Physico-chemical Properties of Milk

Importance of properties

• Helps in detection of adulteration.

• Helps in determining quality of milk.

• Helps in processing of milk & milk products.

• 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 solution.

•  Fat forms Emulsion.

MILK = LIQUID + SOLIDS + GASES

                        (Water)    (Fat + SNF) 

Acidity and pH of milk 

Milk Acidity:

Milk contains natural acids, mainly lactic acid, citric acid, and carbonic acid. Acidity in milk is classified into two types:

1. Natural Acidity (Real Acidity):

   • Comes from milk proteins (casein), dissolved CO₂, citrates, and phosphates.

   • Fresh milk has slight acidity due to these naturally occurring components.

2. Developed Acidity (Titratable Acidity):

   • Occurs due to bacterial fermentation of lactose into lactic acid.

   • Increases as milk gets older or spoiled.

pH of Milk:

• Fresh Milk: Typically has a pH of 6.6 to 6.8 (slightly acidic).

• Sour Milk: As milk ferments, bacteria produce lactic acid, lowering the pH to below 6.0.

• Spoiled or Curdled Milk: The pH can drop further, around 4.5 to 5.0, which causes casein proteins to coagulate.

• Boiled Milk: May have a slightly higher pH due to CO₂ loss.

Milk Acidity Measurement:

• pH Meter: Measures the actual hydrogen ion concentration.

• Titratable Acidity (TA): Expressed as a percentage of lactic acid (usually 0.13% to 0.18% in fresh milk).

Effect of pH on Milk Processing:

• Cheese Making: Requires lowering pH to aid curdling.

• Yogurt Production: Bacteria ferment lactose, reducing pH to around 4.5.

• Pasteurization Stability: Milk with high acidity may curdle during heating.

Density & Specific Gravity of Milk

Density of Milk:

• Definition: Density is the mass per unit volume of milk, usually expressed in g/cm³ or kg/m³.

• Average Density: Fresh cow’s milk typically has a density of 1.028 to 1.034 g/cm³ at 20°C.

• Factors Affecting Density:

  • Fat Content: Higher fat reduces density. Skim milk has higher density than whole milk.

  • Temperature: Higher temperatures decrease density due to expansion.

  • Solids-Not-Fat (SNF): More proteins, lactose, and minerals increase density.

Specific Gravity of Milk:

• Definition: Specific gravity is the ratio of the density of milk to the density of water at a standard temperature (typically 15.5°C or 20°C).

• Formula: Specific Gravity=Density of MilkDensity of Water at the same temperature\text{Specific Gravity} = \frac{\text{Density of Milk}}{\text{Density of Water at the same temperature}}Specific Gravity=Density of Water at the same temperatureDensity of Milk​

• Typical Values:

  • Whole Milk: 1.028 – 1.034

  • Skim Milk: 1.035 – 1.037 (higher due to fat removal)

  • Cream: 1.006 – 1.012 (lower due to high fat content)

Measurement of Specific Gravity:

• Lactometer: A specialized hydrometer used to measure the specific gravity of milk.

• Digital Density Meter: Provides accurate readings by measuring mass and volume.

Importance of Density & Specific Gravity in Milk Industry:

1. Milk Quality Testing: Helps detect adulteration (e.g., dilution with water lowers specific gravity).

2. Standardization: Assists in adjusting milk composition for dairy products.

3. Fat Content Estimation: A lower-than-normal specific gravity may indicate higher fat content, while a higher value suggests skimming.

4. Detection of Adulteration: Adding water lowers specific gravity, while adding sugar or starch increases it.

Color of Milk

Why is Milk White?

Milk appears white due to the scattering of light by its fat globules and casein micelles. These particles are large enough to scatter all wavelengths of visible light, making milk appear white to the human eye.

Factors Affecting the Color of Milk:

1. Fat Content:

   • Higher fat content gives milk a creamier or slightly yellowish tint.

   • Skimmed milk appears bluish due to the absence of fat, which allows shorter (blue) wavelengths to scatter more.

2. Protein & Casein Micelles:

   • Casein micelles contribute to light scattering, reinforcing the white color.

