الفهرس 
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Abstract
There is a steady increase and interest demand for processed meat and poultry products world wide. In Egypt there is a number of processed meat and poultry products commercially available in retail and supermarkets. Processed meat and poultry products are often contaminated by various kinds of spoilage microorganisms which come from different sources. The highly contamination with spoilage microorganisms lead to short shelf-life of these products even at refrigerated temperature. There is also possible presence of many pathogenic microorganisms in these products. It is known that some processed meat products can be consumed without further cooking, others may not have received enough temperature treatment while being processed. This, therefore, could lead to the survival of pathogenic microorganisms and several outbreaks. of food borne illness caused by the consumption of these contaminated products creating a serious public
health problem.
Irradiation of foods for the purpose of killing endogenous microflora and thereby extending shelf-life has been recognized as a preservation technique for several decades. It can eliminate food borne pathogens contaminating meat products efficiently. Sources of irradiation used in food preservation come from gamma rays Co-60 or Cs-137 or from electron beams in the form of high energy electrons. Gamma rays emitted from Co-60 or most commonly used.
This study was carried out to evaluate the microbiological quality of some meat products. The products examined in the survey were meat luncheon, chicken luncheon, meat shawerma, hamburger, chicken shawerma and fish fillet (25 survey samples of each product were tested). The microbiological examination included . the enumeration of the
microbial load together with .the detection of some bacteria of public health significance, i.e. total aerobic bacterial count (TABC), total mold
and yeast counts, S. aureus, E. coli, Enterococcus Faecalis, Salmonella, Listeria monocytogenes and Aeromonas hydrophila.
The effect of different gamma irradiation doses (1.5, 3.0 and 5.0 Kgy) immediately after irradiation and during storage (refrigerated or
frozen) on the microbial, sensory and chemical properties of the selected meat and poultry, namely meat luncheon, chicken luncheon brand 1, chicken luncheon brand 2, meat shawerma and hamburger were studied.
Microbiological, sensorial and chemical quality attributes were used in evaluating the shelf-life and quality of the irradiated products.
The obtained results could be summarized as follow:
1. Survey :
The mean TABC in meat luncheon, chicken luncheon, meat shawerma, hamburger, chicken shawerma, and fish fillet were 6.8x106, 1.0x107, 1.6x106, • 1.2x105, 2.1x105 and 2.4x104 CFU/g, respectively. According to ES, 96% of meat luncheon, 100% of chicken luncheon and 8% of hamburger should be rejected as the TABC is more than the permissible level. The mean value of total mold and yeast counts in meat luncheon, chicken luncheon, meat shawerma, hamburger, chicken shawerma, and fish fillet were 1.4x103, 2.8x103, .3x10,<10 (undetectable), 1.6x103 and 1.1x10 CFU/g, respectively.
E. coli was also found in all products under investigation. The mean counts in meat luncheon, chicken luncheon, meat shawerma, hamburger, chicken shawerma, and fish fillet were 1.4x 102, 7.8, 6.5x10, 2. Experimental (Irradiation of selected products). 2.1 Microbiological aspects
Irradiation of meat luncheon, chicken luncheon brand 1, chicken luncheon brand 2, meat shawerma and hamburger (selected meat products) by using 1.5, 3.0 and 5.0 kGy induced great reduction in the microbial load. This reduction was proportional with the irradiation dose. These irradiation doses also caused great reduction in some pathogens and / or complete elimination of the others. During storage, the counts of the previously mentioned microorganisms increased, but the rate of increase was slower as the irradiation dose increased.
2.2 Sensory attributes
Sensory analysis done by 10 untrained panelists on appearance, odor and texture revealed that unirradiated refrigerated products (meat luncheon, chicken luncheon brand 1, chicken luncheon brand 2) were rejected after only one week of storage at ± 1 ° C due to the slime formation on the surface of these products with average scores < 4 for the criteria studied. Samples irradiated with 1.5 kGy were rejected by the panelists after 2 weeks of refrigerated storage due to slime formation with average score < 4 for the appearance, odor and texture. Samples irradiated with 3.0 kGy were rejected by the panelists after 4 weeks of refrigerated storage due to slime formation with average scores <4.5 (in meat luncheon) and <3.5 (in chicken luncheon brand 1, 2) for the appearance,
odor and texture.
All studied meat products receiving 5.0 kGy irradiation dose had acceptable sensory scores even after 4 weeks of refrigerated storage. In frozen meat products (meat shawerma and hamburger) there was almost no difference between the average scores given for the appearance, odor, texture in unirradiated and irradiated (1.5, 3.0- and 5.0 kGy) samples immediately after irradiation. (0-time). Also there was almost no difference in the average scores given for appearance, odor and texture at 0-time and at the end of frozen (-18 ° C) storage period. (after 3 months) in
hamburger.
In meat shawerma, although the average scores given for the studied criteria decreased slightly after 3 months of frozen storage, the samples were still in a good quality and accepted by the panelists.
2.4 Chemical properties
The moisture percent in unirradiated and irradiated (1.5, 3.0 and 5.0 kGy) refrigerated products (meat luncheon, chicken luncheon brand 2) remained almost unchanged immediately after irradiation (0-time). By refrigerated (4±1 ° C) storage moisture % slightly decreased. In frozen products (meat shawerma, hamburger) the moisture % slightly decreased as the irradiation dose increased. After 3 months of frozen storage, the moisture % decreased in all samples compared with
those at 0-time.
Thiobarbituric acid value (TBA) in refrigerated and frozen meat products immediately after irradiation slightly increased as the irradiation dose increased. The increase in TBA was more evident as the storage
period increased.
Fatty acids methyl esters of lipids extracted from unirradiated meat products samples (meat luncheon, chicken luncheon brand 1, meat shawerma and hamburger) and those exposed to the highest irradiation dose, i.e. (5.0 kGy) were analysed by using GC analysis immediately after irradiation (0-time) and at the end of the storage period (refrigerated or frozen) and the average relative percent of the main fatty acids were determined. The main fatty acids present were palmitic (16:0), palmetoleic (16:1), stearic (18:0), oleic (18:1) and linoleic (18:2). The exposure to 5.0 kGy gamma irradiation induced very little changes in the quantities of various fatty acids compared with unirradiated samples . At the same time the ratio of total saturated fatty acids to total unsaturated fatty acid (Ts:Tu) was almost unaffected in the irradiated samples compared with unirradiated ones. By storage there was slight difference in the relative percent of the main fatty acids composition.
Total amino acids, of unirradiated samples and those exposed to the highest irradiation dose (5 kGy) (rather than tryptophan) were analysed (after hydrolysis with 6.0 N HC1) using dedecated amino acid analyzer immediately after irradiation and at the end of the storage period and the level of each amino acid expressed. as mg amino acid /g protein. The results revealed that 17 amino acids were detected (except for meat luncheon only 15 amino acids) eight of them were considered essential
from the view of nutrition.
Glutamic acid was the predominant amino acid followed by aspartic acid. The exposure to 5.0 kGy gamma irradiation induced a decrease in the amounts of the total amino acids of the previously mentioned meat products except for meat luncheon and chicken luncheon
brand 1 at 0-time which increased.
Finally it could be concluded that the application of 5.0 kGy gamma irradiation greatly reduced the microbial load of studied meat products, hence extended their refrigerated shelf life. This irradiation dose was very effective and sufficient in eliminating all contaminated studied pathogenic bacteria hence rendered these processed meat products free from pathogens and safe for human consumption. In addition, this irradiation dose (5.0 kGy) did not cause adverse effect on the sensory or chemical quality attributes of these products.