3: Lipids

Question 1

Explain the importance of lipids in foods

Answer 1

• Nutrition:
  Lipids bilayers essential for life
  Supply 35-40% of calories
  EFA’s (Linoleic + a-linolenic acids)
  Vitamin carriers - A,D,E
• Palatability:
  Flavour; texture; solubilises; influence appetite
• Processing:
  Heat transfer (in frying) + lubrication (oil)

Question 2

What are lipids?

Answer 2

Compounds that are soluble in non-polar solvents and are insoluble in water

Question 3

What are the 3 classes of lipids?

Answer 3

• Simple lipids (waxes, fatty acids, mono-, di-, triglycerides)
• Compound lipids (phosphoglycerides, glycolipids)
• Derived lipids (sterols, fat soluble vitamins)

Question 4

Define fatty acids, triglycerides and waxes (simple lipids)

Answer 4

• Fatty acids = chains of C + H atoms with a terminal carboxyl group
• Triglycerides = fatty acids linked to glycerol, major food lipid
• Waxes = esters of fatty acids and long chain alcohols (formed by esterification)

Question 5

Explain compound lipids

Answer 5

• Are structural lipids
• Phospholipids in membranes
• Similar structure to triglycerides but contain additional features e.g. C,H,O,N,P
• Polar : have a hydrophilic and hydrophobic (lipophilic) end

Question 6

What is the basic difference between mono, di and triglycerides?

Answer 6

The number of fatty acids attached to the glycerol molecule. E.g. 1 FA = monoglyceride
2 FA = diglyceride
3 FA = triglyceride

Question 7

What’s the difference between simple and mixed triglycerides?

Answer 7

Simple = all R groups (fatty acids) are the same
Mixed = at least 2/3 fatty acids different

Question 8

What is the difference between a cis and trans double bond in FAs?

Answer 8

Cis = methyl groups on same side - normal form
Trans = methyl groups on opposite side - abnormal form. Can be harmful when consumed.

Question 9

What is the structural difference between saturated and unsaturated fatty acids (in relation to a kink in structure)?

Answer 9

Unsaturated fatty acids have one or more double bonds, causing a kink in the molecule. Cis double bonds in FA’s cause a greater kink, impacting the melting point (making it lower).
Trans-unsaturated FA have a smaller kink, meaning they have similar characteristics to saturated FA’s.

Question 10

How does the melting of fatty acids occur?

Answer 10

When the intermolecular bonds (van Der Waals) are broken by heat energy. (NOT intramolecular)

Question 11

What are the key factors influencing melting point?

Answer 11

1. Chain length : The longer the chain, the higher the melting point (increased number of carbons = more intermolecular bonds = more heat energy needed)
2. Unsaturation : The more double carbon bonds, the lower the melting point (increased unsaturation = lower MP)

Question 12

How does the type of unsaturated bonds influence the MP of FAs?

Answer 12

Trans : double carbon bonds introduce a slight kink and lower intermolecular bonds so less energy needed to melt
Cis : double carbon bonds introduce a larger kink and lower intermolecular bonds even more and so even less energy needed to melt

Question 13

Melting range of MP of fats also depends on the arrangement of triglyceride molecules (their crystal structure). Triglycerides are polymorphic. What does this mean?

Answer 13

They can exist in more than one crystalline form

Question 14

What are the 3 main polymorphic forms and why do they occur?

Answer 14

a; B’; B
The forms that grow depend on hoe the liquid is cooled
a= rapidly cooled; low MR; fragile platelets; least stable
B’ = small, needle like crystals
B= slow cooling; high MR; large coarse crystals; most stable

Question 15

Give uses of B’ types and B types

Answer 15

B’ = form soft plastic fats for use in margarine (as they form small needle like crystals)
B = used in cocoa butter & lard (as form large crystals giving grainy texture)

Question 16

What is tempering used for?

Answer 16

Controlled crystallisation in order to induce the desired polymorphic state (e.g. in cocoa butter)

Question 17

Which of the 6 polymorphic states has the desired properties for use in cocoa butter?

Answer 17

Type V / B-3 type (as has MP of 33.8 degrees C) = glossy, firm, best snap, melts near body temp

Question 18

What are the stages of tempering?

Answer 18

1. Liquid cooled to initiate crystallisation
2. Reheated to 32 degrees C, just below MP of B-3 type - and is held, this melts out unwanted crystals
3. Stir at 32 degrees C, allow formation of very small B-3 crystals and then solidify

Question 19

What are the stages involved in fat and oil processing?

Answer 19

1. Lipids extracted from lipid rich commodity = crude lipid (composed of triglycerides + water, protein, phospholipids, vitamins)
2. Lipids are refined (unwanted components removed: water, protein, phospholipids) = refined lipid : triglycerides + vitamins A,D,E

Question 20

What are the 3 states of lipids

Answer 20

Solid, plastic & liquid/oil

Question 21

Name the 2 plant categories of sources of fats and oils

Answer 21

1. Oil seeds (rape, sunflower, palm kernel, soybean, cottonseed, corn germ, peanut, coconut)
2. Fruit pulp (olive, palm fruit)

Question 22

Give examples of animal & fish sources of fats & oils

Answer 22

Lard (rendered from pig kidney fat, fatback, caul fat)
Tallow (the rendered form of fat from a sheep and cattle)
Marine oils (fish, whales)
- Milk fats : butter (produced from churning whole milk/cream)

Question 23

How are fats and oils extracted from lipid rich fruits?

Answer 23

1. Fruits ground to a paste
2. Pressed
3. Separation (decantation or vertical centrifuge)
4. Heating/solvent extraction

Question 24

How is extra virgin olive oil produced?

Answer 24

Produced solely by physical means
- Pressed from the fruit pulp without further refinement
- Other types of olive oil go through a refining process & tend to be lighter in colour

Question 25

How is oil extracted from lipids rich seeds (milling)?

Answer 25

1. Clean seeds to remove impurities 2. Break seeds down 3. Cook (70-100 *C) to denature enzyme lipase (as this would cause rancidity) 4. Expelling 5. Solvent extraction (hexane/heptane) to extract all oil from seeds. Extraction solvent distilled off and reused

Question 26

Explain the process of oil/fat extraction from lipid rich animals (rendering)

Answer 26

1. Wet rendering : * Tissue heated with steam (high temp: 100-110*C) * Lipid melts * Lipid separated by decanting/centrifugation * White lipids with neutral flavour e.g. lard * Process more gentle than dry rendering 2. Dry rendering : * Uses high heat alone * Harsher process than wet rendering * Browner lipid with stronger flavour * Non food uses e.g. soap

Question 27

What are the basic steps in caustic/chemical refining?

Answer 27

1. Degumming (of crude oil) 2. Neutralisation 3. Bleaching 4. Deodorisation = refined oil

Question 28

What are the basic steps in physical refining?

Answer 28

(Crude oil) 1. Degumming 2. Steam distillation under high vacuum 3. Bleaching = refined oil

Question 29

What are the qualities of the final product (after refining)?

Answer 29

Free FAs removed, phospholipids removed, coloured compounds removed, pre-oxidants removed, pesticides & off flavours removed. Bottled oil should last a year

Question 30

Outline the 3 main methods of lipid modification?

Answer 30

1. Hydrogenation : removal of double carbon bonds (less unsaturated); increases melting point; converts cheap veg oils into plastic fats; improves stability & generates hydrogenated vegetable oils (trans fats) 2. Interesterification : rearranged fatty acids on glycerol backbone of triglycerides; changes MP & crystal structure; improves properties (more plastic) 3. Fractionation : selective removal of TGs from lipids based on MP (winterisation); stops clouding at low temps; produces lipids with specific melting properties (cocoa butter substitutes); = clean, clear and cheaper oil

Question 31

Explain the process of hydrogenation

Answer 31

Using refined oil (free of impurities e.g. waxes, proteins, phospholipids, as they may react); in the presence of a nickel catalyst using high temp & pressure, unsaturated FA is converted to saturated FA (or reduce double C bonds). * H atom from the catalyst is transferred to one of the C atoms of the double bond, adsorbed to the catalyst * The other C atom binds with the catalyst (reducing double bonds) * A second H atom transferred from the catalyst produces a saturated product * The loss of a H atom to the catalyst liberates a positional or geometric isomer

Question 32

What is the problem with hydrogenation?

Answer 32

You cannot control it - H atoms can also be released to the catalyst (works both ways) = more double bonds created Hydrogenation is usually only carried out partially

Question 33

What is selectivity ratio (SR)?

Answer 33

The ratio of the rate constants for the two processes; i.e. formation of desirable vs undesirable FAs. e.g the tendency of linoleic acid residues to be converted into oleic (desirable), compared with conversion of oleic into stearic (undesirable)