Optimization and Storage Stability of Milk–Date Beverages Fortified with Sukkari Date Powder

Abstract

This study aims to determine the feasibility of creating a date–milk beverage with nutritional and antioxidant benefits and determine the optimal formulation and storage conditions to preserve its quality. Date powder–milk beverages with 0%, 10%, 15%, 20%, and 25% weight/weight (w/w) dates were refrigerated at 1 °C and 5 °C for 10 days to evaluate their nutritional and antioxidant activities. The investigation showed that response surface methodology models accurately represented experimental data. Date powder concentration, storage period, and storage temperature all negatively affected pH, which ranged from 6.45 to 7.09, close to but surpassing the optimum pH. The beverage’s total dissolved solids (TSS) declined after 10 days, with no notable changes as the storage temperature rose from 1 °C to 5 °C. Increasing date powder concentrations resulted in darker beverages, with a color change (ΔE) ranging from 12.93 to 35.55. All variables showed a considerable increase in dietary fibers in milk–date beverages. Phenolic levels of 9.7 to 10.05 gallic acid equivalent (GAE)/gram dry weight are preserved by colder storage temperatures and greater date concentrations. During storage, antioxidant activity considerably increased (p < 0.001) for all date concentrations, but did not change with temperature. On the tenth day of storage, high-temperature storage and low date powder content increased colony counts (6.22 log10 CFU/mL). This study suggests that adding dates to dairy-based beverages creates nutritional drinks without additives, processed sugars, or preservatives that customers like. Thus, the optimal storage conditions for date–milk drinks were achieved at a date percentage of 25% w/w and a storage temperature of 1 °C for 10 days.

1. Introduction

The date palm (Phoenix dactylifera L.) is a versatile and very resilient plant that is extensively cultivated and utilized. Saudi Arabia ranks second globally in terms of date production, having gathered over 1.61 million tons of dates in 2022 from a cultivated area of 156,000 hectares [1]. Date palm fruits (DPFs) are widely consumed globally and are often regarded as a rich source of bioactive compounds (e.g., carotenoids, polyphenols, tannins, and sterols) that offer multiple health benefits [2]. These compounds act as antioxidants, protecting the body from free radical damage linked to chronic diseases, and they also exhibit antimicrobial and antimutagenic properties [3]. Despite these benefits, DPFs remain a relatively underutilized source of nutrition. Fresh DPFs are subject to certain processing methods besides direct consumption. This includes the production of date syrup (a beverage), which is the prevailing processed date product manufactured in Saudi Arabia from surplus dates. Date beverages are rich in fructose and glucose, as well as calcium, potassium, sodium, and magnesium [4,5].

Fruit and vegetable powders have been used to improve the nutritional and functional properties of dairy products. These powders, derived from various fruits and vegetables, are rich in essential nutrients, bioactive compounds, and dietary fibers, contributing to overall health benefits. Berry powders, high in vitamin C and polyphenols, have been linked to improved immune function and reduced oxidative stress [6,7]. Vegetable powders, particularly those from leafy greens and root vegetables, are rich in dietary fibers, beta-carotene, and other carotenoids, promoting eye health and reducing inflammation [8]. The inclusion of date powder in milk not only increases its antioxidant activity but also contributes to its dietary fiber content, which is crucial for maintaining a healthy digestive system [9].

Date fruit beverage concentrate, known for its exceptional color, flavor, and vitamins, is much sought after in the global market [10]. However, DPF beverages are prone to decay in terms of flavor, vitamins, color, and nutrients when exposed to different storage conditions, both in their original and concentrated form, due to their high contents of reduced and total sugars [11].

Dairy products on the market are typically infused with flavoring agents such as chocolate, strawberry, and banana. Additionally, these milk products often contain refined sugars and preservatives [12,13,14]. These products have gained popularity in recent years due to their nutritional, flavorful, and functional qualities. There is still a growing need for dairy-based beverages that have enhanced nutritional and health benefits. Regarding this matter, it has been discovered that the addition of date beverages can enhance the nutritional, functional, microbiological, and sensory characteristics of whey beverages [15], date–milk drinks [16], and flavored and probiotic yogurt [17].

