MILK

 "Raw milk from healthy animal, that has been produced and handled under hygienic conditions that contains only small numbers of harmless bacteria and that possesses a good keeping quality without being treated by heat." 

‘Milk whole, fresh, clean lacteal secretion obtained by complete milking of one or more healthy milch animals, excluding that obtained within 15 days before calving or 5 days after calving. It should have prescribed percentage of milk fat and SNF’.

 

NUTRITIVE VALUE OF MILK 

Milk is considered as SAMPURNA AHAAR (complete food) i.e. milk is considered the best, ideal and near complete food by virtue of possessing almost all the essential nutritional factors in optimum proportion, viz. fat, protein, sugar, minerals and vitamins. 

If, it is not handled properly, it can serve as a potential vehicle for transmission of many diseases like tuberculosis (TB), brucellosis, diphtheria, anthrax, foot and mouth disease, hepatitis, Q- fever, listeriosis salmonellosis, shigellosis, streptococcal infections, staphylococcal poisoning, E.coli poisoning and botulism.

a.  Proteins  - milk proteins are complete proteins of high quality i.e. they contain all the essential amino acids in appropriate proportion.

b.  Minerals – all the mineral elements essential for nutrition are present in milk. Milk is excellent source of Ca & P. Milk is rather law in iron, Copper and Iodine.

c.  Vitamins – these are accessory food factors, which are essential for normal growth.

Milk is a good source of vit A, vit D, thiamine riboflavin etc., but is deficient in vit C.

d. Fat – plays a significant role in the nutritive value, flavor and physical properties of milk and   milk products. Milk fat imparts a soft body smooth texture and rich taste to dairy products. Milk fat is a rich source of energy and contains significant amounts of essential fatty acids (linoleic and arachidonic acid)

e.  Lactose – Principal function is to supply energy. it also helps to establish a mildly acidic environment in the intestine which check the growth of proteolytic & other undesirable organism.

           Energy value

           Milk fat                          9.3 KC/g

           Milk protein                   4.1 KC / g

           Milk sugar                      4.1 KC/g

 

Effect of  Processing on nutritive value of milk

 

Pasteurization carried out with reasonable care has no effect on vitA, carotene, riboflavin and a number of remaining vitaminB and Vit D.

Of this reminder

• 10 % loss of thiamine and

• 20 % loss of Vit C           

may be expected

Sterilization increases the losses of thiamine and ascorbic acid to 30 – 50% & 50 % respectively.

 

Factors influencing quantity (Yield) and quality (Composition) of Milk

• Species – Each species yields milk of a characteristic composition.

• Bread – breads producing large amount of milk yield milk of a lower fat percentage.

• Individuality – each animal yield milk of a composition that is characteristic of that animal

• Interval of milking

• Completeness of milking

• Frequency of milking

• Irregularity of milking

• Day to day milking

• Disease and abnormal conditions

• Portion of milking

• Stage of lactation

• Yield

• Feeding

• Season

• Age

• Condition of cow at calving

• Excitement

• Administration of drugs and hormones

HUMAN MILK

 

• Lower content of Protein and Ash-better digestibility due to soft curd formation.                        

• Higher content of Lactose- easily digestible due to presence of b-galactosidase.

• Better immunological properties

• Ratio of Casein to Whey Proteins is 60:40.    (80:20 is for Cow or Buffalo Milk)                

COLOSTRUM

 

Secretion of memory gland during first few days after calving.

Physico-chemical properties of colostrums:

• Lactose content is only increases with time after calving, other properties are constant or decreases.

• Acidity of colostrums              0.35-0.41

• Values of Specific Gravity, Acidity and RI are higher for Colostrum than that of Cow or Buffalo milk.

• High content of Whey Proteins.

• High acidity is responsible for the coagulation of Colostrum on heating.

 

MILK PRODUCTION AND CONSUMPTION

 

 

 

MILK COMPOSITION

    MILK     =     LIQUID      +       SOLIDS      +       GASES

                        (Water)              (Fat+SNF)            (CO₂,O₂,N₂)

                 (a)   87% Liquid  (water)

            (b)   13% Solids  

                                4 % Fat

                                 9% SNF  Protein   3.3% casein : whey proteins(4:1) 

                                                  Lactose  5.0%

                                                  Ash         0.7%

MILK FAT

Milk is oil in water type true emulsion. In milk fat exists in form of small globules, size of which ranges from 2 to 5 micron. Surface of fat globule is surrounded by fat globule membrane, which contains phospholipids and lipoproteins in form of a complex.

Milk fat is comprised mostly of triglycerides, with small amounts of mono- and diglycerides, phospholipids, glycolipids, and lipo-proteins. Among Fatty Acids it contains higher portion of low carbon atom chains of fatty acids i.e. C4 to C10. 35-43%-of them are unsaturated fatty acids, having one, two or three double bonds.

