Dairy unit 2. Processing Techniques and naturally occurring preservatives i. Bacteriological aspects of processing techniques like bactofugation, thermisation, pasteurization (in detail process is expected), sterilization and boiling.

 2. Processing Techniques and naturally occurring preservatives :

i. Bacteriological aspects of processing techniques like

bactofugation, thermisation, pasteurization (in detail process is

expected), sterilization and boiling.

BACTOFUGATION

 Introduction

Bactofugation is the process of removal of microorganisms from milk using centrifugal force.

 It is a special form of separation of microorganisms, mainly spore formers (Bacilli/Clostridia) to enable milk to be sterilized at lower temperature-time combinations.

 Most of the microorganisms are inactivated by pasteurization. 

However, the highly heat resistant spores survive pasteurization.

 They can lead to significant quality defects in hard cheese, semi-hard cheese or long-life products due to proteolysis, lipolysis and gas formation. Therefore, bactofugation is mainly used in the manufacture of these products. 

The objectives of bactofugation are as follows:

• To improve hygienic quality of milk

• To avoid heat resistant bacteria without resorting to excessive heating

• To ensure exceptionally high degree of bacteriological purity in milk.

It removes bacteria, both living and dead, from treated substances whereas traditional heat treatment kills bacteria and leaves them in food. 

The microorganisms involved in causing milk spoilage, reducing the quality of powder and butyric fermentation thereby causing late blowing of cheese, are mostly spore formers.

 Bactofugation is important in foodstuffs infected with bacteria containing thermostable endotoxins.

 Bactofuge

Bactofuge are special nozzle clarifying separator with high separation precision that can remove microorganisms from milk based on their density difference (skim milk – 1.036; bacteria – 1.07 – 1.13 g/cm³).

 Design

Separated micro-organisms are concentrated on the periphery of the bowl and the small portion of such discharged milk, highly enriched with the bacteria (bactofugate) is thrown outside via nozzles, and then collected in the sludge discharge pipe. 

The bactofugate which still corresponds to 2 -3 % of the treated milk may be sterilized separately and added back to the bactofuged milk to avoid the losses of milk solids .

 Bactotherm Process

Clarified and standardized milk is pumped into a plate heat exchanger, where it is heated to a temperature of 60-75°C prior to being fed to the bactofuge. 

The centrifugal acceleration is increased to 10,000g. 

The slurry of bacteria (bactofugate) is discharged continuously through nozzles due to their greater specific weight.

 It is about 3% of the feed by volume and represents a reduction in total bacteria by approximately 50-60%. 

The bactofugate stream is UHT processed (130 -140°C for 3-4 seconds) and remixed in the normal stream. 

This time- temperature profile is sufficient to inactivate all spores. 

The sterilized bactofugate is re-chilled in the plate heat exchanger and can be added back to the de-aerated milk or discharged separately for other suitable applications . 

Continuous recycling of the sterilized bacterial concentrate into the milk avoids losses of products.

 There are many other applications of the Bactotherm process such as butter oil separation, high fat cream, and other non-dairy applications. 

For each product, the geometry of the bowl could be altered to suit the product.

Advantages

• Use of bactofuged cheese milk prevents swelling in certain cheeses which may be caused by heat resistant butyric acid bacteria. Removal of bacteria without pasteurization enables cheddar cheese production with more typical cheese flavor.

• In powders, it reduces the count of microbes and allows significant removal of heat resistant bacteria.

• The severity of heat treatment can be reduced in sterilized milks.

• In cream, defects caused by heat resistant Bacillus cereus can be avoided.

                     

 2. Thermization:

Thermization, also spelled thermisation, is a method of sanitizing  of row milk  with low heat.

"Thermization is a generic description of a range of subpasteurization heat treatments (57 to 68°C × 10 to 20 s) that markedly reduce the number of spoilage bacteria in milk with minimal heat damage.

The process is not used on other food products, and is similar to  pasteurization but uses lower temperatures, allowing the milk product to retain more of its original taste.

In Europe, there is a distinction between cheeses made of thermized milk and raw-milk cheeses. 

However, the United States Food and Drug Administration (FDA) places the same regulations on all unpasteurized cheeses.

 As a result, cheeses from thermized milk must be aged for 60 days or more before being sold in the United States, the same restriction placed on raw-milk cheeses by the FDA.

