2024 Volume 30 Issue 1 Pages 107-115
Date fruits supply macronutrients and micronutrients, particularly carbohydrates, amino acids, and minerals. However, the fruits of different date palm cultivars have different nutritional profiles. The aim of this study was to investigate the effects of different pasteurization methods on the antioxidant capacities, polyphenol profiles, and physical properties of the juice of date fruits from two date palm cultivars. The total phenolic and total flavonoid compound contents were higher after thermal pasteurization than after high-pressure processing (HPP) or for the fresh juice. The antioxidant capacity was higher for Khunaizi date palm juice than Barhi date palm juice. SDS-PAGE was performed on the date palm juice samples, and it was found that the juice subjected to HPP retained protein bands but that thermal pasteurization caused the protein bands to disappear. HPP allowed the colour and protein content of the juice to be retained but thermal pasteurization increased the antioxidant capacity and polyphenol content.
Date palm (Phoenix dactylifera L.) is currently grown in large amounts in Thailand, particularly in Kanchanaburi, Lamphun, and Suphanburi provinces. Date palm fruit has recently started to be cultivated as a cash crop in Thailand because of increasing domestic demand, and the area used to cultivate date palms was 66.78 % higher in 2020–2021 than in previous yearsi). Date palm fruit has become popular in Thailand because it is a good source of many essential nutrients, phenolic compounds, flavonoid compounds, and antioxidants (Al-Dashti et al., 2021; Amira et al., 2012; Chaira et al., 2009). Barhi date fruit has oval round shape, hard and crisp texture with yellow-gold colour, while Khunaizi has long oval shape, hard and crisp texture with dark red-brown colour. Date palm fruit juice is a good source of nutrients and energy for many people, including active people (e.g., athletes and labourers) and pregnant women. Consuming date palm fruit or juice increases the concentration of haemoglobin in the blood, so can prevent mild anaemia in pregnant women (Saputri et al., 2021). Date fruit can have a total carbohydrate content of up to 80 % and is a rich source of soluble sugars (glucose and fructose) and dietary fibre. Date fruit has a total sugar content of 50 %–60 % and contains high concentrations of amino acids such as aspartic acid, glutamic acid, and lysine and of minerals such as calcium, iron, manganese, potassium, sodium, and zinc (Hussain et al., 2020). Moreover, dates are rich in carotenoids, anthocyanins, tannins, and isoflavones (Hussain et al., 2020). Date palm fruit might be a useful source of antioxidants.
High-pressure processing (HPP) is a non-thermal process that has been used to preserve the juice of a number of fruits both at the laboratory and commercial scales because it can deactivate microbes without decreasing the nutritional or functional qualities of the juice (Ferreira et al., 2022; Stinco et al., 2019). Conventional thermal pasteurization of fruit and vegetable juice involves applying heat, which affects the colour, bioactive compound contents, taste, and nutritional value of the juice. The ascorbic acid, total phenol, and antioxidant contents of sugarcane juice are retained better by HPP than thermal processing (Sreedevi et al., 2020). HPP has been found to allow the flavonoid and vitamin C contents of cara cara (a type of blood orange rich in anthocyanins) juice to be retained (Ancos et al., 2020).
The aim of this study was to investigate the effects of HPP at 400 and 600 MPa on the physicochemical properties, total phenolic and flavonoid compound contents, antioxidant contents, and soluble protein profiles of date palm juice samples from two date palm cultivars that are popular in Thailand and to compare the results with the results for fresh juice and juice subjected to conventional thermal pasteurization.
Chemicals and reagents Folin-Ciocalteu reagent, sodium carbonate, gallic acid, sodium nitrite, aluminium chloride, sodium hydroxide, quercetin, 2,2-diphenyl-1-picrylhydrazyl (DPPH), trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), glacial acetic acid, 2,3,5-triphenyltetrazolium chloride, ferric chloride hexahydrate (FeCl3·6H2O), 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), fluorescein, 2,2′-azobis(2-amidinopropane) dihydrochloride, sodium dodecyl sulfate (SDS), glycerol, β-mercaptoethanol, bromophenol blue, Tris-HCl buffer, acrylamide, glycine, and coomassie brilliant blue R250 were purchased from Sigma-Aldrich Co. (St. Louis, MO, USA). All chemicals and reagents were analytical grade.