Fat globules typically aggregate in three ways:

• flocculation

• coalescence

• partial coalescence

MILK PROTEINS

 

 

MILK SUGAR: LACTOSE

This exists only in milk, so as called milk sugar. It forms true solution in milk serum. On crystallization it forms hard crystals. It is one-sixths as sweet as sucrose. Under certain conditions, it is responsible for ‘Sandiness’ in ice cream and sweetened condensed milk. Chemically it is composed of one molecule, each of Glucose and Galactose.

Lactose occurs in two forms, both can occur as hydrate or anhydride.

a- lactose

b- lactose 

On fermentation it gives lactic & other organic acids. It is important from both points of view, i.e. production of cultured milk products & spoilage of milk and milk products.

 

MINERALS

VITAMINS

ENZYMES

Alkaline Phosphate

# Hydrolyze the esters of phosphates

# Inactivated by pasteurization

# Used to determine efficiency of pasteurization

Catalase

# Decomposes H2O2

# Inactivated by pasteurization

Peroxidase

# Catalyses the transfer of O2 from peroxides to other substances

# Stable at pasteurization

Xanthineoxidase

# Catalyses the addition of O2 to a substance or the removal of H2

# Stable at pasteurization 

Lipase

# Hydrolyses the ester linage of fat

# Inactivated by pasteurization

# Produces rancidity

       Proteases

# Hydrolyses peptides linkage

# Moderately stable at pasteurization

# Causes proteolyses

 

PHYSICO-CHEMICAL PROPERTIES OF MILK

Physical state

 Acidity & pH 

 Density & specific gravity

 Colour 

 Flavour

 Viscosity

 Surface tension

 Refractive index

 Specific heat

 Electrical conductivity

 Oxidation-reduction potential

 Boiling point

 Freezing point

 Importance of properties:

Ø  Helps in detection of adulteration.

Ø  Helps in determining quality of milk.

Ø  Helps in processing of milk & milk products.

Ø  Helps in evaluating physical changes in milk & milk products during processing.

 

        Physical state of milk

In milk water is present as continuous phase in which other constituents are either dissolved or suspended.

Lactose and Portion of mineral salts form Solution

Protein and Remainder of minerals form Colloidal

Fat forms Emulsion

        Acidity and pH of milk

a.   Natural or apparent acidity

b.   Developed acidity or real acidity

      Titratable acidity =  DA

                       0.13 to 0.14 %      Cow milk

                       0.14 to 0.15 %      Buffalo milk

 Casein, acid phosphate, citrates, whey proteins, CO2 etc of milk        gives­NA

      Produced Lactic acid from Lactose by LAB gives- DA

      TA is measured in terms of present % Lactic acid

      pH of fresh milk        6.4 to 6.6 - Cow milk

                                     6.7 to 6.8 - Buffalo milk

       Density and specific gravity

D =      Mass (Weight) / Volume

SG is the ratio of density of the substance to density of a standard substance(Water). SG of milk is usually expressed at 60⁰F(15.6⁰C). Average SG of milk at 60⁰F ranges from 1.028 to 1.030 for cow milk and 1.030 to 1.032 for buffalo milk. For skim milk it ranges from 1.035 – 1.037

Specific gravity of milk is lowered by addition of water and cream and increased by addition of skim milk or removal of fat.

 

Although buffalo milk contains more fat than cow milk, its specific gravity is higher than the cow milk; this is because buffalo milk contains more SNF with fat which results ultimately results in higher specific gravity. Percentage of TS or SNF in milk is calculated by formula

% TS = 0.25D + 1.22 F +0.72

% SNF = 0.25D + 0.22 F +0.72

D = 100 (d-1)

d= density of sample of milk at 20⁰ C (68⁰ F)

F= fat percentage of sample

        Color of milk

Color of milk is a blend of individual effects produced by

• Colloidal calcium caseinate/phosphate particles and dispersed/emulsified fat globules, both of which scatter light.

• Carotene (to some extent xanthophylls), which imparts a yellowish color.

Color ranges from yellowish creamy white (cow milk) to creamy white (buffalo milk). Intensity of yellow color of cow milk depends on various factors such as breed, feeds, size of fat globules, fat percentage etc.

 

The greater intake of green feed, results in deeper yellow color of cow milk.

Larger fat globules and higher fat percentage also results in increased intensity of yellow color.

Upon heating whiteness increases due to increased reflection of light by coagulate.

Skim milk has a bluish and whey a greenish yellow color (due to presence of riboflavin)

        Flavor of milk

Flavor is composed of small (odor) and taste. Flavor of milk is a blend of the sweet taste of lactose and salty taste of minerals.

