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Effect of drying methods on the phenolic constituents of meadowsweet (Filipendula ulmaria) and willow (Salix alba)
LWT - Food Science and Technology 42 (2009) 1468–1473
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Effect of drying methods on the phenolic constituents of meadowsweet (Filipendula ulmaria) and willow (Salix alba) Niamh Harbourne, Eunice Marete, Jean Christophe Jacquier, Dolores O’Riordan* School of Agriculture, Food Science and Veterinary Medicine, University College Dublin, Belfield, Dublin 4, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 November 2008 Received in revised form 5 March 2009 Accepted 4 May 2009
The objective of this study was to investigate the effect of drying conditions on the phenolic constituents and colour of extracts of organically grown white willow and meadowsweet for incorporation into a functional beverage with potential anti-inflammatory properties. The herbs were freeze-dried, airdried, oven or tray-dried at 30 or 70 C. The drying kinetics of the herbs was first determined. Both drying temperature and method had a significant effect (p 0.05) on the drying rate, the samples traydried had a faster drying rate than those oven-dried. Results show that for meadowsweet and willow, freeze-drying and oven or tray drying at 30 C had no significant effect on the phenolic constituents (e.g. total phenols, salicylates, quercetin) or the colour of the extracts in comparison to traditional air-drying. Although increasing the drying temperature to 70 C resulted in an increase in the drying rate of both herbs it also led to the loss of some phenolic compounds. Also, the extracts from both herbs dried at 70 C were significantly (p 0.05) redder than the other drying methods. Therefore, tray drying these herbs at low temperatures may reduce drying time without having a significant effect on the phenolic content and colour of the extracts. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Drying Meadowsweet Willow Phenolic constituents
1. Introduction Meadowsweet (Filipendula ulmaria L.) and white willow (Salix alba) are medicinal plants indigenous to Europe. They have been traditionally used to treat various ailments due to their antipyretic, analgesic and anti-inflammatory properties (Bruneton, 1995). The efficacy of these plants is mainly due to their phenolic content, which includes salicylates. These salicylates such as salicin in willow and salicylaldehyde in meadowsweet are precursors of salicylic acid. Both meadowsweet and willow played an important role in the development of aspirin (acetylsalicylic acid) as salicylic acid was first isolated from the flowers of meadowsweet and the bark of the willow in 1838 (Blumental, Goldberg, & Brinckmann, 2000; Zeylstra, 1998). Other phenolic compounds present in both herbs include flavonoids (e.g. quercetin) and tannins. In recent years there has been an increasing consumer demand for health related food products which has led to development of novel functional beverages (Katan & De Roos, 2004; Verschuren, 2002). The high phenolic content of these plants and the consumer drive towards natural products demonstrate that these herbal
* Corresponding author. Tel.: þ353 1 7167016; fax: þ353 1 7161147. E-mail address: [email protected] (D. O’Riordan). 0023-6438/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2009.05.005
extracts would be ideal ingredients for incorporation into functional beverages with potential anti-inflammatory properties. Prior to inclusion into beverages these herbs undergo post-harvest processing, including drying, to extend their shelf-life. Previous studies have shown that preservation techniques of medicinal herbs may affect their quality (Harbourne, Jacquier, & O’Riordan, 2009; Julkunen-Tiitto & Sorsa, 2001). For a high quality extract for incorporation into a beverage the level of phenolics should be maximized, in particular the non-tannin fractions which will include the active ingredients with anti-inflammatory properties. Also, the tannin fractions should be minimized as they cause astringency, an undesirable gustative attribute. Drying is an important preservation method for plant material, as it inhibits enzymatic degradation and limits microbial growth. Ambient air-drying is the traditional technique used to preserve medicinal herbs as the low temperatures are thought to protect against degradation of the active components. However, this drying process is slow and metabolic processes may continue longer which may lead to quality loss of the plants and subsequently to the extracts, e.g. colour changes, loss in active ingredients (Fennell, Light, Sparg, Stafford, & Van Staden, 2004; Keinanen & JulkunenTiitto, 1996). Other methods such as freeze-drying, oven drying and tray drying have been previously used to preserve medicinal herbs (Keinanen & Julkunen-Tiitto, 1996; Tanko, Carrier, Soskhansanj, &
N. Harbourne et al. / LWT - Food Science and Technology 42 (2009) 1468–1473
Crowe, 2005), but to date there is little information in the literature on the effect of these drying conditions on the drying kinetics or quality (e.g. colour, phenolic content) of meadowsweet and willow extracts. The purpose of this study was to investigate if drying temperature or method could have an influence on the extract quality, for example the shorter drying times that may be associated with tray drying could have a positive influence. Therefore, the objectives of this study were two-fold, firstly to determine the drying kinetics of meadowsweet and willow. Secondly, to investigate and compare the effect of various drying conditions on the total phenolic content, tannin and non-tannin phenols, bioactive constituents (salicin, salicylic acid and quercetin) and the colour of meadowsweet and willow extracts for possible inclusion into a beverage with potential anti-inflammatory properties. 2. Materials and methods 2.1. Plant material Meadowsweet and willow were organically grown and harvested in Strokestown, Co. Roscommon, Ireland. Debarking of willow stems from young branches was carried out within 48 h of harvesting and the drying procedures for aerial parts (mixture of flowers, stems and leaves) of meadowsweet were set up on the day of harvest. The stems of meadowsweet were approximately 1–3 mm in diameter and the barks of willow were <2 mm in thickness. Initial moisture content was determined by drying approximately 5.0 g of the plant material in the oven (Model No. FD 115/E2, Binder, Germany) at 102 C until a constant weight was achieved. 2.2. Drying The herbs were dried using the following treatments (1) prefreezing in liquid nitrogen followed by freeze-drying (FD) (Edwards Super Modulyo freeze-drier, Sussex, UK), (2) air-drying at ambient temperature (25 C) (AD), (3) drying in a convection oven (Model No. FD 115/E2, Binder, Germany) at 30 C (OD30), (4) drying in a convection oven at 70 C (OD70), (5) drying in a tray drier (Model No. U0P8, Armfield, England) at 30 C (TD30) and (6) at 70 C (TD70) at an air velocity of 0.97 0.03 m/s, which is higher than that in the oven. The herbs were distributed uniformly in a single layer on trays measuring 27 21 cm for oven and air-drying and on trays measuring 28 19 cm in the tray drier. Moisture loss of the air-dried, tray-dried and oven-dried samples was recorded at time intervals and the drying experiments were conducted in triplicate. All the samples were dried to equilibrium moisture content. To evaluate the effect of drying on meadowsweet and willow extracts, the freeze-dried samples were taken as the control. The dried samples were then ground into a moderately fine powder (WHO, 1998) using a lab mill with a sieve size of 3 mm (Christy and Norris Ltd, UK). The Lewis equation (Equation (1)) was used to describe the drying model of the herbs and the drying rate was calculated according to Equation (2) (Doymaz, Tugrul, & Pala, 2006; Tanko et al., 2005).
