Research Article Received: 5 December 2014
Revised: 26 May 2015
Accepted article published: 25 June 2015
Published online in Wiley Online Library: 17 July 2015
(wileyonlinelibrary.com) DOI 10.1002/jsfa.7317
Comparison of different drying methods on the physical properties, bioactive compounds and antioxidant activity of raspberry powders Xu Si,† Qinqin Chen,† Jinfeng Bi,* Xinye Wu, Jianyong Yi, Linyan Zhou and Zhaolu Li Abstract BACKGROUND: Dehydration has been considered as one of the traditional but most effective techniques for perishable fruits. Raspberry powders obtained after dehydration can be added as ingredients into food formulations such as bakery and dairy products. In this study, raspberry powders obtained by hot air drying (HAD), infrared radiation drying (IRD), hot air and explosion puffing drying (HA-EPD), infrared radiation and microwave vacuum drying (IR-MVD) and freeze drying (FD) were compared on physical properties, bioactive compounds and antioxidant activity. RESULTS: Drying techniques affected the physical properties, bioactive compounds and antioxidant activity of raspberry powders greatly. FD led to significantly higher (P < 0.05) values of water solubility (45.26%), soluble solid (63.46%), hygroscopicity (18.06%), color parameters and anthocyanin retention (60.70%) of raspberry powder compared with other drying methods. However, thermal drying techniques, especially combined drying methods, were superior to FD in final total polyphenol content, total flavonoid content and antioxidant activity. The combined drying methods, especially IR-MVD, showed the highest total polyphenol content (123.22 g GAE kg−1 dw) and total flavonoid content (0.30 g CAE kg−1 dw). Additionally, IR-MVD performed better in antioxidant activity retention. CONCLUSION: Overall, combined drying methods, especially IR-MVD, were found to result in better quality of raspberry powders among the thermal drying techniques. IR-MVD could be recommended for use in the drying industry because of its advantages in time saving and nutrient retention. © 2015 Society of Chemical Industry Keywords: raspberry powder; single drying; combined drying; physiochemical quality; antioxidant activity
INTRODUCTION
J Sci Food Agric 2016; 96: 2055–2062
∗
Correspondence to: Jinfeng Bi, Institute of Agro-products Processing Science and Technology, Chinese Academy of Agricultural Sciences/Key Laboratory of Agro-products Processing, Ministry of Agriculture, Beijing 100193, China. E-mail: [email protected]
† These authors contributed equally to the present paper Institute of Agro-products Processing Science and Technology, Chinese Academy of Agricultural Sciences/Key Laboratory of Agro-products Processing, Ministry of Agriculture, Beijing 100193, China
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Raspberry (Rubus idaeus), a member of the Rosaceae family, has attracted great interest owing to not only its good flavor and attractive color but also its abundance of bioactive compounds that have been proved to have beneficial effects on health. Bioactive compounds such as polyphenols, anthocyanins, flavonoids and ellagic acids in raspberries play important roles in antioxidant activity and inhibition effects on obesity, cancer, inflammation, neural degeneration and other diseases.1 – 4 Raspberry is a unique fruit because of its specific flavor and high nutrition value. However, raspberries can only be stored at 0 ∘ C for a few days because of their high moisture content and vulnerable texture.5 Except for fresh and frozen fruits, most raspberries are processed into products such as juices, jams, jellies, wines and freeze-dried fruits.6 Dehydration has been considered as one of the traditional but most effective techniques for perishable fruits, facilitating transportation, storage and product diversification. Raspberry powders obtained after dehydration can be added as ingredients into food formulations such as bakery and dairy products. In recent years, the most common drying techniques applied to raspberries have been convective drying,7 microwave vacuum drying,8,9 freeze drying10,11 and spray drying.12 Although spray drying is commonly
used in raspberry powder production, the drying agent (e.g. maltodextrin) lowers the purity of the powder. In addition, infrared radiation drying, with advantages of low energy consumption, time saving and high efficiency,13 is increasingly being applied in the dehydration of fruits and vegetables. Explosion puffing drying is a relatively new drying technique capable of creating a porous structure and saving on drying time and energy.14 In general, drying processes lead to alterations in the quantity and quality of bioactive compounds. Thus the effects of different drying techniques, including single (hot air drying, infrared radiation drying and freeze drying) and
www.soci.org combined (hot air and explosion puffing drying and infrared radiation and microwave vacuum drying) methods, on the physical properties, bioactive compounds and antioxidant activity of raspberry powders were investigated.
MATERIALS AND METHODS Materials Fruit samples Frozen raspberries (Heritage) from Jinsiwei Agriculture and Food Development Limited Company (Henan, China) were transported by cold chain and stored at −20 ∘ C before the experiments. Reagents CH3 COONa, K2 S2 O8 , NaCl, KCl, AlCl3 , FeSO4 , FeCl3 , Na2 CO3 , NaNO2 , NaOH, ethanol, methanol and acetic acid, all of analytical grade, were purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing, China). Folin–Ciocalteu’s phenol regent (2 mol L−1 ), gallic acid monohydrate (purity ≥98.0%, high-performance liquid chromatography grade), (+)-catechin hydrate (purity ≥98.0%, reagent grade), 2,2′ -azinobis-(3-ethylbenzothiazoline-6sulfonic acid) diammonium salt (ABTS, purity ≥98%), 1,1-diphenyl2-picrylhydrazyl radical (DPPH, purity ≥99.0%), 6-hydroxy-2,5, 7,8-tetramethylchroman-2-carboxylic acid (Trolox, purity ≥99.0%) and 2,4,6-tripyridyl-1,3,5-triazine (TPTZ, purity ≥99.0%) were obtained from Sigma-Aldrich Co. LLC (Shanghai, China). Drying processes The raspberry samples were subjected to the following drying processes: hot air drying (HAD) (DHG-9123A, Jing Hong Laboratory Instrument Co., Ltd, Shanghai, China), infrared radiation drying (IRD) (STC, Senttech Infrared Technology Co., Ltd, Taizhou, China), hot air and explosion puffing drying (HA-EPD) (QDPH10-1, Qin De New Material Technology Co., Ltd, Tianjin, China), infrared radiation and microwave vacuum drying (IR-MVD) (WZD1S-04 microwave vacuum dryer, Sanle Microwave Technology Development Co., Ltd, Nanjing, China) and freeze drying (FD) (Alpha 1-4 LD plus, Marin Christ, Osterode, Germany). Crude powders were obtained by a pulverizer (JYL B060, Joyoung Co., Ltd, Shandong, China) and then micro powders were obtained by a vibration mill (KCW-10, Kunjieyucheng Machinery Co., Ltd, Beijing, China) with a pulverizing time of 5 min at room temperature. The particle size of raspberry powders was determined by a laser particle size analyzer (S3500 BWDL, Microtrac, Montgomeryville, PA, USA). The median diameter D50 (μm) was adopted to express the particle size of powders. Drying conditions for the different drying techniques are presented in Table 1.
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Extractions Fresh raspberry fruits (5 g) or raspberry powders (2 g) were mixed thoroughly with 12 mL of methanol/water (80:20 v/v) solution and placed in the dark for 16 h. The mixtures were then extracted ultrasonically (KQ-500E ultrasonic cleaner machine, Kunshan Ultrasonic Instrument Co., Ltd, Jiangsu, China) for 30 min, followed by centrifugation (Sigma 3 K15 centrifugal machine, SIGMA Laborzentrifugen GmbH, Osterode, Germany) at 9408 × g for 10 min. The supernatants were collected and stored at −20 ∘ C until future analysis.
