الفهرس 
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Abstract
Burger is a famous formed comminuted meat product made from ground meat consumed by millions of people. However, it has limited stability, due to microbial growth and lipid oxidation.
The present study was conducted in an attempt to improve beef burger shelf life and technological properties using lemongrass essential oil. Additionally, lemongrass nanoemulsions were fabricated to optimize the use of this essential oil as a food preservative.
Lemongrass (Cymbopogon citratus) essential oil (LEO) was assessed for its component using GC/MS. Lemongrass oil nanoemulsion “LGNE” was prepared in three methods to produce 3 types of nanofabricates (N-M1, N-M2, and N-M3).
The GC/MS analyses of (LEO) revealed presence of forty-five compounds with considerable amount (> 0.14%). The main components were Citral “citric acid” (28.97 %), Citric acid and Verbenol (24.95 %), β-pinene - á-Myrcene (19.22 %) and Ethyl Acetate (10.32 %).
Pre-experimental work was conducted to identify the optimal technique for preparation of the nanoemulsion and choose the suitable concentration of LEO to be used in the current study depending on the sensory attributes of the burger samples. The results of pre-experimental work showed that the addition of lemongrass nanoemulsion (LGNE) fabricated by the use of acetic acid (N- M3) produced the best burger from sensory point of view. In this context, this method was chosen over the other 2 methods. Furthermore, the addition of raw LEO at different concentrations exhibited drastic effects on the organoleptic attributes of the burger. So, 0.5 % LEO was chosen for the study as an exploratory investigation to compare the effect of the raw oil to the nanoemulsions.
FTIR, PDI, TEM and cytotoxicity were used for LGNE (N-M3) characterization. PDI value of the used nanoemulsion formulation was less than 0.5, indicating the uniformity of the droplet size distribution. Electron microscopy image showed droplets size range from 29 to 49.6 nm in diameter; The droplets had a dark appearance, spherical shape, widely separated from each other, and contained an amorphous core. (IC50) of LGNE determined by Sulforhodamine B (SRB) was 22.38 μg/ml indicating that the prepared nanomaterials exhibit certain cytotoxicity to the cells.
LEO (0.5%) and LGNE at concentrations of 1% and 1.5% were included in the beef burger formula.
In the current study, samples color was examined before cooking whereas texture, odor, taste, and overall acceptability were evaluated after cooking. Burger samples containing 0.5 % LEO (0.5 O) exhibited an unmissable obvious yellow color which was unaccepted. The sensory results showed significant color difference (P<0.05) between samples of 0.5 O treatment (lower scores) and samples of other treatments and control at 0 time, 48 Hrs. and 1st month. On the 2nd month, 1 N samples showed better scores compared to 0.5 O (p>0.05). The samples treated with the nanoemulsion showed no color difference (P>0.05) with control samples throughout the experiment time.
Regarding odor, samples of 0.5 O treatment showed a strong lemon odor that scored lower (P<0.05) than samples of other treatments and control over the period of the first 2 months of examination. By the third month, 0.5 O samples showed significantly lower odor scores compared to nanoemulsion samples (1N and 1.5N). As for taste, results showed degrading values over the storage period. The (0.5 O) burger samples were inedible due to the strong essential oil taste. Moreover, there was significant difference (P>0.05) between the samples of the control and the two other treatments contain lemongrass nanoemulsion “1N and 1.5 N”.
The results of total bacterial count showed that all nanoemulsion and essential oil samples had lower mean values of total bacterial count compared to control samples. Lowest mean values were seen in samples treated with 0.5 % LEO (0.5O) that differ significantly (P<0.05) with control samples over the whole period of examination. Addition of lemongrass nanoemulsion (1 N and 1.5 N) influenced numeral reduction in the total bacterial count compared to the control (P>0.05).
Coliform mean value was the lowest in samples of 0.5 O treatment (i.e. the highest antibacterial effect). Nanoemulsion was observed to have an inhibitory effect that increased with its concentration. After 48 hours a significant difference was observed between control samples and samples of the 0.5 O treatment; and the same was observed for the count at the 2nd month of storage. As well by the 3rd and 4th months, samples of all treatments showed a significant lower count than the controls (P<0.05).
The total yeast and mold count results showed a significant difference was present between control and both treatments (1.5 N and 0.5 O) by the 2nd month. There was a decrease in the mean count values of treatments compared to control samples. where control samples showed the highest mean values, while samples treated with the raw essential oil showed the lowest values over most of the storage period. The samples treated with the nanoemulsion showed as well lower counts with higher antifungal effect when a higher concentration of the nanoemulsion was used.
