ôoff-odor. Autoxidation, which is the reaction involving an oilÆs double bond and groundstate molecular oxygen, is the predominate process that initiates a chain of events leading to the production of off-odors in a fat or oilö (Hannon, 1997, p. 34). Oxidative deterioration leads to a state commonly referred to as rancidity. Hydroperoxides are the primary initial products of lipid autoxidation, and are formed ôaccording to the following reaction: Hydroperoxides have no odor per se, but they rapidly break down to produce ketones, aldehydes, alcohols, hydrocarbons, and other volatile molecules that possess odors associated with rancidityö (Hannon, 1997, p. 34). The factors that determine a vegetable oilÆs oxidative stability are as follows (Hannon, 1997):
1. The first factor is the vegetable oilÆs degree of unsaturation.
2. The second factor is the presence or absence of natural antioxidants (tocopherols) in the vegetable oil.
3. The third factor is the presence or absence of catalysts (iron or other metals) in the vegetable oil.
4. The fourth factor is the process use to refine the vegetable oil.
Gel network theory, which explains how the structure, formation, and stabilities of emulsions containing fatty amphophiles and ionic/nonionic surfactants are controlled by the stability and ôswelling of a crystalline gel network phase. This phase, which sometimes exists in equilibrium with non-swollen crystalline phases, results from surfactant and amphiphile interactions with the continuous-phase waterö (Evison, 1996, p. 36).
The cosmetic emulsions which dominate the personal-care market are (1) macroemulsions [oil-in-water, water-in-oil, and liquid crystal], (2) multiple, or three-phase emulsions [water-in-oil-in-water or oil-in-water-in-oil], and (3) microemulsions [composed of tiny particles to produce an emulsion that typically clear]. Cosmetic emulsions for the care of the skin are designed to restore s...