How Is Natural Antioxidant Rosemary Extract Used for Food Preservation?

With the increasing concern for the safety and nutrition of edible vegetable oils, the efficient maintenance of the nutritional quality and stability of edible vegetable oils has become a key issue in the oil and fat industry. As a natural antioxidant, rosemary extract has good antioxidant ability, which can effectively inhibit the oxidative deterioration of fats and oils and prolong the storage time, so as to maintain the good organoleptic and nutritional qualities of fats and oils, and has better stability in high temperature environments. Since our population mainly consumes fats and oils by thermal processing, rosemary extract, which is naturally non-toxic, highly efficient and resistant to high temperatures, is an ideal antioxidant for dietary oils in China. This study summarizes the progress of the application of natural antioxidant rosemary extract in edible vegetable oils, which is valuable for the antioxidant study of rosemary extract in vegetable oils.

Edible vegetable oils contain a large amount of unsaturated fatty acids, which are an important source of essential fatty acids for our body. However, unsaturated fatty acids are easily oxidized during storage and thermal processing, resulting in the deterioration of oil quality and the production of harmful substances [1]. Currently, small-packed edible vegetable oils in China are generally nitrogen-filled or use endogenous antioxidants to preserve freshness, and oxidation of fats and oils will occur after consumers open the bottles for a certain period of time, while medium-packed and above-packed edible vegetable oils are prevented from oxidation by the addition of tertiary butylhydroquinone (TBHQ). TBHQ has the risks of teratogenicity and carcinogenicity [2-3], which is not in line with the new requirements of high-quality development, therefore, safe and efficient natural antioxidants have received increasing attention.

Edible vegetable oils usually contain trace amounts of nutrient compatibilizers, such as tocopherols, whose molecular structure contains active phenolic hydroxyl groups, which can capture free radicals generated during the oxidation of fats and oils, and thus have antioxidant capacity [4]. Due to oxidation and degradation, tocopherols are severely depleted in heated oils and fats, which reduces their antioxidant capacity [5-7], and since consumers in China mainly consume fats and oils through thermal processing, such as frying and stir-frying, there is an urgent need for safe, efficient, and high-temperature-resistant antioxidants for edible vegetable oils in China.

Studies have shown that rosemary extract is a natural, non-toxic, efficient and high temperature resistant antioxidant [8-9], which is particularly suitable for use as an antioxidant in thermally processed oils [10-11]. In this paper, the active components, domestic and international approvals, antioxidant mechanisms, research progress and problems in the application of natural antioxidants in edible vegetable oils are summarized.

1 Rosemary Extract

Rosemary (Rosmarinus officinalis L.) is an evergreen aromatic small shrub of the genus Rosmarinus of the family Labiatae native to the Mediterranean region, also known as aiguille, sea dew, with an aromatic flavor, is an important medicinal and food plants, with thousands of years of cultivation and food history [12-14].

Rosemary extracts are obtained by solvent extraction or supercritical carbon dioxide extraction using the stems and leaves of rosemary as raw materials.   Rosemary extract contains a variety of chemical components, mainly flavonoids, terpenoids, phenols, organic acids and other substances, with superior antioxidant, antibacterial and anthelmintic properties, as well as a good aroma[15] . In rosemary extract, the main antioxidant components are silymarin, rhamnol and rosmarinic acid (see Figure 1), with silymarin and rhamnol accounting for more than 90% of the antioxidant properties of rosemary extract, of which silymarin is the most powerful active component with the highest antioxidant capacity and content[16-17] . Sage acid and rhamnol are oil-soluble and have significant effects in preventing the oxidation of fats and oils.

Numerous studies have shown that rosemary extract is a highly effective and heat-resistant natural antioxidant, and it is also recognized as a third-generation edible natural antioxidant [18]. Rosemary extract has good antioxidant properties, and its antioxidant capacity is 2-4 times higher than that of synthetic antioxidants, such as butyl hydroxyanisole (BHA) and dibutylhydroxytoluene (Butylated hydroxytoluene (BHT)), and 1-2 times higher than that of tea polyphenols, and it is superior to vitamin E and comparable to the antioxidant effect of TBHQ. The antioxidant effect is comparable to that of TBHQ, and it is natural, non-toxic, highly efficient, and resistant to high temperatures up to 240 ℃, which overcomes the weakness of most natural antioxidants that are unstable at high temperatures, and it is especially suitable for use as an antioxidant in thermal processing oils [8-11,19].

2 Domestic and foreign approvals of rosemary extract as an antioxidant

In 1994, rosemary extract received GRAS approval from the U.S. Food and Drug Administration (FDA) [20] as a natural antioxidant with no safety risks and can be recommended for use; in 2010, the European Union Food Safety Commission approved rosemary extract as a safe and effective antioxidant [21]. In late 2018, Australia approved rosemary extract as a food additive [22]. In late 2018, Australia approved rosemary extract as an antioxidant for food additives. Nowadays, rosemary extract has been used in edible vegetable oils and oil-rich foods in developed countries and regions such as Japan and the European Union [22].

