3.4. Fruit Quality Assessment
The quality of fruits depends on their weight, shape, colour, texture, firmness, and content of acids and sugars. In our experiment, there were considerable differences in the weight of strawberries, which ranged from 16.79 to 21.74 g (
Figure 1).
Geçer et al. [
41] observed smaller differences in this parameter in their experiment—from 11.7 g. The fruits with the lowest weight (16.79 g) were harvested from the plants in the control variant. Sayği et al. [
42] also harvested strawberries with the lowest weight from plants treated with a mineral fertiliser. In our experiment, a significant increase in the average fruit weight was observed in all three variants with the biostimulants. Depending on the biostimulant, the increase ranged from about 18% (Calgreen) to 30% (Vitorg VG), as compared with the control variant. Bogunovic et al. [
43] and Marfa et al. [
44] also observed an increase in the average fruit weight after treating plants with biostimulants.
The differences observed in the biometric parameters of strawberries were smaller than those in the fruit weight. The height and width of the fruit in the variants with the Eranthis and Calgreen biostimulants did not differ significantly from the fruit in the soil fertilisation variant (
Figure 2).
Only the Greit VG biostimulant caused a significant increase in the biometric parameters of strawberries, whose height and width were a few per cent greater than in the control variant (
Figure 2). The positive effect of the Greit VG biostimulant on both the weight of strawberries (
Table 6) and their biometric parameters may have been caused by the amino acids contained in it.
Strawberries can be made more attractive to consumers by modifying some of their quality parameters, e.g., firmness, colour, or aroma. Firmness is particularly important, especially for soft fruits, as they can be delivered even to distant markets while maintaining appropriate quality. In our experiment, the firmness of the fruit after harvesting ranged from 158.8 (soil fertilisation) to 215.1 (Eranthis) g mm
−2 (
Table 7).
These values were similar to the results of the experiments conducted by Kilic et al. [
45] and Sayği et al. [
42]. In the variants with the foliar application of biostimulants, the firmness of the fruit after harvesting was several dozen percent greater than in the soil fertilisation variant. The Eranthis and Greit VG biostimulants increased the fruit firmness by about 36%, whereas Calgreen increased it by about 34%. The Calgreen biostimulant contains calcium, an element that determines the stiffness of cell walls, one that is particularly important for the transport and storage of soft fruits. The use of calcium, especially before harvesting strawberries, improves their firmness and increases their marketable yield [
46,
47]. The significant increase in strawberry fruit firmness under the influence of biostimulants is also reported by Soppelsa et al. [
26].
The firmness of strawberries stored for several days decreased significantly. Again, the fruit harvested from the plants fertilised in the soil were characterised by the lowest firmness (132.1 g mm
−2) (
Table 7). Drobek et al. [
48] observed a significant decrease of about 70% in the firmness of strawberries after their storage for several days. The firmness of stored fruit harvested from the plants which had been treated with the biostimulants several times was significantly higher than the firmness of the fruit in the control variant (
Table 7).
The total soluble solids (TSS) content is a determinant of the taste of fruits [
49]. In our experiment, the TSS content in the strawberries ranged from 8.54 to 10.16 °Brix. The fruits in the control variant were characterised by the lowest TSS value (8.54 °Brix) (
Table 7). In the experiment conducted by Kilic et al. [
45], strawberries with the lowest TSS content were also harvested in the mineral fertilisation variant. There were lower TSS values observed in the experiments conducted by Contessa et al. [
50] (5.5 °Brix), Cvelbar et al. [
47] (8.0 °Brix), and Agehara et al. [
51] (6.32 °Brix). In our experiment, each of the three biostimulants increased the TSS content significantly. The Greit VG biostimulant was the most effective as it increased the TSS content by about 19%. The other two biostimulants were only slightly less effective. The values measured in our experiment were different from those recorded in the experiments conducted by Sayği et al. [
42], who examined the effect of various forms of soil fertilisation on strawberry plantations. The differences may have been caused by the influence of other factors on the content of extract in the fruit. According to Saridas et al. [
52], the quality of berries depends on the cultivation and environmental conditions, cultivar, degree of fruit ripeness, cultivation methods, and storage conditions. Aguero et al. [
53] did not observe any influence of air temperature on the TSS content in several cultivars of strawberries. The analysis of the content of extract in strawberries after storage confirmed the positive effect of biostimulants on this parameter, regardless of the type of biostimulant used.
