MARKET MILK

Module 6. Common dairy operations

Lesson 30
WORKING OF HTST PASTEURIZER

30.1 Introduction

HTST method is also called ‘continuous flow’ or ‘flash’ pasteurization. It is modern method of milk pasteurization and is invariably used where large volume of milk is handled. This system gives a continuous flow of milk, which is heated to 72°C for 15s and promptly cooled to 5°C or below.

30.2 Operation of HTST Pasteurizer (Fig. 30.1)

30.2.1 Initial preparation

1. The plant must be sterilized.

2. All water remaining in the plant must be drained.

3. Clean filter clothes/nylon filters should be fitted in the filter.

30.2.2 Steps for starting the pasteurizer

1. Start the air compressor.

2. Switch on the control panel mains.

3. Fill the hot water tank, start the hot water pump and inspect the tank after 2-3 min for the level.

4. Open the air vents.

5. Start flow of the milk to the float controlled balance tank by starting milk pump.

6. Close the air vents when the milk comes out from them.

7. Set the temperature controller at pasteurization temperature (minimum 71.7°C) and adjust the air reducing valve so that the supply gauge registers 1.76 kg/cm2 pressure.

8. Turn on the steam to the hot water system via ‘solenoid valve’ for controlling steam passage into the heater.

9. Turn on the chilled water/brine as soon as forward flow takes place. Once the chilling temperature is reached, the plant will set itself to forward flow.

Note: The diluted milk that comes out first should not be collected in the balance tank.

30.2.3 Steps for shutting down the plant

1. Ensure availability of sufficient water in the storage tank (approximately, equal to the capacity of the plant).

2. As the last milk leaves the balance tank, tip in the water from the tank.

3. When the last portion of water leaves the balance tank, turn the 3-way valve at the finished milk outlet so that the flow is diverted to the floor.

4. Place a hose in the balance tank and flush the plant thoroughly with water until the discharge from outlet becomes clear.

5. Turn off the chilled water in the cooling sections.

6. Shut off the steam supply.

7. Admit cold water to the hot water tank and run until the plant is cold.

8. Stop the milk and hot water pumps.

9. Turn off the air supply and the main electric switch at the panel.

30.3 Maintenance of Milk Pasteurizers

1. The pasteurizer should be inspected every day for any leakage and for ensuring cleanliness.

2. The filter cloth or filter bag must be changed at regular intervals.

3. Periodical inspection of individual plate surface and gaskets must be done when the pasteurizer is dismantled for manual cleaning.

4. Any loose or broken gasket must be replaced, using proper adhesive.

5. The face of the plate bar and the tightening spindle should be kept lightly coated with grease.

6. All air-operated equipment should be supplied with clean dry air.

7. All recording instruments, thermometers etc. must be checked for accuracy, periodically.

30.4 Efficiency of Pasteurization

Alkaline Phosphatase test is carried out to determine whether milk has been properly pasteurized or not. The test is based on the detection of the activity of enzyme phosphatase, which is present in raw milk, but is completely inactivated at the temperature-time adopted for efficient pasteurization. Enzyme phosphatase is more resistant than the most heat-tolerant vegetative pathogenic bacteria.

When milk containing phosphatase (in active form – raw or underpasteurized) is incubated with, p-nitro phenyl disodium ortho phosphate, it hydrolyses the substrate liberating para-nitro phenol which gives yellow colour under alkaline conditions of the test. The intensity of the yellow color present is directly proportional to the activity of phosphate present in the milk. The presence of yellow color indicates inefficient pasteurization or post-pasteurization contamination with raw milk. Such mixing of raw and pasteurized milk can take place in the regeneration section of HTST pasteurizer.

30.5 Vacuum Pasteurization

The process of heat treatment under vacuum in stainless steel chamber is known as Vacreation. Machine used for vacuum pasteurization is known as Vacreator, which is the registered trademark of M/s. Murray Deodorizers Ltd., New Zealand

30.6 Purpose of Vacuum Pasteurization

1. To kill bacteria including pathogens.

2. Inactivate enzymes.

3. Remove undesirable odours and flavours

4. Remove oxygen

30.6.1 Use of vacreator

This equipment is generally used for vacuum treatment of milk or cream. It was mainly developed for treatment of aged and sour cream meant for butter making. It is an effective means of removing off-flavors particularly due to feed.

30.6.2 Principle of vacuum pasteurization

Principle of vacreation is mixing milk/cream with steam under pressure and immediately followed by sudden and spontaneous expansion into a finely divided state with a chamber maintained at reduced pressure. This process effects separation of off-flavors along with water vapor due to sudden transition from a compressed state to the expanded state. At the same time, instantaneous cooling effect of the product is an important factor in preserving the desirable characteristics of the product, even when high temperature is adopted.

30.6.3 Constructional and operational details

The vacreator consists of three or five stainless steel vessels connected to one another for steam heating and vacuum treatment with continuous product flow (Fig. 30.1). The first chamber is the pasteurizing chamber. This chamber is operated under vacuum (10-23 cm Hg) which maintains a corresponding temperature of 90-96°C. The cream or milk is admitted at the top through a spray pass and falls in a fine shower through expanded steam. Dry saturated steam is passed from the top along with milk or cream. Then, the product and some free steam are passed from the bottom of this chamber to the top of the second chamber. It enters this chamber tangentially and spirals in a thin film to the bottom.

The temperature of this chamber is maintained at 82 - 71°C under a vacuum of 37-50 cm Hg. Under the influence of high vacuum, the product releases a part of the steam as water vapor, which was condensed into it in the pasteurizing chamber. By controlling the amount of steam used, steam distillation occurs. The effect of steam distillation at this stage is to accelerate evaporation and remove undesirable flavor substances. The vapor resulting from evaporation and steam distillation passes over into the condenser, carrying with it volatile off-flavors. From the second chamber, the product is drawn into the third chamber maintained under still higher vacuum through a second uptake pipe. The product temperature in this chamber is between 38-46°C by maintaining a vacuum of 70-67 cm Hg. In this chamber, remaining water and off-flavor are removed, upon reaching the bottom of the third chamber, the product is discharged by means of a multi-stage centrifugal pump.

Fig. 30.2 Vacreator or vacuum pasteurizier