Date palm fruit Fully mature and ripe date palm fruits from the Barhi and Khunaizi cultivars were collected from a farm in Suphanburi Province in Thailand between June and August in 2022 and were transported to the laboratory at 4 °C.
Preparing date palm juice samples Date palm fruits were hand-washed with distilled water and then the seeds were removed and the flesh was chopped using a sharp stainless steel knife. The flesh was then mixed with milli-Q water at a ratio of 1:1 (w/w) and the mixture was homogenized using a household blender for 3 min. The date palm pulp was then passed through a 100-mesh cloth filter to give a date palm fruit juice sample. The juice was immediately packed in 150 mL portions with no headspace and kept at −20 °C until use.
Date palm juice processing and storage A 150 mL aliquot of date palm juice was transferred to a polyethylene terephthalate bottle for HPP. HPP treatments at 400 and 600 MPa and 25 °C for 10 min, which commonly used by food industry to inactivate most of pathogenic microorganism at room temperature (Silva and Evelyn, 2023; Koo et al., 2023), were performed using in a 5 L high-pressure chamber (BaoTou KeFa High-pressure Technology, China). The HPP machine in this study had instantaneous depressurization.
A 150 mL aliquot of date palm juice was placed in a beaker and covered with aluminium foil for thermal pasteurization. The sample was then heated to 100 °C for 1 min, which modified from the study of Liu et al. (2016) and Wang et al. (2018), to maintain the colour, inactivate microorganism growth, and inhibit enzymes. The beaker was then immersed in ice water until the temperature of the juice reached 25 °C, then the juice was transferred to a 150 mL polyethylene terephthalate bottle and stored at −20 °C for analysis. Each test was performed in triplicate and the physical and chemical properties of each date palm juice sample were determined using triplicate aliquots of the sample.
pH and total soluble solids The pH of each juice sample at room temperature was determined using a Seven2Go S7-Field Kit pH meter (Mettler-Toledo, Greifensee, Switzerland). The total soluble solid concentration in each sample was determined using an MA871 digital brix refractometer (Milwaukee Instruments, Rocky Mount, NC, USA).
Colour The colour was determined using methods previously described by Szczepańska et al. (2020). The colour was determined using a colourimeter in reflectance mode at the ambient temperature. The colour was expressed in terms of lightness (L), redness (a), and yellowness (b). The total colour difference (AE) and hue angle were calculated using Eq. (1) and (2).
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where L* represents lightness, a* is red-green values, b* is yellow-blue value, and the variables with subscript ‘0’ represents the initial values of untreated juices.
Turbidity Turbidity was determined using a method described by Wu et al. (2021) with some modifications (Wu et al., 2021). Briefly, 25 mL of date palm juice was transferred to a quartz cuvette, then absorbance at 660 nm was determined using an ultraviolet-visible light spectrophotometer.
Total phenolic compound content The total phenolic compound content was determined using the Folin-Ciocalteu reagent and a method described by Patthamakanokporn et al. (2008). Briefly, 25 μL of a sample was mixed with 50 μL of Folin-Ciocalteu reagent (10 % v/v) and 200 μL of sodium carbonate (7.5 % w/v). Absorbance at 765 nm was then determined using a BioTek Synergy H1 96-well microplate reader (Agilent Technologies, Santa Clara, CA, USA). The standard was gallic acid. Each sample was analysed in triplicate. The total phenolic compound content was expressed as microgrammes of gallic acid equivalent (GAE) per millilitre.
Total flavonoid content The total flavonoid content was determined using a method described by Jingyun et al. (2018) with some modifications (Jingyun et al., 2018). Briefly, 165 μL of a sample, 9 μL of sodium nitrite (5 % w/v), 18 μL of aluminium chloride (10 % w/v), and sodium hydroxide (1M) were mixed, then absorbance at a wavelength of 510 nm was determined using a 96-well microplate reader. The standard was quercetin. Each sample was analysed in triplicate. The total flavonoid content was expressed in microgrammes of quercetin equivalent (QE) per millilitre.
