MARKET MILK

Module 6. Common dairy operations

Lesson 31
STERILIZATION - DEFINITION, PURPOSE AND METHODS

31.1 Introduction

According to Food Safety and Standards Rules-2011, the term ‘sterilization’ when used in association with milk, means heating milk in sealed container continuously to a temperature of either 115°C for 15 min or at least 130°C for a period of one second or more in a continuous flow and then packed under aseptic condition in hermetically sealed containers to ensure preservation at room temperature for a period not less than 15 days from the date of manufacture.

Ultra–high temperature processing has been designed to give a commercially sterile product which is free from pathogens and provides little chance of spoilage during transportation and storage under recommended conditions. In contrast to pasteurized milk which may or may not be homogenized, it is essential to homogenize UHT milk so as to prevent formation of a cream plug during extended storage. Various systems exist for the production of UHT milks, each using a different method of heat transfer.

31.2 Definition

Sterilized milk may be defined as (homogenized) milk which has been heated to a temperature of 100°C or above for such lengths of time that it remains fit for human consumption for at least 7 days at room temperatures.

Commercially sterilized milk is rarely sterile in the strict bacteriological sense. This is because the requirements for complete sterility conflicts with the consumer's preference for normal color and flavor in the product. The spore-forming bacteria in raw milk, which are highly heat-resistant, survive the sterilization temperature-time employed in the dairy and ultimately lead to the deterioration of sterilized milk.

31.3 Requirements

Sterilized milk must:

i). Keep without deterioration, i.e., remain stable and be of good commercial value for a sufficient period to satisfy commercial requirements;

ii). UHT processing can be applied to those milk and milk product which can also be processed using HTST process.

iii). Be free of microorganisms harmful to consumer health, i.e., pathogenic, toxicogenic germs and toxins;

iv). Be free of any microorganisms liable to proli¬ferate, i.e. it should not show signs of bacterial growth.

31.4 Method of In-bottle Sterilization

Fig. 31.1 In-bottle sterilization

The raw milk, on receipt, should be strictly examined by the prescribed physico-chemical and bacteriological tests and only high-quality milk should be used for production of sterilized milk. Care should be taken to accept milk supplies which have no developed acidity and which contain the least number of spore-forming bacteria. The intake milk should be promptly cooled to 5°C for bulk storage in order to check any bacterial growth. Next, it should be pre-heated to 35-40°C for efficient filtration/clarification, so as to remove visible dirt, foreign matters etc., and to increase its aesthetic quality. The milk should again be cooled to 5°C so as to preserve its quality. It should then be standardized to the prescribed percentages of fat and SNF content in order to conform to the legal standards (which may vary from State to State for both cow and buffalo milk). It must be stored at 5°C until processing. The milk should be promptly pre-heated to 60°C for efficient homogenization to prevent any subsequent formation of a cream layer; usually single-stage homogenization is carried out at 2500 psi pressure. The homogenized milk must be clarified so as to remove the sediment formed during the homogenization process. The hot milk from the homogenizer should be filled into the (hot) cleaned and sanitized bottles coming from the bottle washing machine and then sealed with special caps (of the crown seal type). The filled and capped bottles should then be placed in metal crates for sterilization by the Batch Process, or fed into conveyors for the Continuous Process. Usually the milk is sterilized at 110 - 118°C for 15-25 minutes. The sterilized milk bottles should be gradually cooled to room temperature. Any sudden cooling may lead to bottle breakage. Finally the milk-in-bottles should be stored in a cool place.

31.4.1 Advantages

i) Remarkable keeping quality; does not need refrigerated storage

ii) no cream layer/plug

iii) forms a soft digestible curd, and hence useful for feeding of infants and invalids

iv) distinctive rich flavor

v) economical to use

vi) less liable to develop oxidized taints

31.4.2 Disadvantages

i) increased cost of production

ii) more loss in nutritive value than pasteurization (50 per cent of the vitamin C and 33 percent of vitamin B originally present, are destroyed, and there is a slight reduction in the biological value of the milk proteins)

iii) Gerber test by normal procedure not so accurate.

31.5 Types of Sterilization Process

31.5.1. Batch process (In-bottle sterilization)

Milk is first subjected to platform tests, clarified and then standardized. It is then pre-heated and homogenized at 145 kg/cm² pressure. Milk is then filled in glass bottles which are sealed with crown caps and sterilized as shown in Fig. 31.2.

31.5.2 Bottle and bottling

The milk to be sterilized should promptly be cooled and bottled so as to serve the dual purpose of: (a) protecting the milk against contamination, loss, damage or degradation (due to: microorganisms or insects; exposure to heat, light, moisture or oxygen; spillage, evaporation or pilferage), and (b) helping in the sale and distribution of the milk. The glass bottle is still universally used. It is usually transparent, although in some countries brown bottles have been tried (brown bottles prevent light-induced off-flavours in milk; but on the other hand, the product is not visible for ins¬pection). The glass bottle is generally round, but may also be square in shape (as in the USA); the latter is considered to be more economical as it requires less storage space. Though sturdy, the glass bottles also decrease the pay-load of retail vehicles.

