Journal of Industrial Microbiology & Biotechnology, 2007
Absract The plasmid stability of three wild type Lactococcus lactis strains and their mutants was... more Absract The plasmid stability of three wild type Lactococcus lactis strains and their mutants was investigated at different incubation time and temperatures in two different media [M17 broth and reconstituted skim milk (RSM)]. The results showed that both incubation times and temperature are effective on plasmid loss. The plasmid profiles of wild type strains exhibited 8 to 9 distinct plasmid species with molecular weights from 2.1 to 24.0 kb. Lactose fermentation ability was found to be encoded by 22.2 (strain U70), 23.6 (strain U29) and 24.0 (strain U52) kb plasmids in the wild type strains, respectively. The stabilities of the plasmids were explained by applying a second-order polynomial modeling system. Reasonable fittings were obtained for the model and the adjusted regression coefficients (R 2 adj ) were between 0.76 and 0.99 for the overall data. Overall, it was found that incubation time had the most profound effect on plasmid stability, with plasmid loss occurring after 72 h, while temperatures in the range of 15-40°C also induced plasmid instability.
Four lactococcal bacteriophages (fLl6-2, fLl35-6, fLd66-36 and fLd67-42) in M17 broth were pressu... more Four lactococcal bacteriophages (fLl6-2, fLl35-6, fLd66-36 and fLd67-42) in M17 broth were pressurized at 300 and 350 MPa at room temperature and their survival curves were determined at various time intervals. Tailing (monotonic upward concavity) was observed in all survival curves. The resulting non-linear semi-logarithmic survival curves were described by the Weibull model and goodness of fit of this model was investigated. Regression coefficients (R 2 ), root mean square error (RMSE), residual and correlation plots strongly suggested that Weibull model produced a better fit to the data than the traditional linear model. Hazard plots suggested that the Weibull model was fully appropriate for the data being analyzed. These results have confirmed that the Weibull model, which is mostly utilized to describe the inactivation of bacterial cells or spores by heat and pressure, could be successfully used in describing the lactococcal bacteriophage inactivation by high hydrostatic pressure. D
The purpose of this study is to determine the survival and nisin production behaviors of two stra... more The purpose of this study is to determine the survival and nisin production behaviors of two strains of Lactococcus lactis under different stress conditions that represent the food ecosystem. In this respect, the survival ratios of two nisin producers were determined under different pH, temperature, NaCl, and bile salt concentrations. Then, nisin production levels of the strains were determined at each stress conditions. Both strains had similar growth or inactivation patterns under the same stress conditions. NaCl and bile salt stresses on the survival ratio of the strains could be successfully described by the exponential decay function, whereas Gaussian function produced good fits for temperature and pH stresses. The nisin activity of two nisin producers (in their mid-exponential and/or early stationary phase) decreased dramatically under all stress conditions, except osmotic (NaCl) and low temperature applications. The results of this study showed that two nisin producers had similar adaptive responses under severe stress conditions, which could be described by appropriate mathematical equations. Moreover, the effect of harsh environment on the nisin activity of L. lactis strains depends on the stress factors applied.
Ten lactococcal bacteriophages in M17 broth were treated by heat (72 1C, 15 min and 90 1C, 5 min)... more Ten lactococcal bacteriophages in M17 broth were treated by heat (72 1C, 15 min and 90 1C, 5 min), ethanol and isopropanol at concentrations of 10%, 50%, 75% and 100% (v/v). The heat treatments were not sufficient to inactivate all the bacteriophages studied whereas ethanol, which was also more efficient biocide than isopropanol, at a concentration of 75% could inactivate all the phages in 1 min. The 50% and 75% of ethanol were more effective than 100%, which was also observed for isopropanol. Survival curves' deviations from linearity as sigmoidal shapes could be described via an empirical model with four parameters. The parameters of the model could be reduced from four to two with a slight loss of goodness-of-fit. The reduced model also provided good fit to describe the isopropanol inactivation kinetics of the phages at concentrations of 50% and 75%. This demonstration may also be a useful tool for further inactivation studies in which ethanol or other biocides are used with sufficient survival data points at selected time intervals. Although more studies should be carried out with different phages, this study confirmed the lethal effect of ethanol, commonly used to disinfect utensils and laboratory equipment, on lactoccocal bacteriophages.
