Laboratory > Milk
Investigation of different types of drinking milk samples found in market in order to determine their qualities by comparing the results with TSE standards.
Milk, liquid secreted by the mammary glands of female mammals as food for their young. The milk of the cow is most widely used by humans, but the milk of the mare, goat, ewe, buffalo, camel, ass, zebra, reindeer, llama, and yak is also used. The composition of milk varies with the species, breed, feed, and condition of the animal. Milk, an almost complete food, consists of proteins (mainly casein), fat, salts, and milk sugar, or lactose, as well as vitamins A, C, D, certain B vitamins, and lesser amounts of others.
Milk prepared for sale is often homogenized; in this process it is pumped under pressure through small openings to break up the milk-fat globules, thus ensuring an equal distribution of fat throughout the milk rather than permitting it to rise to the top as cream. Pasteurization (partial sterilization by heating) checks bacterial growth, thereby making milk safer to drink and increasing its keeping qualities and range of transport.
Pasteurization is typically associated with milk, first suggested by Franz von Soxhlet in 1886. There are two widely used methods to pasteurize milk: high temperature/short time (HTST), and ultra-high temperature (UHT). HTST is by far the most common method. Milk simply labelled “pasteurized” is usually treated with the HTST method, whereas milk labelled “ultra-pasteurized” must be treated with the UHT method. HTST involves holding the milk at a temperature of 161.5 degrees Fahrenheit (or 72 degrees Celsius) for at least 15 seconds. UHT involves holding the milk at a temperature of 280 °F or 138 °C for at least two seconds.
The HTST pasteurization standard was designed to achieve a 5-log reduction (0.00001 times the original) in the number of viable micro organisms in milk. This is considered adequate for destroying almost all yeasts, mould, and common spoilage bacteria and also to ensure adequate destruction of common pathogenic heat-resistant organisms (including particularly Mycobacterium tuberculosis, which causes tuberculosis and Coxiella burnetii, which causes Q fever). HTST pasteurization processes must be designed so that the milk is heated evenly, and no part of the milk is subject to a shorter time or a lower temperature.
HTST pasteurized milk typically has a refrigerated shelf life of two to three weeks, whereas ultra pasteurized milk can last much longer when refrigerated, sometimes two to three months. When UHT pasteurization is combined with sterile handling and container technology, it can even be stored unrefrigerated for long periods of time.
Common testing of milk:
The organoleptic test permits rapid segregation of poor quality milk at the milk receiving platform. No equipment is required, but the milk grader must have good sense of sight, smell and taste. The result of the test is obtained instantly, and the cost of the test is low. Milk which cannot be adequately judged organoleptically must be subjected to other more sensitive and objective tests.
Clot on Boiling (C.O.B) Test
The test is quick and simple. It is one of the old tests for too acid milk (pH<5.8) or abnormal milk (e.g. colostrum or mastitis milk). If a milk sample fails in the test, the milk must contain many acids or rennet producing microrganisms or the milk has an abnormal high percentage of proteins like colostrum milk. Such milk cannot stand the heat treatment in milk processing and must therefore be rejected.
The test is quick and simple. It is based on instability of the proteins when the levels of acid and/or rennet are increased and acted upon by the alcohol. Also increased levels of albumen (colostrum milk) and salt concentrates (mastitis) results in a positive test.
Bacteria that normally develop in raw milk produce more or less of lactic acid. In the acidity test the acid is neutralised with 0.1 N Sodium hydroxide and the amount of alkaline is measured. From this, the percentage of lactic acid can be calculated. Fresh milk contains in this test also “natural acidity” which is due to the natural ability to resist pH changes .The natural acidity of milk is 0.16 – 0.18%. Figures higher than this signifies developed acidity due to the action of bacteria on milk sugar.
Resazurin test is the most widely used test for hygiene and the potential keeping quality of raw milk. Resazurin is a dye indicator. Under specified conditions Resazurin is dissolved in distilled boiled water. The Resazurin solution can later be used to test the microbial activity in a given milk sample.
The Gerber Butterfat test
The fat content of milk and cream is the most important single factor in determining the price to be paid for milk supplied by farmers in many countries.
Also, in order to calculate the correct amount of feed ration for high yielding dairy cows, it is important to know the butterfat percentage as well as well as the yield of the milk produced. Further more the butterfat percentage in the milk of individual animals must be known in many breeding programmes.