Phospholipids, fatty acids and fat of milk also contribute to the flavor.

Changes in milk flavor may occur due to type of feed, season, stage of lactation, condition of udder, sanitation during milking and subsequent handling during processing. A pronounced flavor of any kind is considered abnormal, source of it may be

• ·      Bacterial growth

• ·      Feed

• ·      Absorbed

• ·      Chemical composition

• ·      Processing & handling

• ·      Chemical changes

• ·      Addition for foreign material

 

Viscosity

Viscosity is the resistance to flow

Milk : 1.5 – 2.0 cp at 20 ⁰ C

Increase in temperature results in decrease in viscosity.

Casein contributes more to viscosity than any other constituent s.

        Surface tension

Surface tension affected by fat content i.e. addition of fat lower down ST.

ST of water : 72 dynes /cm² at 20⁰ C

Milk  :  50 dynes /cm² at 20⁰ C

Skim milk :  52 – 52.5 dynes /cm² at 20⁰ C

        Refractive Index

Refractive Index is the measure of change in direction of light beam in a medium.

γ= Sin i/ Sin r

For water ® 1.33

Milk  ® 1.344 to 1.348

RI is affected by protein, lactose & minerals, not by fat.

Instrument    ® Abbe – refractometer

Specific Heat

Helps in fabrication of equipment and calculating heat requirements to process milk.

specific heat milk ® 0.938 cal at 15⁰ C

Electrical conductivity

 

Measured in terms of specific resistance or specific conductance.

It depends on ions present in milk, mostly Na ⁺ ,K⁺,Cl^- ions are responsible.

Sp. conductivity = sp. conductance

                              sp. resistance

For milk (sp. conductance) ® 4.2 to 6.9

 

Oxi- Red potential

 

Oxi - red potential concerned with the balance in between oxidized and reduced forms of the chemical substances.

Milk  ® + 0.2 to 0.3 ev

Fat, sugar, protein-no affect on ORP

Ascorbic acid, lactic acid ® influences ORP

 

Boiling point

 

Boiling point increases with increase in TS

Pure water ®  100⁰C

Milk  ® 100.17⁰

 

Freezing point

 

Presence of soluble constituents lower or depress freezing point.

For milk ® - 0.525 ⁰C to 0.565 ⁰C

Lactose & minerals affects FP.

Fat & proteins have no effect on FP. Boiling & sterilization increase the value of FP depression but pasteurization has no effect.

Collegative properties

Properties, which depend on number of solute particles in solution i.e. FP, BP, ORP, VP.

Market milk Industry

Term ‘market milk’ refers to fluid whole milk that is sold to individuals usually for direct consumption. It excludes milk consumed on the farm and that used for the manufacture of dairy products.

• Buying & Collection of Milk

A. Buying

     Payment according to weight or volume (flat rate)

     Payment according to the fat content of milk

     Payment according to the use made of milk

     Payment of Premiums

     Payment according to the cost of production

B  Collection

     By Co-operative organization

     By Contractors

     By Individual Producers

‘milk – shed’ is the geographical area from which a city dairy receives its fluid milk supply.

• Cooling and Transportation of Milk

Done on the farm or at the chilling centre. Optimum temperature range for the growth of common milk microorganisms is 20 to 40⁰C.

Effect of storage tamp on bacterial growth in milk:

            Milk held for 18 hrs.                           Bacterial growth

            at tamp (⁰C)                                                    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.

Cooling Methods

In-can or Can-immersion method

Surface cooler

In-tank or Bulk-tank cooler

Milk chilling centers

 

• Receiving of milk

                  Milk Reception operations

a-     unloading

b-     grading

c-     sampling

d-     weighing

e-     testing

 

Bacteriological standards of raw milk:

            SPC/ ml (or gm)                             Grade

            >2x10⁵                                          Very Good    

            2x10⁵  - 1x10⁶                                       Good

            1x10⁶   - 5x10⁵                               Fair

            > 5x10⁵                                          Poor

Pasteurized Milk  < 30,000

 

• Pre-heating – temp. 35-40c

• Filtration/ Classification

• Cooling & storage of raw milk

• Standardization

• Plasteurization

• Homogenization

• Bottling & Packaging

FLUID MILK MANUFACTURING

 

• Receiving of milk(grading, sampling, weighing, testing)

• Pre-heating(35-40⁰C)

• Filtration/Clarification

• Cooling & storage (less than 5⁰C)

• Standardization

• Pasteurization(63⁰C/30min. or 72⁰C/15 sec.)

