1. Introduction
The raspberry (
R. idaeus L.) belongs to the rose family
Rosaceae [
1]. This botanical family includes most cultivated trees (e.g., pears, apples, plums, pears, and cherries), fruit bushes (e.g., raspberries and blackberries—Rubus), ornamental shrubs (e.g., rose—Rosa), and perennials (e.g.,
Fragaria) [
2,
3,
4,
5]. Its natural origins can be found in Europe, North Asia, and North America. In Europe, the raspberry is widespread, especially in the temperate zone. This species has long been cultivated in many European countries, such as Poland, Germany, France, Great Britain, Russia, and Scandinavia. In addition to Europe, the raspberry also occurs naturally in North Asia, in countries such as Russia, Kazakhstan, Mongolia, China, Japan, and Korea. Raspberries are also grown in these regions. In North America, raspberries grow wild. This species is endemic to the continent, with natural ranges from Alaska and Canada to mountainous areas in the United States, including Colorado, Washington, Oregon, and Montana. Thanks to the development of genetics and breeding, the raspberry is now cultivated all over the world in various regions that provide appropriate climatic and soil conditions for this species. The raspberry is very popular among consumers due to its intense taste, attractive appearance, and health benefits [
5,
6]. There are several factors that influence the importance of the raspberry in the consumer market. These are
- –
Taste and quality: Raspberries have a distinct, sweet, and sour taste. The degree of ripeness, the cultivar, and the content of simple sugars, organic acids, and volatile substances determine the taste and aroma of the fruit. Fruit should also be fresh, juicy, and of good quality to meet consumer expectations [
7,
8,
9,
10,
11,
12].
- –
Health benefits: Raspberry fruit is rich in vitamins, minerals, antioxidants, and fiber, which attracts consumers looking for healthy food. They are also low in calories and can be part of a healthy diet [
13,
14,
15,
16,
17,
18,
19].
- –
Naturalness and freshness: Consumers want natural and fresh products, especially when it comes to fruit. Raspberry fruit is often perceived as a natural, seasonal fruit that can be eaten in its original form.
- –
Versatility of use: Raspberry fruit can be eaten in many different ways—as an independent fruit, an addition to desserts, or an ingredient in cocktails, jellies, jams, and casseroles. This versatility attracts diverse groups of consumers.
- –
Availability: Raspberries are available on the market most of the year, both in the form of fresh fruit and processed products. This makes them more accessible to consumers [
8,
10].
- –
Food trends: In recent years, interest in healthy eating and natural products has increased. The true raspberry fits perfectly into these trends, which translates into their increasing importance on the consumer market [
7,
8,
11].
The quality of raspberry fruits and their importance on the consumer market also depend on factors such as region of cultivation, production methods, local availability, and price competitiveness [
4,
7,
10,
11,
12].
The biological value of the raspberry depends to a large extent on the concentration of antioxidants, which include, among others, ascorbic acid and anthocyanins [
4,
11,
12,
14]. Raspberry fruit also contains vitamin C, as well as vitamins from the groups B, PP, A, E, and K, and minerals such as potassium, calcium, magnesium, iron, and, in a smaller amount, manganese, copper, and zinc [
2,
8,
9,
11]. The presence of these ingredients affects the health-promoting properties of the raspberry. Polyphenols have antioxidant, anti-inflammatory, and antimicrobial effects [
13]. They inhibit the formation of free radicals, which adversely oxidize many compounds and damage cell membranes, proteins, lipids, enzymes, and genetic material [
11,
14,
15,
16,
17,
18,
19]. Consumption of products containing these compounds has a beneficial effect on the eyesight and the cardiovascular system [
16,
20,
21]. The fruits of
R. idaeus L. also contain carotenoid compounds that act as natural plant pigments, including β-carotene, zeaxanthin, and lutein [
22,
23]. Zeaxanthin and lutein give color to yellow-fruited cultivars that do not contain anthocyanins. Raspberry is a rich source of phenolic acids, which include ellagic and salicylic acids [
17,
18,
21,
23]. Ellagic acid has anticancer and antioxidant properties, while salicylic acid has analgesic, antipyretic, and anti-inflammatory properties [
13,
14,
16,
19,
24].