Thermization involves heating milk at temperatures of around 145–149 °F (63–65 °C) for 15 seconds, while pasteurization involves heating milk at 160 °F (71 °C) for 15 seconds or at 145 °F (63 °C) for 30 minutes.

Thermization is used to extend the keeping quality of raw milk (the length of time that milk is suitable for consumption) when it cannot be immediately used in other products, such as cheese. 

Thermization can also be used to extend the storage life of  fermented milk products  by inactivating microorganisms in the product.

Thermization inactivates psychotropic bacteria  in milk and allows the milk to be stored below 8 °C (46 °F) for three days, or stored at 0–1 °C (32–34 °F) for seven days.

 Later, the milk may be given stronger heat treatment to be preserved longer.

 Cooling thermized milk before reheating is necessary to delay/prevent the outgrowth of bacterial spores. 

When the milk is first heated, spores can begin to germinate, but their growth can be halted or delayed when the milk is refrigerated, depending on the microorganisms' growth requirements. 

Germinated spores are sensitive to subsequent heating, however since germination is not a homogeneous process, not all spores will germinate or be inactivated by subsequent heating.

 

PASTEURIZATION

 Introduction

The word pasteurization is derived from the name of an eminent French scientist Louis Pasteur (1860), who found that heating certain liquids specially wines to a high temperature improved their keeping quality.

 

Pasteurization came into use on a commercial scale in the dairy industry shortly after 1880 in Germany and Denmark. 

This process is widely employed in all branches of dairy industry. 

Heat treatment destroys microorganisms present in milk.

 Further, a more or less complete inactivation of enzymes occurs, depending on temperature and treatment time. 

In order to retain as many sensory and nutritive properties of the raw materials as possible, different heating methods have been developed to destroy pathogenic organisms (pasteurization) or destroy all microorganisms and inactivate enzymes (sterilization).


Definition

According to International Dairy Federation (IDF), pasteurization can be defined as ‘a process applied to a product with the object of minimizing possible health hazards arising from pathogenic microorganisms associated with milk by heat treatment, which is consistent with minimal chemical, physical and sensory changes in the product’.

In general, the term pasteurization as applied to market milk refers to the process of heating every particle of milk to at least 63°C for 30 min or 72°C for 15s or to any temperature-time combination which is equally efficient, in a properly operated equipment. After pasteurization, the milk is immediately cooled to 5°C or below.

Importance of Pasteurization

• To render milk safe for human consumption by destroying all the pathogenic microorganisms.

• To improve the keeping quality of milk by killing almost all spoilage organisms (88-99%).

Drawbacks of Pasteurization

• It diminishes the cream line or cream volume.

• Pasteurized milk may increase the renneting time.

• It fails to destroy bacterial toxins

• In India, pasteurization is not necessary as milk is invariably boiled on receipt by the consumer

Time-Temperature Combination for Specific Requirements

All pathogenic organisms are destroyed by pasteurization, except spore forming organisms.

The thermal death point of tuberculosis germs (Mycobacterium tuberculosis) is slightly higher than that for inactivation of phosphatase enzyme. 

Pasteurization is carried out at a heat treatment temperature above that for phosphatase inactivation and yet below that for cream line reduction. 

 Methods of Pasteurization

A. Low-temperature long-time (LTLT)/Batch pasteurization

Milk is heated, held and cooled in the inner vessel. 

The space between vessel and the outer casing forms a jacket, through which the heating or cooling medium is circulated. 

To heat the milk, hot water or low-pressure steam is circulated through the jacket and milk is continuously agitated for rapid and uniform heating. 

The heating process could be manually or automatically controlled.

 

The milk is heated to a minimum of 62.7°C and held at this temperature for minimum 30 min. 

It is then cooled as rapidly as possible to 4°C. 

A cooling medium is circulated in the jacket for chilling the milk, but more often the heated milk is discharged to a surface cooler where a film of milk flows down the corrugated metal plates or series of interlocked tubes. 

A cooling medium such as brine or chilled water is circulated on the other side of the plates or through the tubes.

The LTLT pasteurizer may be of three types

1 Water – jacketed vat

This is double-walled around the sides and bottom of the vat in which hot water or steam under partial vacuum circulates for heating, and cold water for cooling. 