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Quality determinations Moisture content and water activity The moisture content of fresh raspberry fruits and raspberry powders was determined using a vacuum oven at 70 ∘ C for 24 h according to AOAC method 934.06.15 The water activity (aw ) of raspberry powders was measured by a water activity meter (Aqualab Series 4TE, Decagon Devices, Inc., Pullman, WA, USA) to an accuracy of ±0.003 aw . Color The color of raspberry powders was evaluated using a colorimeter (D25L, Hunterlab Co., Ltd, Reston, VA, USA). Color was expressed as brightness (L*), redness (+a*), yellowness (+b*), chroma (C*) and hue angle (h∘ ). L*, a* and b* were obtained directly from the reading of the colorimeter. Chroma and hue angle were calculated from the following equations: ( )1∕2 C ∗ = a*2 + b*2
(1)
h∘ = tan−1 (b∗ ∕a∗ )
(2)
Color measurements were performed in triplicate. Soluble solid Raspberry powders (10 g) mixed with 100 mL of distilled water were extracted in a boiling water bath for 30 min under slight stirring with a glass bar. After cooling to room temperature, the samples were centrifuged at 847 × g for 30 min. The supernatant was collected for the determination of soluble solid using a refractometer (MASTER-𝛼 Cat. No. 2311, Atago Co., Ltd, Tokyo, Japan). This method was conducted according to Dawei et al.16 with some modifications. The soluble solid (SS) of raspberry powders was calculated by the following equation: ) ( SS (%) = p × m1 ∕m0
(3)
where p is the soluble solid of solution (%), m1 is the weight of sample after boiling (g) and m0 is the initial weight of sample (g). Water solubility index The method of Syamaladevi et al.17 with some modifications was adopted to determine the water solubility index of raspberry powders. Raspberry powders (2 g) were mixed thoroughly with 25 mL of distilled water in a 50 mL centrifuge tube. After incubation at 37 ∘ C in a water bath (DK-826, Jing Hong Laboratory Instrument Co., Ltd) for 35 min, the mixture was centrifuged at 2352 × g for 10 min at 4 ∘ C. The supernatant was then collected in a pre-weighed aluminum specimen box and dried in an oven at 105 ∘ C. The water solubility index (WSI) of raspberry powders was calculated by the following equation: ) ( ) ( WSI g kg−1 = W∕W0 × 1000
(4)
where W is the weight of dried supernatant (g) and W 0 is the weight of original powder (g). Hygroscopicity The hygroscopicity of raspberry powders was measured using the method of Gallo et al.18 Raspberry powders (∼1 g) were stored in a desiccator at room temperature and 75.29% relative humidity (created by a saturated aqueous solution of NaCl). The powders
© 2015 Society of Chemical Industry
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Table 1. Drying methods and conditions Drying methoda HAD IRD HA-EPD IR-MVD FD
Drying conditions
Dryingtime (h)
Temperature 70 ∘ C, air velocity 2.1 m s−1 Temperature 70 ∘ C, power 675 W Temperature and time of HAD 70 ∘ C and 90 min respectively; explosion puffing temperature 97 ∘ C, vacuum drying temperature 69 ∘ C, absolute pressure 5 kPa, vacuum drying time 150 min Infrared temperature 70 ∘ C (60 min), infrared power 675 W; microwave power 600 W, absolute pressure 15 kPa Temperature −56 ∘ C, vacuum pressure 0.01 kPa
9 4 4 3 36
a
HAD, hot air drying; IRD, infrared radiation drying; HA-EPD, hot air and explosion puffing drying; IR-MVD, infrared radiation and microwave vacuum drying; FD, freeze drying.
were weighed after 7 days. The hygroscopicity was expressed as HG (%) (i.e. g adsorbed moisture per 100 g dry solid) and calculated as follows: ) ( (5) HG (%) = Δm∕m0 × 100 where Δm is the weight increase of powder (g) and m0 is the initial weight of powder (g). Oil absorption index The method of Abdul-Hamid and Luan19 with some modifications was used to determine the oil absorption index of powders. Briefly, 2 g of raspberry powder (M) was mixed with 20 mL of soybean oil (V 1 ) in a 50 mL centrifuge tube. The mixture was stirred for 15 min and then centrifuged at 3000 × g for 10 min. The volume of supernatant oil (V 2 ) was measured. The oil absorption index (OAI) was calculated as follows: ) ( ) ( (6) OAI mL g−1 = V1 –V2 ∕M Total anthocyanin content The total anthocyanin content of fresh raspberries and raspberry powders was measured by a modified version of the pH differential absorbance method.20 The extracts were diluted with pH 1.0 (0.025 mol L−1 KCl) and pH 4.5 (0.4 mol L−1 sodium acetate) buffers to an appropriate dilution factor (DF). The dilutions were then allowed to equilibrate at room temperature in the dark for 15 min. The absorbance values of the dilutions at 510 and 700 nm were determined using a spectrophotometer (UV-1800, Shimadzu, Kyoto, Japan) against a blank with distilled water. The absorbance was calculated as follows: ( ) ) ( (7) A = A510 − A700 pH 1.0 − A510 − A700 pH 4.5
Total polyphenol content The Folin–Ciocalteu method was used to determine the total phenolic content of raspberry powders.20 Extracts (100 μL) were added to 6 mL of distilled water, then 500 μL of Folin–Ciocalteu reagent at a concentration of 100 g kg−1 was added and mixed. After 6 min, 1.5 mL of 200 g kg−1 Na2 CO3 solution was added to the above solution and mixed. The mixture was left to stand for 2 h, then the absorbance at 765 nm was measured using a spectrophotometer. The extract was replaced by the same amount of gallic acid standards to construct a standard curve against a blank with methanol. The total polyphenol content of powders was expressed as g gallic acid equivalent (GAE) kg−1 dry weight (g GAE kg−1 dw). Total flavonoid content The total flavonoid content of samples under five different drying methods was measured by the aluminum chloride colorimetric assay.21 A 1 mL aliquot of extract or catechin standard solution was mixed with 4 mL of distilled water in a 10 mL tube. To the tube, 0.3 mL of 50 g kg−1 NaNO2 solution and 0.3 mL of 100 g kg−1 AlCl3 solution were added. After 6 min, 2 mL of 1 mol L−1 NaOH was added, followed by distilled water to make a total volume of 10 mL. The above solution was mixed thoroughly and the absorbance at 510 nm was measured. The total flavonoid content of fresh fruits and powders was expressed as g catechin equivalent (CAE) kg−1 dry weight (g CAE kg−1 dw)
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Antioxidant capacity by ABTS, DPPH and FRAP (ferric-reducing antioxidant potential) assays Antioxidant capacity was evaluated by three methods: total antioxidant activity by the ABTS method, free radical-scavenging activity by the DPPH method and ferric-reducing ability by the FRAP The anthocyanin concentration in fresh raspberries and raspmethod. berry powders was calculated as cyanidin-3-glucoside by the folIn the ABTS assay, the ABTS radical is scavenged in proportion lowing formula: to the concentration of antioxidant compounds, leading to color variation when reaction occurs between antioxidant and radical. ( ) anthocyanin concentration mg L−1 = (A × MW × DF × 1000) The ABTS method of Re et al.22 was adopted with some modifica−1 ∕ (𝜀 × 1) (8) tions. Specifically, equal quantities of 2.45 mmol L K2 S2 O8 solu−1 tion and 7 mmol L ABTS were mixed to obtain ABTS•+ solution. The solution was allowed to react for 16 h at room temperature in the dark. Before determination, the ABTS•+ solution was diluted where MW is the molecular weight of cyanidin-3-glucoside with 800 g kg−1 ethanol to obtain an absorption of 0.70 ± 0.01 at (449.38 g mol−1 ), DF is the dilution factor and 𝜀 is the molar absorp−1 −1 734 nm. Diluted sample extracts (400 μL) were reacted with 3.6 mL tivity of cyanidin-3-glucoside (26 900 L mol cm ). Finally, the of freshly diluted ABTS•+ solution for 6 min, then the absorbance anthocyanin content of fresh fruits and powders was expressed of the mixture at 734 nm was measured using a spectrophotomeas g cyanidin-3-glucoside equivalent kg−1 sample dry weight (g kg−1 dw). ter. A standard curve was obtained using different concentrations
www.soci.org of Trolox. Results were expressed as μmol Trolox equivalent (TE) g−1 dry weight (μmol TE g−1 dw). In the DPPH assay, the donation of hydrogen from DPPH forms the stable DPPH molecule, resulting in the scavenging of DPPH radical. The DPPH assay followed the method of Brand-Williams et al.23 Diluted sample extracts (2 mL) were mixed with 4 mL of 0.1 mmol L−1 DPPH. The absorbance of the mixture at 517 nm was measured by a spectrophotometer after 30 min in the dark at room temperature. A standard curve was obtained using different concentrations of Trolox. Results were expressed as μmol TE g−1 dw. The FRAP assay measures antioxidant capacity based on the reduction of Fe3+ -TPTZ at low pH to form colored Fe2+ -TPTZ. The FRAP assay was conducted according to the method of Benzie and Strain24 with some modifications. Specifically, FRAP reagent was prepared by mixing 300 mmol L−1 acetate buffer (pH 3.6), 10 mmol L−1 TPTZ solution (acidified with 40 mmol L−1 HCl) and 20 mmol L−1 FeCl3 · 6H2 O solution in the ratio 10:1:1 (v/v/v). The mixture was warmed at 37 ∘ C before use. Diluted sample extracts (100 μL) were reacted thoroughly with 3 mL of FRAP reagent. After incubation at 37 ∘ C for 10 min, the absorbance at 593 nm was recorded. All solutions were prepared on the day they were used. A standard curve was obtained using different concentrations of Trolox. Results were expressed as μmol TE g−1 dw. Statistical analysis Determinations of the investigated indices were carried out in three replicates and the results expressed as mean ± standard deviation (SD). Differences between means were analyzed firstly by analysis of variance (ANOVA) and then by the least significant difference (LSD) test (P < 0.05) using SPSS Statistics Version 19 (IBM Corporation, Chicago, IL, USA). Figures were drawn using Origin 8.0 (OriginLab, MA, USA).
