Changes of Radical Scavenging Activity and Polyphenols Content During Storage of Dried Apples

Abstract

The aim of the present research was to analyze the influence of the heating method used during convection, microwave-convection, and infrared-convection drying of apple tissue on its polyphenols content and radical scavenging activity, as evaluated both directly after drying and after a 12-month period of storage. In the course of drying, a decrease in radical scavenging activity and polyphenols content occurred. At the same time, changes in the former parameter depended on the length of the drying process. The kinetics of the decrease were described by means of first-order reactions. Furthermore, radical scavenging capacity as well as polyphenols content significantly decreased during the storage.

Keywords:

• Dried apples
• Polyphenols content
• Radical scavenging activity
• Storage
• Degradation parameters

INTRODUCTION

Free radicals are groups of atoms or particles having one or more unpaired electrons on the valence shell, which causes them to be chemically unstable and highly reactive. An extremely important role in preventing the effects of free radical activity is performed by antioxidants, which are supplied to the human organism in food. Free radicals can be neutralized by endogenous mechanism of enzymatic protection or by non-enzymatic mechanisms, including agents such as vitamins, β-carotene, some metals like selenium or manganese, and polyphenols.[1,2]

Fruits and vegetables serve as an important source of antioxidants in the human diet[3] and their deficiency leads to various health disorders. Research in biochemistry indicated that diseases, such as atherosclerosis, tumors, accelerated aging, heart attack are caused by free radicals and reactive products of their activity.[4,5] Preservation of plant material by drying opens a possibility of producing quality food, which can be a sources of natural antioxidants in a simple and time-efficient manner.[6] Drying is the longest known method of preserving food, and thus it is important to the food industry.[7,8] Many of its branches employ this process in specific, usually final stages of the production. Drying results in a decrease of mass and volume of the subjected food products, which reduces packaging costs, as well as facilitates transport and storage.[9]

The drying process may result in a considerable reduction of bioactive components of food.[10,11] At the same time, the degradation of antioxidant substances, which already begins during the technological process, follows into the storage period.[12] Storage stability of dried food greatly depends on the final moisture content in the product, as well as on the storage conditions, among which the major impact are temperature, humidity, light exposure, type of surrounding atmosphere, and used packaging. Thus, some of the adverse changes occurring in food may be harnessed both by introducing an appropriate processing procedure, and by optimizing the storage conditions.[13]

With the increase of the storage time, the amount of nutritional substances contained in dried food decreases.[14] Research conducted on prunes,[15] parsley,[16] or red pepper (paprika)[17] has proven that during their storage significant reduction of vitamin C content occurs. Accordingly, due to the falling content of vitamin C, along with the occurring transformations of polyphenolic compounds, storage has a highly unfavorable effect on the antioxidant potential of food products.[18] For instance, after the period of 36 weeks, the content of polyphenols in a batch of pear juice falls on average to the values between 5 and 68% of the original content.[19] Similarly, the content of tocopherols in dried pepper decreases considerably during storage, whereas carotenoids content decrease is minimal.[17]

Modern drying techniques make it possible to produce high quality dried material in a relatively short time. These methods include drying, using infrared radiation[20] or microwaves.[21 ⁻ 24] Drying with the use of microwaves, for example, allows to reduce time required to dry kiwi fruit by 40–89%,[21] bananas by 64%,[22] and apples by 50%,[23] in comparison to convection drying. Moreover, with the increase of used microwave power, drying progresses even more rapidly,[25] which frequently leads to better quality of the final product. Material obtained by microwave drying is superior in terms of color, flavor, nutritional value, and microbiological stability. Furthermore, this drying technique causes enzyme inactivation, and ensures better rehydration properties of the restored product when compared to the fresh material.[26] The infrared drying, however, is regarded as a technique which provides prime quality dried food, particularly fruits, vegetables, and cereals.[6] What is more, infrared radiation is lethal to some microorganisms.[27] It has been proven that convection drying with the use of infrared radiation can reduce the time necessary to dry peaches even 2–2.5 times.[28] Similarly, for carrots and potatoes, drying time was shortened by about 48% when compared to convective drying.[29] Finally, acceleration of the drying process by using microwaves or infrared radiation, reduces the negative effects of biochemical changes taking place in the product due to the shorter period of the direct contact with oxygen.[20]

As a matter of fact, issues related to antioxidant properties of dried fruits and their changes during storage are very rarely discussed in academic literature. This situation calls for scientific research that would investigate the problem more closely, especially devoting attention to non-conventional methods of drying which have a great advantage over infrared radiation and microwaves. Accordingly, the purpose of the present study was to analyze the impact of heating method, i.e., convection, microwave-convection, and infrared-convection drying, on polyphenols content and radical scavenging activity of apple tissue as evaluated directly after drying and after the storage period of 12 months.

