Market Milk

Module 6. Common dairy operations

Lesson 17
CHILLING AND STORAGE OF MILK

17.1 Introduction

The first operation in a dairy plant is reception, chilling and storage of milk. Raw milk is pumped from the dump tank to the storage tank through a filter and chiller. The purpose of storage tank is to hold milk at low temperature so as to maintain continuity in milk processing operations and prevent any deterioration in quality during holding and processing period.

The milk may arrive at a chilling center or dairy plant in cans. After unloading the cans, milk is chilled and stored in storage tanks. Storage tanks are used to store raw or even pasteurized milk. Milk may be held in chilled condition (< 5°C) in the tank for up to 72 hours between reception and processing. Normally the milk storage capacity should be equivalent to one day’s intake.

17.2 Objectives of Storage Tanks

• 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, which will ensure uniform composition

• To allow for uninterrupted operation during processing and packaging

• To facilitate standardization of the milk

17.3 Storage Tank

Storage tanks enable milk to be stored for longer period of holding. They must be designed for easy cleaning and sanitization, preferably through CIP process. Storage tanks consist of a stainless steel inner shell, a layer of insulation, an outer jacket and necessary fittings for inspection control and cleaning. The tanks should be insulated or refrigerated so that they can maintain the required temperature throughout the holding period. Glass wool, Thermocol, Corkboard, Foam glass or Styrofoam can be used for insulation. Corkboard or foam glass is used in the lower portions of the tank where the insulations may carry a part of the load. Agitation must be adequate for homogeneous mixing, but gentle enough to prevent churning and incorporation of air. In many storage tanks, chilled water circulation system is provided to maintain the temperature of milk. All closed type of tanks must be equipped with a manhole round (diameter ~ 450 mm) or oval shaped to permit access to the interior for cleaning and inspection.

For foam-free entry of milk, a curved filling pipe, which guides the milk towards the wall is used. It is better to fill the tank from below, i.e., by the lowest outlet pipe. The storage tanks containing raw chilled milk or standardized pasteurized milk are usually located on the first floor. This allows feeding to the milk pasteurizers or even gravity filling of milk. Now-a-days, big sized silos, usually of > 1.0 lakh litres capacity are installed in the dairies on the ground floor only. They are very useful in storing skim milk for feeding to the powder plants. In the dairy industry, rectangular tanks are less preferred as compared to cylindrical tanks, because cleaning of sharp corners (in rectangular ones) is difficult. Secondly, the agitation effect does not reach the extreme corners of rectangular tanks.

17.4 Types of Storage Tank

Storage tanks can be classified on the basis of their shape and other features.

17.4.1 Insulated storage tanks

These tanks merely stores the milk at a temperature at which it is filled. In most cases, depending upon the quality of insulating material, there is tendency of rise in the temperature of milk with long storage. These tanks are made up of a stainless steel inner shell, a layer of insulation (thermocol and glass wool) and an outer jacket of stainless steel or mild steel ( Fig.17.1, 17.2)

17.4.2 Refrigerated tanks

It has built-in refrigerating facilities so that stored milk is chilled as and when required. This additional feature of maintaining the desired temperature is an added advantage in these tanks. In refrigerated tanks, the hollow space between the inner and outer shells is used for circulating the cooling medium (chilled water or brine solution).

Fig. 17.1 Horizontal milk storage tank

Fig. 17.2 Vertical milk storage tank

17.4.3 Horizontal or vertical tanks

Horizontal tanks require more floor space than vertical ones, but need less headspace. For handling small volumes, horizontal tanks (5,000 to 15,000 litre capacity) may be used. Now-a-days, milk is stored in vertical storage tanks of one lakh litre capacity or more, commonly known as silos. These are vertical cylindrical tanks, installed outside the building. In these silos, milk feeding is from same discharge valve installed near the bottom (Fig 17.3).

Fig. 17.3 Sectional elevation of insulated milk silo

17.5 Chilling Equipments

17.5.1 Surface cooler

It can be either an individual unit or cabinet type. The latter consists of two or more individual units, compactly assembled and enclosed in a cabinet. It is usually larger than those used on the farm/chilling centre.

17.5.2 Plate chiller

It is widely used for large scale cooling of milk of 5000 to 60,000 lit./day at the chilling centers. They are efficient, compact and easily cleanable. In chiller the gasket plates are tightly held between the plates. These plates are so arranged that milk flows on one side of plate and cooling medium (usually chilled water) on the other. There is a counter current flow between the milk and the chilled water through alternate plates. It helps in efficient transfer of heat 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 (IBT) is needed.

17.5.3 Internal tubular cooler

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

17.5.4 Vat/tank cooling

It is suitable for batch cooling, especially of small quantity. It consists of a tank within the tank, with the space between the two being used for circulation of the cooling medium, by either pump or main pressure. An agitator is provided to agitate the milk for rapid cooling.