Dairy Microbiology
Milk and dairy products constitute an important item of our food. These products are very suitable for microbial growth. It thus becomes necessary to know the chemistry of milk, its spoilage, method of preservation, and
1 different dairy products where microbes play a positive rather than negative role.
Milk is considered as a complete food and it contains proteins, fat , carbohydrates, minerals, vitamins and water. It is also a good medium for the growth of microorganisms. It is therefore, important to know the types of microorganisms present in milk, their control and use for beneficial purposes.
Milk contains relatively few bacteria when it is secreted from the udder of an healthy animal. However, during milking operations it gets contaminated from the exterior of the upper and the adjacent areas, dairy untensils, milking machines, the hands of the milkers from the soil and dust.
In this way bacteria, yeasts and molds got into the milk and constitute the normal flora of milk. The number of contaminants added from various sources depends on the care taken to avoid contamination. The presence of these nonpathogenic organisms in milk is not serious but if these organisms multiply quickly, they can cause spoilage of milk, such as souring or putrefaction and develop undesirable odours. Control of their multiplication in milk is therefore, very essential. Milk may also contain pathogenic organisms, derived directly from the animal or from the surroundings. Microorganisms that are harmful and found in milk are Streptococcus cremoris, Pseudomonas sp., Mycobacterium spp. Serratia marcescens, enteric bacteria etc. Normally, milk is pasteurized before use.
However, pasteurization does not kill all the bacteria; the survivors (thermodurics), depending on their initial number.then multiply and if the initial number is high they cause rapid spoilage. It is imporant, therefore, that the milk be refrigerated at. around O°C soon after pasteurization to prevent the growth of these undersirable microorganisms.Pasteurization, either at 145°F for 30 minutes or 161°P for 15"'30 seconds eliminates most of the pathogenic bacteria particularly Mycobacterium tuberculosis.
Boiling of milk destroys all microorganisms except spore formers. Sometimes, on cooling or under improper refrigeration, spores germinate and cause spoilage of boiled milk.
Composition of Milk -
Milk is a complete food, with about pH 7.0,that is an aqueous solution of proteins, fats and carbohydrates with many minerals and vitamins. The following Table 4 shows an average composition of cow milk.
Component
Water
Casein
Percentage
87.0
2.5
Lactalbumin and other proteins
Lactose
0.5
Lipid
5.0
4.0
Sterols, Vitamins A, D, E 0.05
Miscellaneous 0.95
Spoilage of Milk and Milk Products -

Spoilage occurs when microorganisms degrade the carbohydrates, proteins, fats of milk and produce noxious, end products. Milk products as follows;
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Spoilage type
Souring
Oraganisms involved
Lactobacillus sp.
Streptococcus sp.
Sweet curdling Bacillus sp.
Proteus sp.
Micrococcus sp.
Gas production Clastoridium sp.
Ropiness
Red rot
Gray rot
Dairy mould
Signs of spoilage
Sour milk,Curd formation
Alkaline pH
Curd formation
Explosion of curds coliform bacteria
Alcaligenes sp.,Klebsiella sp.,Enterobacter sp. Stringy or slimy milk
Serratia marcescens clotridium sp.
Penicilium sp.,Geotrichum sp.
Red colaration
Gray colaration, Foul smell
Mouldy appearance
Sources of Microorganisms in Milk -
There are several principal sources of contamination of milk. From the time the milk leaves the udder, until it is dispensed into containers, everything with which it comes into contact is a potential source of more microorganisms. Milking performed under hygienic conditions, with strict attention to sanitary practices, will reduce the entry of microorganisms into the milk. Naturally the fewer the organisms that can get into the milk, the fewer have a chance to grow. In the following paragraphs the sources of microorganism in milk and the precautions to minimize their entry into the milk are discussed.
