Total Phenolics Content, Antioxidant Capacity, Physicochemical Attributes, and Browning Susceptibility of Different Apple Cultivars for Minimal Processing

ABSTRACT

The total phenolics content, browning susceptibility, antioxidant capacity, and other physicochemical attributes of five cultivars of apple with different chilling hours requirements were studied to be minimally processed. Granny Smith (GS) and Red Delicious (RD) cultivars (high chill requirement), and Caricia (C), Eva (E), and Princesa (P) cultivars (low chill requirement) were studied. The flesh color, firmness, juiciness, pH, acidity, soluble solids, and flesh browning development, and total phenolics content and antioxidant capacity in flesh and peel were determined. All attributes were significantly different (p ≤ 0.001) among cultivars. RD showed the highest values of soluble solids and pH, and GS, the lowest. GS and P had the highest values of firmness and juiciness. GS, P, and E showed the lowest browning development. RD had the highest phenol content in flesh, followed by E and C. Phenolic content in peel was 2–5 times higher than in flesh for all varieties. The antioxidant capacity of RD flesh was higher than the other four apple cultivars. The antioxidant capacity of the apple peel was 2–4 times higher than the flesh, being RD the highest, followed by C, GS, and P with 50% less. Considering the lower browning development, and higher values of firmness and juiciness, GS and P would be the most suitable cultivars for minimal processing. If fresh-cut apples are not peeled, GS and P would increase their phenolic content in 28–56% and their antioxidant capacity in 65–78%.

KEYWORDS:

• Minimally processing
• apple cultivar
• bioactive compounds
• Malus domestica Borkh
• fruit quality
• total phenolics content
• browning development

Introduction

Minimally processed apples are valued primarily for their convenience and healthy potential. These products are generally defined as fruits that have been washed, peeled, cut, and packaged to retain their natural properties. The nutritional value, safety, and healthy potential of fresh-cut fruits are becoming increasingly more important for a well-informed consumer (Robles-Sánchez et al., 2007). Taking this into account, each process operation must be designed appropriately to obtain a fresh-cut fruit that maintains its quality and provides the maximum of their bioactive potential.

In all cases, the quality of fresh-cut products, considering both its physicochemical attributes and nutritional value or healthy potential, will depend on the quality of the whole fruit, which may change depending on the cultivar used. This is the reason why the cultivar selection is probably one of the most important steps in fresh-cut fruit processing (de Ancos et al., 2009; Piagentini et al., 2012; Pirovani et al., 2015; Wolfe et al., 2003). The elaboration of minimally processed fruits should start with the selection of the most suitable cultivar, evaluating characteristics, such as flesh texture, bioactive compounds, and browning potential, among other physicochemical attributes (de Ancos et al., 2009; Martín et al., 2011; Piagentini et al., 2012; Pirovani et al., 2015).

Apple is one of the most widely cultivated fruits. Although still incipient, the process of varietal change is recorded as a response to global market preferences (Bruzone, 2008).

The main limitation for apple growing in temperate regions of Argentina is the requirement of chilling hours. The apple tree needs a minimum period of cold during the year to breaking of dormancy, that is, to start a new growth cycle in the spring (Dobrzanski et al., 2006). As a result of breeding work, there are cultivars with lower chill requirements (400–500 chill hours per year), such as Caricia, Eva, and Princesa (Castro et al., 2015). These cultivars have the advantage that they are harvested before those of other Argentinian traditional regions, when the supply of apples with high chilling hours decreased. These three apple cultivars, which are grown in much lower volumes, are not yet well known in international markets, and we are not aware of any published data on their suitability for minimal processing, among other characteristics.

The apple (Malus domestica Borkh) intake has always been associated with health benefits. Its composition makes it one of the most complete fruits from a nutritional point of view. It is rich in minerals, vitamins, and sugars. The fiber content of the apple, mainly the pectin content, is considered beneficial for gastrointestinal functions, while helping to balance the level of blood sugar and cholesterol (Dobrzanski et al., 2006; Seipel et al., 2009). Apples contain high levels of biologically active compounds, such as the polyphenols, which not only contribute to color, bitterness, and astringency but also act as antioxidants and may help provide protection against cardiovascular disease and cancer (Dobrzanski et al., 2006; Khanizadeh et al., 2008; Rodrigo-Garcia et al., 2006).

