Application of Lavender and Rosemary Essential Oils (EOs), Their Mixture and Eucalyptol (EOs Main Compound) on Cucumber Fruit Quality Attributes and Microbial Load

Abstract

Cucumber (Cucumis sativus L.), one of the most widely consumed vegetables, presents high perishability during storage and marketing if it is not handled and stored properly. Currently, there is an increased interest of the food industry to reduce waste (due to quality losses) and to utilize natural products for the preservation of fresh commodities. This study’s goal was to evaluate the effects of lavender (Lav) and rosemary (Ros) essential oils (EOs), their mixture (Lav + Ros, 1:1 v/v) and their main compound (eucalyptol) via vapor phase on cucumber’s postharvest quality. The outcomes of this study demonstrated that 200 μL/L of Lav and Ros EOs increased the respiration rate of cucumbers after 10 days of storage at 11 °C, while 100 μL/L of the EOs mixture and Eucalyptol (100 and 200 μL/L) had no effect on respiration, on the same day. The application of Eucalyptol (100 and 200 μL/L) resulted in less acceptable fruits (less pleasant aroma and unpleasant taste). A decrease in fruit firmness was found in cucumbers exposed to Lav 200 μL/L and Ros 100 μL/L. Interestingly, Eucalyptol was found to accelerate the fruit ripening index after five days of storage, and to decrease organoleptic properties of the fruit (i.e., aroma, taste) on the fifth day of storage. The fruit revealed increased oxidative stress (i.e., increased lipid peroxidation), especially at a high concentration (200 μL/L) of Eucalyptol after 10 days. This has resulted in the activation of other non-enzymatic antioxidant mechanisms such as the increase in fruit ascorbic acid content. Notably, no effects on fruit weight loss, total soluble solids and color were observed with the examined treatments. Overall, this study suggests that the investigated products (EOs and their main compound) have a putative role in postharvest storage for the preservation of cucumbers. However, further investigation is needed for the determination of the optimum application conditions (i.e., concentration, time and method of application) on cucumbers and other fresh produce.

1. Introduction

The cucumber (Cucumis sativus L.) belongs to the Cucurbitaceae family, and it is widely consumed around the world. Cucumber fruit is an excellent source of nutrients, i.e., antioxidants, ascorbic acid, magnesium and dietary fibers [1]. Daily consumption of fresh fruits and vegetables has been linked to many health benefits and lower rates of chronic illnesses since vegetables such as cucumbers contain components with anti-inflammatory and antioxidant properties [2,3]. Same as other fresh produce, cucumbers present short shelf life, mainly due to the visible signs of water loss (weight loss), pigment degradation (discoloration) and postharvest diseases [4,5].

The shelf life of fresh commodities (including cucumbers) can be impacted by a series of pre- and postharvest factors. During preharvest, growing conditions such as temperature, light, irrigation and harvesting handling can influence the fruit’s quality after harvesting, processing and storage [6]. Mishandling, storage temperature and relative humidity and inefficient cleaning techniques are postharvest factors that can affect the quality of fresh produce [7,8]. Washing fresh produce with chlorinated water is a crucial step in preserving fruits and vegetables because it minimizes the microbial load. However, the use of chlorine and chlorine-based agents has been linked with adverse health effects due to the formation of harmful compounds [9]. Thus, alternative and natural means and compounds have been investigated for the preservation of fresh produce (in liquid and vapor form).

Natural products such as essential oils (EOs) and other natural compounds have been exploited as alternative means for the postharvest preservation of fresh commodities due to their significant properties (i.e., antioxidant and antimicrobial (among others)) [10,11,12,13]. EOs are mixtures of secondary plant metabolites and a series of them have been characterized as “Generally Recognized as Safe” (GRAS) and can be used in the food industry [14]. Previous studies have shown the beneficial effects of lavender (Lavandula angustifolia Mill.) EO which include antioxidant, antimicrobial and anti-inflammatory properties [14,15,16,17,18]. Similarly, rosemary (Rosmarinus officinalis L.) EO possesses antioxidant, antimicrobial, anti-inflammatory and insecticidal activities [19,20,21]. The Lamiaceae family (Labiatae) is one of the most notable families of medicinal plants, having a wide range of species (over 7000 species in 236 genera) and a global distribution. As they can be purchased as fresh food, dry products, essential oils, and as a wide series of other byproducts, species from this family are economically important on a global scale [22].

A series of earlier studies have examined the effects of the application of EOs on fresh commodities and the reported results are encouraging [11,12,13,23,24]. Lavender and rosemary EOs have been previously applied on fresh produce (i.e., lemons, endives, apples, pears) with various application methods [18,25,26]. For instance, vapor application of mint, basil and lavender EO on lemon fruits showed significant antifungal effects against Penicillium digitatum while at the same time maintained fruit quality attributes (i.e., weight, firmness, pH) [18]. Application of rosemary EO (range: 200–1000 μL/L) on fresh tomatoes resulted in decreased fruit weight loss, while it enhanced fruit flavor [13]. Furthermore, combining different EOs obtained from diverse plants and applying them on fresh produce could preserve sensory attributes (i.e., aroma and flavor) while lowering the possibility of fruit degradation [12].

The postharvest application of EOs alone and/or in combination with other compounds can directly affect the organoleptic characteristics of the produce, resulting in a desirable/acceptable or undesirable product [24]. During the application of EOs, considerations about the concentration, type and time of application, and EOs composition should be made to avoid negative effects on the quality features of the produce [27]. Fruit quality and safety maintenance or improvement is mandatory when vaporized products as the EOs are applied on fresh produce; on top of that, the organoleptic test in such postharvest applications is fundamental. This study aimed to determine the effectiveness of (i) lavender or rosemary EOs vapor application, (ii) their 1:1 (v/v) mixture for evaluating potential synergistic effects and (iii) the application of synthetic eucalyptol (as their common major compound) on the preservation of cucumber’s postharvest quality and spoilage during storage at 11 °C for 10 days. The innovative approach of this study is the combination of two EOs with the same main component, the direct comparison of the role of chemical synthetic eucalyptol to equal levels (as the levels present in the EOs mixtures), with possible synergistic or not effects derived by the main component of an EO. Moreover, the importance of using EOs as a preservation means for fresh produce, rather than the main component (eucalyptol), is important because biocidal properties of an EO are mirrored to the mixture of the components present in the oil profile and not only by one of them.

2. Materials and Methods

2.1. Plant Material and Essential Oil Extraction

Cucumber fruits (Cucumis sativus cv. PS-64; winter parthenocarpic cucumber with cold tolerance, at size of 18–19 cm long) were collected from a local greenhouse farm (coordinates: 34°46′10.61″ N; 33°11′22.08″ E, 217 m; Limassol, Cyprus). The crop was cultivated using common cultivation practices in a clay sandy loam soil, and was drip-irrigated and fertigated according to the crop’s needs. Cucumber plants were trained on a string based on vertical single pruning system (the main stem grew vertically, and lateral shoots were removed) and were grown for approximately 3.5 months. The crop cultivation took place during autumn/winter months in an unheated plastic greenhouse and the temperature ranged between 17 °C and 28 °C. Cucumbers (190 fruits) were randomly selected according to size (uniform), appearance and the absence of any physical defects. After their transfer to the laboratory, fruits were washed with chlorinated water (0.05% NaOCl), rinsed with distilled water and were uniformly divided into each treatment.

