Chemical Quality and Characterization of Essential Oils in Postharvest Hop cv. Cascade: Ventilated Room Temperature as a Sustainable Alternative to Hot-Stove and Freeze-Drying Processes

Abstract

Hop is a key ingredient in beer production, and drying it allows it to be stored before use. Unfortunately, postharvest drying techniques can negatively affect hop quality. In this study, we compared drying using a hot stove (H), freeze-drying (F), and ventilated at room temperature (VRT) drying, focusing on the chemical quality and essential oil composition. To achieve 80% water removal, F and H took two days, while VRT took five days. F and H preserved a high content of total chlorophyll (F 81.89 g/kg dm; H 82.70 g/kg dm) and carotenoids (F 54.02 g/kg dm; H 54.71 g/kg dm). The hop storage index (HSI) increased with all drying techniques, but especially in F and H. The lowest amount of polyphenols was found in the H sample (348.48 g/kg dm), while the highest content was found in VRT (631.11 g/kg dm). Freeze-drying gave the best results, especially in relation to the amount of polyphenols and antioxidant power of the product. Regarding essential oils, in the class of sesquiterpene hydrocarbons, we found α-humulene (F 24.0%; VRT 24.7%; H 25.6%), β-caryophyllene (F 10.5%; VRT 9.4%; H 11.1%), and β-farnesene (F 6.8%; VRT 6.0%; H 7.4%). The total monoterpene hydrocarbon amount increased in the VRT sample. Thus, freeze-drying emerges as an alternative technique to the hot stove; however, the cost is high. Instead, drying at ventilated room temperature represents a sustainable and valid technique for preserving the aromatic characteristics and polyphenols of the product.

1. Introduction

Hop (Humulus lupulus L.) is a fundamental ingredient in beer production, playing a key role in providing bitterness, a characteristic aroma, and extending shelf life [1,2,3,4,5]. Fresh hop inflorescences contain approximately 80% water [6,7,8], requiring dehydration to reduce moisture content to 8–11% before they can be marketed and used in brewing [2,8,9,10,11].

The drying process consists of placing hop inflorescences on a perforated bed, where they are exposed to a continuous flow of forced hot air at 55–60 °C [11,12,13], for 10–12 h [9,14]. Maintaining an appropriate moisture content is crucial, as levels above 13% can promote microbial growth, while levels below 7% may cause excessive cone breakage and lupulin loss [2,15]. To ensure high product quality, drying should take place soon after harvesting [14,15,16].

The structure of hop catkins poses a challenge to the drying process [17,18]. During drying, moisture loss varies between the inner and outer parts of the inflorescence [1,12,13]. This is due to the rachis being covered by bracts and bracteoles, which hinder the penetration of hot air. As a result, moisture content differences of 20–30% between the inner and outer portions can be observed at the end of the process [17,19]. Additionally, the drying temperature can lead to the degradation of specific chemical compounds in the inflorescences [5,11,20]. Notably, the essential oil content in fresh hops decreases by 30–40% after drying due to the volatility of these compounds [3,21,22,23].

In food drying, particularly for fruits, the detrimental impact of high temperatures on aroma is well recognized [13,24,25], as it leads to the loss of characteristic volatile organic compounds (VOCs) and the formation of Maillard-like or other oxidized compounds [26]. To mitigate these effects, alternative techniques such as freeze-drying and microwave–vacuum drying have been explored for hops [27,28]. Tatasciore et al. [29] explored freeze-drying microencapsulation as a technique for producing powdered hop extracts. They found that maltodextrin led to the lowest yield of polyphenols and bitter acid content after processing. However, it offered the highest encapsulation efficiency and effectively protected antioxidant compounds during storage. Addo et al. [28] found that while increasing microwave power enhances the drying rate, it also raises energy consumption. Carbone et al. [30] explored microwave and ultrasound applications for hops, concluding that microwave-assisted extraction with ethanol yielded 2.5 times more xanthohumol than ultrasound treatment.

Beyond this energy-consuming technique, in our experience with grape dehydration, we have observed a significant aroma change due to temperature increases and length of dehydration [31]. For this reason, our attention has focused on dehydrating grape at lower temperatures, but increasing air flow [32,33], testing specific chambers where all the ambient conditions were controlled and, moreover, selecting specific dehydration rates.

Although several studies have explored drying techniques for hops, to our knowledge, no previous research has systematically compared ventilated room temperature (VRT) drying with freeze-drying and hot-stove drying in terms of chemical quality markers and essential oil profile. This work therefore provides novel insights into the potential of VRT as a low-energy, sustainable alternative, particularly valuable for small-scale or craft hop producers.

Building on this premise, this study presents experimental data comparing ventilated drying at room temperature (VRT) drying with freeze-drying (F) and hot-stove (H) methods, focusing on chemical quality markers and essential oil composition in hops. The aim is to test the hypothesis that VRT could serve as a sustainable and energy-efficient alternative for hop drying.

2. Materials and Methods

2.1. Raw Material and Experimental Setup

The raw material, consisting of female hop inflorescences (Humulus lupulus L.) of the Cascade variety, was gathered from the hop field located at 43°35′24.217′′ N, 10°32′19.74′′ E, and owned by Azienda Agricola Opificio Birrario (Crespina-Lorenzana, Pisa, Italy). This hop variety was planted in 2012 with a planting density of 1000 plants per hectare, and irrigation was provided via a micro-irrigation system along the rows. The harvest date was 22 September 2023, which was conducted manually on 70 plants, randomly chosen from the hop-field. The quantity of inflorescences resulting from the removal of leaves, stems and foreign material was 35 kg.

For each water reduction technique, 9 kg of fresh hop inflorescences were used, divided into three sublots of 3 kg each, which were placed in separate perforated trays. An additional 5 kg of raw material were used for the initial analyses (T0 = hops at harvest time).

Three different water content reduction techniques were tested: (i) a freeze-dryer (F); (ii) a hot stove (H); (iii) a ventilated room at room temperature (VRT). The target water reduction in the matrix was at least 90%. The protocols were as follows: (i) in the freeze-dryer (LyoQuest lyophilizer, Azbil Telstar, S.L.U., Terrassa, Spain), two cycles, each lasting a total of 24 h, at a temperature of −52.4 °C and a pressure of 0.072 mBar were used; (ii) in the hot stove (Heratherm^® OMS100, Thermo Fisher Scientific, Milan, Italy) the process was conducted for two days at a temperature of 40 °C; (iii) in the ventilated room at room temperature, the test took place in a room for 5 days at a temperature of 25 ± 2 °C, with an ambient humidity of 30 ± 5%, where small perforated plastic crates with fresh inflorescences were placed in front of floor fans generating 2 m/s of air speed.

All data were reported based on the dry matter (dm) of the sample, after drying at 105 °C until a constant weight was reached in a Hot Air Oven (Thermo Fisher Scientific, Milan, Italy) [34].

2.2. Chemical Analysis of Hop

2.2.1. Chlorophyll and Carotenoids

The hop cones were extracted with a 40% ethanol–water solution (ethanol ACS reagent, ≥99.9%, Sigma-Aldrich, Steinheim, Germany and Milli-Q water), following the procedure previously reported [35], with minor modifications. Specifically, 25 g of hop inflorescences from each tray (with three extractions performed for each sample) were subjected to 500 mL of a 40% ethanol–water solution in a three-step process. In the first step, the matrix was immersed with 200 mL of solvent, stirred in an ultrasonic bath (Elma TI-H-15, Singen, Germany) at 25 °C for 15 min at a frequency of 45 kHz, after which the supernatant was separated. In the second and third steps, the hop sample was extracted with 150 mL of solvent each time. The solutions from the three steps were then combined and filtered (0.20 μm) to yield the extract. The total extraction duration was 45 min. The extract was then dried using a rotary vacuum evaporator (60 °C at 52× g) (Hei-VAP, Heidolph, Schwabach, Germany), redissolved in 25 mL of a 40% ethanol–water solution, and stored at −21 °C in the absence of direct light.

Total chlorophyll, chlorophyll-a (C_a), chlorophyll-b (C_b), and total carotenoids (xanthophylls + β-carotene) were measured based on the absorbance readings at wavelengths (λ) of 440 nm (A₄₄₀), 644 nm (A₆₄₄), and 662 nm (A₆₆₂) [35], using a Cary 60 UV spectrophotometer (Agilent Technologies Inc., Santa Clara, CA, USA). The concentration was measured in the diluted extract, after an additional 1:10 dilution with the 40% ethanol–water solution, by measuring the absorbance of the sample against the blank. The value (g/kg dm) was calculated according to Monacci et al. [36], using the following equations:

$${\mathrm{C}\mathrm{h}\mathrm{l}\mathrm{o}\mathrm{r}\mathrm{o}\mathrm{p}\mathrm{h}\mathrm{y}\mathrm{l}\mathrm{l}}_{-\mathrm{a}}\left(C_a\right)=9.784\text{ × }{\mathrm{A}}_{662}-0.99\text{ × }{\mathrm{A}}_{644}$$

(1)

$${\mathrm{C}\mathrm{h}\mathrm{l}\mathrm{o}\mathrm{r}\mathrm{o}\mathrm{p}\mathrm{h}\mathrm{y}\mathrm{l}\mathrm{l}}_{-\mathrm{b}}{(C}_b)=21.426\text{ × }{\mathrm{A}}_{644}-4.65\text{ × }{\mathrm{A}}_{662}$$

(2)

$$\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{C}\mathrm{h}\mathrm{l}\mathrm{o}\mathrm{r}\mathrm{o}\mathrm{p}\mathrm{h}\mathrm{y}\mathrm{l}\mathrm{l}=1.44{\mathrm{A}}_{662}+24.93{\mathrm{A}}_{644}$$

(3)

$$\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{C}\mathrm{a}\mathrm{r}\mathrm{o}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{o}\mathrm{i}\mathrm{d}\mathrm{s}=4.695\text{ × }{\mathrm{A}}_{440}-0.369(C_a+C_b)$$

(4)

2.2.2. Total Polyphenols and Antioxidant Activity

Total polyphenols and antioxidant activity were determined in an extract [37] prepared using an 80% methanol–water solvent (methanol ACS reagent, ≥99.8%, Sigma-Aldrich, Steinheim, Germany and Milli-Q water)with 5 g of hop in a 1:5 ratio (solid/liquid extraction, w/v) sonicated in an ultrasonic bath (25 °C for 30 min at 45 kHz). The mixture was centrifuged (6869 × g for 15 min) and the supernatant was filtered (0.45 μm).

The Folin–Ciocalteu colorimetric method [38] was applied for the total polyphenols spectrophotometric (λ = 765 nm) analysis, expressing the result as g of gallic acid equivalents (GAE)/kg dm, based on a calibration curve in the range 0–3 g/L of gallic acid (>98%, Sigma-Aldrich, Steinheim, Germany).

The antioxidant activity of the extracts (mmol Trolox equivalents (TE)/kg dm) was measured by ABTS (λ = 734 nm) and DPPH (λ = 515 nm) free radical methods, as previously reported [39], according to different standard curves of Trolox (Sigma-Aldrich, Steinheim, Germany) in the following ranges: 0.1–1.5 mM for ABTS and 0–200 µmol/L for the DPPH.

2.2.3. α-Acids, β-Acids and Hop Storage Index

For the determination of α- and β-acids and the Hop Storage Index (HSI), the method previously reported [40], with slight modifications, was applied. Five grams of hop cones from each tray (three extractions were performed for each hop sample) were shredded using a mortar, placed in a beaker, and 100 mL of toluene (ACS reagent, ≥99.5%, Sigma-Aldrich, Steinheim, Germany) was added. The sample was stirred in an ultrasonic bath (Elma TI-H-15, Singen, Germany) for 30 min at a temperature of 25 °C, then centrifuged at 6869 × g for 5 min. Afterward, the supernatant solution was filtered (0.20 μm) and stored at −21 °C in the absence of direct light.

For the measurement, 0.5 mL of the sample extract was diluted with 9.5 mL of methanol (ACS reagent, ≥99.8%, Sigma-Aldrich, Steinheim, Germany) (solution A). Then, 1.5 mL of solution A was further diluted with 23.5 mL of alkaline methanol prepared by adding 0.02 mL of 6.0 N sodium hydroxide (ACS reagent, ≥98.0%, pellets, Sigma-Aldrich, Steinheim, Germany) to 100 mL of methanol (solution B). The blank was prepared by diluting 0.5 mL of toluene using the same procedure as for the sample preparation. Absorbance readings were taken using a Cary 60 UV spectrophotometer (Agilent Technologies Inc., Santa Clara, CA, USA) in a 1 cm³ quartz cuvette, using the blank as a reference, with absorbance measured at wavelengths (λ) of 275 nm (A₂₇₅), 325 nm (A₃₂₅), and 355 nm (A₃₅₅) of solution B. The concentrations of α- and β-acids (%) and the Hop Storage Index (HSI) were calculated, considering the dilutions, according to what was previously reported in [36] and using the following equations:

$$\mathsf{\alpha }\text{-}\mathrm{acids}(\%)=(-51.56\text{ × }{\mathrm{A}}_{355}+73.79\text{ × }{\mathrm{A}}_{325}\text{ − }19.07\text{ × }{\mathrm{A}}_{275})$$

(5)

$$\mathsf{\beta }\text{-}\mathrm{acids}\left(\mathrm{\%}\right)=(55.57\text{ × }{\mathrm{A}}_{355}-47.59\text{ × }{\mathrm{A}}_{325}+5.1\text{ × }{\mathrm{A}}_{275})$$

(6)

$$\mathrm{H}\mathrm{S}\mathrm{I}={\displaystyle \frac{{\mathrm{A}}_{275}}{{\mathrm{A}}_{325}}}$$

(7)

