose fermentation.
Heterofermentative organisms, however, produce lactic, acetic, propionic, and some other acids, and some alcohols and gases such as CO2 and H2 Organisms continue to form lactic acid until the concentration of acid is itself too great for the organisms to remain live.
Microbacteria, micrococci, coliforn18, etc. also ferment lactose to lactic acid and other products. Many Clostriifiul1J species and, some yeasts such as Torula lactic, and Torula cremoris ferment lactose with acid and gas production.
As the acidity continues to increase and reaches a pH of 4.7, it eventually causes a precipitation of casein. Organisms capable of metabolizing lactic and other acids develop especially acid uric, yeasts and moulds.
The acidity of milk is diminished and the alkaline products of protein decomposition such as amines, ammonia and the like are produced.
This is accomplished by many species of the genera Bacillus, Clostridium, Pseudomonas, Proteus and numerous other forms.
The action of microorganisms does not involve fat as readily as it does lactose and protein. Lipolysis results from the action of lipase produced by bacteria such as Pseudomonas, Achromobacter and by some yeasts and moulds.
Fat is hydrolysed to glycerol and fatty acids. Some of the fatty acids, for example, butyric and caproic acid give milk products, distinctive and usually rancid, odours and flavours.
Several microorganisms also bring about certain objection able changes in the milk which may not be deleterious to health. Rapines in milk is sometimes encountered. The milk become ropy or slimy and may be pulled out into long threads.
It is produced by several organisms but the most important species is Alcaligenes viscolactis. A rapid fermentation of lactose in milk is sometimes observed and is known as stormy fermentation. This is brought about by Clostridium perfringens.
The curd become torn to shreds by the vigorous fermentation and gas production. Several organisms have been isolated from milk which impart brilliant colours. Pseudomonas syncyanea imparts blue colour, pseudomonas synxantha yellow colour and Serratia marcescens red colour to the milk.
Temperature response
Microorganisms found in milk can also be described according their optimum temperature for growth and heat resistance. This is a very practical consideration since milk is preserved by employing low temperatures to prevent changes due to microbial activity and by high temperatures to reduce microbial population and destroy pathogens. All the four types of microorganisms i.e. psychrophilic, mesophilic, thermophilic and thermoduric are found in milk.
Psychrophiles grow at temperatures just above freezing and at refrigeration tempratures. They produce a wide variety of spoilage defects. The defects may result in the production of many "off" flavours and odours. The most commonly encountered psychrophilic bacteria are members of the genera pseudomonas,
Achromobacter, Vibrio, Flavobacterium and Alcaligenes, They arc killed in the pasteurization process, but are sometimes found in pasteurized milk. The contamination takes place after pasteurization from equipment, cans, bottles, and water.
The most important mesophilic bacteria are streptococci, lactobacilli and coliforms, which produce acid and gas and off flavours. They are killed in the pasteurization process
Thermophilic bacteria grow well at the temperature used in pasteurization, specially when the low temperature holding method is followed. Most thermophilic forms are found in two genera, Bacillus and Clostridium.
Thermoduric organisms are regarded as those which survive pasteurization but do not grow at pasteurization temperatures. The most common thermoduric bacteria are found in the genera Microbacterium, Corynebacterium, Micrococcus, Streptococcus and Bacillus.
Excessive numbers of thermoduric bacteria in milk make it difficult to meet the grading standard.
Ability to cause infection and disease
Pathogenic organisms of both bovine and human origin haveĀ¬ been isolated from milk. Milk, therefore, can serve as a carrier of diseases. Many serious epidemics were caused by the consumption of such products before this fact was clearly recognized.
However, this became less common as milk sanitation has improved and pasteurization is being more widely practised. The disease organisms present in milk may be derived from (1) diseased animals or (2) persons collecting and handling milk: Thus the danger is due to the inoculum and not to the growth of organisms in the milk.
The health of animal is an important factor. Several diseases of cattle including staphylococcal and streptococcal infections, tuberculosis, brucellosis, salmonellosis, Q fever and Foot and mouth disease may be transmitted to man.
The organisms causing these diseases may get into the milk either directly from the udder, or indirectly from infected body discharges, which may drop, splash, or be blown into the milk.
Some of the important diseases of human origin that have been transmitted by milk are (1) typhoid fever (2) diphtheria, (3) scarlet fever, (4) dysentery (5) septic sore throat and (6) poliomyelitis.
It is also possible for humans to infect animals. For example, mastitis may be caused by a variety of organisms, including Staphylococcus aureus. The infecting organism, in some cases, has been traced to humans.
Bacteria are the most common, and probably the most numerous of microorganisms with which the dairy processing industry is concerned.
They belong to four main groups: (1) cocci, usually gram positive, (2) gram positive non-sporeforming rods, (3) gram positive sporeforming rods, and (4) gram negative non-spore forming rods.
Yeasts
The yeasts most frequently encountered in milk and milk products act upon the lactose to produce acid and carbon dioxide.
Yeasts are more commonly found in raw, cream during hot weather, but are potential contaminants throughout the year.
Moulds
Moulds often grow in large concentrations and are visible as a fuzzy or fluffy growth. They are sometimes observed on the surface, of butter, old cream, khoa, or cheese.
They are black, grey, green, blue or white. They discolour milk products and often produce undesirĀ¬able, at times repulsive, odours and flavours. Moulds are essential in production of the certain kinds of cheese.
