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Article

Impact of Prolonged Frozen Storage on ‘Mejhoul’ Date Palm Cultivar Based on Selected Qualitative Characteristics

by
Younes Noutfia
*,
Ewa Ropelewska
*,
Justyna Szwejda-Grzybowska
,
Zbigniew Jóźwiak
,
Monika Mieszczakowska-Frąc
and
Krzysztof P. Rutkowski
Fruit and Vegetable Storage and Processing Department, The National Institute of Horticultural Research, Konstytucji 3 Maja 1/3, 96-100 Skierniewice, Poland
*
Authors to whom correspondence should be addressed.
Horticulturae 2025, 11(7), 731; https://doi.org/10.3390/horticulturae11070731
Submission received: 6 May 2025 / Revised: 18 June 2025 / Accepted: 19 June 2025 / Published: 24 June 2025
(This article belongs to the Special Issue Advances in Postharvest Preservation and Quality of Fruits and Vegetables)
Browse Figures
Versions Notes

Abstract

In contrast to previous studies investigating the effect of freezing at low temperatures, this work aimed to evaluate the quality of ‘Mejhoul’ during the long-term storage of 8 months under freezing at −10 °C and −18 °C. Based on numerous physicochemical attributes and image features, the behavior of ‘Mejhoul’ was assessed at 0, 2, 4, 6, and 8 months of frozen storage. The quality characteristics examined included polyphenols, sugars, color parameters, hardness, water loss, defects, and image features. The results exhibited a significant increase in water activity after eight months of frozen storage. pH increased in a similar way for both storage conditions, while titratable acidity decreased significantly. All color attributes (L*, a*, and b*) were not influenced by frozen storage, while hardness decreased significantly after eight months. Phenolic acids decreased significantly at the end of frozen storage for both freezing temperatures. However, total polyphenols, flavonoids, total sugars, glucose, and fructose were not affected by the prolonged frozen storage. Furthermore, this investigation showed a slight water gain at both storage temperatures, with a pronounced occurrence rate of skin separation for ‘Mejhoul’ stored at −18 °C. Finally, the clustering analysis exhibited a high linkage distance between the examined groups at frozen storage at −10 °C compared to −18 °C.

Graphical Abstract

1. Introduction

Dates (Phoenix dactylifera L.) are cultivated in hot, dry, and semi-arid areas. They constitute a component of a healthy diet, and their highly nutritious value is appreciated by many populations and nations around the world. After harvesting, date fruit presented a decreased level of moisture content, an increased concentration of sugars, a consequent respiratory rate, and high metabolic activity. These phenomena often lead to progressive ripening and softening, resulting in quality degradation and weight loss, especially for soft date fruit such as the ‘Mejhoul’ cultivar [1,2].
‘Mejhoul’ is a valuable, commercial, and marketable date cultivar originating from Morocco [3]. This cultivar is widely harvested in several countries, such as Jordan, Palestine, the USA, Mexico, Namibia, and Israel, and was recently introduced in Arabian Gulf countries [4]. ‘Mejhoul’ is among the highest nutritional and commercial quality date fruit categories available in the world of the date market. Compared to other soft date cultivars, ‘Mejhoul’ at an advanced stage of ripeness is characterized by a moisture content ranging from 24% to about 36%, with a sugar composition dominated by glucose and fructose in respective ranges of 27.5–38.4% and 32.2–44.3% of dry mass. ‘Mejhoul’ was also found to be a good source of phenolic compounds, vitamins (especially riboflavin), condensed tannins, and other mineral elements [3,5,6]. Furthermore, the high concentration in terms of reducing sugars (glucose and fructose) as well as the relatively high water content are habitually linked to various postharvest disorders and poor storability indices [7]. Moreover, it was reported that ‘Mejhoul’ is subject to physiological disorders such as “skin separation (puffiness)”, which is considered a major, critical, and irreversible defect. It drastically reduces the visual appearance and compromises the commercial quality [8].
To ensure effective postharvest storage and decrease the occurrence of the above-mentioned undesirable defects and disorders, various preservation techniques were suggested. These include heat treatment, vacuum and modified atmosphere packaging, fumigation, edible coating, ozone treatment, and cold storage [9,10,11]. Among these methods, low-temperature-based techniques allowed for low rates of microbiological and technological disorders, as well as for increased preservation of nutritional and sensory quality [12,13].
In more detail, freezing and frozen storage are applied to date fruit (especially soft cultivars) with the purpose of extending the shelf life, ensuring long-term conservation by stopping enzymatic activity and inhibiting the growth of microorganisms [14]. Various freezing treatments, mainly comprising cryogenic freezing, individual quick freezing, and conventional slow freezing, were evaluated at the same temperature of −20 °C on the ‘Barhi’ date cultivar based on textural characteristics and some limited physicochemical attributes. The results exhibited different changes in assessed quality attributes under frozen conditions for the same cultivar [14,15]. Moreover, the effect of prolonged freezing storage at −18 °C on the physical and chemical parameters of some Northern Omani date cultivars was studied for 6 months. Accordingly, it was mentioned that physical attributes were decreased compared to an increase in total soluble solids (TSSs), titratable acidity, pH, and biomass [16]. Another investigation focused on assessing the overall quality of the ‘Deglet Nour’ Tunisian date cultivar during storage of 10 months at very low temperatures of −20 °C, −40 °C, and −80 °C and did not find any significant change in the overall quality of stored date fruit [17]. For ‘Mejhoul’, the only available study dealing with freezing treatment was limited to determining the effects of freezing rates at −20 °C, −25 °C, and −30 °C on the textural profile, color, TSSs, and electrolyte leakage without any investigation into the behavior of this cultivar under long-term frozen storage [18]. In addition, according to the available literature, it was shown that the assessment of date fruit quality during frozen storage was limited to some physical and chemical characteristics based on destructive analysis. However, non-destructive methods using image features as attributes and artificial intelligence-based techniques as analysis tools could constitute an alternative for monitoring and evaluating the behavior of date cultivars [7,19].
All investigations focused on the storage of Phoenix dactylifera L. were carried out at low temperatures below −18 °C. However, the application of narrow sub-zero temperatures for the storage and preservation of fruit and vegetables is often effective. This is because of the cultivar’s capacity to decrease respiration exchanges and inhibit undesirable reactions and microbiological spoilage with less chilling injury and physical and visual defects [20]. Generally, the temperature that is most used for the storage of food is −18 °C (temperature chosen for our investigation), which corresponds approximately to −0 °F, which is considered the global standard for frozen storage all over the world [21]. Additionally, according to the International Institute of Refrigeration, a food product is considered frozen when the temperature is fixed at −10 °C or lower [22]. With limited exceptions, the existing literature reported that the range of −10 °C to −12 °C is a realistic threshold for microbial growth inhibition, and foodborne pathogens do not proliferate close to 0 °C [21]. In this context, shifting the temperature of storage from −18 °C to lower frozen temperatures has been gaining more interest, and this is motivated mainly by the potential reduction in energy consumption [23].
The innovative aspect of this study is related to the evaluation of the patterns of quality changes in date fruit during storage using a mild freezing temperature (−10 °C). This is a novel and comprehensive study expanding on the storage temperature range of date fruit. In previous investigations, the commonly used temperature for storing date fruits (−18 °C) was compared with a higher temperature. Therefore, the objective of this study was to investigate the effects of two freezing storage conditions at −10 °C and −18 °C during storage for 8 months on the quality of ‘Mejhoul’ date fruit. The main hypothesis in this work assumed an impact of storage time and temperature on the ‘Mejhoul’ samples. Quality assessments included polyphenols, sugars, and other physicochemical attributes such as color, hardness, and water loss. In addition, a hypothesis about the potential relationship between the above-mentioned attributes and image features acquired in a non-destructive manner using a flatbed scanner as an accurate tool for high-resolution image acquisition was stated.

