Effects of microwave and hotair drying methods on colour, carotene ...

International Journal of Food Science and Technology 2013, 48, 1327–1333

Original article Effects of microwave and hot-air drying methods on colour, b-carotene and radical scavenging activity of apricots Donatella Albanese,1 Luciano Cinquanta,2* Gennaro Cuccurullo1 & Marisa Di Matteo1 1 Dipartimento di Ingegneria Industriale, Universita di Salerno, Via Ponte Don Melillo, 84084 Fisciano (SA), Italy 2 Dipartimento di Agricoltura, Ambiente e Alimenti, Universita del Molise, Via F. De Sanctis, 86100 Campobasso, Italy (Received 2 November 2012; Accepted in revised form 3 January 2013)

Summary

The effects of drying by microwave and convective heating at 60 and 70 °C on colour change, degradation of b-carotene and the 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) scavenging activity of apricots were evaluated. Microwave heating reduced significantly the drying time (up to 25%), if compared with convective one, also owing to the higher temperature reached during the last phase of the process, as monitored by infrared thermography. Colour changes of apricot surface, described with lightness and hue angle, in both drying methods followed a first-order reaction (0.927 R2 0.996). The apricots dried by microwave were less affected by the darkening phenomena. The evolution of b-carotene in fresh apricots (61.2 5.6 mg kg1 d.w.) during the drying highlighted a wider decrease (about 50%) when microwave heating was employed for both the temperatures used. Radical scavenging activity increased (P < 0.05) in all dried samples except for hot-air dried apricots at 60 °C.

Keywords

Apricot, b-carotene, colour, drying, infrared thermography, microwave, radical scavenging activity.

Introduction

To extend the shelf life of apricot, perceived as a healthy food and a desirable ingredient for fruit-based snacks, drying is the most commonly used method. Colour is a very important quality attribute of dried apricots, since colour changes affect directly consumer acceptance and can indicate retention of the pigment nutrients of dried fruits. Apricots owe their characteristic colour to carotenoid pigments, mainly b-carotene which is a precursor of vitamin-A. Various processing methods, such as drying, lead to the isomerisation and even degradation of b-carotene (Karabulut et al, 2007). Moreover, carotenoids are susceptible to oxidation when exposed to light (Pesek et al., 1990), oxygen and enzymes (Gregory, 1996). Because of their heat-sensitive nature, even other antioxidants, such as: phenolic compounds and vitamin C, degrade significantly during apricot heating. To preserve nutritional and sensorial quality of fresh apricots, careful selection of drying technique and optimisation of the drying conditions are thus significant (Sablani, 2006; Karatas & Kamısßlı, 2007). Microwave (MW) processing considerably reduces the time needed to reach the set point temperature and hence drying period (Sala*Correspondent: Fax: +39 0874404718; e-mail: [email protected]

zar-Gonzalez et al., 2012). However, critical points arise when performing MW heating, for instance in temperature readout and control where traditional probes failed and uneven temperature patterns arose (Metaxas & Meredith, 1988). The characteristic commonly chosen to criticise MW-assisted dried fruits is generally the surface colour, one of the main sensorial parameters that influence the acceptability of dried apricots. To apply an effective temperature monitoring on apricots surface, an improved MW drying system has been used here. The latter can automatically and continuously switch the power levels by acting on the magnetron duty cycle time, by means of IR thermography, able to control the surface temperature products (Huang & Sites, 2007). The objective of this study was to investigate the effectiveness of microwave plant, compared with hot-air, in terms of time of process, kinetics of colour changes, degradation of b-carotene and inhibition of DPPH scavenging activity during the drying of apricots. Materials and methods

MW prototype

An MW pilot plant was employed (Fig. 1): it housed a magnetron with a nominal power output of 2 kW operating at 2.4 GHz on an insulated metallic cubic

doi:10.1111/ijfs.12095 © 2013 The Authors. International Journal of Food Science and Technology © 2013 Institute of Food Science and Technology

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Quality of microwave dried apricots D. Albanese et al.