MATERIALS AND METHODS

Experimental Procedure

Apples (var.Idared) were collected from the Department of Fruit Culture Experimental Fields of SGGW (Warsaw University of Life Sciences, Warsaw, Poland) and stored for 1 months after harvesting at 5–8°C and at 90% air humidity. The skin was removed from the apples and they were cut into slices 0.005 ± 0.001 m thick and diameter of 0.03 m. The cut material was immersed in the 0.1% citric acid solution to prevent enzymatic browning reactions. Finally, the cubes were blotted with filter paper and spread on the dryer's screens.

Drying Methods

Drying experiments were conducted by three drying methods: convective drying, microwave-convective drying and infrared-convective drying. Convective drying was carried out in a laboratory dryer at air temperature of 70°C and air velocity 2 m·s⁻¹. The dryer was loaded with 0.25 kg (1.92 kg·m⁻²) of apple slices that were spread on screens in a single layer. The air flow was parallel to the screens and the drying process continued until constant mass was reached.

Microwave-convective drying was carried out in a laboratory dryer by Promice Tech Inc. (Wrocław, Poland). The energy emission was 300 W. The samples were placed on a rotating tray in a double layer on shelves and positioned perpendicularly to the flow of air. The air velocity was 3.5 m·s⁻¹ and temperature 40°C. The dryer was loaded with 0.25 kg (2.4 kg·m⁻²) of apple slices. The drying continued until about 10% water content in the dried material was reached.

Infrared-convective drying was performed in a dryer equipped with nine infrared electric bulbs from Philips (Amsterdam, Holland). The distance between the emitters and the heated surface was 0.2 m. The emitters with total power of 7.875 kW·m⁻² were mounted on the rack. The flow of ambient air over the heated surface was set at 1.2 m·s⁻¹. The dryer was loaded with 0.25 kg (1.26 kg·m⁻²) of apple slices spread on a shelf in a single layer. The drying process was carried on until constant mass was achieved.

During drying both mass and temperature of the material were recorded continuously for each method. The parameters of the microwave-convective and infrared-convective drying were chosen in such a way that the temperature inside the slice was similar to the temperature applied during the convective drying. The temperature inside slices of infrared-convective-dried apples was measured by thermocouples from CzakiThermo-Product (Raszyn-Ryble, Poland), which were placed in geometrical center of several slices. During microwave-convective drying the temperature of the slice surface was measured by a pyrometer (RAYTEK, MI, Berlin, Germany) every 5 min. During measurement the microwave energy was not active.

Storage Conditions

Dried apples selected for storage originated from a single batch. The dried material was packed in PE/Al/PE foil bags of high barrier capacity. During the process of packaging, which was carried out with the use of a PP 5.4 packing machine by TEPRO (Koszalin, Poland), first 80% of the air was removed, and then the bags were sealed. Thus, preserved material was then stored for 12 months at three different temperatures: 4, 25, and 40°C. The relevant tests were carried out after 1, 3, 6, and 12 months of storing. For the each of the considered storage temperatures and periods, three portions of the material were prepared (i.e., there were three repetitions of each test). Moreover, 40°C storage conditions was considered as an accelerated storage test (in comparison to the room temperature of 20°C). According to the literature data,[30] the Q ₁₀ approach can be used to evaluate the storage time of the product. Q ₁₀ factor can be defined as a ratio of the reaction rate constants at the reference temperature and other one that is differing by 10°C. Therefore, this concept delivers a simple tool to perform the accelerated shelf-life time tests. Q ₁₀ value for most of reactions that lead to the nutrient loss and degradation of chemical compounds can be considered as equal to 3.10 (for the Ea = 85 kJ/mol and at 20°C). Thus, it means that storage for 12 months at 40°C corresponds to the ca. 115 months, which can be calculated according to the following equation:

(1)

where is reaction rate at T ₂ (s⁻¹); is reaction rate at T ₁ (s⁻¹); Q ₁₀ is temperature dependent reaction rate coefficient, ΔT is temperature difference T ₂ and T ₁. Since the reaction time can be expressed as τ = 1/v (s), the above equation can be defined as:

(2)

Therefore, (storage time at 20°C that corresponds to the 12 months storage time at 40°C), when Q ₁₀ = 3.1 will be equal to 115 months.

Analysis

In fresh and dried material the content of dry matter was measured according to PN-90/A-75101/03 standard by drying to the constant weight at 105°C.[31] For the convection, microwave-convection, and infrared-convection dried material, dry matter content was equal to 94.3 ± 0.07, 92.5 ± 0.6, and 92.1 ± 1.4%, respectively. The contents of polyphenols as well as antioxidant capacity were determined in extracts produced from the raw and dried apples tissue. First, 5 g of raw material was transferred into a flask, and about 50 cm³ of the 80% ethanol was added. The samples were homogenized and boiled for 1 min under the reflux condenser. The extract was filter into a volumetric flask. The amount of the dried material needed for analysis was calculated so that the solids content of dried apples corresponded to the solid content of 5 g of raw apples. Three independent extraction experiments were performed and the arithmetic mean was taken for data interpretation.

Radical scavenging activity was determined by a method that involved assessing the degree to which DPPH radicals were neutralized by the antioxidants present in fresh and dried apple extracts within a 30-min time period, as specified by Brand-Williams et al.[32] For the measurement of the antioxidant capacity, the apples’ extract (0.05, 0.08, 0.12, 0.15, 0.20, 0.25, 0.30, and 0.35 ml) was transferred into separate tubes and filled till 2 ml with 80% aqueous solution of ethanol, then mixed with 2 ml of DPPH solution. The tubes were covered with stoppers and the contents were mixed and stored in the darkness for 30 min. Absorbance measurements were carried out in a spectrophotometer Heλios ThermoSpectronic γ at a wavelength of 515 nm against the sample blank (80% aqueous ethanol solution).

The content of polyphenolic compounds was determined by the Folin-Ciocalteu method, using chlorogenic acid as reference.[1] Initially, 30 ml of distilled water, 1.5 ml of extract, and 2.5 ml of Folin reagent were transferred to flasks. After mixing the contents of the flasks and then waiting 3 min, 5 ml of sodium carbonate solution was added and supplemented with distilled water to a volume of 50 ml. After mixing, the solution was left in the dark for 1 h at room temperature and then absorbance was measured in a spectrophotometer Heλios ThermoSpectronic γ at a wavelength of 750 nm. A reference sample without extract was used as a control. The absorbance values were read on the calibration curve and polyphenol content was expressed in mg of chlorogenic acid/100 ml.

These two procedures, i.e., determination of the radical scavenging activity and polyphenols content, were repeated six times and the results were converted into dry mass basis. Furthermore, changes in the radical scavenging activity and polyphenols content were also being evaluated in the course of drying by scrutinizing the material samples after the removal of 20, 60, and 90% of the moisture. The material used in these tests originated from a different batch than the one used for the storage assay. The kinetics of degradation of polyphenols content and radical scavenging activity during drying was described by means of the following first-order reaction:

(3)

where C is polyphenols or radical scavenging activity after elapsed time ; C ₀ is initial polyphenols or radical scavenging activity; k is rate constant (s⁻¹); and τ is process time (s).

Statistical Analysis

All results were subjected to the analysis of variance (ANOVA) using Statgraphics Plus 4.1 software (StatPoint Technologies, Inc., Warrenton, VA, USA). Individual group differences were identified using Duncan's multiple range tests with the probability level set at 0.05.

RESULTS AND DISCUSSION

Radical Scavenging Activity and Polyphenols Degradation During Drying

In the course of drying, plant material is subjected to changes that usually result in deterioration of its quality. Specifically, the content of compounds with antioxidant capacity falls[10,19,33] due to the occurring oxidation and thermal processes.[34] Regarding the impact of the moisture removal degree on its radical scavenging activity and polyphenols content, it was concluded that during convective and microwave-convective drying a significant initial degradation of radical scavenging activity took place (Table 1). At the relative water content of 0.8 (i.e., after the removal of 20% of moisture), radical scavenging activity decreased by about 22%. With progressing moisture decrease, a substantial reduction of radical scavenging capacity ensued in the convective-dried material (in comparison to the samples obtained by the other techniques). Similar dependency was observed for polyphenols content, i.e., the intensive reduction occurred in the initial phase, regardless of the drying technique used (Table 1). It has to be noted though, that when infrared radiation was in use, at high initial moisture content, the reduction of radical scavenging activity progressed more slowly. However, at the relative moisture content between 0.8 and 0.4 the most intensive reduction was observed around 22 percentage points. As for relative moisture content below 0.4, the radical scavenging capacity of the dried material was comparable with the values achieved in the microwave-convective method.

Table 1  Changes of radical scavenging activity and polyphenols content during drying using different drying methods

Type of material	Relative water content	Changes of radical scavenging activity [%]	Changes of polyphenols content [%]
CD	1.0	100.0 ± 5.0 ^a	100.0 ± 6.1 ^a
	0.6	78.3 ± 3.9 ^b	89.7 ± 4.5 ^ab
	0.4	60.7 ± 3.0 ^c	86.5 ± 4.3 ^b
	0.1	54.9 ± 2.7 ^d	78.5 ± 3.9 ^bc
	0.06	49.7 ± 2.5 ^e	74.0 ± 3.7 ^c
MCD	1.0	100.0 ± 5.0 ^a	100.0 ± 6.1 ^a
	0.6	77.9 ± 3.9 ^b	85.7 ± 4.3 ^b
	0.4	75.1± 3.8 ^b	83.6 ± 4.1 ^b
	0.1	72.3 ± 3.6 ^b	79.1 ± 3.8 ^bc
	0.06	69.2 ± 3.5 ^bc	77.6 ± 3.7 ^bc
IRCD	1.0	100.0 ± 5.0 ^a	100.0 ± 6.1 ^a
	0.6	95.2 ± 4.7 ^a	90.8 ± 4.4 ^a
	0.4	73.2 ± 3.7 ^b	88.1 ± 4.1^ab
	0.1	69.8 ± 3.5 ^bc	85.7 ± 4.2 ^b
	0.06	65.2 ± 3.3 ^c	83.5 ± 4.2 ^b
^a,b,c,d,eThe same letters show homogeneous groups during drying process.
Figures in ± represent standard deviation. Significant differences p < 0.05.

Degradation of antioxidant capacity and polyphenols content in the analyzed material was determined using a first-order equation (Figs. 1 and 2). The equations were adjusted based on regression coefficients R ² values, which were in the range of 0.741–0.919. The effect of drying time on the degradation of antioxidant capacity is expressed by dependencies 4–6:

Figure 1 Effect of drying method on radical scavenging activity in apples.

Figure 2 Effect of drying method on polyphenols content in apples.

(4)

(5)

(6)

where C_CD, C_MCD , and C_IRCD are radical scavenging activity after elapsed time of convective, microwave-convective, and infrared-convective drying, respectively; C ₀ is initial radical scavenging activity; and is process time (s). The value of the rate constant determines how rapidly degradation of radical scavenging activity progressed the drying process. Values of the rate constants for degradation of antioxidants in apple tissue depended on the drying technique. The highest value, equal 1·10⁻⁴ s⁻¹, was observed in the case of microwave-convective drying which was the shortest method used and lasted only 60 min. The total reduction of ensuing antioxidant capacity, despite the rapid pace of the degradation, was much lower than in the case of the other methods. On the other hand, lower values of k in the convective (k = 8·10⁻⁵ s⁻¹) and infrared-convective (k = 7·10⁻⁵ s⁻¹) methods indicate that the rate of the degradation was slower. Nevertheless, in the case of these methods the extended time of the process caused a substantial loss of the antioxidant properties of apple tissue.

Similarly, the kinetics of polyphenols degradation in the course of drying was expressed by means of the following dependencies (Eqs. (7)–(9)):

(7)

(8)

(9)

where C_CD, C_MCD , and C_IRCD are polyphenols after elapsed time of convective, microwave-convective, and infrared-convective drying, respectively; C ₀ is the initial polyphenols; and is process time (s). Value of rate constant determines how rapidly degradation of polyphenols progressed the drying process. The highest value of rate constant for degradation of polyphenols was recorded for microwave-convective drying (k = 7·10⁻⁵ s⁻¹), and a lower value was observed for convective and infrared-convective techniques (k = 3·10⁻⁵ s⁻¹).

Time of drying resulting in relative water content of 0.1 g H₂O/g db depended on the applied technique and the corresponding heating method. In particular, convective drying lasted 160 min, microwave-convective drying took 60 min, and infrared-convective drying proceeded for 116 min. The quicker the drying process the more negative effects of biochemical changes are reduced due to the shorter material exposure to oxygen. When processing data was achieved for all of the analyzed kinds of dried material, an important linear relationship was discovered between radical scavenging activity and drying time (r = −0.618) (Fig. 3). However, no significant correlation between polyphenols content and drying time or between moisture content and antioxidant capacity or polyphenols content in the apple tissue could be discerned during the process of drying.

Figure 3 Dependence between radical scavenging activity and drying time.

In the course of convection drying, as a result of prolonged exposure of apple tissue to high temperatures, antioxidant capacity of the apples diminished notably. Specifically, in order to reduce 50% of DPPH radicals in the convective dried material, about 42% more apple extract was needed in comparison to raw material (Fig. 4). Likewise, during convective drying of Northern Highbush blueberry fruit (Vaccinium corymbosum) at a temperature of 70°C, it was observed that the antioxidant capacity of the material decreased by 41%.[10] Moreover, in convection drying of lime fruit, significant changes in antioxidant capacity could be observed. These changes became more pronounced with the increase of the drying temperature and the elongation of process’ time.[11] As for President plums that were first dried at 85°C and in later stages at 70°C, their antioxidant potential fell by 50% in relation to the plums dried at a constant temperature of 60°C.[35]

Figure 4 Changes of radical scavenging activity of dried material obtained by different drying methods.

The content of polyphenols in the convective dried apples was reduced about 29% (Fig. 5). The phenolic compounds existing in apples are primary substances responsible for the antioxidant properties.[36] Apart from polyphenols, there are other compounds that contribute to the total antioxidant capacity, such as vitamins and natural dyes. Due to that fact, the increased loss of bioactive properties may stem from the depletion of vitamins with antioxidant character.[37]

Figure 5 Changes of polyphenols content of dried material obtained by different drying methods.

Infrared-convective and microwave-convective dried apples resulted in smaller losses of polyphenols and antioxidant capacity compared with convective ones (Figs. 4 and 5). Nonetheless, in comparison to the fresh apple fruit, a statistically significant decrease of polyphenols content and radical scavenging activity was observed for the material from both microwave-convective and infrared-convective drying. There was a 33 and 38% observed decrease in radical scavenging activity, and 20 and 24% decrease in polyphenols content, respectively. Application of microwaves and infrared radiation for drying of fruits, in relation to regular convective drying, allows to reduce negative effects of biochemical transformations, most probably because of a shortened exposure of the material to oxygen.

Microwave-convection dried material, in confrontation with material dried by convection, possessed 25 percentage points higher radical scavenging activity, as well as about 5 percentage points higher polyphenols content (in reference to raw apple tissue), the latter difference being not statistically significant. Likewise, infrared-convective dried material, in the analogous comparison, proved to have radical scavenging activity higher by about 20 percentage points, and the content of polyphenols higher by about 9 percentage points (again, in reference to raw apple tissue). The results quoted above confirm that antioxidant capacity does not rely solely on the content of polyphenols. In the process of drying, dark-colored substances possessing antioxidant features may be formed, which would explain still relatively high radical scavenging activity. The Maillard reactions lead to various chemical changes in the product, and in consequence exert influence on the antioxidant properties.[37] Additionally, shorter time of the process in the case of both microwave-convective and infrared-convective drying facilitated preservation of a greater amount of compounds with antioxidant properties, such as vitamins.

