The effect of herb extracts on the off-odor formation from lipid oxidation

Abstract

Hexanal has been widely used as an index of oxidized odor. However, off-odor components have different threshold values, and the antioxidant mechanisms for lipid oxidation and off-odor formation might differ. This study investigated the antioxidative activity of herb extracts for inhibiting various off-odor components, including hexanal, derived from lipid oxidation. Linoleic acid emulsions to which each herbal extract (spearmint, German chamomile, echinacea or lemongrass) was added were subjected to an oxidation reaction at 37 °C for 5 h. Hydroperoxide was evaluated by the ferric thiocyanate method and the volatiles were analyzed by high-performance gas chromatography. As a result, spearmint (Perillaceae herbs) showed high antioxidant activity for lipid oxidation and off-odor formation. The catechol structure of rosmarinic acid, the main component of spearmint, contributed to its activities. Moreover, the degree of inhibiting off-odor formation differed depending on the precursor of the off-odor component or the antioxidants used.

Introduction

Inhibiting lipid oxidation in food is essential for food quality control because it results in the formation of aldehydes, including hexanal, (E)-2-heptenal, and (E)-2-octenal, which decrease the quality of food with undesirable odors (Zhang et al., 2021; Zhang et al., 2015). Although hexanal formation is widely used as an index of oxidized odor (Mielnik et al., 2003; Ahn et al., 2002), it is known that various compounds other than hexanal also have an oxidized odor. Also, it is an issue that the off-odor of lipids may remain even after adding antioxidants, as each off-odor component has a different threshold (Marco et al., 2007; Feng et al., 2018; Zhang et al., 2022; Qi et al., 2022; Xu et al., 2022). Thus, it is important to select many off-odor components, including hexanal, in comprehensively evaluating the capacity to inhibit off-odors from lipid oxidation.

α-Tocopherol is widely used as an antioxidant to inhibit lipid oxidation (Yamauchi, 1997; Blekas et al., 1995). However, other compounds, such as quercetin or grape seed extract, were also reported to inhibit off-odor formation from lipid oxidation (Wang and Zheng, 1992; Roedig-Penman and Gordon, 1998; Mielnik et al., 2006; Mielnik et al., 2003; Ahn et al., 2002; Nam and Ahn, 2003). Mielnik et al. (2006) investigated the inhibition of lipid oxidation and off-odor formation by the addition of grape seed extract to turkey. As a result, they showed that the inhibiting effect on lipid oxidation and off-odor formation was higher according to the grape extract concentration. Moreover, there was a high correlation between lipid oxidation and off-odor inhibition, more than r = 0.95. However, the relationship between lipid oxidation and off-odor formation might differ according to the antioxidants. Furthermore, the relationships have not been considered sufficiently.

Herbs have been attracting attention as naturally-occurring antioxidants (Parejo et al., 2002; Lamien-Meda et al., 2010; Erkan et al., 2008; Zheng and Wang, 2001; Del Bano et al., 2003). We reported the antioxidant activity of herbs cultivated in Minamiaso, Kumamoto Pref., and evaluated with various methods. As a result, herbs in the Perillaceae family, including peppermint, spearmint, and lemon balm had high antioxidant capacity. Furthermore, rosmarinic acid was acknowledged to be the main antioxidant component of Perillaceae herbs by quantification of polyphenols (Kusaba et al., 2019). The study also found that the Perillaceae family inhibited lipid oxidation by the ferric thiocyanate method; however, the extent to which the off-odor formation was reduced remained unknown.

This study aims to compare the antioxidative activity of herbs for the inhibition of off-odors from lipid oxidation. At the same time, the antioxidant activity of rosmarinic acid, which is the main antioxidant of Perillaceae herbs, was also compared with other polyphenols.

Materials and Methods

Herb extracts    Spearmint (Mentha spicata L.), German chamomile (Matricaria chamomilla), echinacea (Echinacea purpurea), and lemongrass (Cymbopogon citratus) cultivated in Minamiaso, Kumamoto Prefecture (2019), were provided by Kumamoto Keiwa Inc. All herbs were dried at 60-80 °C for less than 10 h and were crushed to smaller than 10 mesh using a crusher (X-TREME, MX1200XTM, Waring Co., Ltd., USA). One gram of each powder was added to 10 mL of MWA solution (methanol: water: acetic acid = 90: 9.5: 0.5). The mixture was vortexed, followed by sonication, and stirring with a stirrer for 10 min, respectively. After centrifugation at 14 000 x g for 15 min at 4 °C, the supernatant was collected. Ten milliliters of MWA solution was added to the remaining residue. The supernatant was prepared using the same procedure described above, and the supernatants were mixed. The solvent of the supernatant was evaporated in an evaporator to obtain the extract.

The antioxidant capacity of each herb extract for lipid oxidation was evaluated by the ferric thiocyanate method as described previously (Kusaba et al., 2019). The results of herb extracts prepared by the MWA solution are shown in Table 1.

Table 1. Inhibition concentration (IC₅₀) for lipid-oxidation of the herb extracts by the ferric thiocyanate method (Mean ± SD, p < 0.05).

Sample	IC50 (mg/ml) ± SD
Spearmint	3.9 ± 0.1 c
German Chamomile	13.0 ± 0.2 a
Echinasea	7.0 ± 0.2 b
Lemongrass	12.7 ± 0.7 a

Materials    Linoleic acid, ammonium thiocyanate, di-potassium hydrogenphosphate, potassium dihydrogenphosphate, caffeic acid, chlorogenic acid, and pyrocatechol were purchased from Nacalai Tesque Inc., Kyoto, Japan. Tween 40 and iron (II) chloride tetrahydrate were purchased from Kanto Chemical Co., Inc., Tokyo, Japan. AAPH (2'2-azobis(2-methylpropionamidine) dihydrochloride), α-tocopherol, quercetin, ferulic acid, and catechin were purchased from Tokyo Chemical Industry, Tokyo, Japan. Kaempferol was purchased from Sigma-Aldrich, Germany. Rosmarinic acid and luteolin were purchased from Wako Pure Chemical Industries, Ltd., Osaka, Japan. All reagents were of special grade. The ethanol grade was 99.5% for all experiments, and ultrapure water was produced by PURIC-Z II (ORGANO, Co., Tokyo, Japan).

Hydroperoxide analysis using the ferric thiocyanate method    The 0.1% linoleic acid emulsion was prepared based on Kusaba et al. (2018); 10 mg of linoleic acid, 0.1 g of Tween 40, and 10 mL of phosphate buffer (pH 7.4) were added to a vial, vortexed for 10 s and sonicated for 10 min. Two hundred microliters of herb extracts or antioxidants dissolved in ethanol and 2 mL of 100 mM AAPH solution were added to the respective emulsions. The control was replaced with 200 µL of ethanol. The solution was incubated at 37 °C for 5 h under shade using a water bath (Lab-Thermo Shaker, TS-20, Advantec Toyo Kaisha, Ltd., Tokyo, Japan). Following incubation, 0.1 mL of sample solution, 0.1 mL of 30% ammonium thiocyanate, and 0.1 mL of 0.02 M iron (II) chloride in 3.5% hydrochloric acid were added to 4.7 mL of 75% ethanol. Absorbance at 500 nm of the mixture was measured after 3 min from the addition of iron (II) chloride in 3.5% hydrochloric using a UV-visible spectrophotometer (UVmini-1240, Shimadzu, Kyoto, Japan). The inhibition rate of antioxidants was calculated using absorbance value according to the following equation:   

Headspace solid-phase microextraction (SPME)    Following incubation, the volatiles of the headspace area above the sample solution were collected by an auto-sampler (HS100, CTC Analytics AG, Zwingen, Switzerland) using an SPME Fiber (50/30 µm DVB/CAR/PDMS SPME Fiber, Supelco, Bellefonte, PA, USA). The vials were treated by heating at 37 °C with agitation at 500 rpm. The volatiles of the sample were extracted for 30 min after pre-incubation for 5 min.

High-performance gas chromatography (GC) analysis    GC analysis was performed using a Heracles II electronic nose (Alpha M.O.S, Toulouse, France). The instrument was equipped with two capillary columns in parallel (MXT-5: polar, 10 m × 0.18 mm × 0.40 µm, RESTEK, Bellefonte; MXT-WAX: slightly polar, 10 m × 0.18 mm × 0.40 µm, RESTEK, Bellefonte), each connected to a flame ionization detector (FID). The injected volume was 1.0 µl, at a speed of 50 µL/s and a temperature of 240 °C. The trapping was performed at 40 °C for 125 s with split mode (10 mL/min). The oven temperature program was: 40 °C for 10 s, 40–250 °C at 1.5 °C/s, and 250 °C for 60 s. The carrier gas was H₂ at 1.6 mL/min in constant flow mode. The temperature of the two detectors was 260 °C. The results are presented as the average of independent experiments performed in triplicate. The inhibition off-odor formation rate of antioxidants was calculated using peak area according to the following equation:   

Identification of volatile compounds    Volatile compounds were identified according to the retention indices (RI) and AroChembase (Ver.6.0, Alpha M.O.S.)

HPLC analysis    The antioxidant compounds of the herbs were determined using HPLC, and the analysis was performed with an Agilent 1100 series. The column was a LiChrospher® 100 RP-18 (5 µm) LiChroCART® 250-4 (Merck, Darmstadt, Germany). The column temperature was set to 37 °C, and the mobile phase was (A) 50 mM H₃PO₄, (B) acetonitrile. Samples were analyzed with a 25-min gradient with the following condition: 0 min (95% A), 25 min (50% A), 25.01 min (5% A). The flow rate was 1.5 mL/min, and the UV spectra was 290 or 325 nm. The herb extracts were dissolved with 50% ethanol. Herb compounds were identified by the retention time compared to those of the standard compounds, and calibration curves of the standard compounds were used for quantification.

Statistical analysis    The results are shown as the mean ± standard deviation (n = 3). Statistical analysis was conducted using the Tukey-Kramer test with JMP Pro15 (Ver.12.2.0, SAS Institute Japan, Inc.).

Results and Discussion

Effects of herbal extracts on the off-odor formation    The effects of 4 herb extracts on the formation of off-odor components (hexanal, (E)-2-heptenal, 1-octen-3-one, (E)-2-octenal, (E)-2-nonenal, (E,E)-2,4-nonadienal, and (E,E)-2,4-decadienal) were investigated. The components were selected in reference to the reports of Warner et al. (2001) and Ulrich and Grosch (1987), and the odor threshold information (Marco et al., 2007; Feng et al., 2018; Zhang et al., 2022; Qi et al., 2022; Xu et al., 2022). As shown in Figure 1, each herb extract inhibited hexanal formation in a concentration-dependent manner. The same results were obtained for other off-odor components (data not shown). In addition, the inhibition of lipid oxidation was highly correlated (r > 0.9) with the inhibition of hexanal formation among all herb extracts (Fig. 2). These results showed that hexanal could be a suitable indicator for the determination of lipid oxidation inhibition. Based on the inhibition curve in Fig. 1, the inhibition concentrations (IC₅₀) of the 7 off-odor components were calculated, and the results are shown in Table 2. The spearmint extract showed a significantly lower IC₅₀ value compared with those of other herb extracts. This result suggests that Perillaceae herbs effectively inhibit the production of off-odor components generated from lipid oxidation. Furthermore, the IC₅₀ value of spearmint varied among the 7 off-odor components (w-z). Compared with the IC₅₀ value of spearmint for hexanal formation, its IC₅₀ value for (E)-2-nonenal was significantly lower, while its IC₅₀ value for 1-octen-3-one was significantly higher. The other herb extracts showed similar results. (E)-2-nonenal was formed by 9-hydroperoxide as a precursor, while 1-octen-3-one was formed by 10-hydroperoxide as a precursor (Lin and Bank, 2003; Zhang et al., 2015; Tsuzuki, 2019). These results suggested that precursors might be contributing, and the inhibitory effect of antioxidants may differ depending on the target off-odor components. It was also suggested that depending on the concentration of antioxidants added, 1-octen-3-one may remain even if hexanal formation was inhibited, which is a crucial issue since the threshold of 1-octen-3-one is extremely low.

Fig. 1.

The inhibition plot of hexanal: a) spearmint, b) German chamomile, c) echinacea, e) lemongrass.

Fig. 2.

The correlation of hydroperoxide inhibition and hexanal inhibition: a) spearmint, b) German chamomile.

Table 2. Inhibition concentration (IC₅₀) of the herb extracts for off-odor formation (Mean ± SD, p < 0.05).