Ten heat-resistant lactococcal phages in M17 broth were inactivated by exposure to a range of dif... more Ten heat-resistant lactococcal phages in M17 broth were inactivated by exposure to a range of different hypochlorite concentrations (2000-5000 mg l À1 ). Deviations from first-order kinetics as sigmoidal shapes were observed in the survival curves of all bacteriophages. Therefore, an empirical model with four parameters was used to describe hypochlorite inactivation. The model provided good fit for all phages; however, it was observed that there was a high correlation between the parameters of this model. That is why, the number of parameters was reduced from four to two with a slight loss of goodness-of-fit and this reduced model produced fits comparable to the original model. Alternative D and z values were also proposed for the model. By comparing the times necessary to reduce the number of phages six-or seven-logs, the most hypochlorite-resistant phages were found as: fpld67 37, fpll47 21 and fpld66 36. Demonstration of this model may also be used for other bacteriophages inactivated by other biocides.
Survival curves of Lactococcus lactis subsp. lactis bacteriophage pll98 inactivated by heat were ... more Survival curves of Lactococcus lactis subsp. lactis bacteriophage pll98 inactivated by heat were obtained at seven temperature values (50-80 °C) in M17 broth and skim milk. Deviations from first-order kinetics in both media were observed as sigmoidal shapes in the survival curves of pll98. An empirical model with four parameters was used to define the thermal inactivation. Number of parameters of the model was reduced from four to two in order to increase the robustness of the model. The reduced model produced comparable fits to the full model. Both the survival data and the calculations done using the reduced model (time necessary to reduce the number of phage pll98 six-or seven-log10) indicated that skim milk is a more protective medium than M17 broth within the assayed temperature range.
Cold smoked salmon were HP treated at 220, 250 and 330 MPa, at 3, 7, 15 and 25°C for 5 and 10 min... more Cold smoked salmon were HP treated at 220, 250 and 330 MPa, at 3, 7, 15 and 25°C for 5 and 10 min. The influences of such treatments on some quality parameters (the changes of colour, TBA and TMA values) were studied. These parameters were determined for cold smoked salmon suitable combinations (at 220-250 MPa, 3°C for 5 min, at 330 MPa, 15°C for 5 min and at 250 MPa, 25°C for 10 min). In the second stage the shelf life of cold smoked salmon HP treated at 250 MPa, 3°C for 5 min and at 250 MPa, 25°C for 10 min and stored at 2°C was investigated by measurement of sensory, chemical and microbiological analyses. Based on the sensory and microbiological results, the control samples were acceptable only up to 6 weeks, compared to 8 weeks in HP treatment cold smoked salmon samples, extending the shelf-life by 2 weeks. Industrial relevance: Little information exists on the effects on physical and biochemical characteristics and shelf life of HP-treated cold smoked samples, compared to other preserved methods. This paper illustrates the changes induced in cold smoked salmon flesh by pressurization at different conditions. HP treatment significantly changed the sensory, chemical and microbiological properties of cold smoked salmons, and in combination with adequate chilled storage, can improve the shelf-life and safety of cold smoked salmons.