3. Carotenoids (β-Carotene):

   • Milk from cows that graze on fresh green grass has a slight yellowish tinge due to β-carotene (a precursor of vitamin A).

   • Buffalo milk appears whiter than cow milk because it has less β-carotene.

4. Riboflavin (Vitamin B₂):

   • Gives a slight greenish tint in whey or skim milk.

5. Adulteration & Processing:

   • Dilution with water makes milk appear whiter and thinner.

   • Boiling or Pasteurization can slightly alter the shade by affecting proteins and fat.

Color Variations in Different Types of Milk:

Type of Milk Color of milk Reason

Whole Cow’s Milk Creamy white to yellowish Contains fat and β-carotene

Buffalo Milk Pure white Higher fat and less β-carotene

Skimmed Milk Bluish white Less fat, more light scattering

Goat’s Milk Slightly whitish-gray Low β-carotene, smaller fat globules

Human Milk Slightly bluish or translucent Lower casein and fat content

Flavor of Milk

The flavor of milk can vary depending on several factors, including its source (cow, goat, buffalo, etc.), processing methods, and any added ingredients. Here are some details about the flavor of milk:

Natural Flavor of Milk

1. Mild & Slightly Sweet – Milk contains natural lactose (a type of sugar), giving it a mild sweetness.

2. Creamy & Rich – The fat content contributes to its creamy texture and richness.

3. Fresh & Clean – Fresh milk has a clean taste with a smooth mouthfeel.

Factors Affecting the Flavor of Milk

Milk's flavor is influenced by a combination of chemical composition, processing methods, and external factors. Fresh milk has a slightly sweet, creamy, and mild taste due to lactose (milk sugar), fats, and proteins. However, various factors can alter its flavor.

1. Composition of Milk

a. Lactose (Milk Sugar)

• Provides a mild sweetness to fresh milk.

• Breakdown of lactose (e.g., in fermented milk) produces lactic acid, making the milk taste sour.

b. Fat Content

• Higher fat content gives a richer, creamier taste.

• Skim milk has a more watery and less flavorful profile.

c. Proteins (Casein & Whey)

• Casein contributes to a slightly chalky texture.

• Whey proteins add a mild umami or cooked flavor when heated.

d. Minerals & Salts (Calcium, Phosphates, Sodium, Magnesium, etc.)

• Affect the balance of sweetness and saltiness.

• Higher sodium or chloride levels (e.g., in mastitis-affected milk) can give a salty taste.

2. Processing & Storage Conditions

a. Heat Treatment (Pasteurization, UHT, Sterilization)

• Pasteurized milk (low temperature, short time) retains more natural flavor.

• Ultra-high-temperature (UHT) milk has a slightly "cooked" or caramelized flavor due to Maillard reactions.

• Boiled milk develops a strong cooked flavor from protein denaturation.

b. Homogenization

• Makes fat globules smaller, distributing them evenly and giving milk a smoother taste.

• Non-homogenized milk has a creamier, more natural flavor with a distinct fatty layer on top.

c. Storage Time & Temperature

• Prolonged storage leads to oxidation, producing stale or rancid flavors.

• Refrigeration slows spoilage but does not completely stop chemical changes.

d. Packaging Material

• Glass bottles preserve fresh taste better than plastic or cartons.

• Plastic packaging can absorb flavors from the environment.

3. Microbial Activity & Fermentation

• Bacterial growth (e.g., lactic acid bacteria) causes sour or tangy flavors as they break down lactose into lactic acid.

• Spoilage bacteria can produce off-flavors like bitter, putrid, or rancid notes.

• Fermented dairy products (yogurt, kefir) develop characteristic tangy flavors due to controlled bacterial action.

4. Feed & Diet of the Dairy Animal

• Grass-fed cows produce milk with a sweeter, richer flavor due to higher fat content.

• Silage-fed cows can produce milk with slight grassy or barny flavors.

• Certain feeds (e.g., wild onions, garlic, fishmeal) may give undesirable flavors to milk.

5. Environmental & External Factors

• Exposure to Light: Causes oxidation of fats, leading to a stale or cardboard-like flavor.

