🧊 Milk Processing, Cooling, and Pasteurization

This lesson covers core livestock production and management concepts for practical farm application and exam-oriented preparation.

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Processing of milk-Cooling

Class 11: Processing of milk-Cooling-Pasteurization-Definition-Various methods-Low

Temperature Long Time, High Temperature Short Time and Ultra High Temperature-

advantages and disadvantages.

Thermal processing :

The main purpose of heat treatment of milk is to render it sage for human consumption

and to enhance its shelf life. Thermal processing is an integral part of all operations/processes of

milk and milk products manufacturing units. The common pathogenic organisms likely to occur

in milk are killed by relatively mild heat treatment. The most resistant organism is the Bacillus

tuberculosis and hence has been made as index organism to achieve complete safety of milk.

Any heat treatment, which may destroy this organism, can be relied upon to destroy all other

pathogens in milk. The thermal death of such pathogenic organisms like Tubercle bacilli,

Typhus and Coliform bacteria of such pathogenic organisms like Tubercle bacilli, Typhus and

Coliform bacteria and Coxiella burnettie (Q fever organism ) has made the basis for time

temperature combinations is also a matter of optimization where both microbiological effects and

quality aspects must be taken into account. Various categories of heat.

Different categories of heat treatment.

Treatments	Process	Temperature (oC)	Time(seconds)
Pasteurization	LTLT	63	1800
-	HTST(milk)	72	15-20
-	HTST(cream)	>80	15
Thermization	-	57-68	15
Sterilization	-	115-121	180-780
Ultra-pasteurization	-	115-130	2-4
UHT	-	135-150	1-6

Pasteurization :

It is the process of heating every particle of milk or milk products, in properly

designed and operated equipment to specified temperature and holding at that

temperature for specified period of time followed by immediate cooling and storing at low

temperatures . Pasteurization can be achieved either by holding method (batch process) or

continuous process. Under batch process the milk is heated to 63 [o] C for 30 minutes in a double

jacketed vat. Heating and cooling is done by spraying or circulating hot water /steam of chilled

water between the inner and outer jacket of the vessel. The milk is kept gently agitated

mechanically to ensure uniform heating/cooling. The process is called low temperature long

time (LTLT) method . This method is suitable for small quantities ranging from 200-1000 litre

requiring low initial cost of equipment.

High temperature short time (HTST) treatment for pasteurization of milk refers to

heating every particle of milk in a continuous flow to a minimum of 72 [o] C for at least 15 seconds

followed by immediate cooling to 4 [o] C. The entire process is automated and is ideal for large

scale handling of 5,000 lph or higher. The complete process of preheating, heating, holding, pre

cooling and chilling is completed in a plate type heat exchanger mounted on a compact frame

with inter connected sections to make the process continuous. The heat exchanger plates are so

designed as to prevent mixing of thin channels of product and heating/cooling medium by

separating the plates with rubber gaskets. The complete equipment consisting of four sections is

called pasteurizer. Each section consists of varying numbers of plates depending on equipment

capacity. The raw cold milk (4-5 [o] C) from balance tank enters the pre-heating/pre-cooling

(regeneration) section, where hot pasteurized milk (72 [o] C) flows counter current to the raw cold

milk, within adjacent plates, thereby, transferring heat for pre-heating of raw milk and pre

cooling of pasteurized milk resulting in energy saving. The pre-heated milk then enters the

heating section where it is heated to a temperature of 72 [o] C, using hot water or steam, passes to

holding section where the temperature of milk is maintained for specified period of time (15

seconds) until it leaves the section. A flow diversion value is placed at the outlet of holding

section that senses the temperature and accordingly diverts the milk either forward or returns to

balance tank if not properly heated. The pasteurized milk thus passes to regeneration section

followed by cooling section where it is chilled using chilled water or glycol solution as a coolant.

Thermization

This process consists of heating milk below pasteurization temperature to temporarily

inhibit bacterial growth. The process is useful where it is not possible to immediately

pasteurization all the milk and some of the milk needs to be stored for hours/days before further

processing. The milk is heated to 63-65 [o] C for 15 seconds and rapidly chilled to 4 [o] C or below to

prevent aerobic spore forming bacteria from multiplying after thermization. Thermization has

favourable effect on spore forming bacteria to revert to vegetative state which are destroyed upon

subsequent pasteurization.

Ultra –pasteurization.

Its objective is to enhance or extend the shelf life of the product (milk) by 15 – 30 days.