The outer wall (lining) is usually insulated to reduce heat loss.

 The heat-exchange takes place through the wall of the inner lining.

 The difference between temperature of the hot water and the milk is kept to a minimum. The milk is agitated by slowly revolving paddles/propellers. 

When heating, the vat cover is left open for escape of off-flavors; and when holding, the cover is closed. 

During the holding period, an air space/foam heater (steam or electrically heated) prevents surface cooling of milk.

Advantage: Flexibility in usage - multipurpose vat.

2 Water–spray type

A film of water is sprayed from a perforated pipe over the surface of the tank holding the product which is continuously agitated. A rapidly moving continuous film of water provides rapid heat transfer.

3 Coil-vat type

The heating/cooling medium is pumped through a coil placed in either a horizontal or vertical position, while the coil is turned through the product. The turning coil agitates the product (but additional agitation may be necessary).
Disadvantage: Coils are difficult to clean.

B. High-Temperature Short-Time (HTST) Pasteurization

This was first developed by A.P.V. Co. in the United Kingdom in 1922. It is the modern method of pasteurizing milk and is invariably used where large volumes of milk are handled. The HTST pasteurizer gives a continuous flow of milk which is heated to 72°C for 15s and then promptly cooled to 5°C or below.

 Advantages

1. Capacity to heat treat milk quickly and adequately, while maintaining rigid quality control over both the raw and finished product

2. Less floor space required

3. Lower initial cost

4. Milk packaging can start as soon as milk is pasteurized

5. Easily cleaned and sanitized (system adapts itself to CIP)

6. Lower operating cost (due to regeneration system)

7. Reduced milk losses

8. Development of thermophiles is not a problem

9. Automatic precision controls ensure proper pasteurization.

 Disadvantages

1. The system is not well-adapted to handling small quantities of liquid milk products

2. Gaskets require constant attention for possible damage and lack of sanitation

3. Complete drainage is not possible (without losses exceeding those from the holder system)

4. Margin of safety in product sanitary control are so narrow that automatic control precision instruments are required in its operation

5. Lethal effect on high-thermoduric bacteria in raw milk is not as great as compared to LTLT system

6. Accumulation of milk-stone in the heating section.

Milk flow

The following steps or stages are involved as milk passes through the HTST pasteurizer:

1. Balance tank

2. Pump

3. Regenerative heating

4. Heating

5. Holding

6. Flow diversion valve (FDV)

7. Regenerative cooling

8. Cooling by chilled water or brine

An arrangement for incorporation of the filter/clarifier, homogenizer, etc., in the circuit is also made possible. There is some variation in the use or order of these steps in different milk processing plants.

 Functions of specific parts

1 Float-controlled balance tank (FCBT)

Maintains a constant head of the milk for feeding the raw milk pump.

2 Pump

Either a rotary positive pump between the regeneration and heating sections , or a centrifugal pump with a flow control device to ensure constant output, is used.

3 Plates

The Plate Heat Exchanger (PHE) (also called Paraflow) is commonly used in the HTST system.

The PHE is a compact, easily cleaned unit

.

 Its plates may be used for heating, cooling and regeneration. 

These plates are supported in a press between a terminal block in each heating and cooling sections. 

The heat moves from a hot to a cold medium through stainless steel plates.

 A space of approximately 3 mm is maintained between the plates by a non-absorbent rubber gasket or seal . 

The plates are numbered and must be properly assembled. 

They are tightened into place, and designed to provide a uniform, but somewhat turbulent flow for rapid heat transfer. 

Raised sections (corrugations) on the plates in the form of knobs, diamonds and channels, help provide the turbulent action. 

Greater capacity is secured by adding more plates.

4 Filter

Filter units are connected directly to the HTST system, placed after the pre-heater or regenerative (heating) section.

 

These units, using 40-90 nylon mesh cloth are usually cylindrical in shape. 

Usually two filters are attached; when one is being used, other can be subjected to cleaning. This permits continuous operation.

5 Regeneration

The raw chilled incoming milk is partially and indirectly heated by the heated outgoing milk (milk-to-milk regeneration).

 This adds to the economy of the HTST process, as the incoming milk requires less heating by hot water to raise its temperature to pasteurization temperature in the heating section.