Bottles should be examined for their colour, capacity and strength, before use. The tests include the following: (i) colour and appearance; (ii) shape; (iii) dimensions (height and neck diameter); (iv) weight; (v) serrations; (vi) mini¬mum wall thickness; (vii) nominal capacity; (viii) strength or durability (these consist of thermal shock test, internal pressure test, impact test, polariscope examination, etc.).

In plants of small capacity, milk may be bottled with hand fillers operated manually with a lever and capable of filling 4 to 12 bottles at a time. The caps are then usually applied with a hand capper. In larger plants, automatic, continuous, mechanical bottle fillers and cappers are used. These are broadly of two types, viz., gravity fillers and vacuum fillers. In the former, the milk flows by gravity into the bottles as they are pressed against the filling valves; in the latter, the bottles are filled by creating a vacuum within them. The milk from the storage tank/pasteurizer usually goes directly into the bowl of the bottle-filling machine, the connecting pipe being equipped with a valve to regulate the flow. Bottle washing operations are so timed in relation to the processing of milk that washed and sanitized bottles arrive at the filler as needed for immediate bottling. There are two principles in filling, viz. level filling and quantity filling. Level filling is quicker and more common; most bottle fillers are designed to fill milk bottles to a pre-determined level; however, a constant low temperature of milk at the time of filling, should be maintained. Quantity filling, although more accurate since it is not affected by either temperature or foam, is slower and hence seldom used.

31.6 Bottle Filling

31.6.1 Gravity fillers

These consist mainly of six different parts, viz., the drive, bowl, filler valves, carrier, capper and star wheels. The circular bowl receives the milk to be bottled. The level of milk is kept constant by a float valve on the inlet pipe. Filling valves are attached radially to the bottom of the bowl. Bottles are fed by hand or directly from the bottle washer by a conveyor. They are mechanically centered into lifters which are located directly under the filling valves and which revolve with them. These lifters rise automatically as the filler revolves and the mouth of the bottle is forced against a tightly fitting rubber valve. The rising bottle pushes up the valve and the milk flows down into the bottle. As the filling valve almost completes a revolution, the lifter on which the bottle is carried is lowered automatically and the valve closes and remains closed until the next bottle opens it. The filled bottle is then transferred to a capper where it is automatically sealed and the lifter, now in the lowered position, is ready to receive another empty bottle for filling. While the bottle is being filled, the air which is displaced by the incoming milk escapes through a vent tube, which extends from the bottom of the valve sleeve to a point above the milk level in the bowl. The height of the bowl tank is adjusted by the operator to suit the size of bottle that is to be filled. Bottles are automatically discharged from the capper onto a conveyor that delivers them to an accumulating table, from where they may be loaded into crates manually or mechanically.

Advantages: Relatively simple to operate; (ii) maintenance not too complicated; (iii) easily and swiftly cleaned.

Disadvantages: Slow filling and hence limited capacity; (ii) leakage losses high (due to badly sealed bottle, bottle with chipped mouth, faulty valve, etc.).

31.6.2 Vacuum fillers

These may be either vacuum-assisted (single-bowl) or straightforward vacuum (double-bowl) types. In the former, the typical gravity bowl, which has open vent tubes and conventional gravity valves, is closed with an air-tight cover. In the latter, there is a rotary bowl and a float bowl. The float bowl is slightly below the level of the tops of the filling head. When the bottle is raised against the rubber ring on the filling head, a seal is formed and air inside the bottle is immediately drawn out through vertical vacuum pipes, and the milk is drawn from the float bowl through the milk pipes to the filling head and is released into the bottle. Foam is drawn off through the vacuum pipes into the vacuum tanks. Excess milk collecting in the vacuum tank automatically goes into the float bowl. Towards the end of the revolution of the filler, the lifters, on which the bottles are carried, are lowered, and the seal is broken. Any milk remaining in the milk pipe siphons back into the float bowl and that remaining in the vacuum line is drawn back into the vacuum tank.

Advantages: (i) Rapid filling; (ii) will not fill a bottle with a chipped mouth or bad seal, thus saving milk; (iii) no milk drip through faulty valves.

Disadvantages: (i) Maintenance complicated; (ii) relatively complicated to operate; (iii) cleaning more time-consuming.

