Introduction

The European hazel (Corylus avellana L.) belongs to the genus Corylus, family Betulaceae (Erdogan and Mehlenbacher, 2000); native to the Black Sea region, its cultivation is now widespread in many countries of the Mediterranean basin (Boccacci and Botta, 2009). Turkey is the world’s largest producer of European hazel with over 67% of global production, followed by Italy (12 %), Azerbaijan (5 %), and the United States (4 %). In Italy, the main production areas are Campania, Piedmont, Lazio, and Sicily regions (Squara et al., 2022).

The fruit is the main product of C. avellana L., consumed whole as an ingredient in various food products and -processed to extract the oil, used in cosmetics and as a food condiment (Alasalvar and Shahidi, 2008). Both the nuts and the oil are known for their chemical composition, which makes these products important for a healthy diet (Maguire et al., 2004; Phillips et al., 2005; Shahidi and Miraliakbari, 2005, 2006; Venkatachalam and Sathe, 2006).

However, hazel by-products, such as the peel, shell, and leaves, are also rich in phytochemicals with recognized antioxidant, free radical scavenging (Alasalvar et al., 2006; Oliver Chen and Blumberg, 2008; Shahidi et al. 2007), anticarcinogenic and antimutagenic properties, and positive effects on coronary heart disease, osteoporosis, and diabetes (Jiang et al., 2002).

The possibility of using dried hazel leaves as ingredients results in their inclusion in the diet as substances with human health benefits. This is supported by several studies showing the potential of stems, leaves, and other parts of different plants to produce herbal teas with high bioactive power (Alasalvar and Shahidi, 2008; Huda et al., 2024; Jimenez-Lopez et al., 2020; Pavlić et al., 2023; Tapsell et al., 2004). In this context, hazel leaves have been used in traditional Iranian medicine as a liver tonic (Akbarzadeh et al., 2015). Today, the leaves are also often used in galenic preparations for the treatment of hemorrhoids, varicose veins, phlebitis, edema of the lower limbs (Amaral et al., 2010), and chronic pain (Bottone et al., 2019).

However, to the best of our knowledge, no study has investigated the use of hazel leaves as ingredients in the preparation of food products and beverages, such as herbal teas.

Today, the European herbal tea market is growing rapidly, with an annual growth rate (CAGR) of 5.49% for the period 2024–2032, oriented toward the use of organic and natural ingredients that combine health and sustainability (Euromonitor International, 2023). A majority of people choose herbal teas not only for their varied taste but also for their beneficial effects, preferring them to traditional drinks such as black or green tea. In this context, herbal teas made from alternative natural products, by-products, or agricultural waste are attracting increasing interest (Verified Market Research, 2025). Their use allows the production of herbal teas with a focus on economic and environmental sustainability (Cincotta et al., 2024, 2025).

The drying process is essential for using leaves in herbal tea preparation and to prolong their shelf life. Drying techniques play a key role in the sustainable valorization of plant resources. Recent studies, conducted on leaves from different species, used Air Drying (AD) and Microwave Drying (MWD) as drying systems, demonstrating the effects of different methods on the final product quality (Cincotta et al., 2024, 2025). Moreover, MWD is characterized by its energy efficiency and reduced processing time compared to traditional methods. This approach contributes to lower energy consumption and emissions, making it a more sustainable process (Nowacka et al., 2024).

In this context, the research aims to evaluate the possibility of using hazel leaves to produce functional and sustainable herbal teas appreciated by consumers. The leaves, used for the production of herbal teas, were dried using different drying technologies, such as AD and MWD, and characterized for their volatile aroma compounds, which are responsible for their sensory characteristics and consumers’ acceptability. This work provides new insights into the valorization of hazel by-products and contributes to the development of environmentally sustainable, functional herbal beverages.

Materials and Methods

Sample collection and drying processes

Fresh leaves of C. avellana L. (1753) were harvested randomly from a hazel grove located in Sicily, Italy, during October 2024, before their natural abscission, and damaged ones were discarded. Following this, the leaves were immediately transported to the laboratory where they were cleaned with cold water to remove surface impurities, split into two batches, and subjected to two different drying treatments: Air Drying (AD) and Microwave Drying (MWD).

