RMRD OF MILK

                  Reception of Milk

Reception includes following operations: Unloading, grading,sampling,testing, weighing or measuring and recording.

Unloading

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.

GRADING :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.

Organoleptic tests: Organoleptic or sensory tests are performed with the help of five sensing organs, viz. eye, nose, tongue, ear and skin. The general appearnce,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.
Temperature: With practice, the grader can judge the temperature of milk with a high degree of accuracy by touching the container. Abnormal to the weather or condition may indicate the prehistory of milk with respect to its cooling or heating,etc.
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 test: 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 correct temperature and combined with milkfat test.
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.

Quality of Milk by Sediment Test

Quality of Milk by Alizarin-Alcohol Test

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.

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.

Characteristics of Milk Sampling Devices

Sampling methods:

Sample should be taken after thorough mixing of milk with the help of plunger or sampler (devices) or some other means.

i) From a small handy batch: Milk is properly mixed before taking sample.

ii) From a large batch:

Milk in can or vat or small tank can be mixed by using a plunger manually. Minimum ten times from top to bottom plungering will ensure adequate mixing before drawing sample. Mechanical stirring can also be done wherever facilities available. Vigorous stirring is avoided as milk may get churned at temperatures between 26.5 and 29.5 0C.

iii) From several containers of different size, shape and type:

If a large vat is available, milk of all the containers are mixed in vat and a sample is drawn.

iv) From bulk units:

Nos. of cans filled from storage tank/bulk unit is not required to be sampled individually. The number of random cans to be sampled shall be as follows:

 v) 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

Milk of initial good quality alone will give products of better quality. Moreover, the payment for milk is based on its constituents such as fat and solids-not-fat (SNF) contents. These tests are conducted in the laboratory.

 i. Chemical Tests

Various chemical and physical tests conducted to assess the quality of milk received in a dairy include titratable acidity, fat content, total solids and solids-not-fat content,and tests for detection of adulterants.

Titratable Acidity: Acidity of milk is expressed in term of its lactic acid content.10 to 50 ml of milk sample is titrated with N/9 or N/10 NaOH solution and phenolphthalein as indicator. Normal acidity of fresh milk varies between 0.10 to 0.16 % lactic acid. The titratable acidity (T.A.) as lactic acid per 100 ml of milk is calculated by using formulae as under:

% Lactic acid = 9 * V1 * N1 / V2

where, V1 = Volume of standard NaOH solution, ml

N1 = Normality of standard NaOH solution

V2 = Volume of milk sample taken, ml

Fat Test: Fat test of milk is done for making payment of the milk. Among several methods, one common method is the acidobutyrometric butterfat test or Gerber test. Fat globule membranes and proteins of the milk are hydrolyzed with concentrated sulphuric acid to break the emulsion and to set the fat free. The volume of fat from a given quantity of milk sample is measured in a specially designed glass recipient, known as butyrometer. The butyrometer reading gives the result directly in fat percentage. For accurate results, the reading has to be taken quickly to avoid cooling of the fat column.

Now, Milko-tester Minor or semi-automatic butterfat analyzer is used for determining the fat content in milk. This is based on the principle of scattering of light by milk fat globules. Light transmission through the milk-mix is measured photometrically and read directly as the fat percentage in milk samples on digital display. Accuracy of the result is approximately the same as for the Gerber fat test.

Determination of total solids (TS) and Solids-not-fat (SNF) content: A lactometer is used to measure the specific gravity of milk. Based on the lactometer reading of milk taken under standard conditions and knowing the fat percentage as determined by the Gerber method, it is possible to calculate TS and SNF by using the well known Richmond’s formula which differs for the types of lactometer, viz. Quevenne, Zeal-and BIS-lactometer.

(i)Quevennelactometer:
% TS = CLR/4 +1.2F + 0.14
% SNF= CLR/4+0.2 F + 0.14

(ii)Zeallactometer

% TS = CLR/4 + 1.2 F + 0.50
% SNF= CLR/4+0.2 F + 0.50

(iii)BISlactometer

% TS = CLR/4 + 1.2 F + 0.60
% SNF= CLR/4+0.2 F + 0.60

Where, CLR = corrected lactometer readings, obtained by applying the specific correction factor to the observed lactometer readings based on temperature of milk.

Determination of TS and SNF of milk can help to detect the adulteration of milk.TS and SNF also form the basis for the pricing of milk.

