Oppfrysning

Oppfrysning, or defrosting, is the process by which something that has been frozen returns to an unfrozen state. The term comes from opp- (up, reversing) and frysning (freezing) and is used in everyday Norwegian as well as in scientific contexts. It describes thawing of ice, snow, and frozen materials, as well as thawing of foods for preparation.

In foods, oppfrysning involves heat transfer from warmer surroundings to the frozen item. The rate depends on the item's size, shape, packaging, and temperature difference. Common thawing methods include: in the refrigerator, which keeps the item at about 4°C or lower and minimizes bacterial growth but can take a long time; cold-water thawing, where the sealed item is submerged in water that is changed regularly to maintain a lower temperature; and microwave thawing, which is fast but can cause uneven heating and partial cooking. Thawing at room temperature is discouraged due to the risk of rapid bacterial growth in the so-called danger zone between roughly 4°C and 60°C. After thawing, perishable foods should be cooked promptly or kept refrigerated.

Quality considerations are important: ice crystal formation during freezing can damage tissue, and thawing can lead to moisture loss and texture changes, particularly in meat and fish. Refreezing thawed foods is generally not recommended because of quality loss; if thawed in the refrigerator, some foods may be refrozen after cooking, but raw thawed items are best used promptly.

Beyond food, oppfrysning also describes the natural melting of ice or snow and, in engineering and climate contexts, the thawing of frozen materials due to heat input.