Lavender (Lavandula angustifolia Mill.) and rosemary (Rosmarinus officinalis L.) plants were collected from the experimental farm of Cyprus University of Technology (coordinates: 34°42′0.50″ N; 32°59′3.44″ E, 98 m; Limassol, Cyprus) in early October with temperature averaged at 27.6 °C and relevant humidity of 48.4%. Plant materials (chosen from 10 plants from each species; ~20 kg of fresh biomass) were transferred to the laboratory and were air dried at 42 °C in an air-ventilated oven. The EOs were obtained with hydrodistilation (Clevenger apparatus for 3 h) and their composition was determined with Gas Chromatography/Mass Spectrometry (GCMS; GC/MSQP-2010 Plus, Shimadzu, Tokyo, Japan) as previously described [28]. The major components found at lavender leaves EO were eucalyptol (59.40%), borneol (8.58%), camphor (7.98%) and β pinene (3.54%), whereas the major components found at rosemary EO were eucalyptol (31.09%), camphor (20.61%), α pinene (12.21%) and camphene (8.49%). The mixture of the two EOs was prepared in the ratio of 1:1 (v/v) from each EO. The major common compound of the two EOs, i.e., eucalyptol (Sigma–Aldrich, Darmstadt, Germany) was used at 45% as the mean value of the eucalyptol of the two EOs mixtures (1:1 v/v).

2.2. Procedure

Fruits were placed in a 5 L polypropylene (PP) plastic container (five fruits/container, two containers per treatment for each of the sampling time point (Day 5 and 10)). An appropriate volume of EOs or eucalyptol (absorbed by a paper strip for slow release of the volatiles) was placed inside the container to reach 100 and 200 μL/L. The EOs levels were selected based on preliminary tests and previous studies [26,29] and targeted to maintain the quality-related attributes during storage conditions. Distilled water was used for the control treatment (0% EO). The containers were sealed with their corresponding lid and were left at room temperature for 1 h to assist the evaporation of the volatile components of the oils. The containers were then stored in an experimental refrigerator at 11 °C and relative humidity of 90% (RH) (achieved by displacing a moist paper inside each container) for the examined storage period of 5 and 10 days [29,30,31]. To avoid any unwanted increase in CO₂ and decrease in O₂ throughout the fruit respiration process, the containers were opened to received aeration every other day. The EOs were applied in a vapor phase as described previously [32].

2.3. Impact on Fruits’ Quality Attributes

A series of quality attributes were evaluated in this study. Fruit weight loss was determined by recording the weight of each single fruit on the tested days (days 0, 3, 5, 7 and 10) and the percentage of total weight loss was computed for each day. The effect of the applications on the respiration rate of the fruit was determined as it has been previously described by Xylia et al. [33] and the results were given in mL of CO₂ produced per kg per h (mL CO₂/kg/h).

In order to evaluate the fruits’ aroma, taste, appearance and marketability on the test days (days 0, 5, and 10), at least eight panelists were used [33]. The evaluation of the aroma and taste were conducted using a 10-point scale (1 interval) as follow: 1: not cucumber-like and quite unpleasant aroma/taste, 3: not cucumber-like and lightly unpleasant aroma/taste, 5: not cucumber-like but pleasant aroma/taste, 8: less cucumber-like aroma/taste and 10: intense cucumber-like aroma/taste. Appearance (visual quality and color) was evaluated using a 10-point scale (1 interval) as follows: 1: yellow color of 50%; 3: yellow–green; 5: light green; 8: green; 10: deep green. To assess the marketability of the fruits (indicating overall quality) a scale of 1–10 (1 interval) was employed, where 1: not marketable quality (i.e., malformation, wounds, infection); 3: low marketability with malformation; 5: marketable with few defects, i.e., small size, decolorization (medium quality); 8: marketable (good quality); 10: marketable with no defects (extra quality).

The color on the surface of the fruits was determined with a colorimeter (Chroma meter CR400 Konica Minolta, Tokyo, Japan) by measuring the L*, a* and b* values (CIELAB uniform color space). Hue (h) value was determined in degrees (°): h = 180 + tan⁻¹(b*/a*) [34,35]. The chroma value (C) was computed as C = (a*² + b*²)^1/2, whereas color index (CI) was computed as CI = (a* × 1000)/(L* × b*) [35]. The browning index (BI) was computed as BI = 100 × (X − 0.31)/0.17 where: X = (a* + 1.75 × L*)/(5.645 × L* + a* − 3.012 × b*), while the yellowing index (YI) was calculated as YI = (142.86 × b*)/L* [36].

Fruit firmness was determined at two distinct sites on each fruit with a texture analyzer (TA.XT plus, Stable Micro Systems, Surrey, UK) equipped with a 3 mm diameter probe (travelling speed: 2 mm/s and penetration depth: 12 mm) [33]. The force needed to crack the pericarp of the fruit was measured in Newtons (N). Drops of the extracted fruit juice were used to assess the total soluble solids content (TSS) with the use of a digital pocket refractometer (Atago, Tokyo, Japan). Results of TSS were expressed as °Brix. Fruit juice was also used to calculate titratable acidity (TA) via titration with 0.1 N NaOH using the aforementioned procedure [37], and the results were expressed as g of malic acid per L of juice. The sweetness of fruits (i.e., ripening index) was determined as the ratio of TSS over TA (TSS/TA). Cucumbers’ ascorbic acid (AA) content was assayed using the 2,6-dichlorophenolindophenol titration method [37] and results were expressed as mg of AA per 100 g of fresh weight (mg/100 g).

2.4. Impact on Fruits’ Polyphenols and Antioxidant Activity

The method for extraction of total polyphenols and antioxidants was assayed as described by Chrysargyris et al. [38]. The determination of total polyphenol content was created using the Folin–Ciocalteu method according to Chrysargyris et al. [28] and results were expressed as μg of gallic acid equivalents per g of fresh weight (μg GAE/g). The antioxidant activity of the methanolic fruit extracts was estimated using three different assays as follows: (i) the 2,2-diphenyl-1-picrylhydrazyl (DPPH), (ii) the ferric reducing antioxidant power (FRAP) and (iii) the 2,2′-azinobis-(ethylbenzothiazoline-6-sulfonic acid) (ABTS) method. The DPPH and FRAP were assayed based on Chrysargyris et al. [28]. The ABTS assay was performed according to Wojdylo et al. [39]. For all three tested methods, results were expressed as μg of trolox per g of fresh weight (μg trolox/g).