Figure 1 Microwave pilot plant heating system. (a) IR camera with computer for aided thermography and ON/OFF control; (b) technical balance; (c) magnetron and standard rectangular WR-340 waveguide with 2:1 aspect ratio; (d) cubic cavity; (e) turning table (f) hole with grid to look inside the chamber; (g) electric motor.

chamber (1 m3) (Cuccurullo et al., 2012). Apricot samples were placed on a porous Teflon rotating plate (50 cm diameter, 22 rpm). The plate carrying the samples was suspended under a technical balance (Libertine EU-C 1200 RS) which was located on the top of the oven where an infrared (IR) camera (thermaCAM Flir P65; FLIR SYSTEMS INC., Portland, OR, USA) was also placed. The camera focused on the rotatingplate plane looking through a circular hole (70 mm diameter) in the oven wall. The measured data, being sample weights, the reflected magnetron energy and duty cycle, were transferred each minute to a personal computer for control and recording purposes. An I/O board (AT MIO 16XE50; National Instruments, Austin, TX, USA) automatically adjusted the magnetron delivered power according to IR thermography temperature readout which was focused on the rotating plate. In particular, a suitably developed Lab View 7.1 code (National Instruments) scanned the actual IR image looking for the maximum temperature and decided to eventually switch on or off the magnetron depending on the position of the selected temperature with respect to a specified differential band (1 °C) over the target temperature. The code facilitated the heating process until the actual moisture content attained a conventional fixed end-value of 20%. Hotair (HA) drying tests were performed in a convective oven (Zanussi FCV/E6L3, Zanussi Professional, Pordenone, Italy); the fan speed and the air change rate were increased until the drying rate was no longer

International Journal of Food Science and Technology 2013

affected (Di Matteo et al., 2003). Weight changes from an initial moisture content of about 85% to a final one of 20% were registered off-line (technical balance: Libertine EU-C 166 1200 RS) during hot air and on line during MW. Experimental procedures

Fresh, fully ripe apricots var. Vitillo (Prunus armeniaca L.) were obtained from the local market (Napoli, Italy). Halved and pitted apricots, about 900 g, were dried in an air convective and microwave oven at 60 and 70 °C. The apricots had a spherical shape with a diameter of about 6.0 cm and a thickness (halved apricot) of 1.2 cm. All experiments were performed in triplicate. Before MW drying, IR camera was calibrated to account for the presence of the shielding grid (Cuccurullo et al., 2012). Colour and kinetic parameters

The colour of the apricot samples was measured offline during the hot-air and MW drying by a CR-200 Chromometer (Minolta, Konica Minolta Optics, Inc., Tokyo, Japan) having an aperture size of 10 mm. Hunter values (L*, a*, b*) were monitored on the surface of fresh and dried samples; nine set of data were obtained from the three replicates with three measurements for each replicate (Albanese et al., 2007). The lightness (L*) of the samples was measured as a per-

© 2013 The Authors International Journal of Food Science and Technology © 2013 Institute of Food Science and Technology


centage decrease during drying. Moreover, the hue angle (h°) parameter (tan1 b/a), that is frequently used to characterise colour in food products, was calculated to evaluate the colour change of dried apricot. The colour degradation during thermal treatment has been reported as zero-order or first-order kinetics (Koca et al., 2007; Saxena et al., 2012). The evolution of colour, described with lightness and hue angle, during hot-air and MW drying at 60 and 70 °C, were modelled by Equations 1–2 for a zero and first-order reaction respectively. C ¼ C0 k0 t

ð1Þ

lnC ¼ lnC0 k1 t

ð2Þ

where C is the concentration at time t; C0, the concentration at time zero; k0, the zero-order rate constant (h1); k1, the first-order rate constant (h1); t, the drying time (h). The goodness of fitted model was determined by the highest correlation coefficient (R2). b-carotene extraction and determination

After sampling, apricots were freeze dried by using a freeze dryer Genesis 25SES (VirTis Co., Gardiner, NY, USA) and carotenoids were extracted according to the method reported by Cinquanta et al. (2010). An aliquot (20 g) was separated (Mouly et al., 1999) by a reverse-phase HPLC system, comprising a Waters Model 600 solvent delivery system (Waters Corp. Milford, MA, USA). Separation was performed by a YMC (Hampsted, NC, USA) stainless steel column (250 9 4.6 mm i.d.), packed with 5 mm silica spheres that were chemically bonded with C30 material, at a flow rate of 1 mL min1. The mobile phase was methanol: methyl tert-butyl ether (MTBE): water (v/v/v), with the following gradient: 0–12 min (90:5:5), 25 min (11:89:0), 40 min (25:75:0), 60 min (50:50:0). The eluted compounds were monitored by a photodiode array detector (Waters 996) set at 430 nm. Chromatographic data and UV-visible spectra (range from 300 to 500 nm) were handled by a Waters Millennium driver station. b-Carotene was quantified on the basis of a commercial standard (Sigma Chemicals, St. Luis, MO, USA), after dilution in methanol: acetone (2:1 v/ v) (Fratianni et al., 2010). The analysis of b-Carotene were replicated three times.

