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    The combined effect of probiotic cultures and incubation final pH on the quality of buffalo milk yogurt during cold storage
    (Wiley, 2018) Akgun, Abdullah; Yazici, Fehmi; Gulec, Haci Ali
    The combined effects of starter culture type (SCT) and incubation final pH (IFpH) on the physicochemical and organoleptic properties of buffalo milk yogurt containing 3g100g(-1) milk fat were investigated throughout 20days of storage at 4 degrees C. The postacidification kinetics fitted to zero-order reaction for all buffalo milk yogurt samples. The reaction rate constants of the buffalo milk yogurt samples containing YC-X11, ABY-2, and ABT-4 cultures were 0.010, 0.007, and 0.004g100g(-1)day(-1), respectively. Regardless of the IFpH, the absence of Lactobacillus delbrueckii subsp. bulgaricus in the starter culture increased the syneresis. L*, a*, and b* values were not affected by the IFpH and the SCT. ABY-2 culture increased the amount of organic acids during cold storage in comparison with the YC-X11, while its effect on the proportions of saturated and unsaturated fatty acids was not significant. The results of sensory evaluation revealed that a more acceptable buffalo milk yogurt can be manufactured by using probiotic ABY-2 culture.
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    Effect of flow behaviour of skim milk on microfiltration performance and cleaning efficiency of polyethersulfone membranes
    (Vup Food Research Inst, Bratislava, 2024) Akgun, Abdullah; Keser, Irem
    The effect of milk rheology on membrane fouling remains a challenging problem for long-term microfiltration. This study aimed to investigate the effects of viscosity changes as a function of weight reduction factor (WRF) and temperature on the performance of commercial polyethersulfone membrane. The retentates were obtained from skim milk in batch concentration mode with WRF of 2.5, 3.5, 4.5 and 5.5, to investigate the effect of change in feed composition on polyethersulfone (PES) membrane performance. The performance of the PES membrane was evaluated with pure water before the microfiltration of milk and it was discovered that using low transmembrane pressure and pH levels above 5.0 resulted in more consistent water flux. The two -fold increase in the transmembrane pressure caused up to ten -fold increase in irreversible fouling resistance. At WRF greater than 2.5, the non -Newtonian pseudoplastic behaviour of the samples obtained became more pronounced, resulting in a more pronounced decrease in flux for the batch concentration mode at 55 degrees C compared to the total recycling mode. The rheological and chemical composition properties of the streams affected by varying microfiltration parameters are important for the dairy products in which they are used, as well as for innovative production processes where quality of whey proteins is required to be preserved.
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    Effect of oleaster (Elaeagnus angustifolia L.) flour addition combined with high-pressure homogenization on the acidification kinetics, physicochemical, functional, and rheological properties of kefir
    (Wiley, 2023) Gul, Latife Betul; Bekbay, Saya; Akgun, Abdullah; Gul, Osman
    In this study, the effects of peeled oleaster flour (OF) addition (0.5% and 1%) with high-pressure homogenization (HPH) at 100 MPa on acidification kinetics, physicochemical, functional, and rheological properties of kefir made from bovine whole milk were investigated. The fermentation kinetic parameters such as Vmax and T-f decreased by 23.63% and 20%, respectively, with 1% OF and application of HPH. The combined use of two treatments had a positive effect on Lactobacillus and Lactococcus counts, reaching a maximum of 9.63 and 9.31 log cfu/mL, respectively. Also, total phenolic contents and antioxidant activity reached maximum values of 85.31 mg GAE/g and 17.22%, respectively. The ?E value was more limited with HPH. The maximum firmness and water-holding capacity values were determined in the sample produced with 1% OF and application of HPH. Rheological analysis revealed that all kefirs exhibited shear thinning behavior, and the Ostwald-de-Waele (R-2 > .99) model was suitable to describe the rheological behavior of all kefir samples. The highest viscosity (0.049 Pa.s, at 50/s shear rate) and consistency index (1.115 Pa.s(n)) were observed in kefir with 1% OF and application of HPH. Kefir samples were characterized as weak gel behavior because storage modulus (G') was much greater than loss modulus (G) and the power-law model was used to characterize the viscoelasticity. The overall quality assessment indicated that the improvement of the fermentation process and the enhancement of textural and functional properties of kefir samples could be achieved with the addition of 1% OF and application of HPH.
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    Effect of reduced fat content on the physicochemical and microbiological properties of buffalo milk yoghurt
    (Elsevier, 2016) Akgun, Abdullah; Yazici, Fehmi; Gulec, Haci Ali
    The objective of this study was to investigate the effect of reduced fat content in buffalo milk on textural, chemical and microbiological characteristics of plain buffalo yoghurt during storage at 4 degrees C. Four yoghurt samples were manufactured with different fat ratios (1.5-6 g/100 g) and analysed on the first, 10th and 20th days of storage. Increasing the fat content of the yoghurt increased the La value, viscosity, and cohesiveness and decreased the a* value, syneresis, hardness, and springiness. Fat reduction did not cause any significant changes in the adhesiveness, gumminess, titratable acidity, b* value, and Streptococcus thermophilus population of buffalo yoghurt samples. Storage time had no significant effect on adhesiveness, springiness, and L* value of buffalo yoghurts. A decrease in S. thermophilus and Lactobacillus delbrueckii subsp. bulgaricus populations during storage was detected. It was concluded that more acceptable buffalo yoghurt could be produced from milk with 3 g/100 g fat. (C) 2016 Elsevier Ltd. All rights reserved.
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    Improving the acidification kinetics, structural and sensory properties of low-fat yoghurt using high-pressure homogenization treatment
    (Croatian Dairy Union, 2024) Akgun, Abdullah
    In this study, the impact of high pressure homogenization at 50, 100 and 150 MPa, on the physicochemical, rheological, textural, microstructural, and sensory properties as well as acidification kinetics of low-fat yoghurt was investigated. The results showed that a shorter fermentation time was obtained for yoghurt samples produced from high-pressure homogenized milk compared to the control sample. Homogenization of milk at high pressures resulted in an increase in the water holding capacity from 42.83 to 52.67 % and firmness from 1.01 to 1.45 N of yoghurt samples. The viscosity (?50) of yoghurt increased up to 0.446 Pa.s by milk homogenization and all yoghurt samples showed shear thinning behaviour. Microstructural analyses revealed that control sample had a typical open porous gel network of casein, while fewer porous and more dense protein network was observed in yoghurts from high pressure homogenized milk. The positive effect of the milk homogenization treatment on the sensory properties of yoghurts was observed as well. Principal component analysis visualized the effect of high pressure homogenization treatment and confirmed the close relationship between structural properties and overall acceptability. Among the tested low-fat yoghurts, high pressure milk homogenization at 150 MPa had the greatest potential for improving yoghurt structure, whereas 100 MPa pressure imparted the most acceptable sensorial properties.
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    Process optimization of a cereal-based fermented beverage (Boza) powder and investigating upscaling conditions
    (Wiley, 2023) Gul, Osman; Atalar, Ilyas; Tornuk, Fatih; Akgun, Abdullah
    Boza is a traditional fermented cereal-based beverage, which should be consumed within 3-5days after production due to its short shelf-life. Spray drying process is an excellent option to overcome this problem to make the product least perishable. The optimal spray drying conditions for boza powder production in a laboratory-scale spray dryer were investigated using, Response Surface Methodology. Air inlet temperature and feed flow rate were the main parameters that affected the responses such as powder yield, moisture content, cell viability, particle size, and solubility. The optimum spray drying conditions were 148 degrees C air inlet temperature, 9 degrees C feed temperature, and 10 ml/min feed flow rate. At these optimum conditions, deviations between predictive and experimental values were found very low. A 1.5 log cfu/g reduction in lactic acid bacteria (LAB) viability was achieved, however instant properties of boza powder should be improved by agglomeration processes. Laboratory and pilot scales were also compared at the determined optimum drying conditions. Approximately 0.4 log cfu/g difference in LAB viability was observed between the two scales. Practical ApplicationsBoza is one of the most valuable fermented cereal products due to its high nutritional value. The short shelf life of Boza limits its consumption and makes it a seasonal product. Spray drying technology could provide overcome these challenges and improve the usability and durability of boza beverages. However, the quality of a spray-dried product highly depends on process conditions. Therefore the effects of the spray drying process conditions on the physicochemical properties of boza powder and the scale-up conditions were investigated for the first time in this study.