Optimization of Infrared Postharvest Treatment of Barhi Dates Using Response Surface Methodology (RSM)

Abstract

Barhi dates are widely consumed at Khalal maturity stage and preserving the freshness quality of Barhi at this stage is a challenging task as this period is short and the fruits are more perishable. In this study, response surface methodology (RSM) was applied to optimize the infrared (IR) treatment and storage conditions for preserving the physicochemical, microbial, and bioactive attributes of fresh Barhi dates. The effect of four factors, IR temperature (50, 70, 90, and 110 °C), IR time (1, 2, 3, and 4 min), storage temperature (1, 5, 15, and 25 °C), and storage time (1, 6, 11, 16, and 21 days), on the responses of total soluble solids (TSS), hardness, total color change (ΔE), total viable count (TVC), total phenolic content (TPC), antioxidant activity (DPPH), and glucose content were evaluated following central composite design (CCD). IR temperature, IR time, storage temperature, and storage time significantly affected the physical, microbial, and bioactive attributes of Barhi dates. The optimal conditions for minimizing the physical changes and microbial load and maximizing the bioactive attributes were IR temperature of 50 °C, IR time of 1.2 min, storage temperature of 1 °C, and storage time of 20 days. At the optimum conditions, the values of TSS, hardness, ΔE, TVC, TPC, DPPH, and glucose were 37.22%, 70.17 N, 11.12, 2.9 log CFU/g, 36.1 mg GAE/g, 65.31%, and 25.38 mg/g, respectively and these values were similar to predicted values. In conclusion, this study identified the ideal IR treatment and storage conditions for maintaining the overall quality attributes of Barhi dates during prolonged storage.

1. Introduction

Barhi date (Phoenix dactylifera L. cv. Barhi) fruits are among the most popular date fruits that are widely consumed at the first edible maturity stage that is known as the Khalal or Bisr stage [1]. This maturity stage is among the main five date fruit developmental and ripening stages; Hababouk (cell division; hard, small green immature), Kimri (cell elongation, unripe green), Khalal or Bisr (unripe full-colored), Rutab (mature soft brwon), and Tamar (firm raisin-like), respectively (Figure 1) [1,2]. The majority of Barhi dates are usually and preferably marketed and consumed at the Khalal stage [3,4]. At this stage, Barhi dates are crispy, have reached their maximum size, and have bright yellow color, and sweet taste due to high content of sucrose [3]. After harvesting Barhi dates, if the handling and storage conditions are not controlled, the dates rapidly convert from Khalal stage to Rutab stage, at which more inverted sugars are formed from sucrose, and subsequently the texture of the fruit becomes softer and the skin color changes to brown [3]. This rapid conversion of Barhi dates from Khalal stage to Rutab stage greatly affect the marketing price of Barhi, as consumers prefer to consume this type of date at the Khalal stage [5,6]. Therefore, prolonging the shelf-life of Barhi dates at the Khalal stage is of high importance from date producers’, traders’, and consumers’ standpoints.

To date, several preharvest and postharvest treatment methods have been applied for maintaining the freshness quality of Barhi dates during handling and storage processes. In preharvest treatment, melatonin and methyl jasmonate [4], a combination of chitosan with calcium chloride and salicylic acid [1], active edible coating with chitosan nanoparticles [7], were found as promising candidates for prolonging the shelf-life and improving the storability of Barhi dates. In postharvest treatments, several methods were applied and found promising in the maintaining the freshness quality of Barhi dates during storage. Controlled atmospheric conditions [8], modified atmospheric packaging [9,10], cryogenic freezing [5,11], and calcium chloride and salicylic acid solutions [2,6] were also found as promising postharvest processing methods for preserving the quality of Barhi dates during storage. In recent years, research on maintaining the freshness quality attributes of Barhi dates during storage has been progressing and there is still need for application of innovative technologies.

Infrared (IR) heating is a powerful technique that is used in numerous food applications due to its versatility, simplicity, environment friendly, power and time saving, heating homogeneity, microorganisms decontamination efficacy, and product quality saving properties [12,13]. Despite high potentials of IR, its application in the preservation of date fruits is scarce, and consequently, optimization of the IR treatment conditions for maintaining the quality of Barhi dates is of high significance. Classical single-factor (CSF) experiment and/or response surface methodology (RSM) are the main methods that are commonly used for the process optimization with the CSF requiring more time and providing insufficient information, while RSM is rapid and provides sufficient information for multiple variables [14,15]. Therefore, in this study, RSM was used to optimize the IR treatment and storage conditions for preserving the physicochemical, microbial, and bioactive attributes of Barhi dates.

2. Materials and Methods

2.1. Materials

Fresh Barhi date fruits at Khalal maturity stage were obtained from a date-producing farm in Riyadh, Saudi Arabia during the harvesting season in 2020. The fruits were transferred to the laboratory of Food Processing at the College of Food and Agricultural Sciences, King Saud University, Riyadh under controlled condition on the same day of harvesting. Soon after arrival to the laboratory, the fruits with consistent shape, size, bright yellow color, and absence of defects and injuries were manually selected and cleaned with moderately compressed air. The initial moisture and total soluble solids (TSS) of the fruits were 74.33 ± 0.74% (fresh wet base) and 14.7 ± 0.6%, respectively. Unless otherwise specified, all chemicals were of analytical grade and were obtained from Sigma Aldrich (Sigma, St. Louis, MO, USA).

