Lucrare publicata in Anuarul FMV Timisoara pe anul 2014
HYGIENIC QUALITY OF MILK, A GREAT AND PERPETUAL DESIDERATUM
RADU MOGA MANZAT
Academia de Ştiinţe Agricole şi Silvice – Filiala Timisoara, Calea Aradului 119, 300645
College of Veterinary Surgeons, Timis, Str. Martir Marius Ciopec nr. 2, Timisoara, Romania.
One of the most debated issues over the time, both in food industry and veterinary medicine, was the quality control of milk. Some european nations have managed to create hundreds of varieties of cheeses, very well appreciated for their qualities and very well sold all over the world. These is not only the result of creative ingenuity of their people, but especially the result of the fact that they always are prepared with a raw milk of the highest hygienic quality(1). From the chemical point of view, was easier to put the quality control of milk into some limits, because was possible to appeal to the specific methods of biochemistry, but from the sanitary point of view, things appear to be more complicated. It is well known that both clinical- and subclinical mastitis cause very important economic damage, but especially the hindmost are important because they are much more numerous then clinical ones and, being clinical unapparent, they are often neglected or even completely ignored. A milk of best quality, good for consumption, is a wholesome milk (WM), obtained hygienically only from perfectly healthy mammary quarters (MQ). Such milk has the following characteristics , organoleptic noticeable : white color or slightly yellowish-white, perfectly homogeneity, with appetizing taste and specific pleasant flavor (not smell!); microscopically may present a small number of somatic cells (SCC) and bacteria. To assess the hygienic quality of milk, in addition to biochemical tests, which became classics and well known, lately are emerging new tests, especially of those which detect the physical changes in milk, with electronic devices.
Key words: hygienic quality of milk, mastitis, portable milk tester, SCC.
Some terminological specifications
First of all, in order to avoid confusions in professional practice, in official documents as in the literature, it is necessary to perceive and properly define the following terms: normal and abnormal milk, compliant and non-compliant milk, healthy (or wholesome) and unhealthy milk, mastitis milk, clinical mastitis and subclinical mastitis. In the case in which a mammary quarter undergoes a severe bacterial infection, this one most often causes an inflammation of the mammary parenchyma, of exudative-purulent type. By mixing this purulent exudate in the milk of the affected compartment follows what is called mastitis (or mastitic) milk (MM). What we must know is that the occurrence of some mastitis in any herd of dairy cows, sooner or later, is inevitable. The question which must concern us is: how frequent, how many and how serious they are, how they occur and what kind of etiological nature they have. To find answers to these questions, we need to determine at least two of the three indicators of hygienic quality of milk: milk somatic cell count (SCC) and the total number of bacteria ( SPC). But of these, SCC is the parameter which gives us the best indication of the frequency and severity of mastitis, because whereas bacteria in milk delivered may originate either from infected udder or the environment, somatic cells always come exclusively from mammary tissue. That is why SCC/ml was adopted worldwide as the main indicator of the hygienic quality of milk and the proportion of mastitis milk included in delivered milk (1) Milk from cows perfectly healthy, with perfectly healthy udder, usually contains between 5’000 – 50’000 SC/ml . This milk is considered normal, is white or yellowish-white , perfectly smooth, has appetizing taste and pleasant aroma. When an infection of the mammary tissue occurs , the number of SC increase to 100’000-200’000, as a defense response , resulting in one of the following situations (10): -either bacteria are destroyed, followed by restitutio in integrity of mammary tissue and reduction of SCC to the normal, milk all the time having characteristic normal appearance; -or the benign inflammation of mammary tissue is developing, with SCC increasing over 200’000/ml, but without altering the organoleptic and directly visual appearance of the milk, which remain normal ; the increasing of SCC is accompanied only by some chemical modification; in this case we are talking about subclinical mastitis; -whether the disease process is more advanced, at the chemical changes in milk are added some physical changes regarding homogeneity, color and transmittance, but detectable only with the help of certain optical instruments; we proposed for them the term of subtle (fruste) mastitis ( fs ); yet this milk, is considered normal – in the sense defined by Rasmussen (2002) and adopted by the European Union-, because its macroscopic appearance is normal; -only in case when very severe mammary tissue alterations appear can talk about clinical mastitis. . Clinical mastitis can be either : a) with macroscopically visible inflammatory alterations of the udder (rubor, tumor, calor, dolor, functio laessa) and obvious physical and organoleptical changes regarding uniformity, color, consistency, smell, taste and flavor of the milk , or b) alterations only in milk uniformity, color, consistency, smell, taste and flavor, without any visible inflammatory alterations of the udder. So, in clinical mastitis, at chemical and cytological changes of the milk, always obvious physical and organoleptic changes of the milk are added. Although subclinical mastitis are in fact the result of microbial infection, the milk produced by them can not be ruled sine ulla exceptione from public consumption due to at least three reasons: 1) Such a goal would require frequent individual control of all lactating cows, which would mean a huge labor and material consumption, greatly raising the cost of production. 