The production of corn distillers dried grains with solubles (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 utilized 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 standardized ileal digestible (SID) amino acids in DDGS.
Experimental design
Ten growing pigs with an average initial body weight of 53.5 kg 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 fiber (ADF), neutral detergent fiber (NDF), lignin, ash, reducing sugars, furosine, and acid detergent insoluble nitrogen (ADIN), and the colors 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 analyzed for protein and amino acidsto determine values for apparent and standardized 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% in DDGS that was not autoclaved to 0.68% 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%, compared with 2.94% 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 color is affected by particle size and the amount of condensed dried solubles added, color 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.
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*.
Table 1. Chemical composition of distillers dried grains with solubles subjected to increasing levels of heat treatment
|
|
Distillers dried grains with solubles |
|||
|
|
Not autoclaved |
Autoclaved at 130°C |
||
|
Item |
- |
10 min |
20 min |
30 min |
|
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 |
32.4 |
|
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 |
1Calculated by expressing the concentration of lysine in each ingredient as a percentage of the concentration of crude protein (CP).
2Reactive lysine (%) = [lysine (%) – (furosine (%) ÷ 0.32 × 0.40)].
3ADIN = acid detergent insoluble nitrogen.
4L* = lightness; a* = redness; b* = yellowness.
Table 2. Standardized 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 |
|||
|
Item |
Not autoclaved |
10 min |
20 min |
30 min |
Linear |
Quadratic |
|
CP, % |
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 |
This report is based on unpublished data by F. N. Almeida, J. Htoo, J. Thomson, and H. H. Stein.