Storage of Milk

i. 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.
•  Intermediate storage between processes.

For Transportation in bulk by road tankers

•  For transportation of pre-cooled milk from collection and chilling centres located in rural areas of production to processing plants located in urban areas of high density consumption.
•  For transportation in container service from process plant to bulk dispensing centres.
• For Transportation in bulk by rail tankers
•  For transportation of pre-cooled milk comparatively large quantity of raw milk from chilling centers located close to rail tracks to processing plants.
•  For transporting processed milk from area of high production to cities located at vast distances.

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.

 

ii. Storage Equipment

Guidelines Deciding Suitability of Storage Facilities: The following guidelines are observed while deciding the suitability of storage facility for specific purposes:

Modern dairy operations necessitate storage facilities to allow continuous(uninterrupted) operation, for extended period of time, during processing. The type of storage facility to be provided depends on various factors. When large volume of milk is processed, the present trend is to install Silo type of tanks of capacity as large as 1,00,000 liters each. This will reduce the cost of building, capital cost of tanks as well as operation, cleaning and maintenance costs, since the number of unit required for any desired capacity is considerably few. The total capacity of raw milk storage tanks will be, under normal circumstances, equal to daily intake at the plant. With a coordinated programme of rate and time of receiving at the plant, and the rate of processing, some raw milk storage capacity could be reduced.Storage facilities at intermediate stages of processing such as for manufacturing operations like separation, standardization, clarification, products manufacturing as ice-creams, butter, cheese, evaporated milk, milk powder, etc. have to be studied for each stage independently. While deciding on the number and size of tanks, for each facility, the length of storage time as well as the time required to clean and sterilize the tank before the next filling starts, has to be taken into consideration.Installing standard sizes, storage for different products, helps in flexibility with respect to alternative use and future expansion. The horizontal or the vertical design consideration must include economy of construction and housing, suitability and space availability.

Material of Construction: The material of construction of the storage equipment and road/rail milk tankers should conform to IS:739-1965. All product contact surfaces must be made of non-corrosive & non-reactive materials such as stainless steel (SS-304 or 316 grades) or aluminum or its suitable alloys. Other parts not coming in the direct contact must be decided based on requirements like appearance,heat transfer properties, strength and surrounding environment. Suitable fittings like ladder (internal and external) for inspection and cleaning should be provided. Suitable provision for milk inlet and out-let along with looking glass or manhole for manual cleaning or maintenance should be considered. The appropriate quality of gasket also needs considerations. The internal ladder could be made of stainless steel. The road /rail milk tankers must have provision of enclosures and locking of valves to prevent theft or sabotage.

Design Considerations: There has been in recent years, an increasing change away from open vats or vats with lids to closed cylindrical tanks. Vats previously in general use, which were rectangular, round or oval, are today used only in some instances such as in milk reception and more commonly in the cheese manufacturing plants. Cylindrical tanks are used either in an upright position or horizontally depending upon the space available. To compensate for irregularities in the floor, when tanks are erected, they are fitted with adjustable legs with levelling screws.

Today tanks, vessels or vats are made almost invariably of stainless steel. Aluminium has largely been replaced in the vessel construction industry. To be suitable for use in the dairy industry, the surfaces of vessels which are in contact with product must be smooth and, the welds in particular have to be made flush and polished during manufacture. Sharp angles where bases and sides meet in flanges and pipe inlets must be avoided. In vessels, corners which connot be properly cleaned must also be avoided. The specific design considerations for cans, tanks and tankers have been discussed below:

Milk can: The centre of gravity of full cans should be conveniently placed. It should be made rollable. The angles of shoulders of the can should allow quick and easy drainage of contents and provide enough strength along with ease of cleaning and inspection. The can should be fitted with handles to assist easy handling by workers. The neck width should allow good stability to hold it up-right in inverted position. Therefore, the design consideration should have angle of balance when upright of 30 degree, spill point greater than 30 degree, shoulder angle 45 degree and angle of balance when inverted 15 to 20 degree with respect to vertical surface.

Storage tanks/vessels: Inner shell is usually made of stainless steel or aluminium alloys with all welded points conforming to IS:2812-1964. Joints are well dressed and ground, smooth and free from visible porosity and brittleness. All inside surfaces are made to drain directly to outlet nozzle. Bottom slope is 1 in 15 to horizontal for vertical tanks and 1 in 12 for horizontal tanks, both in the longitudinal and transverse directions. Internal corners should have a radius of not less than 25 mm. Thickness of plate for shell wall and bottom plate vary according to size of tank and its capacity. A minimum 2 mm thickness for shell and 5 mm thickness for bottom plate is considered for lowest capacity tank/vat.Jacketed space surrounds the inner shell and used for circulation of steam, hot water, chiller water, brine or direct expansion refrigerant for heating in case of vats and for cooling as well as for maintenance of low temperatures of products in storage.

Insulation layer surrounds the inner vessel for ordinary vessels that are not refrigerated or surrounds the outer wall of jacketed space and is provided by vapour barrier on the outer surface. Outer shell surrounds the insulation layer. The material of this shell may be stainless steel, aluminium, galvanized iron or mild steel of not less than 3 mm thickness provided with vent hole to drain any condensation of moisture inside.