Bacteriophages
Bacteriophages are particularly obnoxious in starter cultures used for making cultured milk, butter and cheese.
Phages will, kill the bacterial culture and the whole process of fermentation will be lost.
Microorganisms found in milk can also be described on the basis of the following characteristics;
1. Biochemical activities.
2. Temperature response.
3. Ability to cause infection and disease.
Biochemical activities
If allowed to stand under condition that permit bacterial growth, raw milk of a good sanitary quality will rapidly undergo a series of chemical changes. The principal change is lactose fermentation to lactic acid.
This change is brought about by acid uric lactic organisms, especially Strepotococcus lactis and certain lactobacilli. These include two distinct biochemical types, homo-and heterofermentative. In homofermentation lactic acid is the major product of lactose fermentation.
Heterofermentative organisms, however, produce lactic, acetic, propionic, and some other acids, and some alcohols and gases such as CO2 and H2 Organisms continue to form lactic acid until the concentration of acid is itself too great for the organisms to remain live.
Microbacteria, micrococci, coliforn18, etc. also ferment lactose to lactic acid and other products. Many Clostriifiul1J species and, some yeasts such as Torula lactic, and Torula cremoris ferment lactose with acid and gas production.
As the acidity continues to increase and reaches a pH of 4.7, it eventually causes a precipitation of casein. Organisms capable of metabolizing lactic and other acids develop especially acid uric, yeasts and moulds.
The acidity of milk is diminished and the alkaline products of protein decomposition such as amines, ammonia and the like are produced.
This is accomplished by many species of the genera Bacillus, Clostridium, Pseudomonas, Proteus and numerous other forms.
The action of microorganisms does not involve fat as readily as it does lactose and protein. Lipolysis results from the action of lipase produced by bacteria such as Pseudomonas, Achromobacter and by some yeasts and moulds.
Fat is hydrolysed to glycerol and fatty acids. Some of the fatty acids, for example, butyric and caproic acid give milk products, distinctive and usually rancid, odours and flavours.
Several microorganisms also bring about certain objection able changes in the milk which may not be deleterious to health. Rapines in milk is sometimes encountered. The milk become ropy or slimy and may be pulled out into long threads.
It is produced by several organisms but the most important species is Alcaligenes viscolactis. A rapid fermentation of lactose in milk is sometimes observed and is known as stormy fermentation. This is brought about by Clostridium perfringens.
The curd become torn to shreds by the vigorous fermentation and gas production. Several organisms have been isolated from milk which impart brilliant colours. Pseudomonas syncyanea imparts blue colour, pseudomonas synxantha yellow colour and Serratia marcescens red colour to the milk.
Temperature response
Microorganisms found in milk can also be described according their optimum temperature for growth and heat resistance. This is a very practical consideration since milk is preserved by employing low temperatures to prevent changes due to microbial activity and by high temperatures to reduce microbial population and destroy pathogens. All the four types of microorganisms i.e. psychrophilic, mesophilic, thermophilic and thermoduric are found in milk.
Psychrophiles grow at temperatures just above freezing and at refrigeration tempratures. They produce a wide variety of spoilage defects. The defects may result in the production of many "off" flavours and odours. The most commonly encountered psychrophilic bacteria are members of the genera pseudomonas,
Achromobacter, Vibrio, Flavobacterium and Alcaligenes, They arc killed in the pasteurization process, but are sometimes found in pasteurized milk. The contamination takes place after pasteurization from equipment, cans, bottles, and water.
The most important mesophilic bacteria are streptococci, lactobacilli and coliforms, which produce acid and gas and off flavours. They are killed in the pasteurization process
Thermophilic bacteria grow well at the temperature used in pasteurization, specially when the low temperature holding method is followed. Most thermophilic forms are found in two genera, Bacillus and Clostridium.
Thermoduric organisms are regarded as those which survive pasteurization but do not grow at pasteurization temperatures. The most common thermoduric bacteria are found in the genera Microbacterium, Corynebacterium, Micrococcus, Streptococcus and Bacillus.
Excessive numbers of thermoduric bacteria in milk make it difficult to meet the grading standard.
Ability to cause infection and disease
Pathogenic organisms of both bovine and human origin haveĀ¬ been isolated from milk. Milk, therefore, can serve as a carrier of diseases. Many serious epidemics were caused by the consumption of such products before this fact was clearly recognized.
However, this became less common as milk sanitation has improved and pasteurization is being more widely practised. The disease organisms present in milk may be derived from (1) diseased animals or (2) persons collecting and handling milk: Thus the danger is due to the inoculum and not to the growth of organisms in the milk.
The health of animal is an important factor. Several diseases of cattle including staphylococcal and streptococcal infections, tuberculosis, brucellosis, salmonellosis, Q fever and Foot and mouth disease may be transmitted to man.
The organisms causing these diseases may get into the milk either directly from the udder, or indirectly from infected body discharges, which may drop, splash, or be blown into the milk.
Some of the important diseases of human origin that have been transmitted by milk are (1) typhoid fever (2) diphtheria, (3) scarlet fever, (4) dysentery (5) septic sore throat and (6) poliomyelitis.
It is also possible for humans to infect animals. For example, mastitis may be caused by a variety of organisms, including Staphylococcus aureus. The infecting organism, in some cases, has been traced to humans.