• Contamination: Absorption of strong odors from storage areas can give milk an off-flavor (e.g., garlic, onions, chemicals).

• Antibiotic Residues: Can alter the natural taste of milk.

Common Off-Flavors in Milk & Their Causes

Off-Flavor Possible Cause

Sour Bacterial fermentation (lactic acid)

Bitter Protein breakdown, bacterial spoilage

Rancid Fat oxidation, enzyme activity

Cooked Heat treatment (UHT, boiling)

Metallic Oxidation, storage in metal containers

Barny Absorption of animal/environmental odors

Salty High sodium/chloride (mastitis, contamination)

How to Maintain Fresh & Natural Milk Flavor

• Store milk at 3-5°C (37-41°F) to prevent spoilage.

• Use airtight, opaque containers to avoid light-induced oxidation.

• Consume milk fresh, and avoid long storage.

• Avoid strong-smelling foods near milk to prevent odor absorption.

• Use clean processing equipment to prevent microbial contamination.

Viscosity of Milk

Viscosity refers to the thickness or resistance to flow of a liquid. In the case of milk, viscosity is influenced by several factors, including fat content, temperature, and processing methods.

Typical Viscosity of Milk

• The viscosity of milk is generally 1.5 to 3.0 mPa·s (millipascal-seconds) at 20°C (68°F).

• For comparison, water has a viscosity of 1.0 mPa·s at the same temperature, meaning milk is slightly thicker than water.

Factors Affecting Milk Viscosity

1. Fat Content

   • Higher fat content increases viscosity, making the milk thicker.

   • Whole milk (3.25% fat) has higher viscosity than skim milk (0% fat).

   • Buffalo milk (higher fat content) is more viscous than cow milk.

2. Temperature

   • Viscosity decreases as temperature increases (milk flows more easily when warm).

   • At lower temperatures (e.g., refrigeration at 4°C), milk becomes slightly thicker.

3. Processing Methods

   • Homogenization: Increases viscosity by breaking down fat globules into smaller, uniform particles.

   • Pasteurization: May slightly reduce viscosity due to changes in protein structure.

4. Protein & Solids Content

   • Higher protein (casein & whey) increases viscosity.

   • Solids like lactose and minerals also contribute to thickness.

5. Additives & Modifications

   • Flavored or fortified milk (e.g., chocolate milk) has higher viscosity due to added sugars or stabilizers.

   • Evaporated and condensed milk have much higher viscosity due to reduced water content.

Viscosity of Different Types of Milk

Type of Milk Viscosity (mPa·s at 20°C)

Skim Milk (0% fat) ~1.5 – 1.8

Whole Milk (3.25% fat) ~2.0 – 3.0

Buffalo Milk (Higher fat & protein) ~5.0 – 6.0

Evaporated Milk ~10 – 15

Condensed Milk ~50 – 100

Surface Tension of Milk

Definition:

Surface tension is the property of a liquid that makes its surface behave like a stretched elastic sheet due to cohesive forces between molecules. In milk, surface tension is influenced by water, fat, proteins, and dissolved substances.

Typical Surface Tension of Milk

• The surface tension of milk is 40–50 mN/m (millinewtons per meter) at 20°C (68°F).

• For comparison, pure water has a surface tension of 72 mN/m at the same temperature.

• Since milk contains fats and proteins, which reduce intermolecular forces, its surface tension is lower than water’s.

Factors Affecting Surface Tension of Milk

1. Fat Content

   • Higher fat content lowers surface tension (fat molecules disrupt water's cohesive forces).

   • Skim milk (0% fat) has a higher surface tension than whole milk (3.25% fat).

2. Temperature

   • As temperature increases, surface tension decreases.

   • Cold milk has a slightly higher surface tension than warm milk.

3. Proteins & Solids

   • Proteins (casein, whey) slightly lower surface tension by stabilizing fat droplets.

   • Higher protein milk (e.g., buffalo milk) may have slightly different surface tension properties.

4. Processing & Additives

   • Homogenization reduces surface tension by breaking fat globules into smaller, more evenly distributed particles.

