Introduction

The date palm (Phoenix dactylifera L.) is an important fruit crop with over than two thousand date cultivars which are grown worldwide, especially in the Middle East and North Africa (Chao and Krueger 2007). Date fruits pass through several ripening stages, reaching to full ripeness. These stages include Hababouk, Kimri, Khalal or Besser, Rutab, and Tamer. Tamer stage is the last stage of ripeness where date fruit becomes mature with brown or black color, and relatively higher sugar and lower moisture contents (Hussain et al. 2020). Date fruits are a nutritionally rich food and considered a good source for carbohydrates, including simple sugars and dietary fiber, vitamins such as ascorbic acid, thiamine, and riboflavin, and minerals such as iron, potassium, and phosphorus (Al-Shahib and Marshall 2003; Al-Farsi and Lee 2008). Date fruits have several beneficial health effects, including antimutagenic, antihyperlipidemic, antimicrobial, anti-inflammatory, hepatoprotective, nephroprotective, and gastroprotective activities (Baliga et al. 2011; Tang et al. 2013).

Antioxidants are biologically active compounds able to quench reactive free radicals, such as hydrogen peroxide, superoxide radical, and hydroxyl radical. Thus, they can prevent or slow down body cell damage caused by free radicals (Lee et al. 2004; Liu et al. 2018). Therefore, antioxidants play a vital role in human health as they decrease the risk of many chronic diseases such as cancers, cardiovascular disease, neurodegenerative disorders, liver cirrhosis, atherosclerosis, and diabetes (Neeraj et al. 2013; Barros 2020). In fact, date fruits have high antioxidant properties due to their high content of vitamins such as ascorbic acid and tocopherols, and phytochemicals such as polyphenols and carotenoids (Al-Farsi et al. 2018; Al-Shwyeh 2019; Hussain et al. 2020). Phenolic compounds exhibit an extremely high antioxidant activity (Maqsood et al. 2020). The antioxidant activity of the date fruits refers largely to its phenolic compounds, including gallic, p-coumaric, ferulic, syringic, and caffeic acids, and flavonoid compounds, including rutin, quercetin, quercetin, isoquercetrin, and apigenin (Hong et al. 2006; Biglari et al. 2008; Benmeziane-Derradji 2019).

Considering the high nutritional value of date fruits, studying their antioxidant content is progressively being increased (Kchaou et al. 2014). Numerous factors have been verified to be responsible for the variances in antioxidant content of date fruits such as cultivar type, ripening stage, growing conditions, season, geographic location, fertilizer, type of soil, storage periods, and quantity of sunlight received (Al-Farsi et al. 2007; Al-Mssallem et al. 2020). Gathering detailed information about antioxidant content of various date fruit cultivars could help to reach a better understanding of their health-promoting properties. Unfortunately, the high inconsistency and variability in research designs appear in the studied literature and this area limits their result impact and implications (Allaith 2019). Currently, evidence on characteristics of scientific research conducted to determine antioxidant content in ripe date fruits and their research design consistency is still not known. Therefore, the current scoping review was conducted to identify various characteristics of studies that were carried out to determine antioxidant content in ripe date fruits and to make future recommendations to promote higher research design consistency and, thus, improve research quality in this research area.

Methods

In this scoping review, ripe date fruits are defined as date fruits at full ripeness with a color that was entirely changed from yellow to almost brown or black, and with relatively higher sugar and lower moisture contents. This is equivalent to Tamer stage, the Arabic terms used to describe the full ripening stage of date fruits. Antioxidant content determination research is defined as any original research study with scope related basically to the determination of antioxidant components such as total phenol content and total flavonoid content and related additionally to the determination of antioxidant activity and antioxidant compounds (phenolic compounds and flavonoid compounds) when reported. The framework established by Arksey and O’Malley (2005) was adopted to conduct this scoping review. Furthermore, the current recommendations by Levac et al. (2010) were implemented to enhance the process of searching, screening, and reporting of the scoping review. The applied framework in the present scoping review comprised six stages. These stages include (1) research question identification, (2) relevant study identification, (3) study selection, (4) data charting, (5) result collating, summarizing, and reporting, and (6) experts’ consultation (Arksey and O’Malley 2005). Moreover, the Preferred Reporting Items for Systematic Reviews and Meta-Analysis extension for Scoping Reviews (PRISMA-ScR) was used as a guide throughout the review process (Tricco et al. 2018).

Research Question Identification

The current scoping review was performed to answer five questions. These questions are (1) what are the characteristics of antioxidant content determination research in ripe date fruits, (2) what are the characteristics of collected ripe date fruit samples, (3) what are the characteristics of solvents used to extract antioxidant content, (4) what are the reported information about determining antioxidant content (quantity and quality) and the used analytical methods, citation references for procedures, measurement units, and data presentation methods, and finally, (5) what are the research gaps and future recommendations to promote higher research design consistency and, thus, improve research quality in this research area.