However, raspberry fruits are very susceptible to mechanical damage, water loss, and mold development during transport and storage. For these reasons, the viability of the raspberry after harvest is limited to a few days, usually 3 to 5 days, and only a small percentage of these fruits can be eaten fresh [
10,
17]. To prevent these defects, it is recommended to store the fruit at a temperature close to 0 °C. However, storing fruit at a low temperature is insufficient to extend the shelf life of raspberry fruit to 14 days, and more and more often, storage is carried out in a controlled atmosphere [
25,
26,
27,
28,
29]. Maintaining the concentration of CO
2 at a level of 5–20% and O
2 at a level of 3–10% ensures an increase in the durability of stored
R. idaeus fruits. Reducing the O
2 concentration below 3% may, however, cause deterioration of the taste, while exceeding the CO
2 level above 30% increases the softening of the fruit and causes brown discoloration of the skin [
21,
28,
29].
The yield and quality of
R. idaeus fruits are also determined by cultivars, appropriate cultivation management, including fertilization, especially with nitrogen, and care treatments [
30,
31].
The genetic features of the cultivars studied may have a significant impact on both the yield and the qualitative characteristics of a given species. Genetic characteristics of
R. idaeus L., such as growth potential, ability to form fruit, ripening date, or plant structure, may affect the yield. Cultivars with greater growth potential, more fruit per plant, or a more optimal ripening date may produce higher yields. Important consumption features of raspberries are the taste and aroma of the fruit. For example, some raspberry cultivars may have an intense, sweet flavor, while others may be more acidic. Aromatic properties may also vary depending on the genetic characteristics of the cultivar. Similarly, genetic characteristics determine the size and shape of the fruit. Larger-fruited cultivars may deliver higher yields, while more regular-shaped cultivars may be favored by the market. The genetic characteristics of cultivars can affect fruit consistency and post-harvest shelf life. Appropriate genetic features can ensure greater durability, better protection against mechanical damage, and rapid fruit spoilage. Genetic diversity in plant breeding allows the selection and development of cultivars with desirable characteristics, such as yield, taste, shelf life, and others [
31].
The use of nitrogen fertilizers is very important due to the fact that nitrogen is a component of amino acids that form structural proteins and enzymes important for metabolism, nucleic acids, and chlorophyll [
17].
Consumers purchasing raspberry fruit pay special attention to quality features such as color, size, and shape. The color affects the appearance and attractiveness, as well as freshness and taste. Quality standards for fresh fruit intended for consumption in international trade are defined by the European standard UN/ECE FFV-32, which provides for three quality classes: Extra, I, and II. In each of them, healthy, clean fruit, without foreign smells, with a fresh appearance, free from pests and damage, and not damp, is allowed to be marketed. The size of the fruits of cultivars depends on determining the largest diameter, which for individual fractions is Extra—15 mm, I—12 mm [
32]. In the case of class II raspberries, the size is not specified [
32]. Quality classes differ in the share of fruit with defects and the degree of tolerance of these defects, e.g., separating raspberry fruits after a period of drought and repeated rainfall. Each batch of fruit must be uniform and properly labeled [
33]. Therefore, the aim of this study was to carry out a commodity evaluation of the fruits of two raspberry cultivars bearing fruit on 2-year-old shoots, cultivated under the conditions of differentiated nitrogen fertilization against the background of constant phosphorus-potassium fertilization. The alternative hypothesis assumes that there is a statistically significant difference in the quality of raspberries (
R. idaeus L.) grown under balanced fertilization conditions in relation to the null hypothesis that there is no significant difference in the quality of raspberries grown under balanced fertilization conditions.
3. Material and Methods
The research was based on a field experiment. They were located in the village of Rozbórz Długi (
Figure 1), located in the Przemyśl Foothills (Geographical location: 49°55′45″ N, 22°29′18″ E), on brown, slightly acidic soil.
The experiment was performed using the randomized block method in a dependent split-plot design with 3 repetitions. The first-order factors were 2 raspberry cultivars, ‘Laszka’ and ‘Glen Ample’, bearing fruit on two-year-old shoots. The second-order factor was differentiated nitrogen fertilization (0, 45, 90, 135 kg N·ha−1), against the background of constant phosphorus-potassium fertilization and full dose of manure (40 t·ha−1). The area of one plot was 4000 m2.
3.1. Characteristics of Cultivars
‘Laszka’—a cultivar bred in the Experimental Department of the Institute of Pomology and Floriculture in Brzezna. It was entered into the National Register of Original Cultivars in 2006 (
Figure 2) [
34].
The fruits are large and very large, elongated, intensely red in color, and have a slight mossy appearance. It is resistant to frost, dieback of shoots, and fruit rot. The fruits have a long post-harvest shelf life and are less susceptible to damage during transport [
33]. Its fertility and fruit size are useful in the production of consumption and dessert fruit. It is a cultivar recommended for controlled cultivation for accelerated harvest due to early ripening and high content of ascorbic acid in the fruit [
33,
35].