While previous research has focused on adding various fruit and vegetable powders to dairy products to improve their nutritional and sensory qualities, it has often been limited to a small number of ingredients or has not provided a comprehensive examination of the effects on storage stability, especially under variable conditions. For example, studies on the use of date powder in dairy beverages have shown promising results, but they typically only examine the nutritional value of the product, ignoring the complex interactions between ingredients and how they affect shelf life, antioxidant, and microbiological stability, and overall consumer acceptance. This study presents a novel approach to the development of healthier dairy beverages by investigating the impact of varying Sukkari date powder concentrations on the nutritional, antioxidant, and microbiological profiles of date–milk beverages, differentiating such beverages from existing products that rely on artificial flavors, refined sugars, and preservatives. This research also explores the shelf life of these date–milk beverages under refrigerated storage conditions, contributing to understanding their potential as a natural and nutritious alternative in the market. Thus, this study aimed to investigate the effect of the ratio of ‘Sukkari’ cv. date powder (10%, 15%, 20%, and 25% weight/weight (w/w)) in dairy beverages and their shelf life under refrigerated storage (1 °C and 5 °C) for ten days in terms of the nutritional content, antioxidant qualities, and microbiological activities of the milk–date beverages. For this purpose, the response surface methodology (RSM) model was utilized to model and optimize the storage conditions.

2. Materials and Methods

2.1. Beverage Preparation

‘Sukkari’ cv. date fruits obtained from Al-Qassim, Saudi Arabia, were washed and dried. The moisture content ranged from 20% to 23.5%. The date flesh was minced into a paste, mixed with maltodextrin (40% w/w), and further formed into a paste with 3 mm thickness. The paste was dried in a vacuum dryer (PN: VO49 (Memmert GmbH + Co. KG, Büchenbach, Germany)) at a drying temperature of 70 ± 1 °C and pressure of 10 kPa until the samples reached about 2–2.5% (wet basis (w.b.)). The samples were then ground for 10 s to obtain a date powder, as mentioned in grant number 2-17-04-001-0021 [18]. The particle size distribution of the Sukkari date powder used in the study ranged from coarse particles at 1000 μm to very fine particles at 50 μm, with the majority (52.66%) of the powder being composed of fine particles below 125 μm, which significantly influences the texture and stability of the final date–milk beverage. The milk-based date beverage was created using the Sukkari date powder in liquid milk at four different concentrations (0%, 10%, 15%, 20%, and 25% w/w) as shown in Figure 1. The date powder was incorporated into low-fat cow’s milk and blended using an electric mixer (type: T2, MACAP, Maerne, Italy) for 2 min. The milk-based date powder beverage underwent pasteurization according to the Gulf Standard “Pasteurized Milk” (GSO 984, 2015) [19]. The pasteurization process involved heating the beverage to a temperature of 63 °C for 30 min, using a method known as low-temperature prolonged pasteurization (LTLT). Subsequently, the beverage was promptly chilled to a temperature of no lower than 4 °C. This approach is highly effective in eliminating all vegetative kinds of bacteria and ensuring the optimal preservation of certain biological components in the beverage. The beverage was held at two temperature settings (1 or 5 ± 0.1 °C) in a high-density polyethylene package for 10 days.

2.2. Experimental Design

A series of studies were carried out to ascertain the optimal conditions for storing the beverages consisting of date powder mixed with low-fat liquid cow’s milk. A series of storage experiments were conducted, taking into account several aspects such as the date powder ratio, storage time, and temperature. The storage experiment followed the Box–Behnken experimental design, with three independent variables: the ratio of date powder mixed with low-fat liquid cow’s milk (0%, 10%, 15%, 20%, or 25% w/w) as the addition rate (X₁), the storage time (0, 2, 4, 6, 8, or 10 days; X₂), and the storage temperature (1 °C or 5 °C; X₃). Regarding the shelf life, certain factors such as quality attributes and microbiological development were taken into account while evaluating the results. Response surface methodology was employed to maximize the storage circumstances that impacted the characteristics of the beverage.

Table 1. Variables, codes, and real values for optimized beverage storage conditions.

Variables	Level (Code)
Date powder concentration, % (w/w) (X1)	0 (−1)	10 (−0.2)	15 (0.2)	20 (0.6)	25 (1)
Storage time, number of days (X2)	0 (−1)	2 (−0.6)	4 (−0.2)	6 (0.2)	8 (0.6)	10 (1)
Storage temperature, °C (X3)	1 (−1)	5 (1)

presents the variables and their corresponding levels in both coded and real values.

The multiple regression equation (Equation (1)) was derived by applying a second-order polynomial equation to the experimental data:

$$\gamma ={\beta }_o+{\sum }_{j=1}^k{\beta }_i{X}_j+{\sum }_{j=1}^k{\beta }_{jj}{X}_j^2+\sum {\sum }_{i<j}{\beta }_{ij}{X}_i{X}_j,$$

(1)

where the coefficients of regression for the intercept, interaction, and square are denoted as β_o, β_i, β_jj, and β_ij, respectively; γ represents the expected response; and X_i and X_j are mutually independent variables. The coefficients were interpreted using the F test. The ideal storage conditions were identified by employing surface plotting, regression analysis, and analysis of variance (ANOVA).