Sources of Microorganisms - The Producing Animal -
Unless the producing animal is clean, and her flanks, udder and teats given special sanitary care just before milking, her body can be a source of considerable contamination. The first few streams of milk from each teat should, be collected, separated and discarded. This flushes out the organisms that have entered the teat through the teat opening. Milk from a cow with an infected udder is likely to contain a large number of organisms. The probability of diseases of the udder contaminating the milk is very high. Mastitis, which is a disease causing inflammation of the udder, contributes considerable number of organisms, sometimes even blood cells, into the milk. If all the milk is not drawn out of the udder but remains between milkings, it may spoil within the udder itself and. thus increase the count considerably. Washing and massaging the cow's udder with a warm detergent sanitizer solution before milking serves to clean the area. The hair of all animals harbours organisms. The hair, dirt and dust often fall from the body into the milking pails or the teat cups of milking machines. The modern practice is to keep the flanks clipped to minimize contamination.
Sources of Microorganisms - Milking Area -
The microbial content of air is greatly affected by many conditions and practices. Dried dirt and filth is picked up by all movements and carried about as dust in the atmosphere. For this reason, dust may be the source of almost evey kind of contamination. The sprays which are sometimes used to cut down air contaniination are not very useful. However, the main point of keeping the conditions clean and sanitary, is not to raise dust.

Sources of Microorganisms - Utensils and Equipments -
Utensils and equipments are known to be the greatest sources of contamination. They may account for as much as
.100, 000 to a billion organisms per milliliter. Pails, strainers, milking machines, cans, pipes, bottles, and other
3 equipment used for the handling of milk are sometimes not properly washed and sanitized. Organisms survive in the cracks, corners, crevices, dents, scratches, and other irregularities of the utensils. Such neglect affords ideal conditions for the growth of microorganisms before the utensils are used again. Suitable washing procedures are facilitated by using warm water, a brush, and a detergent satisfactory to the hardness of the local water used for cleaning. Subsequent sanitizing treatments may utilize hot air, hot water, steam, chlorine or quaternary ammonium compounds.
Sources of Microorganisms -
The Personnel - All persons involved in the milking process must be in good health and must be careful in their personal cleanliness.They should wash their hands, clean and rinse them with an effective bactericidal solution, and dry them with a clean towel before starting, and frequently during work, and immediately following every rest stop.They should keep finger nails free of dirt. Each person must always carry a clean handkerchief and use it to prevent the spraying of nasal and oral discharges into the atmosphere, equipment or products. They should, wear a neat and clean uniform. A surgical mask is an effective addition to the uniform. Although workers may not contribute a large number of organisms, these are of considerable importance since they may well be human pathogens.
Probably the majority of milk borne epidemics of disease were started by workers who were carriers, or who had mild cases, or who were in close contact with others so affected. The importance of contamination contributed by the workers is simply that it is the most dangerous.
Sources of Microorganisms Water
Water is continuously required for cleaning and processing operations in a dairy. Special care or treatment must be taken to, supply good quality water. Water quality will vary with the source of supply.Water from surface supplies is contaminated by dust, animals, plants, people, and other agents. The microbiological quality of water is tested before it is used in a dairy plant. The most common test for water destined for domestic or industrial use, is the presumptive coliform test. This test reveals faecal or sewage contamination. Chlorination of water is commonly practiced to assure potability.
Types of Microorganisms in Milk -
The types of micro organisms found in milk vary considerably, and are dependent upon the specific conditions associated with a batch of milk.
Bacteria, yeasts, moulds, and bacteriophages are commonly encountered. Other viruses and Protozoa are seldom observed in milk products, except as occasional contaminants.
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 undesirable, at times repulsive, odours and flavours. Moulds are essential in production of the certain kinds of cheese.
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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 Clostridium 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 activi1y'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 bf 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
5 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 have 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 s9me cases, has been traced to humans.
Milk Spoilage -
Spoilage occurs when microorganisms degrade the carbohydrates, proteins, fats of milk and produce noxious, end products. It may be seen that Lactobacillus or Streptococcus species ferment the lactose to lactic acid and acetic acids turning the mi1k sour. They may produce enough acid to curdle the protein and form sour curd. Attack of milk protein by Micrococcus, Bacillus or Proteus results into sweet curdling.There is little acid formation. If milk becomes contaminated with Gram-negative rods-of coliform group of bacteria, such as E. coli or Enterobacter aerogenes, or Clostridium sp., there is .acid and gas formation from the lactose.This stormy fermentation causes the explosion of curds.
Ropiness, like bread develops from Alcaligenes, Klebsiella and Enterobacter.
Serratia marcescens causes the development of a red pigment.
