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Brief comparative analysis of the methods of Somatic Cells determination (SCC) in milk

Somatic Cells content/number of Somatic Cells in milk is one of the most essential microbiological indicators of milk quality, which value is used for determination of the grades of collected milk. Somatic cells count (SCC) is used in many countries for establishing rates or the price of collected milk, and thus for estimating of the quality of milk.

Direct or reference (arbitration) method of somatic cells count in milk is a microscopic method which involves the counting of the stained somatic cells using a microscope. It is a very time- and labour-consuming method which also requires highly qualified personnel. Besides the errors of the given method depend on several factors such as: the number of test samples, their characteristics as well as performance of the equipment used for SCC. The mean values of errors for the middle SC concentrations (from 200 - 1000) are in the range of 10 - 15% and for the low concentrations (less than 150) are about 20 - 30%.

It is because of the complexity and laboriousness of the microscopic method indirect methods of somatic cells count become widely spread.

Indirect method of SCC are the following:

  • Viscosimetric method;
  • Conductance-measuring method;
  • Optofluoroelectronic method/fluorescence flow cytometry method

At present a number of foreign companies are offering their automated systems for automatic counting of Somatic Cells (where Somatic Cells' nucleus stained with special reagents are counted) which are based on the achievements of optofluoroelectronic method/ fluorescence flow cytometry method. They include an analyzer, PC, specialized software and sometimes a devise for automatic supply of the cuvettes with the samples. The given systems can be applied for big laboratories at milk plants as they require high qualification and special training of lab personnel. Significant disadvantages of such systems are their very high price from Euro 18 000,00 – to 60 000,00 and high-priced consumables.

The more affordable option is DCC Somatic Cells Counter by DeLaval, which is based on the same principle of the automatic direct count of the Somatic Cells number in a milk sample. DCC Somatic Cells Counter allows measurement of the precise number of somatic cells in each quarter of the cow’s udder, as well as in a milk tank. The device is compact and portable, its weight is 4,1 kg. According to the estimate of DeLaval experts the error of this method is 12% for the quantity of 100,000 somatic cells in 1 ml and 7% for the quantity of 1,000,000 somatic cells in 1 ml. The major drawback of the given Somatic Cells Counter is its relatively high price, approx. Euro 3 500,00, the high cost of the consumables (disposable single-use cassettes) and quite a long payback period of the analyzer.

Another indirect method of determining of the somatic cells number is conductance-measuring/conductometric method which is based on the measurement of electrical conductivity of milk. It is well known that inflammation in the udder causes changes not only of the qualitative composition of milk but also its physical and chemical properties change, in particular electric conductivity. Mastitis milk/ milk with high concentration of somatic cells is characterized by increased content of chlorine ions, which leads to the increase of its specific electric conductivity. However the instruments that use this method can rather serve as indicators of the variations in the number of somatic cells in milk, rather than precise somatic cells counters. One of the examples of such instruments is Miltek-1 device for mastitis diagnostics (produced by Sibagropribor Ltd.) which can be used for monitoring of the dynamics or the changes in electrical conductivity of milk and consequently the mastitis dynamics, however it cannot be used to determine the exact content of somatic cells in a milk sample.

The price of the given device is from RUB 13 000.00, measurement time is only a few seconds, the device is very compact (it fits easily in the palm of the hand) and works from a battery.

The devices of this type are useful for veterinarians for fast detection of both clinical and sub-clinical mastitis of animals as well as for fast sorting of healthy and sick animals in the herd.

The devises of indirect action which are based on viscosimetric method (viscosimetric somatic cells analyzers) measure the viscosity of milk. Tested milk sample is mixed up with a special chemical preparation (Mastoprim) which destroys somatic cells’ membranes (leukocytes), resulting in DNA molecules come in a solution, increasing its viscosity.

The higher somatic cells number in a tested milk sample the more viscous (thick and stringy) is the mixture. Viscosimetric somatic cell analyzer measures/determines the time of outflow of the tested sample through a special capillary with a known diameter and displays the results in accordance with the calibration chart that reflects the compliance of the outflow time with the somatic cells number.

The time of milk mixture outflow increases accordingly with the increase of somatic cells concentration, and this dependence is quantitatively determined by the table of the Standards of Russian Federation (GOST RF №23453).

For example, if the outflow time is 12 sec. the index of somatic cells is about 90 thousand somatic cells in 1 cm3 (the minimum value in the chart) and if the outflow time is 58 sec. then the index of somatic cells is 1500 thousand in 1 cm3 ( the maximum value).

Somatos Mini viscosimetric Somatic Cells Analyzer produced by Sibagropribor Ltd. belongs to this type of analytical devices. The analyzer is inexpensive, simple in operation and maintenance and does not require high qualification of the lab personnel. Consumables cost (“Mastoprim” preparation) is quite low and the device can be quickly paid off.

Somatos Mini can be used on all levels of milk production and milk collection from dairy farms and milk collection centers to milk processing plants and enterprises, veterinary laboratories and sanitary epidemiological stations.

Somatos Mini works from the main network and does not require pre-heating, additional calibrations by users, the device is ready for operation immediately after switching on. The measurement time per 1 sample is not more than 4 minutes. The error of the given method is not more than 5%.

According to many end users the valuable advantage of this method and vsicosimetric somatic cells analyzers is that they do not require expensive reagents, consumables and lab equipment. As compared to the costly consumables (such as slides and cassettes with reagents) required for the application of the Optofluoroelectronic method Mastorprim is very affordable.

The inconvenience for users or the drawback of using such somatic cells analyzers include the need for constant thorough cleaning of the glass components of the device, as the poor cleaning of the device affects the accuracy of measurements.

Nevertheless it should be noted that the use of viscosimetric somatic cells analyzers allows not only for monitoring of the health of individual cows and the herd’s health on the whole but also helps avoid conflict situations between milk producers and milk processing plants. According to the feedback received from many end customers the use of the somatic cells analyzers helps avoid possible losses by preventing pouring of the poor quality milk from sick cows into a bulk tank with collective milk, and therefore helps increase the quality of milk and maximize the price of milk and the profit.