Relevant Study Identification

A comprehensive search to identify relevant studies was performed using three electronic databases for academic research: Scopus, PubMed, and Web of Science. The search process targeted any relevant study published in March 2019 or any time before. Specific keywords and synonyms related to this review scope were established as search terms. The used search terms include antioxidants, antioxidants content, antioxidant components, antioxidant activity, antioxidant capacity, antioxidant profile, antioxidant composition, phenolic content, total phenols content, phenolic profile, phenolic composition, date fruits, date palm fruits, ripe date fruits, date cultivars, date varieties, and Phoenix dactylifera. The search process was carried out based on the search terms by experienced researchers. During this process, varied combinations of Boolean operators (OR, AND, NOT) including adjacencies and truncations were used to combine the search terms in order to capture potential studies in the academic resources.

Study Selection

Study selection from potential studies retrieved from academic resources was performed based on previously defined inclusion criteria. The inclusion criteria applied in this scoping review were (1) original research studies that were published in peer-reviewed journals, (2) study language is limited to English, (3) the date fruit samples were collected at full ripeness (Tamer stage), (4) the types of collected ripe date fruit cultivars were specified, (5) the solvent(s) used to extract antioxidant content were specified, and (6) the studies reported determination at least one antioxidant component.

Study selection was completed in two steps. In the first step, the titles and abstracts of identified studies were screened by researchers independently based on the inclusion criteria. Irrelevant studies were excluded. In the second step, full texts of relevant studies were retrieved and screened by researchers independently to assess their eligibility to be included in the qualitative analysis. Collected data from all researchers were compared after each step to ensure the process steadiness. Any discrepancies between the researchers were discussed and refined. The list of selected studies was finalized and adopted for the next stages in this scoping review.

Data Charting

Data extracted from the selected studies were collected in the Microsoft Excel spreadsheet. Actually, the extracted data were organized in a charting table that summarized the general characteristics and information reported in the selected studies. The complete charting table was reviewed by all researchers to ensure consistency in the data charting process.

Result Collating, Summarizing, and Reporting

Data in the charting table were analyzed by using descriptive statistics. The results were summarized in tables and bar graphs that answered the research questions identified in this scoping review. The results were reported to highlight the main findings, address the research gaps, and make useful recommendations for future research. However, the quality of selected studies was not evaluated as it is outside of the current review scope.

Experts’ Consultation

A consultation was conducted with experts and researchers who have a significant research contribution to the area of antioxidant content determination research. The consultation was aimed to ensure the novelty and importance of our findings and usefulness of recommendations for future research.

Results

Search Process Results

We identified two hundred and forty records, twenty-eight of which were excluded as duplicates (Fig. 1). Title and abstract screening was conducted for the remaining two hundred and twelve articles, eighty-seven of which were excluded as they did not answer the research questions that were identified in the current scoping review. We retrieved and screened the full texts of the remaining one hundred and twenty-five articles, ninety-three of which were excluded as they were ineligible based on the inclusion criteria. Finally, a sum of thirty-two articles was selected and included in this scoping review (Mansouri et al. 2005; Benmeddour et al. 2013; Ghiaba et al. 2014; Haimoud et al. 2016; Hachani et al. 2018; Allaith 2008; Abbas et al. 2008; Biglari et al. 2008; Shahdadi et al. 2015; Hemmateenejad et al. 2015; Siahpoosh et al. 2016a; Siahpoosh et al. 2016b; Lemine et al. 2014; Bouhlali et al. 2017; Singh et al. 2012; Al Harthi et al. 2015; Haider et al. 2018; Al-Turki et al. 2010; Al-Najada and Mohamed 2014; Hamad et al. 2015; Ali et al. 2016; Farag et al. 2016; Aldhafiri 2017; Saafi et al. 2009; Chaira et al. 2009; Mrabet et al. 2012; El Arem et al. 2012; Kchaou et al. 2013; Kchaou et al. 2014; Mrabet et al. 2015; Hamza et al. 2016; Souli et al. 2018).