2.2. Infrared Treatment (IR)

A metal box with dimensions of (65 × 56 × 50 cm) was constructed, and eight infrared (halogen type) lamps (500 W, 220 V) and a halogen holder were installed on the top of the box. Height-adjusting screws adjusted the height of the lamps, and the required radiation intensity was obtained by adjusting the lamp height and voltage by a voltage regulator to obtain the temperature levels of 50, 70, 90, and 110 °C on the surface of Barhi dates. Fresh Barhi date samples (3 kg per treatment, total 48 kg) were exposed to IR temperatures (50, 70, 90, and 110 °C) for varying periods of time (1, 2, 3, and 4 min). After that, the treated dates were divided into 0.3 kg portions (25 ± 5 date fruits per portion) and then placed in packages and stored at various temperatures (1, 5, 15, and 25 °C) for different storage periods (1, 6, 11, 16, and 21 days). Measurements of the quality parameters of the samples were taken during 5 days intervals of storage.

2.3. Experimental Design

A response surface methodology model (RSM) with a five-factor, mixed-level experimental design based on central composite rotatable design (CCRD) was used for optimizing the IR treatment and storage conditions for keeping the physicochemical, bioactive, and microbiological quality of fresh Barhi dates. Design Expert software version 6.0.8 (Stat-Ease Inc., Minneapolis, MN, USA) was used for constructing the RSM and analyzing the data. In the CCD, 30 experimental runs with six replicates at the center point were conducted. IR temperature (X₁) (50, 70, 90, and 110 °C), IR treatment time (X₂) (1, 2, 3, and 4 min), storage temperature (X₃) (1, 5, 15, and 25 °C), and storage time (X₄) (1, 6, 11, 16, and 21 days) were designated as the independent factors (

Table 1. Independent variables and their level used for central composite design.

Independent Variables	Level
IR temperature, °C (X1)	50 (−1)	70 (−0.333)	90 (0.333)	110 (1)
IR time, min (X2)	1 (−1)	2 (−0.333)	3 (0.333)	4 (1)
Storage temperature, °C (X3)	1 (−1)	5 (−0.667)	15 (0.167)	25 (1)
Storage time, days (X4)	1 (−1)	6 (−0.5)	11 (0)	16 (0.5)	21 (1)

). Total soluble solids (TSS), hardness, ΔE, bioactive properties (TPC and DPPH radical scavenging activity), total viable count (TVC), and glucose were designated as responses. The complete design matrix of IR treatment and storage conditions variables are shown in Table 1. Second-order polynomial equation was used to express the dependent variables as a function of independent variables, as follows:

$$\mathrm{Y} = {\mathsf{\beta }}_0 + \Sigma {\mathsf{\beta }}_{\mathrm{i}} {\mathrm{X}}_{\mathrm{i}} + \Sigma {\mathsf{\beta }}_{\mathrm{ii}} {\mathrm{X}}_{\mathrm{i}}^2 + \Sigma \Sigma {\mathsf{\beta }}_{\mathrm{ij}} {\mathrm{X}}_{\mathrm{i}} {\mathrm{X}}_{\mathrm{j}}$$

(1)

where Y is the predicted responses, β₀ is intercept, and β_i, β_ii, and β_ij are the regression coefficients of linear, quadratic, and interaction effect terms, respectively. The Xi and Xj are the independent coded variables. Coefficients were interpreted using the F test. Analysis of variance (ANOVA), regression analysis, and surface plotting were performed to establish optimum condition for IR treatment and storage conditions on the quality of Barhi date fruits.

2.4. Determination of Total Soluble Solids (TSS)

The TSS of Barhi date fruits was determined using an ABBA5 refractometer (BS instruments, Jena, Germany). Briefly, 100 g destoned date fruits (n = 3) were pressed and the produced juice was used for measuring TSS.

2.5. Color

The color attributes of Barhi dates (25 ± 5 fruits, 5 readings of each fruit) were assessed using a Hunter Lab-scan XE spectrophotometer (Hunter Lab, Reston, VA, USA). The values of the basic color (L*, a*, and b*) were used to calculate the total color difference (∆E) of the samples, as stated by Maskan [16], using the following equation:

$$\Delta \mathrm{E}=\sqrt{{\left(\Delta {\mathrm{L}}^{\ast }\right)}^2+{\left(\Delta {\mathrm{a}}^{\ast }\right)}^2+{\left(\Delta {\mathrm{b}}^{\ast }\right)}^2}$$

(2)

2.6. Hardness

The hardness of Barhi dates (25 ± 5 fruits) was analyzed using a TA-HDi, Model HD3128 texture analyzer (Stable Micro Systems, Surrey, UK) as described by Alhamdan et al. [11]. Briefly, Barhi dates were compressed using a probe at the velocity of 1.5 mm/s to a depth of 5 mm. The hardness (the maximum force required to compress the fruits) values were obtained from the force–time deformation curves.

