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I realized that butter, when kept cool after buying, can be easily cut and spread when taken out of the fridge. If it gets warm once, however, it will be less soft the next time I take it out of the fridge. If it reaches room temperature once, it cannot even be cut when cooled down again to fridge temperature.

What is causing this phenomenon?

(I refer to real butter from cow milk, not what might be sold as butter in some countries)

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  • $\begingroup$ Even if this is not homework, you should be able to draw out a few ideas about what happens. $\endgroup$
    – Karl
    Dec 9 '17 at 16:14
  • $\begingroup$ Butter is often sold "whipped", i.e. aerated, to make it softer (and to increase he volume, so it appears to be more for the money. Heat it, and your bubbles burst. $\endgroup$ Dec 10 '17 at 2:44
  • $\begingroup$ @DrMoishePippik I googled whipped butter. It's definitely not what I have here. Its a solid block. And there is no volume change. $\endgroup$
    – mhchem
    Dec 10 '17 at 17:45
  • $\begingroup$ Seems to be complicated. In this German FAQ they say that fatty acids have a large impact. With a special temperature treatment, they can make the "hard" fatty acids end up between the "soft" fatty acids, the so called "ball bearing effect". Also, mechanical treatment before packaging has an effect ("break some structures"). So, inverting this knowlege, my guess is that temperatures above fridge temperature allow the fatty acids to rearrange to harder structures. $\endgroup$
    – mhchem
    Dec 10 '17 at 17:53
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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.

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