to centrifugation at 4000 g for 10 min, after the supernatant was filtered by a 0.45 lm filter (Millipore). Successively, 50 lL of the solution was added in a cuvette containing 3 mL of a 6.1 9 105M methanol solution of DPPH. The bleaching of DPPH was recorded, after 2.5 h at 515 nm by a Perkin–Elmer lambda-Bio 40 (PerkinElmer Inc., Waltham, MA, USA) spectrophotometer at 25 °C (Asample). A blank experiment was also carried out applying the same procedure to a solution without the test material and the absorbance was recorded as Ablank. The AA was expressed as percentage inhibition of DPPH and then calculated according to the following equation: Ablank Asample ð%Þ inhibition of DPPH ¼ 100 Ablank The analysis of percentage inhibition of DPPH was replicated three times. The results obtained from fresh and dried apricot samples were reported on dry basis. Statistical analysis

Experiments were performed in triplicate. Data reported are the mean and the standard deviation calculated from three replicates. The analysis of variance (ANOVA) was applied to the data. The least significant differences were obtained using an LSD test (P < 0.05). Statistical analysis was performed using SPSS version 13.0 for Windows (SPSS, Inc., Chicago, IL, USA). Results and discussion

Temperature control during microwave drying

The average temperatures recorded by IR thermography during the MW drying period, were equal to the set value (Table 1). However, fluctuations during the process were observed, shown by the standard deviation that was higher when the higher temperature was set. Moreover, the fluctuations became larger as the process continued (Fig. S1), owing to the increase in sample power density (2.2 W per g at time 0), due to the progressive mass reduction. In particular, the maximum temperature overshoot recorded during the last period of drying at 70 °C was about 18.0 °C (Table 1), thus showing that fruit overheating could

DPPH radical scavenging method

The antioxidant activity (AA) was evaluated by the DPPH scavenging method (Brand-Williams et al., 1995). In its radical form, DPPH absorbs at 515 nm, but upon reduction by an antioxidant or a radical species its absorption disappears. Apricot samples were homogenised in 25 mL of distilled water and subjected

Table 1 Average temperatures, standard deviations (SD), maximum and minimum temperatures during apricots drying by microwave T °C

Tavg (°C)

SD

Tmax (°C)

Tmin (°C)

60 70

59.9 70.1

4.6 7.4

77.3 88.1

47.4 54.8



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isolated mass of liquid, where surface driving forces related to heat and mass transfer are constant, leading to the constant rate period. When the internal resistance to the transport of liquid water to the surface increases, thereby lowering the drying rate, the falling rate period occurs. Hence, internal diffusion may be assumed to be the mechanism responsible for water loss during the drying process. At lower drying temperatures a more uniform moisture distribution exists, that allow the sample to continue to shrink until the last stages of drying with a low porosity. On the contrary, at higher air temperatures, case hardening of the surface may occur and the volume of the sample becomes fixed at an earlier stage, inducing less shrinkage and a more intensive pore formation (Bonazzi & Dumoulin, 2011). Figure 2 Drying curves of apricots by hot air and microwave at 60 and 70 °C.

take place and that temperature control becomes critical at high temperatures and for long periods of heating. Obviously, heating rates realised in setting the magnetron exceeded the radioactive convective cooling rates. Drying curves

As expected, apricot drying was faster at higher temperatures and the reduction time was approximately proportional to the temperature levels both for hot-air and MW heating (Fig. 2). MW significantly reduced the drying time (about 3 h less), in comparison to convective heating. The final moisture content (20% g water per g wet matter) of the apricot was reached after about 12 and 18 h of MW heating set at 70 and 60 °C respectively. This behaviour can be first explained by recalling that apricot surface temperatures recover the target values in a few minutes during MW heating, whereas longer times are required for hot-air heating (To grul & Pehlivan, 2003). Moreover, the highest temperature reached mainly during the last phase of the process MW pointed out as the comparison of the two heating techniques appeared almost as a comparison between the different drying temperatures. Nevertheless, MW absorption provoked internal water heating and evaporation, greatly increasing the internal pressure and concentration gradients and thus the effective water diffusion (Sumnu et al., 2005). In hot-air heating the gap between air and sample temperatures always decreased as the drying process continued because in the early stages sensible heating dominates while, in the later stages, the progressive reduction in the evaporation rates was responsible for the lower drying rates. Once the constant temperature level is attained, the situation is analogues to an