Table 2  Changes of polyphenols content and radical scavenging activity of convective, microwave-convective, and infrared-convective dried apple during storage at different storage condition

		IC50			Polyphenols content [mg·(100g d.m.)^−1]
		Storage temperature [°C]			Storage temperature [°C]
Type of material	Storage time [month]	40	25	4	40	25	4
CD	0	4.94 ± 0.31 ^a	4.94 ± 0.31 ^a	4.94 ± 0.31 ^a	981 ± 76 ^a	981 ± 76 ^a	981 ± 76 ^a
	1	5.05 ± 0.13 ^a	5.01 ± 0.06 ^a	5.05 ± 0.09 ^a	1049 ± 76 ^a	1075 ± 17 ^a	1080 ± 46 ^a
	3	5.00 ± 0.10 ^a	4.99 ± 0.18 ^a	5.10 ± 0.06 ^a	952 ± 26 ^a	1032 ± 75 ^a	940 ± 49 ^a
	6	4.69 ± 0.22 ^ab	4.91 ± 0.07 ^a	4.75 ± 0.08 ^a	1005 ± 66 ^a	959 ± 87 ^a	1130 ± 88 ^a
	12	4.25 ± 0.07 ^c	4.62 ± 0.03 ^b	4.52 ± 0.03 ^b	1086 ± 60 ^a	1090 ± 49 ^a	1095 ± 94 ^a
MCD	0	4.15 ± 0.11 ^a	4.15 ± 0.11 ^a	4.15 ± 0.11 ^a	1044 ± 35 ^b	1044 ± 35 ^b	1044 ± 35 ^b
	1	4.69 ± 0.08 ^b	4.36 ± 0.08 ^ab	4.28 ± 0.10 ^a	935 ± 50 ^bc	1158 ± 42 ^a	1033 ± 39 ^b
	3	4.99 ± 0.20 ^bc	4.55 ± 0.17 ^b	4.72 ± 0.03 ^b	853 ± 58 ^c	1170 ± 35 ^a	942 ± 32 ^bc
	6	5.51 ± 0.08 ^e	4.70 ± 0.12 ^b	4.82 ± 0.06 ^bc	904 ± 58 ^c	1072 ± 91 ^ab	1042 ± 51 ^b
	12	5.34 ± 0.12 ^de	5.07 ± 0.09 ^c	4.67 ± 0.17 ^b	949 ± 47 ^bc	862 ± 44 ^c	944 ± 75 ^bc
IRCD	0	4.30 ± 0.39 ^a	4.30 ± 0.39 ^a	4.30 ± 0.39 ^a	1099 ± 43 ^a	1099 ± 43 ^a	1099 ± 43 ^a
	1	4.38 ± 0.05 ^a	4.42 ± 0.10 ^a	4.35 ± 0.09 ^a	1089 ± 107^a	1038 ± 16 ^a	1079 ± 112^a
	3	4.69 ± 0.12 ^a	4.44 ± 0.11 ^a	4.43 ± 0.14 ^a	997 ± 56 ^ab	918 ± 79 ^b	1112 ± 111^a
	6	4.69 ± 0.08 ^a	4.38 ± 0.17 ^a	4.66 ± 0.13 ^a	991 ± 15 ^a	1157 ± 64 ^a	895 ± 65 ^b
	12	4.42 ± 0.14 ^a	4.32 ± 0.07 ^a	4.66 ± 0.11 ^a	1002 ± 69 ^ab	976 ± 17 ^a	893 ± 123 ^b
^a,bThe same letters show homogeneous groups during storage.
Figures in ± represent standard deviation. Significant differences p < 0.05.

Storage

As a rule, during storage the content of nutrients contained in food diminishes.[14] Still, the character of changes in antioxidant capacity is somewhat different for dried products in comparison to fresh material, and it depends on the type of product and the storage conditions. In the initial stage, due to the oxidation of vitamin C along with the transformations of polyphenolic compounds, the general antioxidant capacity of stored dried food may fall dramatically.[18] However, after an extended storage time, frequently the antioxidant capacity begins to rise, which is caused by the emergence of new compounds with antioxidant properties, for instance as a result of the Maillard reaction or enzymatic browning.[14,35] Lack of air access or modifying the storage atmosphere allows to preserve a greater amount of antioxidants as confirmed, among others, by research concerning storage of dried grapes[38] and prawns.[39]

