Institute of Food Technology
Definations, Nutritive value of milk, Effect of processing on nutritive value, Factors influencing quantity(yield) and quality(composition) of milk, Human milk, colostrum
MILK PRODUCTION AND CONSUMPTION
Milk fat, Milk proteins, Milk sugar( lactose), Minerals, Vitamins, Enzymes
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, Collegative properties
MARKET MILK INDUSTRY
Buying, Collection, Cooling ,transportation and storage of milk, Fluid milk manufacturing
MILK PROCESSING OPERATIONS
Homogenization, Pasteurization, Sterilization, Clarification, Standardization, Separation
MICROBIOLOGY OF MILK & MILK PRODUCTS
Bacteria, Yeast, Mould, Order of dominance of microorganisms in milk, Growth of microorganisms, Factors affecting growth of microorganisms, Microorganisms occurring in milk: LAB, Spoilage causing microorganisms and Pathogens.
Types of Fermentation: acidic fermentation, alcoholic fermentation, citrate fermentation, Mechanism of fermentation processes
NATURAL FERMENTATION OF MILK
4 step sequential process
NATURAL PRESERVATION OF MILK
Immunoglobulins, Lactoferrin, Lactoperoxidase, Lysoenzyme
LP –SYSTEM OF MILK PRESERVATION
Components of LP-system, Mechanism of preservation
"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.
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
Condition of cow at calving
Administration of drugs and hormones
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)
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 = 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)
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:
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.
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.
# Hydrolyze the esters of phosphates
# Inactivated by pasteurization
# Used to determine efficiency of pasteurization
# Decomposes H2O2
# Inactivated by pasteurization
# Catalyses the transfer of O2 from peroxides to other substances
# Stable at pasteurization
# Catalyses the addition of O2 to a substance or the removal of H2
# Stable at pasteurization
# Hydrolyses the ester linage of fat
# Inactivated by pasteurization
# Produces rancidity
# Hydrolyses peptides linkage
# Moderately stable at pasteurization
# Causes proteolyses
PHYSICO-CHEMICAL PROPERTIES OF MILK
Acidity & pH
Density & specific gravity
Importance of properties:
Ř Helps in detection of adulteration.
Ř Helps in determining quality of milk.
Ř Helps in processing of milk & milk products.
Ř 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
· Chemical composition
· Processing & handling
· Chemical changes
· Addition for foreign material
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 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 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
Helps in fabrication of equipment and calculating heat requirements to process milk.
specific heat milk ® 0.938 cal at 150 C
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
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 increases with increase in TS
Pure water ® 1000C
Milk ® 100.170
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.
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
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
By Co-operative organization
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 ( * )
Multiply initial count with this factor to get final count.
In-can or Can-immersion method
In-tank or Bulk-tank cooler
Milk chilling centers
Receiving of milk
Milk Reception operations
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
Cooling & storage of raw milk
Bottling & Packaging
FLUID MILK MANUFACTURING
Receiving of milk(grading, sampling, weighing, testing)
Cooling & storage (less than 50C)
Pasteurization(630C/30min. or 720C/15 sec.)
Homogenization (630C, 2500 psi)
Storage (50C or below)
MILK PROCESSING OPERATIONS
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%)
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.
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
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.
Prevents post – pasteurization contamination.
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
Capacity can be increased.
Equipment can be cleaned easily.
Thermophils can not grow.
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.
Small quantity can not be handled.
Complete destruction is not possible.
Post pasteurization contamination is possible.
Formation of milk stones.
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
3. Regeneration heating
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)
6. Flow controller
7. Regeneration cooling
8. Control panel
9. Hot water set
10. Automatic control devices
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.
Vit.C------oxidation-----> dehydroascorbic acid (unstable to heat)
Riboflavin (sensitizer) Losses on pasteurization
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
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
Mercaptoproptonie acid and plant extracts
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
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 /cm˛
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.
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
microscopic unicellular bodies, generally 1.25 m in size, found nearly everywhere in nature.
unicellular cylindrical or spherical bodies, reproduce by transverse fission.
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
Lag phase- phase of adjustment.
Log phase or exponential phase - exponential increment in cell number.
Stationary phase- neither increases nor decree in cell number.
Death phase- decrease in cell number.
Factors affecting growth:
Food supply/ Kind of food
Air (aerobic / anaerobic)
Acidity or pH – pH 5.6 to 7.5 most favorable for microbial growth
Concentration of Food
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.
o L. delbrueckii subsp. lactis
o Lactococcus lactis subsp. cremoris
o Lactobacillus casei
o L.delbrueckii subsp. lactis
o L. delbrueckii subsp. bulgaricus
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:
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
Brucella abortus causes undulant
fever in human and brucellosis in cow.
Bacillus antracis causes anthrax in
both human and cow.
Salmonella typhi causes
Shigella dysentriae causes
dysentery in infants.
Corynebacterium diphtheria causes
Coxiella burnetti causes Q- fever.
Vibrio comma causes cholera.
Salmonella paratyphi & salmonella
typhimurium causes enteric fever.
Streptococcus pyogenes var.scarlatinae causes scarlet fever.
Escherichia coli O157:H7
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.
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.
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.
Lactose------------------------- Lactic Acid (quantitatively)
Lactose-------------------------LA + ETOH + CO2 (1: 1 :1)
Significance of Fermented milk
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
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
MHC( Major Histocompatibility Complex)
Folate binding Proteins
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.