2.3. pH

To calculate the pH value in the experiments, a pH meter (Jenway, Model 3510, Cambridge, UK) with an error of ± 0.01 was utilized, as indicated in [20,21].

2.4. Total Soluble Solids (TSS)

The beverage’s total soluble solids concentration was measured using a refractometer (ATAGO-28E, Tokyo, Japan) equipped with a sugar % level, providing measurements stated in °Brix, as mentioned in [20,21].

2.5. Crude Fiber Determination

A modified Weende method [22] was utilized to estimate crude fibers in the date–milk mixtures. One gram of a dried sample of date powder with milk was accurately weighed in a glass crucible and designated F₀. Then, 150 mL of 1.25% H₂SO₄ was added to the sample. After boiling with drops of octanol, the acid solution was left to sit for 30 min. Acid was extracted using a vacuum and the solution stirred before washing with 30 mL of hot distilled water. After washing, 150 mL of 1.25% NaOH was added to the alkaline solution, brought to the boil, and allowed to settle for 30 min. A suction device was used to remove the alkaline solution, following which the sample was cleansed with 30 mL of distilled cold water. After three compression air mixes, the sample was washed twice in acetone. The samples were dried in glass containers for 24 h at 110 °C in a drying oven (F₁). The samples were burned in glass crucibles at 550 °C for 3 h until their weight was stable. Samples were weighed in a glass dryer after they cooled (F₂). The crude fibers % formula is as follows (Equation (2)):

$$\% \mathrm{C}\mathrm{r}\mathrm{u}\mathrm{d}\mathrm{e} \mathrm{F}\mathrm{i}\mathrm{b}\mathrm{e}\mathrm{r}=(F_1-F_2 /F_0)\times 100.$$

(2)

2.6. Color Change

To quantify the hue of the sample, a CR-400 colorimeter (Konica Minolta, Japan) was utilized. Three variables were recorded by the colorimeter using the Hunter Lab scale: luminosity (L), redness/greenness (a), and blueness/yellowness (b) [23]. A white plate acted as a reference point during the process of calibrating the apparatus.

The control sample values (standard white color) are denoted as L₀, a₀, and b₀, whereas the stored sample values are represented as L*, a*, and b*. ΔE denotes a more noticeable color change relative to control samples.

The ΔE value, representing the overall color change or variation between the stored and control samples, was computed utilizing the subsequent Equation (4) [24,25]:

$$\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{c}\mathrm{o}\mathrm{l}\mathrm{o}\mathrm{r} \mathrm{c}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{g}\mathrm{e}\left(\Delta E\right)=\sqrt{{\left(L_0-L^{*}\right)}^2+{\left(a_0-a^{*}\right)}^2+{(b_0-b^{*})}^2}$$

(3)

2.7. Total Phenolic Content (TPC)

Polyphenol extraction was performed on 5 g powdered date samples using 100 mL of 80% methanol in a water bath at 60 °C for three hours. Following the filtration process, the resultant liquid (called the extract) was kept at 4 °C. The total phenolic content (TPC) of the extract was calculated using the approach reported in [26] with slight adjustments. A total of 200 mL of extracts was well mixed with 4.0 mL of distilled water and 400 mL of Folin–Ciocalteu reagent. Following vigorous mixing and incubation for 2 h, a 20% Na₂CO₃ solution was introduced. The combination was then allowed to incubate for a further ten minutes at 25 °C. A spectrophotometer (PD-303UV, Apel, Saitama, Japan) was employed to measure the absorbance at 765 nm. The total phenolic content (TPC) was calculated as milligrams of gallic acid equivalent per gram (GAE/g) of extract.