2.2.4. Determination of Yield and Aromatic Composition of Essential Oils

The determination of the hop essential oil was performed in accordance with Van Simaeys et al. [41]. Briefly, 10 g of hops from each tray (three extractions were performed for each sample) and 1 L of deionized water were placed in a 2 L glass flask for the hydro-distillation process using a Clevenger apparatus. The extraction process was conducted for a total duration of 2 h, at the end of which the volume (mL) of extracted oil was quantified. The quantification of the yield of essential oils from the different samples was carried out using the following equation:

$$\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}\mathrm{e}\mathrm{s}\mathrm{s}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{o}\mathrm{i}\mathrm{l}\mathrm{s}\left(\%v/w\right)={\displaystyle \frac{\mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e}\mathrm{h}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{s}\mathrm{o}\mathrm{l}\left(\mathrm{m}\mathrm{L}\right)}{\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\left(\mathrm{g}\right)}}\times 100$$

(8)

This study was designed to qualitatively analyze the aromatic composition of essential oils using GC-MS. Before analysis, the oils were diluted with n-hexane (HPLC-grade > 95%, Sigma-Aldrich, Steinheim, Germany) at 5% in an amount equal to 0.5 mL.

The aromatic profile of hop essential oil was determined using the method detailed by Najar et al. [42]. An Agilent 7890B gas chromatograph system (Agilent Technologies Inc., Santa Clara, CA, USA) was employed, featuring an Agilent HP-5MS capillary column (30 m × 0.25 mm; Santa Clara, CA, USA). Helium served as the carrier gas, with a flow rate of 1 mL/min and a column head pressure set to 13 psi. The injector temperature was maintained at 220 °C, and the split ratio was set to 1:25. The oven temperature was programmed to ramp from 60 to 240 °C at a rate of 3 °C/min. For full scan mass spectrometry (MS) detection, an Agilent 5977B single quadrupole mass-selective detector (Agilent Technologies Inc., Santa Clara, CA, USA) was employed, with an electron impact (EI) ionization energy of 70 eV. The acquisition range for the mass spectra was set from 30 to 300 m/z.

The characterization and identification of the volatiles (relative abundance (%)) were based on the comparison of their retention times (tR) with those of pure reference compounds and their linear retention indices (LRIs) to a series of n-alkanes (C6-C25). Their mass spectra were matched with those found in the commercial libraries NIST [43] and ADAMS [44], as well as in a custom mass-spectral library created from pure substances and components of known samples, alongside MS literature.

2.3. Color Detrmination

The evaluation of the chromatic characteristics of hops before (harvest, T0) and after the 3 different drying techniques (F, H, and VRT) was determined using a Benchtop CLM-196 colorimeter (Eoptis-38121, Trento, Italy). The color values were expressed using the native CIE Lab coordinates (L*, a* and b*) and the calculated cylindrical coordinates (h* and C*) according to Bianchi et al. [45]. Before using the device, it was calibrated by measuring the black and white color standards that are supplied with the instrument.

2.4. Statistical Analysis

A one-way ANOVA (CoStat, version 6.451, CoHort Software, Pacific Grove, CA, USA) followed by Tukey’s Honestly Significant Difference (HSD) test (p ≤ 0.05) for multiple comparisons was performed on the chemical parameters.

The results for the composition of the essential oil, before applying the ANOVA and Tukey’s HSD tests, were transformed with logit transformation performed by JMP 17 software (SAS Institute, Cary, NC, USA). The same program was used to perform the principal component analysis (PCA) between the chemical parameters and the composition of essential oil [46].

3. Results and Discussion

3.1. Chemical Characterization

The residual water content after postharvest drying is shown in

Table 1. Chemical parameters of hop at harvest (T0) and after 3 different drying treatments (F, H and VRT).

Chemical Parameters	Units	T0	F	H	VRT
Water content	%	75.22 ± 1.52 a	0.32 ± 0.20 c	0.21 ± 0.11 c	9.12 ± 0.52 b
Chlorophyll-a	g/kg dm	22.77 ± 0.38 b	27.86 ± 0.41 a	27.99 ± 0.28 a	12.35 ± 0.45 c
Chlorophyll-b	g/kg dm	35.34 ± 0.27 b	54.02 ± 0.60 a	54.71 ± 0.21 a	23.82 ± 0.39 c
Total chlorophyll	g/kg dm	58.11 ± 0.33 b	81.89 ± 0.60 a	82.70 ± 0.39 a	36.11 ± 0.72 c
Total carotenoids	g/kg dm	16.56 ± 0.21 c	54.02 ± 0.27 a	54.71 ± 0.31 a	23.82 ± 0.32 b
Total polyphenols	g GAE/kg dm	524.4 ± 4.5 b	439.61 ± 6.41 c	348.48 ± 1.03 d	631.11 ± 3.47 a
ABTS	mmol TE /kg dm	9.33 ± 0.84 d	35.42 ± 0.49 a	23.52 ± 0.64 b	14.68 ± 0.50 c
DPPH	mmol TE/kg dm	5.60 ± 0.44 d	21.25 ± 0.51 a	14.11 ± 0.23 b	8.81 ± 0.32 c
α-acids	%	6.30 ± 0.08 c	7.07 ± 0.09 b	7.27 ± 0.07 a	7.16 ± 0.11 ab
β-acids	%	6.09 ± 0.06 b	5.82 ± 0.05 c	6.33 ± 0.04 a	5.78 ± 0.09 c
Hop Storage Index (HSI)		0.09 ± 0.03 b	0.20 ± 0.02 a	0.21 ± 0.01 a	0.14 ± 0.02 b
Essential oil yield	% v/w	1.61 ± 0.12 c	4.96 ± 0.09 a	4.97 ± 0.11 a	2.98 ± 0.10 b

Data are the mean (±standard deviation) of 3 samples. Different letters in each row refer to significant differences (Tukey, p ≤ 0.05). T0 = hops at harvest time; F = hops dried using a freeze-dryer; H = hops dried using a hot stove; VRT = hops dried with ventilation at room temperature.

. The VRT sample remained with higher moisture, as expected, but still under the range of residual moisture as mentioned in the literature [9]. The content of total chlorophyll, chlorophyll-a, chlorophyll-b and total carotenoids (xanthophylls + β-carotene) in the inflorescences was strictly dependent on the drying methodology used in the postharvest phase (Table 1). The freeze-drying technique (F) and the usage of a hot stove at 40 °C (H) significantly increased the total chlorophyll (F 81.89 g/kg dm; H 82.70 g/kg dm) and carotenoids (F 54.02 g/kg dm; H 54.71 g/kg dm) in the hop. The reason for this increase is likely the increase in tissue permeability (greater extraction) and also the inactivation of oxidative enzymes [47,48]. The sample dried with VRT at 25 ± 2 °C had total chlorophyll values (36.11 g/kg dm) below T0 (38.11 g/kg dm) due to oxidation (as it is shown by the matrix color in Section 3.3) and an increased total carotenoid content because of chlorophyll degradation, but still significantly lower than the samples dried with the other two methods (F and H).

The different final water content could have affected the final compound concentration, but it was not so linear of a response because the effect of temperature buffered the concentration increase due to the water loss. In the matrix processing, there was a greater increase in α-acids compared to T0 (Table 1). The highest concentration was measured in the H sample (7.27%). In this regard, a significant increase in β-acids compared to T0 was also observed, but only in the H sample (6.33%), while the other two samples had lower contents, at 5.82% and 5.78%, respectively. The minimal variation could be due to different extraction rates. The content of α-acids and β-acids is affected by agronomic factors, climatic conditions, and the maturity of the inflorescences during harvest [36,49,50]. The temperatures at which the various drying processes were carried out were valid in preserving or even increasing the content of the soft resins. As regards HSI, with each of the three drying techniques there was an increase in the storage index. Drying by freeze-drying and hot-stove drying influenced the opening of bracts and bracteoles of the inflorescence (increase in tissue permeability) maintaining the qualitative characteristics of the inflorescences better than VRT, but this technique still maintained a good value for the HSI. The results in HSI specify that the proposed methodologies determine a minimum oxidation of soft resins, in line with what is described in the literature [15,40,51].

The drying techniques used showed a significant influence on the total polyphenol content of the raw material (Table 1). The lowest amount was found in the H sample (348.48 g/kg dm), while the highest content was found in VRT (631.11 g/kg dm). Also, in F, there was a reduction in the concentration of total polyphenols (439.61 g/kg dm), even if the process is performed at temperatures below 0 °C. The temperature at which the drying process is carried out is important to preserve the phenolic fraction of the product. In fact, high temperatures negatively affect the concentration of polyphenols, as thermolabile compounds [52]. In general, the temperature and rate of the drying process determine the reduction in the fraction [53,54].

On the other hand, the total chlorophyl and total carotenoids were very sensitive to the oxidation process [55,56]. F and H maintained higher content due to lower processing temperatures and a faster water loss rate, compared to VRT, where dehydration occurred more slowly at room temperature.

Concerning antioxidant activity, there was no correlation between the content of polyphenols and ABTS and DPPH in the different processing methods (Table 1). In fact, in F the antioxidant power (ABTS and DDPH) is much higher (35.42 mmol TE/kg dm and 21.25 mmol TE/kg dm) than in other drying methods. This value is far higher than T0 (9.33 mmol TE/kg dm and 5.60 mmol TE/kg dm, ABTS and DDPH, respectively). The VRT sample, which had the highest polyphenol content, resulted in the lowest ABTS value (14.64 mmol TE/kg dm) and DPPH value (8.81 mol TE/kg dm). In H, on the other hand, values of 23.52 mmol TE/kg dm and 14.11 mmol TE/kg dm were measured. Hop inflorescences have different classes of phenolic compounds, not all of which are involved in determining the antioxidant power of the product. The scavenging activity of polyphenols against free radicals depends on the number and position of hydroxyl groups (OH) linked to the central aromatic ring [57,58]. The species of polyphenols most involved in conferring antioxidant activity are prenylated flavonoids (xanthohumol and 8-prenylnaringenin), flavonols (quercetin, kaempferol and glycosylated flavonols), flavonoids (catechins, epicatechins and tannins) and phenolic acids (ferulic acid) [59,60,61]. Prenylated flavonoids are found more in hard resins [6]. The temperature used in freeze-drying is assumed to produce less degradation of these molecular classes. This may be correlated with what is described in the literature [53,62], where heat treatments promote the bioavailability of phenolic compounds involved in antioxidant activity. The simultaneous decrease in polyphenols and increase in antioxidant activity (ABTS, DPPH) could be attributed to several factors, such as the enhanced antioxidant capacity of polyphenols at an intermediate stage of oxidation, the increase in reducing sugars, and the formation of Maillard Reaction Products (MRPs) [63], which are known for their significant antioxidant activity, often functioning through chain-breaking and DPPH-type mechanisms [64,65,66]. These findings are consistent with previous data reported on the drying of prunes [67].

3.2. Essential Oils Characterization

The quantity and quality of the essential oils detected are the ones which determine, during the production phase, the aromatic profile of the beer [1,39,49,59]. In our case, the sesquiterpene hydrocarbons were at the highest percentage, followed by monoterpene hydrocarbons (

Table 2. Aromatic composition (relative abundance (%)) of the hop essential oils at harvest (T0) and after 3 different drying treatments (F, H and VRT).

Compounds	LRI 1	Relative Abundance (%)
		T0	F	H	VRT
Limonene	1029	0.30 ± 0.02 a	0.30 ± 0.03 a	n.d.	0.30 ± 0.01 a
Myrcene	991	19.74 ± 1.17 b	19.22 ± 1.46 b	17.72 ± 1.05 b	22.73 ± 1.10 a
β-Pinene	977	0.32 ± 0.01 b	0.42 ± 0.04 a	0.33 ± 0.02 b	0.47 ± 0.02 a
Total Monoterpene Hydrocarbons		20.36 ± 1.20	19.94 ± 1.47	18.05 ± 1.02	23.50 ± 1.13
Linalool	1101	n.d.	0.37 ± 0.02 a	n.d.	n.d.
Geranyl acetate	1385	1.31 ± 0.03 a	1.21 ± 0.04 b	1.23 ± 0.06 ab	1.33 ± 0.01 a
Methyl geranate	1324	0.73 ± 0.01 b	0.62 ± 0.04 c	0.61 ± 0.02 c	0.82 ± 0.04 a
Total Oxygenated Monoterpenes		2.04 ± 0.04	2.20 ± 0.06	1.84 ± 0.08	2.15 ± 0.02
(E,E)-α-Farnesene	1509	0.62 ± 0.04 a	0.66 ± 0.06 a	0.52 ± 0.08 ab	0.44 ± 0.05 b
(E)-β-Farnesene	1458	6.93 ± 0.08 ab	6.83 ± 0.17 b	7.43 ± 0.39 a	6.02 ± 0.05 c
trans-α-Bergamotene	1436	0.41 ± 0.01 a	0.44 ± 0.03 a	0.45 ± 0.03 a	0.30 ± 0.01 b
α-Cadinene	1537	0.34 ± 0.03 a	0.32 ± 0.01 a	0.33 ± 0.03 a	n.d.
α-Copaene	1376	n.d.	0.11 ± 0.07 b	0.32 ± 0.01 a	n.d.
α-Humulene	1453	25.12 ± 0.28 a	24.04 ± 0.59 a	25.61 ± 1.36 a	24.76 ± 0.36 a
α-Murolene	1500	0.82 ± 0.05 a	0.81 ± 0.01 a	0.72 ± 0.09 ab	0.74 ± 0.02 b
α-Selinene	1495	3.28 ± 0.05 bc	3.33 ± 0.01 b	3.63 ± 0.02 a	3.22 ± 0.03 c
β-Bisabolene	1509	3.06 ± 0.07 c	3.30 ± 0.06 b	3.34 ± 0.12 a	2.87 ± 0.11 c
β-Caryophyllene	1419	10.11 ± 0.06 b	10.52 ± 0.29 ab	11.17 ± 0.69 a	9.40 ± 0.14 c
β-Copaene	1429	0.34 ± 0.02 b	0.37 ± 0.01 b	0.44 ± 0.03 a	0.34 ± 0.02 b
β-Selinene	1486	2.55 ± 0.03 bc	2.57 ± 0.04 b	2.82 ± 0.04 a	2.47 ± 0.04 c
γ-Murolene	1477	2.62 ± 0.06 b	2.80 ± 0.01 a	2.50 ± 0.11 cd	2.45 ± 0.04 d
δ-Cadinene	1524	2.63 ± 0.07 c	3.01 ± 0.01 b	3.23 ± 0.11 a	2.50 ± 0.06 c
Total Sesquiterpene Hydrocarbons		58.83 ± 0.59	59.11 ± 0.71	62.51 ± 1.30	55.51 ± 0.88
(E,E)-Farnesol	1722	2.20 ± 0.14 b	2.60 ± 0.09 a	2.31 ± 0.11 b	2.32 ± 0.14 b
Caryophyllene oxide	1582	0.82 ± 0.04 b	1.14 ± 0.08 a	1.13 ± 0.13 a	0.88 ± 0.11 b
Cubenol	1641	0.40 ± 0.04 c	0.52 ± 0.01 b	0.44 ± 0.03 c	0.59 ± 0.01 a
1-Epi-cubenol	1627	0.23 ± 0.26 c	0.54 ± 0.02 a	0.45 ± 0.01 b	0.52 ± 0.04 a
Humulane-1-6-dien-3-ol	1613	0.54 ± 0.06 a	n.d.	0.32 ± 0.13 b	0.63 ± 0.02 a
Humulene oxide II	1608	1.82 ± 0.02 b	1.92 ± 0.16 b	2.63 ± 0.10 a	1.74 ± 0.04 b
T-cadinol	1642	1.14 ± 0.11 ab	1.02 ± 0.07 b	0.91 ± 0.08 b	1.33 ± 0.08 a
Neointermedeol	1655	1.72 ± 0.08 b	1.50 ± 0.10 c	1.40 ± 0.11 c	2.17 ± 0.18 a
Total Oxygenated Sesquiterpenes		8.87 ± 0.23	9.24 ± 0.16	9.59 ± 0.79	10.18 ± 0.61
m-Camphorene	1952	0.63 ± 0.08 b	0.64 ± 0.07 b	0.82 ± 0.07 a	0.42 ± 0.11 c
p-Camphorene	1986	0.16 ± 0.05 c	0.33 ± 0.07 b	0.44 ± 0.02 a	n.d.
Diterpene Hydrocarbons		0.79 ± 0.15	0.97 ± 0.16	1.26 ± 0.10	0.42 ± 0.11
(E,Z)-5,7-Dodecadien-1-ol acetate	1640	1.94 ± 0.10 a	1.92 ± 0.03 a	1.92 ± 0.01 a	1.96 ± 0.04 a
(Z)-2-Heptenal	951	0.16 ± 0.02 c	0.25 ± 0.05 b	0.44 ± 0.04 a	n.d.
(Z)-4-Decanoic acid, methyl ester	1290	0.52 ± 0.02 a	0.37 ± 0.02 b	0.26 ± 0.04 c	0.45 ± 0.06 a
1,8,11,14-Heptadecatetraene	1678	1.84 ± 0.07 b	1.65 ± 0.03 c	n.d.	2.04 ± 0.04 a
Oleic acid	2140	n.d.	n.d.	n.d.	0.32 ± 0.05 a
Palmitic acid	1964	2.57 ± 0.52 a	1.24 ± 0.13 b	0.93 ± 0.14 b	0.43 ± 0.10 c
Total Non-terpene Derivatives		7.03 ± 0.87	5.43 ± 0.16	3.55 ± 0.21	5.20 ± 0.30
Total Identified		97.92 ± 1.11	96.89 ± 1.39	96.80 ± 1.18	96.96 ± 1.07

¹ Linear retention index on a HP-5MS capillary column. Data are the mean (±standard deviation) of 3 samples. Different letters in each row refer to significant differences (Tukey, p ≤ 0.05). n.d. = not detected. T0 = hops at harvest time; F = hops dried using a freeze-dryer; H = hops dried using a hot-stove; VRT = hops dried with ventilation at room temperature.

).

