A critical review of methods used to do
proximate analysis on raw materials
and animal feed and the financial
implications thereof
Evelyn Botha
14 October 2015
Introduction
•
•
•
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Analytical error
Lab result only an estimate of true value
Variation between methods
A lot of accredited methods, particularly
moisture
Accreditation bodies
• AOAC international (Association of Official
Agricultural Chemists)
• AOCS (American Oil Chemist’s Society)
• AACC International (American Association of
Cereal Chemists)
• NFTA (National Forage Testing Association; USA)
• GAFTA (Grain and Feed Association)
• AgriLASA (Agri-Laboratory Association of
Southern Africa)
• EU law: Consolidated TEXT produced by the
CONSLEG system
Moisture
• Important: pricing, prevent mould growth,
accurate feed formulation (affects other
nutrients in matrix)
• Need quick accurate method
• Industry uses method that most convenient
and cost effective
Moisture
• Water in feed: bound or unbound
• Difficult to distinguish and quantify
• Fraction of interest depends on application
Moisture
• Loss on drying / Oven method: most
applicable to animal feeds
• Fast and inexpensive despite fact that prone
to error
• Sample dried in oven: loss in weight =
moisture
• Good method for moisture if volatile content
mostly water
Moisture: accredited methods
• AOAC 934.01: sample dried in vacuum oven
at 95 -100 °C to constant weight (feed)
• AOAC 930.15/ AACC 44-19/ NFTA 2.1.1:
135°C for 2h (feed)
• AOAC 935.29/ AOAC 945.32: 103-106°C for
3h (malt and brewers grains)
• AOAC 945.15: 103-104°C for 3h (cereal
adjuncts)
• NFTA 2.2.2.5: : 105°C for 3h (lab DM)
Moisture: accredited methods
• AOAC 925.09/ AACC 44-40: 98-100 °C,
vacuum oven, to constant weight (usually 5h)
(ground grains and feedstuffs)
• AACC 44-15A: 130°C for 1h (ground grains
and cereal products) or 103°C for 72h (whole
grain)
• AACC 44-31: 130°C for 2h (soy flours)
• NFTA 2.1.2: 100°C for 24h or 105°C for 16h
(forages)
Moisture: accredited methods
• GAFTA 2:1/ CONSLEG: 1971L0393 4.2.1 :
103°C for 4h (ground feedstuffs)
• GAFTA 2:2: 130 - 135°C for 2h (cereals except
maize)
• GAFTA 2:3: 130 - 133°C for 4h (maize and
pulses, ground)
• CONSLEG: 1971L0393 4.2.2: 130°C for 2h
(cereals, ground)
Moisture: accredited methods
• Temperature: varies from 95 to 135 °C (lower
for wet forages)
• Time: varies from 1 hour to 72 hours (to
constant weight or fixed time)
• Sample mass: 2 to 10g
• Sample prep: whole, crushed or
milled/ground
• Oven: air oven, vacuum oven
Moisture
Range in moisture results for DDGS from 23 different laboratories (Thiex 2009)
16
15
14
% moisture
13
12
11
True moisture
10
9
8
7
6
E
A
C
Sample
D
B
Moisture
• Oven moisture results variable because
doesn’t measure water
• Some methods: dry to constant weight. Rare
to really achieve.
• Many labs: standard method: dry for 5h,
usually sufficient.
• Tightly bound water often not detected.
• Difficult to distinguish: free vs bound water or
water vs other volatiles
Moisture
• Varying amounts of volatiles released and
detected as moisture
• In original sample or produced by heating
process
100
98
96
94
Water
Moisture
Moisture
92
90
Other volatiles
88
86
84
DM
DM
Actual
Moisture method 1
DM
82
80
Moisture method 2
Moisture
• Moisture increases with drying temperature
and trying time
Effect of temperature and time in oven on Moisture Content (Corn sample
Experiment 2 of Ahn et al 2014, Asian Australas. J. Anim. Sci.)
13
12.5
LOD (% moisture)
12
95 degrees
11.5
105 degrees
11
115 degrees
10.5
125 degrees
135 degrees for 2 hours
10
KF
9.5
9
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hours in oven
Moisture
• Effect of time in oven: sample milled with retch mill
and dried at 98°C for different times (n=10)
Moisture
Maize (n=3)
Moisture
DFG and chop (n=3)
Moisture
Soya oilcake (n=3)
Moisture
• Ground vs unground: some methods don’t
specify
• AgriLASA recommend ground
• Advantage: sample give off moisture more
readily = shorter drying time
• Risk of pre-grinding: sample lose moisture
during grinding; difficult to quantify
• Many labs: unground; up to 72h
Moisture
• Moisture loss during grinding: Hammer mill
(moisture loss) vs Retch mill (negligible or less
moisture loss??)
