- Email: [email protected]
Stability of quercetin derivatives in vacuum impregnated apple slices after drying (microwave vacuum drying, air drying, freeze drying) and storage
LWT - Food Science and Technology 57 (2014) 426e433
Contents lists available at ScienceDirect
LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt
Stability of quercetin derivatives in vacuum impregnated apple slices after drying (microwave vacuum drying, air drying, freeze drying) and storage Beate Schulze*, Eva Maria Hubbermann, Karin Schwarz Department of Food Technology, Institute of Human Nutrition and Food Science, Christian-Albrechts-University Kiel, Heinrich-Hecht-Platz 10, 24118 Kiel, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 March 2011 Received in revised form 10 November 2013 Accepted 21 November 2013
Microwave vacuum drying (MVD) was investigated for apple slices enriched with quercetin derivatives by vacuum impregnation (VI). Additional freeze drying (FD) and air drying (AD) were conducted. Compared to native apples, the impregnated tissue resulted in higher moisture content, elevation of weight and significant browning, due to the incorporated VI solution. The total quercetin content and quercetin glycoside composition were not affected by MVD and FD. The vacuum conditions protect the polyphenols from oxygen dependent degradation and browning reactions. AD resulted in an average quercetin glycoside loss of 44% and undesirable changes, particularly discoloration. The degradation is caused by both non-enzymatic and enzymatic reactions. The pulsed microwave energy intake improved the drying result in structure and led to a faster drying process of 130 min. The bulk density of MVD apple chips (0.69 g/ml) ranged between 0.33 g/ml for FD and 0.75 g/ml for AD. The final moisture content was the lowest after FD (6.8 g/100 g), followed by 9.0 g/100 g after MVD and 12.7 g/100 g after AD. The shelf life was significantly influenced by storage temperature and time. After 12 month at 20 C, the total quercetin content decreased by 21%. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Apple Vacuum impregnation Microwave vacuum drying Freeze drying Hot air drying
1. Introduction Apples (Malus domestica) are one of the most widely grown and economically important fruit crops all over the world, which are rich in quercetin glycosides. These naturally occurring antioxidative secondary plant products belong to flavonoids, a group of polyphenols with cardio and cancer protective effects (Ekström et al., 2011; Hertog, Feskens, Hollman, Katan, & Kromhout, 1993; Knekt et al., 2002). In apples, quercetin glycosides are located in the apple peel and provide protection against stress induced by UV-B and visible light (Hagen et al., 2007; Solovchenko & Schmitz-Eiberger, 2003). However, during industrial processing apples are often used in their peeled form, thus secondary plant compounds, especially the quercetin glycosides, are removed. Food industry and the food scientists are therefore searching for ingredients rich in quercetin glycosides and innovative methods to incorporate the lost flavonoids. Apple peel extract and apple pomace could potentially be functional food ingredients that are high in quercetin * Corresponding author. Tel.: þ49 431 8805034; fax: þ49 431 8805544. E-mail addresses: [email protected], [email protected] (B. Schulze). 0023-6438/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.lwt.2013.11.021
and beneficial for consumer’s health (Schieber et al., 2003; Wolfe & Liu, 2003). Vacuum impregnation (VI) is a food preparation technique that changes food composition. In recent studies, VI was used to change the properties of food with regards to nutritional, sensory, shelf life and physicochemical characteristics (Chiralt et al., 2001; Guamis et al., 1997; Lin, Leonard, Lederer, Traber, & Zhao, 2006). VI is characterized by replacing the initially occluded air in pores of the fruit or vegetables with a liquid, which is the impregnation solution. With pressure gradients, air of the porous fraction is released until atmospheric pressure conditions are restored and the external liquid phase penetrates the plant tissue. Preserving polyphenols and their bioavailability in spite of drying depends on the preparation technique and the control of food processes such as time and temperature conditions. 3glycosidic bonded sugars with quercetin are especially temperature-sensitive compounds (Rohn, Buchner, Driemel, Rauser, & Kroh, 2007). Sensory characteristics (color, texture, taste) can also be influenced by drying. Apple processing is combined with enzymatic and non-enzymatic browning of fruit tissues. The enzymatic browning reaction in plants is based on polyphenoloxidase (PPO). This copper containing metallo-enzyme
B. Schulze et al. / LWT - Food Science and Technology 57 (2014) 426e433
catalyzes degradation reactions of natural phenols to o-diphenols and o-quinones in the presence of oxygen (Jimenez & GarciaCarmona, 1999). Drying vacuum impregnated fruits may cause undesirable changes. In particularly, fruits containing sugar required special drying methods to decrease the water content. For high-quality products, it is essential to avoid drying methods with high temperatures and long drying times. Freeze drying (FD) is considered as a process that preserves quality characteristics; however it is a time-consuming process that requires high energy. In contrast air drying (AD) is a low cost procedure; however quality parameters such as flavor, color, nutrients, bulk density and rehydration capacity may be negatively affected (Drouzas, Tsami, & Saravacos, 1999; Lin, Durance, & Scaman, 1998; Yongsawatdigul & Gunasekaran, 1996). Microwave vacuum drying (MVD) offers an alternative method to preserve temperature-sensitive compounds (Böhm, Kühnert, Rohm, & Scholze, 2006) and to economize energy and time (Gunasekaran, 1999). In recent years, MVD has been investigated as a potential method for obtaining several highquality dried agricultural products, such as fruits (Böhm et al., 2006; Mousa & Farid, 2002; Yongsawatdigul & Gunasekaran, 1996), herbs (Soysal, 2004; Yousif, Durance, Scaman, & Girard, 2000) and vegetables (Cui, Xu, & Sun, 2004b; Lin et al., 1998). MVD is a technique which combines the microwave heating technique with vacuum drying. The microwave radiation is transformed, directly in the product, to kinetic energy. The vacuum conditions lower the liquid vapor pressure, so that drying of thermo-sensitive products can be performed in the absence of oxygen. Efficient water evaporation can also be used for structural changes and the prevention of shrinkage due to food puffing and induced porous tissues. The objective of this study is to investigate the effect of different drying techniques on various parameters of vacuum impregnated apple slices. To our knowledge, this is the first investigation of flavonoid enriched food dried by MVD. 2. Materials and methods 2.1. Materials For determination and quantification, the quercetin derivatives quercetin dihydrate, quercetin-3-O-glucoside, quercetin-3-O-galactoside, quercetin-3-O-arabinopyranoside, and quercetin-3-O-rhamnoside were used as standards and obtained from Roth (Karlsruhe, Germany). Apples (Malus domestica Borkh. cultivar ‘Braeburn’) were purchased from a local producer and stored at ambient temperature (20 C) until experimental use. The bulk density of whole apple fruits measured by water displacement was 0.913 0.013 g/cm3 (n ¼ 24). The native Braeburn slices contained 87.6 0.5 g/100 g moisture, which equals a water activity of 0.97. The used Braeburn apples had a fruit firmness of 8.2 1.1 kg/cm2 and the soluble solids (11.9 1.0 Brix). 2.2. Sample pre-treatments For physicochemical analysis, the apples were washed with distilled water and peeled. The firmness of the fruit flesh was measured with an Effegi penetrometer (Alfonsine, Italy) fitted with an 11.1 mm tip. The soluble solid concentration (SCC) was determined using a hand-held refractometer (Model HRT 32, Krüss Optronic, Hamburg, Germany) at 20 C. Afterward, the apple core was removed with a core borer and the apples were parted in 6 mm slices. The used Braeburn apple slices had an average initial surface area of 39.9 cm2.
427
2.3. Vacuum impregnation (VI) The impregnation solution consisted of apple juice with 0.3 g/ 100 mL apple peel extract high in flavonoids (hfv) (Val de Vire Bioactives, Condé sur Vire, France) with an SCC of 11.1 0.1 Brix. The total quercetin derivative content of the impregnation solution averaged 96.4 2.5 mg/g dry mass (DM); the pH was 3.38. Apple slices were immersed in the VI solution, high in apple peel flavonoids, and fixed with watch-glasses to avoid floating. The vacuum phase of impregnation was carried out using the vacuum chamber of the microwave vacuum dryer (mWaveVac0150 1c, Püschner, Schwanewede, Germany) at a pressure of 100 hPa for 5 min at room temperature. Afterward, the apple parenchyma remained in the impregnation solution for 10 min at atmospheric pressure. The concentration of quercetin glycosides in apple slices increased with the VI process (Schulze, Peth, Hubbermann, & Schwarz, 2012). 2.4. Drying techniques After vacuum impregnation, three drying techniques were performed; freeze drying (FD), convective hot air drying (AD) and microwave vacuum drying (MVD). For all drying procedures, 6 apple slices of each apple (n ¼ 8) were pre-treated by VI and dried. 2.4.1. Microwave vacuum drying equipment The drying method for convective drying or MVD of nonimpregnated apple slices was not transferable to impregnated apple parenchyma, capable drying conditions and drying parameters for MVD of enriched Braeburn apples were investigated. The MVD were performed with a lab scale MVD dryer (mWaveVac0150 1c, Püschner, Schwanewede, Germany). The system consisted of a vacuum vessel with an integrated wall heater to avoid condensing vapor in the system. The vessel wall was adjusted to 50 C during the drying process. The dryer had a maximal microwave power of 1000 W produced by 1.2 kW/2450 MHz magnetron and a magnetron protection system. During the drying process, the pressure was decreased by a vacuum pump. To avoid overheating and product hot spots, a rotary plate for product movement was used. To measure the drying procedure the drying temperature was checked with a fiberoptical thermometer. In addition, the reflected microwave energy, the drying rate, the product weight and the actual vacuum conditions were controlled via a real-time computer system. 2.4.2. Microwave vacuum drying method The cell load weighed approximately 125 g and the process started at a product temperature of 20 C. Throughout the drying period, the vacuum was set to 20 hPa. The drying started after starting the magnetrons at a vacuum level of 100 hPa. The microwave power entry was set to 500 W for 25 min (period 1). In addition, a microwave stop time of 5 min and a short power-on time of 60 s with an energy entry of 1000 W were programmed (period 2). To reduce high temperature peaks the energy intake was set to 80 W until the end of drying (period 3 and 4). The drying conditions were normally held constant during each experiment (n ¼ 8). 2.4.3. Freeze drying (FD) For FD after VI, the fruit material was immediately shock frozen in liquid nitrogen and freeze dried for 72 h (Christ Gamma 1-20, Osterode am Harz, Germany). 2.4.4. Air drying (AD) The AD of impregnated apple slices was performed continuously using a convection oven under hot air at 50 C for 14 h (Zanussi FCV/ E10L6, Pordenone, Italy) until constant weight was achieved.