2.5. Determination of Fruit Damage Index and Enzymatic Antioxidant Activity

The oxidative degradation of membrane lipids (lipid peroxidation) in cucumbers was assessed in terms of malondialdehyde content using the thiobarbituric acid reaction [40], while hydrogen peroxide content was measured according to the method described by Loreto and Velikova [41]. Both indicators reflected the fruit damage indices in this study. Results were expressed in nmol MDA and μmol H₂O₂ per g of fresh weight, respectively.

The antioxidant enzymes activities of superoxide dismutase (SOD) (EC 1.15.1.1) and catalase (CAT) (EC 1.11.1.6) were determined, as described previously [42], and the absorbance was determined at 560 nm for SOD and at 240 nm for CAT. Peroxidase activity (POD) (EC 1.11.1.6) was determined following the increase in absorbance at 430 nm [42]. Results were expressed as enzyme units per mg of protein. The protein content was determined using the Bradford method and bovine serum albumin was used as reference.

2.6. Impact on Microbial Load

Cucumbers’ microbial load (i.e., total viable count-TVC, yeast and mold) was determined using Plate count agar (PCA, Merck, Darmstadt, Germany) and Dichloran-rose bengal chloramphenicol Agar (DRBC agar, Merck, Darmstadt, Germany), respectively, as previously described by Xylia et al. [43]. After analysis, results were expressed as log CFU per g of fresh weight (log CFU/g).

2.7. Statistical Analysis

The data were subjected to one-way analysis of variance (one-way ANOVA) and a comparison of the treatment on each day was completed using Tukey’s multiple range test (p = 0.05) with IBM SPSS version 25.0. An independent samples t-test was also used for comparing control data on the initial (Day 0) and final (Day 10) days.

3. Results and Discussion

Currently, fresh produce (fruits, vegetables, herbs) are highly consumed due to their highly valued and well-appreciated nutritional properties. Consumers are requesting fresh produce of high quality and safety to be available throughout the year, with the use of less or no synthetic chemicals during plant growth and storage and, of course, at a reasonable cost. It is challenging to effectively satisfy these numerous requirements; however, significant efforts are made in the direction to cover the utmost of them. Preserving the quality of fresh produce is of great importance since it influences the purchasing choices of the consumers. Postharvest treatment of fresh produce with natural products such as EOs, plant extracts and other natural components can maintain the quality-related attributes of fruits and vegetables [44,45]. Previous studies have shown the positive effects of EOs on fresh commodities by enhancing their sensory and organoleptic characteristics and igniting physicochemical processes (i.e., increase in antioxidants) that result in higher nutritional value products [24,38].

3.1. Impacts on Fruits’ Quality Attributes

One of the main causes leading to weight loss of fresh commodities during storage is moisture loss (water loss) due to the processes of respiration and transpiration of the produce. Based on their respiratory profiles, and on how they produce and react to ethylene, fruits are classified into the following two broadly defined categories in relation to their ripening process: climacteric and non-climacteric. The cucumber is classified to the latter one [46]. Non-climacteric fruits, during ripening and senescence, continue their respiration and emit ethylene at basal levels. The impacts of EOs application, their mixture, and their main compound (eucalyptol) on moisture loss (weight loss) and rates of respiration of cucumber fruit are presented in Figure 1. This study shows that the applied treatments did not affect fruit weight throughout storage (Figure 1). Based on previous reports, the maximum weight loss was 7% during storage for cucumbers to maintain a marketable appearance [6], whereas in this study, the maximum weight loss was 2.56% on average. One hypothesis suggests that the hydrophobic nature of EOs decrease in water vapor permeability of the produce surface occurs to prevent water transfer and weight losses. A previous study has also proved that there was no significant weight loss in cucumber fruits during exposure to the vapors of a green-based product made with rosemary and eucalyptus EOs when it was applied at different concentrations [47]. Another study revealed that the vapor application of lavender EO at a concentration of 43 μL/L resulted in significant weight loss of lemon fruits compared to a concentration of 86 μL/L [18]. The same study showed that EOs from mint and basil applied in vapor form also resulted in a decrease in fruit weight [18]. This might be due to the different duration of the storage period for lemons versus cucumbers, as well as due to the different concentrations of the applied EOs; higher EO concentrations could evidently retard fruits’ respiration rates, and accelerate fruits’ senesce [26]. The fruits’ respiration rate was increased with Lav 200 μL/L on the fifth day, while at the end of the storage period (day 10), at all treatments (especially Lav 200 μL/L, Ros 200 μL/L and Lav + Ros 200 μL/L), cucumbers revealed an increased respiration rate. In previous studies, application of rosemary EO mixed with eucalyptus EO (300 μL/L) increased the respiration rates in apple and pear fruits [26], whereas lavender and thyme EOs were found to decrease apples’ respiration and ethylene production [48]. As it appears, different EOs have distinct effects on the respiration process of various commodities which could be related with alterations on the cell wall integrity and gas exchange [49] as well as with the different respiration rates that the non-climacteric fruits have, such as the cucumber, and the climacteric, such as apple and pear. In another study on tomatoes, chlorine application and high levels of EOs vapors (mixture of rosemary and eucalyptus EOs) significantly boosted respiration rates (although these changes were not observed at the dipping application) after 7 days and 14 days. This indicates that the effect of dipping was shorter than that of the vapor application [33]. Therefore, dipping application of EOs is an alternative way of using EOs as a preservation means in postharvest management of fresh produce, and relevant studies should be implemented on cucumber fruits as well.

Quality and sensory characteristics such as dark green color and increased firmness are essential for consumers when purchasing cucumbers. Previous studies showed that EO application during storage of fresh commodities can affect organoleptic characteristics of produce [12,18,24,25,26,50]. The effects of exposure to volatiles on the cucumber’s sensory attributes (aroma, appearance, taste) and marketability are presented in Figure 2 and Figure S1. Exposure to Eucalyptol 100 and 200 μL/L revealed 5.25 (not cucumber and pleasant aroma) and 3.44 (not cucumber and lightly unpleasant aroma) on the fifth day of storage, respectively. Moreover, at the end of the storage period, all treatments resulted in a not cucumber but pleasant aroma. No significant differences were observed regarding fruit appearance after 5 and 10 days of storage at 11 °C (Figure 2). All applied treatments revealed lower scores (especially Eucalyptol 100 and 200 μL/L—not cucumber and very unpleasant) than the non-treated (control) on the fifth day, while even lower scores were observed at the end of the storage period (day 10) by all the applied treatments. In a study on broccoli, it has been showed that the application of rosemary EO (1.82 mg/mL) resulted in less acceptable broccoli florets (less pleasant odor and flavor) as did the application of oregano EO (0.48 mg/mL) and their combination (0.91 and 0.24 mg/mL, respectively) [12]. In our study, after all the applied treatments, especially with Eucalyptol 100 and 200 μL/L (not marketable) on the fifth day, the panelists evaluated the produce as less marketable, while on the last day (day 10) the applied treatments resulted in a low marketable product (scores ranging between 3.50 and 5.25) (Figure 2 and Figure S1). Time of application and concentration of the applied EOs were found to significantly affect the quality-related features of a fresh produce during storage, such as aroma, color and taste, while some studies have reported negative effects on organoleptic characteristics as the concentration of the applied EOs increases [25,51]. Moreover, the impacts on fruit quality characteristics such as aroma and taste can be attributed to the possible interactions of the EOs and the volatile compounds with the food matrix components, which in some cases result in undesirable and unpleasant end products [52].

Consumers are attracted by the dark green color of cucumbers during purchase. However, during long storage and under adverse conditions (high temperature and low relative humidity) the cucumber’s green color turns to yellow due to the degradation of chlorophyll [8].

Table 1. Impacts of lavender (Lav) and rosemary (Ros) EOs, their mixture (Lav + Ros, 1:1 v/v) and eucalyptol vapors on cucumber’s color parameters: L*, a*, b*, hue (h), chroma value (C) and color index (CI) during storage at 11 °C for 10 days.

	Treatment	L*	a*	b*	h (°)	C	CI
Day 0	Control	39.04 ± 0.87	−11.99 ± 0.71	16.30 ± 1.22	126.51 ± 0.53	20.24 ± 1.40	−19.06 ± 0.79
Day 5	Control	36.29 ± 0.48	−11.61 ± 0.16 ab	15.73 ± 0.27	126.44 ± 0.29	19.56 ± 0.29	−20.37 ± 0.38
	Lav 100 μL/L	36.40 ± 1.09	−11.28 ± 0.75 ab	15.15 ± 1.35	126.92 ± 0.66	18.90 ± 1.53	−20.82 ± 1.09
	Lav 200 μL/L	38.12 ± 1.23	−13.41 ± 0.46 b	17.32 ± 1.29	128.13 ± 1.48	21.93 ± 1.26	−21.00 ± 1.88
	Ros 100 μL/L	38.92 ± 1.17	−12.79 ± 0.60 ab	17.79 ± 1.14	125.84 ± 0.52	21.91 ± 1.28	−18.69 ± 0.81
	Ros 200 μL/L	37.31 ± 1.51	−12.48 ± 0.82 ab	17.40 ± 1.59	125.97 ± 0.80	21.42 ± 1.77	−19.75 ± 1.38
	Lav + Ros 100 μL/L	36.05 ± 1.04	−11.40 ± 0.45 ab	14.90 ± 0.76	127.49 ± 0.46	18.76 ± 0.88	−21.43 ± 0.96
	Lav + Ros 200 μL/L	37.41 ± 1.39	−12.85 ± 0.68 ab	17.61 ± 1.28	126.35 ± 0.75	21.80 ± 1.43	−19.89 ± 1.17
	Eucalyptol 100 μL/L	35.68 ± 1.04	−11.59 ± 0.57 ab	15.34 ± 1.00	127.22 ± 0.68	19.23 ± 1.12	−21.41 ± 0.90
	Eucalyptol 200 μL/L	35.24 ± 1.15	−10.28 ± 0.60 a	13.09 ± 0.92	128.24 ± 0.38	16.65 ± 1.09	−22.54 ± 1.07
Day 10	Control	36.66 ± 1.14	−11.97 ± 0.98	16.34 ± 1.74	126.55 ± 0.69	20.26 ± 1.98	−20.41 ± 1.07
	Lav 100 μL/L	39.25 ± 0.94	−13.03 ± 0.55	18.28 ± 1.02	125.59 ± 0.43	22.45 ± 1.15	−18.33 ± 0.73
	Lav 200 μL/L	37.09 ± 1.51	−13.21 ± 0.55	18.10 ± 1.22	126.30 ± 0.66	22.41 ± 1.31	−20.04 ± 1.07
	Ros 100 μL/L	39.55 ± 0.69	−13.09 ± 0.47	18.90 ± 0.81	124.75 ± 0.29	22.99 ± 0.93	−17.58 ± 0.40
	Ros 200 μL/L	38.31 ± 1.13	−12.39 ± 0.77	16.96 ± 1.46	126.43 ± 0.78	21.01 ± 1.62	−19.44 ± 1.03
	Lav + Ros 100 μL/L	39.95 ± 1.23	−12.94 ± 0.80	18.41 ± 1.55	125.32 ± 0.63	22.51 ± 1.73	−17.89 ± 0.94
	Lav + Ros 200 μL/L	38.54 ± 0.48	−11.48 ± 0.05	16.03 ± 0.29	125.63 ± 0.43	19.72 ± 0.25	−18.63 ± 0.48
	Eucalyptol 100 μL/L	36.48 ± 2.15	−11.53 ± 1.39	15.76 ± 2.40	126.90 ± 1.00	19.54 ± 2.76	−21.20 ± 1.98
	Eucalyptol 200 μL/L	38.87 ± 1.00	−13.76 ± 0.69	19.64 ± 1.40	125.23 ± 0.73	27.15 ± 2.72	−19.09 ± 0.36

The presented values are the means (±standard errors) of six biological replicates (per treatment). Values for day 0 refer to the control (non-treated). Different small Latin letters indicate statistically significant differences among treatments on each day (for each column).

illustrates the effects on fruit color of the exposure to volatiles of cucumber fruits after storage for 10 days. A higher a* value was found with Eucalyptol 200 μL/L in comparison to Lav 200 μL/L during the fifth day, while no significant differences were found at the end of storage. Interestingly, other color parameters that were investigated in this study (i.e., L*, b*, hue, chroma value and color index) were not affected by the applied treatments throughout the storage period, indicating that EO-treated produce were able to maintain good color attributes during the 10 days of storage. Similar observations were found in lettuce treated with thyme EO [53]. Moreover, it appears that Ros EO preserved the dark color of cucumber fruits more efficiently than the other treatments. These findings could be explained by the antioxidant activities of rosemary EO and its composition that protected chlorophylls from degradation [54]. The small changes in a* and b* values are good indicators of the absence of oxidative browning of cucumbers [55].

When purchasing fresh produce, consumers frequently select firmer fruits since they preserve better in the refrigerator and/or have longer shelf life. During storage and fruit ripening, fruits’ firmness decreases due to the weakening of the structure of their cell wall and the activation of enzymes such as wall hydrolases and pectinases, resulting in soft fruits that are not accepted by the consumers [56]. On the fifth day of storage, fruits exposed to Ros 200 μL/L revealed higher firmness than the relevant fruits exposed to Lav 200 μL/L and control fruits (

Table 2. Impacts of lavender (Lav) and rosemary (Ros) EOs, their mixture (Lav + Ros, 1:1 v/v) and eucalyptol vapors on cucumber’s firmness, total soluble solids (TSS), titratable acidity (TA), ripening index and ascorbic acid (AA) content during storage at 11 °C for 10 days.

