Regeneration of Used Frying Oils by Selected Metal–Organic Frameworks as Adsorbents

Abstract

The aim of this study was to utilize seven synthesized metal-organic frameworks (MOF) and three natural clays as adsorbents in regenerating used frying oil. First, surface and pore properties, and morphologies of the adsorbents were determined, and used frying oil was treated at 0.1% addition (w/w) level. Then, physicochemical properties, mineral, and fatty-acid compositions of oil were analyzed. Finally, a recycling study was performed over five cycles with three selected adsorbents. Although there were only small improvements in oil color values, oil turbidity was significantly reduced with the adsorbents. Free fatty acidity values were reduced by 50.6% and 42.2% with Ti-MOF and zeolitic-MOF (ZIF-8-MOF) adsorbent treatments. Likewise, peroxide value reductions ranged from 43.6% with the Ti-MOF to 18.1% with natural zeolite. Reductions in the total polar materials (TPM) were the highest (42.5%) with aluminum-MOF (Al-MOF) followed by the ZIF-8-MOF (40.4%). Contents of aluminum, zinc, phosphorous, chromium, magnesium, and potassium were measured in the control and adsorbent-treated oil samples, and it was found that there was no leakage of minerals from the MOF into the oil, but rather some reductions due to adsorption. Furthermore, recycling studies with Ti-MOF, Cr-MOF, and sepiolite showed that the MOF can be used up to five times with only small activity losses compared with sepiolite. Overall, this study demonstrated the great potential of the MOF as adsorbents for frying oil regeneration, and pointed out that more research needed to design and synthesize new MOF structures for applying in the edible oil industry.

Debittering of cold pressed grapefruit seed oil by metal‐organic framework adsorbents

“…Although γ‐CD‐MOF was not effective in improving the L value, it was quite effective in reducing the turbidity. The very same result was observed in our previous study with frying oil regenerations by the MOFs (Yılmaz & Güner, ). Hence, γ‐CD‐MOF could be credited as an effective haze removal adsorbent for cold pressed edible oils.…”

“…Cold pressed fresh oil had very low free fatty acidity (FFA), and adsorbent treatments had slightly reduced it. This is quite an expected finding that adsorbents usually reduce FFA of oils (Vlasova et al, ; Yılmaz & Güner, ). The PVs indicated significant differences, and some treatments caused little increases.…”

“…All treatments were reduced it a little but not significantly. Unsaponifiable matter content of oils includes all minor components like sterols, tocols, hydrocarbons, and previous studies indicated that some of them could be adsorbed by MOF (Vlasova et al, ; Yılmaz & Güner, ). In the previous study (Yılmaz, Aydeniz Güneser, et al, ), saponification number and unsaponifiable matter content of the grapefruit seed oils were reported as 202.53–204.61 mg KOH/g, and 0.79%–0.89%, respectively.…”

“…The most significant reduction was in Zn‐MOF‐treated sample (190.24 mg/kg). Adsorption of sterols on the MOF is a well‐known phenomenon (Yılmaz & Güner, ) and may involve interactions with proton acceptor centers, coordination sites within the MOF structure, or physical adsorption on the surfaces (Vlasova et al, ). In general, the treatments resulted in some sterols losses, but it was not very high.…”

“…The MOFs are synthesized in laboratory from certain metal salts and organic linker molecules under some harsh reaction conditions through strong chemical bonds as open crystalline adsorbent materials, and have been used for gas adsorption and storage, separation, sensors, luminescence, magnetism, drug delivery, and selective adsorption applications (Furukawa, Cordova, Keeffe, & Yaghi, ; Ma et al, ; Stock & Biswas, ). Some MOFs have been used to selectively adsorp and separate herbicides from vegetable oils (Li et al, ), to purify edible oils (Vlasova, Yakimov, Naidenko, Kudrik, & Makarov, ), and to regenerate used frying oils (Yılmaz & Güner, ). To the best of our knowledge, there is no study reporting MOF application to remove bitterness from seed oils.…”