Glen Ample’ is a cultivar bred at the Scottish Institute of Crop Production in Dundee, but it can be grown in Poland (
Figure 3). A high-yielding cultivar with strong growth and compact, bright red fruits [
33,
35]. A typical dessert cultivar with an attractive appearance and very good taste [
36]. It is characterized by frost resistance and very high resistance to diseases and transport [
35].
3.2. Agrotechnical Conditions
The forecrop of the raspberry was white mustard. In the autumn of 2018, deep plowing (35 cm) was performed with manure and mineral, phosphorus-potassium fertilizers in the amounts of 150 kg K and 80 kg P, 75 kg Mg, and 150 kg Ca·ha−1. These fertilizers were introduced in the form of polifoska 6 (NPK—6-20-30), potassium salt 60%, and magnesium lime (CaO, MgO—30-15), then a cultivation unit was used.
Raspberry seedlings, qualifying as grade C, were planted at a spacing of 2.5 m, every 0.5 m in a row. The plant density was 10,000 pcs·ha−1. After the acceptance of the seedlings, nitrogen fertilization was applied in the amount of 10 kg N·ha−1 in three equal doses, every 20 days, in the form of 34% ammonium nitrate. In addition, in order to provide young seedlings with the necessary microelements, Florovit liquid, multi-component foliar, and soil fertilizer were applied.
In 2019, in the spring, white clover was sown in the rows of raspberries in an amount of 8 kg·ha−1 in order to eliminate some weeds and enrich the soil with nitrogen. In the autumn of that year, after examining the abundance of assimilable macronutrients in the soil, the basic mineral fertilization with phosphorus and potassium was established at a dose of 90 kg K·ha−1 (in the form of 60% potassium salt in the amount of 150 kg ha−1) and phosphorus in the form of granulated simple superphosphate (20% P) in the amount of 100 kg ha−1.
In 2020, the plantation was divided into blocks, subblocks, and repeats, leaving drive paths. According to the planned scheme of the experiment, mineral fertilization with nitrogen was introduced. Nitrogen doses above 45 kg ha−1 were divided into 2 or 3 parts, depending on the size of the dose.
The dose of 90 kg N was applied in 2 parts:
- –
The first dose (in the amount of 45 kg N ha−1) was applied in early spring at the beginning of vegetation—in the form of 34% ammonium nitrate;
- –
The second dose—in the amount of 45 kg N ha−1 was applied after the flowering of raspberry shoots in the form of 46% urea.
- –
The dose of nitrogen in the amount of 135 kg N ha−1 was divided into 3 parts:
- –
The first dose (in the amount of 45 kg N ha−1) was applied in early spring at the beginning of vegetation—in the form of 34% ammonium nitrate;
- –
The second dose—in the amount of 45 kg N ha−1 was introduced before the flowering of raspberry shoots in the form of 46% urea;
- –
The third dose—in the amount of 45 kg N ha−1 was applied after flowering of raspberry shoots in the foliar form. An amount of 100 kg of urea was dissolved in 900 dm3 of water and used for 3 periods, every 7 days.
The adopted methods and divisions of nitrogen doses were aimed at optimizing the fertilization of raspberry plantations, taking into account the various stages of plant development and their demand for nutrients. The appropriate amount of ammonium nitrate or urea is dissolved in the water. The dose of nitrogen was adjusted to the needs of the plants and depended on their age and general condition. The appropriate ratio of nitrogen to water was maintained in accordance with the fertilizer manufacturer’s recommendations. The sprayer was prepared and configured to ensure even and thorough spraying of the fertilizer solution on raspberry leaves. Then, foliar spraying with nitrogen was carried out at the appropriate moment of plant development. Foliar fertilization was carried out in the morning or evening to avoid application in full sun. The fertilizer solution was sprayed evenly on raspberry leaves, trying to cover as much of the leaf surface as possible and avoiding too thick a solution that may cause burning of the leaves.
Dosage and frequency of foliar fertilization with nitrogen were adjusted to the needs of plants and their conditions. It should be noted that foliar fertilization was supplemented with basic soil fertilization in accordance with the fertilizer manufacturer’s recommendations and the principles of good agricultural practice. Regular monitoring of plant health and adjustment of fertilization were crucial to maintaining a healthy and productive raspberry plantation.
The quality of raspberry fruit was assessed on the basis of the following: weight of 1000 fruits, organoleptic evaluation of fruit; dry matter, juice yield, fruit storage life, transport resistance, and rheological evaluation. The weight of the harvested fruit was determined using a laboratory balance.
