Assessment of the saponification step in the quantitative determination of carotenoids and provitamins A

Abstract

The saponification step in the determination of carotenoids was reassessed. Synthetic carotenoids (β-apo-8′-carotenal, β-carotene) and food samples (tomato, kale, papaya) were used and different procedures were evaluated. Hot saponification resulted in greater losses and cis- and epoxycarotenoids were formed. The degradation was aggravated by a more direct contact between the carotenoids and the alkali. The AOAC procedure, performed in the presence of acetone, led to complete transformation of β-apo-8′-carotenal to citranaxanthin. Saponification was unnecessary for kale and tomato but was needed for good separation of papaya carotenoids which included carotenol esters. Saponification of the carotenoids dissolved overnight at room temperature in petroleum ether, with equal volume of 10% methanolic KOH, retained β-, γ-carotene, β-apo-8′-carotenal and lycopene and completely hydrolysed the carotenol esters. However, even with this mild saponification, lutein, zeaxanthin and violaxanthin degraded significantly. These losses could be reduced to insignificant levels by using an atmosphere of nitrogen or antioxidant.