• Homogenization (63⁰C, 2500 psi)

• Packaging/Bottling

• Storage (5⁰C or below)

 

 

MILK PROCESSING OPERATIONS

 

 

PASTEURIZATION

Louis pasteure – given heat treatment for preservation of wine at 122-140⁰f (50-60⁰C).

Pasteurization of milk was first attributed to doctor Sozhlet of Zermony in 1886.

 

Definition – Pasteurization refers to the process of heating every particle of milk to at least 63⁰C (145⁰F) for 30 minutes, or 72⁰C (161⁰f)  for 15 seconds (or to any temperature time combination which is equally efficient ), in approve and proper operated equipment. after pasteurization milk is immediately cooled to 5⁰C (41⁰f) or below.

   Objectives / purpose:

• Destruction of cent percent pathogenic microorganisms i.e. ensure safe for consumption.

• Improve keeping quality i.e. destruction of almost all spoilage organisms (85-99%)

Objections:

• Significantly reduces nutritive value of milk.

• Pasteurized milk does not clot with rennet.

• It fails to destroy bacterial toxins in milk.

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

Formulation of standards

Consideration involved in formulation of standards for pasteurization.

1.Bacterial destruction

Complete destruction of pathogens. Mycobacterium tuberculosis being considered the most heat resistant among pathogens is chosen as index organism for pasteurization. Any heat treatment (i.e. time- temperature combination), which killed T.B. germs, also destroy all other pathogens present in milk. 

 

2. Creamline reduction

The cream line or cream volume is reduced progressively with increase in the temperature- time of heating

 

3. Phosphatase inactivation

Complete destruction of enzyme phosphatase. Pasteurization is carried out at a heat-treatment temperature above that for phosphates in activation and yet below that for cream line reduction

 

Pasteurization requirements

             Particulars                     30 minute            15 seconds

Kill TB germs                            138⁰F/58.9⁰C            158⁰F/70⁰C

Inactivate phosphatase             142⁰F/61.1⁰C            160⁰F/71.1⁰C

Pasteurization requirements     143⁰F/61.7⁰C            161⁰F/71.7⁰C

Cream line reduction                 144⁰F/62.2⁰C            162⁰F/72.3⁰C

Methods of Pasteurization

A.         In the – bottle Pasteurization

Bottles filled with raw milk and tightly sealed with special caps are held at 63-66⁰C (145-150⁰F) for 30 minutes. Then the bottles are passed through water sprayers which cool both the product and bottle.

Advantages

• Prevents post – pasteurization contamination.

Disadvantages

• Transfer heat very slowly.

• Risk of bottle breakage.

• Require oversized bottles, for milk expansion during heating.

• Special water tight caps are required.

• Method is at present out – dated, although in the bottle sterilization of  milk is widely  prevalent

B. Batch / Holding pasteurization

Also called as Low Temperature Long Time (LTLT) method . Milk is heated to 63⁰C / 145⁰F for 30 minute and then cooled to 5⁰C or below.

In this system heating is done indirectly, heat moves through a metal wall into the product for heating and out of the product for cooling.

The pasteurizer may be of three types:

1.     water-jacketed vat

2.     water-spray type

3.     coil-vat type

Preferably used for cream & ice cream pasteurization.

 

C. High Temperature Short Time (HTST) Pasteurization

 

First developed by A.P.V. Co. in UK (1922).

Used where large volume of milk is handled.

HTST pasteurizer gives a continuous flow of milk which is heated to 72⁰C (161⁰F) for 15 seconds and then cooled to 5⁰C or below

 

Advantages

• Capacity can be increased.

• Equipment can be cleaned easily.

• Thermophils can not grow.

• Automatically controlled.

• Less time consuming.

• Quality is better than Batch pasteurization.

• Low initial cost.

• Low operating cost.

• Economical due to Regeneration Heating and Cooling.

• Space requirement is less.

 Disadvantages

• Small quantity can not be handled.

• Complete destruction is not possible.

• Post pasteurization contamination is possible.

• Formation of milk stones.

PASTEURIZER:

Equipment is a heat exchanger and is usually of a plate type. In this a number of stainless steel plates separated by rubber jackets are held together in a screw press to form a series  of narrow cavities through which liquid can flow . Each plate has parts to direct the milk and the heating or the cooling medium.

Milk flow in HTST Pasteurization system

Steps or stages, involved as milk passes through HTST system.

1.     Balance tank

2.     Pump

3.     Regeneration heating

4.     Holding

5.     Regeneration cooling

6.     Final cooling by chilled water or brine

 Filter / Clarifier, homogenizer etc. can also be arranged in the circuit of HTST –pasteurization, if required. There may be some variation in the use of order of these steps in different milk processing plants.