As can be seen in
Table 8, the harvest date was the parameter that significantly influenced the quality of strawberries in our experiment. The fruits harvested on the first date, in early June, were the heaviest (25.39 g), but they had the lowest firmness (188.3 g mm
−2) and TSS content (8.53 °Brix). The fruits harvested on the subsequent dates had significantly lower weights but significantly greater firmness (the second date) and extract content, which was over 30% higher (on the third date).
The colour of fruit is an important indicator of the degree of its ripeness [
54]. The average consumer pays attention first to the appearance of the fruit, and then to its taste. Our study showed that the intensity of the colour of strawberries differed significantly in a variant-dependent manner. The analysis of the colour intensity (parameter L) showed that the fruits from the soil fertilisation variant were darker than those from the variants treated with the Calgreen biostimulant, and especially with the Greit VG biostimulant (
Table 9).
Measurements of the ‘a’ coordinate (the colour range from green to red) and the ‘b’ coordinate (the colour range from blue to yellow) showed that the colour of the fruit harvested from the bushes fertilised in the soil was more intense than that of the fruit harvested from the bushes treated with the biostimulants (
Table 9). The higher the value of the ‘a’ coordinate, the higher the intensity of red. The values of the colour intensity of strawberries measured in our experiment were similar to the results obtained by Gündüz et al. [
55] (L = 25.76) and Sayği et al. [
42] (L = 27.4), but they were lower than in the experiments conducted by Kilic et al. [
45] (L = 33.43) and Agehara et al. [
51] (L = 38.9).
Firm fruits, which consumers prefer, are less likely to be damaged by grey mould, caused by the
Botrytis cinerea fungus. In our experiment, the strawberries were slightly infected with this pathogen after harvest. The share of fruits with grey mould symptoms in the control variant did not exceed 3%. In the variants with the biostimulants, there were no fruits attacked by grey mould (
Figure 3). The percentage of infection increased significantly after a week of fruit storage. This was particularly noticeable in the fruit harvested from the plants in the soil fertilisation variant (about 15%). The percentage of fruit infected with grey mould in the variants with the biostimulants was even several times lower—it ranged from 2.5 to 2.8% (
Figure 3). The fruit rot may have been reduced by the positive effect of the biostimulants, which thickened the cell walls and thus limited the penetration of fungi into the cells.
The sugar content determines the taste of fruit [
49]. In our experiment, there were relatively small differences in the total sugar content in the strawberries, which ranged from 11.88 (Calgreen) to 12.79 mg 100 g
−1 (Eranthis) (
Table 10).
Such values were lower than those recorded in the experiments conducted by Paparozzi et al. [
56] at 4.18–9.98% and Gündüz et al. [
55] at 4.48–6.28%. The most common sugars in strawberries are sucrose, glucose, and fructose. In our experiment, the following contents of these sugars were found: fructose—4.85–5.12 mg 100 g
−1, glucose—6.76–7.25 mg 100 g
−1, and sucrose—0.33–0.47 mg 100 g
−1 (
Table 10). These values were greater than those recorded in the experiments conducted by Mahmood et al. [
57] and Kilic et al. [
45]. No significant increase in the fruit sugar content was observed in the variants with the biostimulants. Soltaniband et al. [
58] also found no effect of biostimulants on the sugar content in strawberries.