Degradation of the radiative properties of a 200 lm-polyethylene film caused by exposure to the h... more Degradation of the radiative properties of a 200 lm-polyethylene film caused by exposure to the harsh environment of Riyadh, Saudi Arabia has been investigated over a period of 13 months. Measurements of global solar radiation (GSR), photosynthetically active radiation (PAR), air temperature and relative humidity were made inside and outside two single-polyethylene-covered model structures. Each model structure was 1 m wide, 1 m long, and 0.75 m high. The east side of each model structure was open for natural ventilation. One of the model structures was exposed to the environment during the 13-month period, while the other was kept indoor in a room to protect the polyethylene film from environment and was used as a control when measurements were carried out. Results showed that exposure to the environment reduced the polyethylene film transmittance to GSR and PAR. The reduction in solar radiation transmitted minimized air temperatures inside the structure. The average summer daytime temperature inside the exposed structure was 45.7°C, as compared to 46.9°C inside the control structure, while the average of outside temperature was 38.2°C. The degradation of the polyethylene transmittance progresses rapidly during the first three months of exposure. The relative losses of GSR and PAR transmittance were 9% and 15.3%, respectively, after three months. However, the losses decreased to 5% and 8%, respectively, after 11 months due to rainfall in the area. Radiation transmittance was found to be dependent on season of the year. The averages (standard deviations) of GSR and PAR transmittance for the control film during winter months were 0.8 (0.01) and 0.75 (0.00), respectively. Whereas, during summer months, the averages (standard deviations) were 0.75 (0.02) and 0.71 (0.02), respectively. The averages (standard deviations) of GSR and PAR transmittance for the exposed film during winter months were 0.77 (0.03) and 0.70 (0.03), respectively, as compared to 0.68 (0.01) and 0.61 (0.02), respectively, during summer months.
One of the most common greenhouse covering materials in Saudi Arabia is polyethylene film. Howeve... more One of the most common greenhouse covering materials in Saudi Arabia is polyethylene film. However, polyethylene films are susceptible to mechanical failure due to harsh conditions of high temperature, solar radiation, and wind as occurs in Riyadh, Saudi Arabia. This study examined effects of ambient conditions on the deterioration of mechanical properties of polyethylene films over 14 months, using an experimentally cooled empty greenhouse (3.6 m length, 2.4 m width, and 3.6 m height) covered with a single layer of 200-m thick polyethylene. Three mechanical tests were conducted on the polyethylene samples: penetration, shear, and tension utilizing a SMS texture analyzer. The force-distance curves produced were characterized by two stages, the elastic and plastic regions. Mechanical properties were determined, including modulus of elasticity, rupture point, and total work for each stage of the three tests. Generally, mechanical resistance of the samples decreased with increased exposure time. The results of the tension tests were preferable to penetration and shear tests. For tension tests, work decreased from 21,693 N mm for new samples, to 6658 N mm after 14 months. Based on elongation at break data, the shelf life of polyethylene covers was 12 months under the tested environmental conditions. This illustrates the effect of arid conditions and age on the mechanical deterioration of polyethylene films. The presented data can be utilized to predict the deformation and mechanical behavior of greenhouse polyethylene covers at different exposure times under arid conditions.
Cooling greenhouses is essential to provide a suitable environment for plant growth in arid regio... more Cooling greenhouses is essential to provide a suitable environment for plant growth in arid regions characterized by brackish water resources. However, using conventional cooling methods are facing many challenges. Filtering out near infra-red radiation (NIR) at the greenhouse cover can significantly reduce the heating load and can solve the overheating problem of the greenhouse air. This paper is to review (i) the problems of using conventional cooling methods and (ii) the advantages of greenhouse covers that incorporate NIR reflectors. This survey focuses on how the cover type affects the transmittance of photosynthetically active radiation (PAR), the reflectance or absorptance of NIR and the greenhouse air temperature. NIR-reflecting plastic films seem to be the most suitable, low cost and simple cover for greenhouses under arid conditions. Therefore, this review discusses how various additives should be incorporated in plastic film to increase its mechanical properties, durability and ability to stand up to extremely harsh weather. Presently, NIR-reflecting covers are able to reduce greenhouse air temperature by no more than 5 • C. This reduction is not enough in regions where the ambient temperature may exceed 45 • C in summer. There is a need to develop improved NIR-reflecting plastic film covers.