6 Heating

The preheated milk from regeneration section passes through heating section of HTST, where it is heated to 72°C or more with the help of hot water from hot well.

 Thereafter, the heated milk enters into the holding section (plates/tube).

7 Holding

The holding tube ensures that the milk is held for a specified time, not less than 15s., at the pasteurization temperature of 72°C or more.

8 Flow diversion valve (FDV)

This routes the milk after holding section. 

If the milk is properly pasteurized, it flows forward through the unit. 

In case the milk is not heated to the set heating temperature, it is automatically diverted by the FDV back to the Float Controlled Balance Tank (FCBT) for reprocessing. 

It is usually operated by air pressure working against a strong spring.

 If the temperature of heated milk falls below set temperature, air pressure is released and the valve snaps shut immediately. 

When the temperature is regained, air pressure builds up and the valve opens up for the forward flow to occur.

 The system is so arranged that any failure of electricity moves the valve in the diverted position.

9 Regeneration (cooling)

The pasteurized hot outgoing milk is partially and indirectly cooled by the incoming cold milk (milk-to-milk regeneration).

 This again adds to the economy of the HTST process. 

In fact, when pre-cooled (raw) milk is received, regeneration efficiency is 90% and above which obviates cooling using well water altogether.

10 Control panel

Contains instruments, controls, FDV-mechanism and holding system, all centralized in one moisture-proof panel. 

The lower half of the panel forms an air-insulated chamber which carries the holding tube.

11 Automatic control devices

These include (a) steam pressure controller, (b) water temperature controller and (c) milk temperature recorder.

12 Steam pressure controller

Maintains a constant hot water temperature for heating milk accurately to the required pasteurization temperature. 

It acts as a reducing valve in the steam supply line to give a constant steam pressure.

13 Water temperature controller

Regulates the amount of steam entering the hot water circulating system.

14 Milk temperature recorder

Records the temperature of milk leaving the holding tube/plate.

 This is an electric contact instrument that operates either a FDV or a milk pump, automatically preventing milk from leaving the holding section at temperatures below the one set in the control panel. 

Both the frequency and duration of the flow diversion  and the temperature of milk leaving the heating section are recorded in the thermograph (recording chart) by means of two different colored pens.

15 Hot water

Circulates hot water through the heating section of the machine to maintain the correct milk heating temperature within very fine limits.

16 Pressure in the system

The normal pressures maintained in the HTST system are:

                           

Sterilization

Sterilized milk may be defined as (homogenized) milk which has been heated to a temperature of 100 degree Celsius or above for such lengths of time that it remain 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 requirement for complete sterility conflict with the consumer preference for normal color and flavor in the sterilized 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 

Sterilization of milk is aimed at killing all microorganisms present, including bacterial spores, so that the packaged product can be stored for a long period at ambient temperature, without spoilage by microorganisms.

 Since molds and yeasts are readily killed, we are only concerned about bacteria.

 To that end, 30 min at 110°C (in-bottle sterilization), 30 sec at 130°C, or 1 s at 145°C usually suffices. The latter two are examples of so-called UHT (ultra-high-temperature, short time) treatment. 

Heating for 30 min at 110°C inactivates all milk enzymes, but not all bacterial lipases and proteinases are fully inactivated; it causes extensive Maillard reactions, leading to browning, formation of a sterilized milk flavor, and some loss of available lysine; it reduces the content of some vitamins; causes considerable changes in the proteins including casein; and decreases the pH of the milk by about 0.2 unit.

 Upon heating for 1 s at 145°C chemical reactions hardly occur, most serum proteins remain unchanged, and only a weak cooked flavour develops. 

It does not inactivate all enzymes, e.g., plasmin is hardly affected and some bacterial lipases and proteinases not at all, and therefore such a short heat treatment is rarely applied 

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) Distinction rich  flavor (due to homogenization);

(v)  Economical to use;

(vi)  Less liable to develop oxidized taints.

Disadvantage:

(i) increased cost of production;

(ii) More loss in nutritive value than pasteurization

(iii) Gerber test by normal procedure not so accurate.

Sterilized milk must:

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

(ii) Be free of any micro-organisms harmful to consumer health, i.e., pathogenic toxinogenic germs and toxins

(iii) Be free of any micro-organisms liable to proliferate, i.e. it should not show signs of bacterial growth (which leads, inter alia, to an absence of deterioration).