M Me ¼ expðktÞ Mo Me Drying rate ¼
Mtþdt Mt dt
(1)
(2)
where M, Me, Mo, Mt, Mtþdt, are the moisture content, equilibrium moisture content, initial moisture content, moisture content at t
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and moisture content at t þ dt (g moisture/g dry basis) respectively. k is the drying constant and t is the time in minutes. 2.3. Extraction The ground plant material (2.5 g) was put in a Duran flask containing 100 ml of heated distilled water at 100 C for 20 min using a stirring hot plate (IkaÒ WERKE GmbH and Co., Germany). Extracts were filtered under vacuum through Whatman no. 1 filter paper (Whatman Ltd, England) and cooled immediately on ice. 2.4. Quantification of the total phenolic content in plant extracts The total phenolic content in the extracts was carried out according to the Folin–Ciocalteu method (Singleton & Rossi, 1965). The reaction mixture was composed of 0.2 ml standard/extract, 0.5 ml Folin–Ciocalteu reagent (Merck, Germany), 1.5 ml of 20% sodium carbonate (Merck, Germany) and 7.8 ml of distilled water. The solution was mixed, allowed to stand for 2 h and the absorbance was measured at 760 nm using a UV–Vis spectrophotometer (UV-1240, Shimadzu, Kyoto, Japan). The total phenolic content was calculated as mg of gallic acid equivalents (GAE)/g dry weight. 2.5. Separation of tannin and non-tannin fraction in the extracts The tannin (TT) and non-tannin (NT) fractions in meadowsweet and willow extracts were separated using cinchonine (Sigma Aldrich, MO, USA) precipitation according to Peri and Pompei (1971). The NT and TT fractions were further separated using formaldehyde solution (Sigma Aldrich, MO, USA) containing 0.5 ml of HCl (10%) to yield simple phenols and hydrolysable tannins respectively. After separation all fractions were quantified using the Folin–Ciocalteu procedure. The content of condensed tannins and flavonoids was calculated by difference. 2.6. Analysis of bioactive compounds Meadowsweet and willow extracts were hydrolysed using a modification of the method by Hertog, Hollman, and Venema (1992). Briefly, 4.5 ml of meadowsweet extract, 4.5 ml of methanol and 1 ml of HCl (38%) were mixed and heated at 90 C under reflux for 2 h. After heating, the samples were cooled in an ice-bath and then filtered through Whatman no. 1 filter paper. Additional filtration was done through a 0.2 mm membrane filter (Pall Life Sciences, UK) and 10 ml was injected directly onto the HPLC column. HPLC separation was carried out using an Agilent 1200 HPLC system (Agilent Technologies, Palo Alto, CA) in combination with an Agilent ZORBAX Eclipse XDB-C18 (150 mm 4.6 mm i.d.; 5 mm, particle size) column with a C18 guard column (Phenomenex, Cheshire, UK). The solvents used were (A) 0.025 M phosphoric acid and (B) acetonitrile. The separations were performed at 30 C by gradient elution at a flow rate of 1 ml/min. UV detection was set at 210 nm. The following gradient was used: 0–15 min, from 20 to 40% B; 15–20 min, 20% B. Identification of quercetin and salicylic acid was based on retention times by comparison with a commercial standard. The amount of quercetin and salicylic acid measured represents the total amount of quercetin derivatives and salicylic acid derivatives. The salicin content in willow extracts was separated from nonhydrolysed extracts and quantified using similar HPLC conditions as those discussed above. Salicin standard (Sigma Aldrich) was used to prepare a standard curve. The following gradient was used: 0–33 min, from 5 to 25% B; 33–37 min, 5% B.
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0.30
1.0
0.25
0.8
0.20
0.6
0.15
0.4
0.05
0.2 0.0
70°C
0.00
0
500
2.8. Statistical analysis
3. Results and discussion 3.1. Drying behaviour of meadowsweet and willow The initial moisture content of willow bark and aerial parts of meadowsweet was 61.0 0.2% and 61 1% (wet basis) respectively. To predict the drying kinetics the Lewis model was used, as previous studies (Doymaz et al., 2006; Zanoelo, di Celso, & Kaskantzis, 2007) indicated that this single parameter model allowed the kinetic parameters to be easily determined and it showed a good data fit for the drying of herbs. This is in agreement with the results found in the present study as the Lewis model showed a very good fit between experimental and calculated data (r2 > 0.99) (Fig. 1). Although, the simplistic Lewis model may not give the most accurate approximation of the drying rate (k), use of different models does not seem to affect the k values considerably, as shown by Doymaz et al. (2006). Drying curves of meadowsweet and willow dried at 30 C using both an oven and tray drier are presented in Fig. 1. Drying curves of the air-dried samples at room temperature, oven and tray-dried at 70 C followed the same trend (results not shown). It is clear that for both meadowsweet and
1.8
Moisture content (d.b.)
1000
1500
Time (min)
A one-way analysis of variance (ANOVA) and Tukey’s pair wise comparisons were used to determine significant differences between the various drying treatments. All extractions for each treatment were carried out in triplicate. SAS 9.1.3 was used for analyses (SAS Institute, Cary, NC, USA). Mean values represented by the same letters were considered not significantly different at p 0.05.
1.6
Fig. 2. Influence of oven drying temperature (30 C (squares) and 70 C (diamonds)) on the evolution of dying rate as a function of time on meadowsweet (closed symbols) and willow (open symbols). The left-side y-axis represents the drying rate at 70 C and the right-side y-axis represents the drying rate at 30 C.
willow the moisture content reaches a minimum with drying time. The drying rate of both herbs at temperatures of 30 and 70 C is presented in Fig. 2. It is evident that there was no constant rate period in the drying of the herbs, only the falling rate period was present. In this period the drying rate is controlled by the diffusion of moisture from the interior to the surface. These results are in agreement with previous reports which have studied the drying kinetics of herbs, such as verbena (Belghit, Kouhila, & Boutaleb, 2000), dill, parsley (Doymaz et al., 2006), mint and basil (Akpinar, 2006). As shown in Table 1, the Me for both herbs was below the pharmacopoeia guidelines maxima of 12 and 11% for meadowsweet and willow, respectively (European Pharmacopoeia, 2004), indicating that all treatments lead to well dried herbs suitable for long term storage. The drying rates are also presented in Table 1. It was observed that willow bark dried at a faster rate than meadowsweet, this variation in drying rate may be attributed to the differences in the herbs’ characteristics. At the temperatures studied (30 & 70 C) the drying rate constants (k) for the tray-dried herbs were twice those of the oven-dried herbs (Table 1). In general, drying of meadowsweet and willow at 70 C resulted in significantly higher drying rate than at 30 C (Fig. 2). This has been reported in previous studies which have examined the effect of drying temperature on the drying rate of herbs (Belghit et al., 2000; Doymaz et al., 2006; Tanko et al., 2005). Overall, tray-drying medicinal herbs seem to be the preferred method of drying in comparison to traditional airdrying and oven drying as it results in shorter drying times. As indicated above, higher drying temperatures also resulted in Table 1 Drying rate constant (k) and equilibrium moisture contents (Me) of meadowsweet and willow dried using different methods.
1.4 1.2 1.0 0.8
Plant
Drying treatment
k (103 min1)
Equilibrium moisture content (%)
Meadowsweet
FD AD OD30 OD70 TD30 TD70
– 2.0 0.3 2.0 0.2 14 4 4.0 0.3 20 3
9.6 0.3 9.9 0.7 8.3 0.2 2.2 0.7 93 73
Willow
FD AD OD30 OD70 TD30 TD70
– 5.0 0.1 2.6 0.1 16 5 5.6 0.6 30 8
1.0 0.1 7.8 0.2 6.6 0.1 3.3 0.2 7.4 0.4 2.8 0.1
0.6 0.4 0.2 0.0
0.10
30°C
Drying rate (d.b./dt)
The colour of willow and meadowsweet extracts was determined using a Chroma meter CR-300 (Minolta Ltd, Milton Keynes, UK). Hunter Lab scale was used with L*, a* and b* axes expressing the lightness, redness-greenness and blueness-yellowness respectively. To ascertain the significance of changes of colour the hue angle (H ) and chroma (C*) were calculated from a* and b* colour coordinates according to McGuire (1992). Hue angle is defined as a colour wheel with red-purple at an angle of 0 , yellow at 90 , bluish-green at 180 and blue at 270 .
Drying rate (d.b./dt)
2.7. Determination of colour of the extracts
0
200
400
600
800
1000
1200
1400
1600
1800
Time (mins) Fig. 1. Moisture content (d.b.) of meadowsweet (triangles) and willow bark (circles) as a function of drying time at a temperature of 30 C. Open symbols represent tray drying (OD30) and closed symbols represent oven drying (TD30). (Note: broken lines represent the behaviour predicted by the kinetic model.)
