Why is butter harder if it once got warm?
Butter is a water in oil emulsion that contains butterfat that crystallizes at room temperature. The continuous phase of butter is a network of fat crystals. This network surrounds water and air droplets. One of the unique properties of fats are the interactions of fat crystals, and these interactions are directly related to product hardness. More fat crystal interactions lead to a harder fat. During storage, fluctuations in temperature from refrigerated to room temperature back to refrigerated temperatures result in postcrystallization. This postcrystallization strengthens the fat crystal network since more crystals are formed, leading to a stronger fat crystal network.