3.3. Fruit Picking
It is important to determine the optimal date of fruit harvest, which will reduce losses and obtain good-quality fruit for processing or storage. The fruits of the true raspberry reach harvest maturity when they are fully ripe. Harvest is carried out every 2–3 days as the fruit ripens, but the fruit must not be overripe as it becomes too soft and unsuitable for transport. The fruit harvest on the experimental plantation was preceded by organizational work, securing manual work, and mechanical equipment.
Raspberry fruits were collected in small 0.25 kg packages, which were then placed in collecting containers (
Figure 4). After harvesting, the fruits were cooled to +2 °C to +5 °C in the shortest possible time.
3.4. Meteorological Conditions
The course of air temperature and precipitation during the raspberry growing season in 2019/2020 was varied, as illustrated by the results in
Table 11.
Precipitation in July 2019 was abundant, reaching 235.4 mm, which was 247.3% of the long-term average. The average air temperature in July was 18.9 °C, with a deviation of 0.2 °C, indicating a slight deviation from the norm. August 2019 experienced low precipitation of 30.7 mm, only 45.0% of the long-term average. The average air temperature in August was 20.4 °C, with a deviation of −2.1 °C. September 2019 had very minimal precipitation of 8.3 mm, representing only 15.2% of the long-term average. October 2019 recorded an average precipitation of 29.9 mm, accounting for 72.0% of the long-term average. November 2019 had extremely low precipitation, with only 0.6 mm, just 1.8% of the long-term average. December 2019 experienced average precipitation of 27.2 mm, equivalent to 95.8% of the long-term average. January 2020 had moderate precipitation of 49.8 mm, reaching 145.2% of the long-term average. February 2020 recorded average precipitation of 25.4 mm, representing 86.1% of the long-term average. Finally, in March 2020, there was again average precipitation of 28.1 mm, accounting for 80.1% of the long-term average (
Table 11).
In April 2020, there was low rainfall of 27.6 mm, only 60.1% of the long-term average. The highest precipitation occurred in the third decade, with 18.6 mm. May 2020 experienced an average precipitation of 57.4 mm, representing 86.8% of the long-term average. The highest rainfall was recorded in the second decade, at 33.9 mm. June 2020 had low precipitation of 53.2 mm, accounting for 67.9% of the long-term average. The highest precipitation occurred in the first decade, with 24.0 mm. July 2020 recorded low precipitation of 48.7 mm, only 51.2% of the long-term average. The highest precipitation occurred in the first decade, with 19.3 mm. August 2020 had average precipitation of 57.9 mm, reaching 84.9% of the long-term average. The highest rainfall occurred in the third decade, with 29.3 mm (
Table 11).
3.5. Soil Conditions
The experiment was set up on brown soil with a slightly acidic reaction and moderate humus content. The content of assimilable phosphorus and potassium in the soil was high; magnesium was medium. The content of micronutrients was moderate, except for the low content of boron (
Table 12).
The agronomic category of the tested soil was marked as heavy, valuation class IV (sum of fractions below 0.02 mm: 37.54%) (
Table 13).
3.6. Fruit Quality Assessment
Fruit mass losses were determined after 24 h of storage at 10 °C. The test was performed 3 times for each experimental object. A laboratory scale (accuracy 0.01 g) was used for the measurements [
38].
The weight of 1000 raspberry fruits was determined using a laboratory scale with an accuracy of 0.01 g. The fruits were counted and then weighed. The assay was repeated three times for each harvest date, cultivar, and fertilization combination.
Fruit strength properties were determined using a CT3 texture meter. The forces needed to permanently distort the structure of a compound fruit as a result of squeezing and tearing, i.e., binding individual drupes into a compound fruit, were determined [
38].
The yield of raw juice from fresh raspberry fruit was determined. Juice yield was expressed in ml per 1 kg of fresh fruit. The determination was made using a fruit press. The pressing process lasted 10 min for each sample. Juice was separated from the solid fraction on a sieve with a mesh size of 1.5 × 1.5 mm. The separation process lasted 60 min, followed by the measurement of the juice content in the samples [
38].
Fruit strength during transport was assessed on a 5-point scale, where
- –
Fruits without visible structure damage were rated at 5 points;
- –
A few pieces of slightly damaged fruit—4 points;
- –
Half of the fruit structure is damaged—3 points;
- –
Most damaged and rotting processes started—2 points;
- –
Rotting and mold development on damaged fruit—1 point [
39].
The dry mass of raspberry fruit was determined after the evaporation of water from fresh fruit. The fruit weight of 1 sample intended for determination was 1 kg. The determinations were made on 3 harvest dates. The percentage content of dry matter was calculated using the dryer method [
38]. The organoleptic evaluation was carried out according to a 6-point scale for the following characteristics: taste, smell, color, shape, appearance, and consistency [
39].
