Protein Analysis Methods Flashcards

(35 cards)

1
Q
  • Sample is digested with sulfuric acid in the presence of a catalyst
  • measures total organic N (including ammonia)
  • official method of analysis for crude protein
A

Kjeldahl Method

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2
Q

Advantages:
- applicable to all food types
- inexpensive ( if not using automated system)
- Accurate; official method
- modified to analyze microgram quantities

A

Kjeldahl method

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3
Q

Disadvantages:
- Measures total nitrogen; nitrogen just protein nitrogen
- time consuming
- poorer precision
- corrosive reagent used

A

Kjeldahl method

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4
Q
  • samples combusted @ high temperatures (700-1000 C) with a flow of pure oxygen
  • All carbon is converted to carbon dioxide
  • Total nitrogen released is measured by gas chromatography using a thermal conductivity detector
A

Dumas (Nitrogen combustion) Method

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5
Q

Advantages:
- applicable to all food types
- no hazardous chemicals
- Short time (3 min)
- automated instruments can analyze up to 150 samples without attention

A

Dumas (Nitrogen combustion) Method

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6
Q

Disadvantages:
- Expensive
- measure total nitrogen; not just protein nitrogen

A

Dumas (Nitrogen combustion) Method

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7
Q
  • measure the absorption of radiation by molecules in food sample
  • measures the energy reflected or transmitted by the sample
  • For proteins and peptides: mid-infrared and near infrared characterizes of peptide bond can be used to estimate protein content
A

Infrared Spectroscopy

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8
Q
  • used to infrared milk analyzers to determine milk protein content
A

MID - Infrared spectroscopy

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9
Q
  • applicable to wide range of food products (grains, cereal, meat, and dairy products)
A

NEAR - infrared spectroscopy

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10
Q
  • instruments are expensive
  • samples analyzed rapidly by analysts without minimal training
A

Both mid and near infrared spectroscopy

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11
Q

3 stage process
1) Digestion
2) Neutralization & distillation
3) Titration

A

Kjeldahl method

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12
Q

Reagents used in digestion step & principle

1) sulfuric acid
2) anhydrous sodium sulfate
3) copper catalyst

  • nitrogen is converted into an ammonium ion
A

Reagents used in digestion step & principle

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13
Q
  • Alkali containing sodium hydroxide is added to neutralize the sulfuric acid
  • Ammonium ions are converted into ammonia gas
  • The ammonia is distilled into a boric acid solution containing indicator methylene blue & methyl red
  • ammonium borate complex formed

Note: Ammonia gas liberated during procedure can be titrated via sulphuric acid, boric acid, and HCl in excess

A

Neutralization & distillation step

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14
Q
  • the amount of ammonia trapped in boric acid solution is determined by directly titrating with 0.1 HCl
  • ammonium borate complex yields boric acid and chloride ions
A

Titration step

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15
Q

4 Solvent extraction methods for fats:

A

1) Goldfish - Continuous
2) Soxhlet - Semicontinuous
3) Mojonnier - Discontinuous
4) Chloroform-Methanol - Folch and Bligh & Dyed

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16
Q

Fat content measurement is based off of:

A

Gravimetric analysis

17
Q

Ideal solvents should:

A
  • evaporate readily
  • relatively low building point
  • nonflammable and nontoxic
  • inexpensive
  • nonhygroscopic
  • penetrate sample readily
18
Q
  • Combustion method
  • Nitrogen released measured by built in gas chromatography
19
Q
  • based on energy absorbed when a sample is subjected to a wavelength of infrared radiation
  • near and mid ?
  • samples can be analyzed quickly in half a minute with MINIMAL training
A

Infrared spectroscopy

Near: variety of foods (grains, meat, dairy, cereal)
Mid: Milk analyzers

20
Q
  • measures peptide bonds (at least 2)
  • violet- purple color is produced
  • color intensity is proportional to the protein content
  • does not detect nitrogen from non protein or non peptide sources
A

Biuret method

21
Q
  • measures a combination of peptide bonds and the amino acids tryptophan and tyrosine
  • protein must be isolated from sample first
  • color NOT strictly proportional to protein content
22
Q

Measures only organic nitrogen plus ammonia

A

Kjeldahl method

23
Q

Measures total nitrogen, including the organic fraction

24
Q

This method gives a higher value for products that contain nitrates/nitirites

25
The most rapid method discussed after calibration
Infrared spectroscopy
26
What method has replaced the kjeldahl method for nutrition labeling?
Dumas method
27
Preferred method for high fat samples
Kjeldahl method because fat may cause instrument fire during the incineration procedure in the Dumas method
28
What is the method of choice for multi component food systems?
Acid hydrolysis
29
For fat nutrition labeling purposes, which method is used?
Gas chromatography analysis
30
- continuous - solvent continuously flows over sample - MAY cause channeling - Faster and more efficient than semicontinuous - gravimetric analysis
Goldfish method
31
- semi continuous - solvent builds up in chamber and completely surrounds the sample and then siphons back to the boiling flask - Does NOT cause channeling - takes a long time (~8 hours) - measured by gravimetric analysis
Soxhlet method
32
- discontinuous - does not require removal of moisture from sample - developed for and applied primarily to dairy foods - fat is extracted with a mixture of ethyl and petroleum ether and the extracted fat is dried to a constant weight and expressed as percent fat by weight
Mojonnier method
33
- combination of chloroform and methanol - Modified Folch extraction and Bligh & after procedure
Chloroform - Methanol procedure
34
2 Non solvent wet extraction methods
1) Babcock 2) Gerber Both for milk fat
35
3 instrumental methods
1) infrared 2) specific gravity (Foss-let) 3) nuclear magnetic resonance