Milk Borne Diseases -
The important diseases are, tuberculosis, brucellosis and Q fever. Tuberculosis bacterium.
Mycobacterium bovis is consumed in the milk and passes from human intestine to the blood, from which it spreads to most organs.
Brucellosis, a blood-disease is caused by Gram-negative rod, Brucella abortus. When transmitted to man through cow milk, the bacterium infect the blood
-rich organs.
Q fever, caused by rickettsia, Cxiella burnetii is also a milk borne disease.
Other important disorders associated with milk are primary atypical pneumonia, toxoplasmosis, anthrax, streptococcal infections etc.

Pasterurization -
The process was developed by Louis Pasteur in the 1860s to eliminate bacteria in wines.
The process for milk was adopted in 1895.
Primary object of this process is to eliminate disease-causing bacteria from milk, though the total number of bacteria is also very much reduced during this process.
It reduces the chances of milk-spoilage.
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Temperature Response of Milk Microorganisms-
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.
Holding Method Pasteurization -
This is an old process, in which milk is heated in large tank at 62.9°C for 30 minutes. This method is also known as the LTLT method (low temperature, long time). To ensure uniform heating the milk is constantly stirred during the process.
Flash Method Pasteurization -
This is modem method and also known as HTST (high temperature, short time) method. Raw milk is first warmed using the heat of the previously pasteurised milk. It then passes through a hot cylinder at 71.6°C for a period of 15 to 17 seconds.The milk is then cooled rapidly in part by transferring its heat to the incoming milk.
Ultrapasteurization -
It is also used in some dairy plants. Here the milk and milk products are subjected to heat at 82°C for three seconds only. Pasteurisation destroys the pathogenic, bacteria that may be transmitted by milk.
The pasteurised milk is graded into different grades on the basis of total bacterial counts. In different grades of milk, standards in respect of bacterial content per ml of milk vary from localities to localities. Some bacteria are able to survive pasteurisation, causing milk spoilage. Streptococcus lactis, Lactobacil1s casei and L. cremoris, produce acids, causing the protein to turn into curd.
These bacteria which are able to endure the heat of pasteurisation are called thermoduric. Pasteurisation has no effect on thermophilic bacteria which grow well at 60°C to 70°C.
Sterilization of Milk -
Boiling of milk is still widely used in our country. It kills all the vegetative cells of microorganisms. It does not require refrigerati0n and has a relatively longer shelf life.
Commercial milk sterilization techniques are in the process of being developed. Ultra pasteurization is a more recently perfected process in which milk is heated at 300F for 1 to 2 seconds. In addition the processing includes steps that eliminate any traces of cooked flavour.
The final product is comparable in flavour and nutritional quality to pasteurized milk.
Laboratory Tests of Dairy Products -

There are some tests which ensure the high standards of public health applied to dairy industry. Some of the most commonly used tests are as follows:
7
1.Phosphatase test.
2.standard plate count.
3.Reduction test.
4.Breed counting method.
5.Antibiotic detection.
PhosphateTest -
An enzyme, phosphatase is normally found in milk. This enzyme in respect of heat tolerance is similar to the rickettsia, Cxiella burnetii (Q. fever organism) or Mycobacterium bovis (tuberculosis bacterium).
It thus may be tested whether this enzyme is eliminated during pasteurization, so that it could be assumed that these two microbes are also not present in the milk.Milk sample is combined with disodium phenyl-phosphate.
After few minutes of incubation, a colour reagent (CQC- chloroquinone chloroimide) is added. If phosphatase is present the contents will become blue in colour.
If there is no colour change then it may be assumed that there is no phosphatase present.
Standard Plate Count-SPC -
This determines the total number of bacteria in a specified amount of milk, generally a milliliter (ml). This is used for grading of milk. Under aseptic conditions, one ml of milk is added to 99 ml of distilled water or buffer.
One ml and one-tenth ml samples are then transferred to sterile Petri dishes.. These give 1: 100 and 1: 1000 dilutions of the milk respectively. Other dilutions may also be prepared successively.A growth medium like plate count agar or tryptone glucose yeast extract agar is then added, and the milk samples are mixed with the medium.
The dishes are incubated at 37°C for 24-48 hours.