	Concentration	Firmness (N)	TSS (°Brix)	TA (g Malic Acid/L)	Ripening Index (TSS/TA)	AA (mg AA/100 g)
Day 0	Control	15.24 ± 0.86	2.90 ± 0.15	1.06 ± 0.19	29.23 ± 5.27	2.61 ± 0.23 B
Day 5	Control	14.66 ± 0.47 b	3.10 ± 0.06	1.38 ± 0.24 a	24.10 ± 4.51 c	2.32 ± 0.06 e
	Lav 100 μL/L	14.90 ± 1.00 ab	3.20 ± 0.00	0.78 ± 0.01 b	41.09 ± 0.49 c	2.63 ± 0.16 de
	Lav 200 μL/L	14.31 ± 0.50 b	3.00 ± 0.17	0.53 ± 0.02 bc	56.55 ± 4.77 bc	2.23 ± 0.04 e
	Ros 100 μL/L	15.34 ± 0.32 ab	3.10 ± 0.15	0.60 ± 0.03 bc	51.71 ± 4.36 c	2.27 ± 0.13 e
	Ros 200 μL/L	18.45 ± 1.30 a	3.03 ± 0.07	0.53 ± 0.03 bc	57.84 ± 2.18 bc	3.10 ± 0.06 cd
	Lav + Ros 100 μL/L	15.38 ± 0.53 ab	3.30 ± 0.15	0.31 ± 0.10 c	81.99 ± 5.31 abc	3.65 ± 0.21 bc
	Lav + Ros 200 μL/L	16.14 ± 0.54 ab	2.87 ± 0.13	0.45 ± 0.01 cb	63.65 ± 4.68 bc	4.16 ± 0.16 ab
	Eucalyptol 100 μL/L	16.19 ± 1.19 ab	2.93 ± 0.07	0.29 ± 0.06 c	113.66 ± 32.47 ab	3.74 ± 0.14 abc
	Eucalyptol 200 μL/L	15.25 ± 0.76 ab	2.90 ± 0.20	0.21 ± 0.02 c	139.88 ± 12.52 a	4.40 ± 0.19 a
Day 10	Control	17.27 ± 0.74 a	3.07 ± 0.23	0.72 ± 0.02 a	42.99 ± 4.37	5.58 ± 0.18 abA
	Lav 100 μL/L	16.68 ± 0.66 ab	2.67 ± 0.03	0.31 ± 0.11 b	119.47 ± 51.66	4.88 ± 0.09 bcd
	Lav 200 μL/L	13.91 ± 0.81 b	2.70 ± 0.15	0.21 ± 0.07 b	149.57 ± 36.81	6.37 ± 0.10 a
	Ros 100 μL/L	14.29 ± 0.87 b	2.60 ± 0.10	0.23 ± 0.01 b	112.28 ± 9.43	5.96 ± 0.08 a
	Ros 200 μL/L	15.37 ± 0.66 ab	2.87 ± 0.12	0.33 ± 0.11 b	122.71 ± 58.93	4.45 ± 0.38 cd
	Lav + Ros 100 μL/L	14.74 ± 0.59 ab	2.73 ± 0.15	0.19 ± 0.06 b	112.75 ± 12.31	6.07 ± 0.32 a
	Lav + Ros 200 μL/L	15.88 ± 0.23 ab	2.63 ± 0.09	0.37 ± 0.03 b	73.48 ± 6.17	4.04 ± 0.15 d
	Eucalyptol 100 μL/L	15.90 ± 1.18 ab	3.03 ± 0.12	0.19 ± 0.05 b	178.93 ± 39.01	5.89 ± 0.15 ab
	Eucalyptol 200 μL/L	16.30 ± 0.50 ab	3.00 ± 0.06	0.19 ± 0.04 b	170.81 ± 40.59	5.39 ± 0.20 abc

The presented values are the means (±standard errors) of six biological replicates (per treatment). Values for day 0 refer to the control (non-treated). Different small Latin letters indicate statistically significant differences among treatments on each day (for each column). Different Latin capital letters show significant difference between control on the initial day (day 0) and the last day of storage (day 10).

), and this reflected the fruit crispness and juiciness [57]. Interestingly, a decrease in fruit firmness was found with Lav 200 μL/L and Ros 100 μL/L in comparison to the control at the end of storage. In light of these results, the effects of the applied EOs on fruit firmness may be attributed to the secondary components of the EOs rather than the major one (i.e., eucalyptol). This can be justified if other chemical compounds of the examined EOs could be tested individually or in combination, in order to identify the corresponding effects on the preservation of fruit firmness.

A decrease in TA and an increase in TSS are generally observed during the ripening of fruits and it is expected from respiring fruits as the organic acids such as malic or citric acid are the primary substrates for the respiration process [56]. In this study, no significant changes in TSS content were found throughout storage. Given that cucumber fruit is harvested and eaten while it is still at the breaker stage (immature fruit), this fact could help to explain why we observed unchanged TSS values during the EO treatment in the short period between 5 and 10 days under storage conditions. On the other hand, all applied treatments resulted in decreased TA on both occasions (Table 2). Higher values of ripening indices were reported with Eucalyptol (100 and 200 μL/L) on the fifth day, while no changes were found at the end of storage. A previous study presented a decrease in TA in cucumber fruits exposed to the vapors of a green-based product of eucalyptus and rosemary EOs, while TSS was not affected [47]. Exposure of apples to rosemary and clove EOs resulted in decreased TA and slightly higher TSS compared to cinnamon and citronella grass EOs, even two days after their application [27]. This antithesis could be attributable to the different EOs and their applied concentrations, or to the different tested produce (i.e., climacteric and non-climacteric fruits). Since EOs could interfere with the metabolism of the fruit, they could possibly alter the ratio of TSS and TA and subsequently affect the fruit ripening process [47,56]. Interestingly, a previous study has also mentioned that eucalyptol disrupted metabolism in tomatoes [58]. The AA content of cucumbers was increased after the application of the tested treatments (except Lav 100 and 200 μL/L, and Ros 100 μL/L) on the fifth day. However, a decrease in AA content was found with Ros 200 μL/L and Lav + Ros 200 μL/L on the last day (day 10) compared to the non-treated fruits (control) (Table 2). Interestingly, AA levels were increased at the end of storage for the non-treated fruits compared to the initial day (day 0). Higher AA content was also observed in lemons after their exposure to lavender EO [18]. The increase in AA content might be connected to the antioxidant properties of the applied EOs and their main compound (eucalyptol), which protect the fruit’s components from oxidation while at the same time ignite the fruit’s antioxidant mechanisms, as a response to the applied abiotic stress [14,17,20]. This increase in AA is desirable since it improves the nutritional value of cucumber fruits, which is highly preferred by the consumers.