RESULTS AND DISCUSSION Physical properties Moisture content and water activity The moisture content of raspberry powders (Table 2) was found to be highly dependent on the drying method. Fresh raspberries (moisture content 6.37 g g−1 dry basis (d.b.)) were made into powders with final moisture contents of 0.099, 0.109, 0.099, 0.109 and 0.134 g g−1 d.b. by HAD, IRD, HA-EPD, IR-MVD and FD respectively. The spongy structure of the FD product caused rapid moisture absorption during pulverization, which might lead to the significantly higher moisture content compared with other methods. The following indices were expressed on a dry matter basis and calculated according to the moisture content data. Water activity measures the availability of free water in a food system that is responsible for any biochemical reactions.25 The aw of raspberry powders in this study ranged from 0.178 to 0.264 for different drying methods, which can be considered quite microbiologically stable. The effects of drying techniques on water activity are consistent with their effects on moisture content.
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Water solubility index, soluble solid, hygroscopicity and oil absorption index As can be seen in Table 2, the WSI values of raspberry powders ranged from 385.27 to 452.63 g kg−1 . FD powder presented the highest WSI, 17.48% higher than that of HAD powder which had the lowest WSI. The higher WSI of FD powder could be attributed
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to the good retention of SS during the unheated drying process. There was no significant difference in WSI between IRD and IR-MVD powders, while both were significantly higher compared with HA-EPD powder. The long drying time (up to 9 h) caused the high loss of water solubility in HAD raspberry powder.26 Moreover, it was found that the water solubility of raspberry powders decreased as the particle size decreased, in agreement with Kurozawa et al.27 The HG values of raspberry powders obtained by five different drying methods are shown in Table 2. Drying methods showed significant effects on HG of raspberry powders. The ranking of HG values of powders in descending order was FD > IRD > HA-EPD, IR-MVD > HAD. Hygroscopicity is an important evaluation criterion from which the possibility of stickiness in powders during storage and distribution can be judged.27 It is mainly connected with saccharides of powders. According to Scher et al.,28 monosaccharides such as glucose and fructose facilitate hygroscopicity. As can be seen in Table 2, drying methods showed similar effects on SS and HG. Drying methods with high temperature and/or long heating time, such as HAD and HA-EPD, led to lower SS values. This might be attributed to greater thermal loss of monosaccharides in Maillard reactions. FD raspberry powder had the highest SS (63.46%), which might be the major reason for its high HG. According to Ferrari et al.,29 hygroscopicity values of blackberry powders increased inversely with moisture content. They explained that powders with lower moisture content had greater capacity to absorb ambient moisture. However, no relationship was noted in the present study between moisture content and hygroscopicity. This was supported by the results of Ahmed et al.30 The effect of drying methods on OAI was also determined and the results are presented in Table 2. The OAI of FD powder was the highest, showing no significant difference from that of IR-MVD and IRD powders. The lowest OAI value was found for HAD raspberry powder. Color analysis The effect of drying methods on the color of raspberry powders is presented in Table 3. Significant differences in L*, a* and b* values were noted among raspberry powders. HA-EPD powder had the highest L* value, indicating the brightest character, while FD powder had the lowest L* value. FD powder showed the largest a* value (reddest color), while HAD powder presented the smallest a* value. Additionally, higher b* values were observed in powders prepared by HAD and IRD, representing greater yellowness. The separate analysis of L*, a* and b* parameters is not comprehensive enough to explain changes in color after drying. According to Abers and Wrolstad,31 the chroma value (C*) is a good illustration of the amount of color, distinguishing vivid and dull color. The lower C* value of HAD raspberry powder indicated less saturation and a duller appearance compared with powders prepared by other drying methods. FD powder presented the highest C* value. Besides, a higher hue angle indicates a redder character in the assay.32 The hue angle of FD powder was the highest, being 6.78, 8.97, 28.01 and 31.17% higher than that of powders obtained by HA-EPD, IR-MVD, IRD and HAD respectively. In general, FD, HA-EPD and IR-MVD produced raspberry powders with more redness and better color retention. Anthocyanins are easily degraded and oxidized,33 especially in HAD and IRD powders. Apart from thermal pigment degradation, Maillard reactions may also be responsible for the changes in color of raspberries after drying processes.34
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Table 2. Particle size, moisture content, water activity (aw ), soluble solid (SS), water solubility index (WSI), hygroscopicity (HG) and oil absorption index (OAI) of raspberry powders obtained by five different drying methods Drying methoda
Particle size (μm)
Moisture content (g g−1 d.b.)
HAD IRD HA-EPD IR-MVD FD
37.40 ± 2.02bc 42.96 ± 0.24b 34.79 ± 0.29c 41.80 ± 0.53b 58.71 ± 7.15a
0.099 ± 0.003c 0.109 ± 0.001b 0.099 ± 0.001c 0.109 ± 0.005b 0.134 ± 0.002a
aw
WSI (g kg−1 )
SS (%)
0.178 ± 0.004c 0.198 ± 0.004b 0.183 ± 0.006c 0.206 ± 0.007b 0.264 ± 0.010a
55.49 ± 1.13e 61.29 ± 0.57b 57.16 ± 0.56d 59.32 ± 0.56c 63.46 ± 0.57a
OAI (mL g−1 )
HG (%)
385.27 ± 1.52d 431.80 ± 3.54b 394.08 ± 6.13c 427.70 ± 1.78b 452.63 ± 6.52a
13.45 ± 0.52d 17.31 ± 0.44b 16.01 ± 0.28c 15.85 ± 024c 18.06 ± 0.21a
1.17 ± 0.14c 1.93 ± 0.06a 1.38 ± 0.13b 2.08 ± 0.08a 2.10 ± 0.05a
Results are expressed as mean ± SD of triplicate determinations. Means in the same column with a common letter are not significantly different (P < 0.05). a HAD, hot air drying; IRD, infrared radiation drying; HA-EPD, hot air and explosion puffing drying; IR-MVD, infrared radiation and microwave vacuum drying; FD, freeze drying.
Table 3. Color parameters of raspberry powders obtained by five different drying methods Drying methoda HAD IRD HA-EPD IR-MVD FD
L* 41.18 ± 0.02b 38.83 ± 0.12d 44.10 ± 0.03a 39.88 ± 0.02c 35.26 ± 0.03e
a*
b*
27.66 ± 0.05e 28.72 ± 0.05d 29.03 ± 0.07c 30.84 ± 0.18b 33.20 ± 0.05a
h∘
C*
8.27 ± 0.02b 8.39 ± 0.01a 7.14 ± 0.03e 7.74 ± 0.03c 7.67 ± 0.01d
28.87 ± 0.05d 29.92 ± 0.05c 29.90 ± 0.06c 31.79 ± 0.16b 34.08 ± 0.05a
3.24 ± 0.01e 3.32 ± 0.00d 3.98 ± 0.02b 3.90 ± 0.04c 4.25 ± 0.00a
Results are expressed as mean ± SD of triplicate determinations. Means in the same column with a common letter are not significantly different (P < 0.05). a HAD, hot air drying; IRD, infrared radiation drying; HA-EPD, hot air and explosion puffing drying; IR-MVD, infrared radiation and microwave vacuum drying; FD, freeze drying.
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a
-1
200
150 d
c
bc
d e
100
50
0 Fresh
HAD
IRD HA-EPD Drying methods
IR-MVD
FD
Figure 1. Total polyphenol content of fresh raspberry fruits and raspberry powders obtained by five different drying methods: HAD, hot air drying; IRD, infrared radiation drying; HA-EPD, hot air and explosion puffing drying; IR-MVD, infrared radiation and microwave vacuum drying; FD, freeze drying. Means with a common letter are not significantly different (P < 0.05).
decreases in anthocyanin content of 65.42 and 65.18% respectively. According to Wojdyło et al.,37 the dried product of sour cherry obtained by freeze drying also presented the highest anthocyanin retention among three different drying techniques. No significant difference in anthocyanin retention existed between powders prepared by HA-EPD (46.73%) and IR-MVD (47.06%). Similarly to total polyphenols, the combination of two drying methods
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Total anthocyanin content The effect of drying methods on anthocyanins was investigated and the results are shown in Fig. 2. The content of anthocyanins in the investigated fresh raspberry fruits was 0.54 g kg−1 DW. The processes of drying and pulverization led to great losses of anthocyanins, ranging from 39.30 to 65.42%. Contrary to total polyphenols, the highest anthocyanin concentration (0.33 g kg−1 dw) was found in FD raspberry powder. HAD and IRD induced
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Total polyphenols (g GAE kg dw)
Total polyphenol content The contents of total polyphenols in raspberry fruits and powders are presented in Fig. 1. The total polyphenol content of raspberries decreased significantly (P < 0.05) after dehydration and pulverization. Higher polyphenol concentrations were observed in IR-MVD and HA-EPD powders, with retentions of 61.42 and 58.87% respectively, between which no significant difference was noted. However, FD led to much lower polyphenol retention compared with other drying methods. Harbourne et al.35 found that freeze drying of willow led to a lower total phenolic content than thermal drying methods such as hot air drying, oven drying and tray drying. Utilization of dehydration at low temperature caused a more severe degradation of polyphenols, which was understandable in terms of the long drying time of 36 h. Maybe drying that involved higher temperature created more destruction of the tissue, which in turn allowed more extraction of phenolic compounds or caused changes in other compounds, resulting in more polyphenols. In the case of polyphenols, IR-MVD and HA-EPD resulted in higher retention than any single drying method in this experiment. Mejia-Meza et al.36 also reported that a combined hot air/microwave vacuum drying method performed better in phenolic retention in comparison with single drying methods.