The plates are then placed on a counting device like a Quebec colony counter and the number of bacterial colonies is recorded. The colony count falling between 30 and 300 is selected and multiplied by the reciprocal of the dilution factor to obtain the bacterial count per ml of milk. If 248 colonies appeared on the 1 : 100 plate and 16 colonies on ,the 1:1000 plate, 248 would be selected and multiplied by 100, giving 24,800 total bacteria per ml of milk sample. The 30 to 300 range is used because under 30 the chance of sampling error may be significant, and over 300, due to overcrowding on the plate, there may be formed a single colony by more than one bacteria.
Reduction Test -
This is a rapid test to find relative amount of bacteria in a milk sample. The principle used is that the length of time for a colour change in a specific dye is proportional to the number of bacteria in the sample. A sample of milk is mixed with the dye solution.(methylene blue or resazurin) and this is incubated.Due to growth of bacteria and subsequent fermentation of lactose of milk, the electrons released are passed along to the dye molecules, reducing them. The enzyme involved is a reductase.
On reduction, methylene blue loses its colour and resazurin changes from normal slate blue to pink or colorless form. Good quality milk normally shows minimal colour change even after five to six hours, whereas contaminated milk shows color change rapidly within two hours or so.

Breed Counting Method -
This method does not distinguish between live and dead cells. It is a direct count method in a known area of microscopic field.The sample is spread (about 0.0 I ml) over I cm2 of a microscope slide. Milk smear is dried and
8 stained with Newman - Lampert stain.This stain fixes the smear, dissolves fat globules and stains bacteria with methylene blue. Slide is then scanned under several oil immersion microscopic fields. Then, calculation, are made.
Antibiotic Detection Test-
The test is performed to identify the presence of any antibiotic in milk sample. It is likely that if cow is treated with antibiotics against some diseases as tuberculosis, Q fever, brucellosis etc., the antibiotics may persist in the milk.A paper disc is dipped into the milk and then applied to an agar plate inoculated with an organism like
Bacillus subtilis. The presence of an antibiotic in milk will be indicated by the development of a zone of inhibition around the disc.
Grades of Milk -
There are certain standards by which milk is judged. The grading of milk has become an important factor in maintaining the health of the population. Grades of milk are based upon regulations pertaining to production, processing, and distribution, e. g., sanitation, pasteurization, and holding conditions, as well as upon bacterial standards. The number of organisms permissible in different grades of milk vary some what, depending upon standards set up by local public health authorities.However, the recommendations contained in the U. S. Public Health
Service publication "Milk Ordinance and Code".
Chemical and Other Standards for Grade A Milk and Other Milk Products -
Product
Grade A raw milk for pasteurization
Grad A Pasturized milk products
Grade A Pasteurized cultured products chemical and other
Antibiotics:less than 0.05 unit/ml by Bacillus subtilis method or equivalent
Phosphatasc:less than 1 mg/ml by
Scharer rapid method
Phosphatasc:less than 1 mg/ml by
Scharer rapid method
Bacterial Limit Standards for Grade A Milk and Other Milk Products -
Product
Grade A raw milk for pasteurization
Grad A Pasturized milk products
Grade A Pasteurized cultured products
Bacterial limits
Individual producer milk not to exceed 100,000/ml prior to comming line with other producer milk
Milk and milk products:20,000/ml Coliform
Limit:not exceeding 10/ml.
Exempt.

Product
Cheese
Crème fraîche
Cultured sour cream
Filmjölk
Yogurt
Alternative names creme fraiche sour cream fil
[4] yoghurt, yogourt, yoghourt
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Typical milkfat content
1-75%
30-40%
Fermented Milk Products
Typical shelf life at 4°C
Fermentation agent varies
10 days a variety of bacteria and/or mold naturally occurring lactic acid bacteria in cream
14–18% 4 weeks
Lactococcus lactis subsp. lactis
Description
Any number of solid fermented milk products.
Mesophilic fermented cream, originally from France; higher-fat variant of sour cream.
Mesophilic fermented pasteurized cream with an acidity of at least 0.5%.
Rennet extract may be added to make a thicker product. Lower fat variant of crème fraîche.
