1. Introduction
Date palm fruits (
Phoenix dactylifera Linnaeus) are consumed fresh or dried or are prepared in various ways. Some date products include date honey, date sugar, date vinegar, and date wine (made from date fruit juice). The most popular commercial date products are date juice (primary product), pressed cake (byproduct), and date syrup [
1]. Dates are significant for human nutrition and health due to their vast array of healthy nutrients. Dates are rich in antioxidants, such as selenium, phenolics, and carotenoids [
2]. Furthermore, dates are an excellent source of quick energy due to their high carbohydrate content (70–80%), primarily fructose and glucose, which are readily absorbed by the body [
3]. Many investigations into the functional qualities of dates and their byproducts have been conducted. Studies have discovered that the amount of dietary fiber in 13 kinds of date palms ranges from 8.1% to 12.7% (dry matter) [
4].
Dates are one of the most important fruits in the Kingdom of Saudi Arabia for historical, economic, and consumption purposes. Powders have been introduced into many foods in recent periods, such as pastries, juices, and flavored milks. Dates contain many nutrients, such as vitamins, minerals, sugars, and fiber, hence their importance in the production of powders.
Chocolate-flavored milk is one of the most common and popular milk drinks and is the preferred drink by 90% of children [
5]. Chocolate-flavored milk has also been widely popular among consumers worldwide [
6]. Many milk drinks flavored with chocolate, vanilla, strawberries, and bananas have been developed to feed military personnel [
7]. Date powders contain many nutrients, such as vitamins, minerals, sugars, and fiber, hence their importance. Consumers today tend to choose beverages for their nutritional, functional, and sensory attributes, such as color, aroma, and taste, and the safety of healthy beverages [
8].
One of the best ways to destroy undesirable germs in liquid meals is with thermal pasteurization. Liquid foods that have been pasteurized are safer and have a longer shelf life. The most popular technique for prolonging the shelf life of vegetable and fruit juices is with conventional thermal pasteurization, which uses a mathematical formula to ensure the safety of the final product [
8]. Theoretically, this is a combination of the time–temperature profile and microbial destruction/inactivation. The design of the thermal processing is usually chosen to maximize microbial inactivation with a minimal impact on the product quality [
9]. Apple juice is traditionally thermally pasteurized using plate heat exchangers and tunnel pasteurizers. Thermal pasteurization can effectively extend the shelf life of apple juice [
10]. The contamination of beverages with microbes could take place during the handling of the beverage by operatives. The pathogens commonly found on human skin and tools can be transferred to foods if good hygienic practices are not followed.
Staphylococcus aureus is a gram-positive coccus that produces clusters like grapes. This bacterium is mesophilic, catalase-positive, oxidase-negative, and facultatively anaerobic. It grows at temperatures between 7 °C and 48 °C and pH levels between 4.2 and 9.3. It is distinguished by its halotolerance, corresponding to minimum water activity (a
w) values between 0.83 and 0.85 [
11,
12]. Strains of these bacteria are distinguished by their ability to create heat-stable enterotoxins with a
w values as low as 0.86 under aerobic circumstances.
It is possible to destroy undesirable, harmful, and spoilage microorganisms in food using pulsed electric fields (PEF) technology. Without significantly compromising their nutritional qualities, PEF treatment is effective enough to kill microorganisms in fruit juices at levels comparable to those attained via heat pasteurization [
13]. The inactivation of pathogenic bacteria and the decrease in spoilage microorganisms in milk have demonstrated the benefits of using pulsed electric fields (PEF) technology [
14,
15]. Additionally, it might be possible to prolong the shelf life of milk with PEF by preventing mesophilic bacteria from growing, while very slightly compromising the product’s quality [
16]. A fluid medium that is placed between two electrodes is subjected to high voltage pulses in the pulsed electric field (PEF), which is mostly a non-thermal process [
17].
Therefore, this current study was designed to examine the survival and growth potential of total viable counts (TVC), pH, TSS, and color difference (ΔE) in a stored beverage from milk-based date powder. This study will help to understand the effect of the thermal pasteurization process and non-thermal pasteurization using the pulsed electric fields (PEF) of beverage, and the ratio of date powder to milk on the growth of total counts in a beverage from milk-based date powder without adding any preservatives during shelf life.