In-bottle sterilization

The raw milk, on receipt, should be strictly examined by the physic-chemical and bacteriological test 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, etc. 

The milk should again be cooled to 5°C so as to preserve its quality. 

It should then be standardized to the prescribed percentage of fat and solids-not-fat content in order to conform to legal standards. 

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 cleaned and sanitized bottle coming from the bottle washing machine and then sealed with special caps.

 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 108-111°C for 25-35 minutes.

The sterilized milk bottles should be gradually cooled to room temperature. 

Any sudden cooling may led to bottle breakage. 

Finally the milk-in-bottles should be stored in a cool place .

Sterilizers may be: (i) Batch; (ii) Continuous.

(i) Batch Sterilizers :

These may either be rotary or non-rotary in type.

The batch sterilizers are rectangular, horizontal, boiler shaped retorts with a steam inlet and condensate outlet, fitted with clamp-down covers, into which steam is adjusted for the required temperature and time for sterilization.

Advantages:

Simplicity and flexibility of operation

Less initial capital and recurring expenditure

Disadvantages :

1. Usually produces a brownish appearance and cooked taste in the finished product.

2. Sterilization may be faulty

3. Cooling has to be slow to avoid breakage

4. Economic advantages of large-scale processing are not obtained.

In the batch-rotary type, the filled bottles are put in to holders which are rotated at 6-7 rpm. 

The sterilized milk is of a slightly better quality in rotary-type sterilizers than in non-rotary ones.

(ii) Continuous sterilizers :

In this type, the filled and sealed milk bottles are automatically placed by means of a slat conveyor in to the pockets of carrier cages.

They then passed into water at or near boiling temperature; from there, they enter the sterilizing zone, which consists of a steam chamber at 108-111°C.

Here the bottles remain for a pre-determined time, viz., 25-30 minutes, for milk sterilization

Boiling

Boiling of milk would mean raising  the temperature of milk to its boiling point under atmospheric  temperature and pressure. 

The boiling point of milk is l00.17°C which is decidedly higher than the usual temperature adopted in holding method of HTST pasteurization process.

Boiling of milk certainly destroys all the pathogenic organisms and makes it safe for human consumption, but there is no need for heating the milk for such a higher temperature when the same objective is fulfilled by pasteurization process.

Extra heating of milk will amount to loss of heat energy and nutritive value of milk which otherwise could be avoided. 

Also, in boiling process, there is no cooling involved hence the growth of those microorganisms which survive heat treatment is not retarded and, therefore, boiled milk has lower keeping quality than the properly pasteurized milk.

Boiling of milk is uneconomical and practically impossible in the dairies where large volumes of milk is handled every day.

For individual household consumer boiling of milk prior to consumption is certainly a useful practice. 

The common belief amongst the older generation that drinking of raw milk as it is obtained from the udder of animals is the best way of its consumption has to be contradicted on scientific grounds.

This concept is rather more important in the present days where organized dairies collect milk from various sources and the hygienic practices in handling of milk are still far from satisfactory.

The health of a large population will be at stake if milk is distributed without any resources to heating.

Reference:

https://youtu.be/AoyPkc0reuQ

https://youtu.be/HBDsPfjnLac

https://youtu.be/iAaRs4vM8x8

https://youtu.be/saEzeRUgQ5o

http://ecoursesonline.iasri.res.in/mod/page/view.php?id=6140

http://eagri.org/eagri50/LPM201/lec11.pdf

http://www.ouat.nic.in/sites/default/files/5-pasteurisation_of_milk_dairy_and_food_engineering.pdf

http://ecoursesonline.iasri.res.in/mod/page/view.php?id=6154

http://ouat.nic.in/sites/default/files/6-sterilisation_of_milk_dairy_and_food_engineering.pdf

http://ouat.nic.in/sites/default/files/6-sterilisation_of_milk_dairy_and_food_engineering.pdf

https://www.dairyknowledge.in/article/sterilization

https://en.wikipedia.org/wiki/Thermization

http://www.dairyforall.com/milk-process.php

https://www.healthline.com/nutrition/boiled-milk#benefits-and-downsides

https://krishijagran.com/news/boiling-vs-pasteurization-of-milk/