Fig. 1
figure 1

Flowchart illustrating selection process for the scoping review of antioxidant content determination in ripe date fruits studies

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Characteristics of the Selected Studies

The characteristics of the selected studies that were included in this review are presented in Table 1. The selected studies were published in peer-reviewed journals of varied subject categories. Among thirty-two included studies, eighteen studies (56.3%) were published in journals related to food science. The remaining sixteen studies (44.8%) were published in journals related to subjects other than food science including agricultural sciences (n = 5), chemistry (n = 4), pharmacology (n = 3), nutrition (n = 1), and medical sciences (n = 1). The selected articles were published starting from 2005 up to 2018. Actually, half of the selected studies (n = 16) were published during 2015–2018, while six and ten articles were published during the periods 2005–2009 and 2010–2014, respectively. The highest number of studies (n = 6) was published in 2016 (see Fig. 2). However, none of the included studies was published in the years 2006, 2007, or 2011. The institution of the first authors in the selected studies belongs to one of the fourteen countries. First author’s institution country was Tunisia in eight articles (25%). In the same way, it was Algeria, Iran, and Saudi Arabia in different four articles for each country. The selected studies were carried out for diverse purposes. The main objective was to determine antioxidant content in ripe date fruits in fourteen articles (43.8%). In fact, one study was performed to determine dietary fiber characteristics in ripe date fruits, while another one was done to conduct multivariate statistical analysis. The remaining studies were conducted to determine the effect of specific variables on antioxidant content in date fruits. These variables include ripening stages (n = 6), extraction solvents (n = 5), geographical locations (n = 2), storage periods (n = 1), hydrothermal treatments (n = 1), and parthenocarpy, the process of producing fruits without seeds (n = 1). Funding for conducting research was received from either academic or governmental institutions in about half of the selected studies (n = 17), while no fund was reported in the rest of the articles. “No conflict of interests” phrase had been declared in twelve studies and data are not reported in the remaining studies.

Table 1 Characteristics of selected studies that were included in the qualitative analysis in this scoping review (n = 32)
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Fig. 2
figure 2

Bar graph illustrating the number of selected studies that were published each year during the period 2005 to 2018

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Characteristics of the Collected Ripe Date Fruit Samples

The characteristics of the collected ripe date fruit samples that were included in this review are provided in Table 2. The production country for collected ripe date fruit samples in the selected studies belongs to one or more of different ten countries. The collected ripe date fruit samples were of single country origin in most included studies (n = 30). The production country for collected ripe date fruit samples was Tunisia, Iran, Algeria, and Saudi Arabia in nine, six, five, and five included studies, respectively. However, one study collected samples produced in two different countries (Saudi Arabia and the USA), while another study collected samples produced in three different countries (Bahrain, Saudi Arabia, and Tunisia). The ripening stage of collected ripe date fruit samples was described in several ways in different studies. Most of the selected articles (n = 21) used Arabic terms “Tamer” to describe the ripening stage of collected date fruit samples. The remaining studies reported that they collect ripe fruits (n = 8) or fruits with full ripeness (n = 3). The number of collected ripe date fruit cultivars in each selected study ranged from one to eighteen cultivars. Indeed, the number of collected cultivars was one to five, six to ten, and eleven to eighteen in fourteen, thirteen, and five studies, respectively. Figure 3 shows the number of collected cultivars in different studies. The total number of reported ripe date fruit cultivars in all included studies was one hundred and twenty-nine. Most of them (n = 88) were reported once in all included studies. However, two cultivars (Deglet Nour and Khalas) were reported twelve times, and one cultivar (Zahedi) was reported six times in all studies. Moreover, the cultivars of Sukari, Kabkab, Medjool, and Allig were reported four times in all studies.

Table 2 Characteristics of collected ripe date fruit samples reported in selected studies that were included in the qualitative analysis in this scoping review (n = 32)
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Fig. 3
figure 3

Bar graph illustrating the number of selected studies that reported each number of collected date fruit cultivars (1–18)

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Information about quality, harvest season, local location, source, storage periods, storage temperature, and moisture/dry matter content of collected ripe date fruit samples was reported in thirteen, twenty-two, twenty-nine, fifteen, three, twenty-four, and ten studies, respectively. The collected ripe date fruit samples were mature date fruits with uniform size, and free of physical damage in eleven studies (34.4%), and second-grade date fruits, with texture defect (relatively hard) in another two studies (6.3%). The used ripe date fruit samples were collected during one specific harvest season in twenty studies (62.5%) and during two specific harvest seasons in another two studies (6.3%). The source of collected ripe date fruit samples was local retail markets, and research stations or private farms in eight and six studies, respectively. In addition, one study had collected the samples from local retail markets or private farms, while another study collected samples from the National Clonal Germplasm Repository for Citrus and Dates (USA) and private farms (Saudi Arabia). The storage periods before analysis of collected ripe date fruit samples were 1 day in one study, 1‑2 months in another study, and 1 day, 6 months, and 1 year in a third study. The collected samples were stored before analysis at refrigerator temperature (4 °C), and freezer temperature (− 20 to − 18 °C) in ten and thirteen studies, respectively. However, two studies stored the samples at temperature − 40 °C. The moisture content of collected ripe date fruit samples was reported in eight studies, while two studies reported dry matter content.