The phenolic compounds are also responsible for one of the major color changes that occur on fresh-cut apples, the enzymatic browning. For the occurrence of this reaction, the presence of oxygen and the enzyme polyphenoloxidase (PPO), in addition to the phenolic compounds, are required. Consequently, to obtain the positive aspects of the phenolic compounds without the undesirable browning, those cultivars with a high concentration of phenolic compounds and low PPO activity should be selected for minimal processing (Chang et al., 2000).

Therefore, the objective of this study was to determine the physicochemical characteristics, browning susceptibility, phenolic content, and antioxidant capacity of flesh and peel of five apple cultivars with different chilling hours requirements (Granny Smith and Red Delicious—with recognized commercial importance worldwide and with high chilling hours requirement; and Caricia, Eva, and Princesa—produced in the central-east of the province of Santa Fe and with low chilling hours requirement), together with their suitability for being minimally processed.

Materials and methods

Plant material and sample preparation

Fruits from five apple tree cultivars (Malus domestica Borkh) currently grown commercially and available from two different regions of Argentina, were selected for this study. Granny Smith (GS) and Red Delicious (RD) cultivars, both commercially important worldwide and with a high chill requirement during growing, were harvested from a commercial orchard in Rio Negro province. Caricia (C), Eva (E), and Princesa (P) are low chill requirement apple cultivars cultivated in the central-east region of Santa Fe province. Fruits from the five apple cultivars were harvested at optimum maturity assessed by the starch iodine test (starch index value of 4; Chu, 2000), transported to our Institute, and stored in a cold room (0 to 2 °C and 90–95% RH) until analysis. From the fruits collected, 15 free of defects were selected from each cultivar, and these were divided into three replicate groups of five apples.

Apples were prepared as fresh-cut fruits. Whole fruits were washed with a 100-mg L^–1 solution of NaClO for 2 min. Then, the peels were separated from the flesh with a sharp stainless steel knife obtaining a peel thickness of 1 mm. Fruits were cored and cut in eight wedges with a sharp stainless steel knife. Whole fruits, peels, cores, and waste material were weighted to determine the yield as a minimally processed apple [%Y], expressed as percentage [%Y = (g of wedges 100 g^–1 whole fruit) 100].

Firmness measurement

Apple firmness was determined by measuring the force required by an 11-mm-diameter probe to penetrate the peeled apple flesh to a depth of 7.9 mm using a penetrometer Penefel DFT 14 (Digital Firmness Tester, Agrotechnology, Serqueux, France), expressing the results in newton [N].

Juiciness, soluble solid content, and pH

The juice of peeled and cored apples was extracted with a table centrifuge juicer, expressing the results as g juice 100 g^–1 fresh weight. Measurements of soluble solid content of apple flesh of each fruit cultivar were made with a hand-held digital refractometer model PAL-ALPHA (Atago Co. 124 Ltd, Tokyo, Japan) with automatic temperature compensation. Results were expressed as °Brix. A pH meter (Horiba Cardy Twin B-113, Kyoto, Japan) was used for pH determination.

Total acidity

Potentiometric titration was performed with a Boeco pH-electrode BA17 (Boeckel + Co, Hamburg, Germany). A sample of 10 g of crushed apple flesh diluted 10× with distilled water was titrated with a solution of 0.1 N sodium hydroxide until a pH of 8.1. The analysis was performed by duplicate and results were expressed as g malic acid 100 g^–1 fresh weight (FW).

Color measurement

Color changes of fresh-cut apples were measured on the surface of the wedges of each apple cultivar. Measurements were made at the middle point of the two cut flat surfaces of each fruit wedges during 90 min after cutting the wedges.

Color (CIELAB values) was measured using a Minolta spectrophotometer (Model CM-508d/8, Minolta, Tokyo, Japan), calibrated using the standard white tile. D65/10° was used as the illuminant/viewing geometry and specular component excluded (SCE). L* defines the lightness (0: black, 100: white), a* the red-greenness (a* > 0 or a* < 0, respectively), and b* the blue-yellowness (b* < 0 or b* > 0, respectively).