The drying protocols were adapted from those reported by Cincotta et al. (2025). For the AD process, 10 leaves were placed in a tray dryer (Armfield Ltd., Model UOP8, Hampshire, UK) operating at 50°C for 3 h, with a constant airflow rate of 1.5 m/s. In the MWD process, the leaves were placed on a ceramic plate inside a microwave oven (LG Electronics Inc, Model MH8265DPS.CB1QUESD, Amstelveen, The Netherlands) at 400 W for 4 min.

The drying conditions (temperature, time, and power) were selected based on previous optimization tests (Cincotta et al., 2025) and aimed at achieving optimal sensory characteristics as determined in pretrials by a trained panel. The initial moisture content of the leaves was approximately 73%, and drying was carried out until a moisture reduction of 98% was achieved (final moisture 1.46 %). This was monitored by periodically weighing the samples using an analytical balance (Sartorius, Model QUINTIX 65-1S, Göttingen, Germany) with a precision of 0.0001 g. All drying experiments were performed in triplicate under the same conditions.

Hazel leaf herbal tea preparation

The preparation of hazel leaf herbal tea followed the same protocol used by Cincotta et al. (2025). Briefly, 3 g of finely chopped hazel leaves were infused in 120 mL of mineral water at 95°C for 10 min. The solution was then filtered using standard filter paper (Munktell & Filtrak, Barenstein, Germany) and subsequently cooled to ambient temperature (22°C). All infusions were prepared and analyzed in triplicate to ensure repeatability. The obtained herbal tea was then used for the following analysis.

Total phenolic content and antioxidant capacity

The total polyphenol content (TPC) and antioxidant capacity (AC) of hazel leaf herbal tea were determined through spectrophotometric analysis, employing the Folin–Ciocalteu method and DPPH assay, respectively, following the protocol described by Vinci et al. (2022).

Volatile aroma compound analysis

The analysis of volatile aroma compounds in hazel leaf herbal tea was performed following the method by Cincotta et al. (2025) using headspace solid-phase microextraction coupled with gas chromatography–mass spectrometry (HS-SPME-GC-MS). An 18 mL aliquot of herbal tea was placed into a 40 mL glass vial to which 4 g of NaCl was added, and equilibrated at 30°C for 30 min. A DVB/CARB/PDMS fiber was then exposed to the vial headspace for 30 min to extract volatile compounds, followed by desorption in the GC injector for 3 min at 260°C.

GC-MS analysis was performed using a Shimadzu GC-2010 Plus gas chromatograph coupled with a TQMS 8040 triple-quadrupole mass spectrometer (Shimadzu, Milan, Italy), equipped with a Vf-Wax-ms capillary column (60 m × 0.25 mm i.d., 0.25 μm film thickness). The chromatographic conditions were as follows: injector temperature of 260°C; splitless injection mode; oven temperature of 45°C (held for 5 min) to 110°C at 5°C/min, and ramped to 260°C at 20°C/min; helium as carrier gas at a constant flow rate of 1 mL/min; and transfer line temperature set at 250°C. Mass spectra were acquired over an m/z range of 40–400 with a scan speed of 1250 amu/s.

Compound identification was performed by matching mass spectra with the NIST’24 library (NIST/EPA/NIH Mass Spectra Library, Wiley, Gaithersburg, MD, USA) and the FFNSC 3.0 database, injection of analytical standards, calculation of linear retention indices (LRIs) using the Van den Dool and Kratz equation, and relevant literature. Results were expressed as relative peak area percentages.

Qualitative descriptive analysis

The qualitative descriptive analysis (QDA) of the hazel leaf herbal teas followed the method reported by Cincotta et al. (2025). All participants signed an informed consent under the principles outlined by the Helsinki Declaration. The sensory panel underwent a 4-week training program aligned with ISO 8586:2023 standards. During preliminary sessions, a range of sensory descriptors was generated, from which 17 were selected based on frequency of citation.

