Achieving milking hygiene

SESSION 2 E. Management of milking hygiene

Abstract

The establishment of hygienic procedures during milking has the scope of reducing exchange of pathogens generating mastitis through milking machines and other sources and of reducing the load of occasional contaminating microflora in milk after collection. The aim of this contribution is to provide a very general summary of the consequences of milk contamination on its composition and technological properties.

Pathogens and other environmental bacteria may cause infections to the udder and increase somatic cells. Factors determining infections, increase of somatic cells, changes in milk composition, and reduction in milk production are discussed.

High bacterial counts of microorganisms contaminating milk after collection may generate lipolysis, proteolysis, and other modifications leading to difficulties in coagulation and/or off-flavours which make the milk not suitable for processing.

Economical consequences for the farmer, due to milk reduction and costs for the management of animals affected by mastitis, are discussed.

INTRODUCTION

The establishment of hygienic procedures during and postmilking has the purpose of reducing occasional contaminating microflora in milk and reducing the load of microorganisms generating mastitis in order to increase the safety of dairy products, reduce the severity of milk sanitation, increase animal welfare, increase animal health in general, and protect farmers and processors’ profits.

Other more specific contributions in this symposium will deal with various sources of contamination and their control through appropriate codes of practice. This contribution does not intend to review the enormous amount of literature produced on mastitis and occasional post-milking contamination. The aim of this contribution is to provide, with several approximations, a very general summary of the consequences of milk contamination on its composition and its technological properties, by collecting part of the literature dealing with these items sparsely as an address to the importance of achieving milking hygiene and of adopting specific recommendations relating to milking hygiene.

CONTAMINATION DURING MILKING

Milk is virtually sterile when secreted into the alveoli of the udder [1]. Beyond this stage of milk production, microbial contamination can generally occur from three main sources [2]; from within the udder, from the exterior of the udder and from the surface of milk handling and storage equipment. The health and hygiene of the cow (1), the environment in which the cow is housed and milked (2), the procedures used in cleaning and sanitizing the milking and storage equipment (3), and the temperature and length of time of storage (4) are all key factors in influencing the level of microbial contamination of raw milk.

1. CONTAMINATION FROM THE INTERIOR OF THE UDDER

Microbial contamination from within the udder of healthy animals is not considered to contribute significantly to the total numbers of microorganisms in the bulk milk, while a cow with mastitis has the potential to shed large numbers of microorganisms into the milk supply. The influence of mastitis on the total bacteria count of bulk milk depends on the strain of infecting microorganism(s), the stage of infection and the percentage of the herd infected. Infected cows have the potential to shed in excess of 107 bacteria per ml. If the milk from one cow with 107 bacteria per ml comprises 1% of the bulk tank milk, the total bulk tank count, disregarding other sources, would be 105 per ml [2].

It is well known that traumas, infections, chemical and environmental factors, machine milking, feeding, water, and farm management can influence the insurgence of mastitis, the preservation of the teat end integrity and the risk of bacteria penetration into the teat canal. If these bacteria proliferate in the canal and in the mammary gland generate an inflammation process. The physiological reaction in the mammary gland leads to an increased production of somatic cells with the primary function of eliminating the infection and of repairing damaged tissues. Somatic cells are secreted during milking and somatic cells counts (SCC) are assumed as an indicator of the inflammation of the mammary gland. Extensive reports on mastitis insurgence, control and consequences can be found in publications of FAO and IDF [3, 4, 5].

Cows are very resistant to intramammary infections since the teat duct presents a particular barrier to the penetration of pathogens. Dodd [6] reports that “in herds where no hygiene is practiced the teat ends will be constantly contaminated with mastitis pathogens yet the probability of an uninfected quarter becoming infected in a single milking interval is of the order of 0.003”. If 10 CFU of a pathogen pass through the duct, then the probability of infection jumps up to 0.5. Dodd insists on the relationship between the frequency of infection and level or dose of exposure to pathogens. Not all the mastitis pathogens behave in the same way. Staphylococci and streptococci do not normally persist on healthy teat skin and their presence indicates recent contamination. Therefore, their presence in uninfected quarters should be small if the ducts are not extensively colonised. The presence in milk of the most frequently agents of mastitis (Staphylococcus aureus, Streptoccus agalactiae and Streptoccus dysgalactiae) is the result of significant extent of colonisation of teat skin or ducts rather than contamination from infected quarters transmitted during milking. However, when a quarter clinically affected is milked it will excrete milk containing a very large amount of pathogens which will contaminate either the other unaffected quarters of the same cow or the udder of the next milked animal.

One of the causes generating herd mastitis is the action of the milking machine. More than the transfer of microorganisms from affected cows to unaffected ones, the machine can exert a dangerous action on the teat skin and the teat ducts, which has been said are the most frequently colonised organs. The mechanical action of the teatcups can generate structural damage to teat skin causing lesions or fissures where microorganisms easily colonise. The vacuum applied to the udder can also generate haemorrhages near the teat end and the external orifice of the teat duct. An extensive review of machine milking factors affecting mastitis is reported in Bulletin of IDF 215 [7] and in Bulletin of IDF 381/2003 [5].