The fundamental principle is to reduce main causes of reinfection of the product during

processing and packaging. This is achieved by heating milk to 115-130 [o] C for 2-4 seconds and

cooling it to below 4 [o] C. This requires extremely high level of hygienic practices to be followed

during production and maintenance of temperature lower than 4 [o] C during distribution of such

products. Ultra pasteurized products are packed in pre-sterilized containers aseptically and held

refrigerated to achieve extended shelf life.

Ultra-high temperature treatment (UHT)

It is a technique for preserving liquid food products by exposing them for brief intense

heating. In short the process is termed as UHT treatment. The heating temperature normally

ranges from 135-150 [o] C for 1-6 seconds. The process is continuos which takes place in a closed

system that prevent the product from being contaminated by air-borne microorganisms. The

product passes through heating and cooling stags in quick succession followed by aseptic filling

as an integral part of the process. There exist two methods of UHT treatment indirect heating

and cooling in heat exchangers and direct heating by steam injection or infusion of milk with

steam and cooling by expansion under vacuum. UHT-treated products are packed asceptically in

specially designed multilayer containers, and can be stored at room temperature for extended

period of time (2-6 months) without bacterial growth.

Sterilization :

In this process milk or condensed milk packed in clean containers is usually subjected to

high temperature (115-120 [o] C) for 20-30 minutes. The containers may be tin cans (200-400 g

capacity) for evaporated/sweetened condensed milk or glass bottle for milk. The process of

heating and partial cooling is achieved in a rotary autoclave for batch production or hydrostatic

tower for continuous production. In container sterilization is the original form of sterilization

and is still used.

Microwave heating :

It is a novel method of heating, which greatly reduces the effect of heat penetration lag

associate with traditional process of convection or conduction. Microwaves form part of the

electromagnetic spectrum (frequency range 915 and 2450 MHz). The heating effect is achieved

by transfer of energy to a dipole (in water) within the product. The constant movement of dipole

due to oscillation of molecules generate heat. The high temperature produced in are of high

water concentration transfer heat to other areas of food not absorbing microwave energy so well.

Microwave absorption is inversely proportional to the penetration depth as a function of water

content, salt content and temperature. During microwave heating temperatures at the surface, are

often lower due to evaporative cooling than at the centre of the product. Conduction effects are

only the means of leveling out the temperature imbalance due to microwave heating, Microwave

absorption characteristics change with change in physical phase of the product. In frozen state

molecules are less free to move and therefore less able to interact with electrical field. As the

product melts the areas of water and dissolved salts appear which absorb microwaves rapidly.

Keeping quality of milk – treatment of milk – pasteurization different types – preservation

and storage

Keeping quality of milk is influenced mainly by clean milk production and type of

milking – mecahnical-high, manual-low.

Bacteriological standard milk raw milk. (standard plat count / ml.)

1. spc - not exceeding 2 lacs. very good

2 .2 – 10 lacs. – good

3. Bet 10-50 lacs fair

4. 50 lacs poor

Pasteurized milk – spc should not exceed 30,000

COB (Clot on boiling test) to determine the heat stability.

Treatment of Milk

The main objective of treatment of milk is to increase the keeping quality.

Batch (LTLT) Bottle Continuos

HTST Stassanisation

Cooling Heat treatment

Holding process Flash process

Importance : Milk contains some micro organism when drawn from the udder, that number

increases during subsequent handling. The common milk microorganisms grow best between.

20 and 40 [0] C bacteriological growth is invariably accompanied by deterioration in market

quality due to development of off flavour acidity etc. One method of preserving milk is by

prompt cooling to a low temperature.

The term pasteurization has been coined after the name of Louis Pasteur of France (1860 4) who demonstrated that heating wine at 122 to 140 [o] F (50 - 60 [o] C) killed the spoilage organism

and helped in its preservation. Although Louis Pasteur pioneered studies of heat treatment for

preservation, pasteurization of milk was attributed to Dr.Soxhlet of Germany in 1888.

EFFECT OF TEMPERATURE ON BACTERIAL MULTIPLICATION

Effect of storage temperature on bacterial growth in milk

Milk held for 18 hrs at Bacterial growth

Temperature [0] C Factor *

0 1.00

5 1.05

10 1.80

15 10.00

20 200.00

25 1,20,000.00

• Multiply initial count with this factor to get final count

Pasteurization of Milk

Louis Pasteur found heating the wine to 140 [0] F (60 [o] C) greatly improved keeping quality

by destroying most of the bacteria.