2.8. DPPH Inhibition

The free radical activity of the samples was assessed by analyzing the polyphenol compounds extracted using 1,1-diphenyl-2-picrylhydrazyl (DPPH), as outlined in [27]. A 1 mL extract solution was mixed with 2 mL of a methanolic DPPH solution containing 4 mg of DPPH per 100 mL. As a control, an equal quantity of DPPH and methanol was employed. The optical density (OD) was determined at 517 nm after leaving the mixtures at room temperature for five minutes and aggressively agitating them. The percentage of inhibition of free radicals was determined using Equation (4):

$$\mathrm{D}\mathrm{P}\mathrm{P}\mathrm{H} \mathrm{s}\mathrm{c}\mathrm{a}\mathrm{v}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}=\left({\displaystyle \frac{\mathrm{O}\mathrm{D} \mathrm{o}\mathrm{f} \mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}{\mathrm{O}\mathrm{D} \mathrm{o}\mathrm{f} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}}}\right)\times 100$$

(4)

2.9. Total Bacterial Count (TBC)

As stated by the Association of Official Analytical Chemists [28], a sterile saline solution containing 0.85% NaCl was mixed with 1 mL of the beverage sample. The samples were meticulously shaken. A suitable serial dilution of 10⁻¹–10⁻⁴ was prepared and subsequently mixed with 10 mL of sterile and chilled nutrient agar (45 °C) in sterilized Petri dishes (Oxoid, CM0309, Ottawa, ON, Canada). The plates were then incubated for 24–48 h at 35 °C.

2.10. Statistical Analysis

All experiments were conducted in triplicate, and the results are expressed as mean values ± standard deviation (SD). Data were statistically analyzed using the Statistical Package for the Social Sciences (SPSS) software, version 25.0 (IBM Corp., Armonk, NY, USA). The analysis of variance (ANOVA) was employed to determine the significance of the differences among the means of the different treatments.

Response surface methodology (RSM) was used to model and optimize the effects of different date powder concentrations, storage times, and storage temperatures on the quality attributes of the date–milk beverages. The regression coefficients, as well as the interactions and quadratic terms, were calculated and analyzed using ANOVA to determine their significance. The adequacy of the model was evaluated by the coefficient of determination (R²) and the lack-of-fit test. The results were considered significant when p < 0.05.

3. Results and Discussion

3.1. Fitting the Model

Through utilizing the response surface model technique, it is possible to examine and determine the optimal storage treatment settings. This study examined the impacts of varying concentrations of date powder and different storage conditions on the qualitative attributes of the beverages. The significance of model coefficients was determined through deriving second-order polynomial equation coefficients using experimental data.

Table 2. The components of the process and the responses of the product to calculate regression coefficients (for coded factors).

Variables	ΔE	TSS	pH	TPC	DPPH	Fibers	TBC
Intercept
β0	25.82	18.46	6.77	9.36	30.38	1.32	2.18
Linear
X1 (β1)	8.51 ***	6.67 ***	−0.0431 *	0.5137 ***	19.73 ***	1.22 ***	−1.02 ***
X2 (β2)	2.54 **	0.0164 ns	−0.0489 **	−0.0684 ns	11.35 ***	0.115 *	1.07 ***
X3 (β3)	0.9714	−0.1065 ns	−0.115 ***	0.0196 ns	−0.3394 ns	0.0989 *	0.8665 ***
Interaction
X1X2 (β12)	−0.2948 ns	−1.38 **	−0.0463 *	−0.2885 ***	6.48 **	−0.0628 ns	−1.11 ***
X1X3 (β13)	0.6601 ns	−0.5324 ns	0.0143 ns	0.1533 ***	2.85 ns	0.0522 ns	−0.0945 ns
X2X3 (β23)	0.7289 ns	1.09 *	−0.1237 ***	0.0933 *	−2.75 ns	0.1488 *	−0.0530 ns
Model F-value	50.54	91.99	45.88	75.72	47.39	124.69	34.36
p-value	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001
Mean	25.67	18.34	6.78	9.39	30.04	1.3	2.13
C.V.%	7.00	5.82	0.6357	1.07	15.19	12.32	19.97
Adeq. precision	17.77	25.08	22.63	25.7	21.16	25.52	16.40
R2	0.9681	0.9822	0.9649	0.9785	0.9660	0.9868	0.9537
Adjusted R2	0.9489	0.9715	0.9439	0.9655	0.9456	0.9789	0.9260
Predicted R2	0.7866	0.9303	0.8601	0.9278	0.8226	0.9278	0.7527
Std. dev.	1.80	1.07	0.0431	0.1006	4.56	0.1603	0.4246
F-value (lack of fit)	4.17	303	2.72	2.38	4.65	3214	2.18
p-value (lack of fit)	0.0714 ns	0.1243 ns	0.1483 ns	0.182 ns	0.0586 ns	0.0001 ***	0.2061 ns

* p < 0.05, ** p < 0.01, *** p < 0.001, ^ns (not significant).

presents the polynomial models derived from the experimental data, together with coefficients of multiple determinations (R²) and the coefficient of variance (CV) for ΔE, TSS, pH, TPC, DPPH, and fibers.