3.2. Impact on Fruits’ Polyphenols and Antioxidants

As shown in

Table 3. Impacts of lavender (Lav) and rosemary (Ros) EOs, their mixture (Lav + Ros, 1:1 v/v) and eucalyptol vapors on cucumber’s phenols content and antioxidants (examined by DPPH, FRAP and ABTS assays) during storage at 11 °C for 10 days.

	Concentration	Phenols (μg GAE/g)	DPPH (μg trolox/g)	FRAP (μg trolox/g)	ABTS (μg trolox/g)
Day 0	Control	52.48 ± 0.59 B	17.02 ± 1.40 A	36.83 ± 3.22	93.45 ± 0.73 A
Day 5	Control	63.71 ± 1.30 a	13.47 ± 0.99 b	35.21 ± 0.86 a	78.40 ± 1.02 ab
	Lav 100 μL/L	59.65 ± 0.67 ab	17.88 ± 0.53 a	33.60 ± 1.87 ab	82.93 ± 2.01 ab
	Lav 200 μL/L	50.57 ± 1.22 c	11.64 ± 0.65 b	26.34 ± 1.62 cd	83.52 ± 2.62 a
	Ros 100 μL/L	53.91 ± 2.67 bc	15.67 ± 1.32 ab	28.62 ± 1.40 bc	61.77 ± 1.37 cd
	Ros 200 μL/L	48.80 ± 2.10 c	14.67 ± 0.54 ab	27.61 ± 1.05 bcd	67.27 ± 0.82 c
	Lav + Ros 100 μL/L	53.76 ± 2.32 bc	14.41 ± 1.10 ab	27.66 ± 1.14 bcd	75.38 ± 1.46 b
	Lav + Ros 200 μL/L	46.08 ± 0.43 c	12.37 ± 0.75 b	22.44 ± 1.13 d	60.41 ± 1.44 cd
	Eucalyptol 100 μL/L	49.14 ± 0.96 c	13.18 ± 0.87 b	23.18 ± 0.77 cd	58.32 ± 1.14 d
	Eucalyptol 200 μL/L	53.24 ± 1.92 bc	13.55 ± 0.64 b	25.97 ± 0.71 cd	66.27 ± 1.61 c
Day 10	Control	62.73 ± 0.85 aA	8.45 ± 0.84 bcdB	36.64 ± 1.74 ab	81.37 ± 1.42 bB
	Lav 100 μL/L	51.62 ± 0.21 ab	11.40 ± 0.36 abc	32.55 ± 1.37 bc	89.92 ± 2.45 a
	Lav 200 μL/L	55.08 ± 1.15 ab	12.25 ± 0.62 a	39.07 ± 1.29 a	83.61 ± 1.13 ab
	Ros 100 μL/L	48.79 ± 1.56 c	11.61 ± 0.95 ab	31.53 ± 0.73 cd	69.29 ± 2.20 cd
	Ros 200 μL/L	64.20 ± 1.24 a	9.24 ± 0.53 abcd	36.33 ± 0.62 ab	59.86 ± 1.55 ef
	Lav + Ros 100 μL/L	66.85 ± 1.84 a	7.28 ± 0.93 d	32.76 ± 0.41 bc	56.39 ± 0.72 f
	Lav + Ros 200 μL/L	63.57 ± 1.44 a	8.12 ± 0.48 cd	29.61 ± 0.09 cd	65.23 ± 0.36 de
	Eucalyptol 100 μL/L	50.82 ± 1.60 ab	10.86 ± 0.59 abc	26.97 ± 0.47 d	72.58 ± 0.66 c
	Eucalyptol 200 μL/L	55.39 ± 1.30 b	8.57 ± 0.47 bcd	32.59 ± 0.61 bc	64.62 ± 1.40 de

The presented values are the means (±standard errors) of six biological replicates (per treatment). Values for day 0 refer to the control (non-treated). Different small Latin letters indicate statistically significant differences among treatments on each day (for each column). Different Latin capital letters show significant difference between control on the initial day (day 0) and the last day of storage (day 10).

, all treatments (apart from Lav 100 μL/L) resulted in lower phenolic content in cucumber fruits than the non-treated (control) fruits on day 5. After 10 days, the applications of Lav (100 and 200 μL/L), Eucalyptol (100 and 200 μL/L) and Ros 100 μL/L declined the fruits’ phenols content compared to the control. Other studies on vapor application of rosemary and eucalyptus EO-based products resulted in no significant changes in phenolic content in cucumber fruits [47]. The findings of this study differ from the aforementioned work due to the different EO concentrations and composition of the applied treatments, as well as the duration of the application. On the other hand, during this study, increased antioxidants were observed with Lav 100 μL/L and Lav 200 μL/L on day 5 and 10, respectively (DPPH assay). In contrast, a decrease in antioxidants was found with exposure to Lav 200 μL/L, Ros (100 and 200 μL/L) and Eucalyptol (100 and 200 μL/L) on the fifth day (FRAP assay) (Table 3). In addition, Ros 100 μL/L, Lav + Ros 200 μL/L and Eucalyptol 100 μL/L further decreased antioxidants on the last day (FRAP assay). After five days of exposure to Ros (100 and 200 μL/L), Lav + Ros (100 and 200 μL/L) and Eucalyptol (100 and 200 μL/L), a decrease in antioxidant levels (ABTS assay) was observed. Similarly, those treatments also caused a decrease in cucumbers’ antioxidant levels on the tenth day, whereas Lav 100 μL/L resulted in an increase in antioxidants (ABTS assay) (Table 3), indicating increased nutritive value produce. The decrease in cucumbers’ antioxidant levels might be attributed to the protective effect of the applied EOs that shield plant tissue from biotic and abiotic stress, suppressing defense mechanisms of the plant tissue due to their antioxidant activity [14,59]. The antioxidant capacity of the fruit is not unlimited and might be exhausted after a certain point. In this sense, the cucumber fruit exposed to EO treatment might reveal higher antioxidant status even before the 5 days (where the first sampling took place). Interestingly, AA levels were increased at the end of the storage period for the non-treated fruits, and were compared to the initial day (day 0). The differences in phenolic content and antioxidant capacity of cucumbers during storage in this study might be linked to the different antioxidant activity of the applied EOs. Previous findings showed that a rosemary EO presented greater antioxidant activity than a lavender EO [60]. This could explain the decrease in antioxidants and polyphenols in rosemary-treated cucumber fruits in our case, due to reduced oxidative stress.