Pasteurization is the processing of exposing the milk to a controlled temperature for a

specific time with the object of destroying all the pathogenic bacteria and cooling the milk

immediately to a temperature low enough to retard the growth of the surviving bacteria 161 [o] F for

15 seconds and rapidly cooling to 50 [o] F sufficient to kill the most common disease producing

bacteria .

Advantages :

1. Pasteurization renders milk safe for consumption

2. It destroys all the common disease producing organism eg. TB, Typhoid, Diptheria,

etc. which may be present in milk.

3. Pasteurization destroys approximately 99% of all bacteria and most of the yeast and moulds.

4. Keeping quality is improved facilitating easy transport of milk over long distances.

5. Pasteurized milk or cream – desired type of ripening can be obtained more

effectively.

6. Pasteurization eliminates undesirable taints from milk.

7. Products prepared from Pasteurization milk are of more uniform quality.

8. Natural flavour of milk is not affected by Pasteurization.

9. Pasteurization destroyes lipase enzyme / which is responsible for rancidity of milk .

Objections to Pasteurization

a) Organisms developing in Pasteurized milk form harmful products. streptococcus

theremophilus, S.liquifaciess and M.candidus varian and M.Luteus.

b) Infants do not develop so well on Pasteurized as on raw milk

c) Products of bacterial growth are not destoryed.

d) Pasteurization may be used to mask dirty milk.

e) Pasteurization bring about chemical changes in milk. Cause ppt. of ca and Phosporus

f) Pasteurization - partial destruction of vit.E and K, vit.C is destroyed varying degree

dependent on the system.

Holding process: Low temperature long time (LTLT method milk is heated to 150 [o] F and is held

at that temperature for 30 minutes then it is cooled to a temperature not more than 50 [o] C.

Flash Pasteurization

1. Milk is heated to 161 [o] F held for 15 sec.

2. Plate heat exchanger is the most widely used.

3. It includes a section of regenerative heating and cooling followed by final heating and

cooling.

4. From the balance tank milk is sucked under slight vaccum through regenative section – 120 130 [o] F by the hot Pasteurization milk flowing in pipeline.

5. The partially heated milk them is pumped thro final heating section under 10 lbs pressure,

where it is heated to the required temperature by hot water at temperature 2 to 3 [o] F higher

then final

temperature.

6. Final heating section- the pasteurized milk passes through a holding tube of such

capacity that the holding time is not less then 15 sec. and then regenerative section where it is

partially cooled and final cooling.

Stassanisation : The milk is heated to about 165 [o] F for 7seconds under slight pressure in a thin

layer between two heated surface (in order that all carbonic acid may be returned) the process is

carried out in a tubular heat exchanger consisting 3 concentrate tubes by passing milk between

two water heated pipes thro narrow space of 0.6 to 0.8 mm. It is claimed that there is practically

no milk stone formation, less destruction of vitamins, no evaporation of milk and more economy

in steam utilization than in conventional pasteurization. This device was invented on Dr.Henre

Stassano. Adv.Easy cleaning.

Flow Diagram

ORGANOLEPTIC

Receiving Milk

Preheating (95-105 [o] F)

Filtration

Cooling and storage (40 [o] F or less)

Standardization

Pasteurization (150 [o] for 30 min or 161 [o] F for 30 seconds)

Homogeniation (2500 pss)

Cooling – 50 [o] F

Storage (40 [o] for less)

HTST : This was 1 [st] developed by A.P.V. Co., in the U.K. 1922.

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Summary Cheat Sheet

Topic	Key Point
Main steps	Cooling, pasteurization, and safe handling are core milk-processing operations
Cooling purpose	Rapid cooling slows bacterial multiplication
LTLT	Low Temperature Long Time pasteurization is one standard method
HTST	High Temperature Short Time pasteurization is widely used for fluid milk
UHT	Ultra High Temperature treatment gives long shelf-life product
Thermization	Mild heat treatment used as a preliminary preservation step
Pasteurization purpose	Destroys pathogens and lowers spoilage organisms without major quality loss
Organoleptic quality	Taste, smell, colour, and appearance are important sensory quality factors
Temperature link	Higher temperature generally increases bacterial destruction but may affect quality
Exam trap	Pasteurization is not sterilization; some organisms may survive, but pathogens are controlled