1283
Effect of metabolic acidosis in lactating dairy cows on concentration of milk proteins
Milk casein subunits (α, β and ĸ-casein) play a role to avoid the formation of toxic amyloid fibrils and maintain the milk thermal stability. When milk is produced by healthy cows fibrils are not formed because an alternative aggregation pathway is followed that results in formation of the casein micelle. However, nutritional disorders of lactating cows may negatively influence on composition and quality of milk. Thus, the present study aimed to evaluate the effect of the dietary cation-anion difference on concentrations of milk subunits of casein and whey proteins. Sixteen Holstein cows were distributed in 4 contemporary 4 x 4 Latin Square, consisting of 4 periods of 21 days. Cows were distributed to 4 treatments according to the dietary cation-anion difference (DCAD): 1) +290 mEq/kg of DM; 2) +192 mEq/kg of DM; 3) +98 mEq/kg of DM; 4) -71 mEq/kg of DM. The cows were fed total mixed ration and the DCAD was calculated according to the contents of Na, K, Cl and S of the diets. Individual milk samples were collected for determination of the concentrations of α (α-CN), β (β-CN) and ĸ-casein (ĸ-CN), α-lactalbumin (α-La) and β-lactoglobulin (β-Lg), by high performance liquid chromatography. The results were analyzed using the MIXED procedure of SAS (2001) (α = 0.05). The DCAD reduced linearly the concentration of ĸ-CN [ĸ-CN= 5.02(0.30) – 0.00098(0.0004)*DCAD] as well as increased the concentration of β-LG [β-LG = 1.41(0.16) + 0.00047(0.0002)*DCAD] (Table 1). The ĸ-CN and β-LG have a potential tendency to assemble into toxic amyloid fibrils. Thus, as a consequence of metabolic acidosis due to reduction of DCAD, β-LG may be associated with the ĸ-CN in the micelle and the milk stability during the heat treatment may be reduced.
Table 1 - Effect of dietary cation-anion difference (DCAD) on the milk proteins concentration (mg/ml of milk).
Milk proteins |
DCAD (mEq/Kg of DM) |
SEM |
P |
|||||
-71 |
98 |
192 |
290 |
Linear |
Quadratic |
Cubic |
||
α -CN |
6.38 |
6.36 |
6.52 |
6.17 |
0.12 |
0.404 |
0.165 |
0.118 |
β -CN |
7.34 |
7.29 |
7.38 |
7.10 |
0.19 |
0.423 |
0.482 |
0.449 |
ĸ -CN |
5.07 |
4.96 |
4.95 |
4.68 |
0.15 |
0.031 |
0.361 |
0.478 |
β -LG |
1.41 |
1.45 |
1.46 |
1.59 |
0.08 |
0.040 |
0.330 |
0.539 |
α -LA |
0.84 |
0.76 |
0.74 |
0.77 |
0.05 |
0.115 |
0.172 |
0.815 |
Keywords: Casein. DCAD. Milk Stability.