3.3. Fruit Damage Index and Enzymatic Antioxidant Activity

Essential oils at high concentrations can trigger phytotoxicity and oxidative stress on fresh commodities, and this has been demonstrated by the EO tomato fruit vaporized with sage [24]. During storage, the metabolic rate of fruit increases, and abiotic and biotic stress factors usually occur, resulting in the production and accumulation of reactive oxygen species (ROS) such as hydrogen peroxide (H₂O₂). Fruit alleviates oxidative stress by the activation of both non-enzymatic (phenols, antioxidants, ascorbic acid, proline, etc.) and endogenous enzymatic (SOD, CAT, POD, ascorbate peroxidase-APX, glutathione peroxidase-GPX, etc.) metabolites. However, changes in non-enzymatic or enzymatic metabolites can take place independently or in parallel in order to detoxify fresh produce. The main endogenous (SOD) antioxidant enzymes convert the superoxide anion to H₂O₂, which is a substrate for CAT and GPX. Catalase metabolizes H₂O₂ to water and oxygen, and GPX decreases both H₂O₂ and organic hydroperoxides when reacting with glutathione (GSH) [61]. Thus, molecules such as H₂O₂ and malondialdehyde (MDA) are used for the investigation of plant tissue damage [62]. The impacts of the volatile treatments on cucumbers’ damage indices and the activities of the antioxidant enzymes are presented in

Table 4. Impacts of lavender (Lav) and rosemary (Ros) EOs, their mixture (Lav + Ros, 1:1 v/v) and eucalyptol vapors on cucumber’s hydrogen peroxide (H₂O₂), lipid peroxidation (MDA) and the antioxidant enzyme activity of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) during storage at 11 °C for 10 days.

	Concentration	H2O2 (mmol/g)	MDA (nmol/g)	SOD (units/mg of Protein)	CAT (units/mg of Protein)	POD (units/mg of Protein)
Day 0	Control	0.14 ± 0.00 B	9.63 ± 0.10 A	3.12 ± 0.35	4.27 ± 0.51	10.82 ± 1.72
Day 5	Control	0.15 ± 0.00 a	9.46 ± 0.09 abc	2.23 ± 0.16	3.26 ± 0.22 ab	15.42 ± 2.01 bc
	Lav 100 μL/L	0.14 ± 0.01 ab	8.96 ± 0.15 c	2.46 ± 0.43	3.05 ± 0.21 ab	26.88 ± 4.25 a
	Lav 200 μL/L	0.14 ± 0.00 ab	9.37 ± 0.14 bc	2.74 ± 0.04	5.69 ± 1.01 a	30.32 ± 1.76 a
	Ros 100 μL/L	0.16 ± 0.01 a	9.67 ± 0.33 abc	2.24 ± 0.16	2.81 ± 0.47 ab	14.73 ± 0.23 bc
	Ros 200 μL/L	0.14 ± 0.00 ab	8.54 ± 0.33 c	1.79 ± 0.10	4.29 ± 1.14 ab	29.73 ± 3.81 a
	Lav + Ros 100 μL/L	0.14 ± 0.01 ab	10.09 ± 0.32 abc	2.10 ± 0.09	1.69 ± 0.28 b	9.94 ± 0.24 c
	Lav + Ros 200 μL/L	0.15 ± 0.00 a	10.32 ± 0.29 abc	2.30 ± 0.15	2.50 ± 0.24 b	14.75 ± 1.14 bc
	Eucalyptol 100 μL/L	0.15 ± 0.00 a	11.60 ± 1.17 ab	1.79 ± 0.34	3.45 ± 0.43 ab	12.18 ± 1.44 c
	Eucalyptol 200 μL/L	0.12 ± 0.01 b	11.64 ± 0.07 a	1.51 ± 0.32	2.98 ± 0.62 ab	24.59 ± 3.50 ab
Day 10	Control	0.18 ± 0.01 A	6.98 ± 0.14 dB	3.01 ± 0.71 a	4.45 ± 0.17 de	16.54 ± 2.52 c
	Lav 100 μL/L	0.17 ± 0.01	7.70 ± 0.09 cd	1.90 ± 0.24 ab	9.58 ± 1.12 c	13.69 ± 1.88 c
	Lav 200 μL/L	0.17 ± 0.00	7.82 ± 0.14 cd	1.65 ± 0.18 ab	19.64 ± 0.59 a	37.01 ± 2.26 a
	Ros 100 μL/L	0.17 ± 0.01	8.93 ± 0.28 ab	1.41 ± 0.29 ab	4.99 ± 0.95 de	34.65 ± 7.24 ab
	Ros 200 μL/L	0.18 ± 0.00	8.08 ± 0.31 bc	1.60 ± 0.30 ab	7.70 ± 0.19 cd	26.95 ± 0.91 abc
	Lav + Ros 100 μL/L	0.18 ± 0.01	8.46 ± 0.27 abc	0.95 ± 0.05 b	3.49 ± 0.69 de	25.19 ± 3.81 abc
	Lav + Ros 200 μL/L	0.19 ± 0.01	8.52 ± 0.12 abc	2.83 ± 0.46 a	14.87 ± 1.79 b	30.18 ± 3.94 abc
	Eucalyptol 100 μL/L	0.20 ± 0.01	8.26 ± 0.13 abc	1.80 ± 0.08 ab	3.31 ± 0.20 e	19.57 ± 0.37 bc
	Eucalyptol 200 μL/L	0.17 ± 0.00	9.19 ± 0.20 a	1.52 ± 0.11 ab	5.93 ± 0.46 cde	34.81 ± 1.68 ab

The presented values are the means (±standard errors) of three biological replicates (per treatment). Values for day 0 refer to the control (non-treated). Different small Latin letters indicate statistically significant differences among treatments on each day (for each column). Different Latin capital letters show significant difference between control on the initial day (day 0) and the last day of storage (day 10).

. Exposure of cucumbers for five days to Eucalyptol 200 μL/L resulted in decreased H₂O₂ levels compared to the control, Ros 100 μL/L, Lav + Ros 200 μL/L and Eucalyptol 100 μL/L. On the other hand, no significant differences in H₂O₂ levels were evidenced on the last day (Table 4). The production of MDA decreased with Lav (100 and 200 μL/L), Ros 200 μL/L and Eucalyptol 200 μL/L on the fifth day, whereas Eucalyptol 200 μL/L resulted in increased MDA levels compared to the control, Lav (100 and 200 μL/L) and Ros 200 μL/L (Table 4). The lower MDA levels obtained from fruits treated with EO can be related to the enhanced activation of defense-related enzymes in response to the oxidative stress condition [63] but also to the non-enzymatic metabolites involvement, such as phenols and AA.

After 5 days of storage, SOD activity remained at similar levels for all tested fruits, indicating that the antioxidant capacity of the enzyme has been exhausted since there were changes in the activities of other antioxidant enzymes, as indicated by CAT and POD activities. Therefore, the increased CAT and POD levels were associated with decreased MDA levels, reflecting the antioxidant capacity of CAT and POD to protect fruit from oxidative damage. It has been reported that when the clove EO (0.4%) was applied to citrus fruits, it increased the activity of the enzymes involved in plant defense, including POD, phenylalanine ammonia lyase (PAL), polyphenol oxidase (PPO) and lipoxygenase (LOX) [63]. Similar to this, over a 3-day storage period at 20 °C, strawberries treated with vapors of tea tree EO exhibited increased SOD, PAL and POD activity [64].