The findings indicate that the regression equation accurately represented the results for each of the dependent variables. Additionally, the error analysis showed that the lack of fit was not significantly different for any of the dependent variables. The model’s p-value for all assessed variables revealed that it was statistically different (p > 0.001) for the assessed attributes.

Furthermore, the p-values for the lack of fit fell within the range of 0.0001 to 0.0.2006, suggesting that there were no significant differences (p > 0.05). This indicates that the models used effectively describe the experimental data. In addition, the R² values for the computed variables varied from 0.9475 to 0.9868, while the adjusted R² values ranged from 0.9160 to 0.9789. Hence, the polynomial quadratic models constructed with Equaiton (1) explain more than 95% of the total variation in the experimental data [29]. Optimal models have an accuracy value greater than 4.0, as stated in [30]. The research findings showed that the model was competent in properly interpreting the data, as indicated by adequate precision values ranging from 15.31 to 25.7. The coefficient of variation (CV) allows for the assessment of a model’s precision and validity. It was excellent when the CV was ≤10%, good when the CV was between 10 and 20%, acceptable when the CV was between 20 and 30%, and poor when the CV was > 30% [31]. The coefficient of variation (CV) values for the variables examined in this study ranged from 0.6357 to 19.97%. These values indicate that the trial data were very reliable and predictable, and the RSM models employed in this analysis were found to be both reproducible and sufficient. As such, they can be employed to efficiently optimize the storage conditions and concentrations of date powder blended with milk for the production of beverages, ensuring the preservation of qualitative attributes during storage.

3.2. pH

The pH level has been shown to have a crucial impact on aspects of the production process, quality control, microbial development, and product shelf life. This is because pH directly influences the attributes of beverages. Figure 1 shows a 3D RSM of pH as influenced by the date powder concentration and storage conditions. The independent factors (date powder concentration, storage duration, and storage temperature) had adverse effects on the pH. The following Equation (5) was constructed to estimate the effects of date powder concentration (X₁), storage time (X₂), and storage temperature (X₃) on the pH of date–milk beverages (for actual factors).

$$\begin{gathered} Y_{pH}=6.82236-0.001467X_1+0.036579X_2-0.002804X_3-0.00074X_1{X}_2 \\ +0.000573X_1{X}_3-0.012373X_2{X}_3 \\ (R^2=0.9649; \mathrm{A}\mathrm{d}\mathrm{j}\mathrm{u}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{d} R^2=0.9439; SD=0.0431) \end{gathered}$$

(5)

It is clear from Figure 2a,c that the results revealed statistically significant differences (p < 0.05) between the pH values of beverage samples during various storage periods. At the same time, the addition of more date powder to the beverage resulted in a slow decrease in pH, reaching its lowest value at a concentration of 25% (w/w) (Figure 2a,b). The reason for this observation could be the decreased pH of the powdered dates [20,21]. Furthermore, as the storage temperature increased, the pH of the beverage slightly decreased, with the lowest levels noted at the highest temperature of 5 °C (Figure 2b,c). These results were in agreement with those reported by [32], who observed that the pH gradually decreased with refrigerated storage at 4 °C over a long storage duration of 28 days for probiotic soy milk [33]. In [34], similar results were found for fruit-based beverages. The decrease in pH values may be attributed to the development of acid Maillard products and microbial growth during storage, as previously documented [35,36]. Curiously, the highest pH level that is considered appropriate for beverages has been officially recorded as 4.8 since the year 2008 [37], and beverages that have a pH value exceeding 4.8 can be classified as low-acid beverages [38]. The drinks prepared with all the different treatments in the study approached the desired pH level, and thus, it is recommended to apply pretreatment to slightly decrease the pH level of the prepared beverage.

3.3. Total Soluble Solids (TSS)

Figure 3 shows three-dimensional response surface plots for the effects of storage temperature and duration on the TSS of the date–milk beverage, as well as the effect of the percentage of powdered dates in the milk. The date powder ratio led the beverage’s TSS to peak at 25% (w/w) (Figure 3a,b). Equation (6) was established using actual factors in order to simulate the effects of the date powder rate and storage conditions on the TSS of the considered date–milk beverages.

$$\begin{gathered} \begin{array}{c}{Y}_{TSS}=11.386+0.708189X_1-0.046512X_2-0.330644X_3-0.022109X_1{X}_2\\ -0.021297X_1{X}_3+0.108717X_2{X}_3\end{array} \\ (R^2=0.9822; \mathrm{A}\mathrm{d}\mathrm{j}\mathrm{u}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{d} R^2=0.9715; SD=1.07 \end{gathered}$$