Chroma value [C*_ab = (a*² + b*²)° .⁵], hue angle (h_ab = arctangent b*/a*; 0°: Red, 90°: yellow, 180°: green, 270°: blue), and total color difference [∆E*_ab = (∆L*² + ∆a*² + ∆b*²)° .⁵], were also determined. ∆E*_ab was calculated for each sample at each testing time with respect to its initial value (t = 0 min). The more representative color parameters were included in the results.

Each color parameter was modeled using zero-order kinetic models, in order to compare the browning development rate of each apple cultivar. The following general equation could describe the color parameter change rate:

(1)

where Q = color parameter; t = time (min); n = reaction order, 0; and kq = change rate constant for the color parameter Q (min^–1). The sign (+) corresponds to the parameters a*, b*, C*_ab, and ∆E*_ab that increase with time, and the sign (–) to the parameters h_ab and L*, that decrease with time.

Total phenolic content and antioxidant capacity determination

Extraction procedure

Extractions were made on 5 g of crushed apple flesh or peel, as corresponds in 50 ml of acetone:water (80:20) during 15 min with ultrasound and then were centrifuged at 12,000 g at 4 °C. The extract was used for total phenolic content and antioxidant capacity determination.

Total phenolic content (TPC)

Total phenolic content was determined using the Folin-Ciocalteu reagent method (Singleton and Rossi, 1965). Aliquots of 1 ml flesh or 0.2 ml peel extracts were mixed with 0.5 ml of Folin-Ciocalteu reagent, 1 ml of 10% sodium carbonate solution, and distilled water up to 10 ml using a vortex mixer and allowed to react for 30 min at room temperature before absorbance was measured at 760 nm in a spectrophotometer (Genesis 5, Milton Roy, Ivyland, PA, USA). Total phenolic content analysis was performed by triplicate in each sample and results were expressed as gallic acid equivalents (mg GAE 100 g^–1 FW).

Antioxidant activity (AEAC)

The antioxidant activity of the samples was estimated by determining the free-radical scavenging capacity evaluated with the stable 1,1-diphenyl-2-picryl hydrazyl free radical (DPPH*), according to Brand-Williams et al. (1995). For this purpose, the absorbance decrease of a methanol DPPH* solution in the presence of sample extract was measured at 517 nm. The initial DPPH* concentration was 0.03 gL^–1 and the readings were taken after allowing the reaction mixture to stand for 30 min. The antioxidant activity was expressed as ascorbic acid equivalent antioxidant capacity (AEAC; Lim et al., 2007), using Eq. (2):

(2)

Table 1. Firmness, juiciness, soluble solids, pH, and acidity of the flesh of five apple cultivars.

Cultivar	Firmness (N)	Juiciness (g.100g^–1 FW)	Soluble solids (°Brix)	pH	Acidity(g MA.100g^–1 FW)	SSC:acid ratio (°Brix/g MA.100g^–1 FW)^y
Caricia	48.53c^z	48.58d	13.4c	3.7c	0.32b	41.8b
Eva	44.25c	56.91cd	14.8b	3.8b	0.25b	59.2b
Granny Smith	77.43a	78.18a	12.2d	3.4d	0.52a	23.4c
Princesa	65.70b	68.60ab	13.0cd	3.9b	0.30b	43.3b
Red Delicious	51.66c	64.72bc	16.2da	4.2a	0.18c	90.0a

^zValues in the same column with different letters are significantly different (p ≤ 0.05) by Duncan’s test.

^ySSC: soluble solid content.

where IC_50(AA) was the amount of ascorbic acid (AA) into 1 ml reaction needed to decrease by 50% the initial DPPH* concentration, obtained from the % DPPH* _remaining versus concentration (mg AA ml^–1 reaction) plot.

IC_50(sample) was the amount of sample into 1 ml reaction needed to decrease by 50% the initial DPPH* concentration, obtained from the % DPPH* _remaining versus concentration (mg FW ml^–1 reaction) plot.

The changes in absorbance were measured at a room temperature of about 25 °C. The percentage of remaining DPPH* was calculated as:

(3)

where A_sample = sample absorbance; A_control = absorbance of DPPH* solution.

Statistical analysis

Data were subjected to analysis of variance (ANOVA). Duncan’s multiple range tests at 5% level of significance were performed to determine any significant difference among samples. Analyses of variance and Duncan’s multiple range tests were also made at two evaluation times (0 and 90 min after cutting) for each color parameter among cultivars. Regression analysis of color data and the comparison of the linear regression models obtained for each color parameter and cultivar were assessed stating differences among k values. Correlation analysis among color parameters, physicochemical attributes, AEAC, and TPC, were also done. The statistical analyses were performed with Statgraphics Plus 5.1 software (Statpoint Technologies, Inc., Warrenton, VA, USA).

Results and discussion

The yield obtained for the five apple cultivars processed as fresh-cut product was 80–82% (apple peeled, cored, and cut into eight wedges). The peel (1-mm thick) represented about 12–16% of the whole fruit and 15–17% of the wedges for the five apple cultivars studied.

Physicochemical attributes

A universal constituent of apple quality regardless of cultivar is firmness. The lowest firmness values were found in Caricia, Eva, and Red Delicious apple cultivars (48.53, 44.25, and 51.66 N, respectively), whereas the highest values were found in Princesa (65.70 N) and Granny Smith (77.43 N) apple cultivars (Table 1).

The importance of juiciness has been demonstrated by numerous studies. Apparently, inability of cells to release juice has a greater impact than total moisture content to determine juiciness. For example, water content of juicy and mealy apples was similar, but the last ones have a dry mouth-feel, because cells are separated at the middle lamella, rather than being ruptured and releasing juice during chewing (Dobrzanski et al., 2006).

As seen in Table 1, Granny Smith and Princesa have the highest juiciness values, followed by Red Delicious similarly to what was found for firmness.

Soluble solids (SSC) and total acidity are commonly used to assess fruit quality, and a change in the ratio between these parameters can have a great impact on the taste of the apple (Beaulieu and Gorny, 2002; Hagen et al., 2007). Regarding the taste-related attributes, Table 1 shows that Red Delicious apples have the highest values of soluble solids and pH (16.2 °Brix and 4.2), and Granny Smith has the lowest values (12.2 °Brix and 3.4). Malic acid is the major acid in apple, and it plays a major role in the flavor attribute. It was found that the Granny Smith apples presented the highest value of acidity (0.52 g MA 100 g^–1 FW), which agreed with the characteristics associated with this variety and the results found by other authors (Dobrzanski et al., 2006; Drogoudi et al., 2008). The Caricia, Eva, and Princesa cultivars have lower and similar acidity values (from 0.25 to 0.32 g MA 100 g^–1 FW), and, finally, the least acidic is Red Delicious (0.18 g MA 100 g^–1 FW). Abbott et al. (2004) studied the perceived sweetness of four apple cultivars and the correlation with the SSC:acid ratio. The authors thought that the difference in sensory perception of sweetness probably lies in the acid contents, making less sweet a cultivar with high acid content, and a low acid cultivar would seem sweeter than the SSC levels would indicate. However, the ranking of cultivars by SSC:acid ratio does not agree with the sensory panel’s order of acceptability of flavor for the cultivars, as expected to reflect perceived sweetness. In this study, it was determined that the higher SSC:acid ratio obtained corresponded to Red Delicious cultivar followed in decreasing order, with similar values by Eva, Princesa, and Caricia, with Granny Smith having the lower value (Table 1).

Color and browning susceptibility

With respect to the initial color of the apple flesh (t = 0 min), Figure 1 and Table 2 show that Caricia apples presented a lighter color (higher L* value), followed by Princesa, Granny Smith, Eva, and Red Delicious. The flesh of Princesa apples had the highest b* values (yellow component). The yellow component of Caricia flesh was similar to Eva and Red Delicious, and Granny Smith had the lowest b* value. The chroma values (C*_ab, not shown) followed the same order than b* values. Furthermore, Eva and Red Delicious were the only cultivars that had a very slight reddish hue in the flesh (a* > 0). Granny Smith flesh presented a slight green hue (a* < 0), and in a lesser extent the Princesa and Caricia cultivars (Table 2). The hue angle values (h_ab, not shown) followed an inverse order than a* values.

Table 2. Color parameter values of flesh cut surface of five apple cultivars.

	L*		a*		b*
Variedad	0 (min)	90 (min)	0 (min)	90 (min)	0 (min)	90 (min)	∆E*^z
Caricia	80.9a^y	72.9b	0.0d	4.2a	21.2bc	28.5a	11.4a
Eva	78.9bc	76.6a	2.0a	2.2b	22.0b	23.6c	3.0c
Granny Smith	79.3bc	75.6a	–0.9d	1.0c	13.5d	18.6d	6.1b
Princesa	79.5b	75.7a	–0.1c	1.2c	23.7a	25.6b	4.4c
Red Delicious	78.8c	73.2b	1.5b	4.4a	21.1c	28.9a	10.1a

^zTotal color difference between initial color flesh (t = 0 min) and after 90 min of being cut.