2. Materials and Methods

2.1. Plant Material

The ‘Mejhoul’ date cultivar, known around the world for its high commercial value, was harvested at its full stage of ripeness (called “Tamar”) in a Moroccan orchard, sorted into homogenized batches (Figure 1), and stored at 2–4 °C before freezing experiments.

2.2. Freezing Experiments

The freezing assays consisted of storing the ‘Mejhoul’ date cultivar in two MEDGREE MARECOS freezers (MLF 66 S-ATEX Model, Tremês, Portugal) at −10 °C and −18 °C. Thus, approximately 400 uniform date fruit samples without visual defects and free of diseases were divided into two groups, each corresponding to one freezing temperature. In each group, 100 fruits were fixed and placed into plastic bags for weight loss determination, color assessment, and image acquisition during storage. The remaining fruit samples for each group were divided into subgroups of 20–25 date fruit and used for physicochemical analysis after 0, 2, 4, 6, and 8 months of frozen storage.
Before carrying out (at each frequency) physicochemical analysis, as mentioned in Section 2.3 and Section 2.4, the date fruit samples were defrosted for 24 h at +4 °C.

2.3. Weight Loss Rate and Physical Defects

The weight loss/gain of date fruit was measured on the same fruit samples before and during storage at the above-mentioned frequencies for −10 °C and −18 °C treatments. For each treatment, 100 fruits were used, and the calculation was performed as follows [12]:
Weight loss rate (%) = [(W0 − W1)/W0] × 100%, where W0 corresponds to the initial fruit weight and W1 is the weight after the specified storage period.
The analysis of physical defects included the calculation of the number of fruits with visible sugar spots below the skin, skin separation, and darkening among one hundred date fruits (the same was employed for weight loss) before and after 2, 4, 6, and 8 months of storage. The results were expressed as a % for the ratio of fruit with particular types of defects to the total number of fruit examined.

2.4. Physicochemical Analysis

2.4.1. Total Soluble Solids, pH, Acidity, and Water Activity

The total soluble solids (TSSs) were analyzed using a refractometer RE 50 (Mettler-Toledo, Switzerland), and the results were expressed as °Bx. Titratable acidity, expressed as a percentage of citric acid, and pH were determined based on the analytical protocol described in the PN-EN-12147:2000P standard using an automatic titrator (TitroLine® 7000, SI Analytics, Mainz, Germany). For water activity (aw), sliced date fruit samples were placed into an aw-Therm 40-RS apparatus (Rotronic, Bassersdorf, Switzerland), and the obtained aw result was recorded. All analyses were performed in triplicate.