2.7. Bioactive Properties (TPC and DPPH)

Prior to the analysis of TPC and DPPH, aqueous extracts of destoned Barhi dates were prepared by suspending 1 g of destoned Barhi dates (25 ± 5 fruits) in 100 mL distilled water. Then, the mixture was subjected to 30 min sonication at a temperature of 40 °C, a constant power of 110 W, and a frequency of 40 kHz (Branson 2800 CPX ultrasonic cleaner, St. Louis, MO, USA). The extract was filtered through a Whatman 1 filter paper and stored at −20 °C for further use in the analysis of TPC and DPPH. The TPC of Barhi date extract (n = 3) was assessed using a Folin–Ciocalteu reagent method [17]. Briefly, 100 μL Barhi date extract was mixed with 200 μL of diluted Folin–Ciocalteu reagent (×10), and after 5 min standing at room temperature, 500 μL of 1 M sodium carbonate was added, mixed well, and the mixture was kept for 2 h at room temperature. After that, the absorbance was measured at 765 nm and results were expressed as mg gallic acid equivalent per g sample. DPPH (1,1-diphenyl-2-picrylhydrazyl) radical scavenging activity was measured following the method described by Li et al. [18]. Briefly, 500 μL Barhi date extract was mixed with 1 mL of DPPH methanolic solution (250 μM) and the mixture was allowed to stand for 10 min in the dark at room temperature. The absorbance of the samples (n = 3) and control (prepared in the same way without the extract) was measured at 517 nm, and the percentage of DPPH radical scavenging activity was calculated using the following equation:

$$\mathrm{DPPH} \left(\% \mathrm{inhibition}\right)=\frac{\mathrm{Absorbance} \mathrm{of} \mathrm{control}-\mathrm{Absorance} \mathrm{of} \mathrm{sample}}{\mathrm{Absorbance} \mathrm{of} \mathrm{control}}\times 100$$

(3)

2.8. Glucose

Glucose content of Barhi dates was analyzed using an LC-10 AD high-performance liquid chromatography system (Shimadzu Corporation, Kyoto, Japan) as described by Bouhlali et al. [19] with some modifications. Briefly, 5 g of destoned Barhi dates (25 ± 5 fruits) was homogenized with 100 mL distilled water and the homogenate was incubated in the water bath at 50 °C for 30 min. The homogenate was filtered using Whatman No. 1 filter paper, and prior to injection into HPLC, the filtrate was subjected to further filtration using 0.45 μm membrane filter (Millipore, Burlington, MA, USA). Then, 20 μL filtrate was injected into a Supelcosil LC-NH2 column (25 cm × 4.6 mm × 5 μm) and eluted using a mixture of 75% acetonitrile: 25% water (v/v) at a flow rate of 1 mL/min. Authentic standard of glucose was treated the same way as the samples. The peak retention time of the samples (n = 3) was compared with that of glucose, and quantification was performed by calibration curves generated using known concentrations of glucose.

2.9. Microbial Analysis

The analyses of total viable count (TVC) of Barhi date samples were conducted following the standard procedure [20]. Prior to plating, 25 g of destoned Barhi dates (25 ± 5 fruits) were added to 225 mL sterile saline solution (0.85% NaCl) and subjected to 2 min homogenization (Stomacher^® 400 circulator Seward GmbH, West Sussex, UK). After that, 1 mL of serial dilution (10⁻¹–10⁻⁴) of the sample homogenate was transferred into sterilized Petri dishes and mixed with 10 mL of sterilized and cooled nutrient agar (Oxoid, Basingstoke, Hampshire, UK, CM0309) media. The plates were incubated at 35 °C for 24–48 h, and the number of the colonies were counted. Triplicate samples were analyzed, and the TVC of duplicate plates at each dilution were counted.

2.10. Statistical Analysis

The data of triplicate samples of each treatment were collected and statistically analyzed. The results of physicochemical, bioactive, microbial quality attributes of Barhi dates were subjected to statistical analysis using SPSS software version 18.0 (SPSS Inc., Chicago, IL, USA). The RSM data were analyzed using Design Expert software (version 11.0, Stat-Ease Inc., Minneapolis, MN, USA). The linear, quadratic, and interaction effects of independent variables (IR temperature, IR treatment time, storage temperature, and storage time on the responses (TSS, hardness, TPC, DPPH, glucose, and ΔE) were analyzed using analysis of variance (ANOVA). The adequacy and precision of RSM models were validated using adequate precision, coefficient of variation (CV), coefficient of determination (R²), and adjusted coefficient of determination (adjusted R²). Significance was accepted at p < 0.05, p < 0.01, and p < 0.001 levels.

3. Results and Discussion

3.1. Model Fitting

Barhi dates are favorably consumed at Khalal mature stage, and at Khalal stage the fruits are very perishable and, thus, postharvest handling and processing conditions are very critical factors influencing the shelf life of Barhi dates [5]. Inappropriate handling and processing conditions might significantly affect the freshness of Barhi dates by enhancing the biological processes such as respiration, ripening, and senescence, and permitting microbial contamination, and thereby reduce the shelf life of Barhi dates. In this study, the effects of infrared treatment and storage conditions on the quality attributes of Barhi dates were studied. The results of ANOVA for model validation and adequacy are presented in

Table 2. Regression coefficients for process variables and product responses.