Important parts of a HTST pasteurizer

1.     Float – controlled balance tank (FCBT)

2.     Pump

3.     Plates

4.     Filter

5.     Holding

6.     Flow controller

7.     Regeneration cooling

8.     Control panel

9.     Hot water set

10.   Automatic control devices 

MODERN PASTEURIZER  

The modern electric pasteurization consists of a regenerative pre heater. The milk is brought here at about 120⁰F (48.89⁰C), other part consists of a rectangular, vertical chamber two feet high and about two centimeters in cross section. The two sides are made up of carbon electrodes about 7-8 centimeters apart. The milk flows between these electrodes. The temp of milk is raised to 161⁰F and to 163⁰F (71.7⁰C to 72.8⁰C) by the resistance offered to the passage of a 110 v A.C. The milk is exposed at this temperature for 15 sec after which it is cooled.

 Nutritive valve of HTST Pasteurized milk

• Vit C content of cow’s milk – 2 mg/100 ml. Losses of vitC – 70%.

• Other heat labile vits– vit B₁₂, B₁ (thiamin) vit B₁₂ is heat stable to some extent, it is affected by pasteurization and destroyed by sterilization

• Human milk contains more vit C then the milk of cow or buffalo.

• Human milk contains less thiamine (B₁) then the milk of C & B

• The biological value of milk proteins and vitamins are only slightly lowered down on pasteurization.

Losses in boiling of milk

• Losses of heat labile components.

• By formation of milk stone.

• Losses in the skin or deposit may amount to a loss of 14 percent for Ca & protein and 20 percent for fat.

Photochemical reaction:

        Tropical conditions

Vit.C------oxidation-----> dehydroascorbic acid (unstable to heat)

          Riboflavin (sensitizer)      Losses on pasteurization

                                                                 

Alkaline phosphatase

Plays an important role in the energy transfer mechanism of the living beings.

APase activity in buffalo milk is lower than the cow milk (almost half)

38-40% AP – concentrated in cream where it is absorbed on the fat globules, remaining – distributed through out the milk in the lipoprotein particles.

 Activation of AP:

Boiled milk –         2-3 fold

Dialyzed boiled milk – do not exhibit any action.

k- casein of casein fractions & β– lactoglobulin of whey proteins.

α-lactabumin of whey proteins – no effect

Protease peptone slight effect

Enzyme activity:

Goat < buffalo< cow< ewe

pH for optimum activity of AP –  10

Thermal death point of Mycobacterium tuberculosis and pasteurization coincides with each other hence the activity of the enzymes serve as an indicator for the presence of the bacteria in the milk. 

Alkaline phosphatase test

Alkaline phophatase is an enzyme present in raw milk. This enzyme is heat labile and destroyed by adequate pasteurization. If the milk is under pasteurized it gives positive reaction to phosphatase test.

Enzyme has the ability to liberate phenol from phenol phosphoric acid compounds. Free phenol gives a deep blue colour with certain organic compounds; this is the basis for the phosphatase test.

Disodium phenyl phosphate is used as the source of phenol and 2,6 dichloroquinone – chlorimide is the indicator reagent.

 

Milk is incubated with the DSPP and then indicator reagent is added. Blue colour indicates improper pasteurization

 

DSPP ---- alkaline phophatase------>free phenol

(phenol phosphoric                          +---->indophenol(blue colour)

acid compound)                            2,6 DCQC

                                     (organic compound indicator reagent)

                                                                  

Amount of phenol liberated is proportional to the active enzyme present. Colour intensity is measured by colourimetry.

 

Preservatives, suggested for use in milk (not for commercial use)

• Antibiotics – eg penicillin

• Bromine compounds of acetic acid

• Formaldehyde – 40% formalin solution  in proportion of 1: 20,000 to 1 ; 25,000 

• Ammonium compounds

• Mercaptoproptonie acid and plant extracts

• Peroxides   

• Hydrogen peroxides – 0.05-0.10%

• Solid peroxides thiocynate K₂Cr₂O₇ (30%) + H₂O₂  (30 % w/v)

• Formalin (36%) + H₂O₂(30% w/v)

• Boric acid and borax (oxidizing agent)     1:1000 to 1:2000

• Salicylic & benzoic acid     1:20,000 to 1: 25,000

• Carbonates & bicarbonates

HOMOGENIZATION

Refers to the process of forcing the milk through a homogenizer with the object of sub – dividing the fat globules to 2 µm or less in diameter.

This process insure breakup of the fat globules to such an extent that after 48 hours of storage no visible cream separation occurs on the milk surface. In efficiently homogenized milk, fat percentage of the milk in the stop 100ml & and in the remaining milk does nut differ by more than 10 percent.