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0.6 a
a
-1
Total flavonoids (g CAE kg dw)
-1
Anthocyanin content (g kg dw)
0.5
0.4 b 0.3
c d
0.2
c
d
ab
bc
0.30
cd
de
e 0.25 0.20 0.15 0.10 0.05
0.1 0.00 Fresh
HAD
0.0 Fresh
HAD
IRD HA-EPD Drying methods
IR-MVD
FD
Figure 2. Anthocyanin content of fresh raspberry fruits and raspberry powders obtained by five different drying methods: HAD, hot air drying; IRD, infrared radiation drying; HA-EPD, hot air and explosion puffing drying; IR-MVD, infrared radiation and microwave vacuum drying; FD, freeze drying. Means with a common letter are not significantly different (P < 0.05).
was observed to retain more anthocyanins in raspberries compared with HAD alone. This agreed with the results of Kwok et al.38 Anthocyanins are extremely unstable molecules and vulnerable to heat, oxidation, enzymolysis and other factors.39 Coexistence of high temperature and oxygen was reported to be the most detrimental combination of many factors for anthocyanins.40 Moreover, Jackman and Smith41 recommended short-time heat processing for better anthocyanin retention of foods rich in anthocyanins. Therefore both freeze drying with low temperature and vacuum and combined drying methods with relatively high temperature but short drying time performed better in anthocyanin retention. Total flavonoid content The total flavonoid content of raspberry powders prepared by five drying methods ranged from 0.26 to 0.30 g CAE kg−1 dw (Fig. 3). It was found that IR-MVD caused the least alteration in initial flavonoid concentration of fresh raspberry fruits, showing the highest retention of 97.99%. The powder made by IRD showed no significant difference (P ≤ 0.05) from that made by IR-MVD in flavonoid retention (94.21% for IRD). This may due to the low heat intensity and short heat duration42 of these two methods
IRD HA-EPD Drying methods
IR-MVD
FD
Figure 3. Total flavonoid content of fresh raspberry fruits and raspberry powders obtained by five different drying methods: HAD, hot air drying; IRD, infrared radiation drying; HA-EPD, hot air and explosion puffing drying; IR-MVD, infrared radiation and microwave vacuum drying; FD, freeze drying. Means with a common letter are not significantly different (P < 0.05).
(Table 1). The greatest flavonoid loss (17.35%) was noted for the powder prepared by FD. This result conflicted with a previous statement that freeze drying was a less damaging method for flavonoid retention.43 However, it was supported by Horszwald et al.,44 who also found that freeze drying resulted in relatively less flavonoid preservation compared with thermal drying processes such as hot air and oven vacuum drying. The tendency of different drying methods on flavonoids was similar to that on total polyphenols. Flavonoid losses could be explained by the occurrence of polymerization and oxidation during drying.35,45 Drying processes led to lower loss of flavonoids in fresh raspberries than that of total polyphenols and anthocyanins. This phenomenon could be attributed to the stronger heat stability of flavonoids compared with anthocyanins and other polyphenols.46 Antioxidant capacity The antioxidant capacity data of fresh raspberry fruits and raspberry powders produced by different drying treatments are shown in Table 4. Antioxidant capacity by ABTS assay The ABTS free radical-scavenging ability of raspberry powders can be seen in Table 4. Drying and pulverizing processes led to
Table 4. Antioxidant activity of fresh raspberry fruits and raspberry powders obtained by five different drying methods Drying methoda
ABTS (μmol TE g−1 dw)
DPPH (μmol TE g−1 dw)
Fresh HAD IRD HA-EPD IR-MVD FD
6240.68 ± 48.20a 4532.08 ± 12.06b 4625.42 ± 53.42b 4509.63 ± 65.59b 4586.98 ± 11.23b 4251.83 ± 169.78c
5702.67 ± 70.24a 2786.93 ± 17.65bc 2860.34 ± 35.81b 2733.99 ± 62.94c 2816.03 ± 09.50bc 2587.59 ± 54.45d
FRAP (μmol TE g−1 dw) 8361.29 ± 895.14a 5345.32 ± 52.87c 6230.13 ± 168.42b 5679.39 ± 56.55bc 5918.03 ± 40.57bc 5281.89 ± 101.36 cd
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Results are expressed as mean ± SD of triplicate determinations. Means in the same column with a common letter are not significantly different (P < 0.05). a HAD, hot air drying; IRD, infrared radiation drying; HA-EPD, hot air and explosion puffing drying; IR-MVD, infrared radiation and microwave vacuum drying; FD, freeze drying.
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Effect of drying method on properties of raspberry powders 31.87–25.88% losses of ABTS radical-scavenging capacity in raspberries. Among the five different drying methods, IRD and IR-MVD exhibited noticeable abilities to scavenge ABTS free radicals, with retentions of 74.12 and 73.49% respectively. However, no significant differences (P < 0.05) existed between drying methods, except for FD, which presented the lowest ability to scavenge ABTS free radicals. Antioxidant capacity by DPPH assay The DPPH radical-scavenging capacity of fresh raspberry fruits and raspberry powders is illustrated in Table 4. FD led to the largest change in concentration of compounds capable of scavenging DPPH radicals, losing 54.63% of DPPH radical-scavenging capacity compared with fresh fruits. The retention ratios of DPPH-scavenging capacity of raspberry powders made by IRD, IR-MVD, HAD and HA-EPD were 50.16%, 49.38%, 48.87% and 47.92%, respectively. Among the four thermal drying methods, the value for IRD powder was significantly higher than that for HA-EPD powder. Antioxidant capacity by FRAP assay The FRAP values presented in Table 4 indicate the influence of drying methods on the ferric-reducing antioxidant potential capacity of raspberry powders. Drying techniques caused the degradation of compounds that were responsible for ferric-reducing antioxidant capacity in raspberries to various degrees. Similarly to ABTS and DPPH, raspberry powder obtained by FD was found to show the weakest ferric-reducing power (5281.89 μmol TE g−1 dw), without significant differences except for IRD (6230.13 μmol TE g−1 dw). Through the antioxidant data, it was found that the drying process caused depletions of naturally occurring antioxidants in raw materials, which was in agreement with the statement of Tomaino et al.47 Overall, IRD and IR-MVD performed better in antioxidant capacity retention as evaluated from ABTS, DPPH and FRAP assays. By contrast, FD did not show any advantages in the retention of antioxidant compounds in raspberries compared with the other four techniques. This result contradicted that of Chan et al.,48 who concluded that thermal drying, including microwave, oven and sun drying, caused more serious losses of antioxidant capacity than non-thermal drying (freeze drying). However, in the research of Wojdyło et al.,37 the ABTS radical-scavenging ability of freeze-dried samples was markedly lower than that of vacuum microwave-dried samples under several drying conditions. Antioxidant capacity variation could be attributed to the following reasons. Firstly, the degradation caused by enzymes or heating led to the loss of antioxidant capacity. Moreover, during the thermal process, intense and/or long-lasting thermal treatment may be responsible for the loss of naturally existing antioxidants, most of which are relatively labile.49 Secondly, intermediate products of degradation and Maillard reaction in the heating process can improve the antioxidant ability.50 Polyphenols in an intermediate period of oxidation revealed better antioxidant properties than initially reported by Wojdyło et al.34
CONCLUSION
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capacity of raspberries after dehydration and pulverization, FD was superior to other drying techniques in preserving anthocyanins but inferior in retaining polyphenols, flavonoids and antioxidant capacity. Overall, combined drying methods, especially IR-MVD, were found to present better quality of raspberry powders among the thermal drying techniques. Therefore thermal drying techniques with relatively low temperature and/or short time could be recommended for use in the drying industry.
ACKNOWLEDGEMENT This work was supported by the Special Fund for Agro-scientific Research in the Public Interest (No. 201303073).