2, 2-Diphenyl-1-picrylhydrazyl assay The 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging method described by Fan et al. (2019) with some modifications was used. A 22 μL aliquot of a sample was mixed with freshly prepared DPPH (150 μM, 200 μL), which was used as an indicator of the free radical scavenging activity. The mixture was incubated for 30 min in the dark at room temperature. Absorbance at 517 nm was then determined using a 96-well microplate reader. The standard was trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid). The DPPH value was expressed in micromoles of trolox equivalent (TE) per 100 mL.
Ferric-reducing antioxidant power assay The ferric-reducing antioxidant power (FRAP) assay was performed using a method described by Patthamakanokporn et al. (2008) with some modifications. The FRAP reagent was prepared by mixing acetate buffer (300 mM, pH 3.6), 2,3,5-triphenyltetrazolium chloride solution (40 nM), and FeCl3·6H2O solution (20 mM) at a ratio of 10:1:1. The assay was performed at 37 °C. A 20 μL aliquot of a sample was mixed with 150 μL of the FRAP reagent and the mixture was incubated in the dark for 8 min. Absorbance at 593 nm was determined using a 96-well microplate reader. The standard was trolox. The FRAP value was expressed in micromoles of TE per 100 mL.
2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid assay The 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) assay described by Fan et al. (2019) with some modifications was used. A 50 μL aliquot of a sample was mixed with ABTS (7 mM, 150 μL), which was used as an indicator of the free radical scavenging activity. The mixture was incubated for 5 min in the dark at room temperature. Absorbance at 734 nm was then determined using a 96-well microplate reader. The standard was trolox. The ABTS value was expressed in micromoles of TE per 100 mL.
Oxygen radical absorbance capacity assay The oxygen radical absorbance capacity (ORAC) assay described by Ou et al. (2002) with some modifications was used. A 25 μL aliquot of a sample was mixed with 150 μL of a fluorescein solution (1.14 mM), then the mixture was incubated at 37 °C for 15 min. A 25 μL aliquot of 2,2′-azobis(2-amidinopropane) dihydrochloride (153 mM) was then quickly added to the mixture to start the reaction. The fluorescence intensity at an excitation wavelength of 490 nm and an emission wavelength of 514 nm was monitored for 90 min using a 96-well microplate reader. The results were calculated from the area under the sodium fluorescein decay curve (AUC) using the equation
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where RFU0 is the relative fluorescence value at 0 min, RFUi is the relative fluorescence value at i min. The standard was trolox. The ORAC value was expressed in micromoles of TE per 100 mL.
Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis 7.5 μL of the samples were mixed with 7.5 μL of 2X SDS sample loading buffer which consisted of 1 % SDS, 20 % glycerol, 1 mol/L β-mercaptoethanol, and 0.5 % bromophenol blue in 6.25 mmol/L Tris-HCl buffer, pH 6.8 and heated for 2 min in boiling water. A 12 % separating gel and 4 % acrylamide stacking were prepared follow the method of Laemmli (1970). Then 15 μL of pre-stained SDS-PAGE was loaded in each gel run. Electrophoresis was performed using a constant voltage (100 V) for 2.30 h at room temperature (Mini-PROTEAN® Tetra System, BIO-RAD, Philadelphia, USA) in 1X solution running buffer (Tris-HCl 25 mM, glycine 200 mM, SDS 0.1 % w/v). Coomassie brilliant blue R250 was used to stain the gel protein for 30 min and de-stained overnight. The molecular weight of each protein band was determined using a prestained protein ladder (BIO-HELIX Co., Ltd. New Taipei City, Taiwan) with molecular weights from 10 to 180 kDa. The ChemiDoc MP Imaging System (BIO-RAD) was used for imaging of gels and Image Lab Software (BIO-RAD) was used for band quantity calculation.