   • Detergents, surfactants, or foaming agents can drastically lower surface tension, which is why milk foams well when frothed (e.g., in cappuccinos).

Surface Tension of Different Types of Milk (at 20°C)

Type of Milk Surface Tension (mN/m)

Skim Milk (0% fat) ~50

Whole Milk (3.25% fat) ~45

Buffalo Milk (Higher fat & protein) ~42 – 45

Cream (Higher fat content) ~35 – 40

Refractive Index of Milk

Definition:

The refractive index (RI) of a substance is a measure of how much light bends (refracts) when it passes through it. For milk, the refractive index is influenced by its composition, including water, fat, proteins, and dissolved solids.

Typical Refractive Index of Milk

• The refractive index of milk generally ranges from 1.344 to 1.348 at 20°C (68°F).

• This value is slightly lower than the refractive index of water (1.333), due to the presence of dissolved solids like fats, proteins, and lactose.

Factors Affecting the Refractive Index of Milk

1. Fat Content

   • Higher fat content slightly increases the refractive index.

   • Skim milk (0% fat) has a lower RI than whole milk (3.25% fat).

2. Solids & Protein Concentration

   • More dissolved solids (like lactose and casein) increase the refractive index.

   • Buffalo milk (higher protein and fat) has a slightly higher RI than cow's milk.

3. Temperature

   • As temperature increases, the refractive index decreases due to reduced density.

4. Adulteration & Quality

   • Refractive index measurement is used to detect milk adulteration (e.g., dilution with water lowers RI, while added sugar increases RI).

Practical Applications of Refractive Index in Milk

• Quality Control: Helps check milk purity and detect adulteration.

• Lactose & Fat Content Estimation: Higher lactose or fat content increases RI.

• Milk Processing & Product Development: Used in dairy industries for monitoring milk concentration in various products.

Refractive Index of Different Types of Milk (at 20°C)

Type of Milk Refractive Index (RI)

Skim Milk (0% fat) 1.344 – 1.345

Whole Milk (3.25% fat) 1.346 – 1.348

Buffalo Milk (Higher fat & protein) 1.347 – 1.349

Evaporated Milk ~1.350 – 1.355

Condensed Milk ~1.420 – 1.430

Specific Heat of Milk

Definition:

Specific heat is the amount of heat required to raise the temperature of a unit mass of a substance by 1°C. It is an important property in dairy processing, affecting pasteurization, cooling, and storage.

Typical Specific Heat of Milk

• The specific heat of milk varies based on composition but generally falls within:
  3.85 – 3.95 kJ/kg·°C (at 20°C)

• This is slightly lower than the specific heat of water (4.18 kJ/kg·°C) due to the presence of fats, proteins, and dissolved solids.

Factors Affecting Specific Heat of Milk

1. Water Content

   • Since water has a high specific heat, higher water content in milk increases its specific heat.

   • Skim milk (higher water content) has a slightly higher specific heat than whole milk.

2. Fat Content

   • Fat has a lower specific heat (~2.1 kJ/kg·°C), so milk with more fat has a lower specific heat.

   • Whole milk has a lower specific heat than skim milk.

3. Temperature

   • The specific heat of milk increases with temperature due to changes in molecular interactions.

4. Solids & Proteins

   • More dissolved solids (e.g., lactose, casein) slightly reduce the specific heat.

   • Buffalo milk (higher fat & protein) has a lower specific heat than cow milk.

Practical Applications of Specific Heat in Dairy Processing

• Pasteurization: Helps determine the energy required to heat milk to a safe temperature.

• Cooling & Storage: Affects the rate at which milk cools after processing.

• Evaporation & Drying: Used in dairy product formulation (e.g., making evaporated or powdered milk).

Specific Heat of Different Types of Milk (at 20°C)

Type of Milk Specific Heat (kJ/kg·°C)

Skim Milk (0% fat) 3.93 – 3.95

Whole Milk (3.25% fat) 3.85 – 3.90

Buffalo Milk (Higher fat & protein) 3.75 – 3.85

Cream (High fat content) 2.9 – 3.5

Evaporated Milk ~3.2 – 3.6

Condensed Milk ~3.0 – 3.4

Electrical Conductivity of Milk

Definition:

Electrical conductivity (EC) is the ability of a substance to conduct electricity. In milk, it depends on the presence of ions such as sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), chloride (Cl⁻), and other dissolved minerals.

