Milk, a natural liquid food, is one of our most nutritionally complete foods, adding high-quality protein, fat, milk sugar, essential minerals, and vitamins to our diet. However, milk contains bacteria that––when improperly handled––may create conditions where bacteria can multiply. Most of the bacteria in fresh milk from a healthy animal are either harmless or beneficial. But, rapid changes in the health of an animal, or the milk handler, or contaminants from polluted water, dirt, manure, vermin, air, cuts, and wounds can make raw milk susceptible for spoilage.
Milk brought either in cans or in tankers is unloaded.
Milk cans: As soon as vehicle carrying milk in cans arrives at the reception dock,the cans are unloaded manually and generally placed on the conveyors. If the level of the truck surface is in line with the platform, the unloading of cans requires least effort.
Road/Rail tankers: Tanker, after grading and sampling, is connected with the pump and piping and milk is pumped through a flow meter where the volume of milk is automatically recorded.
Milk cans are placed on gravity roller or power chain conveyor. The lids are removed and each can is subjected to rapid sensory evaluation and some preliminary tests to decide the acceptance or rejection of the milk.
Conveyors: Conveying systems and related devices must be considered in connection with efficient materials handling to provide direct and continuous flow from a point of origin to the ultimate destination. The conveyor system should be simple, direct,flexible, and free from trouble spots. It should have proper elevation at all points,and easy to clean, lubricate and maintain. Sharp curves, sudden changes of elevation switching of various size packages and other tasks can be handled without difficulty.
Pipelines: In a milk plant, liquids are conveyed from point to point by means of pipes that are joined together and attached to equipment by sanitary pipe fittings.Special metals are needed for the construction of pipe fittings coming in contact with dairy products in order to protect flavour and purity. Stainless steel sanitary pipe is now used throughout the industry. The size, length and joints of piping must be optimized in light of quantity of product to be pumped to desired destination in a given period of time.
Examination of Raw Milk
Raw milk has to pass through rigorous examination which may include organoleptic,physical and chemical tests to assess the quality of milk being taken rapidly, and decide for its acceptance or rejection. All these tests known as “platform tests” are performed on each can/tanker to assess the quality of the incoming milk before it is accepted and weighed. These tests must be easy to perform, give quick and reliable results and should not require complicated and elaborate equipment. The classification of milk on the basis of quality is usually referred to “grading of milk”. So grading of milk is done on the basis of platform tests which include organoleptic as well as preliminary tests.
Purpose: Milk received is further subjected to some sort of processing including heat processing for manufacturing products, and finally marketed to consumers. A good quality product cannot be made from bad quality raw ingredients. So, quality and composition of raw milk must be checked for freshness, hygiene and conformity to the legal standards. These can be accomplished by examination of raw milk through platform and other laboratory tests. Accordingly, the main purposes of examination of raw milk at reception dock are assessment of (i) freshness, (ii) hygienic quality, and (iii) composition.
Organoleptic tests: Organoleptic or sensory tests are performed with the help of five sensing organs, viz. eye, nose, tongue, ear and skin. The general appearance,cleanliness, colour, taste and smell of milk are tested before emptying the transport containers.Milk in can is evaluated while moving on the conveyor. Organoleptic tests are quick, cheap and with growing experience of the assessor, very reliable. Since practically no equipment is needed, the sensory test can be carried out anywhere and anytime. However, sensory tests are subjective and would depend on the assessor’s faculty of perception.
Smell or odour: Just after opening the lid of the container, sniffing will detect the smell/odour of the milk. Normal milk should not have any off-or unnatural smell in it. If the smell is slightly sour, the milk might have undergone microbial deterioration.
Appearance: After the odour test, milk in each can is observed for any floating extraneous matters, off-colour, or partially churned milk. Normal milk should be free from these. Presence of visible dirt, straw or manure indicates that milk has been handled in an unhygienic way. The colour of milk of cows and sheep should be slightly yellowish-white, that of buffaloes and goat absolutely white. If the colour is reddish, the milk may contain blood; if it is yellowish, it may contain pus, such milk may be secreted from infected udder (mastitis) and should not be accepted.
Taste: Taste of milk can be noted by taking a spoonful milk in the mouth and rotating inside with the help of tongue. When doubt exists concerning smell, the taste of the milk may help to determine sourness and if so, milk should be rejected.Adulteration of milk with salt, sugar, etc. can also be detected by taste and found positive must be rejected.