Microorganisms most commonly found to generate mastitis are ascribed to S. aureus and S. agalactiae but several other microorganisms are present. It is interesting to note that the species usually found are basically common in very different areas of the world but the incidence of each one varies for several reasons and may change during the years. An example of various microorganisms in infected animals in different areas of the world is reported in Table 1.

TABLE 1
Microorganisms most frequently found in milk from cows affected by mastitis in different areas of the world.

Country	Microorganism	Incidence %	References
Uruguay	Staphylococcus aureus	62.80	[8]
	Streptococcus agalactiae	11.30
	Enterococcus sp	8.00
	Coagulase-negative staphylococci	7.40
	Streptococcus uberis	6.40
	Streptococcus dysgalactiae	1.80
	Escherichia coli	1.50
Brasil	Corynebacterium sp.	44.25	[9]
	Staphylococcus sp.	12.12
	Streptococcus agalactiae	2.13
	Streptococcus dysgalactiae	1.86
	Streptococcus uberis	0.60
	Yeasts	0.19
Ethiopia	Staphylococcus aureus	39.20	[10]
	Streptococcus agalactiae	23.60
	Coliforms	14.10
	Micrococcus sp.	8.00
	Bacillus sp.	8.00
	Corynebacterium	7.00
Turkey	Staphylococcus aureus	43.75	[11]
	Actinomyces pyogenes	19.79
	Streptococcus agalactiae	9.37
	Streptococcus epidermidis	8.33
	Escherichia coli	7.29
	Streptococcus dysgalactiae	4.16
	Bacillus subtilis	3.15
	Bacillus cereus	2.08
	Candida spp.	2.08

Somatic Cell Count and modifications in composition and technological properties

Large debates and experiences have been carried out to assess a threshold of SCC above which a cow can be considered affected by mastitis. A comprehensive summary of this item and the suggestions of the Group of Experts on Mastitis of the International Dairy Federation can be found in Mastitis Newsletter 21 [4].

According to IDF, below a physiological limit of 250 000/ ml SCC milk is considered normal, above this limit there is an indication of a principle of inflammation. An increasing number of SCC above the threshold indicates the severity of the inflammation, which becomes sub-clinical for small deviations or clinical for large increases. Due to the relationship between SCC and the health of the animal, a system of milk payment as a function of the number of SCC is largely diffused, providing incentives for low counts. The European Union has set the legislative limit of ≤ 400 000 SCC/ml as the value above which milk cannot be sold by farmers or used for further processing [12]. This value changes dramatically internationally ranging between 250 000 and 750 000. Scientifically speaking, Hamann [13] clearly reports that above 100 000 SCC, milk constituents “abandon their physiological ranges”.

The importance of studying and discussing the source of infections generating mastitis and of assessing the rate of inflammation is due to the decrease in milk production and in the modifications in milk composition in relation to increasing severity of the disease. There is general consensus in considering still valid the results of research carried out in the early ‘80s and early ‘90s on the reduction in milk production due to mastitis and which will be discussed later. Of no minor importance are the modifications in milk composition and quality.

Lactose content in pathologic milk is decreased for its decreased synthesis in the inflammed udder. The reduction ranges between 1% and up to 16% for severe infections [14].

Lipid content is decreased between 2 and 10% [14], but the major effect is the increase in free fatty acids (FFA) from an average of 0.7 up to 3.9 meq/100 g of fat [15]. This is the combined result of two events: change in milk lipase activity and direct transport of long chain fatty acids (C16- C18) from blood to milk. The lipoprotein lipase, activated by serum lipoproteins, is the main responsible for milkfat lipolysis [16]. The major consequence of the increase in FFA could be the development of off-flavour in milk and increased susceptibility to rancidity of milkfat.

Total protein content is not markedly affected, but severe modifications intervene in the ratio of the various constituents [17, 18]. Beside a reduction in total casein, as-casein plus b-casein show a decrease of about 20% [19]. Major modifications occur in the content of bovine serum albumin (BSA), immunoglobulin (Ig), and lactoferrin (Lf) which in total increase by 3 times. The decrease in the proportion of the two mentioned casein fractions is due to an increase in the proteolytic activity of plasmin which reduces the amount
of b-casein altering the ratio between the two fractions [17-19]. In addition to the modifications induced by plasmin there is also the activity of other proteolytic enzymes arising from the somatic cells [19]. The consequences of these modifications are high production of proteose-peptones and protein breakdown which might lead to bitter flavour in milk, modification in rennetability, and reduced cheese yield. It should be also noted that the recovery of normal protein pattern when SCC are reduced to the normal level is not complete and part of the alteration still remain [19].

An alteration occurs in some anions and cations. Sodium and chloride increase with the increasing number of SCC, while potassium decreases. Decreases occur in total calcium and total magnesium, leading to difficulties in forming a suitable coagulum during cheesemaking [14]. A good summary of these modifications is reported by Hamann [13]. Under the technological point of view these modifications induce several negative consequences, which can be summarised for different products as in Table 2

TABLE 2
Consequences of processing milk from cows affected by mastitis in different products.

