Food Protein Analysis

Question 1

What is food protein analysis?

Answer 1

The use of reliable quantitative methods to determine total protein concentration

Question 2

What are the reasons for quantifying total protein concentration?

Answer 2

• To establish value for trading
• To calculate nutritional value for customer information
• To assess the quality of protein based ingredients
• To evaluate the authenticity of protein-based foods

Question 3

What do the methods used to quantify total protein concentration rely on?

Answer 3

Nitrogen which is used as a marker to estimate protein content
- However it does not differentiate between protein nitrogen and non-protein nitrogen = not totally reliable

Question 4

What is the Kjeldahl method?

Answer 4

1. Digestion converts nitrogen to ammonium sulphate
2. Neutralisation converts ammonium sulphate to free ammonia
3. Distillation of ammonia into boric acid
4. Titration of excess boric acid with standardised alkali

Question 5

How long does the Kjedahl method take?

Answer 5

3 hours

Question 6

What is the safety concern regarding the Kjedahl method?

Answer 6

Use of sulphuric acid

Question 7

How is the total nitrogen content obtained in the Kjedahl method?

Answer 7

The direct nitrogen measurement and subsequent multiplication by conversion factor (6.25)

Question 8

What is the Duman method?

Answer 8

1. Sample is combust at 1000 degrees in the presence of oxygen
2. This produces water, carbon dioxide, nitric oxide, nitrogen gas and sulphur dioxide
3. Carbon dioxide and sulphur dioxide are removed
4. Nitric oxide is reduced to nitrogen gas
5. Total nitrogen concentration is measured by gas chromatography
6. Nitrogen protein conversion factor is used to determine the total protein content from the direct nitrogen measurement

Question 9

How might a seller artificially increase the apparent protein content of a product?

Answer 9

By adding other nitrogen containing compounds as neither the Kjedahl or Duman method distinguish between protein and non-protein nitrogen

Question 10

What are the emerging methods of total protein content determination?

Answer 10

Antibody based methods = ELISA
Chromatographic methods = HPLC 
Electrophoretic methods = PAGE and CE
Mass spectrometry 
High selectivity for individual proteins = only detects protein based nitrogen

Question 11

What does ELISA stand for?

Answer 11

Enzyme linked immunosorbent assays

Question 12

What does HPLC stand for?

Answer 12

High performance liquid chromatography

Question 13

What do PAGE and CE stand for?

Answer 13

PAGE = polyacrylamide gel electrophoresis 
CE = capillary elecrtophoresis