Factors	TSS	Hardness	ΔE	TVC	TPC	DPPH	Glucose
Intercept
β0	63.440	121.345	18,723.132	10.034	34.148	56.140	16.480
Linear
X1 (β1)	−0.676	1.845 **	17,695.333 ***	0.371 **	−0.541 **	2.482 ***	−1.712 ***
X2 (β2)	−2.077 *	−11.794	−9940.188 ***	−0.199 **	−4.363 ***	26.765 ***	−3.133 ***
X3 (β3)	0.343	0.158	−15,610.622 **	−0.103 ***	1.859	−2.644 ***	0.441 **
X4 (β4)	0.315	−3.605 ***	−16,368.966 **	−0.494 **	−0.939 ***	−2.644 ***	1.931 **
Interaction
X1X2 (β12)	−0.0108	0.144	71,706.607 ***	−0.004	0.024 *	−0.012 ***	−0.849 **
X1X3 (β13)	−0.002	0.009	−7040.482	−0.009	−0.006 ***	−0.045 ***	−0.052
X1X4 (β14)	−0.031	0.036	−27,111.007 ***	0.001	−0.006 ***	0.011 ***	−2.343 ***
X2X3 (β23)	−0.047 *	−0.074	−273,690.111 ***	−0.010	−0.165 ***	0.191 ***	2.511 ***
X2X4 (β24)	0.015	−0.352	−7111.538 ***	0.014 *	−0.052 **	−0.280 ***	1.868 ***
X3X4 (β34)	0.013 **	−0.040	6640.601 ***	0.005 **	−0.002 *	−0.055 ***	1.262 ***
Quadratic
X12 (β11)	0.048 ***	−0.0189	46,099.111 ***	−0.021 *	0.005 **	−0.015 ***	6.081 ***
X22 (β22)	0.592	1.624	−15,913.125	0.188	1.387 **	−4.875 ***	1.001
X32 (β33)	−0.018 *	0.032	−35,305.703 ***	0.009	−0.038 ***	0.116 ***	−8.370 ***
X42 (β44)	−0.013	−0.161	−3967.777 ***	0.013 *	0.079 ***	0.131 ***	11.839 ***
Model F-value	3.040	15.630	5.630	37.280	195.87	116.20	320.11
p-value	0.025	0.0012	0.0001	0.0001	<0.0001	<0.0001	<0.0001
Mean	37.05	114.64	34.64	4.150	28.51	60.34	27.45
C.V. %	3.850	2.890	2.660	4.260	1.520	1.752	1.470
Adeq. precision	6.842	11.485	11.485	19.026	9.936	9.336	70.987
R2	0.924	0.963	0.914	0.967	0.999	0.919	0.996
Adjusted R2	0.850	0.921	0.891	0.960	0.994	0.891	0.913
Std. Dev.	1.430	14.35	14.35	0.177	0.433	0.133	0.404
F-value (Lack of Fit)	2.250	0.005	0.005	4.080	1.760	0.254	1.240
p-value (Lack of Fit)	0.194	0.948	0.948	0.099	0.242	0.094	0.316

* p < 0.05, ** p < 0.01, *** p < 0.001. TSS; total soluble solids, ΔE; total color difference, TVC; total viable count, TPC; total phenolic content, and DPPH; 1,1-diphenyl-2-picrylhydrazyl radical scavenging activity.

. The coefficients of determination (R²) of total soluble solids (TSS), hardness, color change (ΔE), total viable count (TVC), total phenolic content (TPC), antioxidant activity (DPPH), and glucose content ranged between 0.914 and 0.999, indicating that >91% of the total variance of the traits were accounted for by the quadratic polynomial models generated using Equation (1). The generated model showed various degrees of significance for the assessed parameter, which was significant for TSS (p < 0.05), highly significant for hardness (p = 0.0012), ΔE, and TVC (p = 0.0001), and extremely significant for TPC, DPPH, and glucose (p < 0.0001). Good statistical models are characterized by comparable values of coefficients of determination R², adjusted R², and predicted R² [15]. In this study, the adjusted R² of TSS, hardness, ΔE, TVC, TPC, DPPH radical scavenging activity, and glucose ranged from 0.850 to 0.994, which are comparable to the R² (0.914–0.999), suggesting that the used model was highly significant. In addition, the values of R² and adjusted R² were close to 1, demonstrating high correlation between predicted and actual values of all parameters [15,21]. The p-values of the lack of fit of TSS, hardness, ΔE, TVC, TPC, DPPH radical scavenging activity, and glucose ranged between 0.094 and 0.948, which were not significant, indicating that selected models adequately described the experimental data. Adequacy precision is another indicator of the model fitting as it shows the signal-to-noise ratio, and high adequacy precision (>4.0) is desirable [15,22]. In this study, the adequacy precision values of all parameters were in the range of 6.842 to 70.987, suggesting that the applied models possessed high signals and they are thus adequate to analyze the data. The coefficient of variation (CV) is an indicator of the precision and reproducibility of the models, where high precision and reproducibility of the models is considered at low CV (<5%) [15,22]. In this study, the CV values of TSS, hardness, ΔE, TVC, TPC, DPPH radical scavenging activity, and glucose were between 1.470% and 4.260%, demonstrating high precision and reproducibility of the experimental data and good fitting of the applied models. Overall, the results of this study revealed that the experimental data are accurate, reliable, and reproducible, and the selected models are adequate and suitable for optimization of the IR treatment and storage conditions for conserving the physicochemical, microbial, and bioactive attributes of fresh Barhi dates.

3.2. Effect of IR Treatment and Storage Condition on the TSS, Hardness, and ΔE Values of Barhi Dates

The experimental data of TSS, hardness, and ΔE of Barhi dates were subjected to multiple regression analysis and coefficients of the models were used to assess the levels of significance (Table 2). The chosen models were found be significant for the assessed parameters (TSS, p = 0.025, hardness, p = 0.0012, and ΔE, p = 0.0001). The independent variables (IR temperature, IR time, storage temperature, and storage time) showed varied effects on the physical properties (TSS, hardness, and ΔE values) of Barhi dates. IR treatment time had negative effect on the TSS, and the values of TSS significantly (p < 0.05) decreased with the increase in the IR treatment time (Table 2).

