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Pomegranate fruit (Punica granatum L.) has a growing popularity worldwide for its unique taste, color and health benefits. Egypt in the 6th leading country in pomegranate fruit production with actual production of 400000 tons/year. Pomegranate is a promising fruit for industrial processing and for export.
The aim of the present work was to maximize the utilization of Egyptian pomegranate varieties in the food industry through:
1- Specifying the physical properties of the major two pomegranate fruit varieties grown in Egypt: Manfalouty and Wonderful,
2- Testing the mechanical properties of the fruits necessary for handling and processing,
3- Application of different juice extraction and treatment methods to get the maximum juice yield with unique chemical and physical properties,
4- Identifying the rheological properties of the pomegranate juices and their change with treatment method and temperature,
5- Testing the functional rheological and thermal properties of pomegranate concentrates necessary for their handling, transport and processing,
6- Assessing the change in aroma profile of pomegranate juices and concentrates as one of the essential consumers acceptability factor.
The obtained results could be summarized as follows:
1- Some physical characteristics of Egyptian pomegranate fruits
The average vertical, horizontal and diagonal dimensions are 79.8 mm, 74.7 and 77.2 mm for Manfalouty and 87.5, 80.7 and 84.1 mm for Wonderful verities. The majority of Manfalouty fruits has their dimensions in the range of 70 to 90 mm, while For Wonderful variety, 82.86% of the fruits showed vertical axis in the range of 80 to 100 mm, while 77.14 and 76.19% of the fruits showed horizontal and diagonal axis length in the range of 80 to 100 mm.
- Manfalouty variety recorded an average weight of 417.045g and a range from 243 to 998 g, while the average weight of Wonderful fruits was 550g and a range from 258 to 940 g. It is clear that Manfalouty fruits are in average lighter than Wonderful variety.
- The average volume of Manfalouty fruits variety was 424.50 cm3 and a range between 245 to 860 cm3, while the recorded corresponding values for Wonderful variety were 565 cm3 and 260 to 1050 cm3.
- The average density of the Manfalouty fruits was 0.97 g/cm3, while that of Wonderful was 0.98 g/cm3.
- The sphericity factor of both fruits was close to that of geometrical sphere (equal 1). The recorded values for Manfalouty variety was 0.97, while that of Wonderful was 0.96.
- The average shape index (L/D) of the Manfalouty fruits was 1.05, while that of Wonderful fruits was 1.07.
- The average peel weight of the fruits was 174 g and 268g, respectively for Manfalouty and Wonderful pomegranate fruits making 49.1% and 47.5% of the fruit average weight, respectively. The separated arils represented an average weight of 180 to 299 g, which makes 50.9% and 52.5%, respectively of the fruit weight.
- Arils of Wonderful variety were whiter than those of Manfalouty, since the L- values were 31.85 for the first and only 20.11 for the later. Also, the Manfalouty variety recorded higher redness (a-values) and chroma levels than those of Wonderful (22.07 versus 15.98) and (25.62 versus 18.40), respectively.
2- Mechanical Tests of Pomegranate Fruits
Penetration and compression (crushing) test
- For fresh Manfalouty fruits, the number of yield points occurred before penetration were in the range of 2 to 3 points with the small pomegranate fruits are internally more integrated (in internal structure) than medium and heavy-weight fruits.
- The maximum force required to penetrate the peel (outer surface of fruit) ranged between 40 and 46.83 N, while the average deformation at the maximum force was in the range of 5.11 to 5.98 mm.
- The energy at maximum force represents the mechanical work necessary to make puncture through the fruit’s skin. The obtained values ranged between 119.43 to 140.83 mJ.
- the total energy for penetration includes all the works done during penetration and pulling out the plunger from the fruit’s skin and their values ranged between 300.55 to 576.54 mJ.
- The average deformation value (approximately corresponding to the peel thickness) was higher for Wonderful variety than those of Manfalouty variety (7.23 mm versus 5.69mm), which agree with the fact that the average weight of the wonderful variety was higher than those of Manfalouty variety (588.5 g versus 555 g).
- Fresh Wonderful variety show corresponding higher number of yield points (2 to 5 yield points) and higher rupture force than those required for the Manfalouty variety (45.84 to 64.84 N).
- The required energy for achieving maximum hardness (maximum force) and total energy of penetration were higher for the fresh Wonderful fruits than those of the Manfalouty variety (199.22 versus 126.62 mJ) and (439.14 versus 408.65 mJ). Storage at room temperature shows that the deformation required for peel penetration was higher than those required for the fresh fruits. Also, the number of yield points occurred during rupture test were increased to the level of 6-7 points. Fruits stored at 4±2 °C under refrigeration showed only one yield point and lower deformation depth (peel thickness) than those observed by the fruits stored at room temperature (5.6mm).
- The deformation required to break the fruits during compression (crushing) test ranged between 26.65 to 35.5 mm depending on the diameter of the tested fruits. The measured fruit diameters for tested fruits were in the range of 91 to 104 mm. Maximum force required to break the fruits ranged between 452.38 to 1049.18 N, while the energy required to achieve the maximum force to break the fruits ranged between 9.08 to 15.99 J. The work necessary to achieve a deformation value of 25 mm was also recorded and their values ranged between 5.084 to 8.83 J.
3- Effect of extraction method on yield, total soluble solid, density and some functional properties of pomegranate juice
- Extraction of pomegranate juice by blending in household mixer gave the lowest juice yield (65.64 to 67.51%), while application of screw press or hydraulic press did increase the juice yield to the level of 76%. Pretreatment of the pomegranate arils by pulsed electric field followed by hydraulic pressing substantially increased the yield to the level of 80%.
- Total soluble solids (°Brix) of Manfalouty juice were higher (16.2° to 17.1° Brix) than those of Wonderful variety (15.12° to 15.63° Brix). Density value ranged between 1.028 to 1.051 g/cm3 for Manfalouty and 1.027 to 1.038 g/cm3 for Wonderful juices, where the highest density value was recorded for the juice extracted by blending and the lowest one was recorded for the juice pretreated by pulsed electric field.
- For Manfalouty fruits juices the acidity of the extracted juice was 1.035%,1.015% and 0.78 % for fresh juices extracted respectively by blending, screw pressing and pulsed electric field treatments. pH value were 3.40, 3.76 and 4.17 for juices extracted, respectively.
- For Manfalouty variety, the total phenolic content of the fresh juices ranged between 286.95 to 903.55 (mg /100ml juice), while the lowest value was recorded for juices extracted by screw press. Similar trend was also found in total tannins content of the juice (134.57 to 381.83 mg /100ml), while the highest flavonoids content was observed in the fresh juices extracted by blending (2.76 mg /100ml). Juice extracted by screw press or pulsed electric field showed lower content in total flavonoids (1.63 mg /100ml and 1.46 mg /100ml), respectively.
- The lowest ascorbic acid value (9.76 mg /100ml) was recorded for the fresh juice extracted by screw press followed by juice extracted by blending (11.27 mg /100ml), while juice extracted by pulsed electric field pretreatment showed the highest ascorbic acid content. Ascorbic acid content of fresh screw pressed Manfalouty and Wonderful juices recorded 9.76 and 11.88 mg / 100 ml, respectively. Extraction by blending and pulsed electric field treatment raised the ascorbic acid content in the obtained juices to higher levels.
- The antioxidative activity was the highest in the juice extracted by blending (63.28%) due to it’s higher flavonoids content, while the antioxidative activity of the juice extracted by screw press was in the level 58.96%
- The total phenolics of Wonderful juice recorded (431.35 mg/100ml) versus (268.95 mg/100ml) for Manfalouty and 256.87 mg/100ml total tannins versus 134.57 mg/100ml. similar trend was also found for the values for total flavonoids, total anthocyanin, ascorbic acid and antioxidant activity.
- Extraction of the juice by blending in a household mixer produced fresh juice with redness (a-value) of 20.75, yellowness (b-value) of 3.42 and lightness (l-value) of 27.44, color saturation (chroma) of 21.03. The Hue angle of the fresh juice showed value of 9.35. which is very close to the red color type.
- Juice extracted with screw press showed higher redness value (26.56), lower b-value (1.58) and lower lightness value (17.48), while the red color saturation of the juice was increased to the level of (26.61) and the Hue angle was decreased to the level of (3.40) which is very close to the pure red color type.
- pulsed electric field followed by extraction in a hydraulic press (100-150 bar) produced a juice with redness of (23.22), yellowness of (6.89) and lightness of (14.28). The corresponding chroma (degree of saturation) was 24.22 and the Hue angle was 16.54°.
- Wonderful pomegranate juice extracted by arils disintegration using a household mixer produced juice with lower redness, b-value and chroma (12.26, 1.31 and 12.33), respectively and higher lightness (33.65) than that of Manfalouty juice extracted under the same condition. The Hue-value recorded 6.11° which was close to that recorded for Manfalouty variety. Extraction of Wonderful pomegranate juice by using a screw press, resulted in juice with a, b and L-values close to those obtained by mixing, but the Hue-angle of the produced juice was slightly higher (9.53°) than that of mixed juice. On other side, Wonderful juice extracted by pulsed electric field pretreatment produced juices with higher redness a-value (13.75) and chroma (14.90) but lower yellowness (5.76) and lightness (19.26). However, the recorded Hue-angle was 22.73°.
- clarification and filtration did improve the redness of the raw juice (increasing a-values). Also, the lightness value of the juice was decreased (lower L-values), while pasteurization of the pulsed electric field juice resulted in lower (a) and chroma values, while b, L and Hue values were increased.
- Hue-values were remarkably influenced by the clarification process, where the color type was changed from pure red (6.106) to the red-blue (Pink) type.
4- Rheological Properties of pomegranate Juices
- Fresh Juices showed higher shear stress response at all applied share rate. For example, the share stress value at 1312 sec-1 for fresh juice measured at 5°c was 59.35 dynes /cm² (5.94 Pa). For clarified juice, this value was decreased to the level of 54.92 (dynes /cm²) (5.49 Pa) and after filtration this value was maintained at the level of 54.92 dynes/cm². (5.49 Pa).
- However, pasteurization of the filtrated juice led to slight increase in the level of the share stress response to 55.94 dynes /cm² (5.59 Pa).
- Rheograms obtained at 5°C were substantially higher than those obtained at 25°C and 40°C in all tested juices sample. The R2 value for the relationship between the shear rate and the shear stress recorded higher level for the linear model. (R2 = 0.9897 to 0.9982), while the R2 value of the power law model were in the range of 0.9632 to 0.9866, which indicate that the flow behavior of the pomegranate juice could be describe as a Newtonian fluid. rather than non-Newtonian fluid.
- The viscosity (µ) of the fresh Manfalouty juice obtained by screw pressing was in the range of 2.49 cp at 40°C to 4.17 cp at 5°C. Clarification and filtration of the Juice reduced its viscosity to the level of 2.2 to 2.47 cp at 40°C and to the level 3.81cp at juice temperature of 5°C. Similar trend was also observed in the clarification, filtration and pasteurization operations of the juices obtained by blending in mixer. The activation energy values for Manfalouty juices were in the range of 6.57 to 9.16 kJ / mole, while the corresponding values for Wonderful juice were slightly higher and ranging between 8.3 to 11.22 kJ / mole.
5- Chemical, rheological and thermal properties of pomegranate juice concentrates
- The total phenolics reached the level of 1440.5 and 1418.48 mg/100ml in concentrates compared with 286.95 and 431.36 for the original juices of Manfalouty and Wonderful juices, respectively. Also, the TSS of juice was concentrated to 4.8 folds of their initial °Brix values (total soluble solids) of the initial juices. The total tannins content was 1601.81 and 1563.70 mg/100g (concentrate), for Manfalouty and Wonderful concentrates, respectively. Also, total flavonoids of pomegranate juice concentrates were increased by 7.15 and 5.1-folds compared with those found in juices of Manfalouty and Wonderful, respectively. The antioxidative activities of the concentrates (71.16% and 77.75%) were higher than those recorded for their fresh juices (58.56% and 61.57%).
- Concentration led to reduction in a-value and an increase in b-value. a-and-b-values of the fresh juice were 26.56 and 1.58, respectively and they were correspondingly changed to the level of 18.23 and 14.02 as the juice was concentrated from 16% to 27%. Also, reduction in L- values, chroma could also be observed. The Hue - value was drastically shifted from the pure red color (Hue = 3.4°) to the red - orange color type (Hue = 37.56°). Increasing the concentration of the Manfalouty pomegranate juice to the level of 52%,68% and 72%. cause a slight decrease in redness (a-value) and chroma, while yellowness and lightness were strongly influenced by the increased concentration and reached much lower values. Also, Hue - value recorded relatively lower values. (14. 89° to 39.44°) as the concentration was increased from 27% to 78%.
- Similar trend was also observed for the Wonderful concentrates, where (a-value) was increased, and b-, L- values were drastically decreased by increasing the final concentration of the evaporated juice. Also, the Hue – values of the concentrates were shifted from red - orange color at concentration of 27% towards the pure red color as the concentration was increased to the level of 78°Brix.
- For Manfalouty juice concentrates, the maximum shear stress response at 1312s-1 shear rate was 60 dynes /cm² for the fresh juices at 5°C and it was slightly increased to 84 dynes /cm2 for the 27% concentrate and then it was shifted to a rapid increase from 566 dynes/cm2 to 20586 dynes /cm2 as the concentration was raised to the level of 52%- and 78%, respectively. Similar results were also obtained for Wonderful juice and it’s concentrates. Higher °Brix concentrates of Wonderful juice (52%, 68%, 72% and 78%) showed shear stress response value at the same shear rates and temperatures and reached the level of 585 dynes /cm2 to 21573 dynes/cm2 as the concentration was increased from 52% to 78%, respectively.
- The R² -values of 27% Manfalouty concentrate data were 0.9894 to 0.9969 for fit as Newtonian fluids and 0.9882 to 0.999 for fitting as non-Newtonian fluids. The viscosity (µ-value) of the Newtonian model was 6.46 cP at 5°C, while the corresponding value for the Wonderful concentrate was 6.66 cP for the Newtonian model. The n-values of the 27% Manfalouty concentrate was 0.78 and the corresponding consistency coefficient value was 0.32 dynes.sn /cm². The obtained apparent viscosity value (µa) at 729s-1 was 7.49 cP which is close to that obtained for the Newtonian fluid type.
- Analysis of 72% concentrate by the two rheological models gave R2-values in favor of the Power law model. The viscosity value of the Newtonian model recorded 2949.3 cP at 5°C, while the n-value remained close to the unity (0.982) but k-value was drastically increased to the level of 34.92 dynes.sn / cm2. The corresponding apparent viscosity values for this concentrate was 3101.4 cP, slightly higher than that obtained for the Newtonian fluid. The 78% concentrate gave crucial higher fit goodness for non-Newtonian fluid compared with the fit as Newtonian fluid. The corresponding viscosity value of the high °Brix concentrate was 14841 cp and the n-value was remarkably deviated from the unity (0.867) and the k-value was 335.69 dynes.sn/cm² and the apparent viscosity value recorded 13878 cP, which was also close to that obtained by the Newtonian model. The increase in the concentration from 72% to 78% increased viscosity by 5-folds and the k-value by 10-folds, Similar behavior was observed for the juices and concentrates of Wonderful variety.
- Increasing temperature of the juices and concentrates lead to loss in viscosity and the loss ratio was increased by increasing the concentration of juices. For example, the viscosity of the 16% juice at 5°C (Manfalouty variety) was 4.17 cP and it decreased by 167% at 40°C, while the corresponding values for the 72% concentrate were 2949.3 cP at 5°C and it was decreased to 1362.5 cP at 40°C.
- For both Manfalouty and Wonderful concentrates, the Ea-values increased from the level of 8. 075 and 10. 817 (kj / mole) for the single strength juice to the level of 71.48 and 79.84 kj / mole For 78% concentrates.
- Each increase by 1%(1°Brix) in the concentration of Manfalouty or Wonderful juice concentrates will result in an increase in the activation energy of the viscosity by 1.035 kj/mol and 1.032 kj/mole for Manfalouty and Wonderful concentrates, respectively.
- The specific heat of the pomegranate juice at 20°C was 3.6235 kj/kg. °C, while it was increased to 3.9330 kj/kg. °C, when the temperature was raised to 25°C.
- The specific heat of the concentrated pomegranate juice was much lower (2.674 kj/kg. °C) than those of the 16% Juice.
- The recorded specific heat for the 65% value concentrate at average 30°C was 2.6746 (kj/kg. °C) and it was increased to the level of 3.322 and 4.0083 (kj/kg. °C) as the temperature was increased to 45°C and 60°C.
- The freezing curve of the pomegranate juice obtained by slow freezing (still air cabinet -23°C and slow temperature recording rate of 2points/minute) was characterized by a moderate supercooling temperature (ts at -2°C) followed by further ice crystal formation at -1.05°C for a period of 85 min after which the freezing point was continuously decreased until 200 minutes due to the increasing concentration of the unfrozen juice and it was continued to decrease to a temperature level of -8°C, after which the remaining juice starts to freeze as a combined crystals (eutectic) until the temperature reached -21°C after 500 minutes . then the freezing curve was flattened indicating the end of freezing process.
- Observation of freezing curve of pomegranate juice during the time from 465 min to 3950 min showed a counties fluctuation between freezing, thawing and refreezing remaining concentrated unfrozen liquid indicate that it is impossible to freeze all the water in the initial juice. Increasing the recording rate of juice temperature during freezing process (from 2 points/min to 60 points/min) resulted in a freezing curve with a sharp and clear supercooling temperature at -8°C.
- Quick freezing tests for Manfalouty and Wonderful juices were carried out in a freezing cabinet with forced air circulating (2m/sec).
- The freezing was very fast so that no clear super cooling point could be observed. However, an apparent supercooling point could be seen at temperature of (-2.9°C) followed by a constant freezing temperature at -2.2°C for the center and (-2.9°C) for the wall. The pure ice formation period lasted for 14.5 min at the wall and about 26 min at the center. The temperature at the center remained almost constant at an average freezing temperature (tf) of -2.2 to -4°C until 26 min. The ice crystals at the wall started to cool linearly until the end temperature of -20°C with a cooling rate of 0.62°C / min. On other side the cooling rate of the combined eutectic crystals was faster than that of pure ice crystals (0.94°C/min) properly because the lower specific heat requirements of the eutectic crystals due to the lower water amount in these mixed crystals. Similar behavior has been observed for the wall and center temperature of the fresh Wonderful juice but with the difference that the time elapsed for formation of pure ice crystals was 8 min at the wall and 20 min at the center.
- The freezing curve of the concentrates consist of three phases without a clear super cooling temperature or a definite freezing temperature. The first phase is linear with a sharp slope until the temperature of the concentrate reached -16°C at the wall and also at the Center for Manfalouty Juice concentrate and -15°C for Wonderful concentrate. The second phase is the phase between -16 and -25°C, which represent the phase of a series of eutectics until the concentrate reached the final crystal formation with a mixture of water and solids. The third phase is the period between -28°C and- 33°C, which represent a cooling phase of the solidified concentrate. The freezer temperature was set at -35°C.
6- chromatographic analysis of pomegranate juices and concentrates
- The major peak of organic acids in the juices of Manfalouty and Wonderful varieties was that of citric acid, with a percentage of 88.73% and 69.58%, respectively. The other organic acids present in the juices were dicarbon acids which makes 11.27% and 6.48% of the organic acid in Manfalouty and Wonderful juices, respectively.
- Sugars of Manfalouty juices are consisting of 53.8% fructose, 42.32% glucose and sugar alcohols (Mannitol) in the ratio of 3.8%. On other side, sugars of Wonderful juices are consisting of 52.23% fructose and 42.06% glucose. Sugar alcohols like mannitol and other sugars like galactose are present in a total ratio of 5.71%. Manfalouty concentrates contains more minor sugars such as ketoses and raffinose. However, fructose and glucose represent the major sugars with a ratio of 94.23% for Manfalouty juice concentrate and 93.41% for Wonderful juice concentrates. The amount of identified active phenolic component in fresh juice of Manfalouty variety was 17.48 µg/ml, while fresh wonderful juice showed 31.12 µg/ml. The major phenolic component in pomegranate juice is gallic acid which makes 54.55% and 43.96% of the total phenolics in juice of Manfalouty and Wonderful, respectively. The second important phenolic component is ferulic acid which makes 16.42% and 23.58% of the phenolic compound in Manfalouty and Wonderful fresh juices, respectively. The third important phenolic compounds is the caffeic acid, which makes 5.03% and 8.19% of the phenolic compounds, respectively in Manfalouty and Wonderful fresh juice. The total amount of identified phenolics for Manfalouty concentrates was 86.75 µg/ml, On the other side, the phenolic compound of Wonderful concentrate showed a value of 119.43 µg/ml. The major compounds of phenolic in pomegranate juice concentrate was gallic acid which makes 35.50% and 42.34% of the total phenolics, respectively in the concentrates of Manfalouty and Wonderful juices. Followed by ferulic acid. (23.1% and 7.72%, respectively).
- A total of 59 volatiles compounds were identified in the tested juices and concentrates of Egyptian pomegranate fruits. They could be, chemically, categorized in 7 groups Aldehydes (9 components); Ketones (3 components); Alcohols (13 components); Acids (2 components); Esters (20 components); Alkanes (2 components); Terpenes and Terpenoids (9 components) and others (1 component), which may make important contribution to the characteristic pomegranate juice flavor. Esters are the major components (33.9%) found in the aroma profiles of pomegranate juices and concentrates followed by alcohols (22%); aldehydes (15.25%) and Terpenes (15.25%).