3. Results and Discussion
The results for the pH values while in storage are shown in
Figure 1a–e for the milk-based beverages made with date powder. Higher percentages of powder led to declines in the pH of the untreated and treated beverage samples. We observed that the decline rates for the pH values of the untreated beverages and those treated with thermal pasteurization (
Figure 1a,b) were greater than the beverages treated with the pulsed electric field, especially at 80 pulses after 6 days (
Figure 1e). The decline rates for the untreated samples were 11.06, 11.58, 12.68, and 11.94, with powder ratios of 10, 15, 20, and 25% (
w/
w), respectively (
Figure 1a). The decline rates for the thermally pasteurized samples (
Figure 1b) were 6.67, 8.45, 10.72, and 14.6, with powder ratios of 10, 15, 20, and 25% (
w/
w), respectively. The decline rates were recorded at 6.59, 8.2, 7.74, and 3.97 for the samples treated with the pulsed electric field (
Figure 1e) at 80 pulses after 6 days, with powder ratios of 10, 15, 20, and 25% (
w/
w), respectively. Samples treated with the PEF at 80 pulses showed a slower decrease in pH compared to samples that were untreated, thermal-treated, and treated with the PEF at 20 and 50 pulses. The independent variable (number of pulses) had a positive impact on the pH, i.e., the pH increased with an increase in the number of pulses (
Table 1). The number of pulses exhibited significant influences on the pH (
p < 0.001), while the powder ratio and storage time had a negative effect on the pH (
p < 0.001), suggesting that increasing the powder ratio and storage time could decrease the pH of the milk–date powder beverage. The storage time had a negative effect on the pH for thermal-treated samples (
p < 0.05), i.e., the pH decreased with an increase in storage time. The reason for this drop could be the growth of lactobacillus and the production of lactic acid in the milk, in addition to other possible chemical reactions. A similar trend of changes in the pH was noted by Mnkeni and Nyaruhucha, Kamaly, and Niimi and Shiokawa [
22,
23,
24]. pH is one of the parameters that affect the survival and development of microorganisms during processing and storage [
25]. Food processors are interested in determining the pH of a food product and maintaining that pH at a specific level to control microbial development and prevent product degradation [
26]. Therefore, the importance of using the pulsed electric field compared to thermal pasteurization for maintaining the pH at the lowest possible rate of change until the end of the shelf life is clear.
The following equation was developed from the results of the pulsed electric field treatment to predict the effects of the number of pulses, powder ratio, and storage time on the pH of milk–date powder beverages:
where ‘
x1’ indicates the number of pulses, ‘
x2’ is the powder ratios, and ‘
x3’ is the storage time.
The results for the TSS values during storage are shown in
Figure 2 for the milk-based beverages made with date powder at different powder ratios and number of pulses. Higher percentages of powder led to increases in the TSS of the untreated and treated beverage samples (
Figure 2a–e). The TSS of the untreated and treated beverage samples decreased with storage time. This may be due to the consumption of some sugars by microbes during the storage period, and this is shown by the increase in microbes over time. We observed that the decline rates for the TSS of the untreated beverages and those treated with thermal pasteurization were greater than those of the beverages treated with the pulsed electric field at all levels, especially at 80 pulses after 6 days. The decline rates for the untreated samples were 16.7, 21.2, 21.7, and 20.8, with powder ratios of 10, 15, 20, and 25% (
w/
w), respectively (
Figure 2a). The decline rates for the thermally pasteurized samples were 17.3, 13.95, 12.09, and 12.75, with powder ratios of 10, 15, 20, and 25% (
w/
w), respectively (
Figure 2b). The decline rates were recorded at 13.7, 12.4, 5.907, and 3.71 for the samples treated with the pulsed electric field at 80 pulses after 6 days, with powder ratios of 10, 15, 20, and 25% (
w/
w), respectively (
Figure 2e). The independent variables (number of pulses and storage time) negatively impacted the TSS, i.e., the TSS decreased with an increase in the number of pulses and storage time (
Table 1). The number of pulses exhibited a non-significant influence on the TSS, but the storage time exhibited significant influences on the TSS (
p < 0.001). The powder ratio had a positive effect on the TSS (
p < 0.001), suggesting that increasing the powder ratio could increase the TSS of the milk–date powder beverage by raising the concentration of sugars and thus total solids. The interaction effect of the number of pulses with the powder ratio positively impacted the TSS (
p < 0.05), implying that increases in the number of pulses and the powder ratio could elevate the TSS of the beverage. In terms of the quadratic effect, the number of pulses showed a negative impact on the TSS (
p < 0.05), implying that a rise in this parameter would lead to a decrease in the beverage’s TSS.
Storage time had a negative effect on the TSS for thermal-treated samples (p < 0.001), i.e., the TSS decreased with an increase in storage time. In contrast, the powder ratio had a positive effect on the TSS (p < 0.001), suggesting that increasing the powder ratio could increase the TSS of the milk–date powder beverage. The interaction effects of the powder ratio with storage time negatively impacted the TSS (p < 0.05), implying that decreasing the powder ratios with storage time could elevate the TSS of the beverage. In terms of the quadratic effect, storage time negatively impacted the TSS (p < 0.01), implying that a rise in this parameter would decrease the beverage’s TSS, while the powder ratio positively impacted the TSS (p < 0.001).
