Difference in Quality and Volatile Flavor Compounds of Zingiber officinale Roscoe with Different Drying Methods

In order to study the efect of diferent drying methods on the quality of ginger ( Zingiber ofcinale Roscoe) in Guizhou Province, the study comparatively analyzed the edible quality, functional components, and volatile favor compounds of dried ginger. Te results revealed that vacuum freeze-drying (VFD) was superior to vacuum microwave drying (WAD), hot air drying (HAD), and natural drying (ND) in terms of retaining ginger color change, rehydration rate, favonoids, and ginger spicy element. Te polyphenol retention in VFD ginger was second only to that in WAD ginger. A total of 86 volatile favor compounds were detected in fresh ginger and all four drying samples. Ester and aldehyde compounds were identifed as the main favor components in dried ginger, with variations observed among the diferent drying methods. Principal component analysis (PCA) and the nearest neighbor algorithm successfully distinguished the ginger samples treated with diferent drying methods. Te aroma activity value (OAV) was calculated based on the threshold, and 12 volatile favor compounds with OAV ≥ 1 were identifed as the key aroma components of ginger. Te aroma profles of ginger were generally similar across the four diferent drying methods, among which WAD ginger exhibited distinct favor characteristics associated with drying. Tis study employed physicochemical properties, active components, and GC-IMS to evaluate the efect of diferent drying methods on ginger, providing valuable insights for the processing and favor quality control of drying ginger.


Introduction
Ginger (Zingiber ofcinale Roscoe), belonging to the genus Zingiber, is commonly known as spicy cloud or indixin.It is a valuable plant with both medicinal and edible uses.Te dried ginger, ginger slices, and ginger leaves are commonly used in traditional medicines [1,2].Ginger is a rich source of essential nutrients such as vitamins, proteins, and minerals [3].Moreover, it contains various bioactive compounds including curcumin, polysaccharides, gingerols, favonoids, and ginger essential oil, which contribute to its diverse pharmacological efects such as antibacterial, anti-infammatory, and lipid-lowering properties [4][5][6].China is the second-largest producer of ginger globally, accounting for approximately 28% of the total ginger cultivation.Ginger varieties in China are mainly classifed into two categories: big seed ginger and small seed ginger.Small seed ginger, also known as Xiao Huang ginger, is highly regarded for its small tubers, vibrant color, and strong aroma [7].Among them, Guizhou small yellow ginger stands out as a prominent variety in the market.
Fresh ginger has a high moisture content, making it susceptible to spoilage during storage [8].Terefore, drying technology for ginger has become a common prestorage treatment method and a focal point of research.Muthukumar et al.'s [9] research showed that drying temperature has an impact on drying characteristics and quality aspects of black ginger dried in an electric dryer, and the increase of drying temperature could improve the product quality of black ginger.Nazmi and Ahmet [10] investigated the efects of convection and microwave drying methods from the perspectives of drying characteristics, color, rehydration, and microstructural properties of ginger, whose result showed that the rehydration rate of ginger was highest and the quality was optimal at 60 °C.Gingerol in ginger spicy element, a nonvolatile pungent compound, is the most abundant bioactive substance in ginger, possessing signifcant nutritional and medicinal value [11,12].
Several studies have shown that gingerol has the ability to inhibit lipid oxidation [13], exhibit neuroprotective efects [14], and promote blood circulation [15].Jung et al. [16] studied the heat-induced conversion of gingerols to shogaols, which were found to be afected by the heating type and sample type, and moist heat treatment at a higher temperature for the preset time is advantageous to obtain the ginger products with high quantity of bioactive components of shogaols.
Te evaluation of the edibility and functional quality of ginger also considers the presence of volatile components, which are commonly analyzed using gas chromatographymass spectrometry (GC-MS) [17][18][19].However, gas chromatography-ion mobility spectrometry (GC-IMS) has emerged as a promising technology for food analysis and quality testing.GC-IMS utilizes gas phase separation for analysis and ofers advantages such as lower detection limits (the detection limit can be as low as ppbv level), higher speed, greater sensitivity, and easier operation compared to other detection methods [20].In a study by Yu et al. [20], the consequence of drying methods on the volatile diferences of dried citrus peel as it is afected by cultivars was analyzed on GC-MS and GC-IMS.Te results showed that there were signifcant diferences in the volatile components of diferent samples, suggesting that sun-drying was the best drying method of preserving total volatiles in dried citrus peel.Bai et al. [21] used GC-IMS to detect the diference of volatile compounds in ginger before and after drying during hot air drying, and the results showed that the alterations of ginger volatile compounds were intimately related to moisture difusion during drying.
In summary, existing literature predominantly focuses on the efects of drying methods on ginger and drying quality, with limited reports on the impact of drying methods on the volatile favor substances of Guizhou ginger.Terefore, this experiment aims to evaluate the efects of four diferent drying methods (vacuum freezing, vacuum microwave, hot air, and natural drying) on the quality and volatile favor substances of Guizhou ginger, as well as to promote the healthy growth of the ginger industry in Guizhou.