The effects of four storage systems on physical and mechanical properties of dates (Khlass variet... more The effects of four storage systems on physical and mechanical properties of dates (Khlass variety) were experimentally measured for a period of 6 months. The first storage was supplied with a refrigerated air cooler that was on/off automatically operated based on inside air temperature. The second storage was equipped with an evaporative cooler unit that was operated automatically by controlling
Degradation of the radiative properties of a 200 lm-polyethylene film caused by exposure to the h... more Degradation of the radiative properties of a 200 lm-polyethylene film caused by exposure to the harsh environment of Riyadh, Saudi Arabia has been investigated over a period of 13 months. Measurements of global solar radiation (GSR), photosynthetically active radiation (PAR), air temperature and relative humidity were made inside and outside two single-polyethylene-covered model structures. Each model structure was 1 m wide, 1 m long, and 0.75 m high. The east side of each model structure was open for natural ventilation. One of the model structures was exposed to the environment during the 13-month period, while the other was kept indoor in a room to protect the polyethylene film from environment and was used as a control when measurements were carried out. Results showed that exposure to the environment reduced the polyethylene film transmittance to GSR and PAR. The reduction in solar radiation transmitted minimized air temperatures inside the structure. The average summer daytime temperature inside the exposed structure was 45.7°C, as compared to 46.9°C inside the control structure, while the average of outside temperature was 38.2°C. The degradation of the polyethylene transmittance progresses rapidly during the first three months of exposure. The relative losses of GSR and PAR transmittance were 9% and 15.3%, respectively, after three months. However, the losses decreased to 5% and 8%, respectively, after 11 months due to rainfall in the area. Radiation transmittance was found to be dependent on season of the year. The averages (standard deviations) of GSR and PAR transmittance for the control film during winter months were 0.8 (0.01) and 0.75 (0.00), respectively. Whereas, during summer months, the averages (standard deviations) were 0.75 (0.02) and 0.71 (0.02), respectively. The averages (standard deviations) of GSR and PAR transmittance for the exposed film during winter months were 0.77 (0.03) and 0.70 (0.03), respectively, as compared to 0.68 (0.01) and 0.61 (0.02), respectively, during summer months.
One of the most common greenhouse covering materials in Saudi Arabia is polyethylene film. Howeve... more One of the most common greenhouse covering materials in Saudi Arabia is polyethylene film. However, polyethylene films are susceptible to mechanical failure due to harsh conditions of high temperature, solar radiation, and wind as occurs in Riyadh, Saudi Arabia. This study examined effects of ambient conditions on the deterioration of mechanical properties of polyethylene films over 14 months, using an experimentally cooled empty greenhouse (3.6 m length, 2.4 m width, and 3.6 m height) covered with a single layer of 200-m thick polyethylene. Three mechanical tests were conducted on the polyethylene samples: penetration, shear, and tension utilizing a SMS texture analyzer. The force-distance curves produced were characterized by two stages, the elastic and plastic regions. Mechanical properties were determined, including modulus of elasticity, rupture point, and total work for each stage of the three tests. Generally, mechanical resistance of the samples decreased with increased exposure time. The results of the tension tests were preferable to penetration and shear tests. For tension tests, work decreased from 21,693 N mm for new samples, to 6658 N mm after 14 months. Based on elongation at break data, the shelf life of polyethylene covers was 12 months under the tested environmental conditions. This illustrates the effect of arid conditions and age on the mechanical deterioration of polyethylene films. The presented data can be utilized to predict the deformation and mechanical behavior of greenhouse polyethylene covers at different exposure times under arid conditions.
The effect of UV-C irradiation on inactivation of spoilage microorganisms and colour of freshly s... more The effect of UV-C irradiation on inactivation of spoilage microorganisms and colour of freshly squeezed orange juice were investigated. Orange juice samples were intentionally fermented in order to increase the natural microflora which were mostly composed of yeasts and then exposed to UV-C irradiation at an intensity level of 1.32 mW/cm(2) and sample depth of 0.153 cm for several exposure times by using a collimated beam apparatus. Applied UV dose was in the range of 0 and 108.42 mJ/cm(2). Resistance of yeast to UV light and existence of suspended particles limited the effectiveness of the process. Survival data obtained for yeasts was either described by the Weibull or traditional first-order model and goodness-of-fit of these models was investigated. Weibull model produced a better fit to the data with higher adjusted determination coefficient (R(2) adj) and lower mean square error (MSE) values which were 0.99 and 0.003, respectively. Time and UV dose of first decimal reduction were obtained as 5.7 min and 31 mJ/cm(2), respectively. The data suggests that biodosimetric studies performed by using inoculated microorganisms for assesment of the efficiency of UV irradiation treatment in the shelf life extension of juices must be carefully evaluated. UV-C irradiation had no influence on the colour of orange juice.