Microbial analysis After HPP and thermal processing, the date palm fruit juices were stored at 4 °C for 15 days. The samples at day 0, 3, 6, 9, 12, and 15 were evaluated. Date palm juice was serially diluted in sterile 0.1 % peptone water and 0.1 mL of each dilution. It was plated onto duplicate plates (for spread-plated) with the appropriate agar (Yuan et al., 2018) for the growth of aerobic microorganism (incubated at 37 °C for 48 h), coliforms (incubated at 37 °C for 24 h), yeasts and molds (incubated at room temperature, 30 °C for 72 h). After incubation, the colonies were counted. The minimum detection level was 1 CFU/mL.
Statistical analysis Statistical analyses were performed using SPSS software for Windows (trial version) (IBM, Armonk, NY, USA). Significant differences (p < 0.05) between samples were identified by performing one-way analyses of variance and Duncan's multiple-range tests.
pH, total soluble solid content, and turbidity The physicochemical properties of the date palm juice samples are shown in Table 1. The pH values of the Barhi and Khunaizi date palm juice samples were 6.53–6.82 and 6.71–6.95, respectively. The total soluble solid contents of the Barhi and Khunaizi date palm juice samples were ∼15 °Brix. The turbidity is an important quality of fruit juice and affects the stability of the juice and acceptance by the consumer. The thermal processing treatment obviously significantly decreased the turbidity of the date palm juice (0.99) (p < 0.05). The turbidity of the date palm juice treated with HPP and untreated ranged from 2.09 to 2.16. The turbidity was significantly lower for thermally pasteurized date palm juice than fresh date palm juice or date palm juice subjected to HPP (p < 0.05). The different turbidities of the different juice samples could be seen with the naked eye. Wu et al. (2021) found that heating decreased the turbidity of pineapple juice by deactivating pectin methylesterase or pectin esterase (Wu et al., 2021). However, HPP does not destroy these enzymes. This explains why the turbidity was higher for untreated date palm juice or date palm juice subjected to HPP than for thermally pasteurized date palm juice.
| pH | TSS (°Brix) | Turbidity | |
|---|---|---|---|
| Barhi | |||
| Fresh juice | 6.53c ± 0.01 | 15.63a ± 0.06 | 2.16a ± 0.18 |
| 400 MPa | 6.78b ± 0.01 | 15.60a ± 0.00 | 2.13b ± 0.00 |
| 600 MPa | 6.82a ± 0.45 | 15.17c ± 0.06 | 2.09c ± 0.12 |
| Heat | 6.74b ± 0.01 | 15.47b ± 0.12 | 0.99d ± 0.01 |
| Khunaizi | |||
| Fresh juice | 6.71d ± 0.01 | 15.67ab ± 0.06 | 2.02b ± 0.18 |
| 400 MPa | 6.87b ± 0.02 | 15.76a ± 0.12 | 2.18a ± 0.00 |
| 600 MPa | 6.95a ± 0.01 | 15.70ab ± 0.17 | 1.91c ± 0.12 |
| Heat | 6.84c ± 0.01 | 15.50b ± 0.10 | 0.10d ± 0.01 |
Data are expressed as the mean ± SD. Different letters in the same column indicate significant differences (p < 0.05).
Colour The colour analysis results for the date palm juice samples are shown in Table 2. Barhi date palm fruit are light brown or amber in the rutab (ripe) state, which is a popular state in which date palm fruit are consumed in Thailand, and Khunaizi date palm fruit are red to dark brown in the rutab (ripe) state. The yellowness value was therefore higher for the fresh Barhi date palm juice (39.96) than the fresh Khunaizi date palm juice (30.92). The redness value was higher for the fresh Khunaizi date palm juice (5.23) than the fresh Barhi date palm juice (3.56).