FD: freeze-dried; AD: air-dried; OD30: oven-dried at 30 C; OD70: oven-dried at 70 C; TD30: tray-dried at 30 C; TD70: tray-dried at 70 C.
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At the drying temperatures studied, the phenolic content of meadowsweet (110 8 to 119 8 mg/g d.b.) was significantly higher than willow (62 3 to 83 1 mg/g d.b.). Overall, drying condition had no significant effect on the total phenol content in meadowsweet and willow extracts (Table 2). Overall, drying condition had no significant effect on the total phenol content in meadowsweet and willow extracts. A decrease in total phenols with increasing drying temperatures has been previously reported for willow. Julkunen-Tiitto (1985) found that an increase in temperature from 48 to 60 C caused a decrease in the total phenolic content of willow leaves. However, Du Toit and Joubert (1998) found that drying temperature (40–70 C) did not affect the total polyphenol content in honeybush tea. As the drying conditions had no significant effect on the total phenol content, the effects of these conditions on the proportion of the phenolic groups (non-tannin and tannin polyphenols) were assessed. 3.3. Effect of drying on the phenolic constituents in meadowsweet and willow bark extracts The effect of drying on the tannin and non-tannin fractions is shown in Fig. 3. It is clear that there was no significant difference between the amount of non-tannins in meadowsweet ðX ¼ 51 6 mg=g d:b:Þ and willow extracts ðX ¼ 48 8 mg=g d:b:Þ at the drying conditions studied. Although, meadowsweet extracts contained a significantly higher level of tannins than willow extracts. Meadowsweet had a higher proportion of simple phenols and hydrolysable tannins than willow, while willow extracts contained a higher proportion of flavonoids and condensed tannins (Table 2). There was no significant difference in the level of tannins or non-tannins in meadowsweet between any of the drying treatments (Fig. 3a). Furthermore, the proportion of simple phenols or hydrolysable tannins in meadowsweet extracts was not significantly affected by drying condition (Table 2). However, the proportion of flavonoids significantly decreased from 32 3% in freeze-dried material to 25 1% in meadowsweet oven-dried at 70 C. Inversely, the proportion of condensed tannins was highest in meadowsweet samples tray and oven-dried at 70 C and lowest
Phenols (mg/g GAE d.b.)
3.2. Effect of drying on the total phenolic content in meadowsweet and willow extracts
a
70 60 50 40 30 20 10 0
FD
AD
OD30
OD70
TD30
TD70
TD30
TD70
Drying Condition
b Phenols (mg/g GAE d.b.)
shorter drying times however it may lead to a loss in quality (i.e. phenols, colour) of the herbs and their subsequent extracts (Julkunen-Tiitto & Sorsa, 2001). Therefore, it is very important to study the effect of drying method on the phenolic constituents and colour of the extracts.
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70 60 50 40 30 20 10 0
FD
AD
OD30
OD70
Drying condition Fig. 3. Effect of drying condition on the tannins ( ) and non-tannins (░) of (a) meadowsweet and (b) willow bark extracts. Freeze-dried (FD), air-dried (AD), ovendried at 30 C (OD30), oven-dried at 70 C (OD70), tray-dried at 30 C (TD30), traydried at 70 C (TD70).
in meadowsweet oven-dried at 30 C. The decrease in flavonoids and increase in condensed tannins (Table 2) in these samples oven and tray-dried at 70 C could possibly be due to the polymerisation of flavonoids to condensed tannins at high temperature. It is possible that these tannins are degradation products brought about by high drying temperatures and may not have the health benefits associated with the condensed tannins synthesised by plants. In the case of willow, the drying condition did not significantly affect the amount of tannins (Fig. 3b). However, drying at 70 C resulted in a significant reduction in the amount of non-tannin phenols in comparison to traditional air-drying or oven drying at
Table 2 Phenolic groups in meadowsweet and willow extracts dried under different conditions as a percentage of the total phenols (mg/g d.b.). Plant
Drying treatment
Total phenols (mg/g d.b.)
Simple phenols (%)
Flavonoids (%)
Hydrolysable tannins (%)
Condensed tannins (%)
Meadowsweet
FD AD OD30 OD70 TD30 TD70
112 2a 119 8a 115 8a 110 6a 119 9a 110 8a
24 1a 23 1a 24 1a 25 1a 23 1a 23.2 0.3a
32 2a 30 1a 30 3ab 25 1c 31 2a 26 2bc
34 3b 37.0 0.3ab 38 3a 36 1ab 34 1b 36 3ab
10 2cd 9 1cd 8 2d 14 1ab 12 1bc 15 1a
Willow
FD AD OD30 OD70 TD30 TD70
62 3a 83 1a 78 8a 68 9a 72 4a 67 6a
14 2x 14 2x 14 2x 19 3x 16 1x 14 3x
60 6x 57 1x 58 5x 43 5y 53 7xy 49 3xy
6 3x 3 2x 6 1x 8 2x 6 4x 5 1x
20 8x 26 4x 23 3x 31 6x 25 9x 33 2x
a–c, x, y
Mean values standard deviation represented by the same letters within the same column are not significantly different at p 0.05. FD: freeze-dried; AD: air-dried; OD30: oven-dried at 30 C; OD70: oven-dried at 70 C; TD30: tray-dried at 30 C; TD70: tray-dried at 70 C.
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30 C. Further separation of tannins to hydrolysable and condensed tannins showed that they were not significantly affected by the drying condition (Table 2). Separation of non-tannins to simple phenols and flavonoids showed that the simple phenols were not affected by the drying condition but drying at 70 C resulted in a decrease in the amount of flavonoids. Therefore, this observed reduction in the non-tannins at 70 C could be solely due to the decrease in the flavonoids. Interestingly, the slight increase observed in the condensed tannins (Table 2) at 70 C which corresponds to the decrease in flavonoids at this temperature could also be an indication of the polymerisation of the flavonoids, which was observed in meadowsweet extracts. 3.4. Analysis of bioactives in meadowsweet and willow extracts 3.4.1. Effect of drying on the salicin content in willow bark extracts (non-hydrolysed extract) The amount of salicin in the dried samples ranged from 5.7 0.9 to 6.6 0.4 mg/g d.b. and indicates that the drying method or temperature did not affect the salicin content significantly. This observation is in agreement with that obtained for the simple phenols, however previous studies found that an increase in drying temperature resulted in an increase in salicin content in willow leaves (Julkunen-Tiitto & Sorsa, 2001). Overall, the amounts of salicin found agree closely with the values of Zaugg, Cefalo, and Walker (1997) who have reported 7.8 and 3.1 mg/g from two S. alba samples. Salicin was not detected in meadowsweet extracts. 3.4.2. Effect of drying on the salicylic acid content in meadowsweet and willow extracts (hydrolysed extracts) As was the case for simple phenols, drying treatment had no statistical effect on the total salicylic acid content in either meadowsweet or willow, probably due to the thermal stability of this compound. The amount of salicylic acid extracted ranged from 0.3 0.1 to 0.7 0.1 mg/g d.b. in meadowsweet and 2.3 0.3 to 2.5 0.1 mg/g d.b. in willow. At all the drying conditions studied the level of salicylic acid was significantly higher in willow than in meadowsweet extracts. 3.4.3. Effect of drying on the quercetin content in meadowsweet extracts (hydrolysed extracts) The highest amount of total quercetin was found in meadowsweet samples which were freeze-dried, although the samples oven-dried at 30 C were not significantly different. A significant decrease in total quercetin concentration was observed in samples oven-dried at 70 C (Fig. 4). This is in agreement with the results
Total Quercetin (mg/g d.b.)
3.5. Effect of drying on the colour of meadowsweet and willow extracts Table 3 shows the effect of drying on the colour of meadowsweet and willow extracts. It is clear that under all drying conditions studied meadowsweet extracts were significantly darker and redder ðXL ¼ 49 2; XH ¼ 31 7Þ than willow extracts ðXL ¼ 59 1; XH ¼ 57 7Þ. This is possibly due to the higher total phenolic content in meadowsweet extracts, which is mainly due to a higher content of tannins. Du Toit and Joubert (1998) also observed a darker extract colour in honeybush tea extracts that had a higher level of polyphenols. The drying method had a significant effect on the colour of both plant extracts. Willow samples dried at 70 C either using a tray drier or a convection oven were significantly redder (H ¼ 49 2) than those dried at lower temperatures. Meadowsweet samples which were freeze-dried or air-dried exhibited the highest hue angle (H ¼ 37), although it was not significantly different from those that were either oven or tray-dried at 30 C. Similar to willow the meadowsweet samples dried at 70 C were significantly redder that those dried using lower temperature. The drying method also affected the chroma and lightness of the willow extracts. The freeze-dried and air-dried extracts were significantly lighter (higher L*) than the oven-dried at 70 C and the chroma for the samples air-dried (8 1) and oven-dried at 70 C (8 1) was lower than for the other drying conditions. The meadowsweet samples dried at 70 C were significantly darker (lower L* value) than those air-dried. Drying treatment also had an effect on the chroma of the meadowsweet extracts. The freezedried samples had the highest chroma or saturation (13 2),
Table 3 Hue angle, chroma and lightness of meadowsweet and willow extracts.
7 6
presented above on the proportion of flavonoids (%) found in meadowsweet extracts (Table 2). Previous studies had conflicting results; therefore the effect of drying on the phenolic constituents of plants seems to be species specific. Cannac, Ferrat, Barboni, Pergent, and Pasqualini (2007) found that freeze-dried Posidonia oceanica samples had significantly lower concentration of flavonols (glycosides of quercetin) than samples oven-dried at 40 C. In contrast, Keinanen and Julkunen-Tiitto (1996) studied the effect of drying treatment on the phenolic content of Birch and found that unlike meadowsweet the content of flavonoid glycosides (which included quercetin glycosides) was significantly higher when freeze-dried or dried at temperatures of 80 C compared to samples dried at 40 C. Quercetin was also analysed in willow extracts but was not detected.
a
Plant
bc
5
ab c
4
bc
0
FD
AD
OD30
OD70
TD30
TD70
Drying Condition Fig. 4. Effect of the drying condition on the total quercetin content of meadowsweet extracts. Freeze-dried (FD), air-dried (AD), oven-dried at 30 C (OD30), oven-dried at 70 C (OD70), tray-dried at 30 C (TD30), tray-dried at 70 C (TD70).
ab
Hue angle (H ) a
Chroma (C*)
FD AD OD30 OD70 TD30 TD70
50 2 51 2a 48 1ab 46.8 0.4b 49 1ab 47.1 0.3b
37 7 37 2a 32 2ab 25 2bc 31 5abc 22 2c
13 2a 10 3ab 9 1ab 6 1bc 6 2bc 5.8 0.4c
Willow
FD AD OD30 OD70 TD30 TD70
60 0x 59 0x 59 1xy 57 1y 59 1xy 59 2xy
66 3x 62 2xy 59 2y 51 2z 60 1y 48 1z
9 1xy 8 1y 9 1xy 8 1y 9 1xy 12 3x
3
1
Lightness (L)
Meadowsweet
bc
2
Drying treatment
a–c, x–z Mean values standard deviation represented by the same letters within the same column are not significantly different at p 0.05. FD: freeze-dried; AD: airdried; OD30: oven-dried at 30 C; OD70: oven-dried at 70 C; TD30: tray-dried at 30 C; TD70: tray-dried at 70 C.
N. Harbourne et al. / LWT - Food Science and Technology 42 (2009) 1468–1473
followed by those air-dried and oven-dried at 30 C, whereas traydried samples at 30 or 70 C and oven-dried at 70 C had a slightly lower chroma. Previous results obtained in our laboratory showed that drying chamomile flowers at high temperatures (80 C) caused a significant decrease in both hue angle and chroma of the extracts in comparison to samples freeze-dried or oven-dried at low temperatures (Harbourne et al., 2009). Du Toit and Joubert (1998) have shown that the drying temperature did not have an effect on the colour of honeybush tea extracts; however the colour of fermented honeybush plant material is dark which may make it difficult to detect a change in colour of the extract. Interestingly, other authors have reported that higher drying temperatures have an effect on the colour of plant material. Julkunen-Tiitto and Sorsa (2001) noticed that willow leaves air-dried at 60 and 90 C turned to a ‘brownish’ colour in comparison to leaves air-dried and freezedried possibly due to quinone formation and decomposition of phenols. Also, Arabhosseini, Huisman, van Boxtel, and Muller (2007) found that increasing the drying temperature of tarragon from 45 to 60 C caused a decrease in the hue angle, lightness and saturation of the dried material.
4. Conclusion At all drying conditions studied willow bark had a higher drying rate than aerial parts of meadowsweet. For both herbs the drying rate increased with drying temperature and tray drying showed a higher drying rate than oven drying. Although drying at higher temperatures resulted in shorter drying times it caused a reduction in the flavonoids and resulted in redder extracts. The decrease in flavonoids and corresponding increase in condensed tannins observed could be probably due to polymerisation during high temperature drying. Freeze-drying, air-drying and oven or tray drying of both herbs at 30 C yielded extracts high in phenols, active ingredients and had a desirable colour for incorporation into a beverage with potential anti-inflammatory properties. Therefore, tray drying these medicinal herbs at low temperatures may decrease the drying time without having any major effects on the total phenols, bioactives and colour of the extracts.
Acknowledgements This work was funded by the Food Institutional Research Measure (FIRM) administered by the Department of Agriculture, Fisheries and Food, Republic of Ireland.
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References Akpinar, E. K. (2006). Mathematical modeling of thin layer drying process under open sun of some aromatic plants. Journal of Food Engineering, 77, 864–870. Arabhosseini, A., Huisman, W., van Boxtel, A., & Muller, J. (2007). Long term effects of drying conditions on the essential oil and color of tarragon leaves during storage. Journal of Food Engineering, 79, 561–566. Belghit, A., Kouhila, M., & Boutaleb, B. C. (2000). Experimental study of drying kinetics by forced convection of aromatic plants. Energy Conservation & Management, 41, 1303–1321. Blumental, M., Goldberg, A., & Brinckmann, J. (Eds.). (2000). Herbal medicine: Expanded Commission E monographs (pp. 253–256). Newton, MA: Integrative Medicine Communications, 408–412. Bruneton, J. (1995). Pharmacognosy, phytochemistry, medicinal plants. (pp. 221–223). Paris: Lavoisier Publishing. Cannac, M., Ferrat, L., Barboni, T., Pergent, G., & Pasqualini, V. (2007). The influence of tissue handling on the flavonoid content of the aquatic plant Posidonia oceanica. Journal of Chemical Ecology, 33, 1083–1088. Doymaz, I., Tugrul, N., & Pala, M. (2006). Drying characteristics of dill and parsley leaves. Journal of Food Engineering, 77, 559–565. Du Toit, J., & Joubert, E. (1998). Effect of drying conditions on the quality of honeybush tea (Cyclopia). Journal of Food Processing and Preservation, 22, 493–507. European Pharmacopoeia. (2004). Directorate for the quality of medicines. Strasbourg: Council of Europe. Fennell, C. W., Light, M. E., Sparg, S. G., Stafford, G. I., & Van Staden, J. (2004). Assessing African medicinal plants for efficacy and safety: agricultural and storage practices. Journal of Ethnopharmacology, 95, 113–121. Harbourne, N., Jacquier, J. C., & O’Riordan, D. (2009). Optimisation of the extraction and processing conditions of chamomile (Matricaria chamomilla L.) for incorporation into beverages. Food Chemistry, 115, 15–19. Hertog, M. G. L., Hollman, P. C. H., & Venema, D. P. (1992). Optimization of a quantitative HPLC determination of potentially anticarcinogenic flavonoids in vegetables and fruits. Journal of Agricultural and Food Chemistry, 40(9), 1591–1598. Julkunen-Tiitto, R. (1985). Phenolic constituents in the leaves of northern willows: methods for the analysis of certain phenolics. Journal of Agricultural and Food Chemistry, 33, 213–217. Julkunen-Tiitto, R., & Sorsa, S. (2001). Testing the effects of drying methods on willow flavonoids, tannins and salicylates. Journal of Chemical Ecology, 27, 779–789. Katan, M. B., & De Roos, N. M. (2004). Promises and problems of functional foods. Critical Reviews in Food Science and Nutrition, 44, 369–377. Keinanen, M., & Julkunen-Tiitto, R. (1996). Effect of sample preparation method on birch (Betula pendula Roth) leaf phenolics. Journal of Agricultural and Food Chemistry, 44, 2724–2727. McGuire, R. G. (1992). Reporting of objective colour measurements. HortScience, 27, 1254–1255. Peri, C., & Pompei, C. (1971). Estimation of different phenolic groups in vegetable extracts. Phytochemistry, 10, 2187–2189. Singleton, V. L., & Rossi, J. A. (1965). Colorimetry of total phenolics with phosphomolybdic–phosphotungstic acid reagents. American Journal of Enology and Viticulture, 16, 144–158. Tanko, H. M., Carrier, D. J., Soskhansanj, S., & Crowe, T. G. (2005). Drying of feverfew (Tanacetum parthenium L.). Canadian Biosystems Engineering, 47, 3.57–3.61. Verschuren, P. M. (2002). Functional foods: scientific and global perspectives. British Journal of Nutrition, 88(2), S125–S130. WHO (World Health Organization). (1998). Quality control methods for medicinal plant materials. (p. 4). Geneva: World Health Organization, ISBN 92-4-154510-0. Zanoelo, E. F., di Celso, G. M., & Kaskantzis, G. (2007). Drying kinetics of mate leaves in a packed bed dryer. Biosystems Engineering, 96, 487–494. Zaugg, S. E., Cefalo, D., & Walker, E. B. (1997). Capillary electrophoretic analysis of salicin in Salix spp. Journal of Chromatography A, 781, 487–490. Zeylstra, H. (1998). Filipendula ulmaria. British Journal of Phytotherapy, 5(1), 8–12.
Contents lists available at ScienceDirect
LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt
Effect of drying methods on the phenolic constituents of meadowsweet (Filipendula ulmaria) and willow (Salix alba) Niamh Harbourne, Eunice Marete, Jean Christophe Jacquier, Dolores O’Riordan* School of Agriculture, Food Science and Veterinary Medicine, University College Dublin, Belfield, Dublin 4, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 November 2008 Received in revised form 5 March 2009 Accepted 4 May 2009
The objective of this study was to investigate the effect of drying conditions on the phenolic constituents and colour of extracts of organically grown white willow and meadowsweet for incorporation into a functional beverage with potential anti-inflammatory properties. The herbs were freeze-dried, airdried, oven or tray-dried at 30 or 70 C. The drying kinetics of the herbs was first determined. Both drying temperature and method had a significant effect (p 0.05) on the drying rate, the samples traydried had a faster drying rate than those oven-dried. Results show that for meadowsweet and willow, freeze-drying and oven or tray drying at 30 C had no significant effect on the phenolic constituents (e.g. total phenols, salicylates, quercetin) or the colour of the extracts in comparison to traditional air-drying. Although increasing the drying temperature to 70 C resulted in an increase in the drying rate of both herbs it also led to the loss of some phenolic compounds. Also, the extracts from both herbs dried at 70 C were significantly (p 0.05) redder than the other drying methods. Therefore, tray drying these herbs at low temperatures may reduce drying time without having a significant effect on the phenolic content and colour of the extracts. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Drying Meadowsweet Willow Phenolic constituents
1. Introduction Meadowsweet (Filipendula ulmaria L.) and white willow (Salix alba) are medicinal plants indigenous to Europe. They have been traditionally used to treat various ailments due to their antipyretic, analgesic and anti-inflammatory properties (Bruneton, 1995). The efficacy of these plants is mainly due to their phenolic content, which includes salicylates. These salicylates such as salicin in willow and salicylaldehyde in meadowsweet are precursors of salicylic acid. Both meadowsweet and willow played an important role in the development of aspirin (acetylsalicylic acid) as salicylic acid was first isolated from the flowers of meadowsweet and the bark of the willow in 1838 (Blumental, Goldberg, & Brinckmann, 2000; Zeylstra, 1998). Other phenolic compounds present in both herbs include flavonoids (e.g. quercetin) and tannins. In recent years there has been an increasing consumer demand for health related food products which has led to development of novel functional beverages (Katan & De Roos, 2004; Verschuren, 2002). The high phenolic content of these plants and the consumer drive towards natural products demonstrate that these herbal
* Corresponding author. Tel.: þ353 1 7167016; fax: þ353 1 7161147. E-mail address: [email protected] (D. O’Riordan). 0023-6438/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2009.05.005
extracts would be ideal ingredients for incorporation into functional beverages with potential anti-inflammatory properties. Prior to inclusion into beverages these herbs undergo post-harvest processing, including drying, to extend their shelf-life. Previous studies have shown that preservation techniques of medicinal herbs may affect their quality (Harbourne, Jacquier, & O’Riordan, 2009; Julkunen-Tiitto & Sorsa, 2001). For a high quality extract for incorporation into a beverage the level of phenolics should be maximized, in particular the non-tannin fractions which will include the active ingredients with anti-inflammatory properties. Also, the tannin fractions should be minimized as they cause astringency, an undesirable gustative attribute. Drying is an important preservation method for plant material, as it inhibits enzymatic degradation and limits microbial growth. Ambient air-drying is the traditional technique used to preserve medicinal herbs as the low temperatures are thought to protect against degradation of the active components. However, this drying process is slow and metabolic processes may continue longer which may lead to quality loss of the plants and subsequently to the extracts, e.g. colour changes, loss in active ingredients (Fennell, Light, Sparg, Stafford, & Van Staden, 2004; Keinanen & JulkunenTiitto, 1996). Other methods such as freeze-drying, oven drying and tray drying have been previously used to preserve medicinal herbs (Keinanen & Julkunen-Tiitto, 1996; Tanko, Carrier, Soskhansanj, &
N. Harbourne et al. / LWT - Food Science and Technology 42 (2009) 1468–1473
Crowe, 2005), but to date there is little information in the literature on the effect of these drying conditions on the drying kinetics or quality (e.g. colour, phenolic content) of meadowsweet and willow extracts. The purpose of this study was to investigate if drying temperature or method could have an influence on the extract quality, for example the shorter drying times that may be associated with tray drying could have a positive influence. Therefore, the objectives of this study were two-fold, firstly to determine the drying kinetics of meadowsweet and willow. Secondly, to investigate and compare the effect of various drying conditions on the total phenolic content, tannin and non-tannin phenols, bioactive constituents (salicin, salicylic acid and quercetin) and the colour of meadowsweet and willow extracts for possible inclusion into a beverage with potential anti-inflammatory properties. 2. Materials and methods 2.1. Plant material Meadowsweet and willow were organically grown and harvested in Strokestown, Co. Roscommon, Ireland. Debarking of willow stems from young branches was carried out within 48 h of harvesting and the drying procedures for aerial parts (mixture of flowers, stems and leaves) of meadowsweet were set up on the day of harvest. The stems of meadowsweet were approximately 1–3 mm in diameter and the barks of willow were <2 mm in thickness. Initial moisture content was determined by drying approximately 5.0 g of the plant material in the oven (Model No. FD 115/E2, Binder, Germany) at 102 C until a constant weight was achieved. 2.2. Drying The herbs were dried using the following treatments (1) prefreezing in liquid nitrogen followed by freeze-drying (FD) (Edwards Super Modulyo freeze-drier, Sussex, UK), (2) air-drying at ambient temperature (25 C) (AD), (3) drying in a convection oven (Model No. FD 115/E2, Binder, Germany) at 30 C (OD30), (4) drying in a convection oven at 70 C (OD70), (5) drying in a tray drier (Model No. U0P8, Armfield, England) at 30 C (TD30) and (6) at 70 C (TD70) at an air velocity of 0.97 0.03 m/s, which is higher than that in the oven. The herbs were distributed uniformly in a single layer on trays measuring 27 21 cm for oven and air-drying and on trays measuring 28 19 cm in the tray drier. Moisture loss of the air-dried, tray-dried and oven-dried samples was recorded at time intervals and the drying experiments were conducted in triplicate. All the samples were dried to equilibrium moisture content. To evaluate the effect of drying on meadowsweet and willow extracts, the freeze-dried samples were taken as the control. The dried samples were then ground into a moderately fine powder (WHO, 1998) using a lab mill with a sieve size of 3 mm (Christy and Norris Ltd, UK). The Lewis equation (Equation (1)) was used to describe the drying model of the herbs and the drying rate was calculated according to Equation (2) (Doymaz, Tugrul, & Pala, 2006; Tanko et al., 2005).
