Module 4. Procurement of milk

Lesson 7

7.1 Introduction

Milk leaves the udder at body temperature of about 38°C. The bacterial load may grow rapidly and bring about curdling and other undesirable changes if milk is held at the ambient temperature. Freshly drawn raw milk should be promptly cooled and held at 4°C till processing to preserve it against bacterial deterioration.

7.2 Importance of Chilling of Milk

Normally milk contains bacteria coming from the animal’s udder, milk vessels and handling persons. When the milk leaves the udder, bacteria grow well at the ambient temperature (20-40°C) and milk starts deteriorating. Bacterial growth factor goes down to 1.05 at 5°C and 1.00 at 0°C. Critical temperature for bacterial growth is 10°C. The growth factor at 10°C is 1.80 which rises to 10.0 at 15°C. Hence freshly drawn raw milk should be promptly cooled to 5°C or below and held at that temperature till it is processed.

7.3 Methods of Chilling

7.3.1 Can immersion

The milk from pails is poured directly into cans through a strainer. The cans of milk are gently lowered into a tank holding cold water. The water level in the tank should be lower than the level of milk in cans to prevent water entering into the milk. In this method, a much smaller refrigeration unit is needed. The cans are kept cooled at the desired temperature (5-7°C) and the capacity of the unit is 200-280 litres of milk.

7.3.2 Surface cooler

The milk is distributed over the outer surfaces of the cooling tubes from the top by means of a distributor pipe and flows down in a continuous thin stream. The cooling medium mostly chilled water is circulated in the opposite direction through inside of the tubes. Cooled milk is collected below in a receiving trough, from which it is discharged. Advantages
  • Transfers heat rapidly and efficiently
  • Relatively in-expensive
  • Aerates the milk and thus improves the flavour Disadvantages
  • Requires constant attention of flow rate.
  • Greater chances for air-borne contamination
  • Cleaning and sanitation is not very efficient.
  • There is slight evaporation loss.
7.3.3 Immersion cooler

Evaporating unit of a refrigeration unit is submerged directly into cans. Evaporator coil is fitted with an agitator. Milk is agitated for quick and proper transfer of heat from milk to refrigerant.

7.3.4 Rotor freeze

In this system, evaporating unit cools the water which in turn cools the milk in can. Several cans of milk can be cooled at a time. The milk cans are placed over the water tank and connected with chilled water circulation system which has specially designed can covers that are attached with chilled water pipe.

7.3.5 Cabinet cooler

It has a series of surface coolers installed close together in a vertical position. Capacity of cabinet cooler to cool the milk depends upon the number of sections in surface coolers. This type of cooler requires very small floor space for installation.

7.3.6 Bulk milk cooler

Bulk tank coolers are run by mechanical refrigeration system which cools the milk rapidly. These coolers maintain the temperature automatically during storage. Milk can be poured directly from milking pails into the tanks. This method is suitable for handling 500-2500 L milk/day. It is widely used at village level milk collection centers in India. From the Bulk milk cooler (BMC), the milk is pumped to the insulated tankers for transportation to dairy plants. The BMC uses horizontal or vertical cylindrical tanks with inner jacket and insulated body on the other side. There is provision of inner shell of the tank or direct expansion refrigerant coil for cooling. Milk is directly poured into the tank or pumped into the tank. Milk remains in contact with the inner shell of the tank cooling it to 4°C. The agitator is provided for uniform cooling.

7.3.7 Plate chiller

It is widely used for large scale cooling of milk (5000 to 60,000 L/day) at the chilling centers. They are highly efficient, compact and easily cleaned. In chiller, the gasketed plates are tightly held between the frames. These plates are so arranged that a flow passage for milk exists on one side of plate and chilled water on the other side. There is a counter- current flow between the milk and chilled water through the alternate plates. It helps in efficient transfer of heat from the milk to the cooling medium resulting in quick chilling of milk. The chilled milk flows from the plate cooler to the insulated storage tank at 4°C. A mechanical refrigeration system with Ice Bank Tank - IBT is needed.

7.3.8 Internal tubular cooler

It is a continuous cooling system consisting of a stainless steel tube of about 2.5 – 5.0 cm in diameter surrounded by a similar tube, forming a concentric cylinder. Several such tubes may be connected in series to obtain sufficient cooling. The cooling medium flows in opposite direction to the milk flow.

7.3.9 Vat/tank cooling

For batch cooling, small volume is desirable. It consists of a tank within the tank, with the space between the two being used for circulating the cooling medium by pump. An agitator is provided for efficient agitation.

7.4 Milk Chilling Centre

Due to scattered milk production by the small farmers and lesser number of organized dairy farms, the milk chilling centers are the alternative solution to the collection and chilling of milk.

7.4.1 Objectives
  • To preserve the quality of raw milk supplies.
  • To enable transportation of milk to the dairy plant without spoilage.
7.4.2 Location

This is guided by:

  • Adequate milk production
  • Adequate potable water supply
  • Proximity to a good road or railway station
  • Electricity supply
  • Sewage disposal facilities
7.4.3 Major items of equipment
  • Milk weighing tank/ pan and electronic weighing balance
  • Drop or dump tank with cover
  • Can washer
  • Milk pump
  • Plate cooler
  • Storage tank
  • Refrigeration unit
  • Cold room
  • Milk testing unit
7.4.4 Operational procedure

On arrival of the milk to the milk chilling center, the milk is graded for acceptance/payment, weighed, sampled for testing, cooled and stored at a low temperature (5°C) till dispatch to the processing dairy plant.

Last modified: Tuesday, 9 October 2012, 9:49 AM