2) The prevalence of subclinical mastitis is often so high that operative removal of all subclinical mastitis in a herd would unacceptably affect farm profitability. 3) The traditional means of detecting individual subclinical mastitis can not be successfully applied to large herds, operated in modern intensive systems and, on the other hand, modern technical means of detecting mastitis online are too expensive and impractical for small dairy herds. In this situation and for these reasons has been accepted a solution of compromise, recommended by the International Meeting in Copenhagen on 27.11.2002, when were launched and defined the terms of normal milk and abnormal milk (Rasmussen 2002) subsequently assimilated also by the European Commission (EC). According to the definition assumed by EC, normal milk (NM) is considered milk having normal aspect, without noticeable macroscopic changes of color or homogeneity, detectable with our sense organs, regardless of whether some chemical, bacteriological or cytological changes, characteristic for inflammation and detectable only by specific tests, are or are not present. Classification as “normal milk” is not conditioned by a certain threshold of SCC or SPC, and therefore this term also includes subclinical mastitis milk, regardless of the value of these parameters. This is why, almost everywhere in the our country, the detection of subclinical mastitis was abandoned, most practitioners wrongly interpreting that in new conditions, it does not matter anymore. In fact, without strict control, as it was already seen hitherto, was not possible to proceed to an exhaustive production of milk conform (in compliance) with EU reglementation. Abnormal milk (AM), most often is the result of bacterial infections of the mammary, in which case it is also called mastitic milk (MM). Other times are non-infectious causes like: injuries, nutrition and colostrum. In any of these situations, the milk as such should be considered inedible. In clinical mastitis, the milk is always abnormal, and usually have much over 200,000 CS. Unlike normal milk, depending on the severity and evolving form of infection (acute, subacute or chronic), abnormal milk in mastitis is not homogeneous, contains small clots or floaters, can have wheyey aspect or, on the contrary, may be more consistent than normal milk, the color can be more or less modified (yellow-brown, reddish or greenish), has a repulsive smell and salty taste. Abnormal milk can and must be observed by the milker, at milking of samples, in order to avoid merging the modified milk with normal milk. To be considered abnormal and excluded from mixing, it is sufficient whether a milker detects the presence of small coarse or floaters in jets milked for sample. Unclear cases can be elucidated even by the milker, using the reference test represented by a 0.1 mm mesh filter: if the milk is abnormal, the mesh will retain small clots. It is important to note that, in the meaning of terminology endorsed by the EC, both the normal and the abnormal milk can be easily recognized visually and labeled as such by milker, with the naked eye, without the need to determine the SCC, SPC or the presence of antibiotics.
The economical importance of mastiti
The economical losses due to cow mastitis are very high, because these disorders, especially those subclinical, are always present in all herds, all over the world. Their prevalence depends on many factors but generally ranges from 5-10% to 50-60% of lactating cows, causing losses to farmers, plus serious losses to processors, without counting the risk of disease to consumers. Reducing losses caused by the infectious mastitis was performed over the time, following two goals: -Prevention, by early diagnosis and treatment of subclinical mastitis, which due to their much higher incidence than those clinically expressed, are much more damaging than the latter . -Correct medical diagnosis and treatment of clinical mastitis. Diagnosis as early as possible of subclinical mastitis seems to be the key of problem, since in this phase there is maximum treatment effectiveness. For this reason, in the last 60-70 years, in various countries worldwide, have been proposed a large number of diagnostic tests based on different principles, according with various modifications produced in mastitis milk, but the progress registered in the recent years for upgrading efficiently methods and techniques for expeditious diagnostic of mastitis in cows, unfortunatelly could be assessed only as modest.