2.4.2. Color and Hardness

Color attributes were determined at each storage period for 20 date fruit samples using a portable spectrophotometer (Konica Minolta CM-2600d, Chiyoda, Tokyo, Japan). Before color measurements, the calibration of the instrument was carried out and then color parameters L* (lightness, 0 (dark)-100 (light)), a* (green (−)–red (+)), and b* (blue (−)–yellow (+)) were automatically recorded. For each fruit, two readings per date fruit were taken on opposite sides. Hardness was determined by subjecting 25–30 pitted date fruit samples to a punch test using the Instron 4303 texturometer (Instron Corp., Norwood, MA, USA), and the maximal load, expressed in Newtons, was considered.

2.4.3. Sugar Analysis

Sugars were analyzed by high-performance liquid chromatography on an Agilent 1200 HPLC system (Agilent Technologies, Waldbronn, Germany), equipped with a differential refractometric detector, using Aminex HPX-87C (300 mm × 7.5 mm) with a mobile phase of 0.1 N edetate calcium disodium (CaNa2-EDTA) at an isocratic flow of 0.6 mL min−1 and a temperature of 80 °C. The samples were homogenized in redistilled water, filtered on filter paper, and purified on a Waters SepPak PLUS C18 filter before quantification using a calibration curve for glucose, fructose, and sorbitol. The results were expressed in g 100 g−1 of dry mass (DM).

2.4.4. Polyphenol Determination

Polyphenols were determined according to a modified high-performance liquid chromatography method using an Agilent 1200 HPLC system (Agilent Technologies, Waldbronn, Germany) equipped with a diode array detector. Date samples were homogenized in 70% methanol, and extracts were filtered through a PTFE filter (0.45 µm, 15 mm) before injection. Separation was performed using a Phenomenex®Fusion-RP 80A column (250 mm × 4.6 mm; particle size 4 µm). The mobile phase consisted of 10.2% (v/v) acetic acid in 2 mM sodium acetate (solvent A) and acetonitrile (solvent B). The elution conditions were as follows: an elution speed of 0.5 mL min−1, a temperature of 25 °C, and wavelengths of 280 nm for flavan-3-ols and hydroxybenzoic acids, 320 nm for hydroxycinnamic acids, and 360 nm for flavonols and flavones.
The calculations of polyphenols were quantified based on calibration curves determined with standards for individual polyphenols from the groups of flavan-3-ols, hydroxybenzoic acids, hydroxycinnamic acids, flavonols, and flavones, and the results were expressed as mg 100 g−1 of DM.
All HPLC solvents, used for sugar and polyphenols analysis, were gradient grade (Mallinckrodt Baker Ltd., Deventer, The Netherlands).

2.5. Image Acquisition and Image-Feature Determination

An Epson Perfection flatbed scanner (Epson, Suwa, Nagano, Japan) was used to acquire high-resolution images of the ‘Mejhoul’ date fruit. The choice of a flatbed scanner as an imaging instrument is justified by its ability to provide high-resolution images and therefore high accuracy rates [19].
At each storage frequency and on a white background, 20 date fruit samples were subjected to scanning on two opposite sides before changing to a black background using the MATLAB software package (MathWorks, Inc., Natick, MA, USA). Then, images were saved in a Windows Bitmap format. The obtained images were processed using Q-MQZDA 23.10 software (Łódź University of Technology, Institute of Electronics, Łódź, Poland) to define regions of interest (ROIs) and extract color, morphological, and textural features. Thus, 40 ROIs were determined at each storage frequency, and from each ROI, 599 features were obtained on the basis of image histograms, gradients, co-occurrence matrices, run-length matrices, autoregressive models, and Gabor and Haar wavelet transforms [19]. From the above-mentioned features, a feature selection was operated using WEKA software (WEKA 3.9 machine learning software, University of Waikato, Hamilton, New Zealand) to select highly discriminative features based on the “BestFirst” algorithm. The selected features were considered for correlation with physicochemical parameters.

2.6. Statistical Analysis

STATISTICA 13.1 (Dell Inc., Tulsa, OK, USA; StatSoft Polska, Kraków, Poland) software was used to assess changes in the quality attributes of ‘Mejhoul’ under the two freezing treatments. The normality of variable distribution and the homogeneity of variance were checked before applying a one-way analysis of variance (ANOVA-I) followed by Tukey’s test to compare means at the significance level of p < 0.05. An ANOVA was perform to compare the means of water activity, pH, acidity, TSSs, hardness, and color parameters (L*, a*, and b*) for samples before and after 2, 3, 6, and 8 months of frozen storage, as well as total sugars, glucose, fructose, total polyphenols, total phenolic acids, total flavonoids before and after 4 and 8 months of frozen storage for each temperature separately.
The means of both physicochemical attributes as well as image features were used to create principal component and clustering analysis with the objective of determining the potential relationship between these destructive and non-destructive parameters for each frozen storage treatment.