Road/Rail Tankers: These are made with or without compartments to transport different quality milk and also reduce churning and foaming during transportation.Maximum number of compartment per tanker is three. If a tank is made with compartments, the baffles require special care in installation and routine cleaning.Inner vessel of stainless steel of not less than 2 mm and outer shell of mild steel of not less than 3 mm thickness is considered in normal design.

Capacity Determination: Capacity of a storage tank is the volume of milk holding when filled to a level of 60 mm below top edge of inner vessel. Determination of capacity of a storage tank is dependent upon the products to be processed and milk operation procedure. This facilitates continuous processing of product, once the operation starts. Adequate storage facilities help independence of unloading of tankers from processing of milk.

 

iii. Parts of Storage Tank

Lid or manhole: Usually located at top or side portion of tank. In case of over head tank, manhole is located at the front end (in-side the process room). In silos,it is positioned near the bottom at suitable working height or at top. Usually the manholes are made of minimum 400 mm diameter, round or rectangular in shape and fitted with gasket around rim.

Product inlet: Inlet provision is made for pouring /loading the product through a bend from top of tank to the side so that milk will gently flow down the sidewall.In vertical silos inlet is given at the bottom to prevent foaming and air incorporation.

Product outlet: Outlet is fitted at the lowest point of bottom and installed to flush and provide proper drainage of product from the tank. In vertical silos, inlet also serves as outlet. Flanges type openings of minimum 50 mm diameter with bend of at-least 55 mm radius are considered most suitable.

Indicating thermometer: Pocket for indicating thermometer of range upto 40°C is provided at least 15 cm above the bottom of inner shell. Remote indicating and automatic temperature control and recording system may be installed for large size tanks and vats.

Opening: An opening is provided at the top of tank for chemical cleaning device of minimum 38 mm size.

Air vent: Air vent at top of tank is provided with screen to prevent entry of foreign materials and of sufficient size to prevent build up of pressure during filling and vacuum condition during rapid draining.

Agitator: Mechanical or pneumatic type agitator devices are provided to prevent cream-line formation during holding. It improves cooling efficiency and maintains uniform temperature and composition. It may be mounted on the lid of the tank near the manhole to facilitate dismantling for cleaning and inspection. Mechanical agitation device consists of motor driven agitator made of stainless steel, whereas pneumatic type has devices for introducing pressurized, clean and moisture/ oil free air into the silo.

Sight and light glass: One each with clear glass and diameter of not less than 100 mm is provided, effectively sealed to prevent ingress of foreign materials from the surroundings. These are shaped and mounted in such a way that inner surfaces drain inversely and is easily removable for cleaning and inspection. Each of these is mounted on opposite ends of top cover.

Sampling cock: size 50 or 63.5 mm

Measuring scale or dip-stick: Made of stainless steel and graduated to tank volume.

Ladder: Ladder, made of stainless steel or aluminum, is used inside and/or outside.It could be made of mild steel for external use.

High and low level cutoff: Large tanks are provided for automatic stoppage of pump motor, together with alarm bells and indicating lamps at the control panel to prevent accidental over filling or low level at outlet and consequent damage to pump material due to dry running.Like storage tanks, the Road/Rail tankers also have provisions of lid or man-hole, vent, inlet connection and ladder for their specific purposes. Outlet is provided at the rear end of a road tanker, whereas in rail tanker it is at the centre of compartment.
Each compartment may have separate outlet. The tanker alignment has sloping towards the outlet point for easy drainage of content. The outlet is provided with two way valve and its size is normally 63.5mm. Preferably flanged on tank end and union connection on the outlet end for hose connection is provided. Other special fittings include:

•  Housing for valve: Stainless steel box with locking arrangement is provided for the safety during transportation.
•  Rear Cabinet: Provision of rear cabinet is made to accommodate milk pump,metering equipment, sampling cock, unloading device and emergency spares such as gaskets, etc.

iv. Cleaning and Sterilization

For effective cleaning and sterilizing, the quality of the construction, finish and installation of equipment is of primary importance. Places such as dead ends, bends,etc. must be dismantled and cleaned manually. It is to be ensured that cleaning/sanitizing solution is not trapped in the equipments and pipe lines. Specific methods for cleaning and sterilizing of storage equipments have been discussed below:

Cans: Immediately after emptying, milk cans are turned upside down on a conveyor or rack and transported through several treatment stages such as drain, pre-rinse,caustic rinse, hot water rinse, hot air drying /steam sterilization and cold air treatment.The liquids are jetted into the can from below. Empty cans are also cleaned manually, steam sterilized and allowed hot drying.

Tanks, vats and vessels: Small tanks and vats can be cleaned manually. After a pre-rinse with lukewarm water, manual cleaning by hand brushes is done. Since there is inadequate lighting and soiling matter removal is dependent on the judgement of individuals, manual cleaning is not so effective. The spray lance of 1m to 1.5m long is directed by hand to the appropriate places of cleaning.

With the development of automatic Cleaning-in-Place (CIP), efficient cleaning and sterilizing results are obtained even in large size vessels. Especially designed spray arms and nozzles (turbine or ball spray) are normally used to ensure uniform spraying of detergent and sanitizing solutions over the surface in programmed manner.

Road/Rail Tankers: In addition to the manual cleaning and sanitization of road/rail tankers, programmed C.I.P. with spray ball or rotating turbine type arrangement for each compartment is used. For 20,000 L tankers, two spray balls and for 40,000 L, four spray balls are provided.