31.7 Caps and Capping

The capping machine is often incorporated into the filler, and in any case, its work must synchronize with it. The milk bottle cap or closure has three main functions: (i) to retain the milk within the bottle; (ii) to protect the pouring lip from contamination; and (iii) to seal the bottle against tampering. (In this case, tampering refers to the removal or replacement of milk from a bottle without this being evident from the appearance of either the bottle or the milk within it.)

The caps may be: (i) Cardboard discs, impregnated with a moisture proof layer (paraffin wax or polythene); (ii) aluminium foil caps; (iii) crown corks. The cardboard discs with separate hoods were the first to be introduced, but are not much used now. The aluminium foil cap is most commonly used. It is either pre-formed or formed-in-place, both types having their advantages and disadvantages. The foil may be 0.05 to 0.15 mm thick and of 50 mm wide. Crown corks generally used for sterilized milk are made of lacquered tin plate, the inner surface of which is lined with water-proof paper/polythene, and are more expensive.

31.8 Inspection of Filled Bottles

Before being (manually) placed in crates, filled milk bottles should be inspected for dirt, etc. by rotating them as they are removed from the machine.

31.9 De-crating and Re-crating of Bottles

Removing dirty bottles from crates (decrating) and refilling them with bottles of pasteurized milk (re-crating) are among the most back-breaking and labor-consuming operations in the dairy. Both decrating and recrating machines look the same. The decrator lifts the empty bottles by vacuum-operated rubber-grippers and rejects any that are damaged. The recrator lifts the filled bottles by compressed-air operated rubber-grippers. In decrating, the crates of empty bottles are at first correctly positioned before lifting, while in recrating, a special marshalling mechanism allows bottles from the filler to assemble in correctly positioned groups, ready to be picked up for transfer to the crates. This stacks crates containing filled bottles, thereby relieving labor of another back-breaking job.

The crates containing the bottles are placed in steam chest and heated at 115°C for 15 min - an equivalent approved temperature-time combination. The method is suitable only for small operations. It results in flavor and color changes in the sterilized milk due to longer heating.

Fig. 31.2 Schematic Diagram of a rotary batch sterilizer

31.10 Semi-continuous In-bottle Sterilization

This method is similar to the one described in 31.5.1 except that an arrangement for rotating the crates is provided. The rotation helps in more uniform and efficient heat transfer, thereby minimizing colour and flavor changes. After the heating process is over, the crates containing the bottles are released into a tunnel of cooling air.

31.10.1 Continuous in-bottle sterilization

In this case sealed milk bottles pass continuously on chain conveyors through the pre-heating water and then to the steam chest for the required temperature and time. Thereafter the bottles move through the cooling water and finally to the atmospheric exit. This method is commonly used by the large scale manufacturers of sterilized milk as it helps in efficient energy usage and also for a comparatively better colour and flavour of finished product.

31.10.1.1 Cooling

After heat-treatment in the batch/tank sterilizers, the milk bottles may be cooled in air or water. If cooling is too rapid, the bottles may crack; if too slow there is a danger of browning due to caramelization. In the continuous system, after leaving the sterilizing zone, the bottles enter a column of hot water where the cooling process begins. This is followed by their passage through another tank of water (at a lower temperature than the previous one) for further cooling, and lastly through a shallow tank of cold water for final cooling. The bottles are then automatically discharged and conveyed to a point where they are placed in crates in which they are transferred to the storage room.

31.10.2 In-can continuous sterilization

This method is the most commonly used throughout the world. It comprises of a pre-heater, a sterilizer and a cooler with a leaking-can detector at the pre-heater and cooler discharge. The sealed milk cans are passed into a pre-heater where the cans are carried through a spiral path inside a drum filled with hot water. The cans then pass through a can detector to observe any leakage and thereafter are subjected to steam under the pressure for sterilization. Finally the milk cans move through the pressure air cooler and discharged. Recently a continuous milk sterilizing line with high temperature short time operation has been developed. It processes milk for 2 min at 127°C with a resultant improvement in quality mainly in color and flavor.

Fig. 31.3 Schematic diagram of a continuous hydrostatic sterilizer

31.10.3. Ultra high temperature (UHT) method of sterilization

In these processes, the milk is heated to 135-150°C for a few seconds, generally in a plate or tubular heat-exchanger. The milk, which is almost sterile, is filled into containers and sealed aseptically in a specially designed aseptic packaging system. The packed milk can be stored at room temperature upto a period of six months.

31.10.3.1 Distribution

Sterilized milk can be distributed once a week. This is why sterilized milk has great scope in tropical countries where household refrigerators are not commonly available.

31.10.3.2 Tests

• Turbidity test – To ascertain sterilization efficiency

• Bacterial count

Note: Phosphatase test is not applicable to sterilized milk.

31.10.3.3 Demerit

The most common is browning. Because of this fault, ‘Plain' sterilized milk is not so popular. Flavored (and simultaneously colored) sterilized milk is more popular.