Kinetics of colour change

The evolution of the apricots’ colour during drying was investigated by colorimetric Hunter values. The browning of the apricots was preliminarily evaluated by the percentage reduction in lightness at the end of drying. L* values have shown significant reductions of nearly 30% and 34.4% in the samples heated by HA at 60 and 70 °C respectively. The apricots dried by MW were less affected by the darkening phenomena: L* values decreased by about 24.4% and 20.1%, when temperatures of 60 and 70 °C were set. The fresh apricots showed positive values for both a* (7.9) and b* (42.8) values, thus indicating an orange-yellow colour. During the drying in all samples, the increase in a* values was conversely proportional to the decrease in b* values (data not reported). The development of discolouration of samples during drying may be related to pigment destruction, ascorbic acid browning, enzymatic and non-enzymatic browning (Abers & Wrolstad, 1979; Ibarz et al., 1999). Versari et al. (2008) reported that in apricot juice the formation of nonenzymatic browning was mainly due to the acid-catalysed dehydration of sugar. In apricots, a strong polyphenol oxidase (PPO) activity has also been reported by Arslan et al. (1998). A modelling approach, which involves the use of various kinetic orders, is widely used to predict changes in fruit colour during drying. As previously reported (Villota & Hawkes, 2007; Saxena et al., 2012), the evolution of Hunter parameters differently elaborated during the drying process of vegetable products can follow a zero-, first-, secondor pseudo first-order reaction kinetics. Zero- and firstorder kinetic models were applied here to describe the colour change in apricots during different drying modes and temperatures. Both models were found appropriate to describe the L* and h° data, with a coefficient of determination (R2) ranging from 0.927 to 0.996 (Table 2). The values of kinetic constant in



(a)

Figure 4 b-carotene changes in apricots during drying by hot air and microwave at 60 and 70 °C. Different letters (a, b, c..) indicate significant differences (P < 0.05) among the apricot samples during the drying by a specific heating method and temperature, different numbers (1, 2….) indicate significant differences (P < 0.05) among apricot samples for a specific drying time.

(b)

Figure 3 Kinetic plots of changes in: (a) Lightness (L*); (b) hue angle (h°) of apricots dried by hot air and microwave at 60 and 70 °C.

first-order model were very close, when considering the L* parameter in MW and HA samples, dried both at 60 and at 70 °C (Fig. 3a). Maskan (2001) reported an increase in the constant rate of L* values in the MW

mode in comparison to the HA mode at 60 °C, during the dehydration of kiwifruit slices. The latter could be justified not only by the different pigments of fruits, but also by the different MW equipment, unable to control the temperatures during the drying process. Instead, by considering the h° parameter, the firstorder kinetic model showed k1 values lower in apricots dried by HA with respect to those dried by MW for both temperatures (Fig. 3b, Table 2). Moreover, the rate constants of lightness and hue angle kinetics increased with the increase in temperature, regardless of drying method. Finally, considering temperatures and drying times used in our trials, it can be argued that changes in colour parameters could be caused mainly by two mechanisms non-enzymatic browning and degradation of carotenoid pigments (Ibarz et al., 1999). b-carotene changes

Table 2 Kinetics parameters for the thermal degradation of Lightness (L*) and hue angle (h°) of apricot samples during microwave (MW) and hot air (HA) drying at 60 and 70 °C

Parameter L*

h°

Zero order model

First order model

Drying method

K0 (h1)

R2

K1 (h1)

R2

HA 60 °C HA 70 °C MW 60 °C MW 70 °C HA 60 °C HA 70 °C MW 60 °C MW 70 °C

1.0162 1.5456 0.8563 1.1140 0.4246 1.0249 0.567 1.0631

0.9565 0.9729 0.9875 0.9723 0.9934 0.9398 0.9809 0.9655

0.0169 0.0278 0.0158 0.0254 0.0053 0.0130 0.0078 0.0179

0.9749 0.9857 0.9821 0.9778 0.9957 0.9267 0.9879 0.9481

Among carotenoid pigments, it is mainly b-carotene which is responsible for the specific colour of apricot varieties. The percentage of b-carotene in total carotenoids has been reported to vary from 39% to 65%. (Ruiz et al., 2005). In our trials b-carotene represented about one half (61.2 5.6 mg kg1 dry matter) of the total carotenoids in fresh apricots, as reported by Kurz et al. (2008). b-carotene is prone to degradation, and more precisely to isomerisation, especially at high temperatures and oxidation. The latter can follow three different pathways: autoxidation, photo-oxidation and enzyme oxidation by lypoxigenases (Penicaud et al., 2011). As expected on the basis of the temperatures recorded on the apricot surface, for both temperatures