 

ii. Microbiological Tests

Microbiological tests provide information on the sanitary condition and keeping quality of milk. The tests are intended to be carried out on samples collected for microbiological analysis. Microbiological standards for cow milk have been presented in below Table.

Microbiological Standards for Cow Milk

Methylene blue and Resazurin reduction tests measure the bio-chemical activity of microorganisms in milk. They get reduced, if added to milk, after a certain time and lose their colour. Quick reduction of a given quantity of dye means high microbiological activity and vice versa.
Half-hour methylene blue reduction (MBR) test: The length of time taken by milk to de-colourize methylene blue is a fairly good measure of its bacterial content.Ten ml milk in sterile test tube is well mixed with one ml methylene blue solution.Tube is closed with sterile rubber stopper and incubated in a water bath maintained at 37.5 + 0.5oC. Tube is inspected after 30 minutes and seen whether milk in the tube is de-colourized at least up to within 5 mm of the surface. Milk is graded as in above Table.

Ten-Minute resazurin test: Since resazurin reduction occurs in two stages, the first from blue to pink, and the second from pink to colourless, the quality of milk can be assessed easily in shorter time. Procedure is the same as described in MBR test. At the end of 10 minutes + 30 second of incubation, the tube is removed from the water bath and immediately the colour is matched with the resazurin disc in the comarator. The result shall be interpreted as follows:

 Direct Microscopic Count (DMC): The direct microscopic count method consists of examination of milk under a compound microscope. It enables the rapid estimation of the total bacterial population of a sample of milk and also reveals useful information for tracing the source of contamination in milk.

 Weighing/Measuring and Recording of Milk

The milk in cans is dumped into the weigh tank/bowl, of single or double compartment,either manually or mechanically. The stainless steel weigh bowl of 250 or 500 Kg capacity is attached to a circular dial weigh scale and the portion towards the outlet valve overhangs with anti-splash stainless steel strainer. The outlet valve of weigh bowl is suited for manual or air actuated operation. The operator makes direct reading of the weight of the milk on the scale and records it separately for individual suppliers. Automatic printing of weight is also possible. The milk in tanker (road/rail) may be measured in volume by passing it through a flow-meter or in weight by using a weigh bridge where tare weight of the tanker is deducted from gross weight of it.

Storage of Milk

Purpose

The purpose of storage of milk at different operational points is described below:
At Production/Collecting/ChillingCentre:

•  For pooling and bulking from small producers until it is transported to chilling centres/processing plants.
•  For chilling and holding at chilling centres until picked up by tankers.
• To handle milk from large producers.

At Processing Centre

•  For storing raw milk as received from chilling centres till taken up for processing.
•  For storing processed milk till taken up for packaging or bulk dispensing.
•  For storing milk products in liquid state during manufacturing.

Modern dairy plants hold both raw and pasteurized milks. Normally the milk storage capacity is equal to one day’s intake. So, large milk tanks & silos are being installed at dairy plants with the following objectives:

• To maintain milk at a low temperature so as to prevent any deterioration in quality prior to processing /product manufacture.
•  To facilitate bulking of raw milk supply to ensure uniform composition.
•  To allow for uninterrupted operation during processing.
•  To facilitate standardization of milk.

•  Intermediate storage between processes.

Types of Storage Materials :

Insulated or Refrigerated : In the former, there are 5 to 7.5 cm. Of insulating material between the inner and outer linings; in the later , the space between the two linings is used for circulation of the cooling medium. Another variation of the refrigerated type is the cold wall tank.

Horizontal or Vertical : While the former requires more floor space and less head space. Modern circulation cleaning methods have made very large vertical storage tanks practical.

Rectngular, Cylindrical or Oval :

Of these , the first suffers from the disadvntage of having dead corners during agitation, while the other two do not.

Parts of a Storage Tank :

• Sight Glass
• Light Glass and Lamp
• Ladder
• Manhole
• Agitator
• Outlet Valve
• Inlet
• Air vent
• Safety Valve
• Legs
• Indicating Thermometer
• Volume Meter.

Filtration and Clarification of Milk

i. Purpose

Raw milk as produced on the farm and transported to the collection centre or a dairy plant generally contains varying amounts of visible, invisible impurities. This foreign matter includes straw and hair pieces, dust particles, leukocytes (somatic cells or white blood cells), insects, etc. If not effectively removed, such extraneous insoluble matter can result in deposits in milk handling equipment such as cooler,etc., and, more importantly, cause unsightly appearance.

