Lipids 2

Question 1

‘Milk is an aqueous solution of ……………., ………………, vitamins and minerals that carries emulsified ……….. …………… and a …………….. dispersion of ……….. micelles’.

Answer 1

Protein, lactose, fat globules, colloidal, casein.

Question 2

Are the fat and casein micelles dissolved or dispersed in milk?

Answer 2

Dispersed.

Question 3

Where is whey protein, lactose and approximately half of the mineral load of milk found?

Answer 3

Dissolved in the aqueous (serum) phase.

Question 4

True or false? Milk contains ALL of the vitamins essential for humans.

Answer 4

True. BUT not necessarily in significant concentrations.

Question 5

Name the molecule…

Beta-D-galactopyranosyl (1-4) beta-D-glucopyranose.

‘The most prominent carbohydrate in milk - a disaccharide of galactose and glucose connected by beta 1,4 glycosidic bond, it is virtually unique to milk!’

Answer 5

Lactose.

Question 6

Lactose is a white ………….., which is fully …………….. in milk.

Answer 6

Solid, dissolved.

Question 7

Can lactose exist in different forms?

Answer 7

Yes, it can exist in the alpha and beta forms due to mutarotation of glucose between its linear and cyclical forms, i.e. it can form two different stereoisomers.

Question 8

When materials crystallise, they tend to take one form. Is this true of lactose?

Answer 8

No, lactose can crystallise in the alpha or beta form, although the beta form is more expensive to produce commercially because it requires a high temperature.

Question 9

Given time, what will happen to the concentration of a solution of pure alpha lactose, pure beta glucose or a mixture of the two?

Answer 9

The concentration will reach an equilibrium due to the mutarotation of glucose.

Question 10

How big are the fat globules in milk (in micrometres)? What do they consist of?

Answer 10

The fat globules in milk have an average size of 3-4 micrometres. They consist mostly of triglycerides but also contain some fat-soluble vitamins.

Question 11

What causes milk to appear white?

Answer 11

Casein micelles dispersed in the milk.

Question 12

What causes milk to turn sour?

Answer 12

Microorganisms can ferment the lactose in milk into lactic acid, causing the milk to sour.

Question 13

What is an organic compound that is required by an organism, but not synthesised by an organism in the required or amounts, of at all?

Answer 13

A vitamin.

Question 14

How many recognised vitamins are there? Can you name them?

Answer 14

There are 13 recognised vitamins:

Vitamin A
Vitamin B1
Vitamin B2
Vitamin B3
Vitamin B5
Vitamin B6
Vitamin B7
Vitamin B9
Vitamin B12
Vitamin C
Vitamin D
Vitamin E
Vitamin K.

Question 15

Which vitamins are water-soluble?

Answer 15

Vitamins B1, B2, B3, B5, B6, B7, B9, B12 and C.

Question 16

Which vitamins are fat-soluble?

Answer 16

Vitamins A, D, E and K.

Question 17

Does it matter whether lactose is in the alpha form or beta form?

Answer 17

Yes, it can. The form of lactose can impact its application. For example, beta-lactose is more soluble than alpha-lactose so it is more useful in infant formulas because it dissolves more readily, even et ambient temperatures.

Question 18

What is ‘caking’ of milk powder?

Answer 18

Milk powder often has lactose in an amorphous, non-crystalline state. On exposure to moisture, the lactose can form hard alpha-monohydrate crystals, described as ‘caking’.

Question 19

Lactose is a reducing sugar. What does this mean?

Answer 19

When the glucose residue of a disaccharide (e.g. lactose) is open in the linear form, it exposes an aldehyde which can react with proteins or amines, donating electrons and oxidising the aldehyde group of the glucose residue.

Question 20

These % represent the various major lipids in the milk fat globule. Name them.

96%-98%
1% (three lipids)
0.5%
Trace (three lipids).

Answer 20

96%-98% - TAG
1% - phospholipids, cholesterol, free fatty acids
0.5% - DAG/MAG
Trace - ceremides (e.g. sphingomyelin and gangliosides), carotenoids, fat soluble vitamins

Question 21

Name two ceremides.

Answer 21

Sphingomyelin and gangliosides.

Question 22

Why are milk fat globues stable in emulsion?

Answer 22

The hydrophobic moieties on the surface of the membranes of the globules reduces interfacial tension between the lipid and water phases of milk, meaning that the lipid droplets disperse easily.

Question 23

What is a glycosphingolipid with one or more sialic acids linked on the sugar chain?

Answer 23

A ganglioside.

Question 24

Describe the biogenesis of milk fat globules in the secretory cells of lactating cells and their secretion into the lumen of the mammary gland secretory duct.

Answer 24

The fat globule is surrounded by a membrane with a hydrophobic inner layer (facing the fat) and a hydrophilic outer layer.

The globules, already covered in a single-unit membrane by the endoplasmic reticulum, emerge from the cell enrobed in a further double-unit membrane. This secondary membrane is formed from the plasma membrane of the secretory cell.

Question 25

Where do milk fat globule membranes contain some protein from?

Answer 25

The plasma membrane of the secretory cell.

Question 26

How many different fatty acids are there in ruminant milk? Where do they all come from?

Answer 26

There are hundreds of different fatty acids in ruminant milk - not all in large quantities!

These fatty acids derive from microbes in the rumen altering the fatty acids found in feed and generating molecules that can be taken up by the ruminant and generated into new fatty acids.

Question 27

What is de novo biosynthesis?

Answer 27

Ruminants can take up the fatty acids generated by microbes in the rumen altering fatty acids in feed and generate them into new fatty acids.

Question 28

In ruminant milk fat, there is a range of short-, medium- and long-chain fatty acids. What affect does this have on the melting temperature of cow fat?

Answer 28

It has a wide range - completely solid at -40c and completely liquid at 40c.

Question 29

How many carbons do short chain fatty acids have?

Answer 29

4-8.

Question 30

How many carbons do medium chain fatty acids have?

Answer 30

10-14.

Question 31

How many carbons do long chain fatty acids have?

Answer 31

16-18.

Question 32

How many types of fatty acids are there in cow’s milk? How types of TAG molecules would there be if the incidence of fatty acids in the TAG molecules were completely random?

Answer 32

There are 20 major types of fatty acids in cow’s milk. If their incidence in TAG molecules was completely random, there would be 4,200 possible TAG molecules!

In reality the incidence of fatty acids in TAG molecules is NOT random.

Question 33

What are the main fatty acids in cow’s milk?

What about goat’s milk?

Answer 33

Cow’s milk - stearic acid, palmitic acid, myristic acid and oleic acid.

Goat’s milk - stearic acid, oleic acid, myristic acid, palmitic acid and capric acid.

Question 34

Which fatty acid is present in much larger quantities in human milk than in ruminant milk?

Answer 34

Linoleic acid.

Question 35

In cow’s milk, short chain fatty acids are found almost exclusively at which position on the glycerol backbone?

Answer 35

SN-3.

Question 36

In cow’s milk, long chain fatty acids are mostly found at which positions on the glycerol backbone?

Answer 36

SN-1 and SN-2.

Question 37

In cow’s milk, medium chain fatty acids are mostly found at which position on the glycerol backbone?

Answer 37

SN-1 and SN-2.

Question 38

Which provides energy more quickly - short, medium or long chain fatty acids? Why?

Answer 38

Short and medium chain fatty acids provide energy more quickly because they are absorbed straight into the portal vein and taken directly to the liver. Long chain fatty acids and monoacylglycerol remnants (MAG) take a longer route to the liver.

Question 39

Which enzyme is specific to fatty acids found at which position on the glycerol backbone?

Answer 39

SN-3.

Question 40

Which enzyme is specific to fatty acids at position SN-1 and SN-3 of the glycerol backbone?

Answer 40

Pancreatic lipase.

Question 41

Name the three sources of fatty acyl groups in ruminant milk.

Answer 41

1. Diet.
2. Action of microbes in rumen (de novo biosynthesis).
3. Mammary fatty acid synthase and desaturase enzymes (de novo synthesis - fatty acid synthase builds up fatty acid molecules, desaturase converts the double bonds to single bonds).

