Effect of cooling rate on the structure and mechanical properties of milk fat and lard

The mechanical and thermal properties of a fat crystal network are determined by factors such as chemical composition, solid fat content (SFC), and crystal habit (polymorphism and microstructure). Processing conditions affect the crystallization of fats, thus having an effect on their functional properties. The effects of cooling rate on the macroscopic properties of a fat crystal network were studied by crystallizing anhydrous milk fat (AMF) and lard either rapidly (5 °C/min, Newtonian cooling) or slowly (0.1 °C/min, stepwise cooling). AMF crystallized rapidly was harder than AMF crystallized slowly and had a higher SFC. Moreover, its solid state was in a more metastable polymorphic form. Upon slow crystallization, AMF had a lower SFC and its solid state was in a more stable polymorphic form. AMF crystallized under fast cooling yielded a higher Avrami constant (k), and a lower Avrami exponent (n) than AMF crystallized under slow cooling. The microstructure was also different between the two treatments. When crystallized rapidly, crystallites were numerous and small, while a smaller number of larger crystallites were observed when crystallized slowly. Similar results were observed for lard crystallized under the same conditions. The mechanical properties of both AMF and lard, expressed as hardness, depend on a variety of factors, which include the SFC, polymorphism of the solid state as well as the microstructure of the fat crystal network.