Option:  Food Chemistry: IB Questions & Answers
1.	Fats and vegetable oils are triesters of glycerol with fatty acids. Many of these acids contain 18 carbon atoms. The table shows the relative percentages of various C18 fatty acid chains in four common fats and oils.

Fat/Oil	C17H35COO–
/ %	C17H33COO–
/ %	C17H31COO–
/ %	C17H29COO–
/ %	
Tallow	52	44	3	1	
Linseed Oil	5	32	18	45	
Olive Oil	2	83	15	0	
Peanut Oil	7	47	46	0	
 
(a)	Deduce which fat or oil from the table could best be described as:
	saturated
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	mono-unsaturated
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	poly-unsaturated.
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(2)
 
(b)	(i)	Explain the meaning of the term shelf life and suggest which fat or oil from the table would have the shortest shelf life.
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(2)

(ii)	Describe two ways in which shelf life could be increased.
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(2)
(Total 6 marks)

2.	(a)	saturated: Tallow;
mono-unsaturated: Olive oil;
poly-unsaturated: Linseed oil;	2 max
3 correct award [2], 2 correct award [1], no marks for just
one correct.
 
(b)	(i)	the time that a food can be stored without the flavour/
smell/colour/texture/appearance becoming unacceptable (to
the consumer);
linseed;	2
 
(ii)	reduce temperature;
store in dark;
remove oxygen / vacuum pack / replace air with inert gas;
reduce water content / drying;
add salt / sodium nitrite;
pickle;
add antioxidants;
genetic modification;	2 max
Do not accept “add preservatives”.
[6]

3.	(a)	State the conditions required for the hydrogenation of unsaturated oils.
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(2)
 
(b)	Hydrogenation can result in the formation of trans fatty acids. Outline the meaning of the term trans fatty acids and explain why their formation is undesirable.
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(2)
(Total 4 marks)

4.	(a)	metal catalyst / Ni, Cu, Zn, Pd, Pt;
high temperature/pressure;	2
 
(b)	fats with trans configuration across the double bond;
not easily digested / accumulate in body tissue / increase LDL
cholesterol levels;	2
Do not accept “bad cholesterol”.
[4]

5.	One property of foods that affects their desirability is their colour.
(i)	State the difference between a dye and a pigment.
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(1)
 
(ii)	Explain, in terms of their effect on light, why these compounds are coloured.
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(1)
(Total 2 marks)

6.	(i)	a dye is a synthetic colouring substance and pigments occur naturally;	1

(ii)	the dye/pigment absorbs some colours/wavelengths/frequencies
of light (whilst reflecting others);	1
[2]

7.	Many food items contain genetically modified ingredients.
(a)	Explain what is meant by the term genetically modified food.
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(1)
 
(b)	Describe two advantages and one concern about the use of genetically modified food.
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(3)
(Total 4 marks)

8.	(a)	a food from an animal or plant in which the DNA/genetic material
has been altered by artificial means / OWTTE;	1

(b)	advantages [2 max]:
quicker growth / reduced maturation time / more harvest per year;
increase resistance to disease/pests / less herbicides/pesticides /
improved plant/animal health;
more tolerant of climate/extending its range / lower water consumption;
increase in the yield/productivity/feed efficiency;
improve flavour;
incorporate beneficial substances;
increased shelf life;
	concerns [1 max]:
increase the risk of allergies;
affect the balance of people’s diets;
escape of modified genes into the environment;
potential harm to natural ecosystem;	3 max
[4]

9.	(a)	The shelf life of vegetable oils is often limited by auto-oxidation, which initially forms hydroperoxides. Outline the mechanism by which hydroperoxides are formed and state a class of compounds, formed from hydroperoxides, that are responsible for the off flavours produced in food.
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(4)

(b)	State the name, and briefly explain the action, of an artificial antioxidant that can be added to reduce the rate at which hydroperoxides are formed.
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(2)
(Total 6 marks)

10.	(a)	fission of C–H bond to produce free radicals;
reaction of alkyl radicals with oxygen to give peroxide radical;
peroxide radical removes H–atom from R–H to give hydroperoxide
and reform the alkyl radical;
Accept appropriate equations for first three marking points.
	aldehydes/ketones;	4
 
(b)	BHA / BHT / TBHQ / tocopherols;
react with free radicals (to form stable products);	2
Accept full names of BHA / BHT / TBHQ.
[6]

11.	The stereochemistry of molecules affects the way they interact with taste and smell receptors in the body.
	State one example of a compound and its enantiomers with different tastes or smells.
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(Total 2 marks)

(b)	BHA / BHT / TBHQ / tocopherols;
react with free radicals (to form stable products);	2
Accept full names of BHA / BHT / TBHQ.
[6]

12.	correct identification of one of the chiral carbon atoms / contains chiral/
asymmetric carbon atom(s);
+(d) and –(l) refer to the direction in which the enantiomers rotate the plane of
plane-polarized light / OWTTE;	2
[2]

13.	The structure of vitamin C is given below.
		
	Refer to the structure to explain why vitamin C can exist in a +(d) form and a –(l) form and explain the difference between the two forms.
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(Total 2 marks)

14.	(a)	An anthocyanin:
beetroot / red cabbage / blackcurrants / cherries / red grapes / named berries;
Accept any other correct answer, but must have specific names
e.g. strawberries instead of berries.
	A carotene:
tomato / pumpkin / capsicum / bananas / squash / mango;	2
Accept any other correct answer but do not accept carrot.
 
(b)	chlorophyll / hemoglobin / heme / myoglobin;	1
 
(c)	labelling/safety regulations vary (considerably) between countries /
a food produced in a country may be considered toxic in another /
a toxic compound may not be banned in all countries / OWTTE;	1
[4]

15.	carvone;
one isomer tastes of caraway, the other of spearmint;
	OR
	limonene;
one isomer smells of lemons, the other of oranges;	2 max
[2]

(b)	chlorophyll / hemoglobin / heme / myoglobin;	1
 
(c)	labelling/safety regulations vary (considerably) between countries /
a food produced in a country may be considered toxic in another /
a toxic compound may not be banned in all countries / OWTTE;	1
[4]

16.	Lycopene, whose structure is shown below, is a carotenoid and is responsible for the red colour in tomatoes. When bromine is slowly added to some tomato juice, the colour of the juice gradually changes from red to yellow. Explain this colour change in terms of changes in bonding in lycopene.

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(Total 3 marks)

17.	Br2 reacts with the double bonds / amount of double bonds in the conjugated
system decreases;
absorbed energy shifts to violet/higher energy in the visible region is absorbed;
resulting in complementary yellow colour;	3
[3]

18.	Different conventions are used to classify enantiomers. Orange and lemon peel each contain different enantiomers of the compound limonene. One of the enantiomers is represented below.

(a)	Identify the chiral centre in this enantiomer with an asterisk (*).
(1)
 
(b)	The (+)d-enantiomer has the characteristic smell of oranges and the (–)l-enantiomer has the characteristic smell of lemons. Explain the meaning of the (+)d and (–)l symbols used in this notation.
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(1)
 
(c)	The R, S notation is also used. The (+)d-enantiomer is often described as R-limonene and the (–)l-enantiomer as S-limonene. Explain what is meant by the R, S notation and state whether the structure shown is R or S.
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(2)
(Total 4 marks)

19.	(a)
	1
 
(b)	dextro/d and levo/l refer to right and left-handed / clockwise
and anti-clockwise rotation of plane polarized light;	1
 
(c)	clockwise and anti-clockwise sequence of prioritized atoms
(working from high to low atomic numbers) / OWTTE;
Allow absolute configuration of enantiomers.
Allow convention for labelling chiral carbon atoms using the
Cahn-Ingold-Prelog notation.
	S;	2
[4]

20.	(a)	(i)	Explain the meaning of the term shelf life.
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(1)
 
(ii)	State two properties which are affected when food has exceeded its shelf life.
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(2)

(b)	Discuss one way, different in each case, in which each of the following factors affect the shelf life and quality of food:
•	water content
•	pH change
•	light.
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(3)
(Total 6 marks)

21.	(a)	(i)	the period that maintains the expected quality desired by the consumer;	1
 
(ii)	Any two of the following:
flavour;
smell;
texture;
colour;
mass;	2 max
 
(b)	water content – loss of nutrients/browning/rancidity/microbial spoilage;
pH change – off flavours/colour change/browning/loss of nutrients;
light – rancidity/vitamin loss/fading of natural colours;	3
[6]

22.	Enantiomers are found in food.
(a)	Distinguish between the two conventions, D / L and + (d) / –(l), used for naming enantiomers.
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(2)
 
(b)	Identify the most common enantiomeric form of naturally occurring amino acids and describe their taste.
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(2)
 
(c)	Most naturally occurring sugars exist in the D form and are sweet. State an example of this.
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(1)
 
(d)	Define the term racemic mixture.
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(1)

(e)	State two examples, other than taste, of properties of food that are affected by the presence of different enantiomers.
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(2)
(Total 8 marks)

23.	(a)	D and L relates to the difference in spatial configuration of the enantiomers;
+(d) and –(l) relates to the direction of rotation of plane polarized light;	2
 
(b)	L form;
tasteless;	2
 
(c)	+(d)-limonene – oranges;	1
 
(d)	a 50:50 composition/equal amounts of two enantiomers;	1
 
(e)	odour;
toxicity;	2
[8]