The taste of fruit was determined on the basis of a 5-point scale, where: very sweet—5; sweet—4; slightly sweet—3; sour—2; very sour—1 [
34]. The smell of fruit was assessed on the basis of a 4-point criterion: typical raspberry flavor—very distinct—5 points; clear raspberry flavor—4 points; slightly distinct but without foreign smells—3 points; slightly distinct, with the smell of mold and bacteria—2 points [
38]. Fruit color was determined according to a 4-point scale: very intense color—5 points; intense—4 points; partially red—3 points; green—2 points [
39]. The fruit shape was determined on the basis of a 5-point scale: shape typical for the cultivar—5 points; very slightly deformed—4; slightly deformed—3; clearly deformed—2; not developed—1 [
34]. The appearance of the fruit was determined on the basis of a 4-point scale: typical appearance for the cultivar (without signs of disease)—5 points; very slightly affected by diseases—4; slightly affected by diseases—3; heavily affected by diseases—2 [
34]. The consistency of the fruit was determined on the basis of a 5-point criterion: very firm (hard) consistency—5 points; firm—4 points; slightly firm—3 points; soft—2 points; very soft with signs of juice spillage—1 point [
38].
The organoleptic evaluation was carried out on a group of 12 properly trained people. The results of an organoleptic evaluation of the raspberry by a group of trained people can provide valuable information on sensory preferences and the evaluation of fruit quality. The results can be used for further improvement of raspberry breeding, selection of the best cultivars, or improvement of raspberry cultivation, harvesting, and storage processes in order to ensure the highest quality of fruit for consumers [
38].
3.7. Statistical Analysis
The test results were statistically calculated and obtained by variance analysis (ANOVA), SAS 9.2 2008 [
39], and multiple Tukey’s
t-tests at the significance level of p
0.05. Multiple comparison tests of Tukey’s
t-tests enable a detailed comparison of mean values by statistically separating individual starting groups of quantities (groups of ‘a’, ‘b’, and ‘c’ sizes) and determining a smaller difference in mean values, which are marked as LSD [
40].
5. Conclusions
Genetic features differentiated the tested cultivars. The cultivar ‘Glen Ample’ had a higher yield and juice yield, while the cultivar ‘Laszka’ had a higher dry weight of fruit. Organoleptic features did not significantly differentiate the tested cultivars, except for their color. The ‘Laszka’ cultivar was distinguished by a better, more intense color of fruit, but only on the first date of harvest. Rheological features of raspberry fruits, such as maximum load, work load, and final load, refer to their mechanical and textural properties. The cultivar ‘Glen Ample’ showed a higher resistance to deformation and crushing, and the cultivar ‘Laszka’, with a higher content of dry matter, showed a higher resistance to transport.
In general, balanced fertilization had a beneficial effect on R. idaeus fruit yield and quality. Increasing nitrogen fertilization up to 135 kg N ha−1, against the background of constant phosphorus-potassium fertilization, resulted in a successive increase in fresh fruit yield but, on the other hand, a decrease in their dry matter content.
The cultivation of this species with balanced fertilization led to the production of fruits with good sensory and rheological properties. The successive increase in nitrogen fertilization against the background of appropriate phosphorus-potassium fertilization did not deteriorate the quality of the raw material, which is the fresh raspberry. The use of appropriate doses of fertilizers can improve the organoleptic characteristics of raspberry fruit, such as taste, aroma, color, appearance, and consistency. Sustainable fertilization may affect the functional properties of raspberry fruits as well as their sensory and rheological properties, resistance to transport, and storage stability.
Raspberry cultivars differed in response to different nitrogen fertilizations. Cultivar ‘Laszka’ responded with a significant increase in the weight of 1000 fruits to nitrogen supply and responded better to this factor of the experiment, while ‘Glen Ample’ turned out to be less sensitive to this component.
Optimal nitrogen fertilization should take into account the balance between the needs of plants and minimizing the negative impact on the environment. It is important to include other nutrients in addition to nitrogen, such as phosphorus, potassium, and micronutrients, to provide plants with comprehensive fertilization. The individual approach to nitrogen fertilization should be adapted to the varietal diversity and growing conditions.
This will allow you to achieve optimal growth, yield, and fruit quality. The dates of fruit harvest determined significantly the weight of 1000 fruits, their dry matter content, and their resistance to transport. The highest value of these features was obtained at the first and earliest date of harvest.
The conclusions suggest that proper fertilization is a key factor affecting the quality of raspberry fruit and should be considered in cultivation practices to ensure optimal results, both in economic and health terms.