الفهرس 
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Abstract
White cheese is very good source of nutritional value for human consumption. This type of cheese produce either by enzymatic or acidic coagulation of fresh milk or reconstituted skim milk powder containing some food additives with vegetable oils. The microbial load of pasteurized white soft cheese is determined by several factors, including the quality of milk, heat treatment, and transportation temperature and storage conditions.
The of aim of this study is to control and achieve microbiological hazards and food safety system in white cheese. The study plan was conducted in three parts as follows.
Part one: Evaluate quality and safety of soft white cheese from some dairy plants.
Results of this part could be summarized as follows.
1. Physiochemical composition assessment
The obtained data for physiochemical revealed that most of the examined samples recorded very low protein, sodium chloride and high fat contents. The means of protein content ranged from 7.83 to 9.50%. The average of fat content ranged from 21.37% to 24.5%. NaCl ranged from 2.5 to 3.6 %.
2. Preservatives substance assessment
Preservative substances (formalin, hydrogen peroxide, beta lactam antibiotics residues, potassium sorbate, sodium benzoate, sulfur dioxide, nitrate, nisin and natamycin were determined in all samples. The mean values of potassium sorbate ranged between 850 to 1100 ppm, potassium benzoate from 450 to 500 ppm, nitrate ranged from 38 to 45 ppm, Nisin from 13 to 16 ppm and natamycin 35 to 55 ppm in cheese samples. Preservatives agents’ content of tested samples were higher than the standard limit. Preservatives content of the samples higher than standard limits.
3. Microbiological Assessment
Total aerobic bacterial counts (ABC) in Double cream cheese samples were ranged from 2.36 to 4.95 log cfu/g. ABC in Paramely cheese samples were ranged from 2.11 to 4.51 log cfu/g. ABC in Istanbuly cheese was ranged between 2.07 and 3.60 log cfu/g. ABC in Feta cheese samples were ranged from 1.09 to 2.69 log cfu/g.
Total yeast and molds count in Double cream samples were ranged from 1.77 to 4.25. Yeast and molds count in Paramely samples were ranged from 2.04 to 4.32 log cfu/g. Yeast and molds count in Istanbuly samples were ranged from 1.69 to 3.96 log cfu/g. Yeast and molds count in Feta samples were ranged from 1.84 to 5.40 log cfu/g. Cheese samples were contaminated by lactic acid bacteria, Coliform group, Staphylococcus aureus, and some pathogenic bacteria were not detected in any sample. According to the soft white cheese standard, almost the mean of microbial counts in all cheese samples were above standard limits. The microbiological and chemical parameters of soft white cheese samples in this study showed unacceptable results for most samples.
Part two: Assessment of some risk preventive controls in soft white cheese production
1. Application of predictive ComBase models
The application aim of ComBase models’ study were to determine the survival kinetics of some pathogens in soft white cheese under different pH, different salt level, and different storage time at 5◦C. The predicted growth parameters indicated that decrease growth of all the studied bacterial strains may occur at temperatures 5°C, even in combination with the high salt levels (2.5 and 3.5%) and a pH as low as 3.5.
The Grmax under different conditions for E. coli was ranged from 0.003 to 0.011 log CFU/ml/h, for Salmonella typhimurium was ranged from 0.003 to 0.006 log CFU/g/h, for Staphylococcus aureus was ranged from 0.004 to 0.005 log CFU/ml/h, for Listeria monocytogenes was ranged from 0.003 to 0.004 log CFU/g/h, for and Bacillus cereus was ranged from 0.016 to 0.004 log CFU/g/h.
2. Using of Nisin producing lactic acid bacteria
The aim of the present study was using lactic acid bacteria as a natural source for Nisin improvement the safety and quality of soft white cheese.
The antimicrobial activity of nisin producing bacteria (Lactococcus lactis ATCC 11454) was tested by agar well diffusion method, it was found that Nisin showed antagonistic effects against pathogenic organisms. They exhibited maximal inhibitory activity against pathogens like Staphylococcus aureus (14 mm), Micrococcus luteus (12 mm), and Bacillus subtillus (10 mm), by using agar Well-Diffusion method. Our study showed that the partially purified Nisin exhibited a molecular weight of 3.5 kDa. Nisin was strongly influenced by factors like pH, salinity, and incubation temperature. This study confirms the probable application of the strain of Lactococcus lactis ATCC 11454 as a bio preservative and probiotic.
3. Using of herbal and medicinal plant extracts and their essential oils as antimicrobial agents.
The aim of the present study was using plant extracts and essential oils as a natural source to improvement food safety of the soft white cheese.