Raw material
Moisture %
(Hammer
mill)
Moisture %
(Retch mill)
Difference
Whole maize (n=3)
9.30 a
10.32 b
1.02
Soya oilcake (n=3)
9.56 a
9.86 a
0.30
8.18
0.88
Sunflower Oilcake (n=1) 7.31
Chop and DFG (n=3)
9.08 a
9.72 b
0.64
Average (n=10)
9.11 a
9.79 b
0.68
Means in the same row not connected by the same letter differ significantly (p<0.05)
Moisture
• By preparing the sample with a hammer mill:
lose about 0.68% moisture before start
analysis! (could be more for high moisture
samples)
Moisture
• Chemical determination of water in sample:
• Karl Fischer titration (AOAC 2001.12) (KF):
viewed by some as the reference method
to which oven methods should be
compared (only on milled sample)
• Preferred for NIR calibrations
• Also calcium carbide and calcium hydride
methods but don’t detect water in interior of
product particles
Moisture
• Other moisture methods
• Distillation with Toluene (AOAC 925.04) or
xylene
• Drying with heat over sulphuric acid (AOAC
920.36)
• Infrared, halogen or microwave drying
• Other indirect methods and moisture meters
Moisture: Karl Fischer
• Expensive: time, expertise, equipment and
reagents
• Not available in most labs
• Find LOD method with smallest bias
compared to KF for that product
• Can’t use same oven procedure for all
products
• Moisture level in sample: affects time need to
dry to get KF result
Moisture
• A standard technique (time and temp of
drying) should be developed for each raw
material based on correlation with KF rather
than drying to constant mass as it is not
known at what stage volatiles are lost and
moisture over-estimated
Moisture
100
98
96
94
Water KF
Moisture over
pred
Moisture = KF
Moisture under
pred
92
90
Other volatiles
88
86
84
DM
DM
DM
DM
82
80
Actual
Moisture method 1 Moisture method 2 Moisture method 3
Moisture: method comparison
• Different oven methods compared to KF
• Note: KF was analysed at a lab that prepared
samples using hammer mill
• Therefore data in the comparison was
corrected for assumed moisture loss in the
hammer mill
Moisture: method comparison
1.46
Whole maize (n=3):
12.00
A
A
11.50
11.00
10.50
10.00
9.50
B
B
B
C
Moisture: method comparison
1.75
Chop and DFG (n=3):
12.00
11.50
11.00
A
AB
BC
C
10.50
10.00
9.50
9.00
8.50
D
BC
Moisture: method comparison
Soya oilcake (n=3):
1.37
11.50
A
11.00
BC
CD
10.50
DE
10.00
9.50
9.00
E
AB
Moisture: method comparison
Bran
0.31
Full fat soya
12.50
10.00
12.00
9.00
11.50
8.00
11.00
0.37
7.00
10.50
6.00
10.00
5.00
9.50
4.00
9.00
Adjusted KF
Unmilled98deg24h
Adjusted KF
Unmilled135deg2h
Sunflower oilcake 0.85
10.00
9.00
Ground maize
1.50
A
8.00
B
12.00
6.00
Unmilled135deg2h
13.00
12.50
7.00
Unmilled98deg24h
11.50
5.00
11.00
4.00
Adjusted KF
Unmilled98deg24h
C
Unmilled135deg2h
10.50
10.00
Adjusted KF
Unmilled98deg24h Unmilled135deg2h
Protein
• Kjeldahl (AOAC 954.01, 988.05, 976.05,
976.06, 990.02, 984.13 and 2001.11; NFTA
3.1 and 3.2; GAFTA 4:1)
• Combustion methods (dumas) (AOAC 968.06,
and 990.03; NFTA 3.3; GAFTA 4:2)
• Statistically equivalent but can differ by 2%
Fat
• Fat also empirical (as moisture and fibre): i.e.
crude fat result is defined by the method
(solvent and extraction conditions)
• Non-lipid substances co-extracted
• Not all lipid material extracted
• Acid hydrolysis vs ether extract: which is
right?
• Could differ by 4%
Practical and financial implications
• Choose appropriate methods according to
application
• E.g. GAFTA methods for maize trading
• Formulation: use same method that was
used when developing specifications i.e.
keep on same basis
Practical and financial implications
Poultry broiler example
Moisture method A
Moisture method B
Oven method
105°C, 3 h
135°C, 2h
Maize moisture
11%
12.1%
Maize energy
41 KJ “more”
Saving (R/t) in
formulation
R35
Bird performance
Who benefits
10g live or 7g dressed wt
Half point in FCR
Feed mill
Poultry company
Practical and financial implications
Dairy cow example
Moisture method A
Moisture method B
Oven method
105°C, 3 h
135°C, 2h
Maize moisture
11%
12.1%
Saving (R/t) in
formulation
R25
Milk production
Who benefits
500ml/cow/day
Feed mill
Dairy farmer
Conclusion
• Confusion: different methods in use leading
to varying results
• Which method is correct?
• Rather be aware of variation and manage
accordingly
• Choose method that suits product and
purpose
Thank you