Temperature of Homogenization

At the time of  homogenization, milk temp should be above the MP of fat I.e. > 33⁰C This is because fat should be in the liquid state fat should be in the liquid state for proper subdivision. The enzyme lipase should be inactivated, preferably prior to homogenization or immediately afterwards. This can be achieved by heating milk to a temp of 55⁰C or above.

In routine practice milk is heated to 65-70⁰C for homogenization on. Danger be avoided during or after homogenization.

Pressure of homogenization

 

    If fat content is < 6 percent, single stage homogenization( 2000 – 2500 psi) is used.

higher pressure may increase the susceptibility of milk to curdle on heating.

 

    if fat content is > 6 per cent

Two – stage homogenization is required to prevent fat clumping.

Ist stage       2000psi

IInd stage     500 psi

1psi = 0.07 kg /cm²

Homogenizer placement

1.     between regeneration heating and final heating section –best placement

2.     between final heating and holding section

3.     between FDV and regeneration cooling

4.     before regeneration heating

 

Possible sequences of various process

• clarification – Pre-heating – homogenization – pasteurization – cooling

• clarification – Pre heating – pasteurization – homogenization – cooling

• Pre- heating – Homogenization – clarification – pasteurization – cooling**

• Pre- heating – clarification - Homogenization – pasteurization – cooling

• Pre- heating – clarification – pasteurization – homogenization – cooling

(**Recommended under Indian condition ) main consideration

 

     Main considerations involved in pasteurization

 

Milk should be pasteurization prior or after homogenization

Milk should be clarified after homogenization

Merits of homogenization

• No formation of cream layer.

• Fat in milk does not churn due to rough handling or excessive agitation

• More palatable due perhaps to brighter appearance heavier body and richer flavor.

• Produces soft curd and is better digested.

• Less susceptible to oxidized flavor development.

FARREL INDEX

Used to measure efficiency of Homogenization. It is obtained by counting number of fat globules over a standard field of hundred micron square. this is observed under a high power microscope. Homogenization is a mechanical process of making a stable emulsion of milk fat and milk serum. Homogenization require pre heating to liquefy fat and to in activate lipase.

 

MICROBIOLOGY OF MILK & MILK PRODUCTS

Microorganisms:       Fungi----------- Yeast and Mould

                                Bacteria                   

                                Viruses

BACTERIA

microscopic unicellular bodies, generally 1.25 m in size, found nearly everywhere in nature.

YEAST 

unicellular cylindrical or spherical bodies, reproduce by transverse fission.

MOULD        

multi-cellular, may be observed as cottony growth of mycelia, moulds are found in soil, feeds, manure & poorly washed utensils.

Activity of microorganisms is responsible for all kind of changes in flavor and appearance after milking.

Order of dominance in milk:

Bacteria >mould>yeast> virus

Microorganisms in milk may be desirable or undesirable or pathogens. 

Growth of microorganisms (i.e. increase in number)

The number multiplied during production & handling of milk is growth of microorganisms, changes in physico-chemical properties in milk are the result of activities of microorganisms.

 Stage of growth: - 4 phases

1. Lag phase- phase of adjustment.

2. Log phase or exponential phase - exponential increment in cell number.

3. Stationary phase- neither increases nor decree in cell number.

4. Death phase- decrease in cell number.

Factors affecting growth:

1. Food supply/ Kind of food

2. Moisture

3. Air (aerobic / anaerobic)

4. Acidity or pH – pH 5.6 to 7.5 most favorable for microbial growth

5. Preservatives

6. Light

7. Concentration of Food

8. Temperature

 Products of microbial growth: - enzymes, pigments, toxins, bacteriocins; degraded products of fats, proteins, sugar, etc. depending on kind of microorganism.

Microorganisms in Milk

Milk is sterile at secretion in the udder but is contaminated by bacteria even before it leaves the udder. Except in the case of mastisis, the bacteria at this point are harmless and few in number. Further infection of the milk by microorganisms can take place during milking, handling, storage, and other pre-processing activities.

Lactic acid bacteria: This group of bacteria are able to ferment lactose to lactic acid. They are normally present in the milk and are also used as starter cultures in the production of cultured dairy products such as yogurt. 

Streptococcus: all the food Streptococci have been changed into Lactococci except Strptococcus salivarious subsp. thermophilus.

Lactococci

o        L. delbrueckii subsp. lactis 

o        Lactococcus lactis subsp. cremoris 

o        Lactobacillus casei

o        L.delbrueckii subsp. lactis 

o        L. delbrueckii subsp. bulgaricus 

Leuconostoc

 

Coliforms: coliforms are facultative anaerobes with an optimum growth at 37° C. Coliforms are indicator organisms; they are closely associated with the presence of pathogens but not necessarily pathogenic themselves. They also can cause rapid spoilage of milk because they are able to ferment lactose with the production of acid and gas, and are able to degrade milk proteins. They are killed by HTST treatment, therefore, their presence after treatment is indicative of contamination. Escherichia coli is an example belonging to this group.