428
B. Schulze et al. / LWT - Food Science and Technology 57 (2014) 426e433
2.5. Flavonoid analysis Flavonoid analysis was carried out by solvent extraction and HPLC. Before extraction, the samples were ground and soaked for 5 h in an extraction solution composed of water with 700 ml/L acetone and 1 g/L citric acid. Afterward, an ultrasonic extraction (Sonopuls HD 2070, MS 73, Bandelin, Berlin, Germany) was accomplished with four extraction steps. The extracts were filtered and analyzed by HPLC-DAD. All analyses were performed in triplicate. Quantitative HPLC-DAD analysis of quercetin derivatives was performed on an HP 1100 HPLC (Agilent Technologies, Waldbronn, Germany) using a Nucleodur Sphinx RP-C18 column (125 4 mm; 5 mm, Machery-Nagel, Düren, Germany) at 30 C. The injection volume was 10 mL. The mobile phase consisted of a binary gradient with 5 ml/L formic acid in water (eluent A) and 100% acetonitrile (eluent B) with a flow rate of 1 ml/min. The gradient program started at 5% solvent B in 20 min to 20%, from 20% to 35% in 8 min, from 35% to 80% in 2 min, from 35% to 80% in 5 min, from 80% to 5% in 5 min and additional 5 min stabilization with 5% of solvent B. The DAD wavelengths were 280 nm and 365 nm for quercetin derivatives, respectively. To qualify the quercetin glycosides, an HPLCESI-MSn HP 1100 (Agilent Technologies, Waldbronn, Germany) was used and compared with standard solutions. 2.6. Surface area The apple slices’ surface area was measured after scanning 4 slices of each apple (n ¼ 8) before and after several enrichment and drying processes with an Epson Perfection 2400 Photo Scanner (Long Beach, CA, USA). Afterward, the apple chips on the scanned images were marked manually and the surface area was calculated by the ImageJ 1.44a software using a defined length scale of 100 mm as reference.
impregnated and dried samples was measured at 25 C using a Novasina aW Sprint TH-500 instrument (Pfäffikon, Switzerland).
X ¼
m md *100 m
2.10. Stability and storage The temperature influence on the flavonoid content of the native apple peel extract and 0.3 g dissolved extract in 100 ml water was studied at 50 C and 95 C. Approximately 5 mg of dry native extract or 5 ml dissolved extract, respectively, were filled into 20 ml GC-Vials which were then tightly sealed. The vials were kept in a water bath at 50 C and 95 C in the dark for 15 h. The apple peel extract was dissolved in water after the temperature influence. All samples were analyzed directly by using HPLC-DAD. To study the shelf life of the freeze dried end product, apple chips were stored at 20 C and 40 C for 12 months in sealed bags (OPA 15 mm e LDPE 15 mm e Aluminum 12 mm e LDPE 20 mm) under light exclusion and the quercetin glycoside levels were determined. Three replicates of the experiments were performed. 2.11. Statistical analysis The statistical analysis was performed with the Statistical Software Package SPSS 17 for Windows (SPSS, Chicago, IL, USA). Mean values were considered significantly different when p < 0.05. To compare the mean values of the quercetin derivatives content, water activity, water content and color measurement 1-way ANOVA were applied. Levene’s test was used to test for homogeneity of variance. 3. Results 3.1. MVD method for vacuum impregnated Braeburn apple slices
2.7. Bulk density The bulk density data were expressed as the mass of dry sample (g) per total volume (ml) of the dried apple chips. The volume was measured with glass bead sediments of known volume, resulting in a weight loss that corresponded to the apple slices. 2.8. Color measurement Surface color of the cut apples was directly measured with an XRite SP62 (Cologne, Germany) colorimeter. Each measurement was taken at 3 locations of 4 apple slices (n ¼ 8) and calibrated using a standard white reflector and black plate. Lightness is reported as L*; the lightness value for perfect white is 100, while L* ¼ 0 corresponds to black. The a* value indicates chromaticity on a green () to red (þ) axis and b* indicates chromaticity on a blue () to yellow (þ) axis. The hue angle (h ) value was calculated using following equation.
1
h ¼ tan
b* a*
!
Apple slices were enriched with quercetin glycosides by vacuum impregnation. The slices were dried by MVD, FD and AD, resulting in apple chips Fig. 1. A typical MVD curve is shown in Fig. 2, representing product parameters during the 130 min drying time. During the first drying period, the product weight decreased rapidly and the majority of the moisture content evaporated. Parallel to weight reduction, the temperature of the apple slices rose from 20 2 C at the beginning to 48 C at the end. The period is characterized by a high drying rate and resulted in a weight decrease of w70%. At the beginning of period 2 the power was set to zero to decrease the product temperature before starting the puffing process, i.e. the product temperature fell to 28 C, in order to avoid product burning during the pulsed puffing period, which is based on a combination of high energy input for a short time with a spontaneous temperature rise. The third MVD period is characterized by a continuous product weight decrease with a moderate drying rate and increased temperature of 37 C. During the last period (period 4) the energy input was low, the product weight decreased only slightly, while the product temperature rose continuously to 50 C (maximum). During the whole MVD process the slices lost 87.3% of the start weight.
2.9. Moisture content and water activity
3.2. Moisture content and water activity
The moisture content (X) of the fresh and treated apples was determined gravimetrically, using the equation below, by drying at 105 1 C until constant weight was achieved. Here, md is the weight of the apple slice after the drying processes and m is the weight of the apple slice. The water activity (aW) of fresh,
The impregnated Braeburn slices exhibited significantly higher moisture contents (88.3% 0.5%) and thus a higher start weight for drying compared to the native apple tissue. Figs. 3 and 4 show the effects of the drying procedures on the moisture content, the weight and the water activity of the apple parenchyma.
B. Schulze et al. / LWT - Food Science and Technology 57 (2014) 426e433
429
Fig. 1. Dried vacuum impregnated Braeburn apple chips after (I) air drying, (II) microwave vacuum drying and (III) freeze drying.
Due to the drying procedures, the moisture content and the water activity fell most significantly. The moisture content decreased to 6.8% after FD, to 9.0% after MVD and to 12.7% after AD. For the first 5 h of the AD process a continuous weight reduction up to 72.4% of the apple slice start weight was observed. The majority of the water removal occurred during the first 7.5 h. The air dried samples lost 84.4% of their start weight. The end drying phase resulted in a minor weight decrease and was stopped after 15 h because the weight remained constant. Parallel to the moisture content the water activity for all drying products was investigated. The lowest water activity was measured after FD. AW evalues of MVD and AD were not significantly different. 3.3. Surface area and bulk density The surface area was measured to investigate changes in size depending on the different drying methods. The enrichment by VI implicated no significant particle size changes in contrast to the drying procedures. All drying procedures resulted in sample shrinkage (Fig. 4). The FD process resulted in the least shrinkage of 8%. Within the MVD process, the apple slices shrank 46.1% from the initial sample size to a surface area of 21.6 cm2 3.3 cm2. The MVD end drying caused 46.4% tissue contraction, when compared to fresh apple slices. AD resulted in the strongest shrinkage of 48.9%. Bulk density measurements showed differences among samples after the drying procedures (Table 1). After FD, the apple chips showed a porous structure which is characterized by a low bulk density. The apple chips of air or microwave vacuum dried samples
Fig. 2. Online plot of drying and product parameters during the developed microwave vacuum drying method of vacuum impregnated apple slices. The dashed black line represents the vacuum pressure.
possessed a compact structure with a higher bulk density compared to the freeze dried material. 3.4. Quercetin derivatives content after drying processes After different drying procedures, the content of the enriched apple parenchyma with quercetin glycosides by VI was determined. FD and MVD only slightly influenced the quercetin glycosides and quercetin aglycone contents. The ratio between quercetin and the glycosides remained constant during the process and corresponded to the original ratio in the apple peel extract (Table 2). The corresponding quercetin glycoside contents are detailed in Fig. 5, which shows marginal but statistically significant degradation of quercetin-3-O-galactoside and quercetin aglycone due to the MVD, when compared with the freeze dried apples. Furthermore, the successive air drying of apple slices at 50 C for 4 h e adjusted to MVD before e did not influence the quercetin glycosides. In contrast, AD procedure of VI apple slices resulted in marked quercetin glycoside degradation. The sum of quercetin derivatives was reduced to 44% in comparison to FD. The highest degradation was detected for quercetin-3-O-xyloside (59%) and the aglycone (54%). Thus, the degradation reaction of the monoglycosidic quercetin derivatives induced no quercetin aglycone accumulation. 3.5. Color measurement data The color of the apple slices was affected in two decisive ways: first by the vacuum impregnation, and second, by the drying
Fig. 3. Effects of drying processes on moisture content and water activity of Braeburn apple slices after vacuum impregnation, data SD (n ¼ 8), bars marked by same letter are not significantly different (p < 0.05 by one-factorial ANOVA with LSD post hoc test).
430
B. Schulze et al. / LWT - Food Science and Technology 57 (2014) 426e433
temperatures. The strongest decrease was found for quercetin-3-Oglucoside (28%) at ambient conditions and for quercetin-3-Oarabinoside (91%) at 40 C. At 40 C storage over 12 months the sum of the quercetin glycosides and quercetin showed an averaged degradation of 70%. The quercetin aglycone was stable at 20 C and showed the lowest decrease (6%) compared to the monoglycosidic derivatives at the higher temperature storage life test. 4. Discussion
Fig. 4. Surface area changes of Braeburn apple slices after vacuum impregnation and drying processes, data are determined in quadruple SD (n ¼ 8) and weight changes are compared to native apple slices as control, data are expressed in percent SD (n ¼ 8), bars marked by different letters are significantly different (p < 0.05 by onefactorial ANOVA with LSD post hoc test).
