Adsorption and oxidation of β-carotene by montmorillonite bleaching clays

The role that clay mineral acidity, specific surface area (SSA) and pore size distribution (PSD) have on the adsorption and oxidation of β-carotene by montmorillonite bleaching clays was investigated. Five commercially available clay minerals were studied, including three acid-activated montmorillonites (AACs) (K10, K30 and 22B), one aluminium-pillared montmorillonite (Al-PC) and an untreated montmorillonite (SWy-3). Each underwent physicochemical characterisation. X-ray diffraction and N₂ gas sorption techniques provided structural and textural information. Titration with n-butylamine in the presence of Hammett and arylmethanol indicators characterised the concentration and strength of clay acid sites. The kinetics of β-carotene adsorption were then measured as a means of comparing their adsorptive power. The tendency of each clay to oxidise β-carotene to a variety of carotenoid-oxidation-products (COPs) was examined by analysing extracts of each clay with ultra-high-performance liquid-chromatography mass-spectrometry (UPLC-MS). The AACs adsorption and oxidation of β-carotene exceeded that of SWy-3 and Al-PC. This was attributed to a greater number of stronger acid sites, and a higher SSA. The clay PSD was also shown to play a significant role. 22B, which had a narrower PSD than K10 or K30, showed a lower tendency to adsorb β-carotene despite having the greatest number of strong acid sites. Lastly, the AACs had a greater oxidative power than SWy-3 or Al-PC and produced COPs in greater abundance than the other clays. They also produced more COPs of a higher oxidation state relative to β-carotene. These findings provide helpful information for the selection of efficient adsorbents for use in vegetable oil bleaching.