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Amino Acid Digestibility of Heat-damaged DDGS Fed to Pigs

11 September 2013

Lysine concentration of is reduced when corn distillers dried grains with solubles (DDGS) is treated with moist heat, as is the digestibility of crude protein and amino acids, according to new research from the Hans H. Stein Monogastric Nutrition Laboratory at the University of Illinois at Urbana-Champaign.

The production of DDGS involves a drying step in which temperature may reach 500°C or greater. The application of heat and moisture to feedstuffs results in the Maillard reaction, which reduces the concentration and digestibility of amino acids. Lysine's chemical structure makes it particularly susceptible to the Maillard reaction. However, during the acid hydrolysis step of amino acid analysis, some lysine is recovered from Maillard products but this lysine cannot be utilised by the animals, and thus, the amount of digestible lysine in a sample may be overestimated. Therefore, methods other than simple lysine analysis must be used when assessing feed that may be heat-damaged.

An experiment was conducted to determine the effects of heat damage on the digestibility of crude protein and amino acids in corn DDGS fed to growing pigs. A second objective of the experiment was to develop regression equations to predict the concentration of standardised ileal digestible (SID) amino acids in DDGS.

Experimental Design

Ten growing pigs with an average initial body weight of 53.5kg were surgically equipped with a T-cannula in the distal ileum and fed one of five diets. Corn DDGS from a single source was separated into four batches; the first batch was not autoclaved, and the others were autoclaved at 130°C for 10, 20 or 30 minutes. The concentrations of acid detergent fibre (ADF), neutral detergent fibre (NDF), lignin, ash, reducing sugars, furosine and acid detergent insoluble nitrogen (ADIN), and the colours of the DDGS determined as L* (lightness), a* (redness) and b* (yellowness) were measured in each batch.

Four diets that contained each of the four batches of DDGS were formulated. DDGS was the only source of crude protein and amino acids in the diets. A nitrogen-free diet was used to determine the endogenous losses of crude protein and amino acids. Ileal digesta were collected after a five-day adjustment period to the diets, and analysed for protein and amino acids to determine values for apparent and standardised ileal digestibility of crude protein and amino acids in each batch of DDGS.

Heat damage decreases lysine concentration and amino acid digestibility

Autoclaving corn DDGS decreased the concentration of lysine, from 0.82 per cent in DDGS that was not autoclaved to 0.68 per cent in DDGS that was autoclaved for 30 minutes. The concentration of crude protein was not significantly changed by heating DDGS. The lysine:crude protein ratio in DDGS that was autoclaved for 30 minutes was 2.51 per cent, compared with 2.94 per cent in DDGS that was not autoclaved.

A linear decrease (P<0.05) in SID was observed for crude protein and all amino acids in corn DDGS as time of heating increased, as well as a quadratic decrease (P<0.05) in the SID of isoleucine, lysine, methionine, phenylalanine and aspartic acid.

SID amino acids can be predicted using regression equations

With the exception of threonine, the concentrations of SID amino acids were well predicted (r2>0.60) by L*. However, because colour is affected by particle size and the amount of condensed dried solubles added, colour can be used as a predictor only in cases where those values are known to be constant.

The concentrations of SID amino acids were also predicted (r2>0.50) by the concentrations of the respective amino acids, again with the exception of threonine. In most cases, the concentration of SID amino acids was best predicted by equations that included the concentration of ADIN in the model.