Drying Methods.
Fresh ginger, free from decay, disease loss, and sprouting, was carefully selected and prepared by washing, peeling, and cutting into slices of 3-4 mm thickness.Te ginger slices, weighing 500 g, were subjected to four diferent drying methods: vacuum freeze-drying (VFD), vacuum microwave drying (WAD), hot air drying (HAD), and natural drying (ND).Te drying parameters were determined based on pre-experiments, following the method outlined by Lv et al. [22] with slight modifcations, to achieve a moisture content of less than 8% in the dried product, indicating the completion of the drying process.Each drying method was performed in triplicate.
For the VFD method, the fresh ginger slices were prefrozen in a tray and evenly placed in a refrigerator set at −80 °C for 24 hours.Subsequently, the prefrozen ginger slices were transferred to a LC-12N-50A vacuum freeze dryer (Shanghai Lichen Bonsey Instrument Technology Co., Ltd.) with the cold hydrazine temperature lowered to −50 °C.Te cold trap temperature was maintained at −60 °C throughout the drying process, with a vacuum level of 2.0 Pa.Te total drying time for VFD was 48 hours.
In terms of the WAD method, the fresh ginger slices were evenly spread on a material tray inside a WBZ-16 microwave drying vacuum (Guiyang Xinqi Microwave Industry Co., Ltd.).Te microwave power was set at 500 w, with a drying interval of 2 minutes every 5 minutes.Te vacuum level was maintained at 0.06 MPa, and the drying temperature ranged between 40 and 45 °C.Te total drying time for WAD was 2.5 hours.
For the HAD method, the fresh ginger slices were evenly placed in a tray inside a 101-2 electric blast dryer (Tianjin Teste Instruments Co., Ltd.).Te electric blast dryer was set at a constant temperature of 50 °C, and the drying process lasted for 36 hours.
As for the ND method, the fresh ginger slices were laid fatly on a material tray and naturally dried at room temperature in a sunny indoors window.Te total drying time for ND was 168 hours.

Quality Changes of Ginger
2.3.1.Browning Degree of Ginger.According to reference [23] and modifcation, ginger powder was homogenized with distilled water at 0∼4 °C at a material-liquid ratio of 1 : 4 for 2 min, centrifuged at 4000 r/min for 5 min, and the absorbance value of the supernatant was measured at 410 nm using an enzyme-labeled instrument (Multiskan Sky Full wavelength microplate reader, Termo, USA)

Color Variation of Ginger.
Te colorimeter (CR-10 colorimeter, Konica Minolta Ltd., China) was used to assess the color variation between freeze-dried ginger and fresh ginger.△E indicates the color diference value.Te measured parameters include brightness, represented by value L; red-green hue, represented by degree a; and yellow-blue hue, represented by degree b.Te subscript 0 (L 0 , a 0 and b 0 ) denote the measurements for fresh ginger, while the superscript asterisk (L * , a * , and b * ) indicates the measurements for freeze-dried ginger.

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(1)  [24,25] was used as reference, and the standard curve was drawn with gallic acid as the standard.Te linear equation Y � 0.0506X + 0.1028, R2 � 0.9948 was obtained, and the total phenolic content was expressed as gallic acid equivalent (mg/g).
According to a previous study, in terms of determination of favonoid content, the sodium nitrite and aluminum nitrate color development method [26] was used to draw the standard curve with rutin as the standard.Te linear equation Y � 4.5308X − 0.0144, R2 � 0.9998 was obtained, and the total favonoid content was expressed as rutin equivalent (mg/g).
According to a previous study, in terms of determination of ginger spicy element content, the vanillin method [27] was used to draw the standard curve with vanillin as the standard.Te linear equation Y � 0.0726X − 0.1933, R2 � 0.998 was obtained, and the ginger spicy element content was expressed as vanillin equivalent (mg/g) × 2.003 (the conversion factor between vanillin and ginger spicy element).