| Fresh juice | HPP 400 MPa | HPP 600 MPa | Heat | |
|---|---|---|---|---|
| Barhi | ||||
| L* | 63.87d ± 0.10 | 65.92b ± 0.04 | 65.60c ± 0.01 | 83.13a ± 0.02 |
| a* | 3.56c ± 0.02 | 3.65b ± 0.01 | 3.85a ± 0.02 | 0.02d ± 0.01 |
| b* | 39.96a ± 0.06 | 39.62b ± 0.01 | 39.54c ± 0.02 | 27.95d ± 0.02 |
| ΔE | - | 2.18b ± 0.04 | 1.90c ± 0.01 | 23.07a ± 0.01 |
| H° | 84.92b ± 0.02 | 84.74c ± 0.02 | 84.44d ± 0.02 | 89.96a ± 0.02 |
| Khunaizi | ||||
| L* | 61.58c ± 0.02 | 59.26d ± 0.02 | 67.87b ± 0.08 | 91.86a ± 0.01 |
| a* | 5.23b ± 0.01 | 5.79a ± 0.01 | 4.69c ± 0.03 | −1.82d ± 0.01 |
| b* | 30.92c ± 0.06 | 32.76a ± 0.02 | 31.05b ± 0.01 | 18.49d ± 0.02 |
| ΔE | - | 2.96c ± 0.03 | 6.33b ± 0.08 | 33.53a ± 0.01 |
| H° | 80.39c ± 0.03 | 79.97d ± 0.01 | 81.41b ± 0.05 | 95.63a ± 0.03 |
Data are expressed as the mean ± SD. Different letters in the same column indicate significant differences (p < 0.05).
Heating markedly changed the colour of the date palm juice samples. For both the Barhi and Khunaizi date palm juice samples, the lightness values were significantly higher for the thermally pasteurized juice than the fresh juice and juice that had been subjected to HPP (p < 0.05). Thermal pasteurization also significantly decreased the redness and yellowness values (p < 0.05). These changes were related to heating decreasing the turbidity (i.e., increasing clarity). Similar results were found in a study of asparagus juice that had been subjected to thermal pasteurization and HPP in which the lightness value was higher after thermal treatment at 121 °C for 3 min than after HPP at 200–600 MPa for 10–20 min (Chen et al., 2015).
The lightness, redness, and yellowness values and the hue angles of the fresh juice and juice subjected to HPP were similar. The total colour difference AE was very low for juice subjected to HPP. This suggested that HPP preserved the colour of the juice well.
Total phenolic and total flavonoid compound contents The total phenolic and total flavonoid compound contents are shown in Figs. 1 and 2. The total phenolic and total flavonoid compound contents of the thermally pasteurized Barhi date palm juice were 326.15 (µg GAE)/mL and 322.38 (µg quercetin)/mL, respectively. The total phenolic and total flavonoid compound contents of the thermally pasteurized Khunaizi date palm juice were 500.55 (µg GAE)/mL and 802.07 (µg quercetin)/mL, respectively. The total phenolic and total flavonoid compound contents were higher after thermal pasteurization than of fresh juice or after HPP. This may have been because phenolic and flavonoid compounds were extracted better and became more soluble during thermal pasteurized at 100 °C for 1 min than during HPP. Stübler et al. (2020) found that thermal treatment extracted polyphenolic compounds from fruit juice but HPP did not affect the polyphenolic compound content of strawberry juice (Stübler et al., 2020). Papoutsis et al. (2018) found that the phenolic and flavonoid compound content of lemons was significantly increased by extracting the lemons with water at 95 °C for 15 min and attributed this to the hot water increasing the solubility of phenolic and flavonoid compounds and causing the compounds to more readily diffuse from the plant matrix to the solvent (Papoutsis et al., 2018). In our study, the total phenolic and total flavonoid compound contents of fresh Barhi date palm juice [283.7 (µg GAE)/mL and 305.61 (µg quercetin)/mL, respectively] and of Barhi date palm juice after HPP at 600 MPa for 10 min [281.1 (µg GAE)/mL and 304.3 (µg quercetin)/mL, respectively] were not significantly different. The total phenolic and total flavonoid compound contents were significantly higher for Khunaizi date palm juice after HPP at 600 MPa [468.09 (µg GAE)/mL and 588.37 (µg quercetin)/mL, respectively] than fresh Khunaizi date palm juice [330.94 (µg GAE)/mL and 533.02 (µg quercetin)/mL, respectively].