0.1-4.5%
0.5–4%
10–14 days
35–40 days
Lactococcus lactis and Leuconostoc
Lactobacillus bulgaricus and
Streptococcus thermophilus
Mesophilic fermented milk, originally from Scandinavia.
Thermophilic fermented milk, cultured with Lactobacillus bulgaricus
Streptococcus thermophilus .
and
Kefir kephir, kewra, talai, mudu kekiya, milkkefir, búlgaros
0-4%
10–14 days
Kefir grains, a mixture of bacteria and yeasts
A fermented beverage, originally from the Caucasus region, made with kefir grains. Can be made with any sugary liquid, such as milk from mammals, soy milk, or fruit juices.
Kumis
Viili
Cultured buttermilk koumiss, kumiss, kymys, kymyz, airag, chigee
4%? filbunke
0.1-
3.5%
1–2%
10–14 days
14 days
10 days
Lactobacilli and yeasts
A carbonated fermented milk beverage traditionally made from horse milk.
Lactococcus lactis subsp.
Mesophilic fermented milk that cremoris , Lactococcus lactis may or may not biovar. diacetylactis ,
Leuconostoc mesenteroides contain fungus on the surface. subsp. cremoris and
Geotrichum candidum
Originally from
Sweden but today is a Finnish specialty.
Lactococcus lactis *
( Lactococcus lactis subsp. lactis *, Lactococcus lactis subsp. cremoris ,
Mesophilic fermented pasteurized milk.

Acidophilus milk acidophilus cultured milk
0.5-2% 2 weeks
Lactococcus lactis biovar. diacetylactis and
Leuconostoc mesenteroides subsp. cremoris )
Lactobacillus acidophilus
Thermophilic fermented milk, often lowfat (2%,
1.5%) or nonfat
(0.5%), cultured with
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Yogurt has been aggressively marketed as a health food. It's been called the "perfect food" and "insurance for good health." The U.S. Department of Agriculture in its yearbook for 1965 makes this unqualified statement: "Yogurt has no food or health values other than those present in the kind of milk from which it is made."
Yogurt has also been advertised as the perfect diet food. Even on this point, yogurt fails. It is high in saturated animal fats, and although plain yogurt has 154 calories per cup, over 80% of all yogurt eaten is the sweetened fruit-flavored variety which has 275 calories a cup.
Research in the last ten years has pointed out another danger of yogurt: cataracts. A cataract is the cloudiness of the lens of the eye. In severe cases, it causes blindness.
In animal experiments, all animals that were fed yogurt exclusively for several months developed cataracts in both eyes. In parts of India where yogurt is a large proportion of the diet, the incidence of cataracts is very high. A coincidence? Doubtful.
Researchers finally decided that some individuals may develop cataracts if they eat foods containing high levels of galactose (a sugar less soluble and sweet than glucose). Yogurt is one of the highest foods in galactose. Most commercial yogurts are 22% to 24% galactose.
People that usually do not eat dairy products sometimes feel obligated to sneak some yogurt into their diet for
"health" reasons. There is nothing magical or healthy about yogurt. Like all milk products, it should not be used in the diet.
Cheese is a very popular fermented food. The harmful effects of this food have already been discussed in an earlier lesson. You may want to consider this fact: most commercial cheeses have their fermentation process started by

the addition of rennet to the milk. Rennet contains the enzyme rennin which is found naturally in the stomach of a cow.
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To get rennet to ferment the cheese, the stomachs of cows are scraped. These stomach extracts are then added to the milk for curdling the cheese. So, can you be a "vegetarian" and still eat cheese which is made with stomach scrapings of cows? Probably not.
Cheese is a food that is always rotting. Leave a piece at room temperature and you'll have blue, green, white and yellow mold growing all over it. Some people even like to eat this mold, but then some people will eat anything.
You don't need "moldy milk" or cheese in your diet.
There are other fermented dairy foods besides cheese and yogurt. Buttermilk and kefir are two popular fermented milk drinks. Sour cream is exactly that: cream that has soured and gone bad.
Be aware that not only are these foods substandard because they are dairy products, but they are often adulterated before being sold. Buttermilk frequently has salt added to it; kefir is usually sweetened, and sour cream will have preservatives to keep it from becoming totally putrid.