Preliminary tests: Preliminary tests as discussed below. These are simple and rapid physical-chemical tests which can be performed easily on the reception dock:
Clot-On-Boiling (C.O.B.): A small portion of milk is heated to boiling point in a test tube for checking whether it withstands heat treatment without clotting. If the milk clots on boiling it is sour or abnormal. This milk cannot be processed any more and hence it should be rejected.
Alcohol test: Milk with increased acidity or of abnormal salt balance or mastitis flocculates after addition of alcohol. This test is used to determine heat stability of milk. If no flocculation occurs after addition of alcohol, the milk is fresh or its acidity is only very slightly increased. If milk flocculates with alcohol, the milk is heat unstable and it should be rejected.
Titratable acidity (T.A.): The titratable acidity of milk is determined by titrating 10 ml of milk with N/9 NaOH solution to assess sourness in milk. The result is either expressed in degrees Dornic, i.e. ml of N/10 NaOH used being equal to 1 degree Dornic(Do) or in % lactic acid, whereby ml of N/10 NaOH used is equal to 0.01% w/v lactic acid. Milk testing more than 0.15% lactic acid should be rejected.
pH: The pH value can be measured with an electronic pH meter or with the help of different pH-indicators to assess the acidity of milk. Normal milk has pH ranging from 6.6 (in cow milk) to 6.8 (in buffalo milk). A higher pH (7.0 to 7.4) means milk from infected (mastitis) udders or it is neutralized by alkali. The major shortcoming of this method is its poor sensitivity since milk is extremely well buffered system,slight change in acidity or alkalinity cannot be detected.
Lactometer Reading: Lactometer is used for determination of specific gravity. However,test result obtained can be misleading due to variation in temperature and fat content in milk etc. This method is correct only when carried out at the specified temperature of milk.
Sediment test: The sediment test is used to check the visible foreign matter contained in the milk. Off the bottom sediment tester or barrel type sediment tester may be used. Test is carried out by allowing a measured quantity of milk (usually 500 ml) to pass through a fixed area of a filter disc and comparing the sediment left with the prepared standard (Table). Any hair flies, pieces of hay or straw or any large particles of dirt are not included in grading sediment. Presence of appreciable sediment indicates careless or insanitary dairy farm practice. However lack of sediment is not always indicative of ideal conditions, since visible sediment may be readily removed by straining at the dairy farm. It may have bacterial contamination.
Raw Milk Reception Dock (RMRD) is the primary entry point of liquid milk entry in dairies in India. It is specifically meant for the reception of milk brought in cans from villages located over a wide area. It is different from the bulk milk collection system, which are also in operation in these dairies. The collection process starts from the villages, groups of which are assigned to distinct Milk Truck Routes. The cans of individual villages from many such routes are unloaded at the RMRD, after which their milk is weighed & tested separately to issue a milk receipt statement according to the measured parameters during the daily shift.
Alizarin-alcohol test: Incorporation of alizarin in alcohol helps to determine both heat stability and approximate percentage of acidity in milk. Milk showing poor heat stability is rejected.
Tilting/Emptying of Milk Cans:
Acceptable milk in cans are lifted manually from the conveyor, rested on floor mounted can tipping cross bar padded with rubber and tilted to drain the milk into weigh bowl. When the can tipping device is used, the operator must hold the can until the milk drains from it and then convey the emptied can by hand to the washer via drip saver. For quick, easy and high speed, a dump grid, suspended over the weigh tank, is used on which the can drains and is automatically moved to the can washer.
From storage tanks/rail/road tankers: Method of sampling is governed by storage/transport conditions. So, no rigid procedure of sampling can be prescribed.However, a recommended procedure described below may be followed.
Milk is thoroughly mixed by using either a sufficiently large plunger or a mechanical agitator or compressed air till a complete agreement is obtained between samples taken at the manhole and the outlet cock with respect to Fat and SNF. Plunger is inserted through manhole and, pushed forward and pulled back, downward and back, and backward and back in cyclic order repeatedly for not less than 15 minutes. Sampling is done through the stopcock in the tank door or from a valve in the discharge line from the tank as it is being emptied.
Sampling of Milk
Samples may be drawn during reception of milk for chemical and microbiological analysis in the laboratory. Only representative sample, without any type of adulteration, dilution and contamination should be drawn using correct technique and appropriate device. While strict precautions regarding sterility of the stirrer,sampler, container, etc. are required for obtaining a microbiological sample and dryness and cleanliness of the above appliances should suffice for a chemical sample. Test cannot be accurate unless the test sample is truly representative of the product to be tested. Samples from milk containers may be drawn with the help of a suitable device. The characteristics of various devices have been listed in Table.