Total phenolic compound contents of fresh (Untreated), high-pressure processed (at 400 and 600 MPa; HPP 400 and HPP 600, respectively), and thermally pasteurized (Heat) date palm juice from two cultivars [Barhi (A) and Khunaizi (B)].
Data are expressed as the mean. Different letters in the same column indicate significant differences (p < 0.05).
Total flavonoid content of fresh (Untreated), high-pressure processed (at 400 and 600 MPa; HPP 400 and HPP 600, respectively), and thermally pasteurized (Heat) date palm juice from two cultivars [Barhi (A) and Khunaizi (B)]. Data are expressed as the mean. Different letters in the same column indicate significant differences (p < 0.05).
Antioxidant capacity The effects of HPP and thermal processing on the antioxidant capacities of the date palm juice samples were investigated using four methods (DPPH, FRAP, ABTS, and ORAC). The antioxidant capacities of the date palm juice samples determined using these methods are shown in Table 3. The antioxidant capacity was higher for Khunaizi date palm juice than Barhi date palm juice. A higher total phenolic compound content and a higher antioxidant capacity were also previously found using the FRAP method for Khunaizi date palm fruit than Barhi date palm fruit (Allaith et al., 2008).
| μmol trolox equivalent/100 mL | ||||
|---|---|---|---|---|
| DPPH | FRAP | ABTS | ORAC | |
| Barhi | ||||
| Fresh juice | 372.80b ± 24.2 | 554.56c ± 18.3 | 835.90b ± 14.8 | 2 070.34b ± 57.3 |
| HPP 400 MPa | 178.59c ± 16.6 | 225.74d ± 14.9 | 326.60d ± 31.4 | 153.68d ± 12.7 |
| HPP 600 MPa | 380.19b ± 13.3 | 800.53a ± 12.6 | 763.97c ± 49.3 | 1 391.66c ± 123.7 |
| Heat | 419.69a ± 21.8 | 673.90b ± 39.6 | 907.08a ± 45.2 | 2 718.43a ± 264.0 |
| Khunaizi | ||||
| Fresh juice | 476.80c ± 32.7 | 759.88c ± 12.6 | 953.54c ± 80.2 | 2 211.29c ± 123.3 |
| HPP 400 MPa | 439.82c ± 31.8 | 711.42c ± 74.4 | 871.78c ± 29.6 | 2 145.68c ± 129.6 |
| HPP 600 MPa | 594.99b ± 64.0 | 1 151.23b ± 45.1 | 1 216.23b ± 36.2 | 2 823.20b ± 123.8 |
| Heat | 783.94a ± 66.0 | 1 306.52a ± 111.2 | 1 455.54a ± 67.8 | 4 441.89a ± 127.5 |
Data are expressed as the mean ± SD. Different letters in the same column indicate significant differences (p < 0.05).
The antioxidant capacities of the Barhi and Khunaizi date palm juice samples determined using the DPPH, FRAP, ABTS, and ORAC assays were significantly higher after thermal pasteurization than for fresh juice and after HPP. For the Barhi date palm juice, the antioxidant capacities determined using the DPPH, FRAP, ABTS, and ORAC assays were 12.6 %, 21.5 %, 8.5 %, and 31.3 % higher, respectively, after thermal pasteurization than for the fresh juice. For the Khunaizi date palm juice, the antioxidant capacities determined using the DPPH, FRAP, ABTS, and ORAC assays were 64.4 %, 71.9 %, 52.6 %, and 100.9 % higher, respectively, after thermal pasteurization than for the fresh juice. The antioxidant capacities were lower after HPP at 400 and 600 MPa than after thermal pasteurization. It was previously found that the antioxidant capacities of strawberry juice and mixed strawberry and kale juice were higher after thermal pasteurization (72 °C for 1 min) than HPP (Stübler et al., 2020). De Souza et al. (2020) found significantly lower ORAC and DPPH values for lemonade and green juice after heating (75 °C for 90 s) than after HPP (De Souza et al., 2020). Ahmadi et al. (2022) found a significantly higher antioxidant activity for goji berries after heating (90 °C) than after HPP (Ahmadi et al., 2022). Increasing the extraction temperature with a long extraction time can affect the antioxidant activity (Hinkaew et al., 2020). However, the antioxidant capacities we found were significantly higher for date palm juice treated at 600 MPa than for date palm juice treated at 400 MPa. Although the techniques and theories used are not the first report, our results would provide useful information on the changes on antioxidant capacity profiles after HPP and heat treatments. This data could explain the use of date fruit of the cash crops of Southeast Asia and South Asia.