In agreement with these findings, ref. [
27,
28] observed a small reduction in the TSS values following the PEF treatment of orange and orange–carrot juice, respectively. The fermentation of carbohydrates by microorganisms could have produced this phenomenon. Increasing the powder ratio brought the beverage’s TSS to its highest level at 25% (
w/
w).
The following equation was developed from the results of the pulsed electric field treatment to predict the effects of the number of pulses, powder ratio, and storage time on the TSS of milk–date powder beverages:
The results for the Δ
E values during storage are shown in
Figure 3 for the milk-based beverages made with date powder at different powder ratios and number of pulses. Higher percentages of powder increased the Δ
E of the beverage for untreated and treated samples (
Figure 3a–e). The Δ
E of the untreated beverage samples decreased with storage time (
Figure 3a), while the Δ
E of the treated beverage samples increased with storage time. We observed that the rate of increase for Δ
E of the beverages treated with thermal pasteurization (
Figure 3b) was lower than that of the beverages treated with the pulsed electric field at all pulse levels (
Figure 3c–e). The decline rates for the untreated samples were 1.4, 4.25, 6.67, and 6.47, with powder ratios of 10, 15, 20, and 25% (
w/
w), respectively. The rates of increase for the thermally pasteurized samples were 0.52, 2.46, 4.33, and 4.14, with powder ratios of 10, 15, 20, and 25% (
w/
w), respectively. The rates of increase were recorded at 5.26, 3.64, 7.98, and 10.5 for the samples treated with the pulsed electric field at 80 pulses after 6 days, with powder ratios of 10, 15, 20, and 25% (
w/
w), respectively.
The starting values for the ΔE for untreated samples were 13.82, 16.36, 17.9, and 18.52 at powder ratios of 10, 15, 20, and 25% (w/w), respectively. On the other hand, the starting values for the ΔE of samples treated with thermal pasteurization were 45.65, 47.13, 48.1, and 48.55, with powder ratios of 10, 15, 20, and 25% (w/w), respectively. The starting values for the ΔE of the samples treated with the pulsed electric field were 17.86, 21.99, 25.7, and 29.9, with powder ratios of 10, 15, 20, and 25% (w/w), respectively, for 80 pulses.
From the above results, we found a significant change in the ΔE values of the heat-treated samples compared to the untreated samples and those treated with a pulsed electric field, and this may be due to the high temperature during the pasteurization process, which affected the color change significantly.
The independent variables (number of pulses, powder ratio, and storage time) had a positive impact on the Δ
E, i.e., the Δ
E increased with an increase in the number of pulses, powder ratio, and storage time (
Table 1). The number of pulses, powder ratio, and storage time exhibited significant influences on the Δ
E (
p < 0.001), suggesting that increasing the number of pulses, powder ratio, and storage time could increase the Δ
E of milk–date powder beverages. The decrease in
L* and
b* values indicated that the beverage color became darker. Interestingly, noticeable increases in
a* values indicated an increase in redness [
29] and thus a decrease in the Δ
E. The interaction effect of the number of pulses with the powder ratio and the powder ratio with storage time negatively impacted the Δ
E (
p < 0.01 and
p < 0.05, respectively), implying that an increased number of pulses with the powder ratio and the powder ratio with storage time could decrease the Δ
E of the beverage. A significant rise in the Δ
E was seen after 15 days of storage in a study by [
30] on fresh carrot juice treated in a high-voltage electrostatic field and then kept at 4 °C, which is broadly consistent with the results of this study. This may be caused by the PEF’s impact on the milk proteins. After synthesizing and binding the dye to the protein, the electric field may break this link, potentially causing the loss of color [
5].
The powder ratio and storage time had a positive effect on the Δ
E for thermal-treated samples (
p < 0.001), i.e., the Δ
E increased with an increase in the powder ratio and storage time [
31]. The interaction effects of the powder ratio with storage time positively impacted the Δ
E (
p < 0.01), implying that an increased powder ratio with storage time could elevate the Δ
E of the beverage. In terms of the quadratic effect, powder ratio and storage time negatively impacted the Δ
E (
p < 0.001), implying that a rise in these parameters would decrease the beverage Δ
E.