- Antimicrobial activity of essential oils: Eleven spice of plant extract and essential oils were tested for their inhibitory effect against some microorganisms. Thyme, cumin, and rosemary extracted, and essential oils had the highest antimicrobials activity against tested microorganisms, while Caraway essential oil showed the least activity. Determine minimum inhibitory concentration (MICs) of essential oils extracted from Cumin, rosemary and Thyme and mixture was assessment against
Listeria monocytogenes ATCC 19115, Escherichia coli ATCC 6933, Staphylococcus aureus ATCC 20231, Salmonella typhimurium ATCC 14028, Bacillus cereus ATCC 33018, Candida spp. and Aspergillus niger. The minimum inhibitory concentrations (MICs) of essential oils (Thyme Cumin, and Rosemary) ranged from 0.063 to 1%. The results revealed that MICs for Thyme and Cumin essential oils were 0.125 % and 0.25% respectively, while MIC for Rosemary essential oil was 0.50%. Thyme and Cumin oils possessed very strong antimicrobial activity compared with the other essential oils. Essential oils of Thyme, Cumin and Rosemary have remarkable antioxidant activity and antimicrobial effect on the growth of pathogenic microorganisms, so they were analyzed by G/C mass for determine volatile compounds.
Antioxidant activity of essential oils and effect of thyme essential oil on ultrastructure morphology of Escherichia coli: Thyme essential oil extract showed the highest antioxidant activity particularly at high concentrations (300 to 400 ppm compared to the same concentrations of Butylated Hydroxy Toluene) BHT(as antioxidant standard. Effect of Thyme essential oil on ultrastructure morphology of Escherichia coli ATCC 6933 was examined by electron microscope. Results showed that treatment of E. coli with Thyme essential oil resulted in partial degradation of cell membrane, increased membrane permeability, leakage of cytoplasm materials, and prominent distortion and shrinkage of bacterial cells. There was a relationship between the chemical structures of the most abundant compounds in the essential oils under investigation and the antimicrobial activity.
4. Combined antimicrobial activity of Lactococcus lactis ATCC11454, thyme oil and cumin oil.
Our study shown survival of L. lactis ATCC 11454 in MRS medium broth contain 0.125% Thyme, 0.25%C oils and combination of them. The results showed a not significant statistical difference for L. lactis ATCC 11454 survival between control and treatment containing various types of essential oils. Results indicated that values of tested pathogenic bacteria indicators in all treatments decreased during incubation time. However, all treatments significantly inhibited microbial growth of pathogenic bacteria when compared to the control.
Part three: Development and validation of a food safety plan for pasteurized soft white cheese with combined risk preventive controls.
The objective of this study was to development and validate of a food safety plan of soft white cheese. Soft white cheese was manufactured with different salt levels, Lactococcus lactis ATCC 11454 as a source of Nisin and essential oils were also used. Ripening periods were after 7, 14, 28, 45, 60 and 90 days in refrigerator at 5° C. The cheese was sampled for microbiological, physiochemical, rheological analysis and sensory evaluation analysis at zero-time,7, 14, 28, 45, 60 and 90 days in refrigerator at 5° C.
Pathogenic bacteria counts of all cheese treatments decreased with the presence of Thyme and Cumin essential oils and combination of them (essential oils mixture) compared with the control cheese effect on which may be attributed to the antimicrobial effect of essential oils and mixture of them under different salt levels at 5°C , Lactococcus lactis (nisin producing) can be used to extend the shelf life of soft cheese for 21 days reach to 90 days at 5 °C under different salt levels (2.5±1 % NaCl) in soft white cheese .
The changes in physicochemical properties during storage of the tested soft white cheese during storage for 90 days at 5° C were revelated that the moisture content of soft white cheese samples were the same as the control sample throughout the storage period without changes. All cheese samples showed slight decrease in pH value and an increase in titratable acidity values during refrigerated storage. Ash content in cheese samples were no meaning.
The changes in textural attributes during refrigerated storage of soft white cheese containing essential oils at 2.5% salt. During storage, all cheese samples showed the same trend of results for hardness, cohesiveness, springiness, gumminess, and chewiness decreased while adhesiveness increased. The texture of cheese sample treated by Thyme and mixture (Cumin, Thyme) essential oil had the lowest values for the same textural attributes comparing to the control treatment.
All cheese samples were analyzed for sensory properties (taste, texture, colour and appearance). The sensory properties results indicated that all the data for the different sensory properties increased with increasing the storage period of soft white cheese samples either untreated or treated with the different essential oils.