Hardness Determination of Ginger Treated with
Different Drying Methods.Te method described in reference [28] was modifed to determine the hardness of the samples on a texture analyzer (TA.XTplus Texture Analyser, Stable Micro Systems, GBR).Te adapted probe P/2 was selected to pierce the ginger slices, and the speed was set to 3 mm/s frstly, 1 mm/s during, and 3 mm/s after the test with a compression distance of 1 mm. 10 measurements were performed on the samples obtained with diferent drying methods, and the average value was recorded.
2.3.6.Microstructure Determination.Te samples were plated with gold spray and fxed on a short column of a scanning electron microscope at an accelerating voltage of 15 kv, and the microstructure was observed and photographed on a scanning electron microscope.

GC-IMS Analysis Conditions.
Te sample was smashed, 0.1 g of dried sample was taken, and 1 g of fresh ginger was sampled and placed in a 20 mL headspace vial with 50 μl of internal standard 100 ppm 2-octanol, and the incubation temperature was 60 °C.Te incubation time had been 20 min.Te incubation speed was 500 rpm.Headspace injection (FlavourSpec ® GC-IMS favor analyzer, G.A.S., Germany) conditions were as follows.Te headspace injection needle temperature was 85 °C.Te injection volume was 500 μl.Column type was polyethylene glycol (30 m × 0.53 mm, 1 μm, RESTEK, USA).Te column temperature was 60 °C.Te analysis time had been 50 min.Carrier gas/drift gas was high-purity nitrogen (purity ≥99.999%).Te drift gas fow rate was always maintained at 150 mL/min.Te initial carrier gas fow rate was 2.0 mL/min.Te initial carrier gas fow rate of 2.0 mL/min had been maintained for 2 min, and the carrier gas fow rate increased to 10 mL/min from 2 to 10 min, reaching 100 mL/min from 10 to 50 min.Te IMS detector temperature was 45 °C.

Evaluation of Odor Activity
Value.Te favor contribution of each volatile favor substance was evaluated by using odor activity value (OAV) [29] (C is the concentration of the volatile favor substance to be measured (μg/g); T is the threshold value of the substance in air [30] (μg/g)).Te following formula is used: 2.6.Data Processing.Volatile favor substances were analyzed and plotted on the analysis software confgured for the GC-IMS instrument, VOCal was used for qualitative and quantitative analysis of spectra and data, and the application software's built-in NIST database and IMS database allow qualitative analysis of volatiles, and Reporter and Gallery Plot plug-ins were used to construct fngerprint and diference spectra of sample volatiles.Dynamic PCA plug-in and GraphPad (version 9.0) were mainly used for dynamic principal component analysis.Signifcance and correlation analysis was performed on SPSS (version 26.0).OriginPro 2021 was used for graphing and analysis of experimental data.

Efect of Diferent Drying Methods on Physicochemical
Properties and Active Ingredients of Ginger.Te results of quality analysis of ginger treated with diferent drying methods are presented in Figure 1 and Table 1.From the graphs, it was evident that the appearance of ginger treated with four diferent drying methods difered signifcantly from that of fresh ginger (Figure 1(a)), which had a white appearance.HAD ginger (Figure 1(d)) and ND ginger (Figure 1(e)) exhibited severe browning, noticeable wrinkling, and a hard texture, with the lowest retention of ginger polyphenols and ginger spicy element.However, ND ginger had a higher favonoid content, possibly due to the disruption of enzyme and substrate distribution during drying with the other three methods.Tis disruption leads to the oxidation or polymerization of some favonoids, resulting in higher favonoid loss [31].WAD ginger (Figure 1(c)) appeared lightly crumpled with a crunchy texture and retained the highest favonoid content, as well as a higher retention of polyphenols and ginger spicy element.VFD ginger (Figure 1(b)) had a porous sponge-like structure, which allows for quicker restoration of its original properties upon rehydration, with a signifcantly higher rehydration rate compared to other drying methods.