Increased H₂O₂ levels were found in untreated cucumbers (control) on the last day, in comparison to the initial day (Day 0). Moreover, decreased MDA levels were found in untreated cucumbers (control) on the last day when compared to Day 0, whereas the activity of the examined antioxidant enzymes did not differ between Day 0 and Day 10, when taking the levels of SOD, CAT and POD into consideration. As rosemary EO has significant antioxidant activity, higher than other essential oils (compared to lavender EO), applying it alone and/or in combination appeared to reduce the oxidative stress of cucumbers while they were being stored [60]. The reduction in the oxidative stress indicators by the vapor of the applied EOs is mainly attributed to their composition (i.e., phenols, flavonoids, etc.), which exhibit free radical scavenging activities while at the same time increasing the antioxidant capacity of the produce they are applied on [59].

3.4. Impact on Microbial Load

Increased microbial load could result in higher decay incidence and increased fresh produce losses [7]. Microbial control, both at preharvest and postharvest level, is of great importance for the preservation of fresh produce with high quality and safety features. As shown in

Table 5. Impacts of lavender (Lav) and rosemary (Ros) EOs, their mixture (Lav + Ros, 1:1 v/v) and eucalyptol vapors on cucumber’s microbial load (total viable count-TVC, yeast and mold) during storage at 11 °C for 10 days.

	Concentration	TVC (log cfu/g)	Yeast and Mold (log cfu/g)
Day 0	Control	5.81 ± 0.10	4.12 ± 0.17
Day 5	Control	4.67 ± 0.03	4.59 ± 0.07 ab
	Lav 100 μL/L	4.71 ± 0.02	4.67 ± 0.18 ab
	Lav 200 μL/L	4.45 ± 0.38	4.47 ± 0.01 ab
	Ros 100 μL/L	4.70 ± 0.07	4.43 ± 0.22 ab
	Ros 200 μL/L	4.43 ± 0.10	4.65 ± 0.08 ab
	Lav + Ros 100 μL/L	4.86 ± 0.05	4.85 ± 0.03 a
	Lav + Ros 200 μL/L	4.58 ± 0.05	4.43 ± 0.04 ab
	Eucalyptol 100 μL/L	4.28 ± 0.12	4.32 ± 0.04 ab
	Eucalyptol 200 μL/L	4.21 ± 0.05	4.08 ± 0.13 b
Day 10	Control	5.38 ± 0.15 d	4.73 ± 0.03 ab
	Lav 100 μL/L	5.69 ± 0.08 cd	3.73 ± 0.00 cd
	Lav 200 μL/L	5.88 ± 0.02 abc	3.30 ± 0.35 d
	Ros 100 μL/L	6.00 ± 0.00 abc	4.88 ± 0.01 ab
	Ros 200 μL/L	4.91 ± 0.05 e	3.11 ± 0.16 d
	Lav + Ros 100 μL/L	5.80 ± 0.05 bcd	4.41 ± 0.09 bc
	Lav + Ros 200 μL/L	6.20 ± 0.03 ab	5.18 ± 0.01 a
	Eucalyptol 100 μL/L	6.19 ± 0.08 ab	4.89 ± 0.07 ab
	Eucalyptol 200 μL/L	6.29 ± 0.15 a	4.56 ± 0.01 ab

The presented values are the means (±standard errors) of three biological replicates (per treatment). Values for day 0 refer to the control (non-treated). Different small Latin letters indicate statistically significant differences among treatments on each day (for each column).

, a decrease in TVC numbers was found with Ros 200 μL/L at the end of the storage period of 10 days. On the other hand, exposure to Lav 200 μL/L, Ros 100 μL/L, Lav + Ros 200 μL/L and Eucalyptol (100 and 200 μL/L) presented higher numbers on the tenth day. As reported previously, if 5 log cfu/mL (or 5.95 log cfu/g, considering the dilution rates in this study) of aerobic plate counts or yeast and mold counts is considered as the critical limit [57], only the Lav 100–200 μL/L, Ros 200 μL/L and Lav + Ros 100 μL/L were within these limits for TVC and decreased the yeast and mold numbers and extended the microbial shelf life of cucumbers after 10 days of storage. The increase in microbial numbers might be attributed to the volatile nature of the EOs components that may evaporate after a long period, and due to their antimicrobial activity weakening. Moreover, this increase might be caused by the possible leaking of nutrients, i.e., calcium, which is responsible for maintaining fruit firmness from potentially damaged fruit cell walls [65]. Yeast and mold were decreased after the exposure to Eucalyptol 200 μL/L, opposite to Lav + Ros 100 μL/L, on the fifth day of storage (Table 5). Moreover, Lav (100 and 200 μL/L) and Ros 200 μL/L also decreased yeast and mold on day 10. According to this study’s results, lavender and rosemary EOs were the main factors responsible for the reduction in cucumbers’ microbial load. This effect does not appear to be solely related to the eucalyptol (their major component), rather, it may also be due to the synergistic effect of other components which are present in lower concentrations in the EOs [26]. The antimicrobial activity (antibacterial and antifungal) of EOs is attributed to the hydrophobic nature of their constitutes that enables them to penetrate the microbial cell wall membrane and interfere with the cell wall permeability and structure, among other vital processes (i.e., quorum sensing) [59,66].

4. Conclusions

The vapor application of lavender and rosemary EOs, their 1:1 (v/v) mixture and their major compound (eucalyptol) on cucumber fruits showed no significant differences in features such as the fruits’ weight loss and color. Notably, fruits’ firmness was maintained throughout storage (exceptions occurred when fruits’ were exposed to Lav 200 μL/L and Ros 100 μL/L). Increased oxidative stress was found after the exposure to Eucalyptol 200 μL/L. On the other hand, the applied treatments resulted in significant increases in antioxidants and AA content (i.e., vitamin C), increasing the nutritional value of the cucumber fruit. The findings of this study indicate that the examined means (the tested EOs, their mixture and the main common compound) could be considered as alternative means for the postharvest preservation of cucumber fruits. As the Ros 200 μL/L treatment revealed increased fruit firmness and AA content after 5 days of application, and decreased TVC and yeast and mold after the 10 days of application, combining high nutritive value and low antimicrobial activity appears to be an effective application when applied during storage. However, the application of such compounds should be performed with caution as it could, in some cases, negatively affect the organoleptic characteristics (i.e., aroma, taste, appearance) of the tested fresh produce and could potentially result in phytotoxicity when used in high concentrations. In addition, the EOs mixture as a preservation means can be explored further, as the combination of the different EO components and the possible synergistic effect on fruit preservation is of high interest. Another factor to consider when using essential oils is their perceived high cost. However, if used in the recommended dosage (concentration x time of application), this could be a cost-effective method. Additional considerations for using EOs include the possibility of phytotoxicity in the applied produce and allergy issues upon consumption.