COMPARISON OF FIELD PERFORMANCE, FRUIT FIRMNESS, AND NUTRITIONAL VALUE OF YELLOW- AND RED-FRUIT POLISH RASPBERRY CULTIVARS

Ireneusz Ochmian¹, Katarzyna Skupień^2
1 Department of Horticulture, West Pomeranian University of Technology in Szczecin
² Laboratory of Plant Raw Materials Processing and Storage, Agricultural University, Szczecin, Poland

ABSTRACT

The objective of the two-year study was to compare a field performance and chemical composition of red-fruit 'Polana' and yellow-fruit 'Poranna Rosa' primocane raspberry cultivars.
Regarding the yield, 'Polana' plants showed better productivity (8.4 t ha^-1) compared to 'Poranna Rosa' (7.3 t ha^-1), although, the latter was characterized by bigger fruits (on average 343 g and 309 g, respectively). Moreover, 'Polana' berries displayed higher fruit firmness, higher soluble solids (10.5%), higher total acid (4.1 g citric acid 100 mL^-1), and higher vitamin C content (43.3 mg 100 mL^-1). Further, 'Polana' berries exhibited higher amount of total polyphenols calculated on HPLC basis (44.98 mg 100g^-1), mainly due to the lack of anthocyanins in 'Poranna Rosa' fruits. The red-fruit 'Polana' showed 40.69 mg anthocyanins per 100 g fresh weight with predominant participation of cyanidin sophoroside (24.54 mg 100 g^-1) and cyanidin glucoside (9.88 mg 100 g^-1). The content of ellagic acid in 'Poranna Rosa' berries was significantly lower (1.46 mg 100 g¹) than in 'Polana' fruit (2.09 mg 100 g^-1). Additionally, red raspberries overrated yellow ones as far as N,P,K and Mg is concerned (16.31, 3.28, 11.73, and 1.45 g kg^-1, respectively), but both cultivars showed the same concentration of Ca, Fe, and Zn (0.43 g kg^-1, 110.0 and 22.0 mg kg^-1, respectively).
In general, red-fruit 'Polana' berries showed better productivity, harder fruits (more resistant for handling), and were found a richer source both of nutritional and biologically active constituents.

Key words: yield, fruit chemical constituents, anthocyanins, flavonols, ellagic acid.

INTRODUCTION

The cultivation of raspberries in Poland estimated at 54,000 tons in 2006, amounts 10% of world production of these berries [9]. The raspberries are characterized by very delicate and perishable fruits. The firmness – a mechanical resistance of particular drupes and the all drupel is presumed to be a good indicator of fruit quality. It indicates fruit suitability for harvest, transport, consumption, and freezing, and moreover, estimates fruit sensitivity toward pathogen infection or the stage of fungi infestation [13]. The soluble solids and acids also contribute to fruit quality imparting sensory attributes for the berries. Rubus idaeus L. is an important berry crop that contains macro- and microelements, essential for the physiological equilibrium of the organism, and phenolic substances with their well known antioxidant properties [19]. Raspberries are a rich source of the antioxidant compounds, such as polyphenols, anthocyanins and ellagitannins [6]. Many of these compounds exhibit a wide arrange of biological effects, including antioxidant, antimicrobial, anti-inflammatory, and vasodilatory actions [15]. Anthocyanins are a group of flavonoids responsible for red, violet and blue colors of most berries and fruits. There is a probable association between consumption of fruit and vegetables and reduced risk of cancer, particularly of the digestive tract [5].

Both the palatability and health-promoting properties of berry plants are affected by cultural, genetic, and processing factors. Increasing data suggests that genotype may have a profound influence on the content of bioactive compounds in berries [3]. A better knowledge of the compositional data on phenolic content of different raspberry varieties is necessary to achieve a good association between their possible antioxidant activity and their effects on human health [8]. The aim of the study was to compare chemical composition and thereby nutritional value of yellow-fruit 'Poranna Rosa' and red-fruit 'Polana' primocane cultivars of Rubus idaeus L.

MATERIALS AND METHODS

The experiment was established in spring 2004 at the Experimental Station Rajkowo belonging to Agricultural University of Szczecin. The raspberries were planted in a brown podsolic soil rich in the nutrients. During experimental period no chemical protection of plants was performed, whereas, every year after fruiting, the shoots were cut down at the ground level. Each year, the plantation was watered and fertigated with ammonium nitrate (120 kg N·ha^-1 in six doses) by means of the permanent drip line type T-Tape. The field experiment was a randomized block (7.5 m²) design with four replications. The fruits were collected at commercial maturity stage, the all experiment covered 60 m².