conditions of the different methods. The observed color changes are characterized by slight to strong browning of apple tissue. The results of color measurements obtained from the vacuum impregnation and drying processes are presented in Table 3 for a* (greenness/redness), b* (blueness/yellowness), chroma, hue and lightness values. Except for the b* values the impregnation process significantly changed the fruit color of Braeburn apple slices. It was observed that within the impregnation step, the a* values for native Braeburn apple slices turned from greenness (negative a* values) to redness (positive a* values). The impregnation step decreased the lightness and hue of the apple parenchyma and chroma rose, compared to the native tissue. FD resulted in a decrease of redness whereas the lightness, the yellowness, the chroma and hue values of the enriched apple parenchyma elevated. The colors of MVD and AD samples showed a different behavior. The air dried apple chips were the darkest and characterized by stronger redness compared to freeze dried samples. Whereas the lightness, the yellowness and chroma values after the MVD application increased in comparison to the VI apple slice, the b*, chroma values and the hue of air dried material were strongly reduced. The measured hue of apple polyphenol enriched Braeburn slices was not significantly influenced after MVD, when compared to the VI apple slice. 3.6. Apple peel extract and VI solution stability To further investigate the degradation of the quercetin derivatives, the apple peel extract was heated at different temperatures as dry powder and solubilized in vacuum impregnation fluid. At 50 C (Tables 4 and 5), the total quercetin content was slightly reduced after 15 h in solution (7%) and in the dry power (10%). In contrast, a strong degradation of quercetin monoglycosides was found at 95 C, resulting in a slight increase of quercetin aglycone (Tables 4 and 5). The highest reduction at 95 C was found for quercetin-3-O-arabinoside in the dry powder extract, as well as in the vacuum impregnation fluid. 3.7. Influence of the storage on quercetin derivatives content The storage life of apple chips enriched with apple peel extract after VI and FD is shown in Fig. 6. Apple chips stored at 20 C in the dark contained higher quercetin derivatives levels than those stored at 40 C. The degradation curves were linear at both
In apples, quercetin glycosides are located predominately in the peel (Burda, Oleszek, & Lee, 1990; Tsao, Yang, Young, & Zhu, 2003). The VI technique was used to increase the apple peel polyphenolics in the fruit parenchyma. Due to vacuum impregnation, moisture and sugar content of apple slices increased markedly (MartinezMonzo, Martinez-Navarrete, Chiralt, & Fito, 1998) before the drying process. This finding requires a thorough investigation of the drying behavior. 4.1. Drying processes In order to evaluate the resulting product and processing parameters of the MVD, two additional different drying regimes, freeze drying (FD) and air drying (AD), were also conducted. The MVD drying process was completed in 130 min and resulted in a moisture content of 9.8%. In contrast, FD required a drying period of 72 h and during air drying at 50 C with continuous air stream the moisture content was still 12.7% after 15 h. The latter drying parameters and results are in accordance with the study of Fernández, Castillero, and Aguilera (2005). The relatively short drying period and the resulting low water content of the MVD apple slices are due to the pulsed energy entry combined with the continuous vacuum. Consequently, the product quality can be improved while pulsed drying shortens the duration time of the drying process (Mousa & Farid, 2002). The process regime leads to a puffing effect of the compact parenchyma structure of the apple slice, thereby enhancing the porosity. In turn, the increased porosity will enhance the release of the water vapor. Furthermore, such short power-off and power-on times can increase energy efficiency compared to continuous MVD (Yongsawatdigul & Gunasekaran, 1996). Unpublished data of our own research showed that the effect of the microwave pulse (1000 W, 60 s) on the parenchyma puffing is dependent on the water content of the apple slices. At water contents above 70%, the puffed structure will collapse again, whereas at markedly lower water contents, the structure of the parenchyma is too firm to allow a puffing effect. Similar to the AD process, the drying rate dropped clearly when the product weight decreased below 30% related to the start weight (100%). Also the product temperature rose from 35 C to 50 C, indicating slow water diffusion from the inner parts of the parenchyma to the surface. 4.2. Surface area and bulk density The bulk density was affected differently by the drying procedures. The low density found for FD apple slices is typical for freeze dried material. The compact structure after MVD and AD shows that the drying was associated with changes in the microstructure and cellular collapse, which is a result of tissue contraction and thus associated with high bulk density. Despite the breakup of the compact structure with the MVD puffing process, the porosity is increased only slightly compared to AD. For a larger puffing-effect, it is necessary to increase energy input, which is also accompanied by a strong temperature increase (data not shown). High temperatures during AD can prevent shrinkage, because the
B. Schulze et al. / LWT - Food Science and Technology 57 (2014) 426e433
431
Table 1 Bulk density of apple chips after freeze drying (FD), microwave vacuum drying (MVD) and air drying (AD), data SD (n ¼ 8). Bulk density (g/ml) 0.33 0.05 0.69 0.09 0.75 0.05
FD MVD AD
outer parts dry quickly, become rigid and their fixed, final tissue volume cannot contract as strongly (Wang & Brennan, 1995). In our study, the maximum temperature of 50 C was not exceeded in order to protect heat-sensitive ingredients. 4.3. Quercetin derivative contents Low degradation of quercetin derivatives was found for apple slices dried by FD and MVD, whereas the AD processes exhibited a strong impact. When AD was conducted after MVD, the quercetin content was not affected. Rupasinghe, Wang, Huber, and Pitts (2008) investigated the retention of quercetin derivatives from apple peel extract in muffins during baking. After 20 min at 175 C, they found 55.9%e73.4% of the quercetin glycosides added to the muffin dough (Rupasinghe et al., 2008). Also, the 3-glycosidic bond is described as more thermo-sensitive than glycosylation at the 40 glycosidic position (Rohn et al., 2007). To further investigate the degradation, the stability of the apple peel extract was investigated in the VI solution at 50 C and 95 C as well as in dried form. The degradation was quite low in all cases (7%e10% loss) and did not explain the strong degradation found in AD apple slices. Therefore, other mechanisms, in addition to the thermal degradation, are responsible for the decrease of quercetin glycoside during AD. The absence of oxygen due to vacuum conditions during the MVD and FD drying process caused the inhibition of oxidation reactions and enzymatic degradation by PPO. The low degradation during AD of apple slices previously adjusted to MVD can be explained by the low moisture content at the beginning of the air drying process. The low water residue affects the mobility of the reactants, therefore the rate of oxidation and enzyme activity is reduced (Lavelli & Vantaggi, 2009). This explanation is also supported by the color change of the apple slices, i.e. the strongest browning was found for AD apple slices. The quercetin glycosides degradation at AD and stability investigation at 50 C and 95 C differs for apple peel extract and VI solution. The combination of temperature and time, as well as the glycosidic bonded sugar influenced the degradation kinetics. At 50 C, the auto-oxidation of quercetin occurs, so that the content
Fig. 5. Quercetin derivative profiles of freeze dried (white bars), microwave vacuum dried (graygray bars), and air dried (dark graygray bars) Braeburn apple slices after vacuum impregnation, data are mean SD (n ¼ 8), bars marked with different letters of the single quercetin derivatives are significantly different (p < 0.05 by one-factorial ANOVA with LSD post hoc test).
decreases, whereas at 95 C, quercetin aglycone content increased, due to a deglycosylation reaction of the monoglycosides. The decreasing content, compared to the initial concentration for the quercetin derivatives, is in descending order quercetin-3-Ogalactoside quercetin-3-O-glucoside > quercetin-3-Orhamnoside > quercetin-3-O-xyloside > quercetin-3-O-arabinoside. The order is similar to stability results in apple juice (van der Sluis, Dekker, & van Boekel, 2005). Under roasting conditions of the pure quercetin derivatives, the order of degraded quercetin monoglycosides differed except for quercetin-3-O-galactoside, which was most stable (Rohn et al., 2007). Our findings demonstrate that based on a moderate temperature profile curve and the absence of oxygen, MVD maintains a high concentration of apple peel flavonoids. These findings are consistent with other MVD development studies that aim to preserve sensitive and valuable ingredients during drying (Böhm et al.,
Table 3 Color measurement data of Braeburn apple slices in native, vacuum impregnated and dried form, data are determined in quadruple SD (n ¼ 8). a*
Table 2 Quercetin derivative profiles of vacuum impregnated Braeburn apples after different drying applications, data are expressed in percent SD (n ¼ 8). Percentage of total quercetin derivatives (%) FD Quercetin-3-O-galactoside Quercetin-3-O-glucoside Quercetin-3-O-xyloside Quercetin-3-O-arabinoside Quercetin-3-O-rhamnoside Quercetin
37.4 10.6 9.2 25.6 11.3 5.9
MVD
1.1 0.5 0.7 0.6 0.9 0.8
36.9 10.9 9.5 26.5 11.6 4.7
MVD-AD 0.8 0.5 0.5 0.5 0.7 0.5
35.6 10.3 10.2 27.3 12.1 4.4
FD: freeze drying. MVD: microwave vacuum drying. MVD-AD: microwave vacuum drying with air end drying. AD: air drying. MVD-AD: microwave vacuum drying with air end drying.
0.7 0.5 0.3 0.6 0.3 0.5
AD 36.2 12.2 6.7 25.9 14.1 4.9
1.0 0.5 0.8 0.4 0.6 0.4
Native VI FD Native VI MVD MVD-AD Native VI AD
3.2 8.5 4.8 2.9 8.7 10.4 13.6 2.8 9.2 16.2
b*
0.5a 1.1b 0.9c 0.4a 0.9b 1.8d 1.8e 0.6a 1.4b 0.6f
33.2 34.9 47.7 32.6 33.9 48.3 50.1 35.1 34.8 15.3
3.6a 2.7a 2.8b 3.0a 3.3a 3.9b 3.5b 3.7a 3.1a 1.0c
33.3 35.8 48.0 32.7 35.0 49.7 52.0 35.1 36.0 22.3
L*
h
C*
3.6a 2.7b 2.8c 2.9a 3.2b 3.9c 2.9c 3.7a 3.2b 0.9d
93.8 84.8 93.6 94.4 84.1 86.5 83.1 94.9 83.6 48.2
0.8a 5.2b 1.2a 0.9a 2.2b 2.6b 2.6b 1.4a 2.0b 1.9c
85.1 53.4 70.8 84.7 52.7 63.9 63.8 83.7 52.4 29.1
1.3a 3.0b 2.1c 1.0a 2.3b 3.4d 2.7d 1.7a 2.1b 0.9e
Letters aef: data with the same letter in columns are significantly not different, p < 0.05 by one-factorial ANOVA with LSD post hoc test. VI: vacuum impregnation. FD: freeze drying. MVD: microwave vacuum drying. MVD-AD: microwave vacuum drying with air end drying. AD: air drying. FD: freeze drying.
432
B. Schulze et al. / LWT - Food Science and Technology 57 (2014) 426e433
Table 4 Quercetin glycoside and aglycone contents of vacuum impregnation solution depending on the heating duration at 50 C and 95 C, data are determined in triplicate SD. Time (h)
Vacuum impregnation solution content (mg/g DM) Quercetin-3-Ogalactoside
Initial 3 6 9 12 15 Initial 3 6 9 12 15
At 50 C 34.84 0.45 34.04 0.30 33.99 0.48 33.50 0.30 32.86 0.39 32.98 0.03 At 95 C 34.84 0.45 30.68 0.44 28.26 0.35 24.91 0.44 21.05 0.57 15.60 0.49
Quercetin-3-Oglucoside
(%)
Quercetin-3-Oxyloside
Quercetin-3-Oarabinoside
Quercetin-3-Orhamnoside
Quercetin
Total quercetin
9.59 9.33 9.10 9.25 9.04 9.08
0.14 0.14 0.16 0.15 0.10 0.02
12.35 11.93 11.59 11.84 11.49 11.52
0.17 0.19 0.15 0.19 0.13 0.03
27.32 26.41 26.19 25.83 24.83 24.78
0.24 0.29 0.22 0.22 0.21 0.07
12.88 12.26 12.20 12.37 12.30 12.18
0.10 0.16 0.15 0.18 0.21 0.03
4.86 4.90 4.83 4.78 4.74 4.64
0.06 0.09 0.17 0.04 0.06 0.02
100 98.9 97.9 97.6 95.3 93.5
9.59 8.57 8.08 7.28 6.20 5.05
0.14 0.13 0.09 0.15 0.15 0.24
12.35 9.52 7.21 5.13 3.24 1.61
0.17 0.11 0.04 0.26 0.13 0.16
27.32 16.38 8.63 4.14 1.68 0.45
0.24 0.09 0.03 0.39 0.19 0.22
12.88 10.68 9.09 7.64 5.68 3.90
0.10 0.09 0.05 0.18 0.22 0.24
4.86 10.78 16.71 19.52 19.79 14.63
0.06 0.33 0.27 0.26 0.26 0.32
100 86.6 78.0 68.6 57.6 40.5
Table 5 Quercetin glycoside and aglycone contents of apple peel extract depending on the heating duration at 50 C and 95 C, data are determined in triplicate SD. Time (h)
Apple peel extract content (mg/g DM) Quercetin-3-Ogalactoside
Initial 3 6 9 12 15 Initial 3 6 9 12 15
At 50 C 33.22 0.63 33.39 0.12 32.86 0.45 32.55 0.50 31.41 1.24 30.68 1.76 At 95 C 33.22 0.63 33.22 0.35 31.91 1.05 31.42 0.69 31.94 0.96 30.29 1.11
Quercetin-3-Oglucoside
(%) Quercetin-3-Oxyloside
Quercetin-3-Oarabinoside
Quercetin-3-Orhamnoside
Quercetin
Total quercetin
9.20 9.28 8.88 8.67 8.38 8.23
0.23 0.12 0.18 0.15 0.26 0.52
11.77 11.94 11.41 11.16 10.86 10.53
0.31 0.32 0.18 0.12 0.45 0.62
26.06 26.62 25.40 25.12 24.36 23.56
0.61 0.45 0.39 0.23 0.99 1.31
12.38 12.63 11.97 11.66 11.35 11.02
0.32 0.21 0.38 0.08 0.36 0.61
5.11 4.86 4.74 4.67 4.48 4.44
0.12 0.04 0.17 0.09 0.15 0.30
100 99.7 94.6 93.8 90.8 89.6
9.20 8.98 8.56 8.46 8.52 8.07
0.23 0.05 0.31 0.22 0.47 0.46
11.77 11.47 10.65 10.56 10.46 9.83
0.31 0.12 0.37 0.19 0.53 0.41
26.06 24.56 21.95 20.73 19.65 18.16
0.61 0.15 0.62 0.40 1.78 1.57
12.38 12.17 11.37 11.12 11.60 10.74
0.32 0.21 0.45 0.40 0.27 0.42
5.11 5.68 6.94 7.82 9.35 9.15
0.12 0.17 0.32 0.72 1.32 1.39
100 96.0 94.3 90.1 91.5 87.4
2006; Clary, Mejia-Meza, Wang, & Petrucci, 2007; Cui, Xu, & Sun, 2004a). 4.4. Color The VI dependent browning is caused by the impregnation solution, which consists of the brown color apple peel extract dissolved in apple juice. The transfer of the brown solution and
occlusion of air from the porous tissue are connected with the elevation in browning and a decrease in lightness. Thus, the color changes of freeze-dried apple chips are mainly due to the vacuum impregnation. Braeburn apple slices without a VI step before FD showed only minor color changes (data not shown). Similar results were reported for VI application in apples and other fruits (Alzamora, Tapia, & López-Malo, 2000). Also, the greenness, which is highly related to color changes of apple flesh
Fig. 6. Stability of quercetin glycosides in vacuum impregnated and freeze dried apple chips during storage with two temperature conditions (n ¼ 3), black dots (dotted line) depict quercetin-3-O-galactoside); dark gray upward triangles (dotted line) depict quercetin-3-O-glucoside; gray diamonds (solid line) depict quercetin-3-O-xyloside; grey downward triangles (solid line) depict quercetin-3-O-arabinoside; black squares (dotted line) depict quercetin-3-O-rhamnoside; grey dots (solid line) depict quercetin.
