Why do manufactures partially hydrogenate oils

Answer

A type of fat formed when liquid oils are made into solid fats (shortening and hard margarine). Partially-hydrogenated oils are made when hydrogen is added to vegetable oil (hydrogenation process) to increase shelf-life and flavor stability of foods.

Explanation:

To increase shelf life and obtain the cooking properties of solid shortenings, oils are partially hydrogenated. That eliminates most of the unstable fatty acids—those with three or two double bonds.

that is correct.

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