^yValues in the same column with different letters are significantly different (p ≤ 0.05) by Duncan’s test.

Figure 1. Color parameter changes on flesh cut surface of five apple cultivars. Bars indicate standard deviation.

One of the main changes in the color of fresh-cut fruits decreasing the visual quality is enzymatic browning. The degree of browning is dependent on the concentration and type of phenols and enzymatic activity (related mainly to the cultivar), and the oxygen concentration. During browning development, the flesh color became mainly darker and redder, and it could be evaluated by the changes in the color parameters, the decrease of L* and h_ab, and by the increase of a*, b*, and C*_ab after the fruit was cut (Limbo and Piergiovanni, 2006; Piagentini et al, 2012). As shown on Figure 1, each color parameter changed in the same way for the five apple cultivars studied. It was found that Eva, Granny Smith, and Princesa cultivars presented higher values of L* and h_ab, and lower a*, b*, and C*_ab values than Caricia and Red Delicious cultivars 90 min after being cut. These results indicated that the degree of browning development was greater for Caricia and Red Delicious cultivars.

The total color difference (ΔE*_ab) indicated the color differences between recently cut apple (t = 0 min) and the apple cut after 30, 60, and 90 min. It was found that ΔE*_ab increased after cutting the flesh for the five apple cultivars studied (Figure 1). Caricia and Red Delicious cultivars showed the greatest color changes after 90 min compared with the other three cultivars (Figure 1). The ΔE*_ab values for Caricia and Red Delicious cultivars after 90 min were 11.4 and 10.1, respectively, and taking into account the scale presented by Limbo and Piergiovanni (2006), these values indicated strong color differences. On the other hand, the total color differences of the other three cultivars after 90 min were perceptible (3 < ΔE*_ab < 6; Figure 1 and Table 2).

To evaluate the potential browning susceptibility of each apple cultivar, the rate of color parameter change was modeled. The zero order kinetic model (n = 0; Eq. 1) adequately represented the changes of all color parameters for the five apple cultivars (R² > 0.85), with the exception of L*, a*, and ∆E*_ab of Eva and h_ab of Eva and Red Delicious cultivars (R² < 0.72; Table 3). Caricia cultivar presented the highest values of the change rate constants (k) for all color parameters. k values of Red Delicious cultivar for L*, a*, b*, and C*_ab parameters were similar to those obtained for Caricia, indicating these two apple cultivars had the greatest browning susceptibility (Table 3). Other authors also found differences in susceptibility to browning development among different apple cultivars (Amiot et al., 1992).

Table 3. Change rate constants (k) of flesh color parameters of five apple cultivars.

Cultivar	Change rate constant (min^–1)
	kL* (R^2)	ka* (R^2)	kb* (R^2)	kC*ab (R^2)	khab (R^2)	k∆E*ab (R^2)
Caricia	–0.0818a^z	0.0462a	0.0784a	0.0821a	–0.0891a	0.1223a
	(0.8869)	(0.8845)	(0.8658)	(0.8716)	(0.8453)	(0.8742)
Eva	–0.0233b	0.0016c	0.0165b	0.0166c	–0.0003c	0.0312c
	(0.6528)	(0.4168)	(0.9376)	(0.9357)	(0.0035)	(0.7160)
Granny Smith	–0.0405ab	0.0213a	0.0556a	0.0555a	–0.0762a	0.0717b
	(0.9455)	(0.9700)	(0.9948)	(0.9949)	(0.9447)	(0.9742)
Princesa	–0.0417a	0.0144ab	0.0225b	0.0229b	–0.0321b	0.0496b
	(0.9854)	(0.9915)	(0.9669)	(0.9678)	(0.9909)	(0.9654)
Red Delicious	–0.0548a	0.0271a	0.0838a	0.0864a	–0.0384b	0.1039b
	(0.9090)	(0.8563)	(0.9473)	(0.9483)	(0.6759)	(0.9279)

^zValues in the same column with different letters are significantly different (p ≤ 0.05) by Duncan’s test.