The interaction term of the IR time and storage temperature and quadratic term of storage temperature also showed negative effects on the TSS of Barhi dates suggesting that increase in IR time and storage temperature resulted in reduced values of TSS in Barhi dates, whereas the interaction term of storage temperature and storage time had showed positive effect on the TSS of Barhi dates. In addition, the quadratic term of IR temperature showed a highly (p < 0.001) positive effect on the TSS values of Barhi dates. These results indicated that increasing the IR temperature, storage temperature, and storage time increased the TSS, and increasing IR time reduced the TSS of Barhi dates. The increase in TSS Barhi dates following IR treatment temperature and storage conditions could be attributed to moisture reduction and conversion of non-reducing sugars into reducing sugars due to respiration and enzymatic processes, whereas the reduction of TSS during prolonged IR treatment time could be attributed to the caramelization [23,24]. Similarly, previous studies indicated that IR drying treatment increased the TSS of mango pulp [23], red dragon fruit [24], and duku peel [25]. In addition, increase in the TSS of Barhi dates during storage was reported and attributed to reduction in moisture content and conversion of polysaccharides into simple sugars [2,6]. IR treatment temperature had a positive (p < 0.01) effect on the hardness of Barhi dates, suggesting that increasing IR temperature increased the hardness of Barhi dates, whereas storage time had negative impact on the hardness of Barhi dates, suggesting that elongating the storage would result in reduced hardness of the dates. The increase in hardness of Barhi dates following increasing IR temperature is likely due to moisture reduction and rapid removal of water from the surface of the fruits [26,27], whereas the reduction of hardness of Barhi dates during elongated storage is probably due to the ripening processes due to the enzymatic breakdown of cellular structure making the fruit softer [9]. The changes in color (ΔE) of Barhi dates were greatly influenced by the treatment variables except the interaction of IR temperature and storage temperature and the quadratic term of IR treatment time. The linear effect of IR temperature, the interactive effect of IR temperature and IR time, and storage time and temperature, and the quadratic effect of IR temperature showed highly (p < 0.001) positive effects on the ΔE values of Barhi dates, indicating that increasing these treatment variables could increase the ΔE values of the dates. The linear effect of IR time, storage temperature and storage time, the interactive effect of IR temperature and storage time, IR time and storage temperature, and IR time and storage time, and quadratic effect of storage temperature and storage time were negative, suggesting that increasing these variables reduced the ΔE values in Barhi dates. The increase in ΔE during the increased IR temperature could be attributed to the changes of the individual color attribute (L*, a*, and b*) due to the browning and decomposition of some sensitive pigments such as carotenoids and chlorophyll [28]. In addition, enzymatic oxidation of phenolic compounds during IR treatment and storage could also affect the ΔE values of Barhi dates [11]. The prediction equations for describing the effect of IR treatment and storage time on the TSS, MC, hardness, and ΔE values of Barhi dates using significant terms are as follows:

$${\mathrm{Y}}_{\mathrm{TSS}}=63.440-2.077{\mathrm{X}}_2-0.047{\mathrm{X}}_2{\mathrm{X}}_3+0.013{\mathrm{X}}_3{\mathrm{X}}_4+0.048{\mathrm{X}}_1^2-0.018{\mathrm{X}}_3^2$$

(4)

$${\mathrm{Y}}_{\mathrm{hardness}}=121.345+1.845{\mathrm{X}}_1-3.605{\mathrm{X}}_4$$

(5)

$$\begin{array}{l}{\mathrm{Y}}_{\Delta E}=18723.132+17695.333{\mathrm{X}}_1-9940.188{\mathrm{X}}_2-15610.622{\mathrm{X}}_3-16368.966{\mathrm{X}}_4+71706.607{\mathrm{X}}_1{\mathrm{X}}_2\\ \hspace{1em}\hspace{1em}-27111.007{\mathrm{X}}_1{\mathrm{X}}_4-273690.111{\mathrm{X}}_2{\mathrm{X}}_3-7111.538{\mathrm{X}}_2{\mathrm{X}}_4+6640.601{\mathrm{X}}_3{\mathrm{X}}_4+46099.111{\mathrm{X}}_1^2\\ \hspace{1em}\hspace{1em}-35305.703{\mathrm{X}}_3^2-3967.777{\mathrm{X}}_4^2\end{array}$$

(6)