Characteristic E. coli colonies could not be identified in any of the examined samples (either for control or treatments).
The TBARS mean value was the lowest in samples treated with 1.5 % nanoemulsion (1.5 N) (0.825, 0.81, 0.84, 0.89, 0.93, and 0.95) compared to the control (0.825, 0.88, 1.18, 1.25, 1.29, and 1.29, respectively) (p<0.05) and other treatments (0.825, 0.84, 0.99, 0.99, 1.02, and 1.03 for 1 N samples ; and 0.825, 1.25, 1.23, 1.23, 1.27, and 1.22 for 0.5 O samples) over the whole period of storage. The mean TBARs values of samples treated with 0.5 % LEO were not considered due to the effect of the intense yellow color of lemongrass oil present in the samples causing false increase in spectrophotometer reading.
The TBARs value exceeded the permissible limit stipulated by Egyptian standards “0.9 mg /kg” for frozen meat, in both control and 1N samples by the 1st month while 1.5 N samples exceeded that limit by the 3rd month.
The TVBN mean values of all beef burger samples had increased during 4 months of frozen storage. The lower rate of increase was for 1.5 N treatment samples; showed lower mean TVBN values (17.27, 16.33, 16.80, 17.27, 17.73, and 20.07) compared to control 17.27, 18.20, 19.60, 20.53, 21.47, and 23.80, respectively) and other treatments (17.27, 16.33, 17.73, 18.67, 20.07 and 21.47 for 1 N samples ; and 17.27, 15.87, 19.13, 21.00, 19.13, and 21.47 for 0.5 O samples) with a significant difference against the control samples in the last 2 months of storage.
The mean values of TVBN exceeded the permissible limits (20 mg/100g) for frozen meat according to the Egyptian standards by the 2nd month of frozen storage in control samples and by the 3rd month in samples of 1 N treatment, while by the 4th month for 1.5 N and 0.5 O treatments samples.
The pH values of all beef burger samples increased during storage time; However, 1 N samples showed the lowest incremental pH values (6.03, 6.03, 6.07, 6.14, 6.16, and 6.24) compared to control samples (6.03, 6.06, 6.10, 6.20, 6.21, and 6.28, respectively) and other treatments (6.03, 6.02, 6.06, 6.16, 6.22, and 6.3 for 1.5 N samples ; and 6.03, 6.06, 6.14, 6.21, 6.23, and 6.35, for 0.5 O samples, respectively). Furthermore, there were significant difference between pH mean values of both nanoemulsion treatments (1 N and 1.5 N) and control samples; revealing the addition of lemongrass nanoemulsion to burger samples was controlling the increase in the pH.
The pH value exceeded the limit set for frozen meat “6.2” by the Egyptian standards; in samples of 0.5 O treatment by the 2nd month; in control and 1.5 N samples by the 3rd month; and in 1N samples by the 4th month of freeze storage.
The WHC mean values (%) of all burger samples assumed in most an increasing pattern throughout storage time. By 48 Hrs of storage there was a significant difference between means of control (lower mean value) and 0.5 O treatments (higher mean value) while by the 1st month, there were significant difference between both 0.5 O samples and 1 N samples with the control.
The cooking loss and yield results were significantly affected by added lemongrass nanoemulsion (P < 0.05), this was revealed by the presence of a significant difference between means of control samples and samples of 1.5 N treatment, however there was no difference (P > 0.05) between control and samples of 1 N treatment. Moreover, there was a significant difference between control samples and samples of 0.5 O treatment by the first month examination. By the end of storage period, the highest mean value of cooking loss was seen in 1N samples (30.45 %) and the lowest was recorded in 0.5 O samples (22.57 %). In sum, compared to control samples, addition of 0.5 % raw essential oil obviously decreased cooking loss and increased mean values of cooking yield % over the period of storage. As well, higher cooking yield were obtained from samples treated with the nanoemulsion (1 N and 1.5 N).
Initial diameter loss mean value was 22.33 %. The highest and lowest diameter loss % (26.67 and 23.00 %) at the end of frozen storage period was found in samples treated with 1 % LGNE (1N) and samples treated with 0.5 % LEO (0.5 O), respectively. Significant differences by 48 Hrs examination were found between means of control samples and samples of 0.5O treatment; as well as, between samples of the 3 different treatments (0.5 O, 1N, and 1.5N) where 0.5 O samples exhibited the lowest diameter loss value (23.00 %). It was noted that despite values of diameter loss % were oscillating. However, the values of 0.5 O samples were undeniably lower than values in control and other treatments.