Phd Degree / Doktora

Permanent URI for this collectionhttps://hdl.handle.net/11147/2869

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Author: search.filters.author.Ünlütürk, SevcanDepartment: search.filters.department.Thesis (Doctoral)--İzmir Institute of Technology, Food Engineering

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Now showing 1 - 5 of 5
  • Doctoral Thesis
    Assesment of the Efficiency of Uv Light Emitting Diodes (uv-Leds) for Fruit Juice Pasteurization
    (Izmir Institute of Technology, 2019) Akgün, Merve; Ünlütürk, Sevcan
    The objectives of this thesis are to evaluate the application of ultraviolet light emitting diodes (UV-LEDs) with various wavelengths for pasteurization of cloudy (CAJ) and clear apple juices (AJ), to assess the disinfection efficiency of combined UV-LED irradiation and mild heat (UV-LED+MH) treatments, to investigate the effect of individual and combined processes on the activity of enzyme, physicochemical and microbiological quality of AJ, to determine the shelf life of UV-LED irradiated juice during storage, to reveal the reactivation potential of target microorganism (E. coli K12) in juice medium with different optical properties, and to elucidate the mechanism of microbial inactivation by UV-LEDs. 280/365 nm UV-LED irradiation assisted by mild heat at 55ºC and 15 min exposure time enhanced the inactivation of E. coli K12 and polyphenoloxidase and satisfied the 5-log reduction pasteurization criterion in apple juice. UV-LED+MH treatment extended the shelf life of CAJ from 3 to 30 days and increased its total phenolic content. However, the color properties were slightly affected by ultraviolet treatment. Subsequent photoreactivation of E. coli K12 after inactivation by UV-LEDs was observed in both CAJ and AJ. Contrarily, dark repair was repressed at 4ºC and 22ºC, furthermore, a decrease in the survival ratio was recorded in both medium. It was revealed that the damage of DNA repair enzymes and the proteins that form the outer cellular membrane of bacteria was highly induced by UV-C light at 280 nm. Additionally, the bactericidal effect of 365 nm (UV-A) wavelength was attributed to enhanced production of reactive oxygen species (ROS) resulting in oxidative damage to cellular lipids, proteins and DNA. The combination of these two wavelengths provided more efficient disinfection than that of UV-C light used alone. UV-LED irradiation assisted by mild heat has a potential to be used as an alternative technique to traditional thermal pasteurization process for juice products. This study leads to useful information for the future design of UV-LED treatment systems.
  • Doctoral Thesis
    Development of Different Koruk (unripe Grape) Products by Using Several Processing Techniques
    (Izmir Institute of Technology, 2018) Kaya, Zehra; Ünlütürk, Sevcan
    Verjuice, obtained by pressing unripe grapes, is mostly used as an alternative to lemon juice and vinegar in salads and traditional meals due to its unique sour taste and flavour. It has a short shelf life due to growth of yeasts and molds when produced at household conditions. Verjuice products in the market are not meeting desirable quality characteristics. The high quality verjuice and its derivatives required to be produced by considering several processing techniques. The objectives of this Ph.D. thesis are to (i) produce long shelf life verjuice with minimum change in its “fresh like” properties by using the combination of UV-C irradiation or Pulsed-UV light (PUV) with mild heating, (ii) produce verjuice powder with a good storage stability by using freeze drying, (iii) produce concentrated verjuice with minimal quality loss by using vacuum evaporation. Verjuice was successfully pasteurized without loosing its desirable quality by a combined UV-C and mild heating (78.0 mJ/mL, 6.2 min, 51.2°C) and PUV assisted with mild heating (6.12 J/cm2, 8.5 min, 47°C) and mild thermal (72°C, 18 s) treatments. 5-log reduction of target S. cerevisiae (NRRL Y-139) was aimed for the pasteurization of verjuice. No microorganism was developed in pasteurized verjuice during 12 and 6 weeks of refrigerated storage after UV+MH and PUV+MH treatments, respectively. Freezedried verjuice powder containing maltodextrin (FD 48h, 20% MD), was highly stable under the accelerated storage conditions (40°C, 90%RH) for 70 days. Concentrated verjuice obtained under vacuum conditions of 45°C/913 mbar, showed minimal quality change and was consumable up to 12 weeks at refrigerated storage.
  • Doctoral Thesis
    Bioactive Compound Retention and Shelf Life Extension of Strawberry Juice by Selected Nonthermal Processing Technologies
    (Izmir Institute of Technology, 2017) Yıldız, Semanur; Ünlütürk, Sevcan; Barbosa-Canovas, Gustavo V.
    Shelf life extension of fruit juices while maintaining quality and functional properties is of great interest. Nonthermal food processing technologies have been emerged as innovative preservation methods alternative to thermal pasteurization of fruit juices. The objectives of this Ph.D. thesis were to (i) ensure microbial safety of strawberry juice (SJ) by thermal and nonthermal technologies, i.e. high pressure processing (HPP), ultrasound (US), pulsed electric field (PEF) and UV-C irradiation; (ii) evaluate physicochemical and phytochemical properties of SJ after processing and during storage; (iii) extend microbial shelf life of SJ without unduly affecting quality. HPP (300 MPa, 1 min), US (120 m, 5.15 J/mL, 55 C, 3 min), PEF (35 kV/cm, 27 s, 155 Hz), and thermal pasteurization (71.7 C, 15 s) conditions were determined based on equivalent inactivation of E.coli (ATCC 11775) in SJ studied at Washington State University. UV-C irradiation (1.01 J/mL, 53.9 C, 6 cycles, 5.8 min) and thermal pasteurization (72 C, 101 s) conditions were determined considering inactivation of E. coli K-12 (ATCC 25253) in SJ studied at IZTECH. Consequently, microbial shelf life of SJ treated by HPP, thermosonