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Article

Spectroscopic Examination and Chemometric Analysis of Essential Oils Obtained from Peppermint Herb (Mentha piperita L.) and Caraway Fruit (Carum carvi L.) Subjected to Pulsed Electric Fields

by
Urszula Sadowska
1,*,
Arkadiusz Matwijczuk
2,
Agnieszka Niemczynowicz
3,
Tomasz Dróżdż
1 and
Andrzej Żabiński
1
1
Institute of Machinery Exploitation, Ergonomics and Production Processes, Faculty of Production and Power Engineering, University of Agriculture in Kraków, Łupaszki 6, 30-198 Kraków, Poland
2
Department of Physics, University of Life Sciences, Akademicka 13, 20-950 Lublin, Poland
3
Department of Analysis and Differential Equations, Faculty of Mathematics and Computer Science, University of Warmia and Mazury, Słoneczna 54, PL-10-710 Olsztyn, Poland
*
Author to whom correspondence should be addressed.
Processes 2019, 7(7), 466; https://doi.org/10.3390/pr7070466
Submission received: 13 May 2019 / Revised: 9 July 2019 / Accepted: 16 July 2019 / Published: 19 July 2019
(This article belongs to the Section Environmental and Green Processes)
Browse Figures
Versions Notes

Abstract

:
In the food industry, the pulsed electric field (PEF) technique is used to support the process of extraction of various substances. The aim of this study was to analyze the effect of a number of PEF pulses applied to peppermint and caraway on both the content of essential oils (EO) and their spectroscopic properties. The examined herb species were placed in a special vessel in the working chamber of the device between two electrodes providing high voltage electric pulses. The pulses were delivered 0, 150, 250, and 350 times per a studied sample of each herb. Essential oils were then obtained by way of hydrodistillation. The infrared spectra for all samples were measured using an FTIR spectrometer in the spectral range of 3700–730 cm−1. The applied electric field of a predetermined number of pulses had no significant effect on the amount of distilled essential oil from caraway fruit, while in the case of peppermint, it caused a slight decrease in relation to the raw material not subjected to PEF exposure. It was found that the analysis of infrared spectra made it possible to compare the quality of the obtained oils with each other and to pre-determine their compositions.

1. Introduction

Peppermint (Mentha pipperita L.) and caraway (Carum carvi L.) are two of the most important herb species grown in Poland [1,2]. Their healing properties are primarily determined by essential oils (EO) [3]. Essential oils from various plant species are widely used in the pharmaceutical, cosmetic, and food industries [4]. In addition to their traditional uses, they may have a potential for additional applications due to their proven inhibitory effects on the development of pathogenic microorganisms [5,6,7,8,9] or their insecticidal activity [10]. Unfortunately, the content of EO in herbal plants is low. Thus, it is important to extract the highest possible amounts of these valuable ingredients. In the case of peppermint pharmaceuticals, raw material comes in the form of leaves, while in the case of caraway, in the form of schizocarps [3]. In pharmaceutics, only raw materials with the required operational parameters and that meet strictly defined standards can be used, including inter alia, the minimum, normatively required amount of the examined active compounds. In accordance with requirements [3], the minimum oil content in peppermint leaves must be 12 mL∙kg−1 for uncrushed substance, while in the case of caraway, it must not be less than 30 mL∙kg–1. The determination of essential oil content in plant substances in the context of pharmaceutical applications is carried out by means of steam distillation, in accordance with a methodology strictly defined for each respective species [3].
At times, the process of substance extraction is preceded by various preliminary procedures. One of the more common and effective entails exposure to a pulsed electric field (PEF). Bazhal et al. [11] showed that PEF application leads to increased efficiency of apple juice pressing processes and changes the physical and optical properties (such as absorbance and transmittance) of the obtained products. Similar results were obtained by Praporscic et al. [12], who used PEF to increase the efficiency of apple and carrot juice pressing. They proved that a pulsed electric field not only increases the pressing capacity, but also positively affects the quality characteristics of the products obtained. Bouzrara and Vorobiev [13] showed that the use of pulsed electric field before sugar beet juice pressing increases juice yield by 18–28%. Inspired by these studies showing beneficial effects of PEF on other food products, we conducted research on the use of PEF treatment to support the secretion of essential oils from herbal plants in the hydrodistillation process.
The aim of the study was to analyze the effect of the number of PEF pulses applied to peppermint and caraway on both the content of essential oils and their spectroscopic properties, which allowed the authors to identify the composition of essential oils from selected plants. The FTIR infrared spectroscopy and chemometric analysis results obtained for selected samples of essential oils from the two plant species were used in the performance of the aforementioned goal.

2. Materials and Methods

The research material consisted of dried peppermint herb (Mentha piperita L.) with a moisture content of 11%, from conventional cultivation, Poland (N 50°30′17″, E 20°21′38″) and Kończewicki caraway fruit (Carum carvi L.) with a moisture content of 9%, both of which were bought at the local seed store.
The study on PEF application was carried out on a prototype research stand. Its basic elements included a high voltage pulse generator, a working chamber (Figure 1), and a computerized control system. The examined herb species were placed in a special vessel in the working chamber of the device between two electrodes providing high voltage electric pulses. The pulses were of rectangular shape and were applied 0, 150, 250, or 350 times per sample of each herb. The duration of a single pulse was ≈25 μs and the time interval between two pulses was 10 s, i.e., the process lasted at least 25 min per sample at the lowest level of intensity (150 pulses).
The intensity of the electric field was 30 kV∙cm−1. The volume of the vessel in the pulse chamber was 20 mL. One vessel contained 4 g of dried peppermint leaves moistened with 9 mL of distilled water, the other, 10 g of caraway schizocarps with 9 mL of distilled water. In each run of the experiment, tests for peppermint were carried out in 15 replicates (15 × 4 g), so as to obtain 60 g of raw material. This mass was necessary to perform the analysis of the content of essential oils in three replicates, in accordance with pharmaceutical requirements [3]. In the case of caraway seeds, three replicates (3 × 10 g) were performed because that was enough to obtain the oil content required [3]. After application of PEF was completed, the temperature of the tested material was measured. The temperature was measured using a K-250PC probe co-operating with a PC720M digital gauge made by the company SANWA.
Due to time constraints, it was impossible to directly analyze the content of oils (a long process), therefore, the obtained material was transferred to an airtight container and placed at 3−4 °C for 24 h. After this time, the content of essential oils was analyzed. The study was carried out in triplicate for each species and experimental condition.
The content of essential oils in the herbal species from individual experimental combinations was determined by 2 h hydrodistillation, in a Clevenger apparatus. A single test sample for the peppermint had a weight of 20 g, and in the case of caraway, 10 g. Hydrodistillation for each species was carried out using 200 mL H2O, in accordance with the pharmaceutical requirements for EO [3]. The tests were performed in three replicates for each species and combination of runs (0, 150, 250, and 300 pulses). The material was tested uncrushed. After distillation, the essential oil was brought to the scaled part of the receiver and its volume was determined. After the reading, the oil was poured into an airtight container and stored at 3–4 °C until spectroscopic analysis. The determinations were carried out in triplicate for each condition.
Infrared absorption spectra were recorded with a 670-IR (Varian) spectrometer. The attenuated total reflection (ATR) configuration was used with 20 internal reflections of the ZnSe crystal plate (45° cut). Typically, 16 scans were collected, Fourier-transformed, and averaged for each measurement. Absorption spectra at a resolution of one data point per 1 cm–1 were obtained in the region between 4000 and 600 cm−1. The instrument was continuously purged with N2 for 40 min. before and during measurements. The ZnSe crystal plate was cleaned with ultra-pure organic solvents from Sigma-Aldrich (Poznań, Poland). The spectral analysis was performed with Grams/AI software from ThermoGalactic Industries (Waltham, MA, USA). All experiments were carried out at 23 °C.
Statistical analyses were carried out using the STATISTICA 13.1 software (Stat Soft, Inc., Kraków, Polska). A one-way analysis of variance (ANOVA) was performed separately for each species. The factor of the experiment was the number of pulses (0, 150, 250, 350). The arithmetic mean as well as the standard deviations (SD) were calculated. When comparing variables between groups, the ANOVA with post-hoc analysis was used to show specific differences between groups. The significance of differences between the mean values was evaluated using Fisher’s test for p = 0.05.
Principal component analysis of the FTIR spectra were performed using OriginPro software (OriginLab, Northampton, MA, USA) using the Principal Component Analysis for Spectroscopy app.

3. Results and Discussion

The results of the chromatographic analysis of the essential oil isolated from the mint herb are presented in Table 1. Fifty-six ingredients were identified, and monoterpenes, menton, and menthol were the dominant compounds. According to many authors, these compounds are the essential components of this plant’s oil [5,14,15,16]. However, in terms of their relative percentages, quite significant differences can sometimes be noticed. Their causes result primarily from the genetic diversity of plants, but they can also have a technological character [17]. Mentone and menthol accounted for more than 60% of the total content in our research. Similar results of the percentages of these compounds were reported by Schmidt et al. [18]. The other essential components of the oil were distinguished by an increased proportion of menthofuran and menthyl acetale (Table 1), which is also consistent with the results obtained by Schmidt et al. [18].
Studies conducted by Seidler-Łożykowską et al. [20,21] indicated little differentiation in the content of the main ingredients of carvone and limonene in oil obtained from the fruit of Kończewicki caraway. This supported the choice of this particular cultivar for the study, particularly coupled with the fact that it is the only cultivar of that species currently registered in the COBORU Register of Agricultural Plant Varieties. It is a variety of plant recommended for cultivation in Poland. The current study pertained to oil extracted from the fruit of Kończewicki caraway.
The chromatographic analysis of oil obtained from the caraway fruit (Carum carvi L.), Kończewicki cultivar, revealed the dominance of two compounds: carvone and limonene, constituting, respectively, 65.60 and 30.27% of the content (Table 2). The contents of other analyzed compounds fluctuated between 0.01 (α-thujone) and 0.16% (Carveol). When analyzing the content of essential oils in caraway fruit obtained from various cultivations and over a number of years, Bosko et al. [22] also observed a negative correlation between the overall oil content and the share of its respective constituents. Specifically, with increasing oil content, the concentration of limonene decreased in favor of carvone. Seidler-Łożykowska et al. [20] also demonstrated, in their studies on various caraway cultivars, a mutual, negative correlation between the two compounds; a higher carvone content in the oil always correlated with a lower content of limonene.
The results of tests on the total content of chemically active compounds in peppermint and caraway are presented in Table 3. A decrease in the content of oils was observed in the case of peppermint after pulsed electric field application, irrespective of the number of pulses used, compared to the material not exposed to PEF. There were no statistical differences between the combinations where a different number of pulses were used. Heating of the substrate subjected to the pulsed electric field was noticed in relation to the herb not subjected to these procedures (Table 3). The highest temperature was recorded for peppermint herb after the application of 250 pulses, slightly lower values were obtained in the case of 350 pulses. However, given the values of the standard deviation for these combinations, the differences should be considered minimal.
It was found that caraway fruit, regardless of the experimental combination, was characterized by a similar amount of essential oil. Similarly to peppermint, heating of caraway fruit subjected to PEF was noted. The fruit reached temperatures in the range of 38–42.3 °C.
In the available literature, there is no data on the impact of pulsed electric fields on the content of essential oils in peppermint herb and caraway fruits. A reduced amount of essential oils isolated from peppermint leaves subjected to PEF application was observed, irrespective of the number of pulses used, in relation to the material not subjected to these treatments. It seems that this may be due to the increasing temperature of the tested material during PEF application. Heating of peppermint above 34 °C may already promote a significant loss of essential oils. Similar observations in the case of sage dried within a comparable temperature range were made by Sellami et al. [23], and by Sadowska et al. [24] for sage and thyme. The results published by these authors indicate 0.3% to 0.26% loss of oil during convective drying at 45 °C compared to natural drying at a temperature of about 22 °C [23], and increase of said loss at drying temperatures between 35 and 40 °C by 0.23 mL∙100 g−1 for sage, and 0.16 mL∙100 g−1 for thyme. According to Vega-Mercado et al. [25] and Ngadi et al. [26], short pulses of electrical energy cause the perforation of cell membranes. Thus, loss of oils can occur during the PEF application itself. In the case of peppermint, the oils are located in glandular hairs located on the leaf surface [27]. It should be noted that the material under investigation was already dried, and, according to the observations previously made by Diaz-Maroto [28] using a scanning electron microscope on the leaves of Mentha spicata L., glandular hairs are already deformed under the influence of drying. Additional damage to cell membranes probably occurs during this treatment, and with simultaneous heating of the material, this leads to the loss of accumulated oils.
The basis for pharmaceutical qualification of caraway is the minimum content of essential oils [3]. In the case of caraway, this should be not less than 30 mL∙kg−1. It should be noted that the tested raw material did not meet the required standards in any experimental combination. The literature on the subject indicates a large span in the content of essential oils in caraway fruits, in the range of 1–7% [2]. The amount of essential oils in herbal plants is determined genetically, but is also modified by environmental or agrotechnical conditions [29]. The obtained results of this research were within the given range.
The PEF application to caraway schizocarps did not cause essential oil loss, as was the case with peppermint. On the contrary, even a small increase in the amount of isolated oils was noticed, however, this dependence was not confirmed statistically. Thus, the results obtained are more promising than in the case of peppermint. Morphological diversity of the location of essential oils in these two species should be noted. In caraway fruits, differently than in peppermint, they are located in oil canals. Studies on the ultrasonic treatment of caraway fruit before essential oil hydrodistillation process, carried out by Assami et al. [30], indicated faster release of the volatile fraction, but, similarly to the results presented in this paper, no major differences in their amount were found.
Figure 2 shows ATR-FTIR spectra for selected essential oil samples obtained for testing from peppermint and caraway fruit after pulsed electric field exposure at the respective doses (mentioned above). Figure 2A presents spectra of peppermint oils for field exposure dosed at 0, 150, 250 and 350 PEF (continuous lines: black, gray, blue, and red, respectively). The spectra obtained for caraway samples are presented in the same order in Figure 2B. The samples were applied onto the ZnSe crystal and tested under N2 (as explained in the Materials and Methods section).
Table 4 presents all characteristic spectra observed in selected samples for essential oils of both species and for individual doses of pulsed electric field stimulation, as well as specific functional groups assigned to particular vibration spectra (with a detailed review of the literature).
Examination of peppermint and caraway essential oil samples after the application of pulsed electric field was performed with the use of FTIR infrared spectroscopy. It should be noted that all infrared spectra (ATR-FTIR) for essential oil samples selected for the study display very intense and distinct bands, which can be attributed to specific vibrations of appropriate functional groups, corresponding in turn to the components contained therein. Essential oils are mixtures of various compounds, for example, carvone (about 50%), limonene, pinene, cymol, and terpenic alcohols in the case of caraway oil. In the case of peppermint essential oils, the main components with a clear contribution to the FTIR spectra include: menthol and menthone (various isomers), piperitone, methyl acetate, and germacrene D.
In many studies, the authors have made appropriate adjustments to specific bands in the spectra of essential oils of plant origin of various species [33,34,35,36] regarding specific vibrations in molecules or their moieties. However, many bands are difficult to properly assign to a specific functional group, which may correspond to the content of a given amount of different substances (the reason is the fact that the substances present in particular oils often have similar chemical compositions). Table 4 presents in detail the frequencies of the characteristic spectra, along with the major extensions of the respective bands in the spectra of essential oil samples selected for the study, their assignment to the appropriate functional groups (with appropriate review and comparison with literature data [33]. The bottom indices also show the intensity of observed bands in typical spectra in the IR region. It should be noted that the assignment of spectra in the present case, corresponding to the tensile vibrations in the infrared spectra of such samples, is usually easier than the assignment of bands corresponding to deformation vibrations (this is often caused by the overlap of the bands responsible for the type of vibrations [33,34,35].
Vibrations of the methylene group, located in the spectral range from 1375 to 1150 cm−1 can be observed in the presented FTIR spectra of two of the species’ essential oil samples selected for testing [33,34]. These are, in this case, the stretching vibrations derived from the vibrations of the group –C–H associated with the group –CH3 and the deformation vibrations in this group (~1140–1180 cm−1 in our case). These bands seemed to be much richer in the case of oils derived from caraway than from peppermint, which was caused by a much greater number of these groups in the ingredients (listed above) in oils from caraway. It is also worth noting that tensile vibrations of the ester bond ν (C–O) consist of two connected asymmetric vibrations, namely vibrations of the C–C (=O)–O and O–C–C groups [35]. The first of these vibrations is usually more intense [36,37]. These bands occur in the region between 1300 and 1200 cm−1 (like C–C(=O)–O, in our case, about 1285 cm−1 (in the case of samples from caraway, practically invisible in peppermint oils), as a sub-band with a maximum at 1236–1238 cm−1), and about 1050–1090 cm−1 (in our case, 1050 to 1100 for these groups for both types of oils). Bands associated with saturated esters C–C(=O)–O are usually observed between 1240 and 1160 cm–1 (in the case of oil varieties selected for testing, ~1245 cm–1 for peppermint, and ~1242 cm–1 for caraway), while for unsaturated esters, the vibrations are more often formed at lower frequencies [33]. However, on the other hand, the O–C–O band originating from primary alcohols appears in the zone between 1090 and 1020 cm−1 (in the present case, the maximum of this band fell around 1059 for peppermint and 1042 cm−1 for caraway), while for secondary alcohols, this band usually appears with a maximum of approximately 1100 cm−1 (in our case, about 1109 for peppermint and 1091 cm−1 for caraway). Both types of vibration described above were present because of the small amount of methyl acetate in peppermint oils, in the case of caraway oils, they may belong to the vibrations of C–H groups. However, some authors often attribute the above-mentioned band (at about 1240 cm−1) only to the bending vibrations of the methylene group beyond the plane [36,37,38]. Subsequently, the next two bands presented in Table 4 (as well as in Figure 1) may cause slight difficulties in their proper recognition: one band with a maximum at about 1439 cm−1 and the second with a maximum at about 1367 cm−1 (for peppermint), and 1454 and 1369 cm−1 (for caraway). The first of these vibrations, with a maximum of about 1440–1460 cm−1 (depending on the sample, Figure 2), can be assigned to vibrations of methyl groups in aliphatic groups of oils selected for testing [33,38]. The second group of bands, with a maximum of about 1360–1370 cm−1 (for all samples), was observed simultaneously with bands with a maximum of about 960–990 cm−1 (for both types of oils), much more intensely for caraway than peppermint samples. It can be noted that the bands below 920 cm−1 (depending on what samples we are dealing with) that appear in all oil samples are related to tensile vibrations of cis-substituted olefinic groups [33], and may be related to vibrations of the vinyl group. These vibrations in peppermint oils come mainly from a high proportion of menthol and menthone, while in the case of caraway oils, mainly from carvone and limonene, as well as other substances containing the abovementioned groups.
The samples of oils selected for testing (in a given type) had quite similar spectra in the infrared area. However, depending on the type of the electric field factor used, they differed in terms of intensity. No band shift was observed (in a given type of oil), which also confirms that the applied electric field did not modify the composition of the tested oils to a negative degree. With an increase in the number of pulses of the electric field in both cases, a decrease in the intensity of all observed bands can be noted, which was much larger for samples of peppermint than caraway oil. The decrease in intensity also indicates a slightly negative effect of the applied electric field factor in the case of peppermint oils on essential oil secretion. In the case of spectra obtained for caraway, the intensity decreased and confirmed the previously observed fact of the small field effect on the amount of essential oils obtained from a given plant.
Other very characteristic areas of vibration included the bands with a maximum at about 1745–1640 cm−1, characteristic of the stretching vibrations of the carbonyl group C=O [33] in menthol esters for peppermint oils (as well as small amounts of piperitone and methyl acetate). In the case of caraway essential oils, the main source of vibrations of the carbonyl group came from the carvone constituting its main component (in this case, a much smaller contribution may come from other substances). In the case of these bands, significant differences were observed between peppermint and caraway samples. In the case of samples of peppermint essential oils, we observed a very intense band with a maximum at approximately 1674 cm−1, with sub-bands at 1710 and 1645 cm−1. In turn, in the case of caraway oils, we observed a very intense band at 1709 cm−1, as well as sub-bands at 1738 and 1676 cm−1. Bands with a maximum at approximately 1710–1715 cm−1, also corresponding to the carbonyl group vibrations, are characteristic for acid groups of oil samples selected for testing [33,39]. Differences in the intensity of these groups (significant ones) indicate different factions of substances such as menthone (in the case of peppermint) or carvone (in the case of caraway) in selected oil samples. The subsequent bands with a maximum at 1552–1561 cm−1 (depending on the type of oil) are the vibrations that originate from the stretching vibrations of the –C=C– group (from the cis- transformation) [33,40]. A very characteristic area also included vibrations with a maximum at 1439 cm–1 (in peppermint) or 1454 cm–1 (in caraway), derived from deformation vibrations of the groups –C–H in the groups CH2 and CH3 (bending vibrations). In addition, it is also worth mentioning the vibrations in the area from 900 to 650 cm–1, which in our case represented characteristic deformation vibrations belonging to –HC=CH– groups (cis- conformation, beyond the plane) and swinging vibrations of the above mentioned groups (δ(–(CH2)n- and –HC=CH– (cis-)) [33].
Next, moving to vibrations in the range of larger wave numbers, very significant stretching vibrations =C–H (trans- transformation) with a maximum at about 3400 cm−1 (Table 3) derived from vibrations of the menthol and menthone fractions (for peppermint) and carvone and limonene in the case of caraway oils should be mentioned [33,34,41,42]. The vibrations from this area also indicated a small amount of water present in both types of oils (or stretching vibrations of the –OH groups). The vibrations from this area can also came mainly from menthol (in peppermint). The vibrations with a maximum at about 2965 to 2850 cm−1 were derived successively from the C–H stretching vibrations in the –CH3, CH2 groups (symmetrical or asymmetrical) belonging to the aliphatic groups for the compounds mentioned above (such as menthol, menthone, or carvone and limonene for caraway) [33,42].
For the majority of the examined samples, a slight enhancement of the band at 1738 cm−1 (responsible for the vibrations of the C=O group, as already described above) (for peppermint about 1710 cm−1) on the side of the lower wave numbers was clearly visible (more clearly for caraway) [43], which can also be attributed to hydrogen bond formation between C=O…H–O–H groups (in the samples tested). The appearance of the band at 1710–1712 or 1738 cm–1 was accompanied by intense bands at about 1350–1370 and below 800 cm−1 [34,40,42], which can also be attributed to stretching vibrations in C–O and C–C groups (described above). These bands, along with the variable amount of water in a given sample and the possibility of hydrogen bond formation between C=O…H–O–H, may show a slight increase in their intensity.
Chemometric analysis including principal component analysis (PCA) was used in order to determine the similarity between the respective samples’ FTIR spectra. PCA is a method that extracts spectral information and uses the maximum variance principle. Furthermore, the data reduction methods which provide decompositions of an FTIR spectrum in terms of principal component loading use the new low-dimensional variables instead of the original high-dimensional variables to achieve data dimension reduction (Figure 3) [44,45].
Figure 4 shows the PCA score plots in PC1 × PC2 space, obtained from the correlation matrix of absorbancies in the region of 4000–600 cm−1.
The oils’ FTIR spectra were rather similar and only small variations could be seen between the different structures, illustrating that although the technique is very efficient at identifying band regions, the mean spectra give little insight into the basis of differentiation at a molecular structure level. PCA can provide further insight into the source of the spectral variability, meaning the differentiation of the different band regions.
PCA results showed that six factors were needed in order to explain almost all variability (98.76%, see Table 5). Based on the percentage of variance criterion, four factors qualified. In contrast, using the Scree plot (Figure 3) provided us with one more factor. In order to simplify interpretation of oil FTIR spectra, this analysis is aimed at understanding the information contained in the first two PCs.
Factor loadings for the PC1 and PC2 are shown in Figure 5. The loading with reference spectra (Figure 5a,b, spectra of K0 and M0) shows that PC1 and PC2 had negative values, which can be attributed to C–H stretching and –C–O stretching vibrations from around 1400–1100 nm, and 3600–3400 nm for a –C=O stretching weak overtone and =C–H or –OH stretching vibrations.
The PC1 respective negative and positive loadings contributed substantially to the differentiation of absorption around 1500–1400 nm and 1700–1600 nm, due to weak stretching deformation vibrations –C–H in CH2 and CH3 groups, bending (scissoring) or weak stretching vibrations (–C–H, cis-), bending (rocking), and very strong stretching vibrations –C=C–, cis-, –C=O attributed to esters or acids.
It is noteworthy that a correlation existed between the values of the loading and the positions of the spectra in the PCA plot (Figure 5a,b). Thus, there was a correspondence between the composition of the analyzed oil samples and the profile of the loading of PC1, which is a representation of the composition of each oil sample.
The score plots (Figure 4) reveal that among the studied samples, K0–K350 were separated on the positive side in PC1, but M0–M350 on the negative side of PC1. The groups were well defined and discriminated on the scores plot, indicating that the PCs are a clear representation of inter-group variance.

4. Conclusions

1. The PEF application does not lead to an increase in the yield of essential oils in dried peppermint herb and caraway schizocarps.
2. FTIR infrared spectroscopy is a method characterized by ease of use and non-invasiveness, and can be successfully used in the assessment of the quality of biological samples, even in small quantities.
3. The analysis of spectra (specific areas of wave numbers) in infrared FTIR gives a quick comparison between the quality of obtained oils by applying the diversified doses of the pulsed electric field. Moreover, changes in the appropriate bands can signal information about the composition (quality) of a specific variety of the obtained essential oils.
4. The changes in the obtained FTIR spectra were significantly emphasized in the chemometric analysis.
5. The greatest differences between the oils could be observed in the vibrations of bands associated with carbonyl groups, which indicates differences in terms of menthone (in the case of peppermint) and carvone fractions (in the case of caraway oils) contained in the oil. In addition, large changes were observed for groups characteristic of stretching vibrations of –CH2 and –CH3 groups, also present in the abovementioned substances.
6. The use of FTIR, combined with chemometrics to analyze the essential oil compounds, effectively classified and identified the samples. It could also suggest reasons for their varying chemical compositions. Moreover, the results obtained in this study can provide a comprehensive evaluation of sample oil quality and an optimization evaluation method for medicinal herb quality control.

Author Contributions

For research articles with several authors, a short paragraph specifying their individual contributions must be provided. The following statements should be used “conceptualization, U.S. and A.M.; methodology, A.N. and U.S., A.M., T.D.; software, A.N., A.M.; validation, U.S., T.D. and A.Ż.; formal analysis, U.S.; investigation, U.S., A.M.; resources, U.S.; data curation, U.S., A.N., A.M.; writing—original draft preparation, U.S., A.M., A.N.; writing—review and editing, U.S., A.M., A.N., T.D., A.Ż.; visualization, U.S.; supervision, U.S, A.M..; project administration, U.S.; funding acquisition, T.Ż.”.

Acknowledgments

This research was financed by Ministry of Science and Higher Education of the Republic of Poland. The research of Agnieszka Niemczynowicz was partially supported by a grant from the University of Warmia and Mazury in Olsztyn no. POWR.03.05.00-00-Z310/17. The authors Agnieszka Niemczynowicz and Arkadiusz Matwijczuk acknowledge the Cost project CA 15126.

Conflicts of Interest

The authors declare no conflict of interest.

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Processes 07 00466 g001 550
Figure 1. Working chamber of pulsed electric field (PEF) research stand.
Figure 1. Working chamber of pulsed electric field (PEF) research stand.
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Processes 07 00466 g002 550
Figure 2. ATR-FTIR spectra of essential oils samples obtained from peppermint (A) and caraway fruit (B) after an application of pulsed electric field at different doses.
Figure 2. ATR-FTIR spectra of essential oils samples obtained from peppermint (A) and caraway fruit (B) after an application of pulsed electric field at different doses.
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Processes 07 00466 g003 550
Figure 3. Scree plot for principal components (PCs).
Figure 3. Scree plot for principal components (PCs).
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Figure 4. The score plot of PC1 and PC2 for all samples.
Figure 4. The score plot of PC1 and PC2 for all samples.
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Processes 07 00466 g005a 550Processes 07 00466 g005b 550
Figure 5. Loading plots of PC1 and PC2 for references spectra K0 (a) and M0 (b).
Figure 5. Loading plots of PC1 and PC2 for references spectra K0 (a) and M0 (b).
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Table
Table 1. Chemical composition of peppermint oil.
Table 1. Chemical composition of peppermint oil.
Peak NameTime (min)Kovats Retention IndexThe Volatile Compounds %
Exp.Lit.
2-(E)-Hexenal4.304830827tr.
1-Hexanol4.346861854tr.
3-Heptanol5.2428898840.02
α-Pinene6.2799389320.29
Sabinene7.4509819690.21
β-Pinene7.5759849740.12
Myrcene7.9679989880.19
3-Octanol8.07910019890.40
Limonene9.33310321024tr.
Eucalyptol9.433103510263.13
Ocimene, (Z)-β9.625104010320.24
Ocimene, (E)-β10.029105010440.04
trans-Sabinene hydrate10.790106910650.03
Linalool12.104110010950.19
2-Methylbutyl-3-methyl butyrate12.450110911110.07
3-Octyl acetate13.121112411230.06
trans-Sabinol13.817114011370.07
cis-p-Mentha-2.8-dien-1-ol13.92511421140tr.
neo-Isopulegol14.142114711440.21
Menthone14.6291152114838.19
Menthofurane15.0081164115911.44
Menthol15.4791172116728.35
Isomenthol15.813118511790.50
neo-Isomenthol16.033119011830.19
Myrtenol16.367119811940.04
cis-Piperitol16.49212021195tr.
trans-Piperitol16.80812101207tr.
Isopipetitenol17.171122012280.10
Isovaleric acid
cis-3-hexenyl ester		18.100123712380.10
Pulegone18.250124012400.24
Piperitone18.925125412491.42
neo-Menthyl acetate19.879127612710.18
Perilla alcohol20.513128912940.08
Menthyl acetate20.775129713048.39
p-Menth-1-en-9-ol20.87512981294tr.
iso – Menthyl acetate21.379131013040.20
Dihydrocarveol acetateτ21.57113141310tr.
α-Copaene24.41313741378tr.
β-Bourbonene24.817138613870.32
β-Elemene25.121139213890.26
Caryophyllene(E)-β26.337142014171.52
β-Copaene26.73714321430tr.
Himachalene27.413144314490.04
unknown sesquiterpene27.4791447-0.12
Humulene27.783145614520.16
E-β-Farnesene27.862145814540.08
cis-Muurola-4(15).5-dieneτ28.18314661465tr.
Germacrene D28.979148214841.81
Bicyclogermacrene29.612150014990.10
γ-Cadinene29.975150815120.14
δ-Cadinene30.696152515220.08
Germacrene D-4-ol32.84615761574tr.
Sapathulenol32.92515801577tr.
Caryophyllene oxide33.167158515820.18
Viridiflorol33.517159415920.24
epi-Cubenol τ34.429162516270.04
neo-Intermedol τ35.971165316580.16
τ -Tentatively identified, tr.—amount less than 0.01%. Prepared based on Sadowska et al. [19].
Table
Table 2. Chemical composition of oil from Kończewicki caraway fruit.
Table 2. Chemical composition of oil from Kończewicki caraway fruit.
Peak NameTime (min)% Composition
Carvone45.1765.60
Limonene15.6930.27
Dihydro-carvone38,040.04
Dihydro-carveol45.500.04
Carveol49.910.16
α-thujone25.250.01
α-pinene9.160.05
β-pinene11.830.02
Prepared based on Seidler-Łożykowska et al. [20,21].
Table
Table 3. The average content of essential oils and temperature of the examined material after pulsed electric field application.
Table 3. The average content of essential oils and temperature of the examined material after pulsed electric field application.
SpeciesNumber of PulsesEssential Oils (mL∙100g−1) ± SDTemperature (°C) ± SD
Peppermint02.25 ± 0.017 a *25.00 ± 1.00 d
1502.03 ± 0.006 b34.06 ± 3.26 c
2502.02 ± 0.006 b44.80 ± 1.93 a
3501.95 ± 0.01 b40.40 ± 2.06 b
Carway01.67 ± 0.006 a25.00 ± 1.00 c
1501.73 ± 0.025 a38.00 ± 1.00 b
2501.73 ± 0.006 a41.00 ± 1.00 a
3501.77 ± 0.006 a42.33 ± 0.58 a
*—averages with the same letter in the same column for the tested species did not differ statistically significantly according to Fisher’s test at the significance level p = 0.05.
Table
Table 4. The location of the maxima of absorption bands FTIR [31,32] with arrangement of appropriate vibrations for selected for sampling: a—Caraway, b—Caraway 150 IMP, c—Caraway 250 IMP, d—Caraway 350 IMP, e—Peppermint, f—Peppermint 150 IMP, g—Peppermint 250 IMP, h—Peppermint 350 IMP made in terms of spectra 3700–730 cm−1.
Table 4. The location of the maxima of absorption bands FTIR [31,32] with arrangement of appropriate vibrations for selected for sampling: a—Caraway, b—Caraway 150 IMP, c—Caraway 250 IMP, d—Caraway 350 IMP, e—Peppermint, f—Peppermint 150 IMP, g—Peppermint 250 IMP, h—Peppermint 350 IMP made in terms of spectra 3700–730 cm−1.
FTIR Type and Origin of Vibrations
Position of Bands (cm−1)
abcdefgh
34343427343134233416342834203432–C=Ow (overtone)
and ν(=C–Hvw, trans-) or ν(–OH)
29732968297229652950295029502954νas(–C–Hm, –CH3, –CH2)
29242923292329202923292629262918νas(–C–Hvst, –CH2)
and νs(–C–Hvst, –CHa) (alifatyczne grupy w trigricerydach)
28862889288628862875281728712871
2845284828522844----
----1737173717351733ν(–C=Ovst) in ester
17051707170717071708170817061706ν(–C=Ovw) in acid
16741674167616761675167516751675νvw(–C=C–, cis-)
16491645163916451648164816501648νvw(–C=C–, cis-) and δ(–OH)
15571559155115511559155715621562/1510δvw(–C–H) in CH2 and CH3 group, bending (scissoring) or νvw(–C–H, cis-) bending (rocking)
1438144014401440145814541454/14211454/1415
13721368/13291368/13201368/13241372/13371368/12811372/13311372/1335νw, m, vw(–C–H, –CH3), banding
12461246124612461285/124412461281/12461281/1246νm(–C–O) or δm(–CH2-)
12051213120312171203120712051203
11411143114311431116111611781112νst(–C–O) or δst(–CH2–)
11081109110911081094109111491094νm(–C–O)
10551059105910571044/10231048/10281091/10441046/1026νm,vw(–C–O)
999999958954991/923986/9231030982δw(–HC=CH–, trans-) bending out of plane
958960891891873869986/925925
893893798802805-871867δvw(–HC=CH–, cis-) bending out of plane
798820---805801799δ(–(CH2)n– and –HC=CH– (cis-) bending (rocking)
761-767769770756770/758768/756
ν—stretching vibrations, δ—deformation vibrations, s—symmetric, as—asymmetric, st—strong, vst—very strong, w—weak.
Table
Table 5. Values of Eigenvalue and percentage of variance with cumulative corresponding to each principal component.
Table 5. Values of Eigenvalue and percentage of variance with cumulative corresponding to each principal component.
Principal Component NumberEigenvaluePercentage of Explained Variance (%)Cumulative Percentage of Explained Variance (%)
1708.8144.1444.14
2343.0521.3665.50
3280.2517.4582.95
4171.0510.6593.60
551.653.2296.81
631.251.9598.76
719.941.24100
85.29 × 10−283.29 × 10−29100

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Sadowska, U.; Matwijczuk, A.; Niemczynowicz, A.; Dróżdż, T.; Żabiński, A. Spectroscopic Examination and Chemometric Analysis of Essential Oils Obtained from Peppermint Herb (Mentha piperita L.) and Caraway Fruit (Carum carvi L.) Subjected to Pulsed Electric Fields. Processes 2019, 7, 466. https://doi.org/10.3390/pr7070466

AMA Style

Sadowska U, Matwijczuk A, Niemczynowicz A, Dróżdż T, Żabiński A. Spectroscopic Examination and Chemometric Analysis of Essential Oils Obtained from Peppermint Herb (Mentha piperita L.) and Caraway Fruit (Carum carvi L.) Subjected to Pulsed Electric Fields. Processes. 2019; 7(7):466. https://doi.org/10.3390/pr7070466

Chicago/Turabian Style

Sadowska, Urszula, Arkadiusz Matwijczuk, Agnieszka Niemczynowicz, Tomasz Dróżdż, and Andrzej Żabiński. 2019. "Spectroscopic Examination and Chemometric Analysis of Essential Oils Obtained from Peppermint Herb (Mentha piperita L.) and Caraway Fruit (Carum carvi L.) Subjected to Pulsed Electric Fields" Processes 7, no. 7: 466. https://doi.org/10.3390/pr7070466

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