Orange peel is two ingredients held together. The colored outer epidermis contains glands of aromatic oil, while the white, spongy albedo beneath is rich in pectin and protective bitter phenolics.1
The bitter compounds are water-soluble but the fragrant oils are not. Repeated cold soaking or brief changes of hot water can therefore leach bitterness while retaining much of the oil; gentle cooking then softens the pectin-rich albedo before sugar is introduced for candying.1 That same separation helps explain why zest, whole peel, and blanched rind behave so differently in sugar, salt cures, vinegar, cheong, kosho, and shio koji.