3. Results

3.1. Weight Loss and Main Physicochemical Attributes

3.1.1. Weight Loss and Physical Defects

The weight loss trend for the ‘Mejhoul’ cultivar is presented in Figure 2 for the two freezing temperatures. It was found that this analytical parameter did not change for date fruit samples stored at 10 °C (M-FRZ10) and −18 °C (M-FRZ18) since, after eight months of frozen storage, the rates of weight gain were, respectively, 0.07% and 0.25%.
This stability in terms of weight loss for frozen date fruit was expected since the freezing treatment acts as a barrier against water loss and transpiration. The packaging employed constitutes a second protective barrier, leading to the stable trend reported in Figure 2.
According to a previous study, it was found that prolonged freezing at −18 °C in a conventional freezer reduced the weight loss of the ‘Khalas’ cultivar at various ripening stages. In addition, a slight decrease was recorded for fully ripe ‘Khalas’ [16]. This partially confirms the findings of this investigation since ‘Mejhoul’ was harvested at an advanced stage of maturity with a low moisture content of 18.4%. The other main indicator for quality assessment is the incidence of physical defects during postharvest storage. In this study, it was found that the main visual and physical defects associated with ‘Mejhoul’ storage are “sugar crystallization below the epidermis” and “skin separation” (Figure 3a). Each fruit in Figure 3a showed both skin separation and clear and visible defects, in addition to some sugar spots below the skin as secondary defects.
According to panels b and c of Figure 3, the “skin separation” defect was observed only for date samples stored at −18 °C, affecting the total occurrence of physical defects, which was higher during storage at −18 °C compared to −10 °C. After eight months of frozen storage, 90% of ‘Mejhoul’ stored at −10 °C was without any visual defect compared to 55% in the case of date samples frozen at −18 °C. Also, Figure 3b,c show a similar occurrence rate of “sugar crystallization below the epidermis” during freezing storage at −18 °C and −10 °C with respective percentages of 15% and 10%. For “skin separation”, this defect occurs at the fourth month of storage and remains stable until the end of storage at the level of 30%. In our investigation, no more than one “skin separation” defect of less than 0.5–0.8 cm was observed in defective fruit.

3.1.2. Physicochemical Attributes

The main physicochemical attributes, including the water activity, pH, acidity, TSSs, hardness, and color parameters of ‘Mejhoul’ date fruit at the beginning of the storage experiment (0 months) and after 2, 4, 6, and 8 months of frozen storage, are presented in Table 1.
As shown in Table 1, water activity increased significantly after eight months of frozen storage to 0.606 and 0.620 for ‘Mejhoul’ samples stored, respectively, at −10 °C and −18 °C compared to 0.586 at the beginning of the storage. However, no significant change was detected in the aw before the sixth and fourth months of storage at −18 °C and −10 °C. Also, it was found that date samples stored at low freezing temperatures exhibited higher water activity at the end of frozen storage.
Moreover, our results could be explained by the trend shown in Figure 2 for water gain that positively correlates with the water activity of ‘Mejhoul’ samples stored at −10 °C and −18 °C. Thus, the slightly higher value observed (at M8) for the aw at −18 °C compared to −10 °C found a reasonable explanation in terms of the rate of water gain shown in Figure 2 at M8 for both storage conditions.
For pH, a significant increase was observed directly after two months of storage for both groups of freezing. Accordingly, pH increased from 6.66 to 6.92 for frozen ‘Mejhoul’ stored at −10 °C and 6.95 for date samples kept at −18 °C. After 2 months, no significant changes were noted, and pH showed very slight changes until the eighth month of frozen storage. In the opposite way, titratable acidity (TA) decreased significantly (p < 0.05) and rapidly only after two months from 1.985% to 1.445% and 1.470% for ‘Mejhoul’ samples frozen at 10 °C and −18 °C, respectively. However, this parameter was still without important changes until the end of storage. TSSs in both frozen groups revealed slight and fluctuating changes during the storage period. Compared with M0, TSSs increased slightly but significantly at the same level of 78.5°Bx after 2 months of storage for −10 °C and −18 °C. Then, the TSS value continues to increase (without significant changes compared to M2) at the fourth month, reaching maximal values of 78.8°Bx and 79.8°Bx for ‘Mejhoul’ groups stored at −10 °C and −18 °C. At the end of storage (M8), TSSs showed similar values to those found at M0.
For hardness, a general decrease can be reported for this physical attribute during storage, especially for −10 °C compared to −18 °C, since the final values obtained at M8 were, respectively, 5.95 N and 6.58 N and were significantly lower than the initial value of 8.49 N. The softening shown in the ‘Mejhoul’ pulp at the end of frozen storage could be attributed to the slight increase in water activity as reported in Table 1.
Regarding color, all assessed attributes (L*, a*, and b*) for the two storage groups did not change significantly (p < 0.05) during the eight months of frozen storage (Table 2). Thus, the L* value fluctuated without any significance in the range of 27.4–28.5 for ‘Mejhoul’ samples stored at −10 °C and in the range of 27.3–28.4 for −18 °C. A slight decrease in a* values was noticed at the end of frozen storage compared to the initial value of 4.95. For yellowness (b*), similarly, it was found that frozen storage at the two temperatures did not significantly affect this color attribute, which remained stable in ranges of 4.18–4.53 and 4.32–4.59 for frozen ‘Mejhoul’ kept at −10 °C and −18 °C. This stability in all color attributes exhibited a conservative effect under the two freezing conditions in terms of keeping the color of ‘Mejhoul’ unchanged during a prolonged frozen storage without any impact from temperature.