Journal of Industrial Microbiology & Biotechnology, 2007
Absract The plasmid stability of three wild type Lactococcus lactis strains and their mutants was... more Absract The plasmid stability of three wild type Lactococcus lactis strains and their mutants was investigated at different incubation time and temperatures in two different media [M17 broth and reconstituted skim milk (RSM)]. The results showed that both incubation times and temperature are effective on plasmid loss. The plasmid profiles of wild type strains exhibited 8 to 9 distinct plasmid species with molecular weights from 2.1 to 24.0 kb. Lactose fermentation ability was found to be encoded by 22.2 (strain U70), 23.6 (strain U29) and 24.0 (strain U52) kb plasmids in the wild type strains, respectively. The stabilities of the plasmids were explained by applying a second-order polynomial modeling system. Reasonable fittings were obtained for the model and the adjusted regression coefficients (R 2 adj ) were between 0.76 and 0.99 for the overall data. Overall, it was found that incubation time had the most profound effect on plasmid stability, with plasmid loss occurring after 72 h, while temperatures in the range of 15-40°C also induced plasmid instability.
Four lactococcal bacteriophages (fLl6-2, fLl35-6, fLd66-36 and fLd67-42) in M17 broth were pressu... more Four lactococcal bacteriophages (fLl6-2, fLl35-6, fLd66-36 and fLd67-42) in M17 broth were pressurized at 300 and 350 MPa at room temperature and their survival curves were determined at various time intervals. Tailing (monotonic upward concavity) was observed in all survival curves. The resulting non-linear semi-logarithmic survival curves were described by the Weibull model and goodness of fit of this model was investigated. Regression coefficients (R 2 ), root mean square error (RMSE), residual and correlation plots strongly suggested that Weibull model produced a better fit to the data than the traditional linear model. Hazard plots suggested that the Weibull model was fully appropriate for the data being analyzed. These results have confirmed that the Weibull model, which is mostly utilized to describe the inactivation of bacterial cells or spores by heat and pressure, could be successfully used in describing the lactococcal bacteriophage inactivation by high hydrostatic pressure. D
The purpose of this study is to determine the survival and nisin production behaviors of two stra... more The purpose of this study is to determine the survival and nisin production behaviors of two strains of Lactococcus lactis under different stress conditions that represent the food ecosystem. In this respect, the survival ratios of two nisin producers were determined under different pH, temperature, NaCl, and bile salt concentrations. Then, nisin production levels of the strains were determined at each stress conditions. Both strains had similar growth or inactivation patterns under the same stress conditions. NaCl and bile salt stresses on the survival ratio of the strains could be successfully described by the exponential decay function, whereas Gaussian function produced good fits for temperature and pH stresses. The nisin activity of two nisin producers (in their mid-exponential and/or early stationary phase) decreased dramatically under all stress conditions, except osmotic (NaCl) and low temperature applications. The results of this study showed that two nisin producers had similar adaptive responses under severe stress conditions, which could be described by appropriate mathematical equations. Moreover, the effect of harsh environment on the nisin activity of L. lactis strains depends on the stress factors applied.
Ten lactococcal bacteriophages in M17 broth were treated by heat (72 1C, 15 min and 90 1C, 5 min)... more Ten lactococcal bacteriophages in M17 broth were treated by heat (72 1C, 15 min and 90 1C, 5 min), ethanol and isopropanol at concentrations of 10%, 50%, 75% and 100% (v/v). The heat treatments were not sufficient to inactivate all the bacteriophages studied whereas ethanol, which was also more efficient biocide than isopropanol, at a concentration of 75% could inactivate all the phages in 1 min. The 50% and 75% of ethanol were more effective than 100%, which was also observed for isopropanol. Survival curves' deviations from linearity as sigmoidal shapes could be described via an empirical model with four parameters. The parameters of the model could be reduced from four to two with a slight loss of goodness-of-fit. The reduced model also provided good fit to describe the isopropanol inactivation kinetics of the phages at concentrations of 50% and 75%. This demonstration may also be a useful tool for further inactivation studies in which ethanol or other biocides are used with sufficient survival data points at selected time intervals. Although more studies should be carried out with different phages, this study confirmed the lethal effect of ethanol, commonly used to disinfect utensils and laboratory equipment, on lactoccocal bacteriophages.
Ten heat-resistant lactococcal phages in M17 broth were inactivated by exposure to a range of dif... more Ten heat-resistant lactococcal phages in M17 broth were inactivated by exposure to a range of different hypochlorite concentrations (2000-5000 mg l À1 ). Deviations from first-order kinetics as sigmoidal shapes were observed in the survival curves of all bacteriophages. Therefore, an empirical model with four parameters was used to describe hypochlorite inactivation. The model provided good fit for all phages; however, it was observed that there was a high correlation between the parameters of this model. That is why, the number of parameters was reduced from four to two with a slight loss of goodness-of-fit and this reduced model produced fits comparable to the original model. Alternative D and z values were also proposed for the model. By comparing the times necessary to reduce the number of phages six-or seven-logs, the most hypochlorite-resistant phages were found as: fpld67 37, fpll47 21 and fpld66 36. Demonstration of this model may also be used for other bacteriophages inactivated by other biocides.
Survival curves of Lactococcus lactis subsp. lactis bacteriophage pll98 inactivated by heat were ... more Survival curves of Lactococcus lactis subsp. lactis bacteriophage pll98 inactivated by heat were obtained at seven temperature values (50-80 °C) in M17 broth and skim milk. Deviations from first-order kinetics in both media were observed as sigmoidal shapes in the survival curves of pll98. An empirical model with four parameters was used to define the thermal inactivation. Number of parameters of the model was reduced from four to two in order to increase the robustness of the model. The reduced model produced comparable fits to the full model. Both the survival data and the calculations done using the reduced model (time necessary to reduce the number of phage pll98 six-or seven-log10) indicated that skim milk is a more protective medium than M17 broth within the assayed temperature range.
Cold smoked salmon were HP treated at 220, 250 and 330 MPa, at 3, 7, 15 and 25°C for 5 and 10 min... more Cold smoked salmon were HP treated at 220, 250 and 330 MPa, at 3, 7, 15 and 25°C for 5 and 10 min. The influences of such treatments on some quality parameters (the changes of colour, TBA and TMA values) were studied. These parameters were determined for cold smoked salmon suitable combinations (at 220-250 MPa, 3°C for 5 min, at 330 MPa, 15°C for 5 min and at 250 MPa, 25°C for 10 min). In the second stage the shelf life of cold smoked salmon HP treated at 250 MPa, 3°C for 5 min and at 250 MPa, 25°C for 10 min and stored at 2°C was investigated by measurement of sensory, chemical and microbiological analyses. Based on the sensory and microbiological results, the control samples were acceptable only up to 6 weeks, compared to 8 weeks in HP treatment cold smoked salmon samples, extending the shelf-life by 2 weeks. Industrial relevance: Little information exists on the effects on physical and biochemical characteristics and shelf life of HP-treated cold smoked samples, compared to other preserved methods. This paper illustrates the changes induced in cold smoked salmon flesh by pressurization at different conditions. HP treatment significantly changed the sensory, chemical and microbiological properties of cold smoked salmons, and in combination with adequate chilled storage, can improve the shelf-life and safety of cold smoked salmons.