Relatively large pieces of such material e.g. straw, hair and insects, are usually removed by ‘straining’ (passing the milk through a fine metal–gauge strainer or metallic sieve on the farm, at the collection centre or at the processing plant.Tubular sieves located in the milk inlet pipe to the processing unit (e.g. pasteurizer) are also used.

However, finer foreign matter to be eliminated requires clarification using a special filter or a centrifuged clarifier. These steps of aesthetic improvement of product are particularly useful for overcoming the problem of sediments in fluid milk and liquid milk products in general, and homogenized milk in particular.

ii. Filtration

Filtration (or, clarification using a filter-bag) refers to making the milk pass through a filter-cloth or filter-pad. The filtering medium has a pore size (25-100 mm) that permits most of the foreign matter to be retained on it. The milk filter consists of a nylon filter-bag or a filter-pad supported on a perforated stainless steel (SS) support held in an SS enclosure with a tight-fitting lid, milk distributor, and inlet- and outlet- connections. Milk usually passes from top to bottom. In case of twin filters,three way valves in the inlet and outlet lines enable switching from one filter to the other when the first is to be cleaned. Sometimes, filters may be provided in the form of cylindrical bags or ‘stockings’ fitted over perforated SS tubes as in the modern continuous pasteurizing plants (high-temperature short-time, or HTST pasteurizers

Filtration can be carried out either on cold milk (about 10oC) or warm milk (40-45oC). Since warm milk filtration is more rapid due to lower viscosity of warm milk,it is universally used. For cold filtration, the filter is located in the line connecting the milk receiving tank or holding tank and the pasteurizer. Since warm filtration requires preheating, the filter of this type is placed between the regenerator and the final heating section of the HTST pasteurizer.

The filter-bag must periodically be cleaned. Accordingly, the operation run may vary from 2 to 10 hours depending on the level of foreign matter and the filter pore size. Generally, twin filters located in parallel are employed to permit cleaning of one filter while the other is in use. This enables continuous process run.We should be able to realize that filtration removes only the gross impurities, and does not remove bacteria from milk. Accordingly, it does not improve the keeping quality of the milk. In fact, bacteria may grow in the filters if they are used for unusually long times before cleaning.

 

iii. Clarification

Definition and objective : As an alternative to filtration, clarification can also be employed to remove insoluble impurities especially the finer ones. It involves the use of a centrifugal machine called ‘clarifier’. Thus, clarification is a process of subjecting milk to a centrifugal force in order to eliminate the finer but heavier particles from milk, somatic cells, dust particles, etc. Although part of bacteria are also removed along with the extraneous matter, clarification cannot be considered an effective means of bacteria removal. Hence, one should be aware that it cannot be a substitute for a suitable heat treatment in order to ensure safety against pathogenic (disease-causing) microorganisms.

Principle of clarification : As we have studied, when milk is introduced between two adjacent rotating conical discs (in a stack of several discs) of a centrifuge bowl, it is subjected to a centrifugal force. This force causes the heavier dirt particles to be thrown out into the sludge space surrounding the discs where it is collected during the run, while the comparatively lighter milk continuously flows inward and upward to the outlet. There is no separation of fat globules (cream) and skim milk in a clarifier.

Operation of a clarifier : Raw milk is made to pass usually under a pump pressure, down a central pipe of a rotating bowl and led to the outer edge of the clarifier discs through a distributor in the bottom and then onto the spinning discs,where milk and dirt are separated. The milk is led to the discharge port at the top of the bowl whereas the dirt is accumulated in the sediment space. The accumulated sludge is removed from the bowl by dismantling the clarifier at regular intervals.The interval may range from 1 to 8 hours depending on size of the clarifier and the amount of impurities in the milk. However, most large-size modern clarifiers are self-desludging or ‘partial desludging’ type in which periodical sludge removal takes place during the clarification process, without interruption of the clarifier operation.Such desludging results in about 0.05-0.10% of milk being lost and the sludge being liquid rather than solid as in the non-self-desludging machines.As for the milk filter, clarifier may be located in the raw milk line between the raw milk tank and pasteurizer. Alternatively, milk may be clarified warm/hot by placing the clarifier at a suitable point in the regeneration section of the HTST unit or between the regeneration and heating sections.The clarifier sludge or clarifier ‘slime’ consists primarily of dust and dirt particles,blood cells, microorganisms and milk protein. Its composition will depend on whether it is liquid (82-86% water, 6-8 % protein), or solid (65-69% water, 24-28% protein).