 

Significance of microorganisms in milk:

 

• Information on the microbial content of milk can be used to judge its sanitary quality and the conditions of production.

• If permitted to multiply, bacteria in milk can cause spoilage of the product.

• Milk is potentially susceptible to contamination with pathogenic microorganisms. Precautions must be taken to minimize this possibility and to destroy pathogens that may gain entrance.

• Certain microorganisms produce chemical changes that are desirable in the production of dairy products such as cheese, yogurt.

Spoilage Microorganisms in Milk

The microbial quality of raw milk is crucial for the production of quality dairy foods. Spoilage is a term used to describe the deterioration of a foods' texture, colour, odour or flavour to the point where it is unappetizing or unsuitable for human consumption. Microbial spoilage of food often involves the degradation of protein, carbohydrates, and fats by the microorganisms or their enzymes.

In milk, the microorganisms that are principally involved in spoilage are psychrotrophic organisms. Most psychrotrophs are destroyed by pasteurization temperatures, however, some like Pseudomonas fluorescens, Pseudomonas fragi can produce proteolytic and lipolytic extracellular enzymes which are heat stable and capable of causing spoilage.

Some species and strains of Bacillus, Clostridium, Cornebacterium, Arthrobacter, Lactobacillus, Microbacterium, Micrococcus , and Streptococcus can survive pasteurization and grow at refrigeration temperatures which can cause spoilage problems.

Pathogenic Microorganisms in Milk

Hygienic milk production practices, proper handling and storage of milk, and mandatory pasteurization has decreased the threat of milkborne diseases such as tuberculosis, brucellosis, and typhoid fever. There have been a number of foodborne illnesses resulting from the ingestion of raw milk, or dairy products made with milk that was not properly pasteurized or was poorly handled causing post-processing contamination. The following bacterial pathogens are still of concern today in raw milk and other dairy products:

Mycobacterium tuberculosis-  Tuberculosis in man and cow, source of this infection is cow.

Micrococcus piogens causes sour throat in man. It is also responsible for mastitis, source of infection is cow.

Brucella abortus causes undulant fever in human and brucellosis in cow.

Bacillus antracis causes anthrax in both human and cow.

Salmonella typhi causes typhoid.

Shigella dysentriae causes dysentery in infants.

Corynebacterium diphtheria causes diphtheria.

Coxiella burnetti causes Q- fever.

Vibrio comma causes cholera.

Salmonella paratyphi & salmonella typhimurium causes enteric fever.

Streptococcus pyogenes var.scarlatinae causes scarlet fever.

Bacillus cereus

Listeria monocytogenes

Yersinia enterocolitica

Escherichia coli O157:H7

Campylobacter jejuni

It should also be noted that moulds, mainly of species of Aspergillus , Fusarium , and Penicillium can grow in milk and dairy products. If the conditions permit, these moulds may produce mycotoxins which can be a health hazard.

 

MILK FERMENTATION

Fermentation is a metabolic process in which chemical changes are brought about on organic substances such as proteins, carbohydrates or fats through the action of enzymes liberated by specific living microorganisms.

or

An anaerobic oxidation of compounds by the enzymes/ action of microorganism, neither gaseous oxygen nor a respiratory chain is involved in this energy leading process, and organic compound is the electron acceptor.

Complex organic molecules/compounds ----------------> simple substances/molecules

Types of Fermentation

 1.     ALCOHOLIC FERMENTATION

Sugar------------   yeast------------------->   ETOH + CO₂

(glucose)                                 (major end products)

minor end products- acid, aldehyde, glycerol, pyruvic acid ,higher alcohols (specially amyl alcohol), succinic acid and tyrosol.

Glycogenesis:           Glucose                           Glycogen

Glycogenolysis:        Glycogen                         Glucose

Glycolysis:                Glucose                           Pyruvic Acid

                      Mechanism of Fermentation

2.    ACIDIC OR LACTIC ACID FERMENTATION

Most important fermentation, used to manufacture various dairy products having distinct flavour and aroma  depending on Inoculums and Incubation conditions. Causes protein denaturation.

Homofermentation:

Lactose------------------------- Lactic Acid (quantitatively)

Heterofermentation:

Lactose-------------------------LA + ETOH + CO₂ (1: 1 :1)

 

Fermented Products

Acidophilus milk

    Lacobacillus acidophilus

Cultured buttermilk

    Leuconostoc lactis-diacetylactis

    Leuconostoc cremoris

    Lactococcus cremoris

Kefir

    Lactococcus lactis

    Lactobacillus bulgaricus

    Saccharomyces sp.