Characteristics of Extraction Solvents

The characteristics of extraction solvents that were used to extract antioxidant content from collected ripe date fruit samples are shown in Table 3. The number of solvents used to extract antioxidant content that were reported in each selected study ranged from one to five solvents. Most of the included studies (n = 27) used one extraction solvent. Nevertheless, using two, three, and five extraction solvents was reported in one, two, and two different studies, respectively. The total number of extraction solvents that were reported in all included studies was forty-five. Actually, six types of extraction solvents were reported in all studies. Methanolic solvents were the most commonly used solvents to extract antioxidant content and reported in twenty-three articles (51.1%). Acetonic solvents, ethanolic solvents, ethyl acetate solvents, aqueous solvents, and methanolic chloroform solvents were reported in eight, seven, three, two, and two studies, respectively. The concentration of used extraction solvents varied in different studies. In fact, methanol with a concentration of 80% was the most frequently used solvent (n = 12) to extract antioxidant content. Absolute methanolic (100%), methanolic (with a concentration 50%), and acetonic (with a concentration 70%) solvents were reported in four, three, and four studies, respectively. However, the concentration of used solvents was not reported in four studies.

Table 3 Characteristics of solvents used to extract antioxidant content from collected ripe date fruit samples in selected studies that were included in the qualitative analysis in this scoping review (n = 32)
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Information About Reported Antioxidant Components, Antioxidant Activity Assays, and Antioxidant Compounds

Information about reported antioxidant components, antioxidant activity assays, and antioxidant compounds is presented in Table 4. The number of antioxidant components that were reported in each selected study ranged from one to four components. Although there was one antioxidant component reported in fifteen studies, there were two, three, and four antioxidant components reported in eight, six, and three studies, respectively. In fact, six types of antioxidant components were reported in all selected studies. Total phenol content was reported in all studies (n = 32). However, total flavonoid content, total condensed tannins/total tannin content, total flavonol content, and oligomeric proanthocyanidin content were reported in seventeen, six, three, and two studies, respectively. Finally, total carotenoid content was reported in one article.

Table 4 Information reported about numbers and types of antioxidant components, antioxidant activity assays, and antioxidant compounds (phenolic compounds and flavonoid compounds) determined for collected ripe date fruit samples in selected studies included in the qualitative analysis in this scoping review (n = 32)
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The number of antioxidant activity assays that were reported in each selected study ranged from one to six assays. While one antioxidant activity assay was reported in nine studies, six antioxidant activity assays were reported in one study. Actually, twelve types of antioxidant activity assays were reported in all selected studies. DPPH radical scavenging capacity assay (DPPH assay), ferric reducing antioxidant power assay (FRAP assay), ABTS radical scavenging capacity assay (ABTS assay), ferrous ion chelating assay (metal chelating assay), and total antioxidant capacity assay (phosphomolybdenum assay) were reported in twenty-seven, eighteen, eleven, six, and five studies, respectively. Three studies were reported each of the following assays: hydrogen peroxide scavenging assay (hydrogen peroxide assay), superoxide anion scavenging assay (superoxide anion assay), and cupric ion reducing antioxidant capacity assay (CUPRAC assay). Similarly, two studies were reported each of the following assays: β-carotene-linoleic acid bleaching assay (β-carotene bleaching assay), hydroxyl radical scavenging assay (hydroxyl radical assay), and lipoperoxyl radical scavenging assay (lipoperoxyl radical assay). Finally, one study reported ascorbic acid equivalent antioxidant capacity assay (AEAC assay). The range information for total phenols and total flavonoids and antioxidant activity (DPPH and FRAP assays) that were reported in selected studies, as well as the name of ripe date fruit cultivars with minimum and maximum values, is presented in Table 5.

Table 5 Total phenols, total flavonoids, and antioxidant activity (DPPH and FRAP assays) of collected ripe date fruits reported in studies that were included in this review (n = 32)
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A total of twenty different phenolic compounds were reported in seven studies. The number of phenolic compounds that were reported in each included study ranged from three to sixteen compounds. Gallic acid and p-coumaric acid were reported in all seven studies, while caffeic acid was reported in six of them. Likewise, a total of fifteen different flavonoid compounds were reported in four studies. The number of flavonoid compounds reported in each included study was either five or thirteen compounds. Rutin, quercetin, and luteolin were reported in all four studies, while isoquercetrin and apigenin were reported in three of them.