Each descriptor was clearly defined and explained to avoid ambiguity. The descriptors were subsequently validated using reference standards, enabling panelists to calibrate their sensory perceptions and maintain consistency in descriptor use across different sessions. The intensity of each sensation was rated on a scale from 1 (absence of sensation) to 9 (extremely intense).

Evaluations took place between 10:00 a.m. and 12:00 p.m. in individual booths illuminated with white light. The sample presentation order was randomized across participants and sessions. Water and unsalted crackers were provided between samples for palate cleansing. Data collection was performed using a computerized registration system (FIZZ Byosistemes ver. 2.00 M, Couternon, France).

Consumers’ acceptability test

The acceptability test involved 80 participants (41 males and 39 females) aged 24–60 years. Participants were randomly recruited by convenience sampling among students and staff of the Department of Veterinary Sciences at the University of Messina who regularly consumed herbal teas. Participation in the survey was completely voluntary. The samples were evaluated for color, appearance, odor, taste, and overall acceptability using a hedonic scale ranging from 1 (extreme dislike) to 9 (extreme appreciation) (Cincotta et al., 2025). Each participant assessed the samples over four separate sessions.

Statistical analysis

Statistical analyses were performed using XLStat software, version 2024.1 (Addinsoft, New York, NY, USA). A one-way analysis of variance (ANOVA) followed by Duncan’s multiple range test was applied to both chemical and sensory data at a 95% confidence level. Principal component analysis (PCA) was used to analyze chemical and sensory data (Tripodi et al., 2025).

Results and Discussions

Total phenolic content and antioxidant activity

Figure 1 reports the TPC (expressed as mg/L gallic acid equivalents) and the percentage of AC in hazel leaf herbal tea, prepared with AD and MWD leaves. The results showed a statistically significant difference between the two drying techniques in terms of TPC, whereas no statistical difference was observed for the AC%.

Figure 1. Total phenolic content and antioxidant activity of hazel leaf herbal teas. Different letters indicate statistically -significant differences with P < 0.05.

The TPC was slightly higher in the microwave-dried leaf herbal tea (0.78 mg/L GAE) compared to the air-dried ones (0.70 mg/L GAE). According to other authors, this suggests that MWD is more effective in preserving or extracting phenolic compounds from hazel leaves (Snoussi et al., 2021). In contrast, the antioxidant -capacity values were similar (35.4 ± 0.2 % for AD and 35.6 ± 0.1 % for MWD).

These findings are consistent with previous studies -highlighting the advantages of MWD in retaining phenolic compounds in plant materials. For example, Sultana et al. (2012) and Lasano et al. (2018) demonstrated that MWD increased the TPC and antioxidant activity in Strobilanthes crispus leaves and in herbal tea from S. crispus leaves, respectively, than conventionally dried samples. Similarly, an increase in TPC was observed in herbal teas prepared with MWD leaves of avocado and fig compared to AD leaves, by using the same conditions used in the present study (Cincotta et al., 2024, 2025). This could be attributed to the fact that MWD offers volumetric heating, which facilitates quick moisture removal while minimizing thermal degradation (Ozdemir and Karagoz, 2024; Salehi et al., 2023).

Differently, AD, due to its longer exposure times, may result in partial degradation or transformation of heat-sensitive phenolic compounds. Moreover, faster enzyme inactivation using microwaves helps preserve antioxidant-related molecules that might otherwise be oxidized during slower drying processes (Dhar and Chakraborty, 2024).

In most cases, polyphenols are the main contributors of the AC (Barimah et al., 2017); however, in our samples, the higher polyphenol content observed in MWD samples has not given an appreciable increase in AC. In agreement with several authors, this result could be related to the fact that the antioxidant power of an extract depends not only on the TPC but also on the specific composition and structure of the compounds present (She et al., 2024). Furthermore, the similar ACs could suggest that both drying methods preserved the bioactive integrity of key antioxidant compounds, including vitamins or volatiles, which are not included by total phenolic assays, but which can contribute to the AC (El-Gamal et al., 2023; Köksal et al., 2006).

Volatile aroma compounds

The volatile aroma compounds of hazel leaf herbal teas revealed that the drying method has a significant influence, resulting in statistically significant differences (P < 0.05) across all major chemical classes (Table 1).

Alcohols, which contribute to the “grassy” and “leafy” odors, were slightly more abundant in herbal teas produced from AD leaves, with a total concentration of 12.67% compared to 9.20% in MWD.

However, notable differences have been observed between saturated and unsaturated alcohols. In particular, (Z)-4-Hexen-1-ol, a grassy and leafy odor compound, was present at high levels in AD (10.41%) and absent in the MWD samples. Conversely, MWD promoted the formation of heavier alcohols such as 1-Octanol and 2-Ethylhexanol, both present in significantly higher amounts than in the AD ones. These differences also stem from the different fatty acids from which they are formed. In fact, (Z)-4-Hexen-1-ol, as well as unsaturated aldehydes, is derived from α-linolenic acid degradation, which is very sensitive to heat (Wang et al., 2024). These differences are likely due to the nature of microwave energy, which induces rapid internal heating and can cause evaporation or degradation of volatile compounds during processing (Qin et al., 2022). The increase of saturated alcohols, such as 1-Octanol and 2-Ethylhexanol, reduces the grassy leaf odor in herbal teas and increases the citrus-like notes. Similar effects have been reported in herbs, such as basil, where MWD has been shown to alter volatile compound profiles due to the thermal impacts (Altay et al., 2024; Di Cesare et al., 2003; Imaizumi et al., 2021). A similar trend was also observed for aldehydes, where a significant difference was observed between saturated and unsaturated ones. The most significant difference resulted in nonanal (orange peel) and octanal (aldehydic, orange) contents, which were present at much higher levels in MWD (41.09 and 7.17%, respectively) than in AD (5.99 and 0.89%, respectively). On the other hand, E-2-Hexenal, a volatile compound strongly associated with grassy and leafy notes, was present in high concentrations in AD (51.17%) but was absent in MWD samples. These findings suggest that MWD promotes the formation of longer-chain saturated aldehydes while reducing the presence of unsaturated aldehydes typically associated with lipid oxidation pathways. This shift is supported by studies on other vegetal food matrices, such as tea, where convective drying determined the formation of unsaturated aldehydes compared to rapid, high-energy methods such as microwaves (Wang et al., 2022).

Esters, known for their fruity and floral notes, prevailed in AD samples (2.30 %) compared to MWD (1.50 %). The loss of esters during MWD may be attributed to their thermal instability and high volatility, as previously observed in other vegetables such as ginger, garlic, and scallion (Okonkwo et al., 2024).

A notable difference was also observed in the ketone fraction. The samples subjected to MWD exhibited almost twice the total ketone content (10.60 %) compared to the air-dried samples (5.25 %). Among these, 6-Methyl-5-hepten-2-one, a compound with fruity and citrus aroma, was particularly higher in MWD samples (8.11% vs 1.14% in AD). This increase could result from microwave--induced degradation of carotenoids or unsaturated fatty acids, as reported by Fratianni et al. (2013).

Terpenes, which contribute significantly to the floral and citrus notes of herbal teas, were better preserved in AD (4.78 %) than in MWD (3.86 %). Estragole and -β--Cyclocitral were completely absent in MWD but present in AD samples. These findings confirm the thermal sensitivity of monoterpenes and phenylpropanoids, which are more likely to degrade or volatilize during rapid heating. Eucalyptol and linalool, on the other hand, were higher in MWD samples, suggesting that MWD may favor the liberation of certain bound terpenes due to cell wall rupture (Di Cesare et al., 2003; Wojdyło et al., 2014).

Furans, which arise from Maillard reactions and sugar degradation, were higher in AD (5.97 %) than in MWD (0.50 %), indicating that prolonged thermal exposure in AD conditions likely favors their formation. Meanwhile, MWD samples showed the presence of styrene (1.45 %), absent in AD, likely formed through the degradation of aromatic precursors under microwave energy (e.g., phenylalanine) (Chen et al., 2022). The presence of styrene in MWD samples could raise additional safety considerations, as it has been classified by the IARC as a probable human carcinogen. Therefore, the choice of the drying method can significantly influence the levels of toxicologically relevant compounds. The compositional differences between AD and MWD samples should be carefully evaluated with respect to potential human exposure.

Overall, as shown in several studies, the volatile fraction results demonstrate that quantitative differences are mainly associated with drying treatment time (Cincotta et al., 2024, 2025; Hu et al., 2023; Shivanna and Subban, 2014). Our results show that AD favors the retention of compounds associated with green, leafy, and herbal notes, such as green alcohols and aldehydes, whereas MWD promotes the formation of aldehydes and ketones contributing to fruity, citrus, and waxy aromas.

Sensory analysis

The sensory evaluation of hazel leaf herbal teas revealed significant differences in most of the key attributes, in relation to the drying method used (Figure 2). Regarding the color, a rich orange and golden color was observed by the panel in AD herbal teas, whereas the MWD teas appeared lighter and greener.

Figure 2. Graphical representation of the sensory profile of AD and MWD hazel herbal tea.

Additionally, MWD samples showed stronger fruity, citrus, and floral notes, along with sweeter taste and aftertaste, but lower intensity in bitter, astringent, and woody taste. On the other hand, AD herbal teas were marked by a more pronounced grassy or leafy, balsamic, and honey-like aromas, as well as a woody, bitter, and astringent taste and aftertaste.

The Principal Component Analysis (PCA) biplot (Figure 3) highlights the sensory differences in hazel herbal teas obtained through MWD and AD. The first two principal components (PC1 and PC2) account for 97.05 % of the total variance. A clear separation was observed along the PC1, with MWD samples clustering on the negative side and AD samples on the positive side. This separation demonstrates that the drying method influenced the sensory profile of the herbal teas. MWD samples were closely associated with floral and fruity, citrus odors, which aligned with the higher quantities of 6-methyl-5-hepten-2-one, 1-Octanol, and 2-Ethylhexanol found in these samples (Di Cesare et al., 2003). Furthermore, the MWD samples had a sweet aftertaste and pale yellow color, which suggested that this method better preserved volatile aromatic compounds and limited thermal degradation. In contrast, AD samples exhibited strong grassy or leafy, balsamic, honey-like, hay, and chamomile odors, as well as bitter, astringent, woody tastes, and a deeper golden to orange hue. The odor notes observed in the AD samples could be associated with the high contents of alcohols and unsaturated aldehydes, in particular (Z)-4-Hexen-1-ol and (E)-2-Hexenal (Wang et al., 2022).

Figure 3. PCA loading plot of AD and MWD hazel herbal tea sensory data.

Microwave drying is known to induce rapid heating at the intracellular level, which promotes cell wall rupture and accelerates moisture removal without exposing the matrix to prolonged high temperatures (Joardder and Karim, 2023). This limits oxidative degradation and the formation of thermally derived bitter or burnt compounds, such as furans or phenolic degradation products, which are more likely to form during slower, and high-oxygen exposure of AD. This phenomenon likely explains the lower intensities of astringent and bitter descriptors in the MWD samples. In contrast, the AD herbal teas, subjected to more gradual thermal exposure, showed higher woody and honey-like sensory notes, which may be related to Maillard reactions and lipid oxidation products, such as furans, occurring over longer periods. This is consistent with the literature findings, indicating that conventional roasting or drying can generate a broader profile of nutty, toasted, and caramelized aromas in hazelnut (Manzo et al., 2017).

Moreover, these findings align with the existing literature, which highlights how conventional drying techniques determine the development of an aroma profile driven by Maillard reactions and lipid oxidation occurring over prolonged heat exposure (Kalkan et al., 2015; Manzo et al., 2017).

Figure 4 shows the PCA biplot integrating both volatile aroma compounds and odor sensory descriptors of herbal teas, demonstrating a strong correlation between the volatile profiles and odor sensory descriptors. A clear separation is once again observed between the two drying treatments. MWD samples are positioned on the positive side of PC1, whereas AD samples in the negative side. This spatial distribution reflects distinct chemical and sensory profiles resulting from the drying methods. MWD herbal teas were strongly associated with compounds such as terpenes, ketones, and saturated aldehydes, typically related to floral and fruity notes, which is highlighted by the proximity of these sensory descriptors. Conversely, AD herbal teas show a stronger correlation with unsaturated aldehydes and alcohols related to grassy and herbaceous notes. Accordingly, grassy or leafy, hay, and chamomile odors are the closest sensory attributes. The presence of furan derivatives and aldehydes in AD samples likely reflects thermal degradation pathways, such as lipid oxidation or Maillard reaction, due to prolonged exposure to heat (Kalkan et al., 2015; Manzo et al., 2017). These findings confirm the critical role of drying technique in shaping both chemical and sensory quality of hazel herbal teas.

Figure 4. PCA loading plot of AD and MWD hazel herbal tea volatile and odor sensory data.

Consumers’ acceptability

Figure 5 shows the consumers’ acceptability results of hazel leaf herbal teas. A clear trend emerges, showing consistently higher consumer preference for herbal teas produced from MWD leaves across all sensory attributes. The color attribute received notably higher scores in MWD samples (~7.2) compared to AD (~5.4), suggesting that MWD leads to a more appealing visual aspect. Attributes related to odor, flavor, and taste followed a similar trend, with MWD samples scoring between 6.2 (like slightly) and 6.5 (like slightly/moderately), while AD samples ranged from approximately 4.0 (dislike slightly) to 5.2 (neither like nor dislike) (Lee et al., 2008; Yang and Lee, 2020).

Figure 5. Consumers’ acceptability of hazel leaf herbal teas. 1: extreme dislike, 9: extreme appreciation. Different letters for the same attribute indicate statistically significant differences at P < 0.05 by Duncan’s test.

These differences may be attributed to the differences observed for volatile aroma compounds and phytochemicals in MWD herbal teas, which tend to change under prolonged high-temperature exposure in conventional AD. This results in a higher overall acceptability for the MWD herbal teas that was significantly higher than AD ones (6.6 vs 4.8), highlighting the influence of the drying technique on consumer appreciation (Cincotta et al., 2025; Radojčin et al., 2021). These findings suggest that MWD is a more suitable method for preserving and/or generating the desirable sensory qualities of hazel leaves, thus enhancing the market potential and consumer appeal.

Conclusions

The present study provides a comprehensive assessment of C. avellana L. leaves as a novel and sustainable raw material for the formulation of herbal teas, with particular attention to the impact of drying technology on their functional, aromatic, and sensory properties. The results demonstrate that MWD is more effective than conventional air drying in preserving total phenolic compounds, without compromising antioxidant capacity. This suggests that MWD may facilitate the retention of specific bioactive molecules while reducing thermal degradation and process time, aligning with sustainability goals in food processing.

From a sensory perspective, the drying method induced significant modifications in the volatile compound profile, which was strongly associated with sensory attributes. MWD samples exhibited higher concentrations of saturated aldehydes and ketones, responsible for fruity and sweet notes and were consistently rated more positively by consumers in terms of color, odor, flavor, and overall acceptability. Conversely, AD samples were characterized by grassy or leafy, balsamic, and astringent descriptors, reflecting the preservation of volatile alcohols and unsaturated aldehydes associated with herbaceous and vegetal notes. These findings underscore the dual role of MWD in enhancing both the functional and hedonic properties of hazel leaf herbal teas.

In conclusion, the development of functional beverages that respond to consumer demands for natural origin, health-promoting properties, and optimized sensory characteristics represents a strategic approach to product differentiation and diversification within the increasingly competitive herbal tea sector. Furthermore, the results obtained can be used as input for the design and optimization of industrial microwave drying systems, enabling the development of new products and better preservation of sensory characteristics and nutraceutical properties. Future research should focus on determining the variation in individual bioactive compounds that affect the antioxidant activity of hazelnut leaves and validating scalability and long-term quality stability.