3.2. Polyphenol Profile and Sugars

Table 3 describes the pattern of sugars and polyphenols in the ‘Mejhoul’ cultivar during frozen storage at −10 °C and −18 °C.
Besides total phenolic acids, all other functional compounds reported in Table 3 did not significantly change during the frozen storage of eight months, both at −10 °C and −18 °C. Compared to fresh groups, slight increases (not significant) of 1 and 1.6 g 100 g−1 DM were observed in the concentration of total sugars at the end of the storage of the ‘Mejhoul’ cultivar that was kept, respectively, at −10 °C and −18 °C. Glucose and fructose were the two principal sugars constituting the sugar matrix in the ‘Mejhoul’ cultivar, and no significant change was exhibited during frozen storage for these two reducing sugars that fluctuate in a restricted range of 41.5–42.4 and 39.8–40.6 g 100 g−1 DM for glucose and fructose, respectively.
Total polyphenols increased slightly after four months to reach maximum values of 34.9 and 34.1 g 100 g−1 DM for −10 °C and −18 °C groups, which then decreased at the eighth month without any significant change during the whole storage period. Similarly, a non-significant difference (p < 0.05) was noted for total flavonoids that fluctuate in the range of 9.63–12.5 g 100 g−1 DM. However, total phenolic acids (TPAs) were significantly affected by freezing storage. Following the same trend for the two freezing storage conditions, TPAs showed a slight increase from 21.9 to 22.4 and 23.4 g 100 g−1 DM after four months for ‘Mejhoul’ date samples frozen, respectively, at −10 °C and −18 °C. Subsequently, TPA content decreased significantly to reach the lower value of 13.9 g 100 g−1 DM after eight months of storage.

3.3. Multivariate Analysis

In this investigation, multivariate statistical analyses were applied to assess the behavior of the ‘Mejhoul’ cultivar under the two frozen storage conditions (Figure 4). Accordingly, principal component analysis (Figure 4a,b) explained more than 92% of the total variability, which was divided, respectively, to 63.9% and 28.9% for PC1 and PC2 in the case of ‘Mejhoul’ frozen at −10 °C. Thus, the first component (PC1) was positively correlated with titratable acidity for both frozen storage conditions, with flavonoids and three textural image features for the −18 °C group, and with hardness for the −10 °C group. High negative correlations were noticed for total sugars, fructose, glucose, pH, and TSSs on PC1 for the two frozen treatments. The second component, which accounted for 28.9% of total variability, was strongly correlated with color attributes (especially for −10 °C), total polyphenols, and total phenolic acids. According to Figure 4a,b, it seems that various image features correlated with particular physicochemical attributes. Thus, a strong correlation was found on PC1 (for samples stored at −18 °C) between flavonoids and titratable acidity and SD8HistSkewness as image features. For the same freezing temperature, according to PC2, total polyphenols, total phenolic acids, hardness, and the b* color attribute correlated with the image feature called uD8HistVariance. For −10 °C, based on PC1, hardness and titratable acidity are positive correlators with BD8HistKurtosis. However, most physicochemical attributes, such as water activity, flavonoids, total phenolic acids, fructose, glucose, and TSSs, are negatively correlated with PC1 and with the two image features YD8DwtHaarS4LH and BD8HistVariance, which can be considered two image-feature predictors.
The PCA allowed for general and potential correlations between classical (destructive analysis) physicochemical attributes and selected image features (obtained using quick and non-destructive analysis). During frozen storage at −10 °C and based on PCA analysis (panel “a” of Figure 4 and PC1), it is possible that the image feature called “BD8HistKurtosis” could be a strong predictor and positive correlator of hardness and acidity. The same image feature seems to be a strong negative correlator of several physicochemical attributes, such as total sugars, glucose, fructose, TSSs, pH, and, to a lower degree, total polyphenols, phenolic acids, and water activity. In the case of −18 °C, the two image features “SD8HistSkewness” and, to a lower degree, “qD8HistVariance” could be chosen as positive predictors of acidity and flavonoids during storage. These two image features might also be used as powerful attributes to estimate TSSs, pH, total sugars, glucose, fructose, and water activity. Thus, an increase in the values of “SD8HistSkewness” and “qD8HistVariance” during the frozen storage of ‘Mejhoul’ at −18 °C could be an indication of an increase in the content of flavonoids and acidity and a decrease in other physicochemical parameters such as total sugars, pH, and water activity.
Furthermore, according to panels c and d of Figure 4, the distance between groups was very high (about 250) at a frozen storage of −10 °C compared to the distance of about 70 recorded for −18 °C. When considering 70 as the average cutting distance between classes, only one additional group (constituted by the outlier feature BD8HistVariance) will not be included in the general clustering for frozen storage at −10 °C. Moreover, Figure 4 showed that the linkage distance was low between the examined groups, both at −10 °C and −18 °C. Thus, the clustering pattern of both the non-destructive features and destructive attributes of the ‘Mejhoul’ cultivar revealed a similar number of clusters under the two frozen storage conditions. The number of clusters could be reduced drastically if the linkage distance decreased to the range of 5–10. Accordingly, distinct clusters built on physicochemical parameters such as aw, pH, and acidity and textural image features such as BD8HistMaxm10, SD8HistSkewness, and YD8Gab4V2Mag could be defined.