When considering the storage of the convective dried material, it turns out that the character of ensuing antioxidant compound transformations was far from unequivocal (Table 2). Initially, regardless of the chosen storage temperature, a slight decrease in radical scavenging activity was observed, but the change had no statistical significance. Incidentally, Caro et al.[35] noticed merely a minor fall of radical scavenging activity in dried plums, even after 4 months of storage. Later, for most of the storage period, the radical scavenging activity of the analyzed convective dried apples did not change significantly from the original value. Nevertheless, the material stored at both 4 and 25°C showed smaller changes of this parameter than the one stored at 40°C. Eventually, towards the end of storing, a significant increase in the antioxidant capacity occurred. Its value could have been incremented by newly appeared compounds with antioxidant character.[40] Caro et al.,[35] in their observations concerning the storage of dried plums in the course of 8 and 12 months, observed an analogous increase of antioxidant capacity and likewise explained it by the formation of new antioxidant compounds.

However, no remarkable changes in polyphenols content were noted during the storage of the convective dried material (Table 2). Nevertheless, there were observed similar changes as in the case of the analyzed antioxidant capacity. The increase noted in the final stages of storage could have been related to the appearance of new non-polyphenolic compounds that were nonetheless being determined by the test method. This fact can be explained by the Folin-Ciocalteu reagent, which was used for the determination, reacting not only with polyphenols, but also with other biochemical compounds present, such as vitamin C, amino acids, proteins, carbohydrates, and aromatic amines.[41] Accordingly, the actual content of polyphenols in the dried material might have been lower than indicated.

During the storage of the microwave-convective dried material, after the period of 1 month a decrease of antioxidant capacity was observed, but it proved to have statistical significance only in the case of the material stored at 40°C (Table 2). However, after 3 months of storage, a statistically significant decrease of radical scavenging activity was found in the material stored at all three temperatures. At the end of storage at temperatures of 40, 25, and 4°C, antioxidant capacity was 29, 38, and 51% of the raw apples, respectively. This significant decrease in radical scavenging activity of dried microwave-convective sample could have been caused by decomposition of chemical compounds with antioxidant properties. Also, the storage caused the reduction of polyphenols content. It was especially noticeable in the case of the samples kept at high temperatures (Table 2).

Finally, both the radical scavenging activity and polyphenols content in the stored material obtained with the use of infrared-convective drying reduced only to a small, statistically insignificant extent (Table 2, Figs. 6 and 7). The most considerable loss of polyphenolic compounds was observed in the sample stored for over 6 months at the lowest storage temperature (4°C), and it was a statistically significant change in relation to the content of polyphenols measured directly after drying (i.e., before storage). However, in infrared dried material stored at 25 and 40°C no significant changes in polyphenols content took place. Admittedly, this result might have been caused by a low selectivity of the test method used for determination of polyphenols. In fact, during storage at higher temperatures, certain compounds might have been formed, which were detected as polyphenols, while the actual content of polyphenols might have been lower.

Figure 6 Changes of radical infrared-convective dried apple scavenging activity during storage at different temperatures.

Figure 7 Changes of polyphenols content of infrared-convective dried material during storage at different storage conditions.

CONCLUSIONS

During the conventionally conducted process of convective drying, both radical scavenging activity and polyphenols content of the investigated apple tissue decreased markedly. By contrast, the material processed by non-conventional drying methods was characterized to possess higher values of the analyzed parameters. In comparison to raw apples, the microwave-convective method decreased the radical scavenging activity and polyphenols content respectively by 33 and 24%, whereas the infrared-convective method by 38 and 20%, respectively. In all of the cases, the decrease progressed in accordance with the kinetics of first-order reactions. The observed values of the radical scavenging activity and polyphenols content decreased also as an effect of storage. Decrease of these parameters depended on the drying method. Still, the obtained research results concerning the radical scavenging activity and content of polyphenols do not allow to draw clear conclusions as to the dependencies between storage temperature and changes in the analyzed characteristics. This is because during storage new compounds might have been formed, which were either antioxidants or revealed certain antioxidant properties, making it difficult to interpret the relationship. Admittedly, in the case of microwave-convective dried material, a dependency was observed (i.e., an increase of the storage temperature corresponds with much quicker decrease of radical scavenging activity). On the other hand, the most stable in terms of preserving radical scavenging capacity was the material obtained with the use of infrared-convection drying.

ABBREVIATIONS

FA	=	Fresh apple
CD	=	Convective drying
MCD	=	Microwave-convective drying
IRCD	=	Infrared-convective drying