M Me ¼ expðktÞ Mo Me Drying rate ¼
Mtþdt Mt dt
(1)
(2)
where M, Me, Mo, Mt, Mtþdt, are the moisture content, equilibrium moisture content, initial moisture content, moisture content at t
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and moisture content at t þ dt (g moisture/g dry basis) respectively. k is the drying constant and t is the time in minutes. 2.3. Extraction The ground plant material (2.5 g) was put in a Duran flask containing 100 ml of heated distilled water at 100 C for 20 min using a stirring hot plate (IkaÒ WERKE GmbH and Co., Germany). Extracts were filtered under vacuum through Whatman no. 1 filter paper (Whatman Ltd, England) and cooled immediately on ice. 2.4. Quantification of the total phenolic content in plant extracts The total phenolic content in the extracts was carried out according to the Folin–Ciocalteu method (Singleton & Rossi, 1965). The reaction mixture was composed of 0.2 ml standard/extract, 0.5 ml Folin–Ciocalteu reagent (Merck, Germany), 1.5 ml of 20% sodium carbonate (Merck, Germany) and 7.8 ml of distilled water. The solution was mixed, allowed to stand for 2 h and the absorbance was measured at 760 nm using a UV–Vis spectrophotometer (UV-1240, Shimadzu, Kyoto, Japan). The total phenolic content was calculated as mg of gallic acid equivalents (GAE)/g dry weight. 2.5. Separation of tannin and non-tannin fraction in the extracts The tannin (TT) and non-tannin (NT) fractions in meadowsweet and willow extracts were separated using cinchonine (Sigma Aldrich, MO, USA) precipitation according to Peri and Pompei (1971). The NT and TT fractions were further separated using formaldehyde solution (Sigma Aldrich, MO, USA) containing 0.5 ml of HCl (10%) to yield simple phenols and hydrolysable tannins respectively. After separation all fractions were quantified using the Folin–Ciocalteu procedure. The content of condensed tannins and flavonoids was calculated by difference. 2.6. Analysis of bioactive compounds Meadowsweet and willow extracts were hydrolysed using a modification of the method by Hertog, Hollman, and Venema (1992). Briefly, 4.5 ml of meadowsweet extract, 4.5 ml of methanol and 1 ml of HCl (38%) were mixed and heated at 90 C under reflux for 2 h. After heating, the samples were cooled in an ice-bath and then filtered through Whatman no. 1 filter paper. Additional filtration was done through a 0.2 mm membrane filter (Pall Life Sciences, UK) and 10 ml was injected directly onto the HPLC column. HPLC separation was carried out using an Agilent 1200 HPLC system (Agilent Technologies, Palo Alto, CA) in combination with an Agilent ZORBAX Eclipse XDB-C18 (150 mm 4.6 mm i.d.; 5 mm, particle size) column with a C18 guard column (Phenomenex, Cheshire, UK). The solvents used were (A) 0.025 M phosphoric acid and (B) acetonitrile. The separations were performed at 30 C by gradient elution at a flow rate of 1 ml/min. UV detection was set at 210 nm. The following gradient was used: 0–15 min, from 20 to 40% B; 15–20 min, 20% B. Identification of quercetin and salicylic acid was based on retention times by comparison with a commercial standard. The amount of quercetin and salicylic acid measured represents the total amount of quercetin derivatives and salicylic acid derivatives. The salicin content in willow extracts was separated from nonhydrolysed extracts and quantified using similar HPLC conditions as those discussed above. Salicin standard (Sigma Aldrich) was used to prepare a standard curve. The following gradient was used: 0–33 min, from 5 to 25% B; 33–37 min, 5% B.
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N. Harbourne et al. / LWT - Food Science and Technology 42 (2009) 1468–1473 1.2
0.30
1.0
0.25
0.8
0.20
0.6
0.15
0.4
0.05
0.2 0.0
70°C
0.00
0
500
2.8. Statistical analysis
3. Results and discussion 3.1. Drying behaviour of meadowsweet and willow The initial moisture content of willow bark and aerial parts of meadowsweet was 61.0 0.2% and 61 1% (wet basis) respectively. To predict the drying kinetics the Lewis model was used, as previous studies (Doymaz et al., 2006; Zanoelo, di Celso, & Kaskantzis, 2007) indicated that this single parameter model allowed the kinetic parameters to be easily determined and it showed a good data fit for the drying of herbs. This is in agreement with the results found in the present study as the Lewis model showed a very good fit between experimental and calculated data (r2 > 0.99) (Fig. 1). Although, the simplistic Lewis model may not give the most accurate approximation of the drying rate (k), use of different models does not seem to affect the k values considerably, as shown by Doymaz et al. (2006). Drying curves of meadowsweet and willow dried at 30 C using both an oven and tray drier are presented in Fig. 1. Drying curves of the air-dried samples at room temperature, oven and tray-dried at 70 C followed the same trend (results not shown). It is clear that for both meadowsweet and
1.8
Moisture content (d.b.)
1000
1500
Time (min)
A one-way analysis of variance (ANOVA) and Tukey’s pair wise comparisons were used to determine significant differences between the various drying treatments. All extractions for each treatment were carried out in triplicate. SAS 9.1.3 was used for analyses (SAS Institute, Cary, NC, USA). Mean values represented by the same letters were considered not significantly different at p 0.05.
1.6
Fig. 2. Influence of oven drying temperature (30 C (squares) and 70 C (diamonds)) on the evolution of dying rate as a function of time on meadowsweet (closed symbols) and willow (open symbols). The left-side y-axis represents the drying rate at 70 C and the right-side y-axis represents the drying rate at 30 C.
willow the moisture content reaches a minimum with drying time. The drying rate of both herbs at temperatures of 30 and 70 C is presented in Fig. 2. It is evident that there was no constant rate period in the drying of the herbs, only the falling rate period was present. In this period the drying rate is controlled by the diffusion of moisture from the interior to the surface. These results are in agreement with previous reports which have studied the drying kinetics of herbs, such as verbena (Belghit, Kouhila, & Boutaleb, 2000), dill, parsley (Doymaz et al., 2006), mint and basil (Akpinar, 2006). As shown in Table 1, the Me for both herbs was below the pharmacopoeia guidelines maxima of 12 and 11% for meadowsweet and willow, respectively (European Pharmacopoeia, 2004), indicating that all treatments lead to well dried herbs suitable for long term storage. The drying rates are also presented in Table 1. It was observed that willow bark dried at a faster rate than meadowsweet, this variation in drying rate may be attributed to the differences in the herbs’ characteristics. At the temperatures studied (30 & 70 C) the drying rate constants (k) for the tray-dried herbs were twice those of the oven-dried herbs (Table 1). In general, drying of meadowsweet and willow at 70 C resulted in significantly higher drying rate than at 30 C (Fig. 2). This has been reported in previous studies which have examined the effect of drying temperature on the drying rate of herbs (Belghit et al., 2000; Doymaz et al., 2006; Tanko et al., 2005). Overall, tray-drying medicinal herbs seem to be the preferred method of drying in comparison to traditional airdrying and oven drying as it results in shorter drying times. As indicated above, higher drying temperatures also resulted in Table 1 Drying rate constant (k) and equilibrium moisture contents (Me) of meadowsweet and willow dried using different methods.