Typical Electrical Conductivity of Milk

• The electrical conductivity of milk generally ranges from 0.4 to 0.7 S/m (Siemens per meter) at 25°C.

• This value is much higher than the conductivity of pure water (0.055 S/m) due to the presence of electrolytes.

Factors Affecting Electrical Conductivity of Milk

1. Ion Concentration (Minerals & Salts)

   • More dissolved salts increase conductivity.

   • Skim milk has slightly higher conductivity than whole milk due to its higher relative water content.

2. Fat Content

   • Fat is a poor conductor of electricity.

   • Higher fat content reduces conductivity (whole milk has lower EC than skim milk).

3. Temperature

   • Electrical conductivity increases with temperature due to higher ion mobility.

   • Conductivity increases by approximately 2% per °C rise in temperature.

4. Milk Adulteration & Quality

   • Water dilution lowers conductivity.

   • Adding salts or contaminants increases conductivity.

5. Mastitis in Cows

   • Milk from cows with mastitis (udder infection) has higher conductivity due to increased sodium and chloride levels.

   • Conductivity measurements are often used in dairy farms for early detection of mastitis.

Practical Applications of Electrical Conductivity in Dairy Industry

1. Milk Quality Testing: Helps detect adulteration (e.g., water dilution, added salts).

2. Mastitis Detection: High conductivity indicates infection in dairy cows.

3. Process Control: Used in dairy processing for monitoring milk composition.

4. Electrolyte Balance in Dairy Products: Important in cheese-making, yogurt fermentation, and rehydration formulas.

Electrical Conductivity of Different Types of Milk (at 25°C)

Type of Milk Conductivity (S/m)

Skim Milk (0% fat) 0.55 – 0.7

Whole Milk (3.25% fat) 0.4 – 0.6

Buffalo Milk (Higher fat & protein) 0.35 – 0.5

Colostrum (Early-stage milk) 0.8 – 1.2

Mastitis-Affected Milk > 1.0

Oxidation-Reduction Potential (ORP) of Milk

Definition:

Oxidation-reduction potential (ORP), also known as redox potential, is a measure of a substance's tendency to gain or lose electrons in oxidation-reduction reactions. It is expressed in millivolts (mV) and indicates whether a substance is in an oxidizing (positive ORP) or reducing (negative ORP) environment.

Typical ORP of Milk

• The oxidation-reduction potential of fresh milk is typically +150 to +300 mV at 25°C.

• The ORP value of milk is lower than that of pure water (+400 to +600 mV) due to the presence of reducing substances like ascorbic acid, sulfhydryl compounds, and proteins.

Factors Affecting ORP in Milk

1. Oxygen Content

   • Fresh milk has a relatively high ORP because it contains dissolved oxygen.

   • When milk is stored, the ORP decreases as oxygen is consumed by bacterial activity or chemical reactions.

2. Fat Content

   • Higher fat content reduces ORP since fats have a lower affinity for oxygen.

   • Skim milk has a higher ORP than whole milk due to lower fat content.

3. Storage Conditions

   • Exposure to air (oxygen) increases ORP.

   • Pasteurization and boiling decrease ORP due to oxygen loss.

   • Long-term storage leads to lower ORP as oxidation reactions take place.

4. Bacterial Activity & Fermentation

   • Microbial growth (e.g., in yogurt or sour milk) lowers ORP as bacteria consume oxygen and produce reducing compounds.

   • Lactic acid bacteria in fermented dairy products create a more reducing environment (ORP can drop below 0 mV).

5. pH & Chemical Composition

   • Lower pH (more acidic milk) is associated with a lower ORP.

   • Presence of antioxidants (e.g., vitamin C, cysteine) reduces ORP.

Practical Applications of ORP in Dairy Industry

1. Milk Freshness Indicator: ORP decreases over time, making it useful for monitoring spoilage.

2. Fermentation Control: ORP monitoring helps in yogurt, cheese, and probiotic product manufacturing.

3. Oxidation Stability: Preventing oxidation (which lowers ORP) is important for milk shelf life and quality.

4. Food Safety: ORP can indicate bacterial contamination and metabolic activity.

ORP of Different Types of Milk

Type of Milk ORP (mV at 25°C)

Fresh Raw Milk +200 to +300

Pasteurized Milk +150 to +250

Skim Milk +250 to +300

Whole Milk +180 to +250

Fermented Milk (e.g., Yogurt) 0 to +100

Spoiled Milk -50 to +50

Boiling Point of Milk

Definition:

The boiling point of a liquid is the temperature at which its vapor pressure equals the surrounding atmospheric pressure, causing it to transition from liquid to gas.

Boiling Point of Milk

• The boiling point of milk is typically 100.2 – 100.5°C (212.4 – 212.9°F) at sea level.

• This is slightly higher than the boiling point of water (100°C or 212°F) due to the presence of dissolved solids like proteins, lactose, and minerals.

Factors Affecting the Boiling Point of Milk

1. Dissolved Solids

   • Milk contains lactose (4-5%), proteins (3-4%), minerals, and fats, which increase its boiling point slightly above water.

   • Higher solid content (e.g., in condensed milk) results in a higher boiling point.

2. Fat Content

   • Fat does not dissolve in water but forms an emulsion, which has minimal effect on boiling point.

   • However, higher fat content may create a layer on top, delaying visible boiling.

3. Atmospheric Pressure (Altitude Effect)

   • At higher altitudes, where atmospheric pressure is lower, milk boils at a lower temperature (just like water).

   • Example: At 2,000 meters (6,561 feet), milk may boil around 98°C (208°F) instead of 100.5°C at sea level.

4. Processing & Heating Conditions

   • Pasteurized milk has a similar boiling point to raw milk.

   • Homogenization does not significantly change boiling behavior.

5. Acidity & Spoilage

   • Spoiled milk with increased acidity (lower pH) may coagulate before reaching the boiling point due to protein denaturation.

Why Does Milk Froth or Spill Over When Boiled?

• Milk forms a surface layer of proteins and fat when heated.

• As boiling starts, steam and air get trapped under this layer, causing sudden overflowing.

• Stirring or heating slowly helps prevent this.

Practical Applications of Boiling Point in Dairy Processing

1. Milk Pasteurization: Heating milk below boiling (72-85°C) to kill bacteria without affecting taste.

2. Evaporated & Condensed Milk: Uses controlled boiling to remove water.

3. Sterilization & UHT Processing: Ultra-high-temperature (UHT) milk is treated at 135-150°C under pressure to extend shelf life.

Boiling Points of Different Types of Milk

Type of Milk Boiling Point (°C) at Sea Level

Skim Milk (0% fat) 100.3 – 100.5

Whole Milk (3.25% fat) 100.2 – 100.5

Buffalo Milk (Higher fat & protein) 100.4 – 100.6

Evaporated Milk ~103 – 105

Condensed Milk ~105 – 110

Freezing Point of Milk

Definition:

The freezing point of a liquid is the temperature at which it changes from a liquid to a solid. For milk, the freezing point is slightly lower than that of pure water due to the presence of dissolved solids like lactose, proteins, and salts.

Freezing Point of Milk

• The average freezing point of milk is -0.540°C to -0.560°C (31.03°F to 30.99°F).

• This is lower than the freezing point of water (0°C or 32°F) because milk contains solutes (lactose, minerals, and proteins), which depress the freezing point.

Factors Affecting the Freezing Point of Milk

1. Dissolved Solids (Lactose, Minerals, Salts)

   • More dissolved solids result in a lower freezing point.

   • Lactose and salts contribute the most to freezing point depression.

2. Fat Content

   • Fat does not significantly affect the freezing point because it is present as an emulsion, not dissolved.

   • Skim milk and whole milk have nearly the same freezing point.

3. Water Content (Adulteration Effect)

   • Adding water to milk raises the freezing point (brings it closer to 0°C).

   • Measuring freezing point is a common method for detecting milk adulteration.

4. Milk Type & Composition

   • Different animal milks have slightly different freezing points due to variations in solids and salts.

5. Processing & Storage

   • Homogenization and pasteurization do not significantly change the freezing point.

   • Storage conditions (temperature fluctuations) may cause freezing point variations due to changes in dissolved gas content.

Why Doesn’t Milk Freeze Like Water?

• When milk freezes, it does not solidify uniformly.

• Water in milk freezes first, leaving behind a concentrated unfrozen portion with higher solute concentration.

• Complete freezing takes time and forms ice crystals along with a concentrated solute phase.

Practical Applications of Freezing Point in Dairy Industry

1. Milk Adulteration Detection: A freezing point above -0.520°C indicates possible water dilution.

2. Storage & Transportation: Prevents accidental freezing, which affects texture and taste.

3. Ice Cream Production: Helps in controlling crystal formation and smooth texture.

4. Dairy Processing: Understanding freezing point is important for freezing dairy products like butter, cheese, and frozen milk.

Freezing Points of Different Types of Milk

Type of Milk Freezing Point (°C)

Skim Milk (0% fat) -0.540 to -0.560

Whole Milk (3.25% fat) -0.540 to -0.555

Buffalo Milk (Higher fat & protein) -0.550 to -0.570

Colostrum (First milk after birth) -0.570 to -0.600

Evaporated Milk -0.650 to -0.700

Condensed Milk -0.750 to -1.000

Foaming of Milk

Definition:

Foaming in milk occurs when air or gas bubbles become trapped in a liquid film, creating a stable or temporary foam structure. This is influenced by milk proteins, fat content, and processing conditions.

Why Does Milk Foam?

Milk foams because of proteins (mainly casein & whey) that stabilize air bubbles when milk is agitated or steamed. The foam forms due to surface tension reduction, allowing air to be incorporated and held within the liquid.

Factors Affecting Milk Foaming

1. Protein Content (Casein & Whey Proteins)

   • Proteins act as foaming agents, stabilizing air bubbles.

   • Whey proteins (β-lactoglobulin, α-lactalbumin) are particularly good at foaming.

2. Fat Content

   • High-fat milk (whole milk, cream) foams less because fat destabilizes the foam.

   • Low-fat or skim milk foams better because there’s less fat interfering with protein structure.

3. Temperature

   • Higher temperatures (55-65°C / 130-150°F) improve foaming by denaturing whey proteins, making them more surface-active.

   • Too high a temperature (above 70°C / 160°F) causes proteins to aggregate, reducing foam stability.

4. Homogenization

   • Homogenized milk foams better because smaller fat globules interact less with proteins, allowing better air incorporation.

5. Milk Freshness & pH

   • Fresh milk foams better than old milk.

   • Acidic conditions (low pH) reduce foaming because proteins lose their ability to stabilize bubbles.

Practical Applications of Milk Foaming

1. Cappuccinos & Lattes: Controlled foaming is essential for creating microfoam in steamed milk.

2. Whipped Dairy Products: Foaming principles are used in whipped cream and dairy-based foams.

3. Dairy Processing: Foam control is important in milk processing, pasteurization, and transportation to avoid excessive foaming.

Foaming Ability of Different Types of Milk

Type of Milk Foaming Ability Foam Stability

Skim Milk (0% fat) High High

Whole Milk (3.25% fat) Moderate Moderate

Cream (High fat) LowVery Low

Lactose-Free Milk Moderate Moderate

Plant-Based Milk (e.g., Soy, Almond) Varies Varies

Colligative Properties of Milk

Colligative properties are properties of solutions that depend on the number of solute particles, not their identity. Since milk is a complex colloidal solution containing water, proteins, lactose, fats, and minerals, its colligative properties differ slightly from those of pure water.

The main colligative properties of milk include:

1. Boiling Point Elevation

2. Freezing Point Depression

3. Osmotic Pressure

4. Vapor Pressure Lowering