Milk Sugar

Milk sugar, commonly referred to as lactose, is the primary carbohydrate in milk and a key source of energy for suckling mammals. Below is a detailed discussion of lactose, covering its structure, composition, properties, occurrence, and significance in milk across species.

Overview of Milk Sugar

• Chemical Name: Lactose (β-D-galactopyranosyl-(1→4)-D-glucose).

• Type: Disaccharide.

• Concentration: Varies by species (e.g., 4.6-4.9% in cow milk, 6.0-6.5% in horse milk, ~4.8% in human milk).

• Role: Provides energy (~4 kcal/g) and supports neonatal growth, particularly brain development via galactose.

Structure

Lactose is composed of two monosaccharides linked by a glycosidic bond:

• Glucose: A six-carbon sugar (aldohexose).

• Galactose: Another six-carbon sugar, differing from glucose in the orientation of the -OH group on carbon 4.

• Bond: β-1,4-glycosidic linkage, where the β-hydroxyl group of galactose attaches to carbon 4 of glucose.

Chemical Formula

• C12H22O11

• In its cyclic form, lactose exists as a pyranose ring structure, with the β-configuration at the anomeric carbon of galactose.

Properties

• Solubility: Moderately soluble in water (~200 g/L at 20°C), less soluble than sucrose, contributing to milk’s smooth texture rather than excessive sweetness.

• Sweetness: ~20-30% as sweet as sucrose, giving milk a mild flavor.

• Reducing Sugar: The free anomeric carbon on glucose allows lactose to reduce compounds (e.g., in Maillard browning during milk heating).

• Crystallization: Can form crystals in concentrated milk products (e.g., sweetened condensed milk), affecting texture if not controlled.

• Stability: Stable in neutral pH but hydrolyzes into glucose and galactose in acidic conditions or with lactase enzyme.

Occurrence in Milk

• Synthesis: Produced in the mammary gland by the enzyme lactose synthase, a complex of:

  • α-Lactalbumin (whey protein): Regulates synthesis.

  • Galactosyltransferase: Transfers galactose to glucose.

• Concentration Across Species:

  • Cow: 4.6-4.9%.

  • Goat: 4.1-4.6%.

  • Sheep: 4.5-5.0%.

  • Buffalo: 4.8-5.2%.

  • Horse/Donkey: 6.0-6.5% (higher due to foal energy needs).

  • Human: ~6.8-7.0% (highest among common species, reflecting infant brain development needs).

  • Camel: 4.0-4.5%.

• Lactation Stage: Highest in early lactation (colostrum has less due to protein dominance), stabilizes in mature milk.

Nutritional and Biological Significance

• Energy Source: Lactose provides ~50% of milk’s calories, broken down into glucose (blood sugar) and galactose (used in glycolipids/glycoproteins).

• Brain Development: Galactose is a precursor for cerebrosides and gangliosides, critical for neural tissue in newborns.

• Calcium Absorption: Enhances intestinal absorption of calcium and magnesium.

• Microbiota: Undigested lactose in the colon ferments into short-chain fatty acids, supporting gut health (though this can cause discomfort in lactose intolerance).

Metabolism

• Digestion: The enzyme lactase (produced in the small intestine) hydrolyzes lactose into glucose and galactose, which are absorbed into the bloodstream.

• Lactose Intolerance: In many adults, lactase production decreases post-weaning, leading to undigested lactose fermenting in the colon, causing bloating, gas, and diarrhea.

• Fermentation: Lactose is fermented by bacteria (e.g., Lactobacillus) into lactic acid in yogurt and cheese production, reducing lactose content.

Factors Affecting Lactose Content

1. Species: Higher in milk adapted for rapid growth (e.g., horse, human) vs. lower in fat-rich milk (e.g., sheep).

2. Diet: Minimal direct impact, as lactose synthesis relies on glucose from blood, not feed carbohydrates.

3. Health: Mastitis reduces lactose due to disrupted mammary cell function.

4. Lactation Stage: Drops in late lactation as milk volume decreases.

5. Genetics: Minor variations within breeds (e.g., Holstein vs. Jersey cows).

Functional Role in Dairy Products

• Texture: Contributes to milk’s viscosity and mouthfeel.

• Browning: Participates in Maillard reactions (e.g., in baked goods or evaporated milk), enhancing flavor and color.

• Fermentation: Essential substrate for lactic acid bacteria in yogurt, kefir, and cheese, lowering pH and aiding preservation.

• Sweetness: Reduced in fermented products as lactose converts to lactic acid.

Challenges and Modifications

• Lactose Intolerance: Affects ~65-70% of adults globally; lactose-free milk is produced by adding lactase.

• Crystallization: In ice cream or condensed milk, large lactose crystals cause sandiness, mitigated by controlling concentration or seeding with fine crystals.