(6)

As shown in Figure 3c, when the storage period increased to 10 days, the total soluble solids in the drink gradually decreased, reaching the lowest value (23.32–24.22 °Brix), without any noticeable significant change (p > 0.05) when the refrigeration temperature increased from 1 °C to 5 °C at the maximum powder concentration. In comparison to our results, the total soluble solids in a date drink were previously found to increase significantly when the storage temperature increased from 1 to 25 °C [39]. The low variability in TSS in this study may be due to the use of low temperatures. Using high temperatures helps to dissolve sugars in the beverage, which is the main reason for the high TSS.

3.4. Color Change

The color features of beverages are regarded as one of the most crucial quality characteristics, as they play an essential role in consumer acceptance and the management of processes in the beverage sector [38,40]. The results showed that beverages with different date powder ratios under both storage temperatures turned darker with time, with lightness (L*) decreasing and color variation (ΔE) increasing over the storage period (Figure 4a–c) [41,42]. The effects of date powder concentration, refrigerated temperature, and storage time on date–milk beverages can be predicted using Equation (7) (derived using actual factors):

$$\begin{gathered} Y_{\Delta E}=15.098+0.625378X_1+0.348533X_2-0.208748X_3-0.004716X_1{X}_2 \\ +0.026402X_1{X}_3+0.072887X_2{X}_3 \\ {(R}^2=0.9681; \mathrm{A}\mathrm{d}\mathrm{j}\mathrm{u}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{d} R^2=0.9489; SD=1.8) \end{gathered}$$

(7)

The value of ΔE under cold storage at 1 °C increased by 28% and 9.75% at 0% and 25% date powder ratios, respectively, on the 10th day; however, beverages stored at 5 °C showed significantly higher variations in ΔE, with increases of 14.05–21.18 and 32.98–38.93 for the 0% and 25% date powder ratios, respectively. Therefore, the maximum level of ΔE in drinks was reached at 5 °C and the lowest level at 1 °C (Figure 4b,c). A notable increase in ΔE (reflecting the decay extent) was observed after 15 days [43] in fresh carrot juice stored at 4 °C, which aligns well with the findings of this study. The color variation (ΔE) may be significantly influenced by non-enzymatic browning, which could impact the change in color characteristics [44]. It has been previously documented that the non-enzymatic browning in kept food products might be influenced by numerous parameters, including storage temperature, organic acids, O₂, and sugars related to TSS [45]. These findings are similar to the results reported in [10] and this study.

3.5. Crude Fibers

The physicochemical features of date powder indicate that it is high in fibers and could be a valuable addition to a nutritious diet [40]. Figure 5a depicts a 3D RSM of fibers percentage based on the storage time (0–10 days) and date powder ratio (0–25% w/w). The fibers’ content for low and high concentrations of dates (0 and 25% w/w) significantly increased (p ≤ 0.05) during storage (Figure 5b). Comparable to the initial value of 0.964% at 1 °C, the cold stored samples in the 5 °C group achieved the highest significant increase, reaching 1.44% by the end of storage for concentration powder of 10% (Figure 5c). All parameters are significant and are represented by the following polynomial Equation (8) for crude fibers (according to actual factors):

$$\begin{gathered} Y_{Fibers}=0.073144+0.096348X_1-0.009085X_2-0.051019X_3 \\ -0.001005X_1{X}_2+0.002086X_1{X}_3+0.014882X_2{X}_3 \\ {(R}^2=0.9868; \mathrm{A}\mathrm{d}\mathrm{j}\mathrm{u}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{d} R^2=0.9789; SD=0.1603) \end{gathered}$$

(8)

The dietary fibers in date fruit correspond to the sugars (and trace lignin) found in their cell walls. The date powder ratio and storage duration had positive significant effects (p < 0.001) on the amount of total dietary fibers as they altered the soluble/insoluble fiber balance through increasing soluble fibers [44]. This has been reported to be due to pectin degradation [41], which is consistent with the current investigation (increased fiber percentage). The proportion of nutritional fibers is typically less than 0.5 g per liter [46], although, under varying storage conditions and date powder concentrations, it was consistently greater than the amount found in recommended beverages.

