QUIZ 5 Example Questions 2017
. What molecular features give rise to the colour in chlorophyll?
. Why might cooking green leaves result in a change in their colour?
. Both real egg yolks and the yolks of chocolate filled cream eggs (which contain tartrazine !) appear yellow.
. What classes of compounds give rise to these yellow colours?
. How could you quantify the exact difference between these two yellows?
. Why is it that if you shine a white light on an egg yolk it still appears yellow?
. Egg yolks may get colour from carotenoids. Explain why feeding chickens carotenoids results in deeper yellow egg yolks but does not turn egg whites yellow?
. How can you explain that fresh meat often has a different colour on the surface compared to its interior ?
. Draw the general structure of an anthocyanin. Where are anthocyanins found and what features of anthocyanins are of importance in food science?
. Explain what is meant by the aglycone structure of anthocyanins. Is this the most water soluble form?
. Explain why the colour of anthocyanins is pH dependent.
. Why do raw sweet potatoes readily turn brown after peeling and cutting?
. Name 3 ways that you can reduce this type of browning
. If these cut-up peeled sweet potatoes are roasted they will turn brown at their surface. What is the cause of this type of browning reaction?
. Anthocyanins are known to be part of the general class of C6-C3-C6 Flavanoids. Explain what is meant by this description and draw the basic anthocyanin (flavan) structure. Why might they be coloured compounds?
. Describe the molecular features that are thought to give rise to sweetness. Give an example of both a natural and synthetic sweetener and outline how each of them contains the molecular feature you have described
. Describe the molecular features that are thought to give rise to colour. Give an example of both a natural food colour and a synthetic food dye and outline how each of them contains the molecular feature you have described
. Explain what gives rise to the colour in chlorophyll. In your answer discuss the various molecular features that most commonly give rise to colour in foods.
. How do chemical leavening agents (which may include salts like sodium bicarbonate) work and what is their function in foods?
. Chemical leavening systems are sometimes mixtures of several ingredients. Why is this necessary and what are the functions of these ingredients?
. What is meant by the odour (or aroma) threshold? How can this value be determined?
. What are the characteristics of aroma active compounds? Give 3 examples of classes of molecules that are well-known odourants found in foods.
. Briefly describe 2 processes that can result in the formation of volatile aldehydes during food processing or storage?
. In what foods would you expect to find the following aroma compounds:
a) Trimethylamine
b) Hexanal
c) 3-methyl butyl acetate (isoamyl acetate)
d) Furfurylthiol
(a) At pH 5 a the predominant anthocyanin structure may look like the structure
below. Explain why this is colourless whereas the flavylium ion and many other flavonoids are coloured
(b) Common anthocyanidins found in foods vary only by the number of hydroxyl and methoxyl groups attached to the basic flavylium ion. Does this affect the colour of these compounds and why?
(a) Why do raw sweet potatoes readily turn brown after peeling and/or cutting? In your answer explain the types of compounds involved.
(b) Name 3 ways that you can reduce this type of browning
(c) If these cut-up peeled sweet potatoes are roasted they will turn brown at their surface. What is the cause of this type of browning reaction?
(a) Describe the molecular features that are thought to give rise to sweetness. Give an example of both a natural and synthetic sweetener and outline how each of them contains the molecular feature you have described
(b) Chemical leavening systems are sometimes mixtures of several ingredients.
Why is this necessary and what are the functions of these ingredients?
(c) What is meant by the odour (or aroma) threshold? How can this value be determined?
1) For each of the following compounds indicate if you would consider it to bean aroma active compounds. For those that are, indicate atypical food source of such an aroma.
(a) Trimethylamine
(b) 3-methylbutyl acetate (isoamyl acetate)
(c) Monosodium glutamate
(d) Limonene
(e) Furfurylthiol
(f) Stevioside
(g) Hexanal
1. Why is spinach green coloured whereas refined sugar is often white? In your answer describe the physical reason why some substances appear as coloured and the molecular features that give rise to colour.
2. If you were to record the visible absorption spectrum of the green pigment from spinach what would be the main features you would see?
3. At pH 5 a the predominant anthocyanin structure may look like the structure below on the left whereas at pH 7 the structure on the right predominates.
(a) Which of them is coloured and which is colourless and why ? (3)
(b) In each case, explain what might be the effect on its colour if some of the hydroxyl groups (-OH) were replaced by methoxy groups (-OCH3)? ( 1)
4. What is the reason that ground beef is typically red and tends to go brown on the surface whereas freshly cut beef is purplish red?
5. Why does blanching fruits/vegetables inhibit enzymatic browning?
1. In what ways do the following types of sweetener differ? (Think of both their chemical form(s) and their sensory or physical effects in foods) (6)
1. Sugar (sucrose) crystals
2. Stevia
3. Sorbitol
4. (a) What are the characteristics of aroma active compounds and what is meant by the odour (or aroma) threshold?
(b) Nonanal has an odour threshold of ~1μg/kg dissolved in water but >10,000 μg/kg dissolved in oil. Explain what might account for this large difference .
5. (a) What classes of aroma active compounds would you expect to find in foods as a result of :
i. Oxidation of ω-6 fatty acids
ii. Fruit ripening
iii. Fish ageing
(b) Which of the following describes 1 part per billion? (1)
i. 1μg/mL
ii. 10pg/g
iii. 1ng/kg
iv. 1μg/kg
v . 1pg/g
6. In eggs, the yellow yolks are usually separated from the whites by a membrane. If this membrane is broken would you expect the yellow colour to migrate into the white? Explain your answer using your knowledge of food chemistry. (hint: lutein is one of the yellow colours)
7. Give examples of 3 distinct types of emulsifier and for each a brief explanation of how they function as an emulsifier.
8. Explain what is meant by HLB value and for each of the 3 types you have described above indicate with reasons whether you expect the HLB value to be >7 or <7
9. How could you form. a nanoemulsion from an oil/water mixture and what would be the advantage in doing over compared to gently stirring the two components together?
10. Explain how emulsifiers help to stabilize emulsions.