Sour cream

    Lactococcus sp.

    Leuconostoc sp.

 Yogurt

    Streptococcus thermophilus

    Lactobacillus bulgaricus

CurdKumis

    Bulgarian buttermik

Significance of Fermented milk

• more palatable

• more nutritive

• more digestive

• therapeutic characteristics

Regular consumption has claimed to increase longitivity of human beings.

Natural fermentation of milk

sequential fermentation (4 steps).

 1.Step first is dominated by Streptococci, results in production of lactic acid, followed by self-inhibition of Streptococci due to excessive production of lactic acid.

 

2.Second step is dominated by Lactobacilli (which can tolerate lower values of pH or high acid), results in excessive or additional production of lactic acid, and causes self-inhibition of Lactobacilli.

 

3.Yeast and moulds dominate third step, moulds oxidize acids and yeast produces alkaline end products from proteolysis, results in decrease in acidity.

 

4.Fourth step is dominated by spore forming bacteria, results further decrease in acidity even below to the original acidity of raw milk and results in development of putrid off flavor.

 

NATURAL PRESERVATION OF MILK

Milk is complex of Proteins, Lipids, and Carbohydrates. Besides these basic nutrients, it also contains a complex spectrum of Biologically Active Molecules, which are the basic protective factors of milk. These are mainly present in form of Proteins. These are

Immunoglobulins

Lactoferrin

Lactoperoxidase

Lysozyme

These components are mainly present in whey fraction of milk as Antimicrobial Proteins, these bioactive proteins have high functional properties.

Non-specific factors highly basic in nature, which give them specific physicochemical properties. All molecules act either individually or in combination with one another to provide an effective defense system.

Specific Antimicrobial Factors

SigA. IgA, IgA2

IgG

IgM

Macrophages

Lymphocytes

MHC( Major Histocompatibility Complex)

Component C1-C9

Non-specific Factors:

Lactoperoxidase

Lactoferrin Lysozyme

Xanthine oxidase

Superoxidedismutase

N-acetyl-B-D-glucosaminidase

Oligosaccharides

Glycopeptides

Folate binding Proteins

Immunoglobulins:

Are major immune system of an animal. A new born calf is born without immunoglobulins in its blood and has to be dependent upon colostrums for the development of its immune system. However a new born human has circulating immunoglobulin in its blood. Structurally all immunoglobulins are 4- chain poly peptide structure consisting of 2- heavy chains (H-chain) and 2 light chains (L -chain), these are catagoriesed on the bassis of their chemical differences in the heavy chain.

Three major classes of immunoglobulins present in colostrum and milk are

IgG, IgM, SIgA 

IgG₁                             Major class of C & B milk

SIgA                            Major class of human milk

         

LP System of milk Preservation

The enzyme Loctoperoxidase is present in milk of various species in adequate quantities to permit  activation of LP –System.

Bovine milk                              3 units/ ml

Buffalo milk                              0.16 to 0.21 units/ml

Ewe milk                                  0.14 to 2.38 units/ml

Goat milk                                 0.05 to 3.55 units/ml

In addition to Lactoperoxidase, SCN^-  and  H₂O₂  act as component of LP- system of milk preservation and there quantities very according to species and breed of animal, and generally there additional amount is required to activate LP system). For activation exogenous addition of up to 21 ppm of SCN^- & 30 ppm of H₂O₂ is required.

In LP –system thiocynate (SCN-)  is oxidized to yield some intermediate oxidation products, which have active antimicrobial  properties.

SCN^- ---+ H₂O₂ ---LP----> OSCN^- + H₂O ----+ H₂O₂--LP --->  O₂SCN^-> + H₂O-----+ H₂O₂^__LP^__>-->

(Thiocynate)          (Hypothiocynate ion)                             (Oxyacids)

Lactoperoxidase catalyses transfer of O₂ from peroxide. Complete oxidation of thiocynate (SCN^-) does not yield any product having antimicrobial effects, these effects are mediated by oxyacides of SCN^- i.e. OSCN^- and O₂SCN^-.These oxyacides exert antimicrobial effects by the oxidation of vital  -SH groups in metabolic enzymes i.e.Hexokinase, Glyceraldehyde-3-phosphate dehydrogenase and/or depletion of reduced Nicotinamide adenine nucleotide (NAD).

Mammalian cells are not affected by LP-system.

LP-system affects both gram +ve and Gram-ve organisms.

LP-system has emerged as an alternative techno-economic alternative to refrigeration  for preservation of milk under tropical climatic conditions.

its activity is nearly 23 % higher in buffalo milk as compare to cow milk.