In our experiment, the strawberries contained the most glucose and the least sucrose. These results were different from those recorded in the experiment by Urün et al. [
1], who found the highest concentration of fructose, especially in ripe fruit. According to Lee [
59], sucrose is the main soluble sugar in strawberry cultivars with low sugar content. Differences in the content of soluble sugars in fruits may be influenced by internal factors (genotypic differences) or external factors (environmental conditions, cultivar, cultivation conditions). According to Kafkas et al. [
60], the degree of fruit ripeness is the most important factor influencing the content of sugars and organic acids in strawberries.
The taste of strawberries is influenced not only by their content of sugars but also by the amount of organic acids in them [
49]. In our experiment, the content of ascorbic acid in the fruit ranged from 94.9 to 139.46 mg 100 g
−1, citric acid—from 15.41 to 23.22 mg 100 g
−1, and malic acid—from 2.61 to 3.10 mg 100 g
−1 (
Table 11).
These values were noticeably greater than those recorded in the experiments conducted by Kilic et al. [
45]. In our experiment, the strawberries had the highest content of ascorbic acid and the lowest content of malic acid (
Table 11). Different results from the one obtained in our experiment were presented by Mahmood et al. [
57], Gündüz et al. [
61], and Urün et al. [
1], who found the largest amount of citric acid in strawberries. Similarly to sugars, external factors may significantly influence the content of organic acids in strawberries. In our experiment, there were no significant differences in the content of either ascorbic or malic acid in the strawberries. However, there were such differences in the content of citric acid. The strawberries harvested from the bushes treated with the biostimulants, regardless of the type, had significantly higher content of this acid than the fruit in the control variant. The differences ranged from 17.7 (Greit VG) to 23.22 mg 100 g
−1 (Calgreen) (
Table 11).
Anthocyanins and phenolic compounds, which can be found in the edible organs of various plants, have anti-inflammatory and anticancer properties [
62,
63], and they prevent DNA damage [
3]. In our experiment, there were no significant differences in the content of phenolic compounds in the strawberries. Depending on the variant, their content ranged from 164.45 to 172.1 mg GAE 100 g
−1 DM (
Table 12).
These values were higher than those recorded in the experiment conducted by Urün et al. [
1] but lower than those observed by Oz et al. [
64] and Agehara et al. [
51] in their experiments. The biostimulants used in our experiment did not have a significant effect on the content of polyphenols in the strawberries. Soltaniband et al. [
58] arrived at a similar conclusion after their experiment.
As mentioned earlier, anthocyanins have antioxidative properties. This fact was confirmed by Wang et al. [
65], who observed a positive correlation between the level of antioxidative activity of strawberries and the content of anthocyanins in these fruits. Their amount depends on the fruit colour intensity. The redder the strawberries are, the more anthocyanins they contain [
51]. Our experiment confirmed this conclusion. The highest content of anthocyanins (7.71 mg 100 g
−1 DM) was found in the strawberries harvested from the bushes fertilised in the soil (
Table 12). They also had the highest colour intensity measured with the L value (
Table 9).
All three biostimulants used in our experiment did not change the content of anthocyanins in the strawberries. The content of these compounds in the variants with the Eratnthis and Greit VG biostimulants was higher (8.36 and 8.03 mg 100 g
−1 DM, respectively) than in the control variant (7.71 mg 100 g
−1 DM). However, the statistical analysis did not confirm the significance of these differences. Cvelbar et al. [
47] analysed the effect of mineral fertilisation on the quality of strawberries and found no differences in the content of anthocyanins in the fruits. According to Agüero et al. [
53], the qualitative characteristics of strawberries (including firmness, the content of extract and anthocyanins) are more influenced by the air temperature and rainfall than by the supply of nutrients available to plants. According to Michalska et al. [
66] and Kruger et al. [
67], high temperatures and exposure to light (photoperiod, wavelength) accelerate metabolic processes in plant cells, thus facilitating the synthesis of anthocyanins in fruits. This fact was confirmed by Balasooriya [
68] and Cervantes [
69], who showed a positive correlation between the content of anthocyanins and air temperature.