The latter include β-myrcene, which was at a significantly higher percentage in VRT, β-pinene, which was higher in VRT and F, and limonene, which did not have a significant difference among samples. The temperature which the H matrix was subjected to determined the volatilization of this compound. The concentration of limonene, in any case, is slightly lower than what can be seen in literature data [68,69]. In the class of sesquiterpene hydrocarbons, we found α-humulene (T0 25.12%; F 24.04%; H 25.61%; VRT 24.76%), β-caryophyllene (T0 10.11%; F 10.52%; H 11.17%; VRT 9.40%), (E)-β-farnesene (T0 6.93%; F 6.83%; H 7.43%; VRT 6.02%) and other compounds in lower concentrations (Table 2). It is interesting that monoterpenes hydrocarbons were higher in percentage in VRT sample while, in the same sample, we measured the lowest percentage in sesquiterpene. The drying temperatures might have affected these changes in percentage indeed H sample had the lowest value in monoterpenes and the highest in sesquiterpene. Due to the larger molecular size of sesquiterpenes compared to monoterpenes, their saturated vapor pressure and aqueous solubility are expected to be significantly lower [70,71]. In fact, the saturated vapor pressure of non-oxygenated sesquiterpenes is more than two orders of magnitude lower than that of non-oxygenated monoterpenes [72,73]. As a result, the volatility of sesquiterpenes is anticipated to increase more with temperature compared to the volatility of oxygenated monoterpenes [71]. It has been seen that the products of the oxidation of sesquiterpenes in gas-phase concentrations remained much lower than those of monoterpene products in aerosols of hemiboreal forests, explained by favorable and effective partitioning of sesquiterpene products into the particle phase [74]. Thus, it could be that the greater surface area of the matrix treated at higher temperature pulls in the sesquiterpenes hydrocarbons.

A principal component analysis (PCA) of all data shown in Figure 1 highlighted that the VRT sample is characterized mainly by total polyphenols and VOCs (monoterpene hydrocarbons, oxygenated monoterpene) because of the use of lower temperature, while F is marked by antioxidant activity, carotenoids and essential oil yield. The H sample clusters for VOCs (diterpene and sesquiterpene hydrocarbon), total chlorophyll and β-acids.

3.3. Color

Finally, the color results confirmed those reported for the chemical analyses. As shown in

Table 3. Color parameters (CIE Lab) of the hops at harvest (T0) and after the 3 different drying techniques (F, H and VRT).

Color Parameters	T0	F	H	VRT
L*	39.00 ± 0.51 d	56.73 ± 0.41 a	52.47 ± 0.34 b	43.75 ± 0.61 c
a*	−11.09 ± 0.41 a	−8.44 ± 0.37 b	−6.17 ± 0.75 c	−1.82 ± 0.54 d
b*	28.27 ± 0.31 ab	28.36 ± 0.21 a	27.70 ± 0.28 b	25.05 ± 0.49 c
C*	30.37 ± 0.31 a	29.58 ± 0.29 b	28.37 ± 0.31 c	25.12 ± 0.51 d
h*	111.42 ± 0.61 a	106.58 ± 0.58 b	102.55 ± 0.47 c	94.14 ± 0.71 d

Data are the mean (±standard deviation) of 3 samples. Different letters in each row refer to significant differences (Tukey, p ≤ 0.05). T0 = hops at harvest time; F = hops dried using a freeze-dryer; H = hops dried using a hot-stove; VRT = hops dried with ventilation at room temperature.

, the a* tends to decrease, reaching the lowest value in VRT; thus, this parameter, together with the decrease in the b*, can be considered as a marker of hop quality.

In addition, both C* and h* values decreased in the VRT samples compared to T0 and the other drying methods (F and H) (Table 3), indicating a loss of color saturation and a shift in the hue angle, which is consistent with chlorophyll degradation and a relative increase in carotenoid content [56]. The change in h*, which is directly related to perceived color tone, supports the shift in color from green to yellow tones (Figure 2).

4. Conclusions

Freeze-drying gave the best results, especially in relation to the antioxidant activity and the amount of polyphenols in the product, thus indicating that it is a viable alternative technique to hot-stove drying. However, the cost of freeze-drying is high. Drying with ventilation at room temperature, on the other hand, could be a valid option for preserving the aromatic characteristics and polyphenols of the product, even though it results in a brown color. To better preserve the final color of the hop, ventilated drying in a cold room could be considered. Thus, this technique can represent an economically sustainable way to produce dried hop for small brewers, especially those seeking to maintain a distinctive sensory profile in their beers. Future studies should focus on the effect of using these dried hops on the aromatic profile of beer, as well as on optimizing drying parameters to further improve the retention of key bioactive compounds. Additionally, the impact of different drying methods on yeast fermentation dynamics and final beer quality should be explored to better understand the potential applications of alternative drying techniques in brewing.