Date powder’s importance in increasing milk-based beverages’ nutritional fiber content is highlighted by the rise in crude fiber content seen during storage, particularly at higher concentrations of Sukkari date powder. Other studies have found that adding fruit and vegetable powders to dairy products increases fiber content over time. For instance, Renard et al. [43] found that pear fibers improved dairy product fiber content, especially during storage, via releasing soluble fibers. Due to pectin and other polysaccharides breaking down into more soluble forms, Colin-Henrion et al. [44] observed that industrially processed apple sauces increased in fiber after storage. These studies support the idea that date powder boosts a beverage’s nutritional profile, stability, and potential health advantages, such as digestive health, which health-conscious customers want.

3.6. Total Phenolic Content (TPC)

Date palm fruits include a significant amount of phenolic compounds, such as flavonoids, phenolic acids, and tannins. These chemicals are essential for the antioxidant properties of these fruits [47]. Phenolic substances have several advantageous features, such as anti-inflammatory, antibacterial, and anti-cancer activities, and have potential for use in the management of diabetes, improvement in cardiovascular health, and facilitation of digestive processes [48]. Figure 6a,c illustrate that the considered date–milk beverages possessed the highest initial phenolic content at 0 days of storage time. Across all ratios of date powder, the phenolic content decreased significantly (p < 0.01) over the ten days of storage. Furthermore, beverages with 20 and 25% w/w date ratios exhibited a higher phenolic content compared to those with other ratios (Figure 6a,b). The polynomial Equation (9) for the total phenolic content (TPC) includes all terms (date powder concentration, storage time, and storage temperature for actual factors):

$$\begin{gathered} Y_{TPC}=8.96707+0.045779X_1+0.016050X_2-0.113535X_3- \\ 0.004617X_1{X}_2+0.006134X_1{X}_3+0.009328X_2{X}_3\dots \\ {(R}^2=0.9785; \mathrm{A}\mathrm{d}\mathrm{j}\mathrm{u}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{d} R^2=0.9655; SD=0.1006) \end{gathered}$$

(9)

The decrease in the total phenolic content (TPC) in ‘Sukkari’ cv. date powder–milk beverages after storage at 5 °C may be due to the breaking of covalent bonds between phenolic compounds and cell walls or alteration of the structure of these due to temperature, thereby increasing the solubility and extractability of phenolic compounds, as has been previously observed for red dragon fruit [49]. This decline matches studies on phenolic component stability in other fruit-based products. Over time, oxidative degradation and phenolic structural breakdown reduced TPC in date palm fruits maintained at ambient temperatures, according to Saafi et al. [45]. Pua et al. [41] found that jackfruit powders held at different temperatures lost TPC, especially at higher temperatures, reducing its antioxidant ability. To preserve phenolic content and keep the health advantages of these compounds throughout the product’s shelf life, optimal storage conditions, such as lower temperatures, are crucial. Thus, Sukkari date powder boosts the beverage’s antioxidant profile, but proper storage is necessary to maximize its advantages.

3.7. Antioxidant (DPPH)

Antioxidants are a specific category of phenolic compounds that, along with other chemicals, can counteract the harmful effects of free radicals and provide protection against oxidative stress [50]. Date fruits are of great interest as they contain a large number of phenolic compounds and have strong antioxidant action. The antioxidant ability of phenolics is dependent on their chemical composition and concentration [51]. Figure 7b,c illustrate the antioxidant activity of date–milk beverages stored under varying temperature conditions. Notably, all date ratios exhibited a statistically significant increase (p < 0.001) in antioxidant content over time (Figure 7a,c). Additionally, no statistically significant difference (p > 0.05) was observed between stored beverages at 1 °C and 5 °C through the ten days of the storage period. All factors were significant, except storage temperatures, as reflected in the following polynomial (Equation (10)) for DPPH antioxidants (for actual factors):

$$\begin{gathered} Y_{DPPH}=6.43272+0.717959X_1+1.80122X_2-0.218379X_3+ \\ 0.103614X_1{X}_2+0.114025X_1{X}_3-0.275330X_2{X}_3\dots \\ {(R}^2=0.9660; \mathrm{A}\mathrm{d}\mathrm{j}\mathrm{u}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{d} R^2=0.9456; SD=4.56) \end{gathered}$$

(10)

The increase in DPPH with increasing date powder ratios over an extended duration may be attributed to de novo synthesis, the disintegration of the cell wall, the liberation of the bound antioxidant chemicals, or to the production of antioxidant molecules such as melanoidins [52,53]. Alternatively, the rise in antioxidant levels during storage may be attributed to the fact that some fruits and vegetables contain precursor compounds that can be converted into active antioxidants during storage. Exposure to air (oxygen) can lead to oxidation, which breaks down antioxidants, resulting in a higher concentration of free radicals (DPPH). Similar to beverages, some food components with antioxidant properties can break down into simpler molecules during storage. These simpler molecules might not have the same free radical scavenging ability as the original ones, leading to a higher DPPH level [54,55,56,57].