Sodium dodecyl sulfate polyacrylamide gel electrophoresis The results of the SDS-PAGE analyses of the total soluble protein profiles of the date palm juice samples are shown in Fig. 3. The protein band was more intense for the Barhi cultivar than the Khinaizi cultivar. This was clearly found for the 26 kDa protein band. SDS-PAGE did not indicate significant differences in the protein patterns or quantities in the fresh date palm juice or juice after HPP, and there were strong bands for 17, 26, 40, 55, and 63 kDa proteins. However, thermal pasteurization caused the protein bands to disappear. The results indicated that thermal pasteurization caused the loss of soluble proteins, possibly through protein coagulation and precipitation. Our results agreed well with the results of a previous study in which the SDS-PAGE protein bands were less intense for strawberry and kale juice after thermal treatment than after HPP (Stübler et al., 2020). In another study, the soluble protein content was significantly lower for heat-treated than untreaded asparagus juice, possibly because heat treatment caused protein denaturation (Chen et al., 2015).
Sodium dodecyl sulfate polyacrylamide gel electrophoresis profiles for fresh, high-pressure processed (at 400 and 600 MPa), and thermally pasteurized (Heat) date palm juice from two cultivars (Barhi and Khunaizi).
Microbial analysis The most important factor affecting pathogenic bacteria, microbial spoilage, and specific enzymes for preservation food quality in low-acid food (pH > 4.5) is acidity and temperature (Agcam et al., 2018). Not only thermal processing but also HPP is an efficiently pasteurization method to inactive microorganism. Less than 100 CFU was found at 10−1 dilution plate of untreated juice. The effects of high pressure at 400 and 600 MPa of 10 min and thermal processing at 100 °C for 1 min of date palm juices (pH = 6.53–6.95) on the counts of total aerobic microorganism, coliforms, yeasts and molds at day 0, 3, 6, 9, 12, and 15 were investigated. Serial dilutions at 10−1 CFU/mL were not found during storage period because the microorganisms were destroyed by HPP and heat treatment. Therefore, these results indicated that date palm juices after pasteurization had a shelf life of 15 days at 4 °C. The study of Liu et al. (2016) also reported that the treatment of high temperature/short time (HTST) at 110 °C for 8.6 s and HPP at 500 MPa for 5 min on cucumber juice (pH = 5.69) reduced total aerobic bacteria, yeasts, and molds (< 1 log cycle) and showed no outgrowth after storage of 20 days (Liu et al., 2016).
Barhi and Khunaizi date palm juices were different colours. The lightness and yellowness values were higher but the redness value was lower for the Barhi date palm juice than the Khunaizi date palm juice. Thermal pasteurization at 100 °C for 1 min changed the colour and turbidity. However, the colour values were similar for juice subjected to HPP at 400 and 600 MPa and for fresh juice. The total phenolic and total flavonoid compound contents and antioxidant capacities were higher for Khunaizi date palm juice than Barhi date palm juice. The total phenolic and total flavonoid compound contents and antioxidant capacities were higher after thermal pasteurization than HPP. SDS-PAGE indicated that thermal pasteurization caused soluble proteins to be lost but HPP allowed soluble proteins to be retained. After HPP and thermal processing, the microorganisms were not detected during storage of 15 days at 4 °C.
Acknowledgements This research was support by the Thammasat University Center of Excellence in Food Science and Innovation and the Faculty of Home Economics Technology, Rajamangala University of Technology Phra Nakhon.
Conflict of interest There are no conflicts of interest to declare.