Degradation of the radiative properties of a 200 lm-polyethylene film caused by exposure to the h... more Degradation of the radiative properties of a 200 lm-polyethylene film caused by exposure to the harsh environment of Riyadh, Saudi Arabia has been investigated over a period of 13 months. Measurements of global solar radiation (GSR), photosynthetically active radiation (PAR), air temperature and relative humidity were made inside and outside two single-polyethylene-covered model structures. Each model structure was 1 m wide, 1 m long, and 0.75 m high. The east side of each model structure was open for natural ventilation. One of the model structures was exposed to the environment during the 13-month period, while the other was kept indoor in a room to protect the polyethylene film from environment and was used as a control when measurements were carried out. Results showed that exposure to the environment reduced the polyethylene film transmittance to GSR and PAR. The reduction in solar radiation transmitted minimized air temperatures inside the structure. The average summer daytime temperature inside the exposed structure was 45.7°C, as compared to 46.9°C inside the control structure, while the average of outside temperature was 38.2°C. The degradation of the polyethylene transmittance progresses rapidly during the first three months of exposure. The relative losses of GSR and PAR transmittance were 9% and 15.3%, respectively, after three months. However, the losses decreased to 5% and 8%, respectively, after 11 months due to rainfall in the area. Radiation transmittance was found to be dependent on season of the year. The averages (standard deviations) of GSR and PAR transmittance for the control film during winter months were 0.8 (0.01) and 0.75 (0.00), respectively. Whereas, during summer months, the averages (standard deviations) were 0.75 (0.02) and 0.71 (0.02), respectively. The averages (standard deviations) of GSR and PAR transmittance for the exposed film during winter months were 0.77 (0.03) and 0.70 (0.03), respectively, as compared to 0.68 (0.01) and 0.61 (0.02), respectively, during summer months.
One of the most common greenhouse covering materials in Saudi Arabia is polyethylene film. Howeve... more One of the most common greenhouse covering materials in Saudi Arabia is polyethylene film. However, polyethylene films are susceptible to mechanical failure due to harsh conditions of high temperature, solar radiation, and wind as occurs in Riyadh, Saudi Arabia. This study examined effects of ambient conditions on the deterioration of mechanical properties of polyethylene films over 14 months, using an experimentally cooled empty greenhouse (3.6 m length, 2.4 m width, and 3.6 m height) covered with a single layer of 200-m thick polyethylene. Three mechanical tests were conducted on the polyethylene samples: penetration, shear, and tension utilizing a SMS texture analyzer. The force-distance curves produced were characterized by two stages, the elastic and plastic regions. Mechanical properties were determined, including modulus of elasticity, rupture point, and total work for each stage of the three tests. Generally, mechanical resistance of the samples decreased with increased exposure time. The results of the tension tests were preferable to penetration and shear tests. For tension tests, work decreased from 21,693 N mm for new samples, to 6658 N mm after 14 months. Based on elongation at break data, the shelf life of polyethylene covers was 12 months under the tested environmental conditions. This illustrates the effect of arid conditions and age on the mechanical deterioration of polyethylene films. The presented data can be utilized to predict the deformation and mechanical behavior of greenhouse polyethylene covers at different exposure times under arid conditions.
Cooling greenhouses is essential to provide a suitable environment for plant growth in arid regio... more Cooling greenhouses is essential to provide a suitable environment for plant growth in arid regions characterized by brackish water resources. However, using conventional cooling methods are facing many challenges. Filtering out near infra-red radiation (NIR) at the greenhouse cover can significantly reduce the heating load and can solve the overheating problem of the greenhouse air. This paper is to review (i) the problems of using conventional cooling methods and (ii) the advantages of greenhouse covers that incorporate NIR reflectors. This survey focuses on how the cover type affects the transmittance of photosynthetically active radiation (PAR), the reflectance or absorptance of NIR and the greenhouse air temperature. NIR-reflecting plastic films seem to be the most suitable, low cost and simple cover for greenhouses under arid conditions. Therefore, this review discusses how various additives should be incorporated in plastic film to increase its mechanical properties, durability and ability to stand up to extremely harsh weather. Presently, NIR-reflecting covers are able to reduce greenhouse air temperature by no more than 5 • C. This reduction is not enough in regions where the ambient temperature may exceed 45 • C in summer. There is a need to develop improved NIR-reflecting plastic film covers.