To understand the interaction of independent variables on physicochemical, microbial, and bioactive properties of Barhi dates, and consequently confirm the optimal level for each variable for better response, three-dimensional (3D) surface blots were constructed [15]. The results indicated that the process variables (IR temperature, IR time, storage temperature, and storage time) affected the TSS, hardness, and ΔE of Barhi dates in different magnitudes (Figure 2, Figures S1 and S2). For TSS, increase in IR temperature resulted in reduction of TSS to lowest values at 80 °C and then increased again to the maximum level at high IR temperature (Figure 2a–c). As the storage temperature and time increased, the TSS level of Barhi dates increased to the maximum level at temperature of 13 °C and time of 11 days and then reduced again as the storage temperature increased and storage time elongated (Figure 2b–f). Increase in IR time showed slight reduction in the TSS of Barhi dates (Figure 2a,d,e). Heat is known to evaporate water from the surface and the cellular architecture of the fruits and decompose the polysaccharides into simple sugars, thereby increasing the total soluble solids of the fruits [23,24]. For hardness, increase in IR temperature increased the hardness of Barhi dates to the maximum at 80 °C and then decreased slightly as the temperature elevated to 110 °C (Figure S1a–c). Increase in storage temperature and IR time showed constant increase in the hardness of Barhi dates, reaching maximum values at 25 °C and 4 min, respectively (Figure S1a,b,d–f). Increase in the storage time possessed constant reduction of hardness of Barhi dates, reaching minimum values at 21 days. Increase in the hardness during increased IR treatment temperature and duration and storage temperature is likely due to the reduction of moisture content and removal of water from the surface of the fruits making the surface layer of the dates more firm and harder [27]. The reduction in hardness during prolonged storage of Barhi dates is likely due to the enzymatic degradation of cellular matrix during ripening processes [9]. For total color change (ΔE), increase in the IR temperature reduced the ΔE of Barhi dates to minimum level at 80 °C and then increased again to maximum values as the temperature elevated to 110 °C (Figure S2a–c). Increase in IR time, storage temperature, and storage time showed constant increase in ΔE of Barhi dates (Figure S2a–f). The increase in ΔE of Barhi dates during IR treatment and storage process conditions is probably due to the enzymatic and non-enzymatic browning in addition to degradation of some natural pigments and oxidation of phenolic compounds in Barhi dates [11,28]. Overall, low IR treatment and storage temperatures could be recommended for keeping the physicochemical quality attributes (TSS, hardness, and ΔE) of Barhi dates, and hence preserve the freshness quality parameters and elongate the shelf life of Barhi dates.

3.3. Effect of IR Treatment and Storage Condition on the Total Viable Count of Barhi Dates

The treatment variables (IR temperature, IR time, storage temperature, and storage time) exhibited varied effects on the total viable count (TVC) of Barhi dates (Table 2). IR temperature had a positive (p < 0.01) effect on the TVC of Barhi dates, suggesting that increase in IR temperature might increase the TVC of Barhi dates. In addition, the interaction effects of IR time with storage time, and storage temperature with storage time on the TVC were also positive, suggesting that increase in these variables could increase the TVC in Barhi dates during storage, whereas IR time, storage time, and storage temperature had negative effect on the TVC, suggesting that increasing these variables will reduce the TVC of Barhi dates. The quadratic effect of storage time on TVC was positive, whereas that of IR temperature was negative. The increase in TVC during IR treatment and storage temperatures increment could be due to the presence of heat-resistant bacteria and bacterial spores on the surface of Barhi dates, for which high temperature is favorable for its growth and development [29,30]. The prediction equation for describing the effect of IR treatment and storage time on the TVC of Barhi dates using significant terms is as follows:

$$\begin{gathered} {\mathrm{Y}}_{\mathrm{TVC}}=10.034+0.371{\mathrm{X}}_1-0.199{\mathrm{X}}_2-0.103{\mathrm{X}}_3-0.494{\mathrm{X}}_4+0.014{\mathrm{X}}_2{\mathrm{X}}_4 \\ +0.005{\mathrm{X}}_3{\mathrm{X}}_4-0.021{\mathrm{X}}_1^2+0.013{\mathrm{X}}_4^2 \end{gathered}$$

(7)

To determine the relationship between the total viable count and experimental data and the interactions between tested variables, 3D surface plots were constructed and are depicted in Figure 2a–f. Increase in IR temperature increased the TVC to maximum counts at 90 °C and then reduced again as the temperature increased to 110 °C (Figure 3a–c), which is likely due to the presence of resistant microbes for which moderate temperature enhances their growth while high temperature eliminates their growth [30]. Increase in the IR time and storage temperature constantly increased the TVC of Barhi dates to maximum values at 4 min and 25 °C, respectively (Figure 3a,b,d–f). This could be due to the formation of more reducing sugars that are considered as carbon sources of the microbes. Increase in the storage time reduced the TVC of Barhi dates to minimum at 11 days and increased again as the time elevated to 21 days (Figure 3c,e,f). The interaction terms of IR time and storage temperature (Figure 3d) and storage time and temperature (Figure 3f) showed high TVC of Barhi dates as these variables increased. Generally, increasing IR temperature to 110 °C or keeping it at the baseline (50 °C) and reducing the storage temperature are favorable conditions for reducing the TVC and hence elongating the shelf life of Barhi dates.

3.4. Effect of IR Treatment and Storage Condition on the Bioactive Properties of Barhi Dates

Date fruits contain several bioactive compounds and possess high antioxidant, antimicrobial, antiviral, anticancer, antidiabetic, antinephrotoxic, and anti-inflammatory activities [31]. In this study, IR treatment and storage conditions affected the total phenolic contents (TPC) and antioxidant activity (DPPH radical scavenging activity) in different manners. The regression analysis showed negative and highly significant (p < 0.01 and p < 0.001) effects of linear term of IR temperature, IR time, and storage time on the TPC, suggesting that increasing these variables will reduce TPC of Barhi dates (Table 2). The interaction terms of IR temperature and storage temperature, IR temperature and storage time, IR time and storage temperature, IR time and storage time, and storage temperature and storage time, and quadratic term of storage temperature and time showed significantly negative effect on the TPC of Barhi dates. The reduction of TPC of Barhi dates during IR treatment and storage conditions could be attributed to decomposition of some thermosensitive phenolic constituents by increased IR temperature, enzymatic oxidation, and degradation of phenolic compounds during prolonged storage at high temperature and reducing extractability of phenolic compound due to complexes formation by treatments [15,32]. The interaction term of IR temperature and IR time and the quadratic term of IR temperature and IR time showed significantly positive effect on the TPC of Barhi dates, indicating increased TPC following increase in these variables. The increase in TPC following IR treatment is likely due to the ability of IR to break down covalent bonds between phenolic compounds and cell walls and to alter the structure of phenolic compounds, thereby increasing the solubility and extractability of phenolic compounds, as observed for IR treatment of red dragon fruit [24].

