Lecture 8 - Protein

Question 1

importane of protein analysis?

Answer 1

• nutrition
• functional properties (ie gliadin and glutenins in wheat flour)
• economic considerations

Question 2

what is the bond in proteins that links AA?

Answer 2

polypeptides (covalent bond)

Question 3

main atoms in proteins?

which is the most important

Answer 3

C H O N S

N

Question 4

N content range in protein?

Answer 4

13 to 19%

Question 5

basic principles of measuring protein content?

Answer 5

1. determination of N
2. peptide bonds
3. aromatic AA
4. dye-binding capacity
5. UV absorptivity

Question 6

%crude protein = ?

Answer 6

%crude protein = total N x conversion factor

Question 7

what are the 2 types of total N in crude protein?

Answer 7

non-protein nitrogen (NPN)

true protein nitrogen (TPN)

Question 8

proximate analysis measures ___ and not ____.

Answer 8

crude protein content and not true protein content

Question 9

what is the conversion factor used in crude protein analysis?

Answer 9

• N:protein conversion factor
• assumes 1kg of plant/animal proteins contains a specific amount of N (13-19%)
• means that N ranges from 130 to 190g/Kg protein

Question 10

if egg proteins contain %16N, what is the conversion factor?

Answer 10

• 1Kg egg proteins contains 160g N

- conversion factor = 1000/160 = 6.25

Question 11

what is the universal conversion factor?

Answer 11

6.25

assumes most proteins contains 16%

Question 12

if rice proteins contains %19.34, what is the conversion factor?

Answer 12

• means that 1kg rice proteins contains 193.4g N

- CF = 1000/193.4 = 5.17

Question 13

2 common methods of measuring crude protein?

Answer 13

1. Kjeldahl method: N is measured as ammonia NH3

2. Dumas Method: N is measured as N2

Question 14

what is the kjeldahl method based on?

Answer 14

the conversion of total N in the food to ammonia

Question 15

what are the 2 steps of the kjeldahl?

Answer 15

1. digestion: converts N to NH3

2. measurement of NH3:

Question 16

3 ways of measuring NH3?

Answer 16

1. distillation followed by titration
2. colorimetric methods
3. ammonium NH4 electrode technique

Question 17

describe the digestion step of the kjeldahl method

Answer 17

• protein N is liberated to form NH4 + (ammonium ions)
• H2SO4 oxidizes organic matter and combines with NH4+ to form nonvolatile ammonium sulfate
• C and H are converted to CO2 and H2O

Question 18

why are catalysts used in the kjeldahl method?

which catalysts are used?

Answer 18

• speeds up digestion and oxidation
• serves as a O carrier in oxidation process

catalysts used:

1. mercury oxide originally, but it is toxic. It has been replaced with:
2. selenium
3. copper

Question 19

what is the function of potassium sulfate?

Answer 19

to increase bp of H2SO4 (since bp of H2SO4 is only 290) so that all protein will be converted to NH3

Question 20

2 reactions in the colorimetric methods in measurements of NH3?

what color compounds do they form?

Answer 20

1. Nessler Reaction: forms red-organge (ammonium dimercuric iodide
2. Berthelot Reaction: forms blue (indophenol)

Question 21

describe ammonium ion electrode measurements of NH3

Answer 21

• measured by ion selective elctrode (ISE) which measures ion activity which depends on ionic strength
• not used frequently, low reliability, expensive