Total phenolics content and antioxidant capacity

Regarding the content of bioactive compounds, the concentration of TPC varied among the apple cultivars. The highest content of phenolic compounds in apple flesh was found in Red Delicious cultivar, followed in a decreasing order with about 30% less by Eva and Caricia, and finally with 45% less by Granny Smith and Princesa. Comparing the total phenolic content of the cultivars with highest and lowest concentration, it was found that Red Delicious flesh had 3.1 times more phenolic content than the flesh of Granny Smith apples (Figure 2). The total phenolic content in apple peel was about 2 to 5 times higher than that found for the apple flesh for all varieties. The highest phenolic content was found for the peel of Caricia and Red Delicious cultivars, containing about 1.80 times more concentration than the peel of Eva, the one with the lowest TPC in peel (Figure 2).

Figure 2. Total phenolic content in flesh, peel, and peel + flesh of five apple cultivars. Different letters over columns of each trait across cultivars are significantly different (p ≤ 0.05) by Duncan’s test. Bars over each column indicate standard deviation.

Correlations between TPC and soluble solids, pH, and acidity (r = 0.98, 0.89, and –0.83, respectively) were found. These results suggested that apple flesh with higher TPC could be sweeter and less acidic. Correlations were also found between a* and h_ab flesh color parameters and total phenolic content of apple flesh (r = 0.82 and –0.86, respectively) suggesting that the apple flesh with higher reddish color (greater a* and lower h_ab values) had greater phenolic content. However, Drogoudi et al. (2008) reported that a more nutritious flesh (with higher TPC) would have a lighter color and lower soluble solids.

Amiot et al. (1992) studied the browning susceptibility of 11 apple cultivars. They reported that the degree of browning was first closely related to the amount of phenols (hydroxycinnamic derivatives and flavan-3-ols) degraded, and then to polyphenol oxidase (PPO) activity. They concluded that a better control of browning would probably consist of a selection of cultivars with first a low content in chlorogenic acid, second the flavan-3-ols content and relative balance of hydroxycinnamic derivatives to flavan-3-ols should be considered in the susceptibility to browning. Third, PPO activity and other minor compounds may also influence browning.

Martín et al. (2011) studied the PPO activities of these five apple cultivars in a previous work, determining that the activity of PPO was higher for Red Delicious cultivar, followed in a decreasing order by Princesa, Caricia, Eva, and finally Granny Smith apples. Taking this into account together with the results obtained in this work, it was found that Red Delicious, the apple cultivar having the highest content of phenolic compounds in flesh and PPO activity and lowest acidity, was the one with greater browning development. Drogoudi et al. (2008) also reported that the apple cultivar with the highest antioxidant activity and TPC became brown quickly when cut into slices, which may be attributed to the high content of phenolic compound that it contained.

With respect to the antioxidant capacity of apple flesh, Red Delicious had the highest values and the other four cultivars showed similar values among each other (p > 0.05). AEAC of Red Delicious flesh was about 1.75 times greater than the AEAC of the flesh of the other four cultivars (Figure 3). The antioxidant capacity of the peel was about 2 to 4 times higher than the flesh for all apple cultivars. The highest AEAC value corresponded to the peel of Red Delicious, followed by Caricia, Granny Smith, and Princesa with 50% less. The peel of Eva cultivar had the lowest antioxidant capacity, 3.1 times lower than the AEAC of the peel of Red Delicious cultivar (Figure 3). Other authors also reported that apple peel contained a higher phenolic compound concentration and antioxidant activity compared to apple flesh (Chinnici et al., 2004; Drogoudi et al., 2008; Khanizadeh et al., 2008; Tsao et al., 2005; Wolfe et al., 2003).

Figure 3. Antioxidant activity (AEAC) in flesh, peel, and peel + flesh of five apple cultivars. Different letters over columns of each trait across cultivars are significantly different (p ≤ 0.05) by Duncan’s test. Bars over each column indicate standard deviation.

These results indicated that peel removal during minimal processing may induce significant nutrient losses. Based on our results (15–17 g of peel 100 g^–1 of flesh), processing fresh-cut apples with peel (flesh + peel in Figures 2 and 3), would increase the phenolic content in about 10 to 56%, and the antioxidant activity in 56 to 87%, depending on apple cultivar. In all of the studied cultivars, both the TPC and AEAC were higher in the peel, followed in a decreasing order by the flesh + peel, and finally by the flesh (Figures 2 and 3). Other researchers found similar results (Wolfe et al., 2003). Hagen et al. (2007) reported that a peel of 1-mm thickness of Aroma apple would represent about 10% of the edible mass but nearly 30% of the antioxidant activity and phenolic compounds. Similar results have been found by McGhie et al. (2005).

A positive correlation was found between TPC and antioxidant activity in flesh and peel tissues (r = 0.80 and 0.88, respectively), suggesting that phenols have a significant contribution to the total antioxidant capacity of apples. Similar correlations were found for different apple cultivars by other authors (Drogoudi et al., 2008).

The difference found among peel AEAC of the five apple cultivars was greater than the difference found among their corresponding TPC values. The opposite happened with the values determined in apple flesh, i.e., there was a greater difference in TPC than among their corresponding AEAC. This would be explained by the possible difference between phenolic compound profile of apple flesh and peel.

Numerous authors reported that the antioxidant activity of apple depended on their phenolic composition in a qualitative and quantitative way. They found that the antioxidant activity of apple was not only correlated to TPC but was also dependent on the polyphenolic composition. Flavan-3-ols/procyanidins contributed the most to the total antioxidant activities of both apple peels and flesh (Chinnici et al., 2004; Tsao, 2005). The peel has additional flavonoids not found in the flesh, such as quercetin glycosides reported also for being one of the major contributors to the peel apple antioxidant activity (Chinnici et al., 2004; Lata et al., 2009; Wolfe et al., 2003).

Conclusions

All of the studied attributes changed significantly (p ≤ 0.001) among the five apple cultivars studied.

Granny Smith, one of the studied cultivars with high chilling hours requirements, presented the lowest values of soluble solids and pH, with higher values of acidity. The three studied cultivars with low chilling hours requirements (Caricia, Eva, and Princesa) had intermediate values in these attributes, between Granny Smith and Red Delicious.

The measured color parameters and the kinetic constants obtained for the zero-order models allowed determining the browning susceptibility of the different apple cultivars. According to the values of the color parameters and the change rate constants, the flesh of Caricia presented the highest rate and degree of browning development, followed by Red Delicious.

Red Delicious showed higher phenolic content in flesh and peel, correlated with the higher antioxidant capacity and browning development. On the other side, Granny Smith and Princesa had the lowest phenolic content, antioxidant capacity, and lower browning development. Similarly, the latter two cultivars showed the highest values of firmness and juiciness.

Correlations between TPC and soluble solids, pH, acidity, AEAC, and a* and h_ab flesh color parameters were found, suggesting that apple flesh with higher TPC could be sweeter, less acidic, with higher antioxidant capacity, and with higher reddish color (greater a* and lower h_ab values).

For the five apple cultivars, it was found that the total phenolic content and antioxidant capacity of apple peel was higher than the flesh. It was found that processing fresh-cut apple with peel would increase the antioxidant activity and the bioactive compounds intake. Moreover, avoiding the peel removal would decrease the flesh surface exposed to air, therefore, reducing the area susceptible to enzymatic browning develops.

Hence, considering the minor browning susceptibility and the highest values of firmness and juiciness of the apples of Granny Smith and Princesa cultivars (both with different chilling hours requirements), it could be concluded that these two apple cultivars would be more suitable for minimal processing. The results also indicated that avoiding apple peel removal could increase their phenolic content in 28 and 56%, and their antioxidant capacity in 65 and 78%, respectively, for Granny and Princesa cultivars, representing a more valuable source of healthy and beneficial compounds.

Acknowledgments

This article is in memoriam of Chem. Eng. Daniel Raúl Güemes. We wish to thank Lic. Maillen Seipel for her technical assistance, and Dr. Norberto Gariglio and Eng. Juan Carlos Favaro (FCA–UNL) for providing the apples of Caricia, Eva, and Princesa cultivars.

Funding

The authors gratefully acknowledge Universidad Nacional del Litoral (Argentina) for financial support through project CAI+D 12/Q103.

Additional information

Funding

The authors gratefully acknowledge Universidad Nacional del Litoral (Argentina) for financial support through project CAI+D 12/Q103.