Comparison of Microstructure of Ginger Treated with
Diferent Drying Methods.SEM images of ginger treated with diferent drying methods are shown in Figure 2. Combined with the hardness evaluation in Table 1, it can be observed that the internal pore size of VFD ginger (Figure 2 Presumably water leaves the product quickly at high temperatures leading to a more compact structure and higher hardness values of 1283.88 g and 1494.66 g, respectively.ND ginger has the highest hardness value of 1759.01 g, and its microstructure (Figures 2(d)) shows severe collapse, with visibly atrophied and varied cell pore sizes and numerous dense pores.

Qualitative and Quantitative Analysis of Volatile Components of Ginger by Diferent Drying Methods.
Te Reporter plug-in program on the LAV analysis software, coupled with the GC-IMS instrument, was used to analyze the volatile favor substances in ginger treated with fve diferent methods.Te spectra of fresh ginger samples were selected as a reference in Figure 3, and the spectra of other drying treatments were subtracted from the signal peaks in the fresh ginger spectra to obtain the GC-IMS diference spectra of ginger treated with diferent drying methods.Te fgure shows that there is not much diference in the composition of volatile substances among ginger samples with diferent treatments.Te migration time mostly falls within the range of 1.0-2.0ms, while the retention time ranges from 250 to  1500 s.In the diference spectrum, white indicates that the substance has the same concentration as in fresh ginger, the blue area indicates a lower concentration in the sample compared to fresh ginger, and the red area indicates a higher concentration.Te darker the color, the greater the variation in substance content.For instance, the volatile substance 2-nonanone, located in the yellow round box area, exhibits considerable variation in content, with the highest concentration in fresh ginger and relatively low concentrations in WAD and ND ginger.
Based on the retention and migration times of volatile favor substances in ginger treated with diferent methods, qualitative analysis of these substances was performed using two databases, NIST and IMS, in the application software.As shown in Figure 4 and Table 2, the same types of volatile substances were detected in all fve ginger samples, with a total of 98 volatile substances identifed.Among them, 85 were detected qualitatively, including 24 terpenes, 21 aldehydes, 13 ketones, 11 alcohols, 11 esters, 2 ethers, 2 pyrazines, and 1 aromatic hydrocarbon substance.Te presence of multiple signal peaks for some compounds may be attributed to different product ions produced by volatile substances depending on their concentration during analysis [32], resulting in the simultaneous detection of monomers, dimers, and trimers of the same compound.2-Octanol was used as an internal standard to calculate the diferences in the content and proportion of volatile favor substances in ginger treated with diferent methods.Table 2 and Figure 5 reveal that the main volatile favor substances in ginger with diferent treatments are esters, terpenes, aldehydes, ketones, and alcohols.Te content of esters and aldehydes signifcantly increases after drying fresh ginger, with ester content increasing due to esterifcation reactions between alcohols and carboxylic acids during drying [33].Te increase in aldehydes can be attributed to the decomposition or oxidation of other compounds at high temperatures [34,35].Te content of alcohols, terpenes, and ketones decreases in fresh ginger after drying.Te decrease in alcohols may be due to the low boiling point of some small molecular alcohols, resulting in less volatility at higher drying temperatures.Terpenes and ketones have active chemical properties.Among them, terpenes are hydrocarbons with relatively strong atmospheric reactivity, which are prone to form particulate aerosols during the drying process, resulting in reduced contents.