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Revised: 26 May 2015
Accepted article published: 25 June 2015
Published online in Wiley Online Library: 17 July 2015
(wileyonlinelibrary.com) DOI 10.1002/jsfa.7317
Comparison of different drying methods on the physical properties, bioactive compounds and antioxidant activity of raspberry powders Xu Si,† Qinqin Chen,† Jinfeng Bi,* Xinye Wu, Jianyong Yi, Linyan Zhou and Zhaolu Li Abstract BACKGROUND: Dehydration has been considered as one of the traditional but most effective techniques for perishable fruits. Raspberry powders obtained after dehydration can be added as ingredients into food formulations such as bakery and dairy products. In this study, raspberry powders obtained by hot air drying (HAD), infrared radiation drying (IRD), hot air and explosion puffing drying (HA-EPD), infrared radiation and microwave vacuum drying (IR-MVD) and freeze drying (FD) were compared on physical properties, bioactive compounds and antioxidant activity. RESULTS: Drying techniques affected the physical properties, bioactive compounds and antioxidant activity of raspberry powders greatly. FD led to significantly higher (P < 0.05) values of water solubility (45.26%), soluble solid (63.46%), hygroscopicity (18.06%), color parameters and anthocyanin retention (60.70%) of raspberry powder compared with other drying methods. However, thermal drying techniques, especially combined drying methods, were superior to FD in final total polyphenol content, total flavonoid content and antioxidant activity. The combined drying methods, especially IR-MVD, showed the highest total polyphenol content (123.22 g GAE kg−1 dw) and total flavonoid content (0.30 g CAE kg−1 dw). Additionally, IR-MVD performed better in antioxidant activity retention. CONCLUSION: Overall, combined drying methods, especially IR-MVD, were found to result in better quality of raspberry powders among the thermal drying techniques. IR-MVD could be recommended for use in the drying industry because of its advantages in time saving and nutrient retention. © 2015 Society of Chemical Industry Keywords: raspberry powder; single drying; combined drying; physiochemical quality; antioxidant activity
INTRODUCTION
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∗
Correspondence to: Jinfeng Bi, Institute of Agro-products Processing Science and Technology, Chinese Academy of Agricultural Sciences/Key Laboratory of Agro-products Processing, Ministry of Agriculture, Beijing 100193, China. E-mail: [email protected]
† These authors contributed equally to the present paper Institute of Agro-products Processing Science and Technology, Chinese Academy of Agricultural Sciences/Key Laboratory of Agro-products Processing, Ministry of Agriculture, Beijing 100193, China
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Raspberry (Rubus idaeus), a member of the Rosaceae family, has attracted great interest owing to not only its good flavor and attractive color but also its abundance of bioactive compounds that have been proved to have beneficial effects on health. Bioactive compounds such as polyphenols, anthocyanins, flavonoids and ellagic acids in raspberries play important roles in antioxidant activity and inhibition effects on obesity, cancer, inflammation, neural degeneration and other diseases.1 – 4 Raspberry is a unique fruit because of its specific flavor and high nutrition value. However, raspberries can only be stored at 0 ∘ C for a few days because of their high moisture content and vulnerable texture.5 Except for fresh and frozen fruits, most raspberries are processed into products such as juices, jams, jellies, wines and freeze-dried fruits.6 Dehydration has been considered as one of the traditional but most effective techniques for perishable fruits, facilitating transportation, storage and product diversification. Raspberry powders obtained after dehydration can be added as ingredients into food formulations such as bakery and dairy products. In recent years, the most common drying techniques applied to raspberries have been convective drying,7 microwave vacuum drying,8,9 freeze drying10,11 and spray drying.12 Although spray drying is commonly
used in raspberry powder production, the drying agent (e.g. maltodextrin) lowers the purity of the powder. In addition, infrared radiation drying, with advantages of low energy consumption, time saving and high efficiency,13 is increasingly being applied in the dehydration of fruits and vegetables. Explosion puffing drying is a relatively new drying technique capable of creating a porous structure and saving on drying time and energy.14 In general, drying processes lead to alterations in the quantity and quality of bioactive compounds. Thus the effects of different drying techniques, including single (hot air drying, infrared radiation drying and freeze drying) and
www.soci.org combined (hot air and explosion puffing drying and infrared radiation and microwave vacuum drying) methods, on the physical properties, bioactive compounds and antioxidant activity of raspberry powders were investigated.
MATERIALS AND METHODS Materials Fruit samples Frozen raspberries (Heritage) from Jinsiwei Agriculture and Food Development Limited Company (Henan, China) were transported by cold chain and stored at −20 ∘ C before the experiments. Reagents CH3 COONa, K2 S2 O8 , NaCl, KCl, AlCl3 , FeSO4 , FeCl3 , Na2 CO3 , NaNO2 , NaOH, ethanol, methanol and acetic acid, all of analytical grade, were purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing, China). Folin–Ciocalteu’s phenol regent (2 mol L−1 ), gallic acid monohydrate (purity ≥98.0%, high-performance liquid chromatography grade), (+)-catechin hydrate (purity ≥98.0%, reagent grade), 2,2′ -azinobis-(3-ethylbenzothiazoline-6sulfonic acid) diammonium salt (ABTS, purity ≥98%), 1,1-diphenyl2-picrylhydrazyl radical (DPPH, purity ≥99.0%), 6-hydroxy-2,5, 7,8-tetramethylchroman-2-carboxylic acid (Trolox, purity ≥99.0%) and 2,4,6-tripyridyl-1,3,5-triazine (TPTZ, purity ≥99.0%) were obtained from Sigma-Aldrich Co. LLC (Shanghai, China). Drying processes The raspberry samples were subjected to the following drying processes: hot air drying (HAD) (DHG-9123A, Jing Hong Laboratory Instrument Co., Ltd, Shanghai, China), infrared radiation drying (IRD) (STC, Senttech Infrared Technology Co., Ltd, Taizhou, China), hot air and explosion puffing drying (HA-EPD) (QDPH10-1, Qin De New Material Technology Co., Ltd, Tianjin, China), infrared radiation and microwave vacuum drying (IR-MVD) (WZD1S-04 microwave vacuum dryer, Sanle Microwave Technology Development Co., Ltd, Nanjing, China) and freeze drying (FD) (Alpha 1-4 LD plus, Marin Christ, Osterode, Germany). Crude powders were obtained by a pulverizer (JYL B060, Joyoung Co., Ltd, Shandong, China) and then micro powders were obtained by a vibration mill (KCW-10, Kunjieyucheng Machinery Co., Ltd, Beijing, China) with a pulverizing time of 5 min at room temperature. The particle size of raspberry powders was determined by a laser particle size analyzer (S3500 BWDL, Microtrac, Montgomeryville, PA, USA). The median diameter D50 (μm) was adopted to express the particle size of powders. Drying conditions for the different drying techniques are presented in Table 1.
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Extractions Fresh raspberry fruits (5 g) or raspberry powders (2 g) were mixed thoroughly with 12 mL of methanol/water (80:20 v/v) solution and placed in the dark for 16 h. The mixtures were then extracted ultrasonically (KQ-500E ultrasonic cleaner machine, Kunshan Ultrasonic Instrument Co., Ltd, Jiangsu, China) for 30 min, followed by centrifugation (Sigma 3 K15 centrifugal machine, SIGMA Laborzentrifugen GmbH, Osterode, Germany) at 9408 × g for 10 min. The supernatants were collected and stored at −20 ∘ C until future analysis.
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Quality determinations Moisture content and water activity The moisture content of fresh raspberry fruits and raspberry powders was determined using a vacuum oven at 70 ∘ C for 24 h according to AOAC method 934.06.15 The water activity (aw ) of raspberry powders was measured by a water activity meter (Aqualab Series 4TE, Decagon Devices, Inc., Pullman, WA, USA) to an accuracy of ±0.003 aw . Color The color of raspberry powders was evaluated using a colorimeter (D25L, Hunterlab Co., Ltd, Reston, VA, USA). Color was expressed as brightness (L*), redness (+a*), yellowness (+b*), chroma (C*) and hue angle (h∘ ). L*, a* and b* were obtained directly from the reading of the colorimeter. Chroma and hue angle were calculated from the following equations: ( )1∕2 C ∗ = a*2 + b*2
(1)
h∘ = tan−1 (b∗ ∕a∗ )
(2)
Color measurements were performed in triplicate. Soluble solid Raspberry powders (10 g) mixed with 100 mL of distilled water were extracted in a boiling water bath for 30 min under slight stirring with a glass bar. After cooling to room temperature, the samples were centrifuged at 847 × g for 30 min. The supernatant was collected for the determination of soluble solid using a refractometer (MASTER-𝛼 Cat. No. 2311, Atago Co., Ltd, Tokyo, Japan). This method was conducted according to Dawei et al.16 with some modifications. The soluble solid (SS) of raspberry powders was calculated by the following equation: ) ( SS (%) = p × m1 ∕m0
(3)
where p is the soluble solid of solution (%), m1 is the weight of sample after boiling (g) and m0 is the initial weight of sample (g). Water solubility index The method of Syamaladevi et al.17 with some modifications was adopted to determine the water solubility index of raspberry powders. Raspberry powders (2 g) were mixed thoroughly with 25 mL of distilled water in a 50 mL centrifuge tube. After incubation at 37 ∘ C in a water bath (DK-826, Jing Hong Laboratory Instrument Co., Ltd) for 35 min, the mixture was centrifuged at 2352 × g for 10 min at 4 ∘ C. The supernatant was then collected in a pre-weighed aluminum specimen box and dried in an oven at 105 ∘ C. The water solubility index (WSI) of raspberry powders was calculated by the following equation: ) ( ) ( WSI g kg−1 = W∕W0 × 1000
(4)
where W is the weight of dried supernatant (g) and W 0 is the weight of original powder (g). Hygroscopicity The hygroscopicity of raspberry powders was measured using the method of Gallo et al.18 Raspberry powders (∼1 g) were stored in a desiccator at room temperature and 75.29% relative humidity (created by a saturated aqueous solution of NaCl). The powders
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Table 1. Drying methods and conditions Drying methoda HAD IRD HA-EPD IR-MVD FD
Drying conditions
Dryingtime (h)
Temperature 70 ∘ C, air velocity 2.1 m s−1 Temperature 70 ∘ C, power 675 W Temperature and time of HAD 70 ∘ C and 90 min respectively; explosion puffing temperature 97 ∘ C, vacuum drying temperature 69 ∘ C, absolute pressure 5 kPa, vacuum drying time 150 min Infrared temperature 70 ∘ C (60 min), infrared power 675 W; microwave power 600 W, absolute pressure 15 kPa Temperature −56 ∘ C, vacuum pressure 0.01 kPa
9 4 4 3 36
a
HAD, hot air drying; IRD, infrared radiation drying; HA-EPD, hot air and explosion puffing drying; IR-MVD, infrared radiation and microwave vacuum drying; FD, freeze drying.
were weighed after 7 days. The hygroscopicity was expressed as HG (%) (i.e. g adsorbed moisture per 100 g dry solid) and calculated as follows: ) ( (5) HG (%) = Δm∕m0 × 100 where Δm is the weight increase of powder (g) and m0 is the initial weight of powder (g). Oil absorption index The method of Abdul-Hamid and Luan19 with some modifications was used to determine the oil absorption index of powders. Briefly, 2 g of raspberry powder (M) was mixed with 20 mL of soybean oil (V 1 ) in a 50 mL centrifuge tube. The mixture was stirred for 15 min and then centrifuged at 3000 × g for 10 min. The volume of supernatant oil (V 2 ) was measured. The oil absorption index (OAI) was calculated as follows: ) ( ) ( (6) OAI mL g−1 = V1 –V2 ∕M Total anthocyanin content The total anthocyanin content of fresh raspberries and raspberry powders was measured by a modified version of the pH differential absorbance method.20 The extracts were diluted with pH 1.0 (0.025 mol L−1 KCl) and pH 4.5 (0.4 mol L−1 sodium acetate) buffers to an appropriate dilution factor (DF). The dilutions were then allowed to equilibrate at room temperature in the dark for 15 min. The absorbance values of the dilutions at 510 and 700 nm were determined using a spectrophotometer (UV-1800, Shimadzu, Kyoto, Japan) against a blank with distilled water. The absorbance was calculated as follows: ( ) ) ( (7) A = A510 − A700 pH 1.0 − A510 − A700 pH 4.5
Total polyphenol content The Folin–Ciocalteu method was used to determine the total phenolic content of raspberry powders.20 Extracts (100 μL) were added to 6 mL of distilled water, then 500 μL of Folin–Ciocalteu reagent at a concentration of 100 g kg−1 was added and mixed. After 6 min, 1.5 mL of 200 g kg−1 Na2 CO3 solution was added to the above solution and mixed. The mixture was left to stand for 2 h, then the absorbance at 765 nm was measured using a spectrophotometer. The extract was replaced by the same amount of gallic acid standards to construct a standard curve against a blank with methanol. The total polyphenol content of powders was expressed as g gallic acid equivalent (GAE) kg−1 dry weight (g GAE kg−1 dw). Total flavonoid content The total flavonoid content of samples under five different drying methods was measured by the aluminum chloride colorimetric assay.21 A 1 mL aliquot of extract or catechin standard solution was mixed with 4 mL of distilled water in a 10 mL tube. To the tube, 0.3 mL of 50 g kg−1 NaNO2 solution and 0.3 mL of 100 g kg−1 AlCl3 solution were added. After 6 min, 2 mL of 1 mol L−1 NaOH was added, followed by distilled water to make a total volume of 10 mL. The above solution was mixed thoroughly and the absorbance at 510 nm was measured. The total flavonoid content of fresh fruits and powders was expressed as g catechin equivalent (CAE) kg−1 dry weight (g CAE kg−1 dw)
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Antioxidant capacity by ABTS, DPPH and FRAP (ferric-reducing antioxidant potential) assays Antioxidant capacity was evaluated by three methods: total antioxidant activity by the ABTS method, free radical-scavenging activity by the DPPH method and ferric-reducing ability by the FRAP The anthocyanin concentration in fresh raspberries and raspmethod. berry powders was calculated as cyanidin-3-glucoside by the folIn the ABTS assay, the ABTS radical is scavenged in proportion lowing formula: to the concentration of antioxidant compounds, leading to color variation when reaction occurs between antioxidant and radical. ( ) anthocyanin concentration mg L−1 = (A × MW × DF × 1000) The ABTS method of Re et al.22 was adopted with some modifica−1 ∕ (𝜀 × 1) (8) tions. Specifically, equal quantities of 2.45 mmol L K2 S2 O8 solu−1 tion and 7 mmol L ABTS were mixed to obtain ABTS•+ solution. The solution was allowed to react for 16 h at room temperature in the dark. Before determination, the ABTS•+ solution was diluted where MW is the molecular weight of cyanidin-3-glucoside with 800 g kg−1 ethanol to obtain an absorption of 0.70 ± 0.01 at (449.38 g mol−1 ), DF is the dilution factor and 𝜀 is the molar absorp−1 −1 734 nm. Diluted sample extracts (400 μL) were reacted with 3.6 mL tivity of cyanidin-3-glucoside (26 900 L mol cm ). Finally, the of freshly diluted ABTS•+ solution for 6 min, then the absorbance anthocyanin content of fresh fruits and powders was expressed of the mixture at 734 nm was measured using a spectrophotomeas g cyanidin-3-glucoside equivalent kg−1 sample dry weight (g kg−1 dw). ter. A standard curve was obtained using different concentrations
www.soci.org of Trolox. Results were expressed as μmol Trolox equivalent (TE) g−1 dry weight (μmol TE g−1 dw). In the DPPH assay, the donation of hydrogen from DPPH forms the stable DPPH molecule, resulting in the scavenging of DPPH radical. The DPPH assay followed the method of Brand-Williams et al.23 Diluted sample extracts (2 mL) were mixed with 4 mL of 0.1 mmol L−1 DPPH. The absorbance of the mixture at 517 nm was measured by a spectrophotometer after 30 min in the dark at room temperature. A standard curve was obtained using different concentrations of Trolox. Results were expressed as μmol TE g−1 dw. The FRAP assay measures antioxidant capacity based on the reduction of Fe3+ -TPTZ at low pH to form colored Fe2+ -TPTZ. The FRAP assay was conducted according to the method of Benzie and Strain24 with some modifications. Specifically, FRAP reagent was prepared by mixing 300 mmol L−1 acetate buffer (pH 3.6), 10 mmol L−1 TPTZ solution (acidified with 40 mmol L−1 HCl) and 20 mmol L−1 FeCl3 · 6H2 O solution in the ratio 10:1:1 (v/v/v). The mixture was warmed at 37 ∘ C before use. Diluted sample extracts (100 μL) were reacted thoroughly with 3 mL of FRAP reagent. After incubation at 37 ∘ C for 10 min, the absorbance at 593 nm was recorded. All solutions were prepared on the day they were used. A standard curve was obtained using different concentrations of Trolox. Results were expressed as μmol TE g−1 dw. Statistical analysis Determinations of the investigated indices were carried out in three replicates and the results expressed as mean ± standard deviation (SD). Differences between means were analyzed firstly by analysis of variance (ANOVA) and then by the least significant difference (LSD) test (P < 0.05) using SPSS Statistics Version 19 (IBM Corporation, Chicago, IL, USA). Figures were drawn using Origin 8.0 (OriginLab, MA, USA).
RESULTS AND DISCUSSION Physical properties Moisture content and water activity The moisture content of raspberry powders (Table 2) was found to be highly dependent on the drying method. Fresh raspberries (moisture content 6.37 g g−1 dry basis (d.b.)) were made into powders with final moisture contents of 0.099, 0.109, 0.099, 0.109 and 0.134 g g−1 d.b. by HAD, IRD, HA-EPD, IR-MVD and FD respectively. The spongy structure of the FD product caused rapid moisture absorption during pulverization, which might lead to the significantly higher moisture content compared with other methods. The following indices were expressed on a dry matter basis and calculated according to the moisture content data. Water activity measures the availability of free water in a food system that is responsible for any biochemical reactions.25 The aw of raspberry powders in this study ranged from 0.178 to 0.264 for different drying methods, which can be considered quite microbiologically stable. The effects of drying techniques on water activity are consistent with their effects on moisture content.
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Water solubility index, soluble solid, hygroscopicity and oil absorption index As can be seen in Table 2, the WSI values of raspberry powders ranged from 385.27 to 452.63 g kg−1 . FD powder presented the highest WSI, 17.48% higher than that of HAD powder which had the lowest WSI. The higher WSI of FD powder could be attributed
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to the good retention of SS during the unheated drying process. There was no significant difference in WSI between IRD and IR-MVD powders, while both were significantly higher compared with HA-EPD powder. The long drying time (up to 9 h) caused the high loss of water solubility in HAD raspberry powder.26 Moreover, it was found that the water solubility of raspberry powders decreased as the particle size decreased, in agreement with Kurozawa et al.27 The HG values of raspberry powders obtained by five different drying methods are shown in Table 2. Drying methods showed significant effects on HG of raspberry powders. The ranking of HG values of powders in descending order was FD > IRD > HA-EPD, IR-MVD > HAD. Hygroscopicity is an important evaluation criterion from which the possibility of stickiness in powders during storage and distribution can be judged.27 It is mainly connected with saccharides of powders. According to Scher et al.,28 monosaccharides such as glucose and fructose facilitate hygroscopicity. As can be seen in Table 2, drying methods showed similar effects on SS and HG. Drying methods with high temperature and/or long heating time, such as HAD and HA-EPD, led to lower SS values. This might be attributed to greater thermal loss of monosaccharides in Maillard reactions. FD raspberry powder had the highest SS (63.46%), which might be the major reason for its high HG. According to Ferrari et al.,29 hygroscopicity values of blackberry powders increased inversely with moisture content. They explained that powders with lower moisture content had greater capacity to absorb ambient moisture. However, no relationship was noted in the present study between moisture content and hygroscopicity. This was supported by the results of Ahmed et al.30 The effect of drying methods on OAI was also determined and the results are presented in Table 2. The OAI of FD powder was the highest, showing no significant difference from that of IR-MVD and IRD powders. The lowest OAI value was found for HAD raspberry powder. Color analysis The effect of drying methods on the color of raspberry powders is presented in Table 3. Significant differences in L*, a* and b* values were noted among raspberry powders. HA-EPD powder had the highest L* value, indicating the brightest character, while FD powder had the lowest L* value. FD powder showed the largest a* value (reddest color), while HAD powder presented the smallest a* value. Additionally, higher b* values were observed in powders prepared by HAD and IRD, representing greater yellowness. The separate analysis of L*, a* and b* parameters is not comprehensive enough to explain changes in color after drying. According to Abers and Wrolstad,31 the chroma value (C*) is a good illustration of the amount of color, distinguishing vivid and dull color. The lower C* value of HAD raspberry powder indicated less saturation and a duller appearance compared with powders prepared by other drying methods. FD powder presented the highest C* value. Besides, a higher hue angle indicates a redder character in the assay.32 The hue angle of FD powder was the highest, being 6.78, 8.97, 28.01 and 31.17% higher than that of powders obtained by HA-EPD, IR-MVD, IRD and HAD respectively. In general, FD, HA-EPD and IR-MVD produced raspberry powders with more redness and better color retention. Anthocyanins are easily degraded and oxidized,33 especially in HAD and IRD powders. Apart from thermal pigment degradation, Maillard reactions may also be responsible for the changes in color of raspberries after drying processes.34
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Effect of drying method on properties of raspberry powders
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Table 2. Particle size, moisture content, water activity (aw ), soluble solid (SS), water solubility index (WSI), hygroscopicity (HG) and oil absorption index (OAI) of raspberry powders obtained by five different drying methods Drying methoda
Particle size (μm)
Moisture content (g g−1 d.b.)
HAD IRD HA-EPD IR-MVD FD
37.40 ± 2.02bc 42.96 ± 0.24b 34.79 ± 0.29c 41.80 ± 0.53b 58.71 ± 7.15a
0.099 ± 0.003c 0.109 ± 0.001b 0.099 ± 0.001c 0.109 ± 0.005b 0.134 ± 0.002a
aw
WSI (g kg−1 )
SS (%)
0.178 ± 0.004c 0.198 ± 0.004b 0.183 ± 0.006c 0.206 ± 0.007b 0.264 ± 0.010a
55.49 ± 1.13e 61.29 ± 0.57b 57.16 ± 0.56d 59.32 ± 0.56c 63.46 ± 0.57a
OAI (mL g−1 )
HG (%)
385.27 ± 1.52d 431.80 ± 3.54b 394.08 ± 6.13c 427.70 ± 1.78b 452.63 ± 6.52a
13.45 ± 0.52d 17.31 ± 0.44b 16.01 ± 0.28c 15.85 ± 024c 18.06 ± 0.21a
1.17 ± 0.14c 1.93 ± 0.06a 1.38 ± 0.13b 2.08 ± 0.08a 2.10 ± 0.05a
Results are expressed as mean ± SD of triplicate determinations. Means in the same column with a common letter are not significantly different (P < 0.05). a HAD, hot air drying; IRD, infrared radiation drying; HA-EPD, hot air and explosion puffing drying; IR-MVD, infrared radiation and microwave vacuum drying; FD, freeze drying.
Table 3. Color parameters of raspberry powders obtained by five different drying methods Drying methoda HAD IRD HA-EPD IR-MVD FD
L* 41.18 ± 0.02b 38.83 ± 0.12d 44.10 ± 0.03a 39.88 ± 0.02c 35.26 ± 0.03e
a*
b*
27.66 ± 0.05e 28.72 ± 0.05d 29.03 ± 0.07c 30.84 ± 0.18b 33.20 ± 0.05a
h∘
C*
8.27 ± 0.02b 8.39 ± 0.01a 7.14 ± 0.03e 7.74 ± 0.03c 7.67 ± 0.01d
28.87 ± 0.05d 29.92 ± 0.05c 29.90 ± 0.06c 31.79 ± 0.16b 34.08 ± 0.05a
3.24 ± 0.01e 3.32 ± 0.00d 3.98 ± 0.02b 3.90 ± 0.04c 4.25 ± 0.00a
Results are expressed as mean ± SD of triplicate determinations. Means in the same column with a common letter are not significantly different (P < 0.05). a HAD, hot air drying; IRD, infrared radiation drying; HA-EPD, hot air and explosion puffing drying; IR-MVD, infrared radiation and microwave vacuum drying; FD, freeze drying.
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a
-1
200
150 d
c
bc
d e
100
50
0 Fresh
HAD
IRD HA-EPD Drying methods
IR-MVD
FD
Figure 1. Total polyphenol content of fresh raspberry fruits and raspberry powders obtained by five different drying methods: HAD, hot air drying; IRD, infrared radiation drying; HA-EPD, hot air and explosion puffing drying; IR-MVD, infrared radiation and microwave vacuum drying; FD, freeze drying. Means with a common letter are not significantly different (P < 0.05).
decreases in anthocyanin content of 65.42 and 65.18% respectively. According to Wojdyło et al.,37 the dried product of sour cherry obtained by freeze drying also presented the highest anthocyanin retention among three different drying techniques. No significant difference in anthocyanin retention existed between powders prepared by HA-EPD (46.73%) and IR-MVD (47.06%). Similarly to total polyphenols, the combination of two drying methods
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Total anthocyanin content The effect of drying methods on anthocyanins was investigated and the results are shown in Fig. 2. The content of anthocyanins in the investigated fresh raspberry fruits was 0.54 g kg−1 DW. The processes of drying and pulverization led to great losses of anthocyanins, ranging from 39.30 to 65.42%. Contrary to total polyphenols, the highest anthocyanin concentration (0.33 g kg−1 dw) was found in FD raspberry powder. HAD and IRD induced
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Total polyphenols (g GAE kg dw)
Total polyphenol content The contents of total polyphenols in raspberry fruits and powders are presented in Fig. 1. The total polyphenol content of raspberries decreased significantly (P < 0.05) after dehydration and pulverization. Higher polyphenol concentrations were observed in IR-MVD and HA-EPD powders, with retentions of 61.42 and 58.87% respectively, between which no significant difference was noted. However, FD led to much lower polyphenol retention compared with other drying methods. Harbourne et al.35 found that freeze drying of willow led to a lower total phenolic content than thermal drying methods such as hot air drying, oven drying and tray drying. Utilization of dehydration at low temperature caused a more severe degradation of polyphenols, which was understandable in terms of the long drying time of 36 h. Maybe drying that involved higher temperature created more destruction of the tissue, which in turn allowed more extraction of phenolic compounds or caused changes in other compounds, resulting in more polyphenols. In the case of polyphenols, IR-MVD and HA-EPD resulted in higher retention than any single drying method in this experiment. Mejia-Meza et al.36 also reported that a combined hot air/microwave vacuum drying method performed better in phenolic retention in comparison with single drying methods.
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0.6 a
a
-1
Total flavonoids (g CAE kg dw)
-1
Anthocyanin content (g kg dw)
0.5
0.4 b 0.3
c d
0.2
c
d
ab
bc
0.30
cd
de
e 0.25 0.20 0.15 0.10 0.05
0.1 0.00 Fresh
HAD
0.0 Fresh
HAD
IRD HA-EPD Drying methods
IR-MVD
FD
Figure 2. Anthocyanin content of fresh raspberry fruits and raspberry powders obtained by five different drying methods: HAD, hot air drying; IRD, infrared radiation drying; HA-EPD, hot air and explosion puffing drying; IR-MVD, infrared radiation and microwave vacuum drying; FD, freeze drying. Means with a common letter are not significantly different (P < 0.05).
was observed to retain more anthocyanins in raspberries compared with HAD alone. This agreed with the results of Kwok et al.38 Anthocyanins are extremely unstable molecules and vulnerable to heat, oxidation, enzymolysis and other factors.39 Coexistence of high temperature and oxygen was reported to be the most detrimental combination of many factors for anthocyanins.40 Moreover, Jackman and Smith41 recommended short-time heat processing for better anthocyanin retention of foods rich in anthocyanins. Therefore both freeze drying with low temperature and vacuum and combined drying methods with relatively high temperature but short drying time performed better in anthocyanin retention. Total flavonoid content The total flavonoid content of raspberry powders prepared by five drying methods ranged from 0.26 to 0.30 g CAE kg−1 dw (Fig. 3). It was found that IR-MVD caused the least alteration in initial flavonoid concentration of fresh raspberry fruits, showing the highest retention of 97.99%. The powder made by IRD showed no significant difference (P ≤ 0.05) from that made by IR-MVD in flavonoid retention (94.21% for IRD). This may due to the low heat intensity and short heat duration42 of these two methods
IRD HA-EPD Drying methods
IR-MVD
FD
Figure 3. Total flavonoid content of fresh raspberry fruits and raspberry powders obtained by five different drying methods: HAD, hot air drying; IRD, infrared radiation drying; HA-EPD, hot air and explosion puffing drying; IR-MVD, infrared radiation and microwave vacuum drying; FD, freeze drying. Means with a common letter are not significantly different (P < 0.05).
(Table 1). The greatest flavonoid loss (17.35%) was noted for the powder prepared by FD. This result conflicted with a previous statement that freeze drying was a less damaging method for flavonoid retention.43 However, it was supported by Horszwald et al.,44 who also found that freeze drying resulted in relatively less flavonoid preservation compared with thermal drying processes such as hot air and oven vacuum drying. The tendency of different drying methods on flavonoids was similar to that on total polyphenols. Flavonoid losses could be explained by the occurrence of polymerization and oxidation during drying.35,45 Drying processes led to lower loss of flavonoids in fresh raspberries than that of total polyphenols and anthocyanins. This phenomenon could be attributed to the stronger heat stability of flavonoids compared with anthocyanins and other polyphenols.46 Antioxidant capacity The antioxidant capacity data of fresh raspberry fruits and raspberry powders produced by different drying treatments are shown in Table 4. Antioxidant capacity by ABTS assay The ABTS free radical-scavenging ability of raspberry powders can be seen in Table 4. Drying and pulverizing processes led to
Table 4. Antioxidant activity of fresh raspberry fruits and raspberry powders obtained by five different drying methods Drying methoda
ABTS (μmol TE g−1 dw)
DPPH (μmol TE g−1 dw)
Fresh HAD IRD HA-EPD IR-MVD FD
6240.68 ± 48.20a 4532.08 ± 12.06b 4625.42 ± 53.42b 4509.63 ± 65.59b 4586.98 ± 11.23b 4251.83 ± 169.78c
5702.67 ± 70.24a 2786.93 ± 17.65bc 2860.34 ± 35.81b 2733.99 ± 62.94c 2816.03 ± 09.50bc 2587.59 ± 54.45d
FRAP (μmol TE g−1 dw) 8361.29 ± 895.14a 5345.32 ± 52.87c 6230.13 ± 168.42b 5679.39 ± 56.55bc 5918.03 ± 40.57bc 5281.89 ± 101.36 cd
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Results are expressed as mean ± SD of triplicate determinations. Means in the same column with a common letter are not significantly different (P < 0.05). a HAD, hot air drying; IRD, infrared radiation drying; HA-EPD, hot air and explosion puffing drying; IR-MVD, infrared radiation and microwave vacuum drying; FD, freeze drying.
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Effect of drying method on properties of raspberry powders 31.87–25.88% losses of ABTS radical-scavenging capacity in raspberries. Among the five different drying methods, IRD and IR-MVD exhibited noticeable abilities to scavenge ABTS free radicals, with retentions of 74.12 and 73.49% respectively. However, no significant differences (P < 0.05) existed between drying methods, except for FD, which presented the lowest ability to scavenge ABTS free radicals. Antioxidant capacity by DPPH assay The DPPH radical-scavenging capacity of fresh raspberry fruits and raspberry powders is illustrated in Table 4. FD led to the largest change in concentration of compounds capable of scavenging DPPH radicals, losing 54.63% of DPPH radical-scavenging capacity compared with fresh fruits. The retention ratios of DPPH-scavenging capacity of raspberry powders made by IRD, IR-MVD, HAD and HA-EPD were 50.16%, 49.38%, 48.87% and 47.92%, respectively. Among the four thermal drying methods, the value for IRD powder was significantly higher than that for HA-EPD powder. Antioxidant capacity by FRAP assay The FRAP values presented in Table 4 indicate the influence of drying methods on the ferric-reducing antioxidant potential capacity of raspberry powders. Drying techniques caused the degradation of compounds that were responsible for ferric-reducing antioxidant capacity in raspberries to various degrees. Similarly to ABTS and DPPH, raspberry powder obtained by FD was found to show the weakest ferric-reducing power (5281.89 μmol TE g−1 dw), without significant differences except for IRD (6230.13 μmol TE g−1 dw). Through the antioxidant data, it was found that the drying process caused depletions of naturally occurring antioxidants in raw materials, which was in agreement with the statement of Tomaino et al.47 Overall, IRD and IR-MVD performed better in antioxidant capacity retention as evaluated from ABTS, DPPH and FRAP assays. By contrast, FD did not show any advantages in the retention of antioxidant compounds in raspberries compared with the other four techniques. This result contradicted that of Chan et al.,48 who concluded that thermal drying, including microwave, oven and sun drying, caused more serious losses of antioxidant capacity than non-thermal drying (freeze drying). However, in the research of Wojdyło et al.,37 the ABTS radical-scavenging ability of freeze-dried samples was markedly lower than that of vacuum microwave-dried samples under several drying conditions. Antioxidant capacity variation could be attributed to the following reasons. Firstly, the degradation caused by enzymes or heating led to the loss of antioxidant capacity. Moreover, during the thermal process, intense and/or long-lasting thermal treatment may be responsible for the loss of naturally existing antioxidants, most of which are relatively labile.49 Secondly, intermediate products of degradation and Maillard reaction in the heating process can improve the antioxidant ability.50 Polyphenols in an intermediate period of oxidation revealed better antioxidant properties than initially reported by Wojdyło et al.34
CONCLUSION
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capacity of raspberries after dehydration and pulverization, FD was superior to other drying techniques in preserving anthocyanins but inferior in retaining polyphenols, flavonoids and antioxidant capacity. Overall, combined drying methods, especially IR-MVD, were found to present better quality of raspberry powders among the thermal drying techniques. Therefore thermal drying techniques with relatively low temperature and/or short time could be recommended for use in the drying industry.
ACKNOWLEDGEMENT This work was supported by the Special Fund for Agro-scientific Research in the Public Interest (No. 201303073).
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