Question 42

Ruminants have a specialised digestive system to break down cellulose. What are the four chambers of their stomachs?

Answer 42

1. Rumen - food is processed mechanically and exposed to microbes that can break down cellulose.
2. Reticulum - allows for regurgitation and reprocessing (‘chewing the cud’).
3. Omasum - finer particulate is processed mechanically.
4. Abomasum - the ‘true stomach’ where digestive enzyme lysozyme breaks down bacteria to release nutrients.

Question 43

Do ruminants use plant material directly or indirectly?

Answer 43

Indirectly, primary processing is by bacteria in the rumen.

Question 44

What are the sources of C16 and smaller fatty acids in milk?

Answer 44

Either from diet or de novo biosynthesis by microbes in rumen or enzymes in the udder.

Question 45

What are the sources of C18 fatty acids in milk?

Answer 45

Mostly from the diet, some can be altered by biohydrogenation (formation of rumenic acid).

Question 46

What are the two proteins found in cow’s milk? In what proportions?

Answer 46

80% casein, 20% whey.

Question 47

Describe the characteristics of whey.

Answer 47

It is a mostly globular protein, dissolved in milk serum.

65% beta-lactoglobulin, 25% alpha-lactoalbumin, 8% bovine serum albumin and 2% immunoglobulin and enzymes.

Question 48

Describe the secondary structure of casein.

Answer 48

Mostly short lengths of alpha-helix and beta-sheets interspersed with proline.

Distinct hydrophilic and hhydrophobic regions enriched by amino acid proteins.

Question 49

How many of the essential amino acids are contained in casein?

Answer 49

All eight of them.

Question 50

What is the ambient pH of milk?

Answer 50

pH 6.7.

Question 51

How many phospherine groups do alpha-casein, beta-casein and kappa-casein have?

Answer 51

8, 4 and 1 respectively.

Question 52

What makes kappa-casein unique?

Answer 52

It only has one phospherine but unlike alpha and beta casein it does have one or more threonine(s) carrying a trisaccharide.

Question 53

How big is a casein micelle (in nanometres)? How many proteins does it contain?

Answer 53

50-300nm, containing 20,000 proteins.

Question 54

What are the salts associated with casein?

Answer 54

Citrate, calcium and phosphate.

Question 55

In which phase of milk are the majority of citrate ions found? What effect does this have on the other salts?

Answer 55

Most of the citrate salts in milik are in the serum phase. They prevent calcium and phosphate combining to form solid calcium phosphate.

Therefore there is solid calcium phosphate in the micelles but not in the phosphate.

Question 56

What intervenes in the interaction between calcium and phosphate inthe serum pahse of milk, do that solid calcium phosphate is only found in the micelles?

Answer 56

Citrate ions.

Question 57

Casein proteins form casein micelles following which hypothesis?

Answer 57

The nanocluster hypothesis.

Question 58

Describe the structure of casein nanoclusters.

Answer 58

Alpha and beta casein together with phospherine residues form electrostatic bonds with calcium in nanoclusters. Kappa casein is found at the surface of the micelle because it has limited phospherine residues to form associations with nanoclusters.

Question 59

Where nanoclusters are absent, …………………. interactions between casein micelles exist.

Answer 59

Hydrophobic.

Question 60

Does casein denature on heating?

Answer 60

No!

Question 61

Does whey denature on heating?

Answer 61

Yes!

Question 62

Name the forces that keep casein micelles dispersed.

Answer 62

Electrostatic repulsion and steric repulsion.

Question 63

There is a net …………….. charge on the surface of casein micelles at ambient milk pH of 6.7. What does this mean for the casein micelles’ isoelectric point and the forces that keep them dispersed?

Answer 63

The casein micelles have an IEP of pH 4.6, so there is electrostatic repulsion between the micelles keeping them apart.

Question 64

How does kappa casein on the surface of casein micelles keep them dispersed?

Answer 64

The kappa casein’s trisaccharides extend out into the aqueous phase (hairy), these sticking out bits repel the micelles before they can get close to one another.