PHYSICO-CHEMICAL PROPERTIES OF MILK

 Importance of properties of milk

• Helps in detection of adulteration
• Helps in determining quality of milk
• Helps in processing of milk and milk products
• Helps in evaluating physical changes in milk & milk products during processing
• Helps in pricing of milk and milk products

​

• 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

 Physical state of milk

• In milk, water is present as continuous phase and other constituents are either dissolved or suspended on it
• Lactose, whey protein and portion of mineral salts form the true solution
• Casein protein and some minerals form colloid
• Fat forms emulsion

Acidity and pH of milk

Natural or apparent acidity

Due to the presence of casein, acid phosphate, citrates, whey proteins, CO₂ etc. in milk; the higher the milk SNF, higher is the NA and vice-versa

Developed or real acidity

Due to lactic acid formed in milk as a result of bacterial action on lactose

Titratable acidity =  NA + DA

                       0.13 to 0.14 %      Cow milk

                       0.14 to 0.15 %      Buffalo milk

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

Higher pH values indicate udder infection (mastitis) while lower pH values indicate bacterial action

0.18%

Relationship of Titrable acidity and pH of milk

• Titratable acidity and pH are inversely related; as TA ↑es, pH ↓es
• TA measures the total amount of acid in milk, while pH is a log measure of H⁺ concentration
• TA is more sensitive to small changes in acidity than pH, so it is preferred in the dairy industry for monitoring the progress of acidification 

• When LAB convert lactose to lactic acid, the total TA goes up, and the pH value goes down
• A change of one full pH unit means a tenfold change in acidity. This makes pH less sensitive to small changes compared to TA
• It is easier to detect a small increase in acidity indicating a culture is active by measuring a change in TA than by observing a small change in pH

​

Eg: In fresh milk, the pH is around 6.5−6.7 with a titratable acidity of 0.14−0.18%. As milk ages and ferments, its pH might drop to 4.5−4.6, while its TA increases to 0.75−0.90%

Density and Specific gravity

D = Mass (Weight)/Volume

SG is the ratio of density of the substance to density of water. SG of milk is usually expressed at 60^°F(15.6 ^° C) or 68 ^° F (20 ^° C)

• Cow milk= 1.028-1.030; Buffalo= 1.030-1.032 and Skim milk= 1.035-1.037
• SG 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 cow milk; as buffalo milk contains more SNF with fat resulting a higher SG
• TS and SNF in milk is calculated by

% TS = 0.25D + 1.22 F +0.72

% SNF = 0.25D + 0.22 F +0.72

D = 1000 (d-1)

d= density of sample of milk at 20^°C (68^°F)

f= fat percentage of sample

• The SG of a fluid varies with its temperature. The rise in SG is more rapid at low temp. than at higher ones and average to 0.001 and is attributed to:
• Change in SG of fat due to partial cooling and solidification
• Hydration of the proteins
• Loss of carbon dioxide
• Presence of air bubbles
• Water reaches its maximum SG at 39^°F/3.9 ^°C, while milk attain its maximum SG at 31.01 ^°F/–0.55 ^°C, the FP of milk
• As fat is the lightest constituent of milk, the more it is, the lower the SG is and the greater the SNF content, the heavier the milk will be

Color of milk

Color of milk is a blend of individual effects produced by colloidal Ca caseinate/phosphate and emulsified fat globules, both of which scatter light.

• Carotene (to some extent xanthophylls), 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 % etc.
• The greater intake of green feed results in deeper yellow color of cow milk
• Larger fat globules and higher fat content 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 smell (odor) and taste. Flavor of milk is a blend of the sweet taste of lactose and salty taste of minerals, moderated by proteins
• Phospholipids, fatty acids and milk fat also contribute to 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, it’s sources may be
• Bacterial growth
• Feed
• Absorbed
• Chemical composition
• Processing & handling
• Chemical changes
• Addition for foreign material

​

Viscosity

• Viscosity is the resistance to flow of liquid due to the internal frictional force between molecule
• The viscosity of milk and cream create an impression of richness to consumer and contributes to mouthfeel and flavor release from organoleptic stand point
• Milk: 2.0– 2.1 cP, skim milk: 1.5- 1.8 cP, whey: 1.2 cP at 20^°C
• Increase in temperature decrease in viscosity and vice versa
• Casein contributes more to viscosity than any other constituents
• Homogenization increases viscosity

​

Viscosity of milk and milk products depends on:

• Temperature
• Extent of the state of dispersion of solids
• pH

Salient contributors:

• Caseinate micelles
• Fat globules
• Lactose has inverse effect
• An increase in temperature causes marked reduction in viscosity. For e.g., at 20°C, milk is about half viscous as at 0°C and at 40°C, it has approx. one-third value of 0°C

• pH affects viscosity of milk  primarily through its influence on protein aggregation
• At 6.7 to neutral pH, milk is more stable and has a lower viscosity
• As pH is lowered, Ca activity increases, leading to larger casein micelles and increased viscosity, especially after heat treatment
• Conversely, increasing the pH beyond 6.7 also increases viscosity, due to different protein interactions