4. Discussion

In the literature, it was mentioned that the observed defects (in the case of the ‘Mejhoul’ cultivar) significantly reduced the quality value of date fruit, and these defects are attributed to the mechanical properties of the skin, some environmental conditions, and preharvest stresses [24]. Likewise, it was demonstrated that genetics is a significant and strong factor that contributes the most to skin separation (or puffiness). Accordingly, the analysis of genetically similar cultivars grown in different regions exhibited a range of skin separation values between date fruit of the same cultivar [25]. Nevertheless, no indication was cited regarding the link between storage and this defect in the case of Phoenix dactylifera L species. Skin puffiness was previously attributed to the “albedo cell rupture” in peel secretory cavities [26], to some metabolic disorders of endogenous hormones [27], and to the ratio between soluble sugars and organic acids in the skin [28]. Moreover, one of the potential hypotheses explaining skin separation was attributed to the weakness of the peel cell wall, due to the lignification of parenchyma cells close to the skin secretory cavities. The second possible hypothesis concerned a premature senescence of the peel caused by AsA deficiency-induced oxidative damage [28]. In this perspective, it could be concluded that at −18 °C, ‘Mejhoul’ is more predisposed to albedo cell damages, causing a high percentage of “skin separation”. Also, the mechanical, biochemical, and metabolomic properties of the skin of some date samples chosen for storage at −18 °C could be significantly different from the other samples. The main characteristic of ‘Mejhoul’ samples showed that “skin separation” could be due to the reduced flexibility and increased rigidity of the skin, resulting in a greater predisposition to damage [29]. Also, the higher freezing temperature of −18 °C could further increase the incidence of skin separation. For other stone fruit, it was outlined that chilling injury was a result of fruit exposure to freezing temperatures, which leads to the formation of ice crystals in their intercellular spaces and inside the cells [30].
For physicochemical parameters, it was reported that freezing treatments crystallize water and thereby reduce water activity [31]. During the prolonged frozen storage of date fruit (as in our investigation), no investigation has focused on monitoring the variation in the aw. Nevertheless, it was concluded that water activity increased for several date cultivars from 0.49 to 0.64 at the sixth month of cold storage at +5 °C [32].
The general increase in pH was globally in accordance with the evolution of the same parameter under the frozen storage of 10 months at −18 °C for the ‘Khalas’ cultivar [16]. Also, similarly to our findings, a significant decrease in titratable acidity from 2.02 to 1.78 g citric acid 100 g−1 FW was indicated for the ‘Deglet Nour’ cultivar after 10 months of storage at −20 °C [17]. This decrease in TA was also reported for apricot fruit nearing seven weeks of frozen storage [33]. Under cold storage conditions, it was suggested that the decrease in TA can be justified by the use of organic acids in the respiration process [34] or their transformation into sugars [35]. During the respiration process, organic acids are subjected to oxidative breakage and used as substrates for the release of simple molecules such as carbon dioxide and water, automatically leading to an increase in water content and, accordingly, an increase in water activity [36]. This hypothesis could partially explain the increase observed for water activity in this investigation. Regardless of the storage temperature, it was determined that oxidation is also indicated as the main cause of organic acid reduction in several fruits and vegetables during storage [37].
The same pattern in TSS content during storage, like that observed in our study, was reported for the ‘Khalas’ cultivar, which showed an increase in TSS values after 6 months of storage before decreasing by the 10th month of storage at −18 °C [16]. TSS content is known as an important attribute for fruit quality, and fluctuations in this parameter are partially linked to sugar content, the respiratory process, and some biochemical reactions, mainly comprising the decomposition of organic matter into sugars [38].
Similarly to our results, it was indicated that the hardness of the ‘Barhi’ cultivar decreased after 3 months of quick and conventional slow freezing and remained stable until 9 months [14,39]. Hardness is a critical attribute in consumer acceptability and contributes to the overall quality of the fruit [40,41]. The general softening of ‘Mejhoul’ at M8 of storage could be attributed to the semirigid nature of cells and fruit tissues. Fruits, in contrast to vegetables, do not possess a fibrous structure, allowing them to resist the freezing treatment and making their tissues lose their crispness and become soggy [41].
Compared to previous studies, it was reported that L* and b* values of fresh date fruit cultivated at the early stage of maturity showed a continuous and notable decrease under conventional and cryogenic freezing, while the a* value increased during storage [14].
In a comparable study regarding sugars, a fluctuating variation in glucose content and a stable evolution in fructose content were observed during ultra-freezing at −40 °C and conventional freezing at −20 °C for the ‘Barhi’ cultivar [14,15]. For ‘Deglet Nour’, glucose and fructose concentrations showed a slight but significant decrease after 10 months of frozen storage at −18 °C [17]. In comparison with our findings, these discordances could be attributed to the effect of the cultivar, maturity stage, and packaging used for the preservation of date fruit during storage.
It was observed that maintaining the concentration of sugars, including glucose and fructose, at high levels is a pro-osmolarity factor that contributes to decreasing water loss [42,43], as was found for this investigation. However, the increased variations in sugar compounds have been reported to be linked to cold stress and water deficits in fruits and vegetables, which could constitute a protection mechanism for cell membranes for chilling stress.
In contrast to our findings, total phenolics and flavonoids increased significantly for ‘Khalas’ and ‘Khunaizi’ cultivars after storage at −20 °C for one month [44]. However, our results related to total polyphenols are in agreement with [17], since no significant change was observed in the levels of polyphenols in ‘Deglet Nour’ after 10 months at −20 °C. The pattern of changes in TPAs follows similar behavior reported for total phenolic compounds of guava and Makiang fruit during 3 months of storage at −20 °C [45]. In addition, the decrease in the TPAs shown in this investigation is in broad agreement with the decrease in phenolic acid content reported for strawberries stored at −20 °C for a long-term storage of 360 days [46]. In the case of ‘Mejhoul’ used in this study, the TPA profile was mainly represented by chlorogenic acid derivatives, ferulic acid, and gallic and vanillic acid [47]. Even if the mechanisms of polyphenols and especially TPA variation during frozen storage remained unclear [48], it was hypothesized that the oxidation of chlorogenic acid derivatives could explain the loss of TPAs at low storage temperatures since these phenolic acids are potential substrates for endogenous polyphenol-oxidase enzymes [49]. Moreover, cell rupture during freeze–thaw procedures leads to the depletion of phenolic acids [50], which explains the obtained results in this investigation.
Taken together, image features obtained in rapid and non-destructive ways could have a potential application at the industrial scale to replace tedious, long, and complex quality analyses with the objective of monitoring the behavior of date fruit under storage conditions. Thus, the use of image analysis in the specific case of “date palm storage and preservation” could effectively contribute to the prediction of several biochemical components.