	IC50 (mg/ml) ± SD
	Hexanal	(E)-2-Heptenal	1-octen-3-one	(E)-2-Octenal	(E)-2-Nonenal	(E,E)-2,4-Nonadienal	(E,E)-2,4-Decadienal
Spearmint	x 3.3±0.3 c	x 3.4±0.4 c	w 5.0±0.7 c	xy 2.4±0.5 b	y 1.5±0.4 b	xy 2.7±0.4 b	xy 2.6±0.5 c
German Chamomile	w 11.9±1.7 a	w 13.8±1.8 a	w 15.4±0.2 ab	wx 10.0±2.0 a	x 5.5±1.3 a	wx 10.8±1.9 a	x 5.9±1.1 ab
Echinasea	x 7.3±0.3 b	x 8.0±0.1 b	w 15.2±0.8 b	xy 4.8±1.1 b	x 7.3±0.3 a	xy 6.6±1.3 ab	z 4.2±0.2 bc
Lemongrass	xy 12.6±1.3 a	wx 15.5±1.4 a	w 20.4±2.2 a	xy 10.5±1.3 a	z 3.5±1.8 ab	xy 10.7±1.7 a	yz 7.8±1.1 a

*The letters a-c show the significant difference among herb extracts, w-z show the significant difference among off-odors.

Effects of rosmarinic acid and other antioxidants on the off-odor formation    Rosmarinic acid is the main component of spearmint (Wang et al., 2004; Farahbakhsh et al., 2021). The spearmint used in this study contained 32.3 mg/g-dry herb extract of rosmarinic acid, and, as mentioned earlier, the spearmint extract effectively inhibited the formation of off-odor components. The effect of rosmarinic acid, the main component of spearmint, on the generation of off-odor formation was compared with that of α-tocopherol, caffeic acid, or 7 other antioxidants (9 antioxidants in total). Figure 3 compares the inhibitory effect of rosmarinic acid on hexanal formation with α-tocopherol. Both showed a concentration-dependent effect, but rosmarinic acid had a significantly greater effect than α-tocopherol at low concentrations. It was clear that hexanal formation was sensitively affected by the concentration of rosmarinic acid, as the slope of the inhibition curve of rosmarinic acid is 5 times greater than that of α-tocopherol. The same trend was seen for other off-odor components (data not shown). Figure 4-a shows the IC₅₀ values of rosmarinic acid and α-tocopherol calculated from the inhibition curve for the formation of 7 off-odor components. Compared with the IC₅₀ value of α-tocopherol, that of rosmarinic acid was 3-5 times lower for all 7 off-odor components. Rosmarinic acid had a greater inhibitory effect on the off-odor formation compared to α-tocopherol, which is conventionally used as an antioxidant for lipid oxidation, suggesting that rosmarinic acid is a highly practical antioxidant for the inhibition of lipid oxidation. The IC₅₀ value of rosmarinic acid for the formation of all 7 off -odor components was about half of that of caffeic acid (Fig. 4-b). Since rosmarinic acid is a dimer of caffeic acid, it was suggested that the antioxidant activity of these two compounds derives from their catechol structure (Del Bano et al., 2003; Lu and Foo, 2001; Guitard et al., 2016), and the number of catechol structures affected the level of off-odor inhibition.

Fig. 3.

The inhibition curve of hexanal formation: a) α-tocopherol, b) rosmarinic acid.

Fig. 4.

Off-odors' inhibition concentration (IC₅₀) of rosmarinic acid and a) α-tocopherol, b) caffeic acid.

In order to compare the effect of antioxidants on the off-odor formation of all 9 antioxidants with that of rosmarinic acid, principal component analysis (PCA) was performed based on Table 3, which listed the IC₅₀ values of the off-odor formation by the 9 antioxidants (Fig. 5). The total explained variance was 90.5% (81.0% by PC1 and 9.5% by PC2). According to PCA analysis, the inhibitory effect of rosmarinic acid on the off-odor formation was similar to that of quercetin or luteolin, suggesting that these antioxidants possess high antioxidant activity and effectively inhibited the formation of off-odors. Catechin and caffeic acid, also located to the right side of PC1, were likely to show a similar trend with rosmarinic acid. The catechol structure of these antioxidants could greatly contribute to the inhibition of off-odor formation. Figure 6-b also suggested that rosmarinic acid has a high inhibitory effect on the formation of off-odor components. The loading plot of the 7 off-odor components was positioned to the left with α-tocopherol. Among the off-odor components, 1-octen-3-one and (E)-2-heptenal were positioned to the lower side of PC2, and (E)-2-nonenal was positioned to the upper side of PC2. This distribution implied that α-tocopherol was ineffective in inhibiting the formation of 1-octen-3-one and (E)-2-heptenal, and ferulic acid and kaempferol were ineffective in inhibiting (E)-2-nonenal. These results showed that the inhibitory effect of antioxidants on the off-odor formation differed depending on the structure of the antioxidants. Since Fig. 6 implied that the catechol structure in antioxidants may contribute to the inhibition of off-odor formation, the same procedure was conducted using pyrocatechol to confirm the inhibitory effect of compounds containing the catechol structure. Figure 6 compares the IC₅₀ value of pyrocatechol and rosmarinic acid. Pyrocatechol had an IC₅₀ value of about 2 times higher than that of rosmarinic acid, which was similar to the result of caffeic acid, suggesting that the inhibitory effect of rosmarinic acid on the off-odor formation was heavily dependent on the catechol structure.

Table 3. Inhibition concentration (IC₅₀) of antioxidants for off-odor formation (Mean ± SD, p < 0.05)

	IC50 (mol/ml) ± SD
	Hexanal	(E)-2-Heptenal	1-octen-3-one	(E)-2-Octenal	(E)-2-Nonenal	(E,E)-2,4-Nonadienal	(E,E)-2,4-Decadienal
Rosmarinic acid	w 1.01±0.05 e	v 1.42±0.08 d	u 1.81±0.05 e	w 0.80±0.14 d	w 0.80±0.20 c	w 0.82±0.18 e	w 0.75±0.05 e
Caffeic acid	w 2.12±0.02 d	v 2.83±0.07 c	u 3.59±0.15 c	x 1.63±0.24 cd	x 1.60±0.11 bc	w 2.12±0.15 cd	x 1.57±0.08 cd
Ferulic acid	v 4.30±0.12 ab	vw 3.76±0.02 b	u 5.43±0.11 b	wx 3.24±0.12 a	x 2.97±0.24 a	wx 3.31±0.35 ab	x 2.92±0.10 b
Chlorogenic acid	u 3.13±0.50 c	u 3.03±0.50 bc	u 3.89±0.00 c	u 2.47±0.40 ab	u 1.88±0.50 abc	u 2.77±0.40 bc	u 2.53±0.50 b
Quercetin	uv 1.74±0.16 de	uv 1.72±0.09 d	uv 1.70±0.08 e	wx 1.09±0.16 cd	vw 1.44±0.08 bc	uv 1.56±0.05 de	x 0.96±0.10 de
Kaempferol	v 4.12±0.16 b	vw 3.30±0.41 bc	v 3.86±0.37 c	x 1.91±0.27 bc	wx 2.24±0.12 ab	v 3.62±0.26 ab	wx 2.21±0.14 bc
Catechin	w 1.91±0.11 d	v 2.93±0.05 c	u 3.87±0.07 c	wx 1.44±0.14 cd	x 0.65±0.36 c	wx 1.61±0.11 de	wx 1.27±0.11 de
Luteolin	w 1.47±0.10 de	v 1.91±0.02 d	u 2.43±0.06 d	x 1.10±0.09 cd	w 1.37±0.08 bc	w 1.50±0.09 de	x 0.95±0.12 de
α-Tocopherol	wx 4.96±0.30 a	vw 6.14±0.28 a	u 8.06±0.23 a	yz 2.87±0.30 a	z 2.43±0.69 ab	xy 4.13±0.26 a	xy 3.88±0.40 a

*The letters a-c show the significant difference among antioxidants, w-z show the significant difference among off-odors.

Fig. 5.

The principal component analysis is based on the inhibition concentration (IC₅₀) of the antioxidants for off-odor components.: a) PCA, b) Loading plot. Symbols: ●, Rosmarinic acid; ○, Caffeic acid; ■, Ferulic acid; ◆, Chrologenic acid; △, Quercetin;▼,Kaempferol; ▲, Catechin; *, Luteolin; □, α-Tocopherol.

Fig. 6.

Off-odors' inhibition concentration (IC₅₀) of rosmarinic acid and pyrocatechol.

Conclusion

The inhibitory effects of herb extracts on lipid oxidation and off-odor formation were investigated. Spearmint, one of the Perillaceae herbs, showed high antioxidant activity, and further tests suggested that rosmarinic acid greatly contributes to its antioxidant activity. It was found that the antioxidants with the catechol structure, e.g., rosmarinic acid, possessed high activity, which was confirmed by the result that pyrocatechol has half of the inhibitory effect of rosmarinic acid. These results established that the catechol structure contributes to the inhibition of off-odor formation of rosmarinic acid.

Conflict of interest    There are no conflicts of interest to declare.

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