The effects of four storage systems on physical and mechanical properties of dates (Khlass variet... more The effects of four storage systems on physical and mechanical properties of dates (Khlass variety) were experimentally measured for a period of 6 months. The first storage was supplied with a refrigerated air cooler that was on/off automatically operated based on inside air temperature. The second storage was equipped with an evaporative cooler unit that was operated automatically by controlling
Degradation of the radiative properties of a 200 lm-polyethylene film caused by exposure to the h... more Degradation of the radiative properties of a 200 lm-polyethylene film caused by exposure to the harsh environment of Riyadh, Saudi Arabia has been investigated over a period of 13 months. Measurements of global solar radiation (GSR), photosynthetically active radiation (PAR), air temperature and relative humidity were made inside and outside two single-polyethylene-covered model structures. Each model structure was 1 m wide, 1 m long, and 0.75 m high. The east side of each model structure was open for natural ventilation. One of the model structures was exposed to the environment during the 13-month period, while the other was kept indoor in a room to protect the polyethylene film from environment and was used as a control when measurements were carried out. Results showed that exposure to the environment reduced the polyethylene film transmittance to GSR and PAR. The reduction in solar radiation transmitted minimized air temperatures inside the structure. The average summer daytime temperature inside the exposed structure was 45.7°C, as compared to 46.9°C inside the control structure, while the average of outside temperature was 38.2°C. The degradation of the polyethylene transmittance progresses rapidly during the first three months of exposure. The relative losses of GSR and PAR transmittance were 9% and 15.3%, respectively, after three months. However, the losses decreased to 5% and 8%, respectively, after 11 months due to rainfall in the area. Radiation transmittance was found to be dependent on season of the year. The averages (standard deviations) of GSR and PAR transmittance for the control film during winter months were 0.8 (0.01) and 0.75 (0.00), respectively. Whereas, during summer months, the averages (standard deviations) were 0.75 (0.02) and 0.71 (0.02), respectively. The averages (standard deviations) of GSR and PAR transmittance for the exposed film during winter months were 0.77 (0.03) and 0.70 (0.03), respectively, as compared to 0.68 (0.01) and 0.61 (0.02), respectively, during summer months.
One of the most common greenhouse covering materials in Saudi Arabia is polyethylene film. Howeve... more One of the most common greenhouse covering materials in Saudi Arabia is polyethylene film. However, polyethylene films are susceptible to mechanical failure due to harsh conditions of high temperature, solar radiation, and wind as occurs in Riyadh, Saudi Arabia. This study examined effects of ambient conditions on the deterioration of mechanical properties of polyethylene films over 14 months, using an experimentally cooled empty greenhouse (3.6 m length, 2.4 m width, and 3.6 m height) covered with a single layer of 200-m thick polyethylene. Three mechanical tests were conducted on the polyethylene samples: penetration, shear, and tension utilizing a SMS texture analyzer. The force-distance curves produced were characterized by two stages, the elastic and plastic regions. Mechanical properties were determined, including modulus of elasticity, rupture point, and total work for each stage of the three tests. Generally, mechanical resistance of the samples decreased with increased exposure time. The results of the tension tests were preferable to penetration and shear tests. For tension tests, work decreased from 21,693 N mm for new samples, to 6658 N mm after 14 months. Based on elongation at break data, the shelf life of polyethylene covers was 12 months under the tested environmental conditions. This illustrates the effect of arid conditions and age on the mechanical deterioration of polyethylene films. The presented data can be utilized to predict the deformation and mechanical behavior of greenhouse polyethylene covers at different exposure times under arid conditions.
The effect of UV-C irradiation on inactivation of spoilage microorganisms and colour of freshly s... more The effect of UV-C irradiation on inactivation of spoilage microorganisms and colour of freshly squeezed orange juice were investigated. Orange juice samples were intentionally fermented in order to increase the natural microflora which were mostly composed of yeasts and then exposed to UV-C irradiation at an intensity level of 1.32 mW/cm(2) and sample depth of 0.153 cm for several exposure times by using a collimated beam apparatus. Applied UV dose was in the range of 0 and 108.42 mJ/cm(2). Resistance of yeast to UV light and existence of suspended particles limited the effectiveness of the process. Survival data obtained for yeasts was either described by the Weibull or traditional first-order model and goodness-of-fit of these models was investigated. Weibull model produced a better fit to the data with higher adjusted determination coefficient (R(2) adj) and lower mean square error (MSE) values which were 0.99 and 0.003, respectively. Time and UV dose of first decimal reduction were obtained as 5.7 min and 31 mJ/cm(2), respectively. The data suggests that biodosimetric studies performed by using inoculated microorganisms for assesment of the efficiency of UV irradiation treatment in the shelf life extension of juices must be carefully evaluated. UV-C irradiation had no influence on the colour of orange juice.
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