Fingerprint Analysis of Ginger Treated with Diferent
Drying Methods by GC-IMS.Te GC-IMS fngerprint profles of ginger with diferent treatments were constructed using the Gallery Plot plug-in in LAV software.Tis analysis provided a deeper understanding of the changes in volatile components of ginger treated with diferent drying methods.Te results are presented in Table 2 and Figure 6.Each treatment of ginger had three parallel samples, with each row representing the signal peaks selected from one sample, and each column representing the signal peaks of the same VOC in diferent samples.Tis information provided a comprehensive view of the volatiles present in diferent treatments of ginger, as well as the diferences between the dried ginger samples.
Te concentration of volatile substances in region B was higher than that in vacuum freeze-dried (VFD) ginger.It included substances like 2-octanone, (E)-beta-ocimene, 1,8cineole, 1,4-cineole, and diethyl acetal.Under vacuum conditions, water evaporated directly into the gaseous state during ginger drying, which might have led to the release of these volatile substances due to cell rupture.
Te concentration of volatile substances in region D was higher in vacuum microwave drying (WAD) ginger.It included substances like myrcene, alpha-thujene, alphaterpineol, terpinen-4-ol, tetrahydro-linalool, and others.Te increase in myrcene could be due to the thermal decomposition of beta-pinene during the drying process.Te volatile substances in region were the characteristic favor substances of high-temperature air drying (HAD) ginger.Tey included 2-methyl-1-propanol, linalool, propyl acetate, pentanal, acetone, and others.Te increase in the content of propyl acetate and linalool was particularly signifcant.Te rise in propyl acetate might be attributed to the oxidation or cleavage of unsaturated fatty acids in ginger under high temperature and aerobic conditions [36].Te increase in linalool might be a result of the longer drying time of HAD compared to WAD, leading to further reactions of myrcene to produce linalool.
Region C showed relatively stable changes in the content of volatile substances during ginger drying.Several substances, including citral, phenylethyl 2-methylpropanoate, alpha-terpinene and alpha-phellandrene were detected in all fve samples.Tis indicates that drying had less efect on the main favor substances of ginger.Te concentration of volatiles in region E, including ethyl butanoate and citronellyl formate, was higher than that in fresh ginger.Tis could be attributed to the increase in temperature during drying, which favors the formation of esters.

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Journal of Food Quality

Cluster Analysis of Ginger Treated with Diferent Drying
Methods.Te principal component analysis (PCA) results are presented in Figure 7. Te 1st principal component (PC1) contributed 43%, the 2nd principal component (PC2) contributed 23%, and the cumulative contribution of the frst 2 principal components was 66%.Te ginger samples treated with diferent drying methods were clearly separated in the fgure, with fresh ginger samples grouped on the left and ginger samples with diferent drying methods on the right.Fresh ginger and VFD ginger showed the greatest diference and were furthest apart, with fresh ginger being closer to WAD ginger, HAD ginger, and ND ginger, which means they had similar favors.Te diferences between the diferent treatments of ginger mainly stem from the contribution of diferent volatile favor substances, with fresh ginger having a richer favor component compared to dried ginger.
By calculating the Euclidean distance between each pair of samples, we obtained the nearest neighbor-Euclidean distance plots for ginger samples treated with diferent drying methods.Te results, shown in Figure 8, indicated that the fve diferent treatments of ginger could be clearly distinguished.Notably, ND ginger and HAD ginger had the closest distance and highest similarity, which could be attributed to the similar principles underlying these two drying methods.
Te favor of ginger became more similar after drying, likely due to the increase in hexanal, p-xylene and diethyl succinate resulting from these two drying methods.Tis demonstrated that the nearest neighbor-Euclidean distance plot aligned with the results of PCA analysis.

Analysis of Characteristic Aroma Substances in Ginger
Treated with Diferent Drying Methods.OAV is often used to evaluate the contribution of volatile compounds [38].To determine the contribution of each volatile favor substance to the overall favor characteristics of ginger, we calculated the odor activity value (OAV).By referring to Table 2, we identifed volatile substances with OAV >1.Twelve volatile substances with signifcant aroma characteristics in ginger were retained for aroma characterization.As depicted in Figure 8 and Table 2, there were noticeable diferences in the aroma profles of ginger treated with diferent treatments, in which there were fve main aroma components, namely, grassy aroma contributed by 2-decenal and (E, Z)-2,6nonadienal, nutty aroma contributed by 2-methylpropanal and 3-methylbutanal, fruity aroma contributed by 2undecanone, 2-nonanone, citral, and rthyl butanoate, pungent aroma contributed by 2-heptanone and cineole, and creamy aroma contributed by diacetyl.Journal of Food Quality       of Food Quality As observed in Figure 9, the aroma profles of ginger were generally similar across the fve diferent treatments.Fresh ginger exhibited more grassy, fruity, and pungent aromas.However, the diferent drying methods leaded to a reduction in 2-decenal content in ginger, resulted in a signifcant reduction of grassy aroma of gingers.Tis reduction had the greatest impact on the volatile components.Te aroma of VFD, HAD, and ND ginger also diminished due to water evaporation during processing.Among these, the aroma characteristics of HAD ginger and ND ginger were more similar, with grassy and pungent scented being the main characteristic.On the other hand, VFD ginger experienced the most aroma loss, possibly due to the vacuum environment during drying, which leaded to strong water transpiration.Although some volatile favor substances were carried away with the water vapor [39], the lower drying temperature limited their transformation and results in weaker aromatic odor.WAD ginger, on the other hand, exhibits the highest levels of nutty and creamy aromas.Tis drying method imparts distinct favor characteristics to the dried ginger, retaining pungent and fruity aromas to some extent.