The raw milk is poured onto a pre-filter in order to eliminate the largest of any solid particles. The milk is stored in a tank specially designed for this purpose which has a capacity of 150 litres. This deposit tank has probes installed to detect both maximum and minimum levels. When the minimum level is detected the machine will start working. Another filter situated in the lower part of the unit undertakes a second elimination of solid particles, this time finer ones.
How do microorganisms enter the milk supply?
Our environment contains an abundance of microorganisms that find their way to the hair, udder, and teats of dairy cows and can move up the teat canal. Some of these germs cause an inflammatory disease of the udder known as mastitis while others enter the milk without causing any disease symptoms in the animal. In addition, organisms can enter the milk supply during the milking process when equipment used in milking, transporting, and storing the raw milk is not properly cleaned and sanitized. All milk and milk products have the potential to transmit pathogenic (disease- causing) organisms to humans. The nutritional components that make milk and milk products an important part of the human diet also support the growth of the organisms. Drinking raw milk causes foodborne illness, and dairy producers selling or giving raw milk to friends and relatives are putting them at risk. What are common pathogens in milk? Illnesses from contaminated milk and milk products have occurred worldwide since cows have been milked. In the 1900s it was discovered that milk can transmit tuberculosis, brucellosis, diphtheria, scarlet fever, and Q-fever (a mild disease characterized by high fever, chills, and muscular pains) to humans. Fortunately, the threat of these diseases and the incidence of outbreaks involving milk and milk products has been greatly reduced over the decades due to improved sanitary milk production practices and pasteurization.
Rapid and efficient milk cooling is essential for preserving milk quality. Milk leaves the udder at approximately 35°C and the heat in fresh milk must be quickly removed. Milk retains a natural resistance to bacteria immediately after extraction, but only rapid cooling to a storage temperature of around 4°C to 6°C, prevents or minimizes further micro-organism growth.
What is pasteurization?
Pasteurization destroys most disease producing organisms and limits fermentation in milk, beer, and other liquids by partial or complete sterilization. The pasteurization process heats milk to 161 degrees Fahrenheit (63 degrees centigrade) for 15 seconds, inactivating or killing organisms that grow rapidly in milk. Pasteurization does not destroy organisms that grow slowly or produce spores. While pasteurization destroys many microorganisms in milk, improper handling after pasteurization can recontaminate milk. Many dairy farms use a home-pasteurizing machine to pasteurize small amounts of milk for personal use. Raw milk can also be pasteurized on the stovetop. Microwaving raw milk is not an effective means of pasteurization because of uneven heat distribution. For more information on purchasing and caring for home pasteurization machines, contact your county Extension agent.
The process of heating EVERY PARTICLE of milk and milk products to the minimum required TEMPERATURE (for that specific milk or milk product), and holding it continuously for the minimum required TIME in equipment that is PROPERLY DESIGNED and
Pasteurization has also been described as a heat treatment or thermal process used to kill part but not all of the vegetative microorganisms present in the food.
The two main forms of industrial pasteurisation are:
Batch pasteurisation – where the product is held in a specific temperature range for a long time, e.g. for milk 63º C for 30 minutes
High temperature and short time or HTST pasteurisation – where the product is heated to a higher temperature but for a shorter time, e.g. 72 º C for 15 seconds for milk, using a plate heater exchange.
Standardization of milk refers to the adjustment which means rising or lowering of fat and solids not fat levels of milk. The standardization of milk is commonly done in case of marketmilk supply and also in case of manufacture of milk products. e.g. condensed milk, milk powder, ice-cream and cheese etc. the standardization is mostly done to have a uniform milk fat content in the finished dairy product. In India, milk is toned to 3 % fat or double toned 1.5 % by standardizing the fat content of milk and thus volume of milk is increased so that the milk can be supplied to the consumers at low price and to a large population.
Correct calculations by Pearson’s method regarding ingredients to be used for standardization.
1) Whole milk,
2) skim milk/cream
3) Butter oil
4) Skim milk
Is an entirely separate process that occurs after pasteurization in most cases. The purpose of homoginization is to break down fat molecules in milk so that they resist separation. Without homoginization, fat molecules in milk will rise to the top and form a layer of cream. Homoginizing milk prevents this separation from occurring by breaking the molecules down to such a small size that they remain suspended evenly throughout the milk instead of rising to the top.