Vegetable oil comes from many different types of grains and plants, including corn, safflower, sunflower, canola, soybean and peanut. Products labeled "vegetable oil" are made from blends of different oils and are generally have a very slight taste and clear to slightly less yellow than pure corn oil. Vegetable oil is well suited for use in cake and brownie mixes, as it imparts no flavor of its own to the final product.

Olive oils vary from the strong, fruity taste of extra virgin oil to the neutral, mild flavor of pure olive oil and light olive oil varieties. The lightest types are made by straining the oils until no solids are left and most of the acidity has been removed. Pure or light olive oil can be used in brownie mixes without affecting their flavor. Use the same amount of light olive oil as you would any other type of vegetable oil.

All fats, solid or liquid, play an important role in baking. When flour is combined with liquids, gluten is produced, and it's what gives the dough in yeast breads its characteristic elastic quality. In other baked goods adding fats, including vegetable oil, interferes with the formation of gluten, which develops as strands in the dough. The word "shortening" comes from the action fats take in coating flour molecules so they produce shorter gluten strands and more finely grained texture in baked goods such as brownies.

Packaged brownie mixes are a combination of ingredients that include flour, sugar, cocoa powder, baking soda, salt, flavorings and stabilizers. They also contain a small amount of vegetable oil, often in the form of soybean oil, and stirring in the amount of oil called for in the package directions adds moisture to the batter, producing a tender texture.