Product	Defects	References
Milk for consumption	Bitter flavour due to the action of proteases	[19]
	Gelation in UHT milk	[19]
	Rancid off-flavour induced by lipases	[16]
	Flocculation for altered salts equilibria	[13]
Fermented milk and butter	Bitter flavour Rancid off-flavour Rancidity susceptibility Colour modifications Fragility of the membrane of milkfat globules Difficulties in acid production by lactic cultures	[20, 21]
Cheese	Difficulties in natural fat separation in semi-skimmed hard cheeses	[22]
	Increased rennet clotting time	[22, 23]
	Reduction of coagulum strength	[23- 25]
	Loss of constituents in whey	[23- 25]
	Reduction in cheese yield and quality	[22-25]

As described, alteration of milk composition and of milk technological properties might be very severe and lead to extensive loss of profit. To reduce to the minimum the incidence of mastitis some precautions can be adopted: 1- keep the teat skin healthy and elastic. This can be achieved by housing cows under hygienic conditions and by controlling the performances of milking machines. 2- improvement of herd management which can reduce the proliferation of pathogens living in or near the teat duct. This will include avoiding teat lesion of any type, renewing bedding materials frequently, avoiding sawdust where microorganisms proliferate, using decontaminated water to wash udders. 3- post-milking teat disinfection. This would reduce the risk that the orifice stressed by the milking process and enlarged for milk suckling will be penetrated by bacteria. The adoption of teat dips disinfectant greatly reduces the infection by S. aureus, S. agalactiae and S. dysgalactiae, principal agents of mastitis. 4- wash udders carefully and flush cluster of milking machine after each milking. 5- application of the codes of practice in the different areas of farm management and herd management.

2. MICROBIAL CONTAMINATION FROM THE EXTERIOR OF THE UDDER

The exterior of the cow’s udder and teats can contribute to microorganisms that are naturally associated with the skin of the animal as well as microorganisms that are derived from the environment in which the cow is housed and milked. In general, natural inhabitants of the skin have not an important influence on the total bulk milk count and they do not grow competitively in milk. More important are microorganisms from teats soiled with manure, mud, feeds or bedding. Teats and udders of cows inevitably become soiled while they are laying in stalls or when allowed in muddy barnyards [26].

If this dirt is not removed beforehand, the microorganisms associated with it are washed into the milk during milking. The influence of dirty cows on total bacteria counts depends on the extent of soiling of the teat surface and the wash procedures used immediately before milking. Milking heavily soiled cows could potentially result in bulk milk counts exceeding 104 per ml. Several studies have investigated pre-milking udder hygiene techniques in relation to the bacteria count of milk [27, 28]. Cleaning of the teat with a sanitizing solution followed by drying with a clean towel is effective in reducing the numbers of microorganisms in milk contributed from soiled teats. However, under clean conditions, individual teats were occasionally heavily soiled with dung. Milk from washed teats also had variable counts, probably because it was difficult to ensure that teat ends were clean in the limited time available for washing. The difficulty of controlling both environmental conditions and effectiveness of teat washing probably explains why studies have generally failed to show a clear relationship with bacterial counts in the milk [29, 30].

The influence of various pre-milking practices applied to both teats and udders on bacteria levels in milk is illustrated in Table 3. The data clearly shows that drying before milking causes an additional significant decrease in bacterial contamination [32].

Organisms associated with bedding materials that contaminate the surface of teats and udders include streptococci, staphylococci, spore-formers, coliforms and other Gram-negative bacteria. For housed cows, standard plate count (SPC) range from 105-107 CFU per teat. Micrococci (including coagulase-negative staphylococci) and streptococci (mainly faecal types) are the predominent groups present. Gram-negative bacteria, including coliforms, are less numerous [33]. Both psychrotrophic and thermoduric bacteria are commonly found on teat surfaces [2].

In raw milk quality problems arise when psychrotrophic contaminants are numerous and/or they have opportunities to grow and release their enzymes. The more important conditions follow: (1) Dirty equipment and dirty cows. (2) Delayed cooling and elevated storage temperatures. (3) Long term storage of milk. (4) Excessive handling.

Most of the psychrotrophic bacteria are gram-negative rods. In milk the majority of them belong to the genus Pseudomonas, and the major species is P. fluorescens. Names of other psychrotrophs often mentioned are Alcaligenes, Flavobacterium and certain coliforms. Psychrotrophs are defined as those microorganisms that are able to grow at temperatures of 7º C and below. In refrigerated milk products stored at 7º C range, once these bacteria have passed through into the logarithmic growth phase, they are able to increase their numbers about 1000-fold per day. Psychrotrophs commonly produce large amounts of extracellular enzymes which break down proteins, fats, phospholipids, glycoproteins and glycolipids of milk and milk products. However, psychrotrophs have only limited abilities to use lactose. The enzymes which psychrotrophs produce determine the types of endproducts which we find in milk and milk products. Thus they determine off flavors and odors and changes in body, texture, and color [34]. One of the most important effects of psychrotrophs and their enzymes in cheese manufacture is reduction in yields. Mohamed and Bassette [35] observed complete failures in attempts to produce recoverable curd under certain conditions of growth of selected highly proteolytic psychrotrophs in skim milk. When counts of the psychrotrophs reached about 2 x 106 /ml and the milk was separated before cheese manufacture, vat failures occurred. Cousin and Marth [36] used electrophoresis to follow the hydrolysis of milk proteins by proteolytic psychrotrophs. The psychrotrophs preferentially attack casein instead of whey protein. Depending on the isolate either a- or b-casein or both forms of casein are attacked. Nelson and Marshall [37] found that there was a significant decrease in yield of curd as numbers of proteolytic psychrotrophs increased to more than 107/ml. However not all psychrotrophs were equally effective in decreasing the yield. 

