Why do ice crystals form when melted ice cream is refrozen?

Ice cream is usually made from milk (which contains water, lipids, proteins and lactose), flavourings and sugar. Separate ice crystals are always present in ice cream, however they are so small and numerous that our tongues can’t detect them. Ice cream gets its smooth texture in the manufacturing process by being stirred slowly while cooling. Larger ice crystals will form upon refreezing melted ice cream due to the preference of water droplets to join together, and thus form bigger droplets. Mixing while refreezing would help keep the droplets as small as possible, however you should never return thawed ice cream to the freezer anyway as this is a health risk.

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