B. Schulze et al. / LWT - Food Science and Technology 57 (2014) 426e433
(Goupy et al., 1995), decreased with the impregnation step to redness. These results were consistent with other apple VI investigations (Alzamora et al., 2000). Beside the redness, the yellow color of the apple product increased during FD and MVD. The data were consistent with other drying studies (Fernández et al., 2005; Krokida, Maroulis, & Saravacos, 2001). Under the experimental drying conditions, AD produced drastic changes in color, especially for lightness, b* value, hue and chroma. The dark browning is caused by non-enzymatic and enzymatic browning reactions as confirmed by the strong degradation of quercetin derivatives. The drying under vacuum, the short drying time and the low thermal load during MVD and FD, as compared to AD, caused overall lower color changes of the apples slices. 4.5. Storage Investigation of the stability of freeze-dried apple chips over a period of one year showed that the levels of quercetin derivatives were significantly influenced by storage temperature and storage time. It is assumed that enzymatic and non-enzymatic reactions are responsible for the degradation. Storage under moderate temperature conditions was associated with better preservation of quercetin glycosides. Enriched apple chips stored at 20 C for 12 months still showed significant levels of quercetin derivatives. 5. Conclusion This study showed that similar to freeze drying (FD), gentle microwave vacuum drying (MVD), of vacuum impregnated apple slices maintained high quercetin contents in the parenchyma. In contrast, air drying (AD) resulted in significant losses of quercetin derivatives and marked browning, indicating extensive oxidation reactions. Due to the moderate temperature regime (50 C max) during drying, strong structural changes occurred in MVD and AD apple slices. MVD apples slices showed slightly higher porosity than AD apple chips and both drying procedures resulted in high shrinkage. In contrast, FD apple slices showed significantly higher porosity and the lowest shrinkage. Acknowledgments This work was financially supported by the German Federal Ministry of Education and Research (BMBF 0313856A) within the project ‘Functional Foods for Vascular Health e from Nutraceuticals to Personalised Diets’. The authors would like to thank the company Püschner Microwave Power Systems for providing equipment for vacuum microwave drying. References Alzamora, S. M., Tapia, M. S., & López-Malo, A. (Eds.). (2000). Aspen food engineering series. Minimally processed fruits and vegetables: Fundamental aspects and applications. Gaithersburg, Md: Aspen Publishers. Böhm, V., Kühnert, S., Rohm, H., & Scholze, G. (2006). Improving the nutritional quality of microwave-vacuum dried strawberries: a preliminary study. Food Science and Technology International, 12(1), 67e75. Burda, S., Oleszek, W., & Lee, C. Y. (1990). Phenolic compounds and their changes in apples during maturation and cold storage. Journal of Agricultural and Food Chemistry, 38, 945e948. Chiralt, A., Fito, P., Barat, J. M., Andrés, A., González-Martínez, C., Escriche, I., et al. (2001). Use of vacuum impregnation in food salting process. Journal of Food Engineering, 49(2e3), 141e151. Clary, C. D., Mejia-Meza, E., Wang, S., & Petrucci, V. E. (2007). Improving grape quality using microwave vacuum drying associated with temperature control. Journal of Food Science, 72(1), E023eE028. Cui, Z. W., Xu, S. Y., & Sun, D. W. (2004a). Effect of microwave-vacuum drying on the carotenoids retention of carrot slices and chlorophyll retention of Chinese chive leaves. Drying Technology, 22(3), 563e575.
433
Cui, Z. W., Xu, S. Y., & Sun, D. W. (2004b). Microwaveevacuum drying kinetics of carrot slices. Journal of Food Engineering, 65(2), 157e164. Drouzas, A. E., Tsami, E., & Saravacos, G. D. (1999). Microwave/vacuum drying of model fruit gels. Journal of Food Engineering, 39(2), 117e122. Ekström, A. M., Serafini, M., Nyrén, O., Wolk, A., Bosetti, C., & Bellocco, R. (2011). Dietary quercetin intake and risk of gastric cancer: results from a populationbased study in Sweden. Annals of Oncology, 22(2), 438e443. Fernández, L., Castillero, C., & Aguilera, J. M. (2005). An application of image analysis to dehydration of apple discs: IV Iberoamerican Congress of Food Engineering (CIBIA IV). Journal of Food Engineering, 67(1e2), 185e193. Goupy, P., Amiot, M., Richard-Forget, F., Duprat, F., Aubert, S., & Nicolas, J. (1995). Enzymatic browning of model solutions and apple phenolic extracts by apple polyphenoloxidase. Journal of Food Science, 60(3), 497e501. Guamis, B., Trujillo, A. J., Ferragut, V., Chiralt, A., Andres, A., & Fito, P. (1997). Ripening control of Manchego type cheese salted by brine vacuum impregnation. International Dairy Journal, 7(2e3), 185e192. Gunasekaran, S. (1999). Pulsed microwave-vacuum drying of food. Drying Technology: An International Journal, 17(3), 395e412. Hagen, S. F., Borge, G. I. A., Bengtsson, G. B., Bilger, W., Berge, A., Haffner, K., et al. (2007). Phenolic contents and other health and sensory related properties of apple fruit (Malus domestica Borkh., cv. Aroma): effect of postharvest UV-B irradiation. Postharvest Biology and Technology, 45(1), 1e10. Hertog, M. G. L., Feskens, E. J. M., Hollman, P. C. H., Katan, M. B., & Kromhout, D. (1993). Dietary antioxidant flavonoids and risk of coronary heart disease: the Zutphen Elderly Study. The Lancet, 342(8878), 1007e1011. Jimenez, M., & Garcia-Carmona, F. (1999). Oxidation of the flavonol quercetin by polyphenol oxidase. Journal of Agricultural and Food Chemistry, 47(1), 56e60. Knekt, P., Kumpulainen, J., Järvinen, R., Rissanen, H., Heliövaara, M., Reunanen, A., et al. (2002). Flavonoid intake and risk of chronic diseases. The American Journal of Clinical Nutrition, 76(3), 560e568. Krokida, M. K., Maroulis, Z. B., & Saravacos, G. D. (2001). The effect of the method of drying on the colour of dehydrated products. International Journal of Food Science & Technology, 36(1), 53e59. Lavelli, V., & Vantaggi, C. (2009). Rate of antioxidant degradation and color variations in dehydrated apples as related to water activity. Journal of Agricultural and Food Chemistry, 57(11), 4733e4738. Lin, T. M., Durance, T. D., & Scaman, C. H. (1998). Characterization of vacuum microwave, air and freeze dried carrot slices. Food Research International, 31(2), 111e117. Lin, D. S., Leonard, S. W., Lederer, C., Traber, M. G., & Zhao, Y. (2006). Retention of fortified vitamin E and sensory quality of fresh-cut pears by vacuum impregnation with honey. Journal of Food Science, 71(7), S553eS559. Martinez-Monzo, J., Martinez-Navarrete, N., Chiralt, A., & Fito, P. (1998). Mechanical and structural changes in apple (Var. Granny Smith) due to vacuum impregnation with cryoprotectants. Journal of Food Science, 63(3), 499e503. Mousa, N., & Farid, M. (2002). Microwave vacuum drying of banana slices. Drying Technology: An International Journal, 20(10), 2055e2066. Rohn, S., Buchner, N., Driemel, G., Rauser, M., & Kroh, L. W. (2007). Thermal degradation of onion quercetin glycosides under roasting conditions. Journal of Agricultural and Food Chemistry, 55(4), 1568e1573. Rupasinghe, H. P. V., Wang, L., Huber, G. M., & Pitts, N. L. (2008). Effect of baking on dietary fibre and phenolics of muffins incorporated with apple skin powder. Food Chemistry, 107(3), 1217e1224. Schieber, A., Hilt, P., Streker, P., Endreß, H.-U., Rentschler, C., & Carle, R. (2003). A new process for the combined recovery of pectin and phenolic compounds from apple pomace. Innovative Food Science and Emerging Technologies, 4(1), 99e107. Schulze, B., Peth, S., Hubbermann, E. M., & Schwarz, K. (2012). The influence of vacuum impregnation on the fortification of apple parenchyma with quercetin derivatives in combination with pore structures X-ray analysis. Journal of Food Engineering, 109(3), 380e387. van der Sluis, A. A., Dekker, M., & van Boekel, M. A. J. S. (2005). Activity and concentration of polyphenolic antioxidants in apple juice. 3. Stability during storage. Journal of Agricultural and Food Chemistry, 53(4), 1073e1080. Solovchenko, A., & Schmitz-Eiberger, M. (2003). Significance of skin flavonoids for UV-B-protection in apple fruits. Journal of Experimental Botany, 54(389), 1977e 1984. Soysal, Y. (2004). Microwave drying characteristics of parsley. Biosystems Engineering, 89(2), 167e173. Tsao, R., Yang, R., Young, J. C., & Zhu, H. (2003). Polyphenolic profiles in eight apple cultivars using high-performance liquid chromatography (HPLC). Journal of Agricultural and Food Chemistry, 51(21), 6347e6353. Wang, N., & Brennan, J. G. (1995). Changes in structure, density and porosity of potato during dehydration. Journal of Food Engineering, 24(1), 61e76. Wolfe, K. L., & Liu, R. H. (2003). Apple peels as a value-added food ingredient. Journal of Agricultural and Food Chemistry, 51(6), 1676e1683. Yongsawatdigul, J., & Gunasekaran, S. (1996). Microwave-vacuum drying of cranberries: Part I. Energy use and efficiency. Journal of Food Processing and Preservation, 20(2), 121e143. Yousif, A. N., Durance, T. D., Scaman, C. H., & Girard, B. (2000). Headspace volatiles and physical characteristics of vacuum-microwave, air, and freezedried oregano (Lippia berlandieri Schauer). Journal of Food Science, 65(6), 926e930.