5. Conclusions

Storage at −10 °C preserved the quality of ‘Mejhoul’ in a manner comparable to storage at −18 °C, with a lower incidence of defects such as skin separation and sugar spots under the epidermis. Generally, total polyphenols, flavonoids, total sugars, glucose, and fructose were not affected during storage. However, phenolic acids, pH, titratable acidity, water activity, and hardness showed significant changes throughout the frozen storage period. Imaging characteristics showed potential for non-destructive monitoring, while a reduction in phenolic acids suggests oxidative degradation. Overall, −10 °C is recommended to balance quality and energy efficiency. Future studies should evaluate intermediate temperatures and include sensory analyses.

Author Contributions

Conceptualization, Y.N., E.R. and K.P.R.; Methodology, Y.N., E.R., J.S.-G., Z.J., M.M.-F. and K.P.R.; Validation, Y.N. and E.R.; Formal analysis, Y.N., E.R. and J.S.-G.; Investigation, Y.N., E.R. and M.M.-F.; Writing—original draft, Y.N.; Writing—review & editing, E.R. and J.S.-G.; Visualization, Z.J.; Supervision, E.R., M.M.-F. and K.P.R.; Project administration, Y.N.; Funding acquisition, Y.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research is part of the project no. 2022/45/P/NZ9/03904 that was co-funded by the National Science Centre and the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement no. 945339, project title: “A novel approach to the assessment of date fruit quality (Phoenix dactylifera L.) under different storage conditions, using innovative models based on image analyses and machine learning” (M-LEARN4DATE). For the purpose of open access, the author has applied a CC-BY public copyright license to any Author Accepted Manuscript (AAM) version arising from this submission.Horticulturae 11 00731 i001

Data Availability Statement

Data is contained within the article.

Acknowledgments

The authors are grateful to the technical staff of the storage facilities at the Fruit and Vegetable Storage and Processing Department of the National Institute of Horticultural Research in Skierniewice, Poland, for their time, effort, and support in the installation and monitoring of the freezing experiments.

Conflicts of Interest

The authors declare no conflict of interest.