Information About Reported Analytical Methods Used to Determine Antioxidant Components

Information about reported analytical methods that were used to determine antioxidant components is shown in Table 6. Generally, antioxidant component determination in all selected studies was carried out by using colorimetric methods. Contrarily, high-performance liquid chromatography (HPLC) was used in a limited number of studies to determine antioxidant components. Colorimetric method using folin–ciocalteu reagent was used to determine total phenol content in twenty-nine studies, while it was determined by using HPLC in the remaining three studies. In the same way, colorimetric method using aluminum chloride reagent was used to determine total flavonoid content in fifteen studies, while it was determined by using HPLC in two studies. Total condensed tannins/total tannin content was determined by colorimetric methods, but different reagents were used, which include vanillin reagent (n = 4), folin–ciocalteu reagent (n = 1), and ferric reagent (n = 1). Finally, colorimetric methods were used to determine total flavonols, oligomeric proanthocyanidin, and total carotenoids by using aluminum chloride reagent (n = 3), ammonium ferric sulfate dodecahydrate reagent (n = 2), and butylated hydroxytoluene reagent (n = 1), respectively.

Table 6 Information reported about analytical methods used to determine antioxidant components for collected ripe date fruit samples in selected studies that were included in the qualitative analysis in this scoping review (n = 32)
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References Used to Cite Procedures Related to Solvent Extraction, Antioxidant Component Determination, and Antioxidant Activity Assays

The references that were used to cite procedures that were related to solvent extraction, antioxidant component determination, and antioxidant activity assays are presented in Table 7. Many selected studies did not use any references to cite procedures that were related to solvent extraction, antioxidant component determination, and antioxidant activity assays that were applied on collected ripe date fruit samples. However, a wide range of references were used to cite these procedures in the remaining studies. Although there were no references used to cite solvent extraction procedures in about half of the included studies (n = 17), two primary references (Biglari et al. 2008; and Al-Farsi et al. 2005) were cited in nine studies. Two references (Singleton and Rossi 1965; and Al-Farsi et al. 2005) were cited mainly in fifteen studies for total phenol determination procedures. Likewise, two references (Kim et al. 2003; and Zhishen et al. 1999) were cited mainly in ten studies for total flavonoid determination procedures. One reference (Julkunen-Tiitto 1985) was cited mainly in two studies for total condensed tannins/total tannin determination procedures. Moreover, a single reference was used to cite the procedures used to determine total flavonols (Jimoh et al. 2010), oligomeric proanthocyanidin (Quettier-Deleu et al. 2000), and total carotenoids (Al-Farsi et al. 2005).

Table 7 Citation references for procedures related to solvent extraction, antioxidant component determination, and antioxidant activity assays applied on collected ripe date fruit samples in selected studies that were included in the qualitative analysis in this scoping review (n = 32)
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While some antioxidant activity assays were cited mainly by a specific reference, other antioxidant activity assays were cited by several references in different studies. A specific reference was used mostly to cite procedures for DPPH assay (Brand-Williams et al. 1995), FRAP assay (Benzie and Strain 1996), ABTS assay (Re et al. 1999), and phosphomolybdenum assay (Prieto et al. 1999). On the other hand, different references were used to cite the procedures for the remaining antioxidant activity assays (metal chelating assay, hydrogen peroxide assay, superoxide anion assay, CUPRAC assay, β-carotene bleaching assay, hydroxyl radical assay, lipoperoxyl radical assay, and AEAC assay) in different studies.

Measurement Units Used to Quantify Values of Antioxidant Components, Antioxidant Activity, and Antioxidant Compounds

The measurement units that were used to quantify values of antioxidant components, antioxidant activity, and antioxidant compounds are provided in Table 8. Various selected studies used several types of measurement units to quantify the values of antioxidant components, antioxidant activity assays, and antioxidant compounds (phenolic compounds and flavonoid compounds) that were determined for collected ripe date fruit samples. Total phenol content values were reported by using nine different measurement units in all studies (n = 32). However, most of these studies (n = 22) used one of the two measurement units: mg gallic acid equivalents (GAE)/100 g fresh weight (FW) or mg GAE/100 g dry weight (DW). Values of total flavonoid content, total condensed tannins/total tannin content, and total flavonol content were reported by using eight, four, and two different measurement units in different studies that reported these antioxidant components, respectively. Actually, four studies used mg catechin equivalents (CE)/100 g DW, and three studies used mg quercetin equivalents (QE)/100 g DW as measurement units to quantify total flavonoid content values. Four studies used either mg CE/100 g FW or mg CE/100 g DW as measurement units to quantify total condensed tannins/total tannin content values. Furthermore, total flavonol content values were reported by using one of the two measurement units: mg rutin equivalents (RE)/100 g FW or mg RE/100 g DW. Values of oligomeric proanthocyanidin content and total carotenoid content were quantified by using mg of cyanidin equivalents (CYE)/100 mg dry extract and mg/100 g FW as measurement units, respectively.