3.8. Total Bacterial Count (TBC)

The total bacterial count (TBC) as a function of storage time and date powder ratio at various temperatures (1 °C and 5 °C) is displayed in Figure 8. Furthermore, the interactive impact of storage time with storage temperature on TBC was positive (p < 0.01; p < 0.05). At the end of the ten-day storage period, the high temperature (5 °C) and low concentration (0%) of date powder inside beverages contributed to the highest colony counts (6.22 log10 colony-forming units (CFU)/mL) observed. Equation (11) presented here was derived from the results, which allows for forecasting the impacts of the storage temperature, date powder ratio, and storage period on the total bacterial count (TBC) in beverages derived from milk and date powder (according to actual factors).

$$\begin{gathered} Y_{TBC}=-0.498185+0.018439X_1+0.452156X_2+0.506987X_3- \\ 0.017767X_1{X}_2-0.003778X_1{X}_3-0.005301X_2{X}_3\dots \\ {(R}^2=0.9537; \mathrm{A}\mathrm{d}\mathrm{j}\mathrm{u}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{d} R^2=0.9260; SD=0.4246) \end{gathered}$$

(11)

These findings align with the preservation benefits observed for solutions containing high concentrations of sugars—such as date syrup—through reducing the activity of water [49]. Previous studies have demonstrated that date syrup includes significant quantities of phenolic chemicals, which possess antibacterial properties [57,58]. Additionally, the growth of molds throughout the storage period of beverages can be attributed to the heightened activity of molds that thrive in media with low water activity [44]. The bacterial counts in this investigation were lower than those reported in conventionally created fermented milk [59] and fermented milk enriched with wheat germ [60].

3.9. Optimization of Beverage Formulation and Storage Conditions

The optimization of the date–milk beverage formulation and storage conditions was performed using response surface methodology (RSM) to identify the optimal levels of date powder concentration, storage temperature, and storage duration that would maximize the nutritional, antioxidant, and microbiological stability of the beverage. The optimal date powder concentration of 25% w/w was found to significantly enhance the nutritional profile of the beverage, particularly in terms of dietary fiber and phenolic content. This concentration also contributed to an increase in antioxidant activity, which is crucial for the beverage’s ability to combat oxidative stress and maintain its quality over the storage period. The storage temperature of 1 °C was determined to be optimal for preserving the beverage’s quality attributes. At this temperature, the beverage’s pH remained relatively stable, and microbial growth was minimized, resulting in a longer shelf life. Higher storage temperatures, such as 5 °C, were associated with increased microbial counts and a faster decline in pH, which could compromise the safety and sensory qualities of the beverage. The 10-day storage duration was chosen as the optimal period, balancing the need for a sufficient shelf life with the maintenance of the beverage’s nutritional and sensory properties. Although antioxidant activity increased over the storage period, the phenolic content and dietary fiber levels began to decline slightly after 10 days, suggesting that longer storage might lead to a reduction in the beverage’s overall quality.

4. Limitations and Further Research Perspectives

The study did not investigate the sensory aspects of the date–milk beverage, such as taste, aroma, and overall acceptability, which are crucial factors affecting consumer preference (a future study was planned after determining the optimal and safe storage conditions from the microbial side).

5. Conclusions

This study highlighted the potential of creating a nutritious and antioxidant-rich milk beverage through incorporating date powder as a natural sweetener. The preservation behavior of milk beverages under different date concentrations was simulated using response surface methodology. The results demonstrate that while the storage duration and temperature negatively impact the pH and color, they do not significantly affect the total soluble solids. Notably, the dietary fibers and antioxidant content of the beverages increased with a higher date concentration and colder storage, underscoring the importance of these factors for product quality. Furthermore, higher storage temperatures and lower concentrations of date powder in beverages led to higher bacterial colony counts after ten days. These findings are valuable for food processing engineers, as they provide insights into optimizing the formulation and storage conditions for date–milk beverages (in particular, a date powder ratio of 25% w/w and storage temperature of 1 °C for 10 days) in order to ensure stability, nutritional value, and consumer appeal. Response surface methodology (RSM) is a useful tool for optimizing various processes as it allows for the simultaneous optimization of multiple variables and helps in predicting the optimal conditions for achieving desired outcomes.