Contents lists available at ScienceDirect
LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt
Stability of quercetin derivatives in vacuum impregnated apple slices after drying (microwave vacuum drying, air drying, freeze drying) and storage Beate Schulze*, Eva Maria Hubbermann, Karin Schwarz Department of Food Technology, Institute of Human Nutrition and Food Science, Christian-Albrechts-University Kiel, Heinrich-Hecht-Platz 10, 24118 Kiel, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 March 2011 Received in revised form 10 November 2013 Accepted 21 November 2013
Microwave vacuum drying (MVD) was investigated for apple slices enriched with quercetin derivatives by vacuum impregnation (VI). Additional freeze drying (FD) and air drying (AD) were conducted. Compared to native apples, the impregnated tissue resulted in higher moisture content, elevation of weight and significant browning, due to the incorporated VI solution. The total quercetin content and quercetin glycoside composition were not affected by MVD and FD. The vacuum conditions protect the polyphenols from oxygen dependent degradation and browning reactions. AD resulted in an average quercetin glycoside loss of 44% and undesirable changes, particularly discoloration. The degradation is caused by both non-enzymatic and enzymatic reactions. The pulsed microwave energy intake improved the drying result in structure and led to a faster drying process of 130 min. The bulk density of MVD apple chips (0.69 g/ml) ranged between 0.33 g/ml for FD and 0.75 g/ml for AD. The final moisture content was the lowest after FD (6.8 g/100 g), followed by 9.0 g/100 g after MVD and 12.7 g/100 g after AD. The shelf life was significantly influenced by storage temperature and time. After 12 month at 20 C, the total quercetin content decreased by 21%. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Apple Vacuum impregnation Microwave vacuum drying Freeze drying Hot air drying
1. Introduction Apples (Malus domestica) are one of the most widely grown and economically important fruit crops all over the world, which are rich in quercetin glycosides. These naturally occurring antioxidative secondary plant products belong to flavonoids, a group of polyphenols with cardio and cancer protective effects (Ekström et al., 2011; Hertog, Feskens, Hollman, Katan, & Kromhout, 1993; Knekt et al., 2002). In apples, quercetin glycosides are located in the apple peel and provide protection against stress induced by UV-B and visible light (Hagen et al., 2007; Solovchenko & Schmitz-Eiberger, 2003). However, during industrial processing apples are often used in their peeled form, thus secondary plant compounds, especially the quercetin glycosides, are removed. Food industry and the food scientists are therefore searching for ingredients rich in quercetin glycosides and innovative methods to incorporate the lost flavonoids. Apple peel extract and apple pomace could potentially be functional food ingredients that are high in quercetin * Corresponding author. Tel.: þ49 431 8805034; fax: þ49 431 8805544. E-mail addresses: [email protected], [email protected] (B. Schulze). 0023-6438/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.lwt.2013.11.021
and beneficial for consumer’s health (Schieber et al., 2003; Wolfe & Liu, 2003). Vacuum impregnation (VI) is a food preparation technique that changes food composition. In recent studies, VI was used to change the properties of food with regards to nutritional, sensory, shelf life and physicochemical characteristics (Chiralt et al., 2001; Guamis et al., 1997; Lin, Leonard, Lederer, Traber, & Zhao, 2006). VI is characterized by replacing the initially occluded air in pores of the fruit or vegetables with a liquid, which is the impregnation solution. With pressure gradients, air of the porous fraction is released until atmospheric pressure conditions are restored and the external liquid phase penetrates the plant tissue. Preserving polyphenols and their bioavailability in spite of drying depends on the preparation technique and the control of food processes such as time and temperature conditions. 3glycosidic bonded sugars with quercetin are especially temperature-sensitive compounds (Rohn, Buchner, Driemel, Rauser, & Kroh, 2007). Sensory characteristics (color, texture, taste) can also be influenced by drying. Apple processing is combined with enzymatic and non-enzymatic browning of fruit tissues. The enzymatic browning reaction in plants is based on polyphenoloxidase (PPO). This copper containing metallo-enzyme
B. Schulze et al. / LWT - Food Science and Technology 57 (2014) 426e433
catalyzes degradation reactions of natural phenols to o-diphenols and o-quinones in the presence of oxygen (Jimenez & GarciaCarmona, 1999). Drying vacuum impregnated fruits may cause undesirable changes. In particularly, fruits containing sugar required special drying methods to decrease the water content. For high-quality products, it is essential to avoid drying methods with high temperatures and long drying times. Freeze drying (FD) is considered as a process that preserves quality characteristics; however it is a time-consuming process that requires high energy. In contrast air drying (AD) is a low cost procedure; however quality parameters such as flavor, color, nutrients, bulk density and rehydration capacity may be negatively affected (Drouzas, Tsami, & Saravacos, 1999; Lin, Durance, & Scaman, 1998; Yongsawatdigul & Gunasekaran, 1996). Microwave vacuum drying (MVD) offers an alternative method to preserve temperature-sensitive compounds (Böhm, Kühnert, Rohm, & Scholze, 2006) and to economize energy and time (Gunasekaran, 1999). In recent years, MVD has been investigated as a potential method for obtaining several highquality dried agricultural products, such as fruits (Böhm et al., 2006; Mousa & Farid, 2002; Yongsawatdigul & Gunasekaran, 1996), herbs (Soysal, 2004; Yousif, Durance, Scaman, & Girard, 2000) and vegetables (Cui, Xu, & Sun, 2004b; Lin et al., 1998). MVD is a technique which combines the microwave heating technique with vacuum drying. The microwave radiation is transformed, directly in the product, to kinetic energy. The vacuum conditions lower the liquid vapor pressure, so that drying of thermo-sensitive products can be performed in the absence of oxygen. Efficient water evaporation can also be used for structural changes and the prevention of shrinkage due to food puffing and induced porous tissues. The objective of this study is to investigate the effect of different drying techniques on various parameters of vacuum impregnated apple slices. To our knowledge, this is the first investigation of flavonoid enriched food dried by MVD. 2. Materials and methods 2.1. Materials For determination and quantification, the quercetin derivatives quercetin dihydrate, quercetin-3-O-glucoside, quercetin-3-O-galactoside, quercetin-3-O-arabinopyranoside, and quercetin-3-O-rhamnoside were used as standards and obtained from Roth (Karlsruhe, Germany). Apples (Malus domestica Borkh. cultivar ‘Braeburn’) were purchased from a local producer and stored at ambient temperature (20 C) until experimental use. The bulk density of whole apple fruits measured by water displacement was 0.913 0.013 g/cm3 (n ¼ 24). The native Braeburn slices contained 87.6 0.5 g/100 g moisture, which equals a water activity of 0.97. The used Braeburn apples had a fruit firmness of 8.2 1.1 kg/cm2 and the soluble solids (11.9 1.0 Brix). 2.2. Sample pre-treatments For physicochemical analysis, the apples were washed with distilled water and peeled. The firmness of the fruit flesh was measured with an Effegi penetrometer (Alfonsine, Italy) fitted with an 11.1 mm tip. The soluble solid concentration (SCC) was determined using a hand-held refractometer (Model HRT 32, Krüss Optronic, Hamburg, Germany) at 20 C. Afterward, the apple core was removed with a core borer and the apples were parted in 6 mm slices. The used Braeburn apple slices had an average initial surface area of 39.9 cm2.
427
2.3. Vacuum impregnation (VI) The impregnation solution consisted of apple juice with 0.3 g/ 100 mL apple peel extract high in flavonoids (hfv) (Val de Vire Bioactives, Condé sur Vire, France) with an SCC of 11.1 0.1 Brix. The total quercetin derivative content of the impregnation solution averaged 96.4 2.5 mg/g dry mass (DM); the pH was 3.38. Apple slices were immersed in the VI solution, high in apple peel flavonoids, and fixed with watch-glasses to avoid floating. The vacuum phase of impregnation was carried out using the vacuum chamber of the microwave vacuum dryer (mWaveVac0150 1c, Püschner, Schwanewede, Germany) at a pressure of 100 hPa for 5 min at room temperature. Afterward, the apple parenchyma remained in the impregnation solution for 10 min at atmospheric pressure. The concentration of quercetin glycosides in apple slices increased with the VI process (Schulze, Peth, Hubbermann, & Schwarz, 2012). 2.4. Drying techniques After vacuum impregnation, three drying techniques were performed; freeze drying (FD), convective hot air drying (AD) and microwave vacuum drying (MVD). For all drying procedures, 6 apple slices of each apple (n ¼ 8) were pre-treated by VI and dried. 2.4.1. Microwave vacuum drying equipment The drying method for convective drying or MVD of nonimpregnated apple slices was not transferable to impregnated apple parenchyma, capable drying conditions and drying parameters for MVD of enriched Braeburn apples were investigated. The MVD were performed with a lab scale MVD dryer (mWaveVac0150 1c, Püschner, Schwanewede, Germany). The system consisted of a vacuum vessel with an integrated wall heater to avoid condensing vapor in the system. The vessel wall was adjusted to 50 C during the drying process. The dryer had a maximal microwave power of 1000 W produced by 1.2 kW/2450 MHz magnetron and a magnetron protection system. During the drying process, the pressure was decreased by a vacuum pump. To avoid overheating and product hot spots, a rotary plate for product movement was used. To measure the drying procedure the drying temperature was checked with a fiberoptical thermometer. In addition, the reflected microwave energy, the drying rate, the product weight and the actual vacuum conditions were controlled via a real-time computer system. 2.4.2. Microwave vacuum drying method The cell load weighed approximately 125 g and the process started at a product temperature of 20 C. Throughout the drying period, the vacuum was set to 20 hPa. The drying started after starting the magnetrons at a vacuum level of 100 hPa. The microwave power entry was set to 500 W for 25 min (period 1). In addition, a microwave stop time of 5 min and a short power-on time of 60 s with an energy entry of 1000 W were programmed (period 2). To reduce high temperature peaks the energy intake was set to 80 W until the end of drying (period 3 and 4). The drying conditions were normally held constant during each experiment (n ¼ 8). 2.4.3. Freeze drying (FD) For FD after VI, the fruit material was immediately shock frozen in liquid nitrogen and freeze dried for 72 h (Christ Gamma 1-20, Osterode am Harz, Germany). 2.4.4. Air drying (AD) The AD of impregnated apple slices was performed continuously using a convection oven under hot air at 50 C for 14 h (Zanussi FCV/ E10L6, Pordenone, Italy) until constant weight was achieved.
428
B. Schulze et al. / LWT - Food Science and Technology 57 (2014) 426e433
2.5. Flavonoid analysis Flavonoid analysis was carried out by solvent extraction and HPLC. Before extraction, the samples were ground and soaked for 5 h in an extraction solution composed of water with 700 ml/L acetone and 1 g/L citric acid. Afterward, an ultrasonic extraction (Sonopuls HD 2070, MS 73, Bandelin, Berlin, Germany) was accomplished with four extraction steps. The extracts were filtered and analyzed by HPLC-DAD. All analyses were performed in triplicate. Quantitative HPLC-DAD analysis of quercetin derivatives was performed on an HP 1100 HPLC (Agilent Technologies, Waldbronn, Germany) using a Nucleodur Sphinx RP-C18 column (125 4 mm; 5 mm, Machery-Nagel, Düren, Germany) at 30 C. The injection volume was 10 mL. The mobile phase consisted of a binary gradient with 5 ml/L formic acid in water (eluent A) and 100% acetonitrile (eluent B) with a flow rate of 1 ml/min. The gradient program started at 5% solvent B in 20 min to 20%, from 20% to 35% in 8 min, from 35% to 80% in 2 min, from 35% to 80% in 5 min, from 80% to 5% in 5 min and additional 5 min stabilization with 5% of solvent B. The DAD wavelengths were 280 nm and 365 nm for quercetin derivatives, respectively. To qualify the quercetin glycosides, an HPLCESI-MSn HP 1100 (Agilent Technologies, Waldbronn, Germany) was used and compared with standard solutions. 2.6. Surface area The apple slices’ surface area was measured after scanning 4 slices of each apple (n ¼ 8) before and after several enrichment and drying processes with an Epson Perfection 2400 Photo Scanner (Long Beach, CA, USA). Afterward, the apple chips on the scanned images were marked manually and the surface area was calculated by the ImageJ 1.44a software using a defined length scale of 100 mm as reference.
impregnated and dried samples was measured at 25 C using a Novasina aW Sprint TH-500 instrument (Pfäffikon, Switzerland).