Table 8 Measurement units used to quantify values of antioxidant components, antioxidant activity assays, and antioxidant compounds (phenolic compounds and flavonoid compounds) determined for collected ripe date fruit samples in selected studies that were included in the qualitative analysis in this scoping review (n = 32)
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Higher variability was observed in measurement units used to quantify the values of antioxidant activity assays. Values of DPPH assay, FRAP assay values, ABTS assay, and metal chelating assay were reported by using eleven, thirteen, five, and four different measurement units in different studies that reported these assays, respectively. DPPH assay values were essentially reported in different studies by using three measurement units: scavenging capacity (inhibition %), inhibition concentration (IC50) (μg/mL), or antiradical efficiency (AE). FRAP assay values were mainly reported in different studies by using two measurement units: IC50 (mg/mL) or μmol/100 g DW. ABTS assay values were mostly reported in different studies by using two measurement units: μmol trolox equivalents (TE)/100 g FW or μmol TE/100 g DW. In the same way, metal chelating assay values were mainly reported in different studies by using two measurement units: chelating activity (inhibition %) or IC50 (μg/mL). However, phosphomolybdenum assay values were reported in five different studies by using five measurement units. Values of the remaining antioxidant activity assays (hydrogen peroxide assay, superoxide anion assay, CUPRAC assay, β-carotene bleaching assay, hydroxyl radical assay, lipoperoxyl radical assay, and AEAC assay) were quantified by using three different measurement units or less. Finally, two different measurement units (mg/100 g FW or mg/100 g DW) were used to report the values of antioxidant compounds (phenolic compounds and flavonoid compounds).

Data Presentation Methods Used to Report Values of Moisture/Dry Matter Content, Antioxidant Components, Antioxidant Activity Assays, and Antioxidant Compounds

Data presentation methods that were used to report values of moisture/dry matter content, antioxidant components, antioxidant activity assays, and antioxidant compounds are presented in Table 9. Two data presentation methods (tables and bar graphs) were used to report the values of moisture/dry matter content, antioxidant components, and antioxidant activity assays in different studies. Contrarily, values of antioxidant compounds (phenolic compounds and flavonoid compounds) were reported by using tables. Values of moisture/dry matter content, antioxidant components, and antioxidant activity assays were reported by using tables in seven, twenty-six, and twenty-four studies, respectively, and bar graphs in three, six, and eight studies, respectively.

Table 9 Data presentation methods used to report values of moisture/dry matter content, antioxidant components, antioxidant activity assays, and antioxidant compounds (phenolic compounds and flavonoid compounds) determined for collected ripe date fruit samples in selected studies that were included in the qualitative analysis in this scoping review (n = 32)
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Discussion

This scoping review is the first attempt that was carried out to map the available evidence from studies conducted to determine antioxidant content in ripe date fruits. We called to this work by huge inconsistency, and high variability in research design in studies that investigated this research area. This work will enable researchers in the field to identify the gaps and drawbacks in the current literature and the relevant practices and appropriate research designs in future studies. Moreover, it could help researchers to conduct potential systematic reviews and meta-analyses in this research area.

Determination of antioxidant content in natural foods such as ripe date fruits is a growing area of research which is important to increase the knowledge and understanding of the health benefits of these foods. The findings of this scoping review have several implications for future research practices. Firstly, it seems that there is an emerging trend in research publications that investigated antioxidant content determination in ripe date fruits during the past two decades. Based on our findings, available studies in this research area are still growing up in numbers and were conducted in a few countries (most of them from the Middle East and North Africa). Many of the selected studies were carried out mainly to determine antioxidant content in ripe date fruits. However, many studies were conducted to understand the effectiveness of certain variables such as ripening stages and extraction solvents. Therefore, more academic efforts, especially from researchers in the field of food science, are required to develop the research activities in this area of research.

Secondly, collected ripe date fruit samples in different studies are likely to be highly variable in their characteristics. It was noted that not only the number of collected ripe date fruit cultivars in each selected study was variable, but also the type of collected cultivars was highly variable in different studies. Although the information about quality, harvest season, local location, source, storage periods and temperature before analysis, and moisture/dry matter content of collected ripe date fruit samples was missing in many studies, there were variations in reported information about collected ripe date fruit samples among different studies. For example, information about storage periods before analysis was missing in twenty-nine selected studies (90.6%). In addition, there was an obvious variation in this period between the three studies that reported this information. Al-Najada and Mohamed (2014) observed that total phenolic content and flavonoid content of fresh date fruits of two Saudi cultivars (Khalas and Shishi) were increased, while antioxidant activity, measured by DPPH and FRAP assays, was decreased during storage for 6 and 12 months at 4 °C. Unfortunately, the absence of important information about characteristics of collected ripe date fruit samples in many studies may have a negative effect on the impact of their results. In addition, high variability in these characteristics makes it hard to compare results of different studies. Therefore, more focus on reporting all characteristics of collected ripe date fruit samples in future studies is needed. Furthermore, attempts to limit differences in these characteristics and producing standard characteristics to follow are highly recommended to enhance the research quality and reproducibility in this area of research.