Butterfat tests are also done on milk and milk products in order to make accurate adjustments of the butterfat percentage in standardised milk and milk products.
Addition of water to milk can be a big problem where we have unfaithful farm workers, milk transporters and greedy milk hawkers. A few farmers may also fall victim of this illegal practice. Any buyer of milk should therefore assure himself/herself that the milk he/she purchases is wholesome and has not been adulterated. Milk has a specific gravity. When its adultered with water or other materials are added or both misdeeds are committed, the density of milk change from its normal value to abnormal. The lactometer test is designed to detect the change in density of such adulterated milk. Carried out together with the Gerber butterfat test, it enables the milk processor to calculate the milk total solids (% TS) and solids not fat (SNF).
Freezing Point Determination
The freezing point of milk is regarded to be the most constant of all measurable properties of milk. A small adulteration of milk with water will cause a detectable elevation of the freezing point of milk from its normal values of -0.54ºC. Since the test is accurate and sensitive to added water in milk, it is used to detect whether milk is of normal composition and adulterated
Milk collected from producers may contain drugs and/or pesticides residues. These when present in significant amounts in milk may inhibit the growth of lactic acid bacteria used in the manufacture of fermented milk such as Mala, cheese and Yoghurt, besides being a health hazard.
Principle of the method: The suspected milk sample is subjected to a fermentation test with starter culture and the acidity checked after three (3) hours. The value of the titratable acidity obtained is compared with titratable acidity of a similarly treated sample which is free from any inhibitory substances.
The Efficiency of Pasteurisation – The Phosphatase Test
The Phosphatase test is a chemical measure for the efficiency of milk processing, revealing whether the products have been adequately heat-treated and the possible causes of under-pasteurisation or whether they have been contaminated with raw products. The Phosphatase test is valuable as a rapid test, designed to detect minor processing faults after an incubation period as short as 30 minutes, compared with incubation of up to 24 hours for alternative methods.
TOTAL SOLID (%)
TURKISH STANDARDS FOR COW MILK
Total acidity as Lactic Acid (%)
Fat Content (%)
Solid Non-Fat Content (%)
In this experiment we performed a series of analysis for both raw milk and different labelled processed milk found in market. In order to obtain quality milk, some standards are published by TSE and it is advised to obey these values to produce healty food for people to consume safetly. With respect to this milk producing farms should be kept under control and they must forced to feed their cows with proper foods, to be concerned with cows health and maintain hygenic conditions during milking and transportation.
Basic analysis performed on milk prior to accept the milk by factory are protein, SNF, acidity, density, fat, lactose,ash and dirth analysis. In our experiment five groups performed analysis on raw milk. When we compare the result with TSE values, they are said to be acceptable unless the % acidity result of Group B4 which is lower than the acceptable value and results of % ash are lower than the accepted minimum % ash value stated on TSE standards.There is a point to be mentioned about the raw milk analysed in the experiment. That is all raw milk samples analysed were obtained from the same source so the results must be closed to each other. However the results of Group B4 significantly differ from the results of other groups. Group B4 might make mistakes during performing the analysis. There is another point to be mentioned is that the turbidity results of raw milk samples which were obtained positive. Turbidity test indicates if the pasteurization process is done properly or not. So a positive result for raw milk is plausible since it was not processed yet.
In the experiment six different labelled processed milk were analysed. When we compare experiment results with TSE standard we see that % ash content and % protein contents are lower than the values stated on TSE standards for cow milk. Protein content may be lower if excess pasteurization process applied which cause denatuation of proteins. % ash content problem which was seen in raw milk samples also can be caused by the procedure applied. Procedure of analysing % ash content is described in TSE1018 at session 2.3.16 is different than the procedure we performed in the laboratory. In the laboratory we only determined the chloride in ash, not total ash(?). The result of % acidity for İçim labelled milk (B7) is lower than the accepted value. This result may be caused by addition of excess NaOH to the milk to decrease its acidity. The turbidity test results for Danone (B9) and Halk (B10) labelled milks are positive which indicates that they are not pasteurized enough.
EtiketlerClot on Boiling Laboratory > Milk Resazurin test The Alcohol Test The Gerber Butterfat test