X ¼
m md *100 m
2.10. Stability and storage The temperature influence on the flavonoid content of the native apple peel extract and 0.3 g dissolved extract in 100 ml water was studied at 50 C and 95 C. Approximately 5 mg of dry native extract or 5 ml dissolved extract, respectively, were filled into 20 ml GC-Vials which were then tightly sealed. The vials were kept in a water bath at 50 C and 95 C in the dark for 15 h. The apple peel extract was dissolved in water after the temperature influence. All samples were analyzed directly by using HPLC-DAD. To study the shelf life of the freeze dried end product, apple chips were stored at 20 C and 40 C for 12 months in sealed bags (OPA 15 mm e LDPE 15 mm e Aluminum 12 mm e LDPE 20 mm) under light exclusion and the quercetin glycoside levels were determined. Three replicates of the experiments were performed. 2.11. Statistical analysis The statistical analysis was performed with the Statistical Software Package SPSS 17 for Windows (SPSS, Chicago, IL, USA). Mean values were considered significantly different when p < 0.05. To compare the mean values of the quercetin derivatives content, water activity, water content and color measurement 1-way ANOVA were applied. Levene’s test was used to test for homogeneity of variance. 3. Results 3.1. MVD method for vacuum impregnated Braeburn apple slices
2.7. Bulk density The bulk density data were expressed as the mass of dry sample (g) per total volume (ml) of the dried apple chips. The volume was measured with glass bead sediments of known volume, resulting in a weight loss that corresponded to the apple slices. 2.8. Color measurement Surface color of the cut apples was directly measured with an XRite SP62 (Cologne, Germany) colorimeter. Each measurement was taken at 3 locations of 4 apple slices (n ¼ 8) and calibrated using a standard white reflector and black plate. Lightness is reported as L*; the lightness value for perfect white is 100, while L* ¼ 0 corresponds to black. The a* value indicates chromaticity on a green () to red (þ) axis and b* indicates chromaticity on a blue () to yellow (þ) axis. The hue angle (h ) value was calculated using following equation.
1
h ¼ tan
b* a*
!
Apple slices were enriched with quercetin glycosides by vacuum impregnation. The slices were dried by MVD, FD and AD, resulting in apple chips Fig. 1. A typical MVD curve is shown in Fig. 2, representing product parameters during the 130 min drying time. During the first drying period, the product weight decreased rapidly and the majority of the moisture content evaporated. Parallel to weight reduction, the temperature of the apple slices rose from 20 2 C at the beginning to 48 C at the end. The period is characterized by a high drying rate and resulted in a weight decrease of w70%. At the beginning of period 2 the power was set to zero to decrease the product temperature before starting the puffing process, i.e. the product temperature fell to 28 C, in order to avoid product burning during the pulsed puffing period, which is based on a combination of high energy input for a short time with a spontaneous temperature rise. The third MVD period is characterized by a continuous product weight decrease with a moderate drying rate and increased temperature of 37 C. During the last period (period 4) the energy input was low, the product weight decreased only slightly, while the product temperature rose continuously to 50 C (maximum). During the whole MVD process the slices lost 87.3% of the start weight.
2.9. Moisture content and water activity
3.2. Moisture content and water activity
The moisture content (X) of the fresh and treated apples was determined gravimetrically, using the equation below, by drying at 105 1 C until constant weight was achieved. Here, md is the weight of the apple slice after the drying processes and m is the weight of the apple slice. The water activity (aW) of fresh,
The impregnated Braeburn slices exhibited significantly higher moisture contents (88.3% 0.5%) and thus a higher start weight for drying compared to the native apple tissue. Figs. 3 and 4 show the effects of the drying procedures on the moisture content, the weight and the water activity of the apple parenchyma.
B. Schulze et al. / LWT - Food Science and Technology 57 (2014) 426e433
429
Fig. 1. Dried vacuum impregnated Braeburn apple chips after (I) air drying, (II) microwave vacuum drying and (III) freeze drying.
Due to the drying procedures, the moisture content and the water activity fell most significantly. The moisture content decreased to 6.8% after FD, to 9.0% after MVD and to 12.7% after AD. For the first 5 h of the AD process a continuous weight reduction up to 72.4% of the apple slice start weight was observed. The majority of the water removal occurred during the first 7.5 h. The air dried samples lost 84.4% of their start weight. The end drying phase resulted in a minor weight decrease and was stopped after 15 h because the weight remained constant. Parallel to the moisture content the water activity for all drying products was investigated. The lowest water activity was measured after FD. AW evalues of MVD and AD were not significantly different. 3.3. Surface area and bulk density The surface area was measured to investigate changes in size depending on the different drying methods. The enrichment by VI implicated no significant particle size changes in contrast to the drying procedures. All drying procedures resulted in sample shrinkage (Fig. 4). The FD process resulted in the least shrinkage of 8%. Within the MVD process, the apple slices shrank 46.1% from the initial sample size to a surface area of 21.6 cm2 3.3 cm2. The MVD end drying caused 46.4% tissue contraction, when compared to fresh apple slices. AD resulted in the strongest shrinkage of 48.9%. Bulk density measurements showed differences among samples after the drying procedures (Table 1). After FD, the apple chips showed a porous structure which is characterized by a low bulk density. The apple chips of air or microwave vacuum dried samples
Fig. 2. Online plot of drying and product parameters during the developed microwave vacuum drying method of vacuum impregnated apple slices. The dashed black line represents the vacuum pressure.
possessed a compact structure with a higher bulk density compared to the freeze dried material. 3.4. Quercetin derivatives content after drying processes After different drying procedures, the content of the enriched apple parenchyma with quercetin glycosides by VI was determined. FD and MVD only slightly influenced the quercetin glycosides and quercetin aglycone contents. The ratio between quercetin and the glycosides remained constant during the process and corresponded to the original ratio in the apple peel extract (Table 2). The corresponding quercetin glycoside contents are detailed in Fig. 5, which shows marginal but statistically significant degradation of quercetin-3-O-galactoside and quercetin aglycone due to the MVD, when compared with the freeze dried apples. Furthermore, the successive air drying of apple slices at 50 C for 4 h e adjusted to MVD before e did not influence the quercetin glycosides. In contrast, AD procedure of VI apple slices resulted in marked quercetin glycoside degradation. The sum of quercetin derivatives was reduced to 44% in comparison to FD. The highest degradation was detected for quercetin-3-O-xyloside (59%) and the aglycone (54%). Thus, the degradation reaction of the monoglycosidic quercetin derivatives induced no quercetin aglycone accumulation. 3.5. Color measurement data The color of the apple slices was affected in two decisive ways: first by the vacuum impregnation, and second, by the drying
Fig. 3. Effects of drying processes on moisture content and water activity of Braeburn apple slices after vacuum impregnation, data SD (n ¼ 8), bars marked by same letter are not significantly different (p < 0.05 by one-factorial ANOVA with LSD post hoc test).
430
B. Schulze et al. / LWT - Food Science and Technology 57 (2014) 426e433
temperatures. The strongest decrease was found for quercetin-3-Oglucoside (28%) at ambient conditions and for quercetin-3-Oarabinoside (91%) at 40 C. At 40 C storage over 12 months the sum of the quercetin glycosides and quercetin showed an averaged degradation of 70%. The quercetin aglycone was stable at 20 C and showed the lowest decrease (6%) compared to the monoglycosidic derivatives at the higher temperature storage life test. 4. Discussion
Fig. 4. Surface area changes of Braeburn apple slices after vacuum impregnation and drying processes, data are determined in quadruple SD (n ¼ 8) and weight changes are compared to native apple slices as control, data are expressed in percent SD (n ¼ 8), bars marked by different letters are significantly different (p < 0.05 by onefactorial ANOVA with LSD post hoc test).
conditions of the different methods. The observed color changes are characterized by slight to strong browning of apple tissue. The results of color measurements obtained from the vacuum impregnation and drying processes are presented in Table 3 for a* (greenness/redness), b* (blueness/yellowness), chroma, hue and lightness values. Except for the b* values the impregnation process significantly changed the fruit color of Braeburn apple slices. It was observed that within the impregnation step, the a* values for native Braeburn apple slices turned from greenness (negative a* values) to redness (positive a* values). The impregnation step decreased the lightness and hue of the apple parenchyma and chroma rose, compared to the native tissue. FD resulted in a decrease of redness whereas the lightness, the yellowness, the chroma and hue values of the enriched apple parenchyma elevated. The colors of MVD and AD samples showed a different behavior. The air dried apple chips were the darkest and characterized by stronger redness compared to freeze dried samples. Whereas the lightness, the yellowness and chroma values after the MVD application increased in comparison to the VI apple slice, the b*, chroma values and the hue of air dried material were strongly reduced. The measured hue of apple polyphenol enriched Braeburn slices was not significantly influenced after MVD, when compared to the VI apple slice. 3.6. Apple peel extract and VI solution stability To further investigate the degradation of the quercetin derivatives, the apple peel extract was heated at different temperatures as dry powder and solubilized in vacuum impregnation fluid. At 50 C (Tables 4 and 5), the total quercetin content was slightly reduced after 15 h in solution (7%) and in the dry power (10%). In contrast, a strong degradation of quercetin monoglycosides was found at 95 C, resulting in a slight increase of quercetin aglycone (Tables 4 and 5). The highest reduction at 95 C was found for quercetin-3-O-arabinoside in the dry powder extract, as well as in the vacuum impregnation fluid. 3.7. Influence of the storage on quercetin derivatives content The storage life of apple chips enriched with apple peel extract after VI and FD is shown in Fig. 6. Apple chips stored at 20 C in the dark contained higher quercetin derivatives levels than those stored at 40 C. The degradation curves were linear at both
In apples, quercetin glycosides are located predominately in the peel (Burda, Oleszek, & Lee, 1990; Tsao, Yang, Young, & Zhu, 2003). The VI technique was used to increase the apple peel polyphenolics in the fruit parenchyma. Due to vacuum impregnation, moisture and sugar content of apple slices increased markedly (MartinezMonzo, Martinez-Navarrete, Chiralt, & Fito, 1998) before the drying process. This finding requires a thorough investigation of the drying behavior. 4.1. Drying processes In order to evaluate the resulting product and processing parameters of the MVD, two additional different drying regimes, freeze drying (FD) and air drying (AD), were also conducted. The MVD drying process was completed in 130 min and resulted in a moisture content of 9.8%. In contrast, FD required a drying period of 72 h and during air drying at 50 C with continuous air stream the moisture content was still 12.7% after 15 h. The latter drying parameters and results are in accordance with the study of Fernández, Castillero, and Aguilera (2005). The relatively short drying period and the resulting low water content of the MVD apple slices are due to the pulsed energy entry combined with the continuous vacuum. Consequently, the product quality can be improved while pulsed drying shortens the duration time of the drying process (Mousa & Farid, 2002). The process regime leads to a puffing effect of the compact parenchyma structure of the apple slice, thereby enhancing the porosity. In turn, the increased porosity will enhance the release of the water vapor. Furthermore, such short power-off and power-on times can increase energy efficiency compared to continuous MVD (Yongsawatdigul & Gunasekaran, 1996). Unpublished data of our own research showed that the effect of the microwave pulse (1000 W, 60 s) on the parenchyma puffing is dependent on the water content of the apple slices. At water contents above 70%, the puffed structure will collapse again, whereas at markedly lower water contents, the structure of the parenchyma is too firm to allow a puffing effect. Similar to the AD process, the drying rate dropped clearly when the product weight decreased below 30% related to the start weight (100%). Also the product temperature rose from 35 C to 50 C, indicating slow water diffusion from the inner parts of the parenchyma to the surface. 4.2. Surface area and bulk density The bulk density was affected differently by the drying procedures. The low density found for FD apple slices is typical for freeze dried material. The compact structure after MVD and AD shows that the drying was associated with changes in the microstructure and cellular collapse, which is a result of tissue contraction and thus associated with high bulk density. Despite the breakup of the compact structure with the MVD puffing process, the porosity is increased only slightly compared to AD. For a larger puffing-effect, it is necessary to increase energy input, which is also accompanied by a strong temperature increase (data not shown). High temperatures during AD can prevent shrinkage, because the
B. Schulze et al. / LWT - Food Science and Technology 57 (2014) 426e433
431
Table 1 Bulk density of apple chips after freeze drying (FD), microwave vacuum drying (MVD) and air drying (AD), data SD (n ¼ 8). Bulk density (g/ml) 0.33 0.05 0.69 0.09 0.75 0.05
FD MVD AD
outer parts dry quickly, become rigid and their fixed, final tissue volume cannot contract as strongly (Wang & Brennan, 1995). In our study, the maximum temperature of 50 C was not exceeded in order to protect heat-sensitive ingredients. 4.3. Quercetin derivative contents Low degradation of quercetin derivatives was found for apple slices dried by FD and MVD, whereas the AD processes exhibited a strong impact. When AD was conducted after MVD, the quercetin content was not affected. Rupasinghe, Wang, Huber, and Pitts (2008) investigated the retention of quercetin derivatives from apple peel extract in muffins during baking. After 20 min at 175 C, they found 55.9%e73.4% of the quercetin glycosides added to the muffin dough (Rupasinghe et al., 2008). Also, the 3-glycosidic bond is described as more thermo-sensitive than glycosylation at the 40 glycosidic position (Rohn et al., 2007). To further investigate the degradation, the stability of the apple peel extract was investigated in the VI solution at 50 C and 95 C as well as in dried form. The degradation was quite low in all cases (7%e10% loss) and did not explain the strong degradation found in AD apple slices. Therefore, other mechanisms, in addition to the thermal degradation, are responsible for the decrease of quercetin glycoside during AD. The absence of oxygen due to vacuum conditions during the MVD and FD drying process caused the inhibition of oxidation reactions and enzymatic degradation by PPO. The low degradation during AD of apple slices previously adjusted to MVD can be explained by the low moisture content at the beginning of the air drying process. The low water residue affects the mobility of the reactants, therefore the rate of oxidation and enzyme activity is reduced (Lavelli & Vantaggi, 2009). This explanation is also supported by the color change of the apple slices, i.e. the strongest browning was found for AD apple slices. The quercetin glycosides degradation at AD and stability investigation at 50 C and 95 C differs for apple peel extract and VI solution. The combination of temperature and time, as well as the glycosidic bonded sugar influenced the degradation kinetics. At 50 C, the auto-oxidation of quercetin occurs, so that the content
Fig. 5. Quercetin derivative profiles of freeze dried (white bars), microwave vacuum dried (graygray bars), and air dried (dark graygray bars) Braeburn apple slices after vacuum impregnation, data are mean SD (n ¼ 8), bars marked with different letters of the single quercetin derivatives are significantly different (p < 0.05 by one-factorial ANOVA with LSD post hoc test).
decreases, whereas at 95 C, quercetin aglycone content increased, due to a deglycosylation reaction of the monoglycosides. The decreasing content, compared to the initial concentration for the quercetin derivatives, is in descending order quercetin-3-Ogalactoside quercetin-3-O-glucoside > quercetin-3-Orhamnoside > quercetin-3-O-xyloside > quercetin-3-O-arabinoside. The order is similar to stability results in apple juice (van der Sluis, Dekker, & van Boekel, 2005). Under roasting conditions of the pure quercetin derivatives, the order of degraded quercetin monoglycosides differed except for quercetin-3-O-galactoside, which was most stable (Rohn et al., 2007). Our findings demonstrate that based on a moderate temperature profile curve and the absence of oxygen, MVD maintains a high concentration of apple peel flavonoids. These findings are consistent with other MVD development studies that aim to preserve sensitive and valuable ingredients during drying (Böhm et al.,
Table 3 Color measurement data of Braeburn apple slices in native, vacuum impregnated and dried form, data are determined in quadruple SD (n ¼ 8). a*
Table 2 Quercetin derivative profiles of vacuum impregnated Braeburn apples after different drying applications, data are expressed in percent SD (n ¼ 8). Percentage of total quercetin derivatives (%) FD Quercetin-3-O-galactoside Quercetin-3-O-glucoside Quercetin-3-O-xyloside Quercetin-3-O-arabinoside Quercetin-3-O-rhamnoside Quercetin
37.4 10.6 9.2 25.6 11.3 5.9
MVD
1.1 0.5 0.7 0.6 0.9 0.8
36.9 10.9 9.5 26.5 11.6 4.7
MVD-AD 0.8 0.5 0.5 0.5 0.7 0.5
35.6 10.3 10.2 27.3 12.1 4.4
FD: freeze drying. MVD: microwave vacuum drying. MVD-AD: microwave vacuum drying with air end drying. AD: air drying. MVD-AD: microwave vacuum drying with air end drying.
0.7 0.5 0.3 0.6 0.3 0.5
AD 36.2 12.2 6.7 25.9 14.1 4.9
1.0 0.5 0.8 0.4 0.6 0.4
Native VI FD Native VI MVD MVD-AD Native VI AD
3.2 8.5 4.8 2.9 8.7 10.4 13.6 2.8 9.2 16.2
b*
0.5a 1.1b 0.9c 0.4a 0.9b 1.8d 1.8e 0.6a 1.4b 0.6f
33.2 34.9 47.7 32.6 33.9 48.3 50.1 35.1 34.8 15.3
3.6a 2.7a 2.8b 3.0a 3.3a 3.9b 3.5b 3.7a 3.1a 1.0c
33.3 35.8 48.0 32.7 35.0 49.7 52.0 35.1 36.0 22.3
L*
h
C*
3.6a 2.7b 2.8c 2.9a 3.2b 3.9c 2.9c 3.7a 3.2b 0.9d
93.8 84.8 93.6 94.4 84.1 86.5 83.1 94.9 83.6 48.2
0.8a 5.2b 1.2a 0.9a 2.2b 2.6b 2.6b 1.4a 2.0b 1.9c
85.1 53.4 70.8 84.7 52.7 63.9 63.8 83.7 52.4 29.1
1.3a 3.0b 2.1c 1.0a 2.3b 3.4d 2.7d 1.7a 2.1b 0.9e
Letters aef: data with the same letter in columns are significantly not different, p < 0.05 by one-factorial ANOVA with LSD post hoc test. VI: vacuum impregnation. FD: freeze drying. MVD: microwave vacuum drying. MVD-AD: microwave vacuum drying with air end drying. AD: air drying. FD: freeze drying.
432
B. Schulze et al. / LWT - Food Science and Technology 57 (2014) 426e433
Table 4 Quercetin glycoside and aglycone contents of vacuum impregnation solution depending on the heating duration at 50 C and 95 C, data are determined in triplicate SD. Time (h)
Vacuum impregnation solution content (mg/g DM) Quercetin-3-Ogalactoside
Initial 3 6 9 12 15 Initial 3 6 9 12 15
At 50 C 34.84 0.45 34.04 0.30 33.99 0.48 33.50 0.30 32.86 0.39 32.98 0.03 At 95 C 34.84 0.45 30.68 0.44 28.26 0.35 24.91 0.44 21.05 0.57 15.60 0.49
Quercetin-3-Oglucoside
(%)
Quercetin-3-Oxyloside
Quercetin-3-Oarabinoside
Quercetin-3-Orhamnoside
Quercetin
Total quercetin
9.59 9.33 9.10 9.25 9.04 9.08
0.14 0.14 0.16 0.15 0.10 0.02
12.35 11.93 11.59 11.84 11.49 11.52
0.17 0.19 0.15 0.19 0.13 0.03
27.32 26.41 26.19 25.83 24.83 24.78
0.24 0.29 0.22 0.22 0.21 0.07
12.88 12.26 12.20 12.37 12.30 12.18
0.10 0.16 0.15 0.18 0.21 0.03
4.86 4.90 4.83 4.78 4.74 4.64
0.06 0.09 0.17 0.04 0.06 0.02
100 98.9 97.9 97.6 95.3 93.5
9.59 8.57 8.08 7.28 6.20 5.05
0.14 0.13 0.09 0.15 0.15 0.24
12.35 9.52 7.21 5.13 3.24 1.61
0.17 0.11 0.04 0.26 0.13 0.16
27.32 16.38 8.63 4.14 1.68 0.45
0.24 0.09 0.03 0.39 0.19 0.22
12.88 10.68 9.09 7.64 5.68 3.90
0.10 0.09 0.05 0.18 0.22 0.24
4.86 10.78 16.71 19.52 19.79 14.63
0.06 0.33 0.27 0.26 0.26 0.32
100 86.6 78.0 68.6 57.6 40.5
Table 5 Quercetin glycoside and aglycone contents of apple peel extract depending on the heating duration at 50 C and 95 C, data are determined in triplicate SD. Time (h)
Apple peel extract content (mg/g DM) Quercetin-3-Ogalactoside
Initial 3 6 9 12 15 Initial 3 6 9 12 15
At 50 C 33.22 0.63 33.39 0.12 32.86 0.45 32.55 0.50 31.41 1.24 30.68 1.76 At 95 C 33.22 0.63 33.22 0.35 31.91 1.05 31.42 0.69 31.94 0.96 30.29 1.11
Quercetin-3-Oglucoside
(%) Quercetin-3-Oxyloside
Quercetin-3-Oarabinoside
Quercetin-3-Orhamnoside
Quercetin
Total quercetin
9.20 9.28 8.88 8.67 8.38 8.23
0.23 0.12 0.18 0.15 0.26 0.52
11.77 11.94 11.41 11.16 10.86 10.53
0.31 0.32 0.18 0.12 0.45 0.62
26.06 26.62 25.40 25.12 24.36 23.56
0.61 0.45 0.39 0.23 0.99 1.31
12.38 12.63 11.97 11.66 11.35 11.02
0.32 0.21 0.38 0.08 0.36 0.61
5.11 4.86 4.74 4.67 4.48 4.44
0.12 0.04 0.17 0.09 0.15 0.30
100 99.7 94.6 93.8 90.8 89.6
9.20 8.98 8.56 8.46 8.52 8.07
0.23 0.05 0.31 0.22 0.47 0.46
11.77 11.47 10.65 10.56 10.46 9.83
0.31 0.12 0.37 0.19 0.53 0.41
26.06 24.56 21.95 20.73 19.65 18.16
0.61 0.15 0.62 0.40 1.78 1.57
12.38 12.17 11.37 11.12 11.60 10.74
0.32 0.21 0.45 0.40 0.27 0.42
5.11 5.68 6.94 7.82 9.35 9.15
0.12 0.17 0.32 0.72 1.32 1.39
100 96.0 94.3 90.1 91.5 87.4
2006; Clary, Mejia-Meza, Wang, & Petrucci, 2007; Cui, Xu, & Sun, 2004a). 4.4. Color The VI dependent browning is caused by the impregnation solution, which consists of the brown color apple peel extract dissolved in apple juice. The transfer of the brown solution and
occlusion of air from the porous tissue are connected with the elevation in browning and a decrease in lightness. Thus, the color changes of freeze-dried apple chips are mainly due to the vacuum impregnation. Braeburn apple slices without a VI step before FD showed only minor color changes (data not shown). Similar results were reported for VI application in apples and other fruits (Alzamora, Tapia, & López-Malo, 2000). Also, the greenness, which is highly related to color changes of apple flesh
Fig. 6. Stability of quercetin glycosides in vacuum impregnated and freeze dried apple chips during storage with two temperature conditions (n ¼ 3), black dots (dotted line) depict quercetin-3-O-galactoside); dark gray upward triangles (dotted line) depict quercetin-3-O-glucoside; gray diamonds (solid line) depict quercetin-3-O-xyloside; grey downward triangles (solid line) depict quercetin-3-O-arabinoside; black squares (dotted line) depict quercetin-3-O-rhamnoside; grey dots (solid line) depict quercetin.
B. Schulze et al. / LWT - Food Science and Technology 57 (2014) 426e433
(Goupy et al., 1995), decreased with the impregnation step to redness. These results were consistent with other apple VI investigations (Alzamora et al., 2000). Beside the redness, the yellow color of the apple product increased during FD and MVD. The data were consistent with other drying studies (Fernández et al., 2005; Krokida, Maroulis, & Saravacos, 2001). Under the experimental drying conditions, AD produced drastic changes in color, especially for lightness, b* value, hue and chroma. The dark browning is caused by non-enzymatic and enzymatic browning reactions as confirmed by the strong degradation of quercetin derivatives. The drying under vacuum, the short drying time and the low thermal load during MVD and FD, as compared to AD, caused overall lower color changes of the apples slices. 4.5. Storage Investigation of the stability of freeze-dried apple chips over a period of one year showed that the levels of quercetin derivatives were significantly influenced by storage temperature and storage time. It is assumed that enzymatic and non-enzymatic reactions are responsible for the degradation. Storage under moderate temperature conditions was associated with better preservation of quercetin glycosides. Enriched apple chips stored at 20 C for 12 months still showed significant levels of quercetin derivatives. 5. Conclusion This study showed that similar to freeze drying (FD), gentle microwave vacuum drying (MVD), of vacuum impregnated apple slices maintained high quercetin contents in the parenchyma. In contrast, air drying (AD) resulted in significant losses of quercetin derivatives and marked browning, indicating extensive oxidation reactions. Due to the moderate temperature regime (50 C max) during drying, strong structural changes occurred in MVD and AD apple slices. MVD apples slices showed slightly higher porosity than AD apple chips and both drying procedures resulted in high shrinkage. In contrast, FD apple slices showed significantly higher porosity and the lowest shrinkage. Acknowledgments This work was financially supported by the German Federal Ministry of Education and Research (BMBF 0313856A) within the project ‘Functional Foods for Vascular Health e from Nutraceuticals to Personalised Diets’. The authors would like to thank the company Püschner Microwave Power Systems for providing equipment for vacuum microwave drying. References Alzamora, S. M., Tapia, M. S., & López-Malo, A. (Eds.). (2000). Aspen food engineering series. Minimally processed fruits and vegetables: Fundamental aspects and applications. Gaithersburg, Md: Aspen Publishers. Böhm, V., Kühnert, S., Rohm, H., & Scholze, G. (2006). Improving the nutritional quality of microwave-vacuum dried strawberries: a preliminary study. Food Science and Technology International, 12(1), 67e75. Burda, S., Oleszek, W., & Lee, C. Y. (1990). Phenolic compounds and their changes in apples during maturation and cold storage. Journal of Agricultural and Food Chemistry, 38, 945e948. Chiralt, A., Fito, P., Barat, J. M., Andrés, A., González-Martínez, C., Escriche, I., et al. (2001). Use of vacuum impregnation in food salting process. Journal of Food Engineering, 49(2e3), 141e151. Clary, C. D., Mejia-Meza, E., Wang, S., & Petrucci, V. E. (2007). Improving grape quality using microwave vacuum drying associated with temperature control. Journal of Food Science, 72(1), E023eE028. Cui, Z. W., Xu, S. Y., & Sun, D. W. (2004a). Effect of microwave-vacuum drying on the carotenoids retention of carrot slices and chlorophyll retention of Chinese chive leaves. Drying Technology, 22(3), 563e575.
433
Cui, Z. W., Xu, S. Y., & Sun, D. W. (2004b). Microwaveevacuum drying kinetics of carrot slices. Journal of Food Engineering, 65(2), 157e164. Drouzas, A. E., Tsami, E., & Saravacos, G. D. (1999). Microwave/vacuum drying of model fruit gels. Journal of Food Engineering, 39(2), 117e122. Ekström, A. M., Serafini, M., Nyrén, O., Wolk, A., Bosetti, C., & Bellocco, R. (2011). Dietary quercetin intake and risk of gastric cancer: results from a populationbased study in Sweden. Annals of Oncology, 22(2), 438e443. Fernández, L., Castillero, C., & Aguilera, J. M. (2005). An application of image analysis to dehydration of apple discs: IV Iberoamerican Congress of Food Engineering (CIBIA IV). Journal of Food Engineering, 67(1e2), 185e193. Goupy, P., Amiot, M., Richard-Forget, F., Duprat, F., Aubert, S., & Nicolas, J. (1995). Enzymatic browning of model solutions and apple phenolic extracts by apple polyphenoloxidase. Journal of Food Science, 60(3), 497e501. Guamis, B., Trujillo, A. J., Ferragut, V., Chiralt, A., Andres, A., & Fito, P. (1997). Ripening control of Manchego type cheese salted by brine vacuum impregnation. International Dairy Journal, 7(2e3), 185e192. Gunasekaran, S. (1999). Pulsed microwave-vacuum drying of food. Drying Technology: An International Journal, 17(3), 395e412. Hagen, S. F., Borge, G. I. A., Bengtsson, G. B., Bilger, W., Berge, A., Haffner, K., et al. (2007). Phenolic contents and other health and sensory related properties of apple fruit (Malus domestica Borkh., cv. Aroma): effect of postharvest UV-B irradiation. Postharvest Biology and Technology, 45(1), 1e10. Hertog, M. G. L., Feskens, E. J. M., Hollman, P. C. H., Katan, M. B., & Kromhout, D. (1993). Dietary antioxidant flavonoids and risk of coronary heart disease: the Zutphen Elderly Study. The Lancet, 342(8878), 1007e1011. Jimenez, M., & Garcia-Carmona, F. (1999). Oxidation of the flavonol quercetin by polyphenol oxidase. Journal of Agricultural and Food Chemistry, 47(1), 56e60. Knekt, P., Kumpulainen, J., Järvinen, R., Rissanen, H., Heliövaara, M., Reunanen, A., et al. (2002). Flavonoid intake and risk of chronic diseases. The American Journal of Clinical Nutrition, 76(3), 560e568. Krokida, M. K., Maroulis, Z. B., & Saravacos, G. D. (2001). The effect of the method of drying on the colour of dehydrated products. International Journal of Food Science & Technology, 36(1), 53e59. Lavelli, V., & Vantaggi, C. (2009). Rate of antioxidant degradation and color variations in dehydrated apples as related to water activity. Journal of Agricultural and Food Chemistry, 57(11), 4733e4738. Lin, T. M., Durance, T. D., & Scaman, C. H. (1998). Characterization of vacuum microwave, air and freeze dried carrot slices. Food Research International, 31(2), 111e117. Lin, D. S., Leonard, S. W., Lederer, C., Traber, M. G., & Zhao, Y. (2006). Retention of fortified vitamin E and sensory quality of fresh-cut pears by vacuum impregnation with honey. Journal of Food Science, 71(7), S553eS559. Martinez-Monzo, J., Martinez-Navarrete, N., Chiralt, A., & Fito, P. (1998). Mechanical and structural changes in apple (Var. Granny Smith) due to vacuum impregnation with cryoprotectants. Journal of Food Science, 63(3), 499e503. Mousa, N., & Farid, M. (2002). Microwave vacuum drying of banana slices. Drying Technology: An International Journal, 20(10), 2055e2066. Rohn, S., Buchner, N., Driemel, G., Rauser, M., & Kroh, L. W. (2007). Thermal degradation of onion quercetin glycosides under roasting conditions. Journal of Agricultural and Food Chemistry, 55(4), 1568e1573. Rupasinghe, H. P. V., Wang, L., Huber, G. M., & Pitts, N. L. (2008). Effect of baking on dietary fibre and phenolics of muffins incorporated with apple skin powder. Food Chemistry, 107(3), 1217e1224. Schieber, A., Hilt, P., Streker, P., Endreß, H.-U., Rentschler, C., & Carle, R. (2003). A new process for the combined recovery of pectin and phenolic compounds from apple pomace. Innovative Food Science and Emerging Technologies, 4(1), 99e107. Schulze, B., Peth, S., Hubbermann, E. M., & Schwarz, K. (2012). The influence of vacuum impregnation on the fortification of apple parenchyma with quercetin derivatives in combination with pore structures X-ray analysis. Journal of Food Engineering, 109(3), 380e387. van der Sluis, A. A., Dekker, M., & van Boekel, M. A. J. S. (2005). Activity and concentration of polyphenolic antioxidants in apple juice. 3. Stability during storage. Journal of Agricultural and Food Chemistry, 53(4), 1073e1080. Solovchenko, A., & Schmitz-Eiberger, M. (2003). Significance of skin flavonoids for UV-B-protection in apple fruits. Journal of Experimental Botany, 54(389), 1977e 1984. Soysal, Y. (2004). Microwave drying characteristics of parsley. Biosystems Engineering, 89(2), 167e173. Tsao, R., Yang, R., Young, J. C., & Zhu, H. (2003). Polyphenolic profiles in eight apple cultivars using high-performance liquid chromatography (HPLC). Journal of Agricultural and Food Chemistry, 51(21), 6347e6353. Wang, N., & Brennan, J. G. (1995). Changes in structure, density and porosity of potato during dehydration. Journal of Food Engineering, 24(1), 61e76. Wolfe, K. L., & Liu, R. H. (2003). Apple peels as a value-added food ingredient. Journal of Agricultural and Food Chemistry, 51(6), 1676e1683. Yongsawatdigul, J., & Gunasekaran, S. (1996). Microwave-vacuum drying of cranberries: Part I. Energy use and efficiency. Journal of Food Processing and Preservation, 20(2), 121e143. Yousif, A. N., Durance, T. D., Scaman, C. H., & Girard, B. (2000). Headspace volatiles and physical characteristics of vacuum-microwave, air, and freezedried oregano (Lippia berlandieri Schauer). Journal of Food Science, 65(6), 926e930.