Thirdly, the current review found that the solvents used to extract antioxidant content from collected ripe date fruit samples varied in their types and their concentrations among different studies. Despite that the methanolic solvents, especially solvent with 80% concentration, were commonly used in selected studies, other solvents and other concentrations were still used to extract antioxidant content from collected ripe date fruit samples. According to Kchaou et al. (2013), the total phenolic content and antioxidant activity varied greatly when five different solvents were used to extract antioxidant content from six ripe Tunisian date fruit cultivars which indicate a possible effect of extracting solvent on results. In addition, antioxidant content in three ripe Saudi date fruit cultivars (Sefri, Sari, and Ruzeiz) was extracted by using five different solvents. They noted that wide ranging of total phenolic content and antioxidant activity were reported when different solvents are used (Ali et al. 2016). Similarly, antioxidant content in two ripe Iranian date fruit cultivars (Berhi and Kabkab) was extracted by using three different solvents. Results indicate that the contents of total phenols, total flavonoids, and oligomeric proanthocyanidins in addition to antioxidant activity were highly varied based on the used solvent (Siahpoosh et al. 2016a; Siahpoosh et al. 2016b). The same finding was reported by Hachani et al. (2018) in five ripe Algerian date fruit cultivars concerning total phenolic content, total flavonoid content, condensed tannin content, antioxidant activity, phenolic compounds, and flavonoid compounds (Hachani et al. 2018).

Kchaou et al. (2013) reported that acetone 70% was found to be the most effective solvent compared with other used solvents to extract total phenols (199.43–576.48 mg GAE/100 g FW). Contrarily, ethanol 100% was found to be the least efficient solvent for extracting total phenols (54.73–93.82 mg GAE/100 g FW). The antioxidant activity was measured using DPPH, FRAP, and phosphomolybdenum assays. Both acetone 70% and methanol 50% extracts had strong DPPH free radical scavenging activities. Results of FRAP assay showed that the best reducing power was obtained by acetone 70% for the six studied cultivars compared with methanol 50%. When antioxidant activities of different five solvent extracts were measured using phosphomolybdenum assay, methanol 50% was found with the highest total antioxidant for all studied cultivar date, followed by methanol 88% and acetone 70%. On the other hand, absolute ethanol and water gave the lowest antioxidant activity among the used solvent. Their results suggest that acetone 70% and methanol 50% could be the solvents of choice in all subsequent experiments (Kchaou et al. 2013). Unfortunately, antioxidant content results drawn from different studies cannot be compared when different extracting solvents are used. Therefore, more research to investigate the possible effect for extracting solvents on antioxidants is needed. Furthermore, producing a standard solvent extraction procedure to follow is highly recommended to improve the research quality and reproducibility in this area of research.

Fourthly, very high inconsistency and variability were noted in reported information about determining antioxidant content and the used analytical methods, citation references for procedures, measurement units, and data presentation methods. The first place to locate a high inconsistency among selected studies is information reported about numbers and types of antioxidant components, antioxidant activity assays, and antioxidant compounds determined for collected ripe date fruit samples. Although total phenol content was reported in all selected studies, there was high variability in numbers and types of antioxidant components that were reported in each included study. The variability looks like more noticeable in the information reported about numbers and types of antioxidant activity assays that were reported in each included study. While DPPH assay was reported in the vast majority of selected studies, the appearance of other assays is still highly variable among different studies. Even though information about phenolic and flavonoid compounds was reported in a limited number of studies, inconsistency is still observed in numbers and types of compounds determined in different studies. The second place to find a high inconsistency among selected studies is information reported about analytical methods used to determine antioxidant components for collected ripe date fruit samples. Colorimetric methods were the common analytical methods used to determine antioxidant components. However, some studies determined total phenol content and total flavonoid content by using HPLC. Moreover, several types of reagents were used through colorimetric methods to determine total condensed tannins/total tannins. The third place to detect a high inconsistency among selected studies is citation references for procedures related to solvent extraction, antioxidant component determination, and antioxidant activity assays applied on collected ripe date fruit samples. Actually, some references were frequently used to cite certain analytical procedure in selected studies. For example, one reference (Singleton and Rossi 1965) was used commonly to cite total phenol determination procedure, and another reference (Brand-Williams et al. 1995) was used commonly to cite DPPH assay procedure. However, high variability in used references still can be seen in selected studies. It is important to cite the source(s) followed to run any analytical procedure in order to allow other researchers to track down and reproduce them, especially when some details of these analytical procedures are omitted in published articles. In addition, choosing the best source(s) to cite is crucial to avoid high dispersion and disparity in the used analytical procedure. The best source(s) to cite could be the article(s) that contribute significantly to develop or improve the analytical procedure (Bryson 2012; Santini 2018). Therefore, following good citation practices for any used analytical procedures when writing future articles is highly recommended.

