Free radicals induce oxidative damage in vivo causing aging and various diseases. They are also the major cause for deterioration, quality loss, and shelf life reduction in oils/fats and fat-containing systems. There are many studies on the antioxidative effects of natural extracts derived from rapeseedm, vegetables and olives. Most of these extracts contain water-soluble components as active ingredients. Thus, there is a need for naturally derived lipid-soluble antioxidants for utilization in oil/fat products as well as in vivo. Examples of lipid-soluble antioxidants include butylhydroxyanisole (BHA), butylhydroxytoluene (BHT), tert-butylhydroquinone (TBHQ) and propylgallate (PG). Their safety has been extensively questioned and attempts to eliminate them from human diet continued. Consequently, there is a need for developing highly active fat-soluble antioxidants from natural sources, especially from the under-utilized by-products such as meals.
2,6-Dimethoxy-4-vinylphenol (known as canolol or 4-vinylsyringol) is a well-known lipid-soluble potent antioxidant and antimutagenic compound formed in canola oil via sinapic acid decarboxylation during oil pressing at high temperature and pressure. The antiradical scavenging activity of canolol is much greater than that of well-known antioxidants, including vitamin C, bcarotene, a-tocopherol, rutin and quercetin. Unluckily, canolol is almost completely lost during oil refining which advocates its isolation from meal and adding it back to the oil. The lipophilic characteristics of canolol might account for its high affinity to the cell membranes and other biological membranes and hence its reactivity inside the body where water-soluble antioxidants are hard to react, thus establishing its outstanding role. The industrial demand of vinyl phenols and canolol is relatively satisfied by chemical synthesis and not from natural sources. Investigations have explored large scale synthesis of 4-vinylphenols through microbial or chemical decarboxylation of cinnamic acids such as p-coumaric acid, ferulic acid, sinapic acid and caffeic acid from plant sources including barley, wheat bran and sunflower seeds.
Investigations to obtain this bioactive compound from the natural sources are rare and not really successful. Although many attempts were tried, the amounts obtained from this compound from either natural sources or chemical synthesis were not able to meet the ever-increasing industrial demand due to the limited amount of this compound in plant sources and the less yield and high cost in the chemical synthesis. Consequently, there is a growing interest in developing alternative natural sources and more economic procedures to obtain this phenol in significant amounts to fulfill the industry demand.
This paper reports a new procedure for the production of canolol from under-utilized canola meal through a sequence of alkaline/enzymatic hydrolysis followed by decarboxylation of the hydrolyzed substrates under microwave irradiation. The conversion of sinapine and other sinapic acid derivatives to canolol will pave the way for new and unlimited opportunities, especially for byproduct utilization, canola oil refining and value-added processing of canola meal.
The phenolic extracts were hydrolyzed with NaOH to release the esterified phenolics. When the pH was below 2, the released phenolics were available as phenolic acids instead of ionic forms and could be extracted with diethyl ether/ethyl acetate mixture. Insoluble-bound phenolics were not extractable using the normal common extraction procedures and potentially retained in the meal. Their proportion is small, but they can be released from canola by alkaline hydrolysis. As sinapine, the choline ester of sinapic acid, is the major phenolic compound in canola meal extracts, the major role of the alkaline treatment is to break the ester linkage in sinapine thereby liberating sinapic acid and choline. The released phenolics were extracted with either diethyl ether, or ethyl acetate, or both. The conversion efficiency and the yield obtained were higher with alkaline hydrolysis compared to other methods including enzyme hydrolysis. Alkaline hydrolysis was conducted on the methanolic extracts as well as on the meal itself. Sinapine was the predominant phenolic compound in rapeseed meal, while the amount of sinapic acid was considerably less. Concerning the methanolic extracts, the total phenolic content was 10.6 ± 0.01 mg/g meal (sinapic acid equivalents; SAEs) estimated using the HPLC analysis from the total area under all peaks. The results are in accordance with previous work. After hydrolysis, it ranged from 8.1 to 11.0 mg/g reaching its highest value after hydrolyzing for 2 h using 20 mL of 4 M NaOH for 20 mL of the extract. The amount of released sinapic acid showed how efficient different hydrolyzing conditions were in releasing sinapic acid from its esters. Hydrolysis is considered complete if the value of released sinapic acid is 100 % of the total phenolic content in the original extract. Under optimum conditions all phenolics were hydrolyzed to sinapic acid which increased from 0.14 to 10.2 mg/g representing 94.8 % of the total phenolic content in the original extract.
Using canola meal as a substrate for alkaline hydrolysis, the released sinapic acid content was lower compared to using the methanolic extracts. The original meal contained 6.8, 1.1, 0.3 and 8.5 mg/g sinapine, sinapoyl glucose, sinapic acid and total phenolics, respectively. After hydrolysis, the released sinapic acid content was 7.1 mg/g (81.0 and 93.9 % of the total phenolics in the original and hydrolyzed meal, respectively).
Rabie Y. Khattab • Michael N. A. Eskin • Usha Thiyam-Hollander. J Am Oil Chem Soc (2014) 91:89–97