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Horticulturae 11 00731 g001
Figure 1. ‘Mejhoul’ cultivar used for freezing experiments.
Figure 1. ‘Mejhoul’ cultivar used for freezing experiments.
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Horticulturae 11 00731 g002
Figure 2. Weight loss observed during the 8-month frozen storage of ‘Mejhoul’. M-FRZ10: ‘Mejhoul’ frozen at −10 °C; M-FRZ18: ‘Mejhoul’ frozen at −18 °C (each result is for 100 fruits).
Figure 2. Weight loss observed during the 8-month frozen storage of ‘Mejhoul’. M-FRZ10: ‘Mejhoul’ frozen at −10 °C; M-FRZ18: ‘Mejhoul’ frozen at −18 °C (each result is for 100 fruits).
Horticulturae 11 00731 g002
Horticulturae 11 00731 g003aHorticulturae 11 00731 g003b
Figure 3. Illustration of “skin separation” and “sugar crystallization below the epidermis” for ‘Mejhoul’ stored at −18 °C (a), and the incidence of physical defects during ‘Mejhoul’ frozen storage at −18 °C (b) and −10 °C (c) (measurements for 100 fruits).
Figure 3. Illustration of “skin separation” and “sugar crystallization below the epidermis” for ‘Mejhoul’ stored at −18 °C (a), and the incidence of physical defects during ‘Mejhoul’ frozen storage at −18 °C (b) and −10 °C (c) (measurements for 100 fruits).
Horticulturae 11 00731 g003aHorticulturae 11 00731 g003b
Horticulturae 11 00731 g004
Figure 4. Multivariate analysis of data obtained for frozen ‘Mejhoul’: principal component analysis of frozen ‘Mejhoul stored at −10 °C (a) and −18 °C (b); hierarchical cluster for frozen ‘Mejhoul stored at −10 °C (c) and −18 °C (d).
Figure 4. Multivariate analysis of data obtained for frozen ‘Mejhoul’: principal component analysis of frozen ‘Mejhoul stored at −10 °C (a) and −18 °C (b); hierarchical cluster for frozen ‘Mejhoul stored at −10 °C (c) and −18 °C (d).
Horticulturae 11 00731 g004
Table 1. Evolution of main physicochemical attributes of ‘Mejhoul’ cultivar during frozen storage.
Table 1. Evolution of main physicochemical attributes of ‘Mejhoul’ cultivar during frozen storage.
Quality AttributeStorage Time (Months)−10 °C−18 °C
Water activity 00.586 a0.586 a
20.589 a,b0.596 a
40.584 a0.592 a
60.616 c0.587 a
80.606 b,c0.620 b
pH06.66 a6.66 a
26.92 b6.95 c
46.89 b6.93 b,c
66.84 b6.77 a,b
86.91 b6.98 c
Acidity (%)01.985 a1.985 a
21.445 b1.470 b,c
41.530 b1.495 b,c
61.440 b1.630 b
81.475 b1.290 c
TSS (°Bx)077.7 a77.7 a
278.5 a,b78.5 a,b
478.8 b79.8 b
678.3 a,b77.8 a
877.7 a78.1 a
Hardness (N)08.49 a8.49 a
27.02 a,b6.08 b
46.14 b6.81 a,b
66.46 b7.95 a,b
85.95 b6.58 b
For each attribute, values with different letters in the columns are significantly different according to Tukey’s test (p < 0.05); mean values for water activity, pH, acidity, and TSSs were obtained from experiments carried out in triplicate, while for hardness, it was 30 repetitions.
Table 2. Comparison of color parameters of ‘Mejhoul’ cultivar during frozen storage.
Table 2. Comparison of color parameters of ‘Mejhoul’ cultivar during frozen storage.
Quality AttributeStorage Time (Months)−10 °C−18 °C
L*028.4 a28.4 a
227.4 a27.3 a
427.6 a27.9 a
627.9 a27.9 a
828.5 a28.3 a
a*04.95 a4.95 a
24.96 a4.82 a
45.00 a4.58 a
64.69 a4.98 a
84.56 a4.81 a
b*04.49 a4.49 a
24.33 a4.70 a
44.53 a4.32 a
64.18 a4.32 a
84.32 a4.59 a
For each attribute, values with different letters in the columns are significantly different according to Tukey’s test (p < 0.05); mean values for color parameters L*, a*, and b* were 40 repetitions, with 20 date fruit samples for two opposite sides.
Table 3. Polyphenols and sugars of ‘Mejhoul’ cultivar during frozen storage.
Table 3. Polyphenols and sugars of ‘Mejhoul’ cultivar during frozen storage.
Functional Compound (g 100 g−1 DM)Storage Time (Months)−10 °C−18 °C
Total sugars 082.3 a82.3 a
482.9 a83.7 a
883.3 a83.9 a
Glucose 041.5 a41.5 a
441.7 a42.4 a
842.2 a42.0 a
Fructose039.8 a39.8 a
440.0 a40.1 a
839.8 a40.6 a
Total polyphenols 033.9 a33.9 a
434.9 a34.1 a
829.5 a31.1 a
Total phenolic acids021.9 a21.9 a
422.4 a23.4 a
813.9 b13.9 b
Total flavonoids 012.1 a12.1 a
412.5 a10.7 a
89.63 a10.6 a
For each attribute, values with different letters in columns are significantly different according to Tukey’s test (p < 0.05), and mean values were obtained from experiments carried out in triplicate.
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Noutfia, Y.; Ropelewska, E.; Szwejda-Grzybowska, J.; Jóźwiak, Z.; Mieszczakowska-Frąc, M.; Rutkowski, K.P. Impact of Prolonged Frozen Storage on ‘Mejhoul’ Date Palm Cultivar Based on Selected Qualitative Characteristics. Horticulturae 2025, 11, 731. https://doi.org/10.3390/horticulturae11070731

AMA Style

Noutfia Y, Ropelewska E, Szwejda-Grzybowska J, Jóźwiak Z, Mieszczakowska-Frąc M, Rutkowski KP. Impact of Prolonged Frozen Storage on ‘Mejhoul’ Date Palm Cultivar Based on Selected Qualitative Characteristics. Horticulturae. 2025; 11(7):731. https://doi.org/10.3390/horticulturae11070731

Chicago/Turabian Style

Noutfia, Younes, Ewa Ropelewska, Justyna Szwejda-Grzybowska, Zbigniew Jóźwiak, Monika Mieszczakowska-Frąc, and Krzysztof P. Rutkowski. 2025. "Impact of Prolonged Frozen Storage on ‘Mejhoul’ Date Palm Cultivar Based on Selected Qualitative Characteristics" Horticulturae 11, no. 7: 731. https://doi.org/10.3390/horticulturae11070731

APA Style

Noutfia, Y., Ropelewska, E., Szwejda-Grzybowska, J., Jóźwiak, Z., Mieszczakowska-Frąc, M., & Rutkowski, K. P. (2025). Impact of Prolonged Frozen Storage on ‘Mejhoul’ Date Palm Cultivar Based on Selected Qualitative Characteristics. Horticulturae, 11(7), 731. https://doi.org/10.3390/horticulturae11070731

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