Another major source of inconsistency and variability among selected studies is measurement units used to quantify the values of antioxidant components, antioxidant activity assays, and antioxidant compounds determined for collected ripe date fruit samples. Measurement refers to a technique used to determine a specific property of an object by comparing it to a standard. Accordingly, the measurement unit refers to a fixed magnitude of a quantity used as a standard for quantitative measurement of the same kind. Both of them are important because, without correct measurement and suitable units to express them, qualitative assessment for any property cannot be done (Humphry 2013; Mari et al. 2018). Furthermore, when more than one measurement unit are used to measure a specific property, it is important to be able to convert between them easily and quickly in order to make comparing results from different sources possible and effective. In the selected studies, wide range of measurement units were used to express amounts of antioxidant content in collected ripe date fruit samples. While converting quantities from certain measurement unit to another can be handled easily and quickly, the conversion between many reported measurement units could be complex and difficult. For example, total phenol content was reported by using nine measurement units in different selected studies; two of them are mg GAE/100 g DW and μg GAE/mg DW. Conversion between them can be simply done. Another measurement unit used to report amounts of total phenol content was mg GAE/100 g FW. Conversion of this unit to mg GAE/100 g DW can be completed only if the moisture or dry matter content of the collected date fruits is known. In another example, total flavonoid content was reported by using eight measurement units in different selected studies; two of them are mg QE/100 g DW and mg CE/100 g DW. Indeed, conversion between them is more complex and problematic. Therefore, reporting moisture or dry matter content in future articles and adopting standard measurement units to follow are highly recommended to advance the research quality and reproducibility in this area of research.

The last source of inconsistency among selected studies is data presentation methods used to report the values of moisture/dry matter content, antioxidant components, antioxidant activity assays, and antioxidant compounds determined for collected ripe date fruit samples. Despite that most of the values of moisture/dry matter content, antioxidant components, and antioxidant activity assays were reported by using tables, many of these values in many studies were reported only by using bar graphs. Bar graph or bar chart is a graphical display of data using bars of different heights to represent actual values. Bar graphs are considered an effective method for data presentation. However, a major issue with bar graphs is how to extract the actual numerical values, especially when these values were not reported numerically in the article text. Usually, researchers extract the numerical values from bar graphs either manually by using a ruler or automatically by using specialized programs or online tools such as Web Plot Digitizer (Rohatgi 2019). Data errors are still expected when numerical values are extracted from bar graphs. In addition, numerical value extraction, in some cases, becomes almost unavailable due to poor bar graph presentation. An example is extracting values of total phenol content from bar graph in one of the selected studies (Farag et al. 2016). Therefore, presenting result values in future articles by using tables to make them easily available for other researchers is highly recommended. When bar graphs are used, it is important to use clear and easy to understand bar graphs and to ensure that the values that were reported in bar graphs are numerically available in the bar graphs themselves or in text or provided in an attached supplementary document.

A few challenges and limitations were met during the synthesis of the current scoping review. Firstly, only three reputable electronic databases (Scopus, PubMed, and Web of Science) were searched to ensure selecting articles of good quality. Limiting the search of sources to these databases could affect our results. Secondly, many studies were excluded during the study selection stage based on inclusion criteria for various reasons such as missing information about the maturation stage or cultivar type of collected samples in these articles. The high rate of study exclusion could affect our findings. Thirdly, some authors published several selected articles. These articles could be based on the same study, which may also affect our results. However, the finding informed in this scoping review is still valuable and will contribute significantly to promote higher research design consistency and thus, improve research quality in the area of antioxidant content determination in future articles.

Conclusions

In the light of emerging research trend in the area of antioxidant content determination, high inconsistency and variability were observed in various characteristics of the selected studies, collected ripe date fruit samples, and extraction solvents, as well as information about determining antioxidant content (quantity and quality) and the used analytical methods, citation references for procedures, measurement units, and data presentation methods. Furthermore, the research gaps and future recommendations to promote higher research design consistency and improve research quality in this area of research have been described and discussed carefully.

While it was common to determine antioxidant components such as total phenols and total flavonoids in ripe date fruits by using colorimetric methods, which is a simple, rapid, and low-cost method, there is a growing tendency to use chromatographic methods such as HPLC which is more sophisticated, reproducible, and costly methods to determine both antioxidant components and antioxidant compounds (phenolic compounds and flavonoid compounds). Ripe date fruits are one promising food source of valuable components such as antioxidants and dietary fiber. Many attempts try to use these components after utilization with necessary pre-treatments as nutraceuticals or supplements for various applications in the food industry. The future trends go to investigate the proper procedures to extract and utilize these active components in date fruits.