1.4 1.2 1.0 0.8
Plant
Drying treatment
k (103 min1)
Equilibrium moisture content (%)
Meadowsweet
FD AD OD30 OD70 TD30 TD70
– 2.0 0.3 2.0 0.2 14 4 4.0 0.3 20 3
9.6 0.3 9.9 0.7 8.3 0.2 2.2 0.7 93 73
Willow
FD AD OD30 OD70 TD30 TD70
– 5.0 0.1 2.6 0.1 16 5 5.6 0.6 30 8
1.0 0.1 7.8 0.2 6.6 0.1 3.3 0.2 7.4 0.4 2.8 0.1
0.6 0.4 0.2 0.0
0.10
30°C
Drying rate (d.b./dt)
The colour of willow and meadowsweet extracts was determined using a Chroma meter CR-300 (Minolta Ltd, Milton Keynes, UK). Hunter Lab scale was used with L*, a* and b* axes expressing the lightness, redness-greenness and blueness-yellowness respectively. To ascertain the significance of changes of colour the hue angle (H ) and chroma (C*) were calculated from a* and b* colour coordinates according to McGuire (1992). Hue angle is defined as a colour wheel with red-purple at an angle of 0 , yellow at 90 , bluish-green at 180 and blue at 270 .
Drying rate (d.b./dt)
2.7. Determination of colour of the extracts
0
200
400
600
800
1000
1200
1400
1600
1800
Time (mins) Fig. 1. Moisture content (d.b.) of meadowsweet (triangles) and willow bark (circles) as a function of drying time at a temperature of 30 C. Open symbols represent tray drying (OD30) and closed symbols represent oven drying (TD30). (Note: broken lines represent the behaviour predicted by the kinetic model.)
FD: freeze-dried; AD: air-dried; OD30: oven-dried at 30 C; OD70: oven-dried at 70 C; TD30: tray-dried at 30 C; TD70: tray-dried at 70 C.
N. Harbourne et al. / LWT - Food Science and Technology 42 (2009) 1468–1473
At the drying temperatures studied, the phenolic content of meadowsweet (110 8 to 119 8 mg/g d.b.) was significantly higher than willow (62 3 to 83 1 mg/g d.b.). Overall, drying condition had no significant effect on the total phenol content in meadowsweet and willow extracts (Table 2). Overall, drying condition had no significant effect on the total phenol content in meadowsweet and willow extracts. A decrease in total phenols with increasing drying temperatures has been previously reported for willow. Julkunen-Tiitto (1985) found that an increase in temperature from 48 to 60 C caused a decrease in the total phenolic content of willow leaves. However, Du Toit and Joubert (1998) found that drying temperature (40–70 C) did not affect the total polyphenol content in honeybush tea. As the drying conditions had no significant effect on the total phenol content, the effects of these conditions on the proportion of the phenolic groups (non-tannin and tannin polyphenols) were assessed. 3.3. Effect of drying on the phenolic constituents in meadowsweet and willow bark extracts The effect of drying on the tannin and non-tannin fractions is shown in Fig. 3. It is clear that there was no significant difference between the amount of non-tannins in meadowsweet ðX ¼ 51 6 mg=g d:b:Þ and willow extracts ðX ¼ 48 8 mg=g d:b:Þ at the drying conditions studied. Although, meadowsweet extracts contained a significantly higher level of tannins than willow extracts. Meadowsweet had a higher proportion of simple phenols and hydrolysable tannins than willow, while willow extracts contained a higher proportion of flavonoids and condensed tannins (Table 2). There was no significant difference in the level of tannins or non-tannins in meadowsweet between any of the drying treatments (Fig. 3a). Furthermore, the proportion of simple phenols or hydrolysable tannins in meadowsweet extracts was not significantly affected by drying condition (Table 2). However, the proportion of flavonoids significantly decreased from 32 3% in freeze-dried material to 25 1% in meadowsweet oven-dried at 70 C. Inversely, the proportion of condensed tannins was highest in meadowsweet samples tray and oven-dried at 70 C and lowest
Phenols (mg/g GAE d.b.)
3.2. Effect of drying on the total phenolic content in meadowsweet and willow extracts
a
70 60 50 40 30 20 10 0
FD
AD
OD30
OD70
TD30
TD70
TD30
TD70
Drying Condition
b Phenols (mg/g GAE d.b.)
shorter drying times however it may lead to a loss in quality (i.e. phenols, colour) of the herbs and their subsequent extracts (Julkunen-Tiitto & Sorsa, 2001). Therefore, it is very important to study the effect of drying method on the phenolic constituents and colour of the extracts.
1471
70 60 50 40 30 20 10 0
FD
AD
OD30
OD70
Drying condition Fig. 3. Effect of drying condition on the tannins ( ) and non-tannins (░) of (a) meadowsweet and (b) willow bark extracts. Freeze-dried (FD), air-dried (AD), ovendried at 30 C (OD30), oven-dried at 70 C (OD70), tray-dried at 30 C (TD30), traydried at 70 C (TD70).
in meadowsweet oven-dried at 30 C. The decrease in flavonoids and increase in condensed tannins (Table 2) in these samples oven and tray-dried at 70 C could possibly be due to the polymerisation of flavonoids to condensed tannins at high temperature. It is possible that these tannins are degradation products brought about by high drying temperatures and may not have the health benefits associated with the condensed tannins synthesised by plants. In the case of willow, the drying condition did not significantly affect the amount of tannins (Fig. 3b). However, drying at 70 C resulted in a significant reduction in the amount of non-tannin phenols in comparison to traditional air-drying or oven drying at
Table 2 Phenolic groups in meadowsweet and willow extracts dried under different conditions as a percentage of the total phenols (mg/g d.b.). Plant
Drying treatment
Total phenols (mg/g d.b.)
Simple phenols (%)
Flavonoids (%)
Hydrolysable tannins (%)
Condensed tannins (%)
Meadowsweet
FD AD OD30 OD70 TD30 TD70
112 2a 119 8a 115 8a 110 6a 119 9a 110 8a
24 1a 23 1a 24 1a 25 1a 23 1a 23.2 0.3a
32 2a 30 1a 30 3ab 25 1c 31 2a 26 2bc
34 3b 37.0 0.3ab 38 3a 36 1ab 34 1b 36 3ab
10 2cd 9 1cd 8 2d 14 1ab 12 1bc 15 1a
Willow
FD AD OD30 OD70 TD30 TD70
62 3a 83 1a 78 8a 68 9a 72 4a 67 6a
14 2x 14 2x 14 2x 19 3x 16 1x 14 3x
60 6x 57 1x 58 5x 43 5y 53 7xy 49 3xy
6 3x 3 2x 6 1x 8 2x 6 4x 5 1x
20 8x 26 4x 23 3x 31 6x 25 9x 33 2x
a–c, x, y
Mean values standard deviation represented by the same letters within the same column are not significantly different at p 0.05. FD: freeze-dried; AD: air-dried; OD30: oven-dried at 30 C; OD70: oven-dried at 70 C; TD30: tray-dried at 30 C; TD70: tray-dried at 70 C.