Changes in mastitis milk and diagnostic tests
Let‘s do enumerate a few categories from the most important changes, which indicates installing a mastitis infection, that may be determined by different tests: -Increasing the somatic cell count in milk. -Increasing the percentage of neutrophils. -Increasing the pH of milk. -Increasing the concentration of th chloride. -Increasing the electrical conductivity of milk. In fact, the last three categories are influenced by the same phenomenon: the increasing concentration of salt in mastitis milk. One of the latest tests performed, which determine electrical conductivity of milk, is based on properties acquired of electrolytic substances present in milk (salts, glucose) to conduct electricity between two electrodes. Conductivity, measured by a special device, increases in mastitis milk proportional to their concentration. In this test were set high expectations, because it is very fast, posible to run with a portable device and does not require any kind of reagents but, unfortunately, has some problems regarding its specificity and sensitivity. At present the experience and use of spectrometry in assessing the health status of milk, represents a promise and a hope, but researches in this domain still are are not finalized (10,12). The number and variety of tests proposed for diagnosis of subclinical mastitis range very much just because none of them responds simultaneously to all desiderata. As a rule, each of these tests are able to detect only a single or at most two changes which appear in mastitis milk: the presence of microorganisms, increasing the total number of somatic cells, the percentage of neutrophils, increasing the concentration of salts, pH changes etc. Results obtained by different tests are not well correlated, because each of these parameters are varying in different way by different dynamics in time, along the evolution of infection, depending on a lot of factors, like: the phase of infection, nature of etiological agent, the particular environmental circumstances, individual status of health etc. Each parameter measured and each test provides some advantages and some disadvantages in comparison with the others. The correlation between results of these tests is often random and therefore no test can be considered perfect or taken as a reference test for the others, nor even bacteriological one, whose results are relevant only when is positive. An increasing number of somatic cells, or neutrophils, may indeed betray an evolutionary mastitis, but equally well might only reflect a former mastitis, meanwhile healed spontaneously or by medication, and therefore it also could not be taken as reference test. A special mention deserves California Mastitis Test (CMT), which is based on the changes of two parameters, simultaneuslly measured: increased somatic cell count and milk pH modification, test currently recognized as the most valuable of all tests, the test that gives the fewest false results, and therefore are on the first place in preferences of practitioners. However, because this test also has some disadvantages that limit its applicability in very large herds, with modern technologies, now all hopes are turning to the new emerging tests, using electronic devices (10,11,12).
Conditions of acceptability for a good diagnostic test
A new test conceived for subclinical mastitis detection is useful and well received in practice, only if show a number of advantages and as few as possible disadvantages in comparison with those already known. Among disadvantages of different diagnostic tests could be included: -A large dose of subjectivity in reading and interpreting the results (as happens with CMT , Bernburg Schneltest, R-mastitest); -Detecting a profusion of false positives or negatives results (as happens with all tests); -Needs expensive equipment ( like automatic direct counting of cells); -High-cost of reagents and other consumables (microbiological and cytological examinations); -Require laboratories and qualified staff (bacteriological and cytological examinations) -Long-time of expectation for results (bacteriological examination. In this context, an important step forward could only be made in the future by designing new tests, based upon new principles, meeting the following conditions: -To detect simultaneously multiple changes that usually occurs from the very beginning of installation, in mammary infection. -To provide the most reliable results, preferably numerical quantifiable, closely correlated with those of the main diagnostic tests known and eventually taken as reference tests. -To be efficient, requiring low-cost per sample. – To be simple, quick, requiring no particularly skilled labor. – Possible to be performed in farms, with portable device. Already numerous tests and devices, designed taking into account the wishes mentioned above, appeared lately, new ones emerging today, some completed, others in the training course, described in a growing number of publications (2,3,4,5,6,7,8,9,10,11,12,13). However none of them simultaneously satisfy all the above desiderata. Therefore some of them are preferred in some circumstances and others are preferred in different circumstances.
Hygienic quality of cow’s milk remains a very important issue of public catering, insufficiently resolved until now, although real progress were lately registered. Final resolution of the matter depends largely of improving techniques and apparatus used for operative diagnosis of subclinical and subtle mastitis. This would allow continuous monitoring of individual health status of the milk from each cow, and operative establishment of appropriate measures.
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