2. Lactose and other saccharides in milk — biosynthesis, content, factors influencing the content of lactose, significance

Question 1

1) Lactose

Answer 1

• lactose = main sugar of the milk
• consists of one molecule of glucose and one of galactose i.e. disaccharides
• 2-8% of milk
• amount varies among species and individuals
• important in technology of dairy products — special bacteria called lactic acid bacteria, normal
  part of starter cultures that are commercially produced that are put into the milk to create dairy
  products e.g. yoghurt and cheese
• cow:4,8%
• low casein type of milk: mare, donkeys, human — concentration is usually higher - Ho:7%
• Eq:8%
• found in milk in all mammals, unique to milk
• synthesised only in the mammary gland

Question 2

Synthesis

Answer 2

• need to get the precursors (glucose) into the secretory cells
• all the synthesis is performed in the golgi apparatus
• need special enzymatic system = lactose synthase enzymes:
• 2 proteins: A and B
• A=galactosyltransferase
• B = a-lactalbumin
• i.e. need to synthesise from aa
• also uses energy
• glucose enters cell via specific transport mechanism:
• some converted to galactose, others to glycerol —> golgi apparatus to form lactose
• lactase enzymatic system: complex system consisting of 2 proteins, one of the being a-
  lactalbumin —> lactose —> goes to milk
• lactose secreted via secretory vesicles with milk proteins

Question 3

Significance

Answer 3

• source of energy
• provides sweet taste of milk — lactose free milk is sweeter than normal as the lactose in this
  milk is cleaved to glucose and galactose, they have sweeter taste
• supports resorption of calcium
• influences osmotic pressure, freezing point and boiling point
• essential ingredient in production of dairy products
• contributes to nutritional value
• lactose intolerance: inability to digest lactose due to insufficient levels of enzyme lactase
• affects texture of some thickened and frozen products
• contributes to colour, flavour and aroma — can cause problems in heat treated dairy products
• after heat treatment, 2 types of reactions that can change lactose to other molecules

Question 4

Factors influencing the content of lactose

Answer 4

• during lactation, lactose content is reduced in the colostrum
• shortly after, there is an increase which continues until the lactation peak is reached
• after peak, there is slight decrease
• this decrease is in contrast to lactation trends for fat and protein, which after dropping at the start
  of lactation during the second lactation phase, are growing
• contrary to fat and protein, the lactose content can only be very poorly influenced by the feeding
  of dairy cows

Question 5

Chloride/lactose ratio

Answer 5

• can be used to detect mastitic milk
• in case if mastitis: significant decrease of lactose and in order to maintain osmotic pressure in
  milk the chloride and sodium content increases
• change in ratio is expressed as chloride/lactose ration, used as an indicator of the health status
  of mammary gland
• significant changes in lactose content also occur with some metabolic disorders
• Cl (mg/100g) / lactose (g/kg) = opt. 1,7-2,2
• more than 2,6 = mastitis

Question 6

2) Other saccharides

Answer 6

• cows milk also contains other saccharides:

- monosaccharides: D-glucose, D-galactose, D-mannose, D-fructose

Question 7

Lactose intolerance

Answer 7

• defined as the inability to completely break down lactose due to a lack or reduced production of lactase enzyme by the small intestines
• also referred to as lactase deficiency
• most common type of carbohydrate intolerance
• most common genetic disorder affecting more than half the world population