The regression analysis also showed positive and highly significant (p < 0.001) effect of linear terms of IR temperature and IR time, interaction terms of IR temperature and storage temperature, and IR time and storage time, and the quadratic terms of storage temperature and storage time on the DPPH radical scavenging activity of Barhi dates (Table 2). These findings suggest that increasing IR temperature and duration increased the DPPH of Barhi dates which is likely due to increased liberation of bond bioactive compounds and formation of Millard reaction products that possess antioxidant activity [15,24], whereas there were highly significantly (p < 0.001) and negative effects of the linear terms of storage temperature and time, interaction terms of IR temperature and IR time, IR temperature and storage temperature, IR time and storage time, and storage temperature and storage time, and the quadratic terms of IR temperature and IR time on the DPPH radical scavenging activity of Barhi dates. These findings indicate that increase in storage temperature and time result in reduced DPPH radical scavenging activity of Barhi dates, which could be due to enzymatic oxidation and decomposition of bioactive compounds during ripening of Barhi dates, as reported previously [1]. The prediction equations for describing the effect of IR treatment and storage conditions on the TPC and DPPH of Barhi dates using significant terms are as follows:

$$\begin{gathered} {\mathrm{Y}}_{\mathrm{TPC}}=34.148-0.540{\mathrm{X}}_1-4.363{\mathrm{X}}_2-0.939{\mathrm{X}}_4+0.024{\mathrm{X}}_1{\mathrm{X}}_2-0.006{\mathrm{X}}_1{\mathrm{X}}_3-0.006{\mathrm{X}}_1{\mathrm{X}}_4-0.165{\mathrm{X}}_2{\mathrm{X}}_3 \\ -0.052{\mathrm{X}}_2{\mathrm{X}}_4-0.002{\mathrm{X}}_3{\mathrm{X}}_4+0.005{\mathrm{X}}_1^2+1.387{\mathrm{X}}_2^2-0.038{\mathrm{X}}_3^2+0.079{\mathrm{X}}_4^2 \end{gathered}$$

(8)

$$\begin{gathered} {\mathrm{Y}}_{\mathrm{DPPH}}=56.140+2.482{\mathrm{X}}_1+26.765{\mathrm{X}}_2-2.644{\mathrm{X}}_3-2.644{\mathrm{X}}_4-0.012{\mathrm{X}}_1{\mathrm{X}}_2-0.045{\mathrm{X}}_1{\mathrm{X}}_3+0.011{\mathrm{X}}_1{\mathrm{X}}_4 \\ +0.191{\mathrm{X}}_2{\mathrm{X}}_3-0.280{\mathrm{X}}_2{\mathrm{X}}_4-0.055{\mathrm{X}}_3{\mathrm{X}}_4-0.015{\mathrm{X}}_1^2-4.875{\mathrm{X}}_2^2+0.116{\mathrm{X}}_3^2+0.131{\mathrm{X}}_4^2 \end{gathered}$$

(9)

The 3D surface plots of the effects of treatment variables on the bioactive properties of Barhi dates are shown in Figure 3 and Figure 4. For TPC, increasing IR temperature and storage time reduced the TPC of Barhi dates to minimum values at 70 °C and 11 days and then increased to maximum values as the IR temperature and storage time raised to 110 °C and 21 days, respectively (Figure 4a–c,e,f). Increasing IR time continually increased the TPC of Barhi dates (Figure 4a,d,e). Increasing storage temperature increased the TPC of Barhi dates to maximum values at 15 °C and then reduced as the temperature raised to 25 °C (Figure 4b,d,f). For DPPH radical scavenging activity, increasing IR temperature and time increased DPPH of Barhi dates to the maximum at 90 °C and 3 min and then slightly reduced as the temperature and time elevated to 110 °C and 4 min, respectively (Figure 5a–e). However, at the high IR temperature and long time, the DPPH percentage was higher than that at low temperature and short time, suggesting that IR treatment enhanced the antioxidant activity of Barhi dates. Increasing the storage temperature and duration reduced the DPPH to minimum levels at 15 °C and 11 days and then increased again to the maximum levels as the temperature and time increased to 25 °C and 21 days, respectively (Figure 5b–f). These findings indicate that moderate IR treatment conditions and low storage conditions are favorable for improving and conserving the bioactive properties of Barhi dates. The enhancement of bioactive properties (TPC and DPPH radical scavenging activity) with moderate IR temperature and duration is likely due to easing the release of phenolic compounds from cellular matrix; however, at extreme temperature and long exposure, sensitive phenolic compounds might be decomposed, thereby reducing their bioactivity [15,24,32]. High storage temperature might also adversely influence the bioactive properties of Barhi dates, which is mostly due to the enhancement of the ripening process at high storage temperature which, in turn, increases the enzymatic degradation and oxidation of bioactive compounds, thereby reducing their bioactivity, which is comparable to other studies [1,10]. Thus, moderate IR treatment conditions and low storage temperature could be ideal for enhancing and conserving the bioactive properties of Barhi dates without major effect on the physicochemical properties of the product.