Surface Tension

Surface tension is the tendency of the surface film of a liquid to minimise surface area caused by the attraction of particles in surface layer by the bulk of liquid

• Surface tension affected by fat content i.e. addition of fat lower down ST
• ST of water : 72 dynes/cm at 20 ^°C
• Milk :  50 dynes/cm at 20 ^°C
• Skim milk:  52 – 52.5 dynes/cm at 20 ^°C
• Cream: 46- 47 dynes/cm at 20 ^°C

​

​

The principal surface active components of milk:

~Proteins

~Milk fat

~Phospholipids

~FFA

• Powerful depressants of tension include: Fat, Casein, Lactalbumin, Lactglobulin
• Lactose, salts, heating, homogenization, etc. increase ST

Refractive Index

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

γ= Sin i/ Sin r (Snell’s law)

• Water- 1.33
• Milk- 1.344 to 1.348
• RI is affected by protein, lactose & minerals, not by fat.
• Instrument= Abbe – Refractometer

​

Used as a means of determining total solids or added water in milk

​

• RI of water = 1.33299
• Cow milk = 1.3440- 1.3485
• Buffalo milk= 1.3440- 1.3485
• Goat, ewe, human= higher than above
• Milk fat = 1.4537 to 1.4552 at 20°C

Specific Heat

Specific heat is the amount of heat energy required to raise the temperature of one unit mass of a substance by 1 degree Celsius (one Kelvin). 

• Milk has a specific heat of around 3.93 kJ/kg^°C, which means it takes approx. 3.93 kilojoules of energy to raise the temperature of 1 kilogram of milk by 1^°C
• Helps in fabrication of equipment and calculating heat requirements to process milk
• Specific heat milk: 3.93 kJ/kg^°C  (or 3.93 J/g^°C)

• SH of milk determines the energy required for heating and cooling, directly affecting the processing time, equipment design, and energy efficiency
• Understanding milk’s SH is crucial for controlling the temperature accurately during processes like pasteurization

Electrical Conductivity

• EC increase with temperature
• Contents of dissolved salts influence the EC
• Mastitis markedly increases EC
• Normal milk EC: 4.0 - 5.5 mS/cm at 25°C

EC is considered as an index of:

• Udder infection
• Added water
• Added neutralizers
• As a means of controlling solids concentration and composition in dairy processing

EC measures how well a substance conducts electricity and in milk, it's influenced by presence and concentration of dissolved ions, particularly Na, K, and Cl ions. 

• The significance of EC of milk is its use as a rapid and inexpensive indicator of udder infection and for assessing overall milk quality
• During mastitis, the mammary gland is damaged, leading to increased permeability for ions between the blood and milk. This results in an increase in Na and Cl concentrations and a decrease in K and lactose concentrations in the milk
• As EC is directly related to the concentration of free ions in a solution, the conductivity of milk from an infected quarter rises significantly compared to milk from a healthy udder

Oxi- Red potential

• Oxi-red potential of milk is a measure of its ability to gain or lose electrons. Fresh milk has ORP around +250 to +350 mV, indicating an oxidized state
• ORP is concerned with the balance in between oxidized and reduced forms of ions in milk
• Fat, sugar, protein- no affect on ORP
• Ascorbic acid, lactic acid and riboflavin contributes to ORP
• High ORP in milk is associated with the presence of dissolved oxygen, while low, negative values are due to fermentation, where oxygen is consumed and acidic by-products are formed 

​

Freezing Point

• Presence of soluble constituents lower or depress freezing point
• For milk - 0.516^°C to - 0.565 ^°C
• Lactose and minerals affects F. P.
• Fat and proteins have no effect on F. P. but addition of water raises it
• Boiling or sterilizing milk causes a slight increase in its freezing point, means it freezes at a temperature closer to 0°C than raw milk but pasteurization has no effect 

​

• Skim, whole milk or cream have same FP
• Addition of water increase freezing points
• The magnitude of the rise in freezing point is roughly proportional to the amount of water added. This relationship is used to identify and measure the amount of added water in milk samples
• Addition of 1% water increase freezing point 0.0056 ^°c
• Measure by cryoscopy

Boiling Point

• Water boils at 212 ^°F (100 ^°C) at sea level, while milk boils at 212.3 ^°F (100.17 ^°C)
• Milk boils at a temperature slightly higher than water
• Milk is slightly heavier than water because of its solute content and boiling point of a liquid is influenced by factors responsible for its Sp. gravity
• Boiling point increases with increase in TS
• Determined by thermometer

Why does milk rise and overflow when it boils?

The reason is because milk is an emulsion of fat and water. When milk is boiled, the fat separates to the top and forms a layer on top. The water below boils and vaporizes to form steam, but is trapped by the layer of fat above. It pushes the layer up, causing milk to rise. This condition is favoured by foaming of milk protein during boiling.

Nutritive value of milk