TABLE 3
Bacteria counts in milk associated with use of water hose in wetting both the teats and udder during pre-milking preparation procedures [31].

Water Hose	Wash Sanitizer	Manual Drying	Bacteria in Milk, % Change*
X			+13
X	X		-10
X	X	X	-68

*Percent change of bacteria in milk compared to no preparation.

The aerobic thermoduric organisms on teat surfaces are often spores which are typically found in soil. A spore is a bacterial survival mechanism that is resistant to many agents. When they enter the bulk milk they may survive normal pasteurization processes and cause post-pasteurization problems. Dirty cows and/or dirty surfaces may be the source. If these bacteria gain entry into cracks in old rubber they may create problems if not removed during the cleaning cycles. The Bacillus spores are the more frequently found, whose counts range from 102-105 per teat depending on environmental conditions [29]. Teat surfaces are also source of anaerobic clostridial spores in milk. They have been detected in cows’ fodder, bedding and faeces, and decline markedly in numbers when cows go out to pasture. Clostridial spore counts are highest in winter, because they are mainy derived from silage and from bedding materials. Spores of lactate fermenting clostridia (Cl. tyrobutyricum), which cause late blowing faults in hard cheeses, may be transmitted via faeces or silage fed cows to the cows’ teats and then to milk unless the faecal material is washed from the teats [38].

3. INFLUENCE OF EQUIPMENT CLEANING AND SANITIZING PROCEDURES

Once milk leaves the cow, the retention or preservation of milk quality requires cleanliness, sanitation and careful handling. To consistently produce high quality milk with low bacteria counts requires continual attention to numerous details. Maximum benefits are derived only when these traits are applied to all aspects of the milk production system: cows, cow environment, milking system, milking practices or procedures and milk storage or cooling system. A deficiency in any part of the overall system will result in decreased milk quality.

The milk contact surfaces of milking and cooling equipment are a main source of milk contamination and frequently the principal cause of consistently high bacterial counts.

Milk residue left on equipment contact surfaces supports the growth of a variety of microorganisms. Organisms considered to be natural inhabitants of the teat canal, apex and skin are not thought to grow significantly on soiled milk contact surfaces or during refrigerated storage of milk. This generally holds true for organisms associated with contagious mastitis (i.e. S. agalactiae) though it is possible that certain strains associated with environmental mastitis (i.e. coliforms) may be able to increase to significant numbers. In general, environmental contaminants (i.e. from bedding, manure, feeds) are more likely to grow on soiled equipment surfaces.

It is virtually impossible with practical cleaning systems to remove all milk residues and deposits from the milk contact surfaces of milking equipment. Except in very cold, dry weather, bacteria can multiply on these surfaces during the interval between milkings, and their numbers may increase more rapidly than visible residues. The essential requirements are, to use milking equipment with smooth milk contact surfaces with minimal joints and crevices, an uncontaminated water supply, detergents to remove deposits and milk residues and a method of disinfection to kill bacteria.

Cleaning and sanitizing procedures can influence the degree and type of microbial growth on milk contact surfaces by leaving behind milk residues that support growth, as well as by setting up conditions that might select for specific microbial groups. More resistant and/or thermoduric bacteria may endure in low numbers on equipment surfaces that are considered to be efficiently cleaned with hot water. If milk residue is left behind (i.e. milk stone) growth of these types of organisms, though slow, may persist.

Less efficient cleaning, using lower temperatures and/or the absence of sanitizers tends to select for the faster growing, less resistant organisms, principally Gram-negative rods and lactic streptococci. Psychrotrophic bacteria tend to be present in higher count milk and are often associated with occasional neglect of proper cleaning or sanitizing procedures and/or poorly cleaned refrigerated bulk tanks [39-42].

As might be expected the microorganisms found on milk contact surfaces of equipment are similar to those found in fresh raw milk. Certain species, notably mastitis pathogens, have not been reported as forming any appreciable part of the microflora of milking equipment, although large numbers of streptococcal and staphylococcal mastitis organisms can be present in milk passing through the equipment. These organisms may be transferred on udder washing cloths and on milkers’ hands, but they are probably unable to multiply on surfaces of milking equipment between milkings [29].