The following equation was developed from the results of the pulsed electric field treatment on the pH and TSS and the low Δ
E values of the pulsed electric field compared to thermal pasteurization. The equation can be used to predict the effects of the number of pulses, powder ratio, and storage time on the Δ
E of milk–date powder beverages:
Figure 4 shows the total viable count (TVC) of untreated and thermal-treated samples as affected by the powder ratios and storage time as well as the samples treated with a pulsed electric field as affected by the number of pulses, powder ratios, and storage time. The TVC decreased by increasing the powder ratio (as shown in
Figure 4a) for untreated beverages, in which the values for the TVC decreased from 6.2 to 3.94 log10 CFU/mL when the powder ratios increased from 10 to 25% (
w/
w) after 6 days. This could be due to the increase in the concentration of sugars and thus total soluble solids, which also affects microbial growth. The increase in storage time led to an increase in the TVC for untreated samples from 4.11 to 6.2 log10 CFU/mL and from 2.82 to 3.94 log10 CFU/mL, with powder ratios of 10 and 25% (
w/
w) after 6 days, respectively.
Figure 4b shows the TVC as affected by powder ratios and storage time. The TVC decreased by increasing the powder ratio for thermal-treated samples, where the values for the TVC decreased from 4.021 to 3.314 log10 CFU/mL when the powder ratios increased from 10 to 25% (
w/
w) after 6 days. The increase in storage time led to an increase in the TVC for thermal-treated samples from 2.06 to 4.021 log10 CFU/mL and from 0.123 to 3.314 log10 CFU/mL for powder ratios 10 and 25% (
w/
w) after 6 days, respectively.
Figure 4c–e shows the TVC as affected by the number of pulses, powder ratios, and storage time. The TVC decreased by increasing the powder ratios for beverages treated with the pulsed electric field. The values for the TVC decreased from 5.2 to 3.69 log10 CFU/mL, 3.99 to 3.01 log10 CFU/mL, and from 1.52 to 0.512 log10 CFU/mL, with the number of pulses at 20, 50, and 80 pulses, respectively, when the powder ratios increased from 10 to 25% (
w/
w) at the end of the shelf-life period of 6 days. The increase in storage time led to an increase in the TVC, as shown in
Figure 4c–e. From the previous results, it is evident that the TVC reached 1.527, 1.108, 0.665, and 0.512 log10 CFU/mL with powder ratios of 10, 15, 20, and 25% (
w/
w), respectively, at 80 pulses at the end of the shelf-life period of 6 days.
The storage time positively affected the TVC for thermal-treated samples (p < 0.001), i.e., the TVC increased with an increase in storage time, while the powder ratio negatively affected the TVC (p < 0.05), suggesting that increasing the powder ratio could decrease the TVC of milk–date powder beverages. The interaction effects of the powder ratio with storage time positively impacted the TVC (p < 0.05), implying that an increased powder ratio with storage time could elevate the TVC of the beverage. In terms of the quadratic effect, storage time negatively impacted the TVC (p < 0.05), implying that a rise in this parameter would decrease the beverage TVC, while the powder ratio positively impacted the TVC (p < 0.05).
The independent variables (number of pulses and storage time) positively impacted the TVC, i.e., the TVC increased with an increase in the number of pulses and storage time. The number of pulses and storage time positively influenced the TVC (
p < 0.001). The powder ratio negatively affected the TVC (
p < 0.001), suggesting that decreasing the powder ratio could increase the TVC of milk–date powder beverages. This could be due to the decrease in the concentration of sugars and thus total soluble solids, which also affects microbial growth. The interaction effect of the number of pulses with the powder ratio positively impacted the TVC (
p < 0.01), implying that the increased number of pulses with the powder ratio could elevate the TVC of the beverage. The interaction effect of the number of pulses with storage time had negative impacts on the TVC (
p < 0.01), implying that a decreased number of pulses with the powder ratio could elevate the TVC of the beverage. In terms of the quadratic effect, the number of pulses and storage time negatively impacted the TVC (
p < 0.001), implying that a rise in this parameter would decrease the beverage’s TVC. The powder ratios positively impacted the TVC (
p < 0.05), implying that the increased powder ratio could elevate the TVC of the beverage. Microorganism inactivation is associated with alterations in and the electromechanical instability of the cell membrane [
32]. The primary impact of an electrical field is an increase in the permeability of the membrane due to membrane compression and pore formation. Cell inactivation is believed to be caused by unusual membrane porosity [
33]. According to the research described by [
34], cell surfaces become rougher as the electric field intensity rises. The pulsed field may generate an electrical–mechanical compression that results in a separation of charges between the interior and exterior of microbial cells in milk, which could be one mechanism for the breakdown of the cell wall. Small holes in the membrane may then result from this separation, allowing the contents of the cell to flow out. Because increasing electrical–mechanical compression causes more cell wall disintegration, an increase in field intensity may, therefore, enhance microbial inactivation.
The following equation was developed from the results of the pulsed electric field treatment to predict the effects of the number of pulses, powder ratio, and storage time on the TVC of milk–date powder beverages: