الفهرس 
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Abstract
The objective of the present study was to shed light on the assessment of quality and safety of frozen green beans, pasteurized orange juice and refrigerated bolti fish by evaluating:
1. The effect of blanching on green beans with different methods i.e. (microwave, steam and boiling water) and freezing followed by frozen storage at -18±2°C for 6 months on physicochemical properties, stability of chlorophylls, color characteristics, microbiological assessment as well as the activity of peroxidase, catalase and ascorbic acid oxidase enzymes. Also, the sensory evaluation of fresh and frozen green beans was investigated.
2. The effect of pasteurization on orange juice by different methods i.e. (heat and microwave) for various times followed by refrigeration storage at 4±1°C for 60 days on physicochemical properties, browning index, microbiological quality as well as the activity of pectin methyl-esterase and ascorbic acid oxidase enzymes. The sensory evaluation of tested orange juice was also undertaken.
3. The effect of refrigeration at 4±1°C on bolti fish either fresh or treated with 1% acetic acid, 3% citric acid and mixture of the two on physicochemical properties, chemical indices (TBA and TVN), microbiological attributes and the activity of catalase and protease enzymes. The sensory evaluation of bolti fish was also studied.
The obtained results could be summarized as follows:
Part 1. Quality and safety attributes for green beans:
Green beans was blanched with different methods and freezing followed by frozen storage at -18±2°C for 6 months. Blanching methods before freezing included, boiling water blanching for 2 and 4 min. in boiling water with and without a mixture of chemicals containing (0.5% potassium metabisulphite, 0.1% magnesium oxide and 0.1% sodium bicarbonate), steam blanching for 3 and 5 min. with and without pretreatment with the mixture of chemicals and; microwave blanching for 30 and 60 sec. with and without pretreatment with the mixed chemicals. The obtained results showed that:
1.1. Physicochemical analysis:
1.1.1. Moisture content:
Blanching in boiling water caused to increase the moisture content of green beans at initial time of blanching, especially for samples treated with the mixture of chemicals through blanching which recorded 0.90 to 1.11% increasing after 2 and 4 min., respectively. However, by extending the time of microwave and steam blanching, the moisture content was reduced. On the other hand, with the progressive time of storage at -18±2˚C, the moisture contents of green beans were significantly (p≤0.05) decreased for all frozen blanched samples particularly for samples untreated with the mixture of chemicals.
1.1.2. Ash content:
There was a decremental pattern in ash contents after blanching especially for green beans blanched in boiling water. Consequently, by increasing the time of blanching, the loss (%) of ash contents were decreased for microwave and steam blanched green beans, while, boiling water blanched samples recorded a decremental trend in ash contents by increasing the time of blanching. Also, there was an increase of ash contents by using the mixture of chemicals. On the other hand, by increasing the storage time at -18±2˚C, ash contents were significantly (p≤0.05) decreased in all blanching methods used, particularly for samples that had not been treated with the mixture of chemicals.
1.1.3. Retention of ascorbic acid:
The ascorbic acid contents were reduced for all tested green beans samples after blanching. Ascorbic acid was comparatively higher in microwave blanched samples (95.2-92.1%), moderate in steam blanched samples (92.1-84.1%) and lower in the boiling water blanched samples (87.3-66.7). However, with increasing the time of blanching, a further reduction in ascorbic acid was observed. Also, the ascorbic acid retention was comparatively higher for all blanching methods used pretreatment with the mixture of chemicals before blanching of green beans comparing with that blanched without the chemicals. On the other hand, the loss in ascorbic acid was increased by increasing the time of frozen storage during 6 months at -18±2°C.
1.1.4. Retention of chlorophylls:
Blanching treatments caused a decrease in chlorophylls contents of green beans within the range of 74.39 to 92.08% among all blanching methods used in this investigation. The highest retention of the total chlorophylls was observed in the boiling water followed by microwave and steam, respectively. However, the presoaked green beans in the solution of the mixture of chemicals had a better retention of chlorophylls, where, boiling water blanched green beans for 2 min. with the mixture of chemicals had the highest retention of chlorophylls comparing with other used treatments. Also, the loss of chlorophylls were increased by increasing the time of blanching in all samples. On the other hand, the boiling water blanched samples had the lowest degradation percentage of chlorophylls followed by microwave and steam blanched samples, respectively, after 6 months of storage at -18±2˚C.
1.1.5. pH values:
A significant increase (p≤ 0.05) in pH values was observed after blanching treatments for all green beans samples. Generally, the highest pH values were observed in green beans blanched in boiling water followed by microwave and steam, respectively. On the other hand, a significant reduction (p≤ 0.05) in pH values was noticed after 6 months of frozen storage at -18±2˚C, for all frozen blanched green beans which were blanched by different methods.
1.1.6. Color characteristics:
The color of green beans samples were changed after blanching (to bright green color) and during frozen storage correlated well with the selected blanching method. However, the longer the blanching time, the higher losses in color values were. Also, using the mixture of chemicals led to obtain the better retention of color. The lightness (L* value) of green beans was decreased after blanching depending on the blanching method used. The blanching of green beans in boiling water contained mixing of chemicals for 4 min. improved the green color and maintained L* parameter with high level (47.13) comparing with the microwave blanched green beans for 30 sec. which recorded the greatest reduction in L* value (40.25). Moreover, there was an incremental pattern in a*values for blanched frozen green beans. On the other hand, blanching of green beans caused a reduction in the yellowness (b* value). The highest retention of b* values was achieved by blanching in boiling water for 4 min. in the presence of the mixing chemicals (28.98). Meanwhile, after 6 months of storage at -18±2°C, there was a reduction in L* and b* parameters and an increase in a*values for all samples.
1.2. Microbiological evaluation:
All blanching methods that were used in this investigation caused a reduction in the counts of various spoilage microorganisms; including the total viable bacteria, psychrophilic population, coliforms count and, yeasts and molds with different levels affecting with the blanching method used, the pretreatments before blanching and the time of blanching. Where, the blanching in boiling water had more reduction effect than other blanching methods. While, the frozen microwave blanched green beans recorded the lowest reduction of microbial counts comparing with other frozen blanched samples under investigation, particularly when microwaved for 30 sec. However, the reduction effect was enhanced by increasing the time of blanching and / or using of the mixture of chemicals. On the other hand, a gradual decreasing in microbial populations was noticed by extending the shelf-life of blanched green beans up to 6 months at -18±2˚C. Moreover, the microbial counts for different tested blanched green beans samples were lower than the maximum level as proposed in the Egyptian Standards (ES: 1743/ 2005).
1.3. Activity of peroxidase, catalase and ascorbic acid oxidase enzymes in blanched green beans and throughout frozen storage:
1.3.1. Peroxidase (POD) activity:
The loss of POD activity increased with increasing the time of blanching for all blanching methods used. The blanching in boiling water for 4 min. and water blanching for 2 and 4 min. in the presence of the mixture of chemicals could be enough to inactivate the POD activity for green beans after blanching. Also, using the mixture of chemicals caused an increase in the inactivation of the POD. On the other hand, there was a gradual decline in the POD activity, by increasing the period of frozen storage at -18±2°C for blanched frozen green beans up to 6 months. On the other hand, the reactivation of POD was noticed for frozen green beans which previously blanched with microwave for 60 sec., microwave blanched green beans for 30 sec. which presoaked in the mixture of chemicals, green beans blanched with steam for 5 min, and steam blanched green beans for 3 min. which presoaked in the mixture of chemicals, respectively.
1.3.2. Catalase (CAT) activity:
The blanching of green beans caused to reduce the activity of CAT with different percents depending on the blanching method used and exposure time. The best blanching method used to ensure the CAT inactivation was blanching in boiling water followed by steam and microwave, respectively. Whereas, the highest residual activity of CAT just after blanching was observed in microwave blanched green beans for 30 sec. (9.8%) followed by steam blanched samples for 3 min. (9.0%) and microwave blanched green beans for 30 sec. which presoaked in the mixture of chemicals (5.5%). However, the activity of CAT was minimized by increasing the time of blanching. Also, using of the mixture of chemicals in the blanching media was more effective for inactivation of CAT comparing with other blanching methods in this investigation. Consequently, there were no activities of CAT for frozen green beans after 4 months of frozen storage at -18±2˚C. For instance, after 6 months of storage, the CAT activity for all frozen blanched green beans under investigation was not detected.
1.3.3. Ascorbic acid oxidase (AAO) activity:
A noticeable reduction in AAO activity occurred depending on the blanching method, the time of the operation and the pretreatments before the blanching treatment. However, by increasing the time of blanching the loss of AAO was. The best blanching method for insuring the inactivation of AAO was the blanching in boiling water. Also, using the chemical mixture caused to increase the enzyme inactivation. In contrast, the highest activity of AAO (8.50×10-4 O.D/min.) was recorded for green beans blanched with microwave for 30 sec. Subsequently, a gradual reduction of AAO was observed by increasing the storage period up to 6 months of storage at -18±2˚C, where, a complete inactivation of AAO was occurred in all tested samples, after 6 months of frozen storage at -18±2˚C.
1.4. Sensory evaluation:
All blanching methods used in this investigation introduced changes in the sensory attributes for green beans after blanching and during 6 months of storage at -18±2°C. However, the tested sensory attributes were significantly (p≤0.05) decreased with increasing the time of blanching. The samples blanched with microwave for 30 sec. or boiling water in the presence of the mixture of chemicals for 2 min. recorded better scores of color than other frozen blanched samples after 6 months of storage at -18±2°C. Also, presence of the mixture of chemicals either in blanching media or as presoaking treatment caused obtaining a better color than other samples blanched in the absence of the mixture of chemicals under the same conditions. Moreover, a reduction in texture scores was found in frozen blanched green beans samples after 6 months of frozen storage at -18±2°C compared with fresh (un-blanched) green beans. While, there were no significant differences (p≤0.05) in the texture of un-blanched and green beans blanched in boiling water for 2 min. in the presence of the mixture of chemicals. On the other hand, the flavor of green beans blanched in boiling water with the mixture of chemicals for 2 min. was enhanced by frozen storage at -18±2°C.
Part 2. Quality and safety attributes for orange juice:
Orange juices were pasteurized by heat pasteurization (HP) at 90°C for 2 and 4 min. as well as microwave pasteurization (MP) at (2450 MHz, 850 W) for 30 and 60 sec. Pasteurized juices were stored under refrigeration at 4±1°C for 60 days. The obtained results indicated that:
2.1. Physicochemical properties:
2.1.1. Moisture and ash contents:
The moisture contents of the pasteurized juices were little significantly (p≤0.05) lower than fresh ones, just after obtaining the juice. The highest loss in moisture content was found in the heat pasteurized orange juice for 4 min. (1.2%). While, the pasteurization caused an increase in ash contents of orange juices in comparing with control sample. Meanwhile, no significant differences were observed between ash contents of different pasteurized orange juices just after pasteurization. The reduction of moisture and ash contents were increased, by increasing the time of pasteurization. On the other hand, the moisture and the ash contents were decreased gradually by increasing the time of storage up to 60 days at 4±1°C. The heat pasteurized orange juice for 4 min. had the highest loss of moisture content (1.7%), after 60 days of storage at 4±1°C,
compared with that for fresh orange juice. Whereas, the microwave pasteurized orange juice for 30 sec recorded the lowest value of ash content (0.68%).
2.1.2. Reducing sugars:
The reducing sugar contents were increased in all orange juices, just after pasteurization, compared with that of control sample. The heat pasteurized orange juice for 2 min. recorded the highest quantity of reducing sugars than that in other tested juices. On the other hand, a decremental trend of the reducing sugar contents was noticed, by increasing the refrigerated storage time of the tested juices. The lowest reductions in reducing sugars (7.71%) was recorded in the microwave pasteurized orange juices in comparing with 9.24% reduction in control juice, while the highest content of reducing sugars (5.1%) was observed in the heat pasteurized orange juice for 2 min.
2.1.3. Retention of ascorbic acid:
A destruction of ascorbic acid contents was observed in the pasteurized orange juices by increasing the time of storage at 4±1°C. The ascorbic acid contents were less affected when the microwaves were applied to pasteurize the juice. On the other hand, after 60 days of storage at 4±1°C, the heat pasteurized orange juice for 4 min. had the highest loss in ascorbic acid (46.6%) comparing with the microwave pasteurized juice for 30 sec. which had the highest retention of ascorbic acid (79.61%).
2.1.4. Titratable acidity (T.A.):
The titratable acidity of orange juices was increased owing to pasteurization. However, with extending the storage period of refrigerated orange juices, the T.A. was increased gradually for all tested orange juices. Where, the maximum content of the T.A. (0.69%) was recorded in the microwave pasteurized orange juice for 30 sec., while it was the minimum (0.65%) in the heat pasteurized juice for 4 min, after 60 days of refrigerated storage at 4±1°C.
2.1.5. pH values:
There were no significant differences (p≤0.05) between untreated (fresh) and pasteurized orange juice in terms of pH values, within the range between 5.14-5.17. Also, there was a gradual reduction trend in pH values for all orange juices during storage period at 4±1°C up to 60 days.
2.1.6. Total soluble solids (TSS):
A slight significant (p≤0.05) effect on TSS values of orange juices was observed for various pasteurization methods of tested orange juices. Furthermore, the heat pasteurized orange juice for 4 min. had the highest TSS content comparing with other pasteurized juices. Consequently, there was a significant reduction (p≤0.05) trend on the TSS contents for the pasteurized orange juice by increasing the storage time up to 60 days at 4±1°C. Where, the microwave pasteurized orange juice recorded the highest TSS content. Also, no significant differences (p≤0.05) were observed of TSS between both the heat pasteurized and the microwave pasteurized orange juices, after 60 days of storage at 4±1°C. 2.1.7. Color characteristics:
The general trend for the color of pasteurized orange juices showed that lightness (L* value) decreased, redness (a* value) increased, and yellowness (b* value) decreased, Just after pasteurization, compared to unpasteurized juice (control). The highest values of L* and a* were noticed for the orange juice pasteurized by microwave for 60 sec. However, the b* value of microwave pasteurized orange juices was significantly (p≤0.05) lower than b* values of other tested pasteurized juices. Also, a significant increase (p≤0.05) in L* values was observed in pasteurized orange juices after 12 days of storage at 4±1°C. On the other hand, there was a decrease in L* and a* values. While, b* values were increased for all pasteurized orange juices especially in heat pasteurized juices, after 60 days of storage at 4±1°C. The microwave pasteurized juice for 60 sec. and the heat pasteurized juice for 2 min. showed greater L* than other tested juices. However, a* values of pasteurized orange juices were higher than that of unpasteurized juice. Moreover, the microwave pasteurized orange juice for 60 sec. had the lowest b* value.
2.1.8. Browning index (BI):
The values of browning index of orange juices were increased just after pasteurization on comparing to control juice. Consequently, a gradual increase in the browning values for all tested orange juices were observed, by increasing the storage time up to 60 days of storage at 4±1°C. Meanwhile, the untreated orange juice had the highest level of browning rate, after 60 days of storage at 4±1°C. However, the orange juice pasteurized by microwave recorded the lowest rate of browning index comparing with other orange juices pasteurized by heat.
2.2. Microbiological evaluation:
The different pasteurization methods used in this study caused to reduce the microbial counts in orange juice. This reduction was correlated well with the pasteurization method used and holding time. Where, the total viable, psychrophilic bacterial counts as well as yeast and mold counts of orange juices were completely inactivated by the heat pasteurization at 90°C for 2 and 4 min. as well as the microwave pasteurization for 60 sec. Whereas, the microwave pasteurization for 30 sec. couldn’t completely inactivate the microbial counts but caused to reduce the total microbial counts comparing with that for untreated orange juice. On the other hand, an incremental pattern in microbial populations was observed by extending the shelf-life of both control and pasteurized orange juices up to 60 days of storage at 4±1°C. Such increases depending on the species of microorganisms which showed different levels of heat resistance. Where, the heat pasteurization for 4 min. was more effective for minimizing of microbial counts of orange juices followed by the microwave pasteurization for 60 sec., after 60 days of storage at 4±1°C.
2.3. Pectin methyl-esterase and ascorbic acid oxidase activity:
2.3.1. Pectin methyl-esterase (PME) activity:
The decline of PME was observed for pasteurized orange juices owing to the used pasteurization method and the holding time of pasteurization. Where, the heat pasteurization for 4 min. caused the highest loss of PME activity (93.8%) comparing with 86.2%, 83.4% and 82.6% losses for the microwave pasteurized juice for 60 sec., the heat pasteurized juice for 2 min. and the microwave pasteurized juice for 30 sec., respectively. On the other hand, there was a gradual loss in PME activity for tested orange juices by extending the time of storage at 4±1°C up to 60 days. Moreover, the heat pasteurized orange juice for 4 min. had the lowest residual activity of PME (0.49%), after 60 days of storage, followed by the microwave pasteurized orange juice for 60 sec. (2.04%), the microwave pasteurized orange juice for 30 sec. (4.38%) and the heat pasteurized orange juice for 2 min. (6.05%), respectively.
2.3.2. Ascorbic acid oxidase (AAO) activity:
The fresh (control) orange juice recorded a higher activity of AAO (61.04×10-3 O.D/min) than the other pasteurized samples. While, the heat pasteurized orange juice for 4 min recorded the lowest activity of AAO (1.26×10-3 O.D/min.) which represented 97.94% reduction compared with that of control orange juice. On the other hand, the activity of AAO for all pasteurized orange juices was decreased gradually by extending the refrigerated storage. Where, after 60 days of storage at 4±1°C, the residual activity of AAO for the heat pasteurization for 4 min was 0.16% (99.84% loss) and 0.84% (99.16% loss) for the microwave pasteurization for 60 sec. comparing with 2.81% (97.19% loss) for the heat pasteurization for 2 min. and 4.28% (95.72 %loss) for the microwave pasteurization for 30 sec.
2.4. Organoleptic properties for fresh and pasteurized orange juices:
Analysis of variance showed significant differences (p≤0.05) in color, flavor and overall acceptability for all tested juices. However, fresh orange juice recorded the highest scores for all selected parameters of sensory evaluation compared with pasteurized orange juice. On the other hand, the microwave pasteurized orange juice for 30 sec. recorded the best score for color followed by the heat pasteurized orange juice for 2 min. and the microwave pasteurized juice for 60 sec., respectively. Whereas, the heat pasteurized orange juice for 4 min. recorded the lowest score of color. Also, the heat pasteurized orange juice for 2 min. had the highest flavor score comparing with the other pasteurized orange juices. Meanwhile, there were no significant differences (p≤0.05) in flavor scores between the microwave pasteurized juice for 30 sec and 60 sec.
The sensory scores indicated that, the highest overall acceptability was for the microwave pasteurized orange juice for 60 sec. in comparing with other tested juices, after 60 days of storage at 4±1°C. Moreover, there were no significant differences (p≤0.05) in overall acceptability for the microwave pasteurized juice for 60 sec., the heat pasteurized orange juice for 2 min. and the microwave pasteurized orange juice for 30 sec. Therefore, color and flavor seemed to have a good correlation with the acceptance for all tested orange juices under the investigation.
Part 3. Quality and safety attributes for Nile bolti fish:
Whole Nile bolti fish were stored under refrigeration at 4±1°C for 12 days, after immersing in aqueous solutions of 1% acetic acid (AA), 3% citric acid (CA) and; mixing of 1% AA and 3% CA, for 5 min.
The obtained results revealed that:
3.1. Physicochemical analysis:
3.1.1. Proximate chemical composition:
The mean compositional contents of moisture, protein, lipid, and ash (% on dry weight basis) in the fresh bolti fish analyzed were 82.89, 84.15, 10.20, and 5.50%, respectively. However, the suggested treatments were significantly affected for the chemical composition, particularly for the lipid and protein contents. Also, significant (p≥0.05) decremental patterns in moisture, protein and ash contents during storage at 4±1°C in different samples were observed. Inversely, lipid content increased significantly (p≤0.05) during 12 days of storage at 4±1°C.
3.1.2. Physical analysis:
3.1.2.1. Changes in pH values:
pH values of all treated samples were significantly (p≤0.05) lower than that of the control sample. Also, samples dipped in 1% AA solution achieved the lowest significant value of pH after 12 days of refrigerated storage at 4±1°C.
3.1.2.2. Water holding capacity (WHC) and plasticity:
Fish samples treated with 1% CA had the highest values of WHC during 12 days of storage followed by AA-treated samples and finally its combination with CA (1% AA and 3%CA), respectively, this may be related to the pH of fish samples. On the other hand, there was a significant (p≤0.05) decremental pattern in the plasticity of all tested samples during storage at 4±1°C. While, the samples immersed in 3% CA recorded higher plasticity value than tested samples.
3.1.3. Chemical quality indices:
3.1.3.1. Thiobarbituric acid reactive substances (TBARS):
The values of TBA for all tested fish samples were increased significantly (p≤0.05) with the progressive of storage time. However, there was a significant (p≤0.05) reduction in TBA concentration in all treated fish samples when compared with the control. Where, the TBA values for different samples treated with organic acids, after 12 days of storage at 4±1°C, were lower than the maximum level which is 4.5 mg malonaldehyde/ kg as proposed in the Egyptian Standards ES: 3494/ 2005. Moreover, fish samples immersed in 3% CA has a potent antioxidant effect more than 1% AA. Also, dipping of bolti fish in CA did not cause any significant reduction in the TBA values (p≤0.05) in comparison with the samples treated with the combination of AA and CA.
3.1.3.2. Total volatile nitrogen (TVN):
A significant increase in TVN of bolti fish was noticed, with the progression of storage at 4±1°C. However, a significant (p≤ 0.05) high value of 29.54 mg N/100 g was detected for TVN in control comparing with those in the different treated samples, after 12 days of refrigerated storage. The TVN values in all analyzed samples throughout storage period were below the maximum value of 30 mg N/100g flesh specified by the Egyptian Standards ES: 3494/ 2005 for chilled fish.
3.2. Microbiological evaluation:
A low initial bacterial counts (< 5 log10 CFU/g) were determined for fresh bolti used in this study which considered as a good initial quality. Meanwhile, the microbial counts were gradually increased, by increasing the storage period of fresh bolti fish at 4±1°C. On the other hand, dipping of fish in aqueous solution of AA, CA and; mixture of AA and CA caused a reduction in various categories of spoilage microorganisms; including aerobic and psychrophilic populations, yeast and molds, and coliform group also, extended the shelf life of the product. The general order of antimicrobial activity of the different organic acids used for immersing fresh bolti fish was; the combination of AA and CA followed by AA- and CA- treated fish samples.
3.3. Assessment of catalase and protease enzymes activity:
3.3.1. Catalase (CAT) activity for tested fish samples:
Catalase activity for fresh Nile bolti fish was 5.39×10-2 O.D/min. However, catalase activity was gradually increased, by increasing the storage period at 4±1°C. Meanwhile, samples treated with different organic acids recorded less activity of CAT, where, the residual activity of CAT was approximately 85.5, 42.8 and 39.6% for fish treated with AA, CA and; a mixture of 1% AA and 3% CA, respectively compared with that in control fish sample. Therefore, the usage of the combination of AA+CA was more effective in reduction of CAT activity during storage period up to 12 days comparing with other treatments.
3.3.2. Protease activity of tested fish samples:
The activity of protease enzyme for bolti fish was affected by the used treatments before refrigeration and the storage time. Where, the activity of protease enzyme was more less by the treatment with aqueous organic acids used in this study in comparing with untreated (control) samples. The highest proteolytic enzyme activity was found in control samples, which showed a sharp increase by increasing the storage time at 4±1°C. Whereas, proteolytic enzymes extracted from samples treated with a mixture of 1% AA and 3% CA had the lowest activity, followed by enzymes extracted from fish treated with CA and AA, respectively.
3.4. Organoleptic evaluation of boti fish samples:
Sensory evaluation indicated that, there were no significant differences (p≤0.05) in appearance, odor, texture and eye’s lustrous between all tested fish samples under the study, at initial time of storage at 4±1°C. On the other hand, a gradual decrease (p≤0.05) in sensory analysis was observed as the storage time prolonged, for all tested samples. Control fish samples had the highest deteriorative rate for all sensory parameters, after 12 days of storage at 4±1°C, comparing with other treatments. Also, there were significant differences (p≤0.05) in appearance, eye’s lustrous, gills color, odor, texture and overall acceptability between the different treated bolti fish. Meanwhile, fish samples immersed in 3% CA solution expressed more acceptability followed by 1% AA- and; the combination of 1% AA and 3% CA treated samples, respectively. These findings are confirmed with microbial results. For instance, there are a good correlations between the log microbial counts and the organoleptic scores.