An extensive review of Practices for cleaning and sanitation of milking machines is reported in Bulletin of the IDF N° 381/2003 [5]

Water supplies

Water used on the farm might also be a source of microorganisms, especially psychrotrophs, that could seed soiled equipment and/or the milk [2]. Water used in the process of milk production should be of a bacteriologically potable quality. Even if the purity is assured, bacterial contamination can be introduced from storage tanks which are not properly protected from animals and dust. Bacteria may also come from dirty wash troughs, carrying buckets and hoses.
Untreated water supplies from natural sources (e.g. wells, lakes, etc.) may be contaminated with faecal microorganisms, e.g. coliforms, faecal streptococci and clostridia. In addition, a wide variety of saprophytic bateria derived from the soil, or from vegetation may be present, including Pseudomonas spp., coliforms and other Gram-negative rods, Bacillus spores, coryneform bacteria and lactic acid bacteria. Even if heavily contaminated water gains access to milk, it may be insignificant in terms of CFU/ml of milk. However, multiplication of some of the water-borne bacteria in any residual water in the equipment will result in more serious contamination and may lead to the establishment and development of some undesirable types of microorganisms, e.g. psychrotrophic Gram-negative rods, in milk equipment [29].

Aerial contamination

Air in a milking area contains dust, moisture, and bacteria. Air admission should be minimized. However, aerial contamination of milk by bacteria is insignificant under normal production conditions. Air is not an important source of microorganisms in milk. Micrococci, coryneforms, Bacillus spores, streptococci, and Gram-negative rods can be present. Calculations based on aerial contamination indicate that, normally, airborne bacteria would account for <1 or < 5 CFU/ml of milk produced [43]. Such level of contamination are negligible in comparison with those derived from teat surfaces. In addition to possible introduction of bacteria, excessive air causes foaming which increases rancidity and off-flavors, and decreases shelf life. The foaming of milk may affect its bacterial count. Generally, the more air that is incorporated into milk, the faster the bacteria grow.

Personnel

It is probable that the actions of the milker, in dislodging dust and dirt particles, by increasing aerial contamination in the environment of the udder or by contact with hands, may add microorganisms to milk. Risks of contamination from milker are higher when cows are hand milked, and much less with machine milking. Dirty clothes and dirty hands increase the risk of contamination of the cow and milking system. Wear clean clothes during milking. Wash hands prior to starting milking and frequently during milking. Be sure to wash hands after handling any cow known or suspected of being infected and after being in contact with any part of the cow or its environment.

Detergents and disinfectants

The milking system should be sanitized immediately prior to each milking, within one hour of milking time. Failure to sanitize prior to every milking is a common cause of erratic bacteria counts.

Detergents increase the ‘wetting’ potential over the surfaces to be cleaned, displace milk deposits, dissolve milk protein, emulsify the fat and aid to the removal of dirt. Detergent effectiveness is usually increased with increasing water temperature, and by using the correct concentration and time of application. Hard water (i.e. high levels of dissolved calcium and other salts) will cause surface deposits on equipment and reduce cleaning effectiveness. In such cases, it is necessary to use de-scaling acids such as sulphamic or phosphoric, periodically. Omitting the acid rinse hastens the deterioration of rubber goods and allows minerals from the wash water to accumulate in the milking system. Both situations make cleaning and sanitizing of the milking system more difficult.

Disinfectants are required to destroy the bacteria remaining and subsequently multiplying on the cleaned surfaces. The alternatives are either heat applied as hot water or chemicals. Heat penetrates deposits and crevices and kills bacteria, providing that correct temperatures are maintained during the process of disinfection. The effectiveness of chemicals is increased with temperature but even so, they do not have the same penetration potential as heat and they will not effectively disinfect milk contact surfaces which are difficult to clean.

When hot water alone is used, it is best to begin the routine with water at not less than 85°C, so that a temperature of at least 77°C can be maintained for at least 2 minutes. Many chemicals are suitable as disinfectants, some of them combined with detergents (i.e. detergent-sterilisers).

The temperatures of solutions used for cleaning and disinfection influence the microflora. Application of hot solutions can determine a survival of thermoduric organisms, as non-sporeforming Gram-positive rods (e.g. Microbacterium spp.), micrococci, streptococci and Bacillus spp.

It is important with any method of cleaning that the equipment is drained as soon as possible after washing for storage between milkings. Bacteria will not multiply in dry conditions but water lodged in milking equipment will, in suitable temperatures, provide conditions for massive bacterial multiplication. Equipment with poor milk contact surfaces, crevices and large number of joints, remaining wet between milkings in ambient temperatures above 20°C, should receive a disinfectant rinse before milking begins. A complete wash cycle with appropriate cleaners or chemicals have to be run after every milking.

4. MILK STORAGE TEMPERATURE AND TIME

The temperature and duration of storage, the numbers and types of bacteria in the milk influence the increase in bacterial numbers which occurs in stored milk. Because of the wide variation in the initial microflora, and in the conditions under which milk is stored, only generalizations can be made concerning changes in the microflora of milk occurring during storage and transport.

The growth of many kinds of bacteria can be reduced or stopped by refrigeration of milk. This is best illustrated by the fact that at normal refrigerated temperatures, milk will still, in time, turn sour. Refrigeration masks the effects of unhygienic production conditions which, in poorly cooled milk in warm weather, result in souring and other obvious form of spoilage of milk.

Refrigeration storage, while preventing the growth of non-psychrotrophic bacteria, will select for psychrotrophic microorganisms that enter the milk from soiled cows, dirty equipment and the environment. Minimizing the level of milk contamination from these sources will help prevent psychrotrophs from growing to significant levels in the bulk tank during the storage period on the farm or at the dairy plant. In general these organisms are not thermoduric and will not survive pasteurization. The longer raw milk is held before processing, the greater the chance that psychrotrophs will increase in numbers. Though milk produced under ideal conditions may have an initial psychrotroph population of less than 10% of the total bulk tank count, psychrotrophic bacteria can become the dominant microflora after 2-3 days at 4.4°C [44]. Colder temperatures of 1-2°C will delay this shift, though not indefinitely.

Under conditions of poor cooling with temperatures greater than 7.2°C, bacteria other than psychrotrophs are able to grow rapidly and can become predominant in raw milk. Though incidents of poor cooling still occur, this defect is not as common as when milk was held and transported in cans. Streptococci have historically been associated with poor cooling of milk. These bacteria will increase the acidity of milk, and certain strains are also responsible for a “malty defect” that is easily detected by its distinct odour. Storage temperatures greater than 15°C tend to select for these types of contaminants [44]. The types of bacteria that grow and become significant will depend on the initial microflora of the milk [2].

Failure to operate the bulk tank agitator during milking can result in “pockets” of warm milk in which bacteria can multiply rapidly. Keeping warm and cooled milk blended will result to be more uniform. According to the European regulation [12], the milk, immediately after milking, must be placed in a clean place which is so equipped as to avoid adverse effects on the milk. If the milk is not collected within two hours of milking, it must be cooled to a temperature of 8°C or lower in the case of daily collection or 6°C or lower if collection is not daily. While the milk is being transported to the treatment and/or processing establishment, its temperature must not exceed 10°C.

Farm collection tankers are insulated, and the temperature of the large volumes of milk they carry is unlikely to rise much during transport. At 5°C, the results of storing samples of daily collected milk having a mean initial psychrotroph count of 104 CFU/ml showed an increase to over 106 CFU/ml in 3 days [29]. It is not clear to what extent bacterial contamination from transport tankers, hoses, pumps and, where fitted, meters and automatic samplers contributes to the increased bacterial content of milk arriving in tankers for transfer to bulk storage at processing dairies. Because of the large volumes of milk involved, any significant increase in count of milk indicates very heavy contamination of the milk contact surfaces. Generally, within 2-3 days after transfer from transport tankers, the microflora of the milk is dominated by psychrotrophs, whereas the thermoduric microflora does not increase, and changes little in composition. Spore of psychrotrophic Bacillus spp. could be present, but germination of spores and outgrowth is unlikely to occur to any significant extent at the temperatures, and for the time, raw milk is stored before use. The predominant psychrotrophic genera and species will vary, but will be derived from those present in raw milk initially. Milk treatment at the dairy may destroy the psychrotrophic bacteria, but not necessarily the products of their metabolism or their enzymes that can adversely affect product yield and quality. Therefore, good handling practices will avoid high bacterial counts and possible presence of undesirable bacterial enzymes or other effects.

ECONOMICAL ANALYSIS OF CONTAMINATION EFFECTS

It’s quite difficult to quantify the gross economical impact of producing and processing variously contaminated milk.

This evaluation should include the loss of profit due to selling milk with a microbial load above the quality standard agreed with the milk purchaser. This computation is in practice impossible due to the diversities in the contracts in diffenrent countries and region by region. A rough estimation of cost due to mastitis can, on the contrary, be forwarded.

It has been already mentioned that above 100 000 SCC/ml milk leaves its physiological state and this deviation is increasing with the increase of the severity of the inflammation. The modifications in milk composition and technological properties are accompanied by a reduction in milk production, estimated in losses varying between 3 and 6%, and up to 11% for severe infections in heifers at the early lactation period. It is estimated that the limit of ≤ 400 000 SCC set by the EU for milk commercialisation leads to a loss of about 2 litres per pluriparous cow/day, which means about 550 Kg per cow/per lactation. Considering an average value of 0.32 Euro per Kg, each cow considered “normal” can loose 176 Euros per year, equal to approximately 7 100 Euros for an average herd of 40 lactating cows.

Beside the straight loss of milk production other factors have to be taken into consideration in evaluating the consequences of mastitis. Reduction of milk quality, up to the impossibility of using the milk for processing, early cow elimination for udder tissue alteration, slowing down of the programmes in genetic improvement, increase in manpower for animal treatment, increase in expenses for the veterinarian, cost of medicines, elimination of milk containing antibiotic residues. Furthermore it has to be taken in consideration that animals affected by clinical mastitis do not fully recover their standard milk production and the incidence of this factor becomes more severe if the infection happens at early lactation stage. The turnover of cows affected by mastitis is 2-3 times higher than the average in the herd.

Even if quite old, still interesting appears the study by Hoblet et al. [45] in computing the costs of mastitis. He reports a cost of $ 107 for each outbreak, 17 of which due to medical cost and drugs. The increase in manpower is computed in 0.4 hours/day per cow.

For all these reasons, a 2003 USDA report [46] estimates an annual loss in US of more than $ 2 billion due to mastitis.

On the basis of the above, the balance in the costs for the adoption of all the codes of practice in the various stages involved in milk production, handling, and processing and the costs, also in socio-economical terms, of not achieving hygiene has to be carefully analysed.

CONCLUSIONS

In general, farmers are not very keen on adopting modifications in their usual management and are not inclined in investing, unless there is a clear possibility of increasing their income.

There is enough knowledge on the causes of milk contamination either from disease or from environment, on the consequences of processing contaminated milk, and, from what above, on the economical consequences.

Thecnical instruments for controlling and minimising the effects of contamination are also available from international organisations. Every possible effort in producing guidelines and recommendations has been made.

It’s now up to the individual farmer the decision of adopting or not the proposed documents and up to the individual state, if the case, to recall in their legislation these documents.

REFERENCES

1. Tolle, A. 1980. The microflora of the udder. p 4. In Factors Influencing the Bacteriological Quality of Raw Milk. International Dairy Federation Bulletin, Document 120.

2. Bramley, A.J., McKinnon, C.H. 1990. The microbiology of raw milk. pp. 163-208 In Dairy Microbiology, Vol. 1. Robinson, R.K. (ed.) Elsevier Science Publishers, London.

3. FAO 1989. Milking, milk production hygiene and udder health. FAO Animal Production and Health Paper 78. Rome,Italy (1989).

4. IDF Mastitis Newsletter N° 21. Newsletters of the International Dairy Federation N° 144. IDF, Brussel, Belgium(1996).

5. IDF Mastitis Newsletter N° 25. Bulletin of the International Dairy Federation N° 381. IDF, Brussel, Belgium (2003).

6. Dodd, F.H. 2003. Bovine mastitis - The significance of levels of exposure to pathogens. In Bulletin of the International Dairy Federation N° 381. Mastits Newsletter 25, 3-6. IDF, Brussel, Belgium.

7. Bulletin of the International Dairy Federation N° 215. IDF, Brussel, Belgium (1987).

8. Giannechini, R., Concha, C., Rivero, R., Delucci, I., Moreno Lopez, J. 2002. Occurrence of clinical and sub-clinical mastitis in dairy herds in the West Littoral Region in Uruguay. Acta Vet. Scand. 43 (4): 221-230.

9. Costa, E., Garino Junior, F., Watanabe, E.T., Silva, J.A.B., Ribeiro, A.R., Horiuti, A.M. 2001. Patogenos de mastite bovina isolados de glandulas mamarias negativas aos testes de Tamis e CMT. Napgama 4 (2): 12-15.

10. Dego, O.K., Tareke, F. 2003. Bovine mastitis in selected areas of Southern Ethipia. Trop. Anim. Health Pro. 35 (3):197-205.

11. Beytut, E., Aydin, F., Ozcan, K., Genc, O. [Pathological and bacteriological investigations on bovine mastitis in Kars Region and its surrounds]. In DSA 65 (9): 835 (2003).

12. Council Directive 92/46/EEC. 1992. Health rules for the production and placing on the market of raw milk, heattreated milk and milk-based products. Official Journal L 268, 14/09/1992.

13. Hamann, J. 2003. Definition of the physiological cell count threshold based on changes in milk composition. In Mastitis Newsletter N° 25. Bulletin of the International Dairy Federation N° 381. 9-12. IDF, Brussel, Belgium.

14. K itchen, B.J. 1981. Review of the progress of Dairy Science: bovine mastitis; milk compositional changes and related diagnostic tests. J. Dairy Res. 48: 167-188.

15. Needs, E.G., Anderson, M. 1984. Lipidic Composition of Milks from Cows with Experimentally Induced Mastitis. J. Dairy Sci. 51: 239-249.

16. Salih, A.M.A.. Anderson, M. 1979. Observation on the influence of high cell count on lipolysis in bovine milk. J. Dairy Res. 46: 453-462.

17. Verdi, R.J., Barbano, D.M., Dellavalle, M.E., Senyk, G.F. 1987. Variability in True Protein, Casein, Nonprotein Nitrogen, and Proteolysis in High and Low Somatic Cell Milks. J. Dairy Sci. 70: 230-242.

18. Senik, G.F., Barbano, D.M., Shipe, W.F. 1985. Proteolysis in Milk Associated with Increasing Somatic Cell Counts. J. Dairy Sci. 68: 2189-2194.

19. Saeman, A.I., Verdi, R.J., Galton, D.M., Barbano, D.M. 1988. Effect of Mastitis on Proteolytic Activity in Bovine Milk. J. Dairy Sci. 71: 505-512.

20. Hampton, O., Randolph, H.E. 1969. Influence of Mastitis on Properties of Milk. II. Acid Production and Curd Firmness. J. Dairy Sci. 52:1562-1565.

21. Randolph, H.E. 1969. Influence of Mastitis on Properties of Milk. III. Lactic Culture Inhibitory Activity and Inhibition Titers. J. Dairy. Sci. 52: 1566-1568.

22. Zecconi, A. 1996. Somatic cells and their significance for milk processing (technology). In Mastitis Newsletter 21. Newsletters of the International Dairy Federation N° 144:11-14 . IDF, Brussel, Belgium.

23. Politis, I.. Ng-Kwai-Hang, K.F. 1988. Association Between Somatic Cell Count of Milk and Cheese-Yielding Capacity. J. Dairy Sci. 71: 1720-1727.

24. Grandison, A.S.. Ford, G.D. 1986. Effects of variation in somatic cell count on the rennet coagulation properties of milk and on the yield, composition and quality of Cheddar cheese. J. Dairy Res. 53: 645-655.

25. Politis, I., Ng-Kwai-Hang, K.F. 1988. Effects of Somatic Cell Count and Milk Composition on Cheese Composition and Cheese Making Efficiency. J. Dairy Sci. 71: 1711-1719.

26. Hogan, J.S., Smith, K.L., Hoblet, K.H., Todhunter, D.A., Schoenberger, P.S., Hueston, W.D., Pritchard, D.E., Bowman, G.L., Heider, L.E., Brockett, B.L., Conrad, H.R. 1989. Bacterial counts in bedding materials used on nine commercial dairies. J. Dairy Sci. 72:250.

27. McKinnon, C.H., Rowlands, G.J., Bramley, A.J. 1990. The effect of udder preparation before milking and contamination from the milking plant on the bacterial numbers in bulk milk of eight dairy herds. J. Dairy Res. 57:307.

28. Pankey, J.W. 1989. Premilking udder hygiene. J. Dairy Sci. 2:1308.

29. Cousins, C.M., Bramley, A.J. 1984. The microbiology of raw milk. pp. 119-163 In Dairy Microbiology, Vol. 1. Robinson, R.K. (ed.) Elsevier Science Publishers, London.

30. Panes, J.J., Parry D.R., Leech, F.B. 1979. Report of a survey of the quality of farm milk in England and Wales in relation to EEC proposals. Ministry of Agriculture, Fisheries and Food, London.

31. Galton, D.M., Peterson, L.G., Merrill, W.G. 1988. Evaluation of udder preparations on intramammary infections. J Dairy Sci.; 71 (5): 1417-1421.

32. Bodman, G. R., Rice, D. N. 2001. Bacteria in Milk Sources and Control. University of Nebraska, Communications and Information Technology, NU Institute of Agriculture and Natural Resources. Lincoln, NE (http://ianrpubs.unl.edu/dairy/)

33. Cousins, C.M. 1978. Milking techniques and the microbial flora of milk. XXth Int. Dairy Congress, Paris.

34. Marshall, R. T. 1979. Psychrotrophic bacteria: their relationship to raw milk quality and keeping quality of cottage cheese. Marschall Italian & Specialty Cheese Seminars. Madison, Wisconsin, 9/13/1979 (http://www.marschall.com/
marschall/proceed/pdf/79_52.pdf)

35. Mohamed, F. O., Bassette, R. 1979. Quality and yield of cottage cheese influenced by psychrotrophic microorganisms n milk. J. Dairy Sci. 62:222-226.

36. Cousins, M. A., Marth, E. H. 1977. Changes in milk protein caused by psychrotrophic bacteria. Milchwissenschaft 32:337-340.

37. Nelson, P. J., Marshall, R. T. 1977. Microbial proteolysis sometimes decreases yield of cheese curd. J. Dairy Sci. 60, Suppl 1:35.

38. Bergere, J.-L. 1979. Développement de l’ensilage: ses conséquences sur la qualité du lait et des produits laitiers. Revue Laitiere Francaise, 378, 19-25.

39. MacKenzie, E. 1973. Thermoduric and psychrotrophic organisms on poorly cleaned milking plants and farm bulk tanks. J. Appl. Bacteriol. 36:457.

40. Murphy, S. C., Boor, K. J. 2000. Trouble-shooting sources and causes of high bacteria counts in raw milk. Dairy Food Environ. Sanit. 20:606-611.

41. Olson, J.C. Jr., Mocquat, G. 1980. Milk and Milk Products. P.470. In Microbial Ecology of Foods. Vol. II. J.H. Silliker, R.P. Elliott. A.C. Baird-Parker, F.L. Bryan, J.H. Christion, D.S. Clark, J.C. Olson, and T.A. Roberts (eds.). Academic Press,
New York, NY.

42. Thomas, S.B., Druce, R.G., King, K.P. 1966. The microflora of poorly cleansed farm dairy equipment. J. Appl. Bacteriol. 29:409.

43. Underwood, H.M., McKinnon, C.H., Davies, F.L., Cousins, C.M. 1974. XIXth International Dairy Congress, 1E, 373.

44. Gehringer, G. 1980. Multiplication of bacteria during farm storage. In Factors influencing the bacteriological quality of raw milk. International Dairy Federation Bulletin, Document 120.

45. Hoblet, K.H., Schnitkey, G.D., Arbaugh, D., Hogan, J.S., Smith, K.L., Schoenberger, P.S., Todhunter, D.A., Hueston, W.D., Pritchard, D.E., Bowman, G.L., Heider, L.E., Brockett,B.L., Conrad., H. R. 1990. Economic losses associated with episodes of clinical mastitis in nine low somatic cell count
herds. J. Am. Vet . Med. Assoc. 199:190.

46. ARS-USDA. Researchers Develop Effective Mastitis Treatments. Healthy Animals Newsletter. 2003. Issue 15T

IDF/FAO international symposium on dairy safety and hygiene Cape Town, March 2–5, 2004, South Africa