Conclusions
In this study, the quality of processed ginger was analyzed using diferent drying methods, revealing signifcant efects on the physicochemical properties and active ingredients of ginger.Among the methods, VFD demonstrated the best quality, closely resembling the appearance of fresh ginger and retaining high levels of favonoids, polyphenols, and ginger spicy element.VFD ginger also exhibited the highest rehydration rate, with a porous and full internal structure and minimal shrinkage.
Te volatile favor substances of fresh ginger and ginger treated with the four drying methods were analyzed using GC-IMS.A total of 86 volatile substances were identifed in the fve samples, including terpenes, aldehydes, ketones, alcohols, esters, ethers, pyrazines, and aromatic hydrocarbon substances.Many of these substances existed as monomers, dimers, and trimers.By constructing GC-IMS fngerprint profles, the characteristic favor substances of ginger treated with diferent drying methods could be determined.Te results indicated that the content of terpenes, ketones, and alcohols relatively decreased during the drying process, while ethers remained more stable.Esters and aldehydes increased to varying degrees in all four dried samples.
Te volatile favor substances of ginger treated with different drying methods also exhibited noticeable diferences.VFD treatment resulted in a signifcant loss of ginger's volatile components, while the diferences between HAD and ND treatments were less pronounced.Principal component analysis and nearest neighbor algorithm were employed to analyze the samples, efectively diferentiating ginger samples treated with diferent drying methods.Te variations in the content of volatile favor substances contributed to diferences in aroma composition among the fve samples.Fresh ginger exhibited a distinct grassy, fruity, and pungent aroma.WAD ginger retained the fruity and pungent aroma characteristics of fresh ginger, while also displaying a more prominent nutty and creamy aroma in terms of favor.
Te application of GC-IMS in dried ginger provides a novel reference method for studying the efect of diferent drying methods on the volatile favor substances of ginger.Tis approach also ofers a theoretical basis for quality control of ginger under various drying conditions.
(a)) is mostly round or oval, with uniform and fufy pores.Te porous and full structure, along with the low relative density, results in a low hardness of ginger (791.38 g).Te microstructures of WAD ginger (Figure 2(b)) and HAD ginger (Figure 2(c)) both exhibit varying degrees of collapse, with signifcantly contracted internal pore sizes compared to VFD ginger.Te pores in WAD and HAD ginger are numerous and disorganized.

Figure 1 :
Figure 1: Changes of appearance of Zingiber ofcinale Roscoe treated with diferent drying methods.

Figure 4 :
Figure 4: Location points of characteristic peaks of volatile substances in Zingiber ofcinale Roscoe treated with diferent drying methods.

Figure 7 :
Figure 7: Principal component analysis (PCA) and loading diagram of volatile matter in Zingiber ofcinale Roscoe treated with diferent drying methods.

Figure 6 :
Figure 6: Fingerprint of volatile compounds of Zingiber ofcinale Roscoe treated with diferent drying methods by GC-IMS.

Figure 5 :
Figure 5: Relative content of compounds of Zingiber ofcinale Roscoe treated with diferent drying methods.

Table 2 :
Qualitative and quantitative analysis of characteristic favor substance of Zingiber ofcinale Roscoe treated with diferent drying methods.

Table 2 :
Continued.substance monomer, D indicates volatile substance dimer, T indicates volatile substance trimer; "-" indicates threshold values that cannot be queried and OAV values that cannot be calculated; count 86-98 are unknown compounds; 1,8-cineole and 1,4-cineole are the same as cineole, the threshold values are the same, and OAVs are combined.Te same letter indicates that the diference is not signifcant and diferent letters indicate signifcant diferences (P < 0.05).