In 2014, China approved rosemary extract as a food additive antioxidant, and the national standard GB2760-2014 "National Standard for Food Safety, Standard for the Use of Food Additives" stipulated the scope of use and limited values, of which the maximum use level in edible vegetable oil was 0.7 g/kg. The national standard GB1886.172-2016 "National Standard for Food Safety, Food Additives, Rosemary Extract", which came into effect in 2017, stipulated that the content of total antioxidant components (in terms of Salvia divinorum acid and Salvia divinorum phenol) in rosemary extract was 0.7 g/kg. GB1886.172-2016 "National Standard for Food Safety Food Additives Rosemary Extract" stipulates that the content of total antioxidant ingredients in rosemary extract (in terms of Salvia divinorum acid and Salvia divinorum phenol) should not be less than 10%, and the content of rosemarinic acid should not be less than 5%.

3 Antioxidant mechanism of rosemary extracts

Rosemary extract is a lipophilic antioxidant, which can scavenge unilinear oxygen, hydroxyl radicals and lipid peroxyl radicals, thus preventing lipid peroxidation and rupture of biological membranes [3]. The main components of rosemary extract have two hydroxyl groups in the neighboring position on the benzene ring, which is conducive to the formation of semiquinone structure after the oxidation reaction, and the mono electrons form large τ-bonds with the benzene ring through the resonance effect, so that the semiquinone structure is more stable, and the quinone structure is more stable if the other hydroxyl group is oxidized as well [23].

Therefore, rosemary extract has high antioxidant capacity. Rosemary extract also contains a large number of terpenoids, phenolic hydroxyl groups and unsaturated double bonds, which can effectively capture free radicals and inhibit the oxidation of vegetable oils [24]. Some of the diterpenoids are structurally stable and are not easily volatilized or decomposed at high temperatures. Therefore, rosemary extract has not only good antioxidant properties but also high thermal stability in edible vegetable oils.

4 Application of Rosemary Extracts in Edible Vegetable Oils Study

4.1 Rosemary extract as a single antioxidant

Zhang et al. showed that rosemary extract exhibited strong inhibitory effects on lipid oxidation of sunflower oil during accelerated storage, significantly reducing free fatty acid, peroxide and anisidine values of sunflower oil during storage. The antioxidant effect was superior to synthetic antioxidants BHT and BHA [3]. Liu Fengxia et al. compared the effect of adding rosemary extract and chemical antioxidant to gardenia oil after storing at 63 ℃ for 20 d. The results showed that the addition of rosemary extract could significantly inhibit the generation of conjugated dienes and peroxides in gardenia oil, slow down the decrease of DPPH free radicals and the rate of ABTS free radicals scavenging, and showed antioxidant activity superior to that of the chemically synthesized antioxidants, BHA+BHT [ 25]. 25].

Jin et al. investigated the antioxidant properties of rosemary extract and a mixture of TBHQ, ascorbyl palmitate and citric acid in the formation of 4-hydroxy-2-trans-nonenal (HNE) in corn oil heated for 6 h at 185 ℃. The results showed that the mixture of TBHQ, ascorbyl palmitate and citric acid reduced the formation of HNE by 27% after heat treatment of corn oil for 6 h, while the rosemary extract reduced the formation of HNE by 29% under the same conditions[26] . The results of Yang et al. showed that the addition of rosemary extract increased the scavenging capacity of DPPH and ABTS radicals in soybean oil, rice bran oil and cottonseed oil during the storage process, significantly prolonged the oxidation induction period of the oils and reduced the loss of unsaturated fatty acids, suggesting that rosemary extract can be used as an antioxidant for the oxidation of oils and fats. This suggests that rosemary extract can effectively improve the antioxidant capacity of oils and fats during storage [27].

The above studies have shown that rosemary extract can effectively slow down the oxidation of unsaturated fatty acids in vegetable oils, and has obvious effects in free radical scavenging ability, inhibiting the generation of oxidized products in oils and fats, and has good antioxidant properties. Moreover, in high temperature environment, rosemary extract can significantly slow down the oxidation of vegetable oils, reduce the loss of nutrients, and effectively prolong the shelf-life of vegetable oils.

4.2 Compounding of rosemary extract with other antioxidants

Ibsch showed that rosemary extract in combination with ascorbyl palmitate showed better antioxidant properties than the synthetic antioxidants BHA and BHT, and was more effective than the synthetic antioxidants BHA and BHT, even at high temperatures, and effectively slowed down the oxidation rate of soybean oil[28] . Yang Jinfang et al. found that the stability of walnut oil was best when vitamin E, rosemary extract and tea polyphenols were added at the levels of 880, 690 and 340 mg/kg, and the shelf-life was 5.6 times longer than that of the control walnut oil[29] .

Wang et al. found that the induction period of sunflower oil was 3.63 and 1.91 times higher than that of rosemary extract and teat polyphenol palmitate alone at a ratio of 5:3, which was confirmed by electron spin resonance spectroscopy (ESRS) analysis, and this was attributed to the fact that the combination of antioxidants greatly reduced the number of free radicals involved in the oxidation of the oils and fats [30]. Wei Jianlin et al. showed that the antioxidant properties of tea polyphenol palmitate and rosemary extract were synergistic, and when tea polyphenol palmitate was 0.45 g/kg and rosemary extract was 0.15 g/kg, they had the strongest antioxidant properties in sunflower oil, and the effects of slowing down the acidity and peroxide were similar to those of TBHQ and better than those of BHA and BHT, while their effects of slowing down the increase of malondialdehyde were better than those of TBHQ [31]. The ability to slow down the rate of increase of malondialdehyde content was better than that of TBHQ [31]. Wang Kai et al. found that the combination of rosemary extract and tea polyphenol palmitate showed the best synergistic effect, which effectively prolonged the oxidation induction time of sunflower oil, efficiently inhibited the production of primary and secondary oxidation products, and reduced the generation of heating volatile products [32].

The above studies have shown that rosemary extract in combination with other antioxidants can increase its antioxidant capacity, reduce the production of volatiles in high temperature environments, and maintain good organoleptic and nutritive qualities. The antioxidant activity of rosemary extracts, both as a single antioxidant and in combination with other antioxidants in vegetable oils, has been shown to be favorable.

5 Problems in the use of rosemary extracts in edible vegetable oils.

5.1 Interaction of rosemary extract with other antioxidants

Zhao et al. found a synergistic effect of rosemary extract and TBHQ in antioxidant activity based on the changes in acid value and peroxide value in radish seed oil. The antioxidant effects of 0.01% rosemary extract and 0.01% TBHQ complex were superior to those of 0.07% rosemary extract and 0.02% TBHQ, respectively[33] . Jaswir et al. demonstrated that by comparing the changes in fatty acid content during frying of palm oil, rosemary extract had a good synergistic effect with citric acid, and the retention of essential fatty acids was optimal in palm oil. In the same study, Malihe et al. found that rosemary extract and citric acid significantly delayed the oxidation of olive oil and increased the oxidative stability of olive oil showing synergistic effects[34-35] .

Hras et al. found that the antioxidant capacity of sunflower oil stored at 60 ℃ was characterized by peroxide and anisidine values, and the antioxidant capacity of rosemary extract showed an additive effect when synergized with citric acid and ascorbyl palmitate, respectively, and an antagonistic effect when used with α-tocopherol[36] . Hwang et al. studied the interactions of amino acids with natural antioxidants in soybean oil. Hwang et al. investigated the interaction between amino acids and natural antioxidants in soybean oil and characterized the antioxidant capacity in terms of polyglycerides and free radicals, and showed that methionine showed an additive effect and lysine showed an antagonistic effect with rosemary extract in soybean oil [37].

The above studies have shown that when rosemary extract acts together with other antioxidants, it exhibits two effects: one is to enhance its antioxidant capacity in vegetable oils, which has a synergistic effect and effectively slows down the oxidation of fats and oils; and the other is to inhibit its antioxidant capacity and reduce its antioxidant effect in vegetable oils. Therefore, further studies are needed to analyze the mechanism of the interaction between rosemary extract and other antioxidants and their effects on the oxidative stability of vegetable oils from a deeper perspective.

5.2 Transformation of Rosemary Extracts in the Antioxidant Process of Edible Vegetable Oils

The main antioxidant component of rosemary extract is silymarin, which protects the oil from oxidation by interrupting the oxidative chain reaction through the hydrogen atoms provided by its own hydroxyl group. When it is consumed, it will be degraded into rhamnol and other intermediates [38]. Rhamnol is a fat-soluble substance with antioxidant capacity, but there is no in-depth study on whether its intermediates also have antioxidant capacity, and whether it is hazardous to human beings.

6 Conclusion

With the widespread concern for food safety, natural and healthy antioxidants with no safety risks to human body are increasingly favored by consumers. As a natural antioxidant, rosemary extract has significant antioxidant effects in edible vegetable oils, and it still exerts good antioxidant effects under high temperature conditions. However, the interactions between rosemary extract and other antioxidants and their transformation in the antioxidant process of edible vegetable oils deserve attention and further investigation.

In recent years, the increasing scale of rosemary cultivation in China has promoted the rapid development of rosemary extract as an antioxidant in the edible vegetable oil industry, which has a broad application prospect, and is expected to lead the edible vegetable oil industry to better meet the people's demand for safe, nutritious and healthy consumption upgrading, and to enrich the content of the "Quality Grain Project".

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