Study guide: Milk Biosynthesis and Composition

📋 Course Outline

1. Lactase Persistence and Human Evolution
2. Dairy Through History and Nutrition
3. Milk Production and Lactation
4. Bovine Udder Structure
5. Mammary Secretion Pathways
6. Major Milk Components
7. Lactose Biosynthesis
8. Milk Exosomes and Research

📖 1. Lactase Persistence and Human Evolution

★ Must-know

• In most mammals, lactase production decreases after weaning, reducing the ability to digest lactose.

• Some human populations carry genetic mutations that allow lactase production to continue into adulthood.

Further detail

• Lactase persistence varies widely among populations and is believed to have evolved in response to dairy-animal domestication and milk consumption.

• Lactase-persistence mutations are described as among the fastest-spreading mutations in human history.

💡 Memory Hook

Dairy animal domestication → milk consumption → selection for continued lactase production

📖 2. Dairy Through History and Nutrition

★ Must-know

• Historically, dairy supplied calories, protein and fat, and fermented milks such as piimä and viili helped preserve milk and improve its safety.

• Milk contains calcium, phosphorus, high-quality protein, vitamins B2 and B12, iodine and potassium.

Further detail

• Dairy-animal domestication is placed at approximately 6000–4000 BCE, after the agricultural revolution.

• Milk has historically supplied calcium, protein and energy in low-sunlight regions, including during harsh winters.

• The course lists bone health, muscle maintenance and metabolic regulation as health associations of dairy, and probiotics and gut health as associations of fermented products.

• The Nordic nutritional recommendations cited in the lecture advise 350 ml to 500 ml of dairy per day, including milk, yogurt and cheese, with low-fat options encouraged.

• The lecture reports inverse associations between fermented and low-fat dairy and cardiometabolic risk factors such as total and LDL cholesterol.

• The World Cancer Research Fund concluded that evidence supports a protective association between dairy and colorectal cancer.

💡 Memory Hook

Survival food in harsh winters; modern dairy is a context-dependent nutrient source

📖 3. Milk Production and Lactation

★ Must-know

• A dairy cow is bred at 15–18 months of age, has a gestation period of about 9–10 months, and produces milk for about 10 months after calving.

Further detail

• A dairy cow has a non-lactating period of up to 60 days before the next calving.

• A dairy cow is economically productive for 3–5 years, although it can remain productive longer.

• Milk composition and production vary with genetics, diet and stage of lactation.

📖 4. Bovine Udder Structure

★ Must-know

• The bovine udder consists of four mammary glands, corresponding to four udder quarters.

Further detail

• A double-sheeted medial suspensory ligament supports the bovine udder.

• A fine membrane divides the udder into two halves, with each half containing two quarters.

📖 5. Mammary Secretion Pathways

★ Must-know

• During lactation, mammary epithelial cells secrete milk components through exocytosis, fat secretion, vesicular transcytosis, transporter-mediated movement and the paracellular pathway.

Further detail

• The named secretion routes are:
  • cytoplasmic lipid droplet pathway
  • microlipid droplet pathway
  • secretory pathway for skim-milk components

📖 6. Major Milk Components

★ Must-know

• Milk lipids consist mainly of triglycerides and phospholipids.

• The four main casein subgroups are:

  • αs1-casein
  • αs2-casein
  • κ-casein
  • β-casein

Further detail

• The listed processes in milk-fat synthesis are: de novo fatty-acid synthesis, uptake of long-chain fatty acids from blood, long-chain fatty-acid transport and desaturation, triacylglycerol synthesis, lipid-droplet secretion

• Milk proteins comprise caseins and whey proteins, with whey proteins described as about four-fifths of the milk proteins.

• Caseins are heterogeneous phosphoproteins with several genetic variants.

• Named whey-protein components include:

  • β-lactoglobulin
  • α-lactalbumin
  • immunoglobulins
  • serum albumin
  • lactoferrin
  • lactoperoxidase

📖 7. Lactose Biosynthesis

🔑 Key Concepts & Definitions

• Lactose: A milk sugar and a disaccharide composed of glucose and galactose; it is found only in milk.

📝 Essential Points

• Lactose content in milk varies from 3.6 to 5.5%.

• Lactose biosynthesis proceeds as glucose enters the cell and Golgi through GLUT1, is converted to UDP-galactose in the cytoplasm, and UDP-galactose is transported into the Golgi, where the lactalbumin–β4-galactosyl transferase complex catalyzes lactose formation.

📖 8. Milk Exosomes and Research

🔑 Key Concepts & Definitions

• Milk exosomes: Small extracellular vesicles measuring 30–150 nm and containing proteins, lipids and RNAs, including microRNAs.

★ Must-know

• Evidence on milk-exosome effects is mainly from in vitro and animal studies, and human health effects are not fully established.

Further detail

• Milk exosomes are naturally secreted by mammary epithelial cells.

• Milk exosomes may participate in intercellular communication by modulating the immune system and cell growth, and may have roles in gut health and development.

• Milk exosomes may carry bioactive molecules that resist digestion and may modulate inflammation and metabolism.

• The stability of milk exosomes during processing, including pasteurization and fermentation, remains under investigation.

💡 Memory Hook

Picture a tiny 30–150 nm vesicle carrying proteins, lipids and RNAs through milk