Milk Lipids - Physical Properties

 The physical properties of milkfat can be summerized as follows:

  • density at 20° C is 915 kg m(-3)*
  • refractive index (589 nm) is 1.462 which decreases with increasing temperature
  • solubility of water in fat is 0.14% (w/w) at 20° C and increases with increasing temperature
  • thermal conductivity is about 0.17 J m(-1) s(-1) K(-1) at 20° C
  • specific heat at 40° C is about 2.1kJ kg(-1) K(-1)
  • electrical conductivity is <10(-12) ohm(-1) cm(-1)
  • dielectric constant is about 3.1

*the brackets around numbers denote superscript

At room temperature, the lipids are solid, therefore, are correctly referred to as "fat" as opposed to "oil" which is liquid at room temperature. The melting points of individual triglycerides ranges from -75° C for tributyric glycerol to 72° C for tristearin. However, the final melting point of milkfat is at 37° C because higher melting triglycerides dissolve in the liquid fat. This temperature is significant because 37° C is the body temperature of the cow and the milk would need to be liquid at this temperature. The melting curves of milkfat are complicated by the diverse lipid composition:

trans unsaturation increases melting points
odd-numbered and branched chains decrease melting points

Crystallization of milkfat largely determines the physical stability of the fat globule and the consistency of high-fat dairy products, but crystal behaviour is also complicated by the wide range of different triglycerides. There are four forms that milkfat crystals can occur in; alpha, ß , ß ' 1, and ß ' 2, however, the alpha form is the least stable and is rarely observed in slowly cooled fat.