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Figure 5 Antioxidant activity changes of apricots during drying by hot air and microwave at 60 and 70 °C. End drying = 21, 18, 15 and 12 h, for HA 60 °C; MW 60 °C, HA 70 °C and MW 70 °C respectively. Different letters (a, b, c..) indicate significant differences (P < 0.05) among the apricot samples during the drying.

set MW has caused a higher b-carotene decrease (P < 0.05), in comparison to hot-air method (Fig. 4). For instance, convective drying at 60 °C caused a significant decrease (P < 0.05) in b-carotene, equal to about 20% after 8 h of process, while a similar decrease was attained after only 2 h in samples dried by MW at 70 °C. In hot-air dried samples the reduction in b-carotene was significantly lower (P < 0.05) at 60 °C (20%), than at 70 °C (40%), while no differences were shown in MW dried samples, in which b-carotene decrease was equal to about 50% (Fig. 4). Lastly, no correlation between b-carotene content and colour values was found during drying, probably because of the neo-formation of dark pigments due to non-enzymatic browning. Radical scavenging power

Antioxidant activity (AA) in fresh fruits is due to the presence of carotenoid, polyphenol and vitaminic compounds (Wang et al., 2011), that act as free-radical scavengers and chain breakers, complexers of pro-oxidant metal ions and quenchers of singlet-oxygen formation. In our study the antioxidant activity measured as inhibition of DPPH scavenger activity was related only to hydrophilic compounds because of extraction method used for the analysis. A significant (P < 0.05) decrease in AA during drying was observed only for HA samples dried at 60 °C (Fig. 5). An opposite behaviour was observed for MW heating and HA samples dried at 70 °C in which the AA was significantly (P < 0.05) higher than the fresh samples. Similar results were previously obtained by Igual et al. (2012). Piga et al. (2003) observed that the decrease in


antioxidant capacity of dried plums in contrast to fresh fruit was linked to drying temperature: AA decreased at 60 °C and increased at 85 °C. Madrau et al. (2009) found antioxidant values four times higher in apricots dried at 75 °C than in fresh apricots. This behaviour could be explained by two factors: (i) oxidation of polyphenols that leads to the formation of stable intermediates which exhibit strong antioxidant capacity (Cheigh et al., 1995); (ii) formation of Maillard Reaction Products (MRPs) known to exhibit various antioxidant properties (Morales & Perez, 2001). The highest inhibition of DPPH was found for apricot samples dried by MW at 70 °C, whereas no significant differences (P < 0.05) were found between apricot samples dried by hot air at 70 °C and those by MW at 60 °C (Fig. 5). This behaviour may be associated with the increase in temperature during the end phase of the MW (Fig. S1) that, in this case, speeds up the non-enzymatic browning phenomena. Since the quantification of radical scavenging activity of carotenoids requires lipophilic extraction no correlation between the decrease in b-carotene and increase in antioxidant activity can be studied. Conclusions

Microwave heating reduced significantly the drying time in comparison to convective heating at set temperatures. The apricots’ surface temperature fluctuations became larger while the MW process progressed, owing to the increase in sample power density (2.2 W per g at time 0). A lower lightness (L*) loss in apricots dried by MW in comparison with those dried by hotair was observed. The discolouration of apricots during drying, measured by L* and hue values, followed both a zero- and a first-order kinetics. MW caused a higher decrease in b-carotene respect to hot-air mode, and significant differences were recorded by comparing the different temperatures in convective drying mode. Moreover, the change in radical scavenging power found in MW and HA at 70 °C dried samples, was significantly (P < 0.05) higher than in fresh fruit. References Abers, J.E. & Wrolstad, R.E. (1979). Causative factors of colour deterioration in strawberry preserves during processing and storage. Journal of Food Science, 44, 75–78. Albanese, D., Cinquanta, L. & Di Matteo, M. (2007). Effects of an innovative dipping treatment on the quality of minimally processed Annurca apples. Food Chemistry, 105, 1054–1060. Arslan, O., Temur, A. & Tozlu, I. (1998). Polyphenol oxidase from Malatya Apricot (Prunus armeniaca L.). Journal of Agricultural and Food Chemistry, 45, 2861–2863. Bonazzi, C. & Dumoulin, E. (2011). Quality changes in food materials as influenced by drying processes. In: Modern Drying Technology Volume 3: Product Quality and Formulation, 1st edn, (edited by



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Supporting Information

Additional Supporting Information may be found in the online version of this article: Figure S1. Temperature fluctuations on apricots during microwave drying at 60 and 70 °C detected by infrared thermography.



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