lecture quiz 9: proximate analysis continued

Question 1

to analyze DM components:

Answer 1

• cannot use DM that has been heated at 105ºC → high heat destroys nutrients
• for wet/high moisture samples: dry @ 55-60ºC (e.g. fresh forage, wet cat food, silage)
• grind (1mm particle size) → makes a homogenous representative sample
  
  • wiley mill used in labs
  • should not be powder

Question 2

proximate analysis: ash

Answer 2

• represents total mineral content (inorganic portion)
• use muffle furnace
• high purity alumina (Al2O3) lab crucibles have high melting point, high hardness, & good chemical stability
• sources of error: high temp may volatilize some minerals (e.g. Cl, Zn, Se, & I)
• tells you nothing about concentration of indiv materials
• may indicate soil contamination
• acts as diluent of other nutrients (& energy) → high ash is not good (>12%)
• method:
  
  1. weigh sample
  2. combust sample at 500-600ºC for 4h
  3. weigh ash
• calculations:
  
  • % ash = ash remaining weight/sample weight x 100
  • % organic matter = 100 - % moisture - % ash

Question 3

proximate analysis: crude protein (CP) & kjeldahl method

Answer 3

• digest small dried sample in concentrated sulfuric acid until all organic matter is destroyed
• N from feed is now in form of ammonium sulfate (NH4)2SO4
• ammonia is freed by adding NaOH to digest, distill off, & collected in standard acid
• quantitative content of N is determined by titration w/ H2SO4 or HCl
• this determines amount of N in sample
• CP = %N x 6.25
• wet chemistry method
• takes 3h per sample
• dangerous

Question 4

dumas combustion method

Answer 4

• alternative to kjeldahl method for determining crude protein in a feed
• sample undergoes oxidation by flash combustion (combustion w/ O2)
• converts all organic & inorganic substances into gases (N₂, CO₂, H₂O)
• detector measures amount of N gas
• total CP is calculate from N content of feed material (%N x 6.25)
• not wet chemistry ∴ no nasty chemicals
• less labor intensive
• faster

Question 5

limitations w/ kjeldahl & dumas procedures

Answer 5

1. do not differentiate between true protein & NPN (to find NPN: must look at ingredient label/feed tag)
   
   • bad N ingredients: urea (cheap, high N content) → added to feed to inflate %N
2. gives no indication of AA composition or protein quality (if you want to know either must know about indiv ingredients)

Question 6

is it possible to have CP values greater than 100%?

Answer 6

• yes → there are NPN sources that contain >16% N ∴ CP >100%
• e.g. urea (CH₄N₂O)
  
  • can be fed to ruminants as economical replacement for protein in diet (they can use low quality N)
  • feed grade urea contains ~45%N or 287% CP

Question 7

melamine pet food recall of 2007

Answer 7

• melamine = N-based compound used for manufacturing (e.g. laminate flooring, countertops, dinnerware)
• contains ~67% N or 419% CP
• in 2007: cat & dog deaths from kidney failure
• FDA found that melamine was illegally added to inflate CP content in certain pet foods