Table 1. Chemical composition of distillers dried grains with solubles subjected to increasing levels of heat treatment
 Distillers dried grains with solubles
 Not autoclavedAutoclaved
Item 10 mins20 mins30 mins
Dry matter % 93.21 92.43 90.91 94.47
Ash % 5.25 5.08 5.17 5.06
Crude protein % 27.91 27.44 26.51 27.05
Lysine:CP ratio1 2.94 2.37 2.75 2.51
Furosine % 0.015 0.009 0.006 0.008
Reactive lysine2 0.80 0.64 0.72 0.67
ADF % 7.96 11.05 9.85 10.89
NDF % 31.29 33.23 33.32 32.40
Lignin % 0.88 2.06 1.73 2.57
ADIN3 % 0.12 0.53 0.42 0.55
Reducing sugars% 0.78 0.60 0.88 0.65
L* 4 59.70 43.90 46.92 45.01
a* 4 11.79 10.31 10.77 10.27
b* 4 30.22 11.25 14.51 12.01
INDISPENSABLE AA %        
Arginine 1.24 1.10 1.19 1.10
Histidine 0.71 0.67 0.70 0.67
Isoleucine 0.97 0.91 0.96 0.93
Leucine 2.92 2.82 2.89 2.78
Lysine 0.82 0.65 0.73 0.68
Methionine 0.53 0.49 0.52 0.50
Phenylalanine 1.24 1.17 1.21 1.17
Threonine 1.02 0.98 1.01 0.98
Tryptophan 0.22 0.20 0.20 0.20
Valine 1.26 1.19 1.26 1.23
DISPENSABLE AA %        
Alanine 1.90 1.83 1.87 1.82
Aspartic acid 1.75 1.66 1.73 1.66
Cysteine 0.55 0.50 0.52 0.50
Glutamine 4.43 4.31 4.41 4.26
Glycine 1.08 1.04 1.09 1.04
Proline 2.21 2.06 2.13 2.04
Serine 1.31 1.26 1.29 1.23
1 Calculated by expressing the concentration of lysine in each ingredient as a percentage of the concentration of crude protein (CP).
2 Reactive lysine ( per cent) = [lysine ( per cent) – (furosine ( per cent) ÷ 0.32 × 0.40)].
3ADIN = acid detergent insoluble nitrogen.
4L* = lightness; a* = redness; b* = yellowness



Table 2. Standardised ileal digestibility of crude protein (CP) and amino acids in distillers dried grains with solubles subjected to increasing levels of heat treatment by growing pigs
 Distillers dried grains with solubles 
  Autoclaved at
130°C
P-value
ItemNot autoclaved10 mins20 mins30 minsLinearQuadratic
Crude protein % 77.9 72.1 66.1 68.5 <0.01 0.25
INDISPENSABLE AA %            
Arginine 87.9 81.3 81.0 78.5 <0.01 0.11
Histidine 78.1 71.2 70.1 70.3 <0.01 0.27
Isoleucine 77.3 71.1 71.4 68.2 <0.01 0.03
Leucine 86.7 83.3 83.1 81.8 <0.01 0.12
Lysine 66.8 54.9 55.3 51.9 <0.01 0.04
Methionine 86.1 80.4 80.5 78.5 <0.01 <0.01
Phenylalanine 83.7 79.0 79.2 77.5 <0.01 0.05
Threonine 70.2 64.0 63.1 61.3 <0.01 0.33
Tryptophan 65.9 55.5 54.5 53.2 <0.01 0.20
Valine 77.2 71.0 70.7 67.9 <0.01 0.09
MEAN 80.7 75.2 74.9 73.1 <0.01 0.09
DISPENSABLE AA %            
Alanine 83.3 79.1 77.1 76.8 <0.01 0.89
Aspartic acid 72.3 62.1 62.1 60.2 <0.01 0.03
Cysteine 77.7 70.8 66.8 68.9 <0.01 0.55
Glutamic acid 85.2 80.1 79.0 78.7 <0.01 0.33
Glycine 73.4 66.0 58.1 61.7 <0.01 0.39
Proline 76.9 72.4 71.3 70.5 <0.01 0.48
MEAN 78.1 71.7 69.0 69.4 <0.01 0.91

Key Points

  • When DDGS is treated with moist heat, the concentration of lysine is reduced.
  • The digestibility of crude protein and amino acids, particularly lysine, in DDGS is reduced as a result of heat damage.
  • The concentrations of SID amino acids may be accurately predicted from regression equations including values of total amino acids, ADIN, and sometimes also L*.

September 2013

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