3.5. Effect of IR Treatment and Storage Condition on Glucose Content of Barhi Dates

The regression analysis showed that IR treatment and storage conditions affected the glucose content in Barhi dates (Table 2). The effect of treatment variables on the glucose content of Barhi dates showed some similarity. In linear terms, IR temperature and IR time showed significantly (p < 0.001) negative effect, whereas storage temperature and time showed significantly (p < 0.01) positive effect on the glucose of Barhi dates. In interaction terms, IR temperature and storage time showed negative effect, whereas IR time and storage time, IR time and storage temperature, and storage time and storage temperature showed positive effect on the glucose of Barhi dates (p < 0.01, p < 0.001). In quadratic terms, IR temperature and storage time possessed positive effect, whereas the storage temperature showed negative effect on the glucose of Barhi dates (p < 0.01, p < 0.001). Generally, it can be observed that IR treatment conditions negatively affected the glucose content, indicating that increasing IR temperature and time reduced the glucose content of Barhi dates. The reduction in glucose content in Barhi dates following IR treatment could be attributed to intense and rapid heating of IR inside the cellular matrix of Barhi dates, consequently leading to the cleavage of glucosidic bonds in polysaccharides chain and acceleration of the decomposition of monosaccharides, as reported in IR-dehydrated papaya fruits [33] and shiitake mushrooms [34]. On the other hand, storage conditions positively affected the glucose content, suggesting that increasing storage temperature and duration increased the glucose content of Barhi dates. The increase in glucose content at increased storage temperature and duration is probably due to the enzymatic conversion of polysaccharides and disaccharides into monosaccharides, as reported in other studies [10,35]. The prediction equations for describing the effects of IR treatment and storage conditions on the glucose of Barhi dates using significant terms are as follows:

$$\begin{gathered} {\mathrm{Y}}_{\mathrm{Glucose}}=16.140-1.712{\mathrm{X}}_1-3.133{\mathrm{X}}_2+0.441{\mathrm{X}}_3+1.931{\mathrm{X}}_4-0.849{\mathrm{X}}_1{\mathrm{X}}_2-2.243{\mathrm{X}}_1{\mathrm{X}}_4+2.511{\mathrm{X}}_2{\mathrm{X}}_3 \\ +1.868{\mathrm{X}}_2{\mathrm{X}}_4+1.262{\mathrm{X}}_3{\mathrm{X}}_4+6.081{\mathrm{X}}_1^2-8.370{\mathrm{X}}_3^2+11.839{\mathrm{X}}_4^2 \end{gathered}$$

(10)

The 3D surface plots of the effects of treatment variables on the glucose content of Barhi dates are shown in Figure 5. Increasing IR temperature and storage time reduced the glucose content to minimum level at 85 °C and 11 days and then increased at high temperatures and long storage time (Figure 6a–c,e,f). Increasing IR time constantly increased the glucose content of Barhi dates (Figure 6a,d,e). Increasing the storage temperature increased the glucose content of Barhi dates to maximum values at 13 °C and then reduced again as the temperature elevated to 25 °C (Figure 6b,d,f). Overall, extreme IR and storage conditions increased the glucose contents of Barhi dates; the increment could be due to increased conversion of complex starch and polysaccharides into simple sugars such as glucose, thereby increasing the concentration of this sugar in Barhi dates [10,35]. Therefore, to avoid increased levels of glucose contents in Barhi dates while preserving its quality for a long time, low IR treatment conditions (temperature and duration) and low storage temperature are recommended.

3.6. Optimization of IR Treatment and Storage Conditions

In this study, an attempt was made to optimize the IR treatment and storage conditions for preserving the quality attributes of Barhi dates at the most perishable maturity stage (Khalal). The optimal conditions for minimizing the microbial load and physical changes and maximizing the bioactive properties of Barhi dates were achieved using the applied RSM models and generated equations. Based on that, the optimal IR treatment and storage conditions for maintaining the freshness quality of Barhi dates were IR temperature of 50 °C, IR treatment time of 1.2 min, storage temperature of 1 °C, and storage duration of 20 days. Under these optimal conditions, the experimental values for the assessed responses agreed with the predicted values, giving high desirability level.

4. Conclusions

Preserving the quality attributes of fresh Barhi dates at Khalal maturity stage remains the main aim for date producers and consumers, as at the preferable consumption maturity stage, this date type is very perishable and its quality quickly changes during storage. In this study, infrared treatment and storage conditions were optimized using response surface methodology (RSM) for maintaining the physicochemical, microbial, and bioactive attributes of Barhi dates at Khalal stage. Infrared conditions (temperature and duration) and subsequent storage conditions (temperature and duration) were found to affect the quality attributes of Barhi dates in both positive and negative manners. The optimal conditions for minimizing the physical changes and microbial load and maximizing the bioactive properties of Barhi dates were IR temperature of 50 °C, IR treatment time of 1.2 min, storage temperature of 1 °C, and storage duration of 20 days. Overall, IR can be used as a postharvest treatment for preserving the quality of Barhi dates and the treated dates could be stored for up to three weeks at 1 °C without major decay of the freshness and the physicochemical and bioactive quality attributes of the product.