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30 C. Further separation of tannins to hydrolysable and condensed tannins showed that they were not significantly affected by the drying condition (Table 2). Separation of non-tannins to simple phenols and flavonoids showed that the simple phenols were not affected by the drying condition but drying at 70 C resulted in a decrease in the amount of flavonoids. Therefore, this observed reduction in the non-tannins at 70 C could be solely due to the decrease in the flavonoids. Interestingly, the slight increase observed in the condensed tannins (Table 2) at 70 C which corresponds to the decrease in flavonoids at this temperature could also be an indication of the polymerisation of the flavonoids, which was observed in meadowsweet extracts. 3.4. Analysis of bioactives in meadowsweet and willow extracts 3.4.1. Effect of drying on the salicin content in willow bark extracts (non-hydrolysed extract) The amount of salicin in the dried samples ranged from 5.7 0.9 to 6.6 0.4 mg/g d.b. and indicates that the drying method or temperature did not affect the salicin content significantly. This observation is in agreement with that obtained for the simple phenols, however previous studies found that an increase in drying temperature resulted in an increase in salicin content in willow leaves (Julkunen-Tiitto & Sorsa, 2001). Overall, the amounts of salicin found agree closely with the values of Zaugg, Cefalo, and Walker (1997) who have reported 7.8 and 3.1 mg/g from two S. alba samples. Salicin was not detected in meadowsweet extracts. 3.4.2. Effect of drying on the salicylic acid content in meadowsweet and willow extracts (hydrolysed extracts) As was the case for simple phenols, drying treatment had no statistical effect on the total salicylic acid content in either meadowsweet or willow, probably due to the thermal stability of this compound. The amount of salicylic acid extracted ranged from 0.3 0.1 to 0.7 0.1 mg/g d.b. in meadowsweet and 2.3 0.3 to 2.5 0.1 mg/g d.b. in willow. At all the drying conditions studied the level of salicylic acid was significantly higher in willow than in meadowsweet extracts. 3.4.3. Effect of drying on the quercetin content in meadowsweet extracts (hydrolysed extracts) The highest amount of total quercetin was found in meadowsweet samples which were freeze-dried, although the samples oven-dried at 30 C were not significantly different. A significant decrease in total quercetin concentration was observed in samples oven-dried at 70 C (Fig. 4). This is in agreement with the results
Total Quercetin (mg/g d.b.)
3.5. Effect of drying on the colour of meadowsweet and willow extracts Table 3 shows the effect of drying on the colour of meadowsweet and willow extracts. It is clear that under all drying conditions studied meadowsweet extracts were significantly darker and redder ðXL ¼ 49 2; XH ¼ 31 7Þ than willow extracts ðXL ¼ 59 1; XH ¼ 57 7Þ. This is possibly due to the higher total phenolic content in meadowsweet extracts, which is mainly due to a higher content of tannins. Du Toit and Joubert (1998) also observed a darker extract colour in honeybush tea extracts that had a higher level of polyphenols. The drying method had a significant effect on the colour of both plant extracts. Willow samples dried at 70 C either using a tray drier or a convection oven were significantly redder (H ¼ 49 2) than those dried at lower temperatures. Meadowsweet samples which were freeze-dried or air-dried exhibited the highest hue angle (H ¼ 37), although it was not significantly different from those that were either oven or tray-dried at 30 C. Similar to willow the meadowsweet samples dried at 70 C were significantly redder that those dried using lower temperature. The drying method also affected the chroma and lightness of the willow extracts. The freeze-dried and air-dried extracts were significantly lighter (higher L*) than the oven-dried at 70 C and the chroma for the samples air-dried (8 1) and oven-dried at 70 C (8 1) was lower than for the other drying conditions. The meadowsweet samples dried at 70 C were significantly darker (lower L* value) than those air-dried. Drying treatment also had an effect on the chroma of the meadowsweet extracts. The freezedried samples had the highest chroma or saturation (13 2),
Table 3 Hue angle, chroma and lightness of meadowsweet and willow extracts.
7 6
presented above on the proportion of flavonoids (%) found in meadowsweet extracts (Table 2). Previous studies had conflicting results; therefore the effect of drying on the phenolic constituents of plants seems to be species specific. Cannac, Ferrat, Barboni, Pergent, and Pasqualini (2007) found that freeze-dried Posidonia oceanica samples had significantly lower concentration of flavonols (glycosides of quercetin) than samples oven-dried at 40 C. In contrast, Keinanen and Julkunen-Tiitto (1996) studied the effect of drying treatment on the phenolic content of Birch and found that unlike meadowsweet the content of flavonoid glycosides (which included quercetin glycosides) was significantly higher when freeze-dried or dried at temperatures of 80 C compared to samples dried at 40 C. Quercetin was also analysed in willow extracts but was not detected.
a
Plant
bc
5
ab c
4
bc
0
FD
AD
OD30
OD70
TD30
TD70
Drying Condition Fig. 4. Effect of the drying condition on the total quercetin content of meadowsweet extracts. Freeze-dried (FD), air-dried (AD), oven-dried at 30 C (OD30), oven-dried at 70 C (OD70), tray-dried at 30 C (TD30), tray-dried at 70 C (TD70).
ab
Hue angle (H ) a
Chroma (C*)
FD AD OD30 OD70 TD30 TD70
50 2 51 2a 48 1ab 46.8 0.4b 49 1ab 47.1 0.3b
37 7 37 2a 32 2ab 25 2bc 31 5abc 22 2c
13 2a 10 3ab 9 1ab 6 1bc 6 2bc 5.8 0.4c
Willow
FD AD OD30 OD70 TD30 TD70
60 0x 59 0x 59 1xy 57 1y 59 1xy 59 2xy
66 3x 62 2xy 59 2y 51 2z 60 1y 48 1z
9 1xy 8 1y 9 1xy 8 1y 9 1xy 12 3x
3
1
Lightness (L)
Meadowsweet
bc
2
Drying treatment
a–c, x–z Mean values standard deviation represented by the same letters within the same column are not significantly different at p 0.05. FD: freeze-dried; AD: airdried; OD30: oven-dried at 30 C; OD70: oven-dried at 70 C; TD30: tray-dried at 30 C; TD70: tray-dried at 70 C.
N. Harbourne et al. / LWT - Food Science and Technology 42 (2009) 1468–1473
followed by those air-dried and oven-dried at 30 C, whereas traydried samples at 30 or 70 C and oven-dried at 70 C had a slightly lower chroma. Previous results obtained in our laboratory showed that drying chamomile flowers at high temperatures (80 C) caused a significant decrease in both hue angle and chroma of the extracts in comparison to samples freeze-dried or oven-dried at low temperatures (Harbourne et al., 2009). Du Toit and Joubert (1998) have shown that the drying temperature did not have an effect on the colour of honeybush tea extracts; however the colour of fermented honeybush plant material is dark which may make it difficult to detect a change in colour of the extract. Interestingly, other authors have reported that higher drying temperatures have an effect on the colour of plant material. Julkunen-Tiitto and Sorsa (2001) noticed that willow leaves air-dried at 60 and 90 C turned to a ‘brownish’ colour in comparison to leaves air-dried and freezedried possibly due to quinone formation and decomposition of phenols. Also, Arabhosseini, Huisman, van Boxtel, and Muller (2007) found that increasing the drying temperature of tarragon from 45 to 60 C caused a decrease in the hue angle, lightness and saturation of the dried material.
4. Conclusion At all drying conditions studied willow bark had a higher drying rate than aerial parts of meadowsweet. For both herbs the drying rate increased with drying temperature and tray drying showed a higher drying rate than oven drying. Although drying at higher temperatures resulted in shorter drying times it caused a reduction in the flavonoids and resulted in redder extracts. The decrease in flavonoids and corresponding increase in condensed tannins observed could be probably due to polymerisation during high temperature drying. Freeze-drying, air-drying and oven or tray drying of both herbs at 30 C yielded extracts high in phenols, active ingredients and had a desirable colour for incorporation into a beverage with potential anti-inflammatory properties. Therefore, tray drying these medicinal herbs at low temperatures may decrease the drying time without having any major effects on the total phenols, bioactives and colour of the extracts.
Acknowledgements This work was funded by the Food Institutional Research Measure (FIRM) administered by the Department of Agriculture, Fisheries and Food, Republic of Ireland.
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