Immediately after each harvest during vegetative season the yield and fruit mass was determined. The yield was expressed as tons per hectare. Fruit mass was measured on 100 berries with RADWAG WPX 4500 electronic scales (0.01 g accuracy). The fruit firmness was measured with a FirmTech 2 apparatus (BioWorks, USA). The firmness of 50 randomly selected berries from every replicate was expressed as a gram-force causing fruit surface to bend 1 mm.

Also, in each term of harvest in fresh raspberry juice soluble solids content was determined with an Abbé refractometer (PN-90/A-75101/02). Titratable acidity was determined by titration of a water extract of raspberry homogenate with 0.1 N NaOH to an end point of pH 8.1 (measured with an Orion 720 A pH meter; Orion Research Incorporated, USA) according to PN-90/A-75101/04. Vitamin C content was measured with a RQflex 10 requantometer (Merck) and expressed as mg per 100 mL. The chemical analyses were carried out in three replications.

Having completed fruit collecting, fruit samples were kept frozen (-32°C) in polyethylene bags (200-250 g) until analyzed on HPLC.

The 2 g aliquots of fruit (after thawing) were extracted three times with approx. 8 ml of 80% MeOH acidified with a glacial acetic acid (1 ml of 100% acetic acid per 1 l 80% MeOH) in an ultrasonic bath for 15 min. The samples were filtered and transferred to the flasks and made up to the final volume 25 ml. Further, the extracts were centrifuged twice at 12.000× g and 20 µL of supernatants were injected into the HPLC system. The HPLC apparatus consisted of a Merck-Hitachi L-7455 diode array detector (DAD) and quaternary pump L-7100 equipped with D-7000 HSM Multisolvent Delivery System (Merck-Hitachi, Tokyo, Japan). The separation was performed on a Cadenza CD C18 (75×4.6 mm, 5 µm) column (Imtakt, Japan). Column oven temperature was set at 30°C. The mobile phase was composed of solvent A (4.5% formic acid), pH 2.2) and solvent B (acetonitrile). The program began with a linear gradient from 0% B to 21% B (0-30 min), followed by washing and reconditioning the column. The flow rate was 1 ml·min^-1 and the runs were monitored at the following wavelengths: p-coumaric acid derivatives at 320 nm, flavonols and ellagic acid derivatives at 360 nm, and anthocyanin glycosides at 520 nm. The Photo Diode Array spectra were measured over the wavelength range 200-600 nm in steps of 2 nm. Retention times and spectra were compared to those of pure standards within 200-600 nm. Standards of cyanidin glycosides were obtained from Polyphenols Laboratories (Norway), while, p-coumaric acid, kaempferol 3-glucuronide, quercetin 3-glucoside, and ellagic acid from Extrasynthese (France). The chemical analyses were carried out in duplicate.

The combined fruit samples were thawed at room temperature, and then dried (initial temperature 70°C, the final 105°C). The dried fruit were pulverized (Lab mill WŻ-1). The macro- and microelements content was determined according to Polish Standards guidelines. The dried plant material was mineralized in the mixture of H₂SO₄ and H₂O₂ for nitrogen determination, whereas, mineralization in the mixture of HNO₃ and HClO₄ (3:1) was performed for P, K, Ca, Mg, Zn, and Fe estimation. After mineralization total nitrogen content was determined with the Kjeldahl method. The content of K and Ca was measured with the atomic emission spectrometry, whereas Mg, Zn, and Fe content with the flame atomic absorption spectroscopy using Solaar S AA. Phosphorus content was determined with the Barton method at wavelength 470 nm employing spectrophotometer Marcel s 330 PRO. The macro- and microelements analyses were carried out in triplicate.

The results obtained were subjected to statistical analysis using Statistica 7.1 (Statsoft, Poland). The analysis of variance in a form of 2-year synthesis for fixed model was used. The values were evaluated by the Duncan test and the differences between the means at P < 0.05 were considered significant.

RESULTS AND DISCUSSION

One of the most important features determining cultivar suitability for growing is its productivity. 'Polana' and 'Poranna Rosa' are primocane cultivars, bearing fruits on 1-year old shoots, in contrast to floricane cultivars bearing fruits on 2-year shoots. The cropping of primocanes starts approximately 3 weeks after the floricanes [12]. Two-year study showed that 'Polana' yielded much higher 8.4 t·ha^-1 compared to 'Poranna Rosa' 7.3 t·ha^-1 (Fig.1). Apart from the yield, fruit quality also plays an important role. Fruit quality encompasses, inter alia, fruit size and firmness as well as their chemical composition. In the course of the experiment, diminishing of fruit size at the end of harvest occurred. Similar correlations were observed by Grajkowski and Ochmian [11] for other autumn-bearing raspberry cultivars. However, considerably bigger fruits during the all cropping season were obtained from 'Poranna Rosa' (Fig. 2). The average weight of 100 fruit for 'Poranna Rosa' was 343 g, whereas, that of 'Polana' 309 g. In the study carried out by Mikos-Bielak [18] 'Polana' cv yielded better compared to our experiment (9.9 t·ha^-1), however its fruit were smaller – 286 g. On the other hand, in USA only 4.5 t of berries per ha was obtained and an average fruit weight was only 200 g [14]. The fruits of 'Poranna Rosa' and 'Polana' were also much bigger compared to the reports for 'Summi' (110 g) and 'Willamette' (max. 299 g) cultivars [10].

The fruit firmness is one of the features determining berry suitability for transport. In this finding, fruit firmness was measured both with floral disks (toruses) and instantaneously after floral disks removal. 'Polana' was characterized by much harder fruits (Fig. 3). The berries of 'Poranna Rosa' were softer both with the toruses and without them. Noteworthy, the fruits without floral disks were 3-fold lesser firm compared to berries with floral disks. It is in agreement with the observations for other raspberry cultivars tested by Grajkowski and Ochmian [11].

The suitable level of soluble solids and acids and their ratio substantially affect fruit taste. The results obtained in this experiment show that 'Polana' fruits had significantly higher amount of soluble solids, total acid, and vitamin C (Table 1). The soluble solids content (9.5-10.5%) was similar to that reported for other raspberry cultivars by Maloney et al. [17] (9.0-11.0%). On the other hand, the amounts of vitamin C in 'Polana' (43.3 mg 100 ml) and 'Poranna Rosa' berries (32.7 mg 100 ml) were higher compared to the values obtained by Agar and Streif [2] (29 mg) and Wieniarska et al. [25] ('Polana' 27.6 mg).

Further, total acid content for 'Polana' and 'Poranna Rosa' (4.10 and 3.27 g citric acid per 100 ml, respectively) was higher in comparison to data reported by Danek and Markowski [7]. The variations observed may result from not only genetic factors but also from weather conditions during vegetative season of each field trial [16]. Regarding macro-and microelements content, 'Polana' fruit surpassed 'Poranna Rosa' berries as far as N, P, K and Mg content is concerned, whereas, Ca, Fe, and Zn concentration were exactly the same. The nitrogen concentration (12.88-16.31 g·kg^-1) observed in this study responded to the data obtained by Strik [23] for ripe fruit of red raspberry 1.4-1.7%. Whereas, phosphorus content determined in 'Poranna Rosa' (2.54 g·kg^-1) and 'Polana' (3.28 g·kg^-1), was much higher than reported by Ağaoğlu and Eyduran [1] – 44 mg·100 g^-1. The amount of potassium found in this study ranged from 10.79 ('Poranna Rosa') to 11.73 g·kg^-1 on dry weight basis ('Polana') and the values were similar to the average K level (11.24 g·kg^-1 after recalculation) determined by Plessi et al. [19] in six raspberry genotypes. Mg concentrations in 'Poranna Rosa' and 'Polana' berries (respectively 1.20 and 1.45 g·kg^-1) were less disparate compared to 1.30 and 1.80 g·kg^-1 (after recalculation) as estimated by the above authors for bred varieties. In the present study, both cultivars showed the same Ca concentration (0.43 g·kg^-1), however, much lower compared to the data obtained by Plessi et al. [19] 1.53-2.67 g·kg^-1 (after recalculation) for wild raspberries and cultivars. Fe content for 'Poranna Rosa' and 'Polana' amounted 110 mg·kg^-1, whereas, the above authors report a scope 70.2-141.6 mg·kg^-1 (after recalculation) for wild and bred genotypes, respectively. Further, the authors determined slightly higher Zn concentrations (28.3-32.9 mg·kg^-1 after recalculation) compared to our findings (22.0 mg·kg^-1). In general, apart from Ca and P, the results obtained in this experiment are in sufficient accord with the literature data.

'Poranna Rosa' and 'Polana' are primocane cultivars fruiting until autumn (October). However, the cultivars differ in fruit color. 'Poranna Rosa' has yellow berries, whereas, 'Polana' yields red fruits (Phot.1). The fruit color disparity is distinctly reflected in phenolic composition of both genotypes. 'Poranna Rosa' berries have no anthocyanins what can be seen in Fig. 4 and Table 2. In contrast,  red-fruit 'Polana' showed 40.69 mg anthocyanins per 100 g fresh weight with dominating participation of cyanidin sophoroside (24.54 mg·100 g^-1) followed by cyanidin glucoside (9.88 mg·100 g^-1). Benvenuti et al. [4] determined 29.2 mg·100 g^-1 of total anthocyanins for 'September' berries and 41.2 mg·100 g^-1 for 'Sumner' fruits. On the other hand, de Ancos et al. [8] found much higher anthocyanin content ranging from 122.8 mg·100 g^-1 ('Ceva') to 31.13 mg·100 g^-1 ('Autumn Bliss'). Similarly to this study, cyanidin sophoroside and cyanidin glucoside were found predominating anthocyanins in four cultivars tested.

Both cultivars analyzed in this finding, showed the presence of neochlorogenic acid (the compound belonging to hydroxycinnamic acids group). In 'Polana' berries neochlorogenic acid amounted 0.47 mg·100 g^-1 compared to 0.12 mg·100 g^-1 in 'Poranna Rosa' fruit. On the other hand, neochlorogenic acid was not detected in 'Polka' raspberries (data not published). However, in comparison to chokeberries (74.60-99.76 mg·100 g-1) as reported by Skupień and Oszmiański [21] the amounts of neochlorogenic acid determined in raspberries are negligible thus the taste of these fruits is not astringent.

Regarding p-coumaric acid content, berries of 'Poranna Rosa' showed significantly higher values (0.18 mg·100 g^-1) than 'Polana' fruits (0.08 mg·100 g^-1). However, these amounts were very small compared to p-coumaric acid level determined in strawberries 32.19-42.03 mg·100 g^-1 [22].

Total ellagic acid derivatives determined in 'Polana' berries (2.09 mg·100 g^-1) overrated that of 'Poranna Rosa' fruits (1.46 mg·100 g^-1). On the other hand, Anttonen and Karjalainen [3] found under their experimental conditions much higher ellagic acid amounts in raspberry fruit from 38.0 to 118.0 mg·100 g^-1. Thus, the discrepant findings of ellagic acid content reported for raspberries must have resulted from genetic factors, environmental, and cultural conditions as well as lab procedures applied.

Quercetin 3-glucuronide and kaempferol 3-glucuronide were the two flavonols identified in both cultivars tested. 'Polana' berries showed more than twice as much of quercetin 3-glucuronide (0.27 mg·100 g^-1) compared to 'Poranna Rosa' fruit (0.12 mg·100 g^-1). Versari et al. [24] found similar quercetin concentration in control raspberry juice (27 mg·l^-1). The kaempferol 3-glucuronide content was very similar for both cultivars (1.31 and 1.38 mg·100 g^-1 for 'Poranna Rosa' and 'Polana', respectively) and corroborated by literature data presented by Rommel and Wrolstad [20] – 2.31 ppm for raspberry juice.

In general, 'Polana' berries showed much higher total polyphenol content (44.98 mg·100 g^-1) than 'Poranna Rosa' (3.19 mg·100 g^-1) mainly due to the lack of anthocyanins in this yellow-fruit cultivar.

CONCLUSIONS

1. The two-year observations of field performance showed that red-fruit 'Polana' cultivar yielded higher than 'yellow-fruit' 'Poranna Rosa'. However, the average fruit mass of 'Poranna Rosa' was higher than that of 'Polana'.

2. The berries of 'Polana' displayed higher fruit firmness (with and without floral disks) compared to 'Poranna Rosa'.

3. The berries of 'Polana' exhibited higher soluble solids, total acids, and vitamin C content.

4. The berries of 'Polana' showed higher amount of total polyphenol, mainly due to the presence of anthocyanins, in contrast to 'Poranna Rosa' fruits.

5. Higher concentration of N, P, K, and Mg was found in 'Polana' berries, however, both cultivars showed the same level of Ca, Zn, and Fe.

6. Comparison of yellow- and red-fruit raspberry cvs showed that red-fruit 'Polana' berries exhibited better productivity, harder fruits (more resistant for handling), and were found a richer source both of nutritional and biologically active constituents.

REFERENCES

Accepted for print: 23.10.2008

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