*4.3. Food Decay Prevention: Antioxidant Activity*

Stored food products are subject to oxidation, involving a loss of quality, alteration of the organoleptic properties and nutritional value, as well as of food safety problems. Synthetic antioxidant additives commonly used to avoid this process are under controversy currently, which has led to an increased interest in the agri-food industry to use the preservative properties of plant products.

Several essential oils and their components have already demonstrated their potential role in overcoming storage losses and enhancing food shelf-lives in the near future [243–245]. Some have even been approved as flavour or food additives, and others are under validation. Nowadays, the encapsulation of essential oils is also being studied to try to stabilise their antioxidant activity and even enhance it [246].

In general, *C. longa* and its products have shown their antioxidant potential as biopreservatives of physicochemical and organoleptic properties of food items, such as paneer, white hard clams, rainbow trout, cuttlefish and mashed potatoes, either alone or in combination with other plant products. This property can be improved even more with the help of nanotechnology that may control the aqueous solubility and stability of turmeric derivatives [245,247–255].

The antioxidant properties of the turmeric essential oils have been widely studied. The leaf essential oil with 22.8% β-sesquiphellandrene and 9.5% terpinolene as the main compounds has been proposed as a potential option to prevent the oxidative deterioration of fat-containing food products because of its hydrogen-donating properties and reducing power [256] (Table 3). Likewise, *C. longa* rhizome oil is able to decrease lipid peroxidation and other processes related to free-radical formation, achieving the extending shelf-lives of food products. In fact, it has exhibited the lowest peroxide value with respect to oleoresins and synthetic antioxidants, meaning a more efficient inhibitory effect of the formation of the secondary oxidation product malondialdehyde [164] (Table 3). This effect has been corroborated by means of diverse methods that evaluate both the scavenging capacity for different free radicals and the metal ion-chelating ability of the essential oil. Particularly, the essential oil obtained from the fresh rhizomes of *C. longa* (α-turmerone (42.6%), βturmerone (16.0%) and ar-turmerone (12.9%)) has exhibited satisfactory dose-dependent DPPH (2,2-diphenyl-1-picrylhydrazyl) and ABTS (2,2 -azino-bis-3-ethylbenzothiazoline-6 sulphonic acid) radical-scavenging activities (IC50 10.03 and 0.54 mg/mL, respectively), as well as reducing power [34] (Table 3). Both the DPPH and ABTS methods are between the most carried out antioxidant capacity assays [257]. These are good estimators of the antioxidant activity of any extract in general, using a simple redox reaction between the antioxidant and reactive oxygen species (ROS), being considered the DPPH method as the first line for evaluating the ability of a compound and extract or other biological source to act as a free-radical scavenger or hydrogen donor because of its accuracy, simplicity and low cost [258]. On the other hand, ABTS has been observed as especially useful to track changes in the antioxidant system itself during the storage and processing steps [257]. Reducing power is usually a complementary test to the previous ones to further evaluate the antioxidant activity [259].

In this way, the antioxidant activity of *C. longa* rhizome essential oil stood out over 10 other different essential oils. Its free radical-scavenging potential was twice higher than that of Trolox (~60% vs. 28.2%, respectively), and the antioxidant activity (72.4%) was near the values of the reference essential oil *Thymus vulgaris* (90.9%) and butylated hydroxyanisole (BHA) (86.74%) [217] (Table 3). Similarly, the reducing potential of *C. longa* rhizome oil was also highlighted over *Eucalyptus* spp., such as *E. sideroxylon*, *E. tereticornis* and *E. citriodora* [130.5 ± 1.2, 122.1 ± 1.4 and 95.8 ± 1.0 μM ferric reducing antioxidant power (FRAP) equivalents, respectively], with 138.4 ± 1.1 μM FRAP equivalents. This value was even higher than the one of other *Curcuma* spp.—for instance, *C. aromatica* (130.6 ± 1.5 μM FRAP equivalents) [260]. This antioxidant potential has also been demonstrated in vivo. A starch/carboxymethyl cellulose (CMC) edible coating including *C. longa* oil suppressed the oxidase enzyme activity of fresh-cut "Fuji" apples by 9% [261] (Table 3).

The luminol-photochemiluminiscence (PLC) assay corroborated afterwards the high antioxidant activity of *C. longa* essential oil [217]. It results in an easy, fast and sensitive method to know the scavenging activity of antioxidants against the radical anion superoxide, especially for hydrophobic-like essential oils [217]. This property may be due to the total phenolic content of *C. longa* essential oil that also highlights over more than 15 essential oils from different plant species [235]. However, the phenolic compounds of *C. longa* essential oil and, consequently, the antioxidant activity can vary depending on the cultivation conditions. Especially, the substrate type, together with the presence of fungi, have significantly influenced the composition and activity of *C. longa* leaf essential oil [169,170]. The antioxidant activity can also change according to many other factors, such as the degree of dryness of *C. longa* rhizome. Specifically, the essential oil from the fresh rhizomes (24.4% arturmerone, 20.5% α-turmerone and 11.1% β-turmerone) exhibited higher DPPH radical scavenging, as well as Fe2+-chelating abilities, than the dry ones (21.4% ar-turmerone, 7.2% α-santalene and 6.6% ar-curcumene). The antioxidant activity of both essential oils was significantly higher than the commercial antioxidants BHA and butylated hydroxytoluene (BHT) [164] (Table 3). Nevertheless, other authors reported a different trend. Gounder et al. demonstrated throughout several tests that dried and cured rhizomes had higher antioxidant activity than the fresh ones (ar-turmerone (21.0–30.3%), α-turmerone (26.5–33.5%) and β-turmerone (18.9–21.1%)). Specifically, ABTS radical cation scavenging [Trolox equivalent antioxidant capacity (TEAC) 68.0, 66.9 and 38.9 μM at 1 mg/mL]; ferric-reducing antioxidant potential (TEAC 276.8, 264.1 and 178.4 μM at 1 mg/mL); total antioxidant capacity by phosphomolybdenum assay (686, 638 and 358 ascorbic acid equivalents per 1 mg of oil) and reducing power were stronger in dried and cured rhizome than in fresh ones, respectively [165]. These differences are mainly due to the different compositions reported by the authors [164,165] in the fresh and dry rhizome essential oils used in the test.

Several research carried out with turmeric rhizome essential oil without β-turmerone among the main compounds [105,262] reported lower DPPH bleaching potential and ferric-reducing antioxidant power of *C. longa* rhizome oil (45.5% ar-turmerone and 13.4% αturmerone) principally, in comparison to those of Trolox (IC50 14.5 ± 2.9 mg/mL *vs.* 0.012 ± 0.004 mg/mL and 389.0 ± 112.0 *vs.* 402.3 ± 20.1 μM ascorbic acid equivalents, respectively) [105], as well as negligible DPPH radical scavenging activity (38.7% arturmerone and 14.2% α-turmerone) with respect to other, different essential oils, among which were cinnamon, clove, green tea, lemon eucalyptus, rosemary, oregano and its main compound carvacrol [262] (Table 3).

**Table 3.** Antimicrobial, herbicidal and antioxidant activities of *C. longa* essential oil in the agri-food industry. DPPH: 2,2 diphenyl-1-picrylhydrazyl, ABTS: 2,2 -azino-bis-3-ethylbenzothiazoline-6-sulphonic acid, BHA: butylated hydroxyanisole and BHT: butylated hydroxytoluene.



**Table 3.** *Cont.*

On the other hand, other *Curcuma* spp. essential oil with very different chemical compositions have also demonstrated strong and dose-dependent antioxidant abilities. In this sense, *C. zedoaria* (17.72% curzerenone, 15.85% γ-eudesmol acetate and 6.50% germacrone) and *C. angustifolia* (29.62% epicurzerenone, 10.79% curzerenone and 6.12% *trans*-β-terpineol) rhizome essential oils showed higher DPPH (IC50 2.58 ± 077 μg/mL and 12.53 ± 0.14 μg/mL) and ABTS (IC50 1.28 ± 0.05 μg/mL and 5.53 ± 0.29 μg/mL) radical scavenging ability, as well as reducing power (EC50 4.77 ± 0.14 μg/mL and 5.68 ± 0.11 μg/mL) than BHT and ascorbic acid (DPPH: 19.07 ± 0.17 and 5.31 ± 0.2 μg/mL, ABTS: 14.19 ± 0.21 and 1.51 ± 0.32 μg/mL and reducing power: 9.61 ± 0.18 and 5.21 ± 0.13 μg/mL, respectively) [194]. The leaf essential oil of *C. angustifolia* (33.2% curzerenone, 18.6% 14-hydroxy-δ-cadinene and 7.3% γ-eudesmol acetate) showed even higher DPPH and ABTS free-radical scavenging (4.06 ± 0.06 and 1.35 ± 0.14 μg/mL, respectively), as well as reducing (EC50 2.62 ± 0.25 μg/mL) activities, than the rhizome oil and the standard references [128]; *C. amada* rhizome oil (40% β-myrcene, 11.78% β-pinene and 10% ar-curcumene) and the essential oil obtained from the pulverized rhizome of *C. petiolata* (83.99% 2-methyl-5-pentanol) presented moderate antioxidant activity in comparison to the extracts and standard references [133,263].

*C. longa* rhizome essential oil has also exhibited strong antioxidant potential when combined with other essential oils—for instance, *Z. officinale*. In this case, the combination of both showed higher DPPH radical scavenging activity (IC50 3.75 μL/mL vs. 4.28 and 7.19 μL/mL), as well as stronger β-carotene–linoleic acid bleaching (65.24% vs. 59.88 and 55.82%) than *C. longa* and *Z. officinale* oils alone, respectively [235]. This last test has been commonly used to compare the lipid peroxidation inhibitory activity of either individual compounds or mixtures, despite possible scattered results due to different factors like the chemical composition and extracting solvent [264].

Overall, the genus *Curcuma* and its derived products have been popularly used as food additives to confer special beneficial properties, which include colouring, preservation and healthy effects. Particularly, the biopreservative properties of *C. longa* rhizome oil can meet the needs of the agri-food industry. Its suitability as a natural alternative to synthetic antioxidants has been broadly corroborated through many in vitro and in vivo tests, obtaining interesting results replacing the reference synthetic antioxidants. So much so that this essential oil is being included in food coatings to keep them much longer. Moreover, further investigation regarding the most appropriate application of *C. longa* rhizome oil, as well as combinations with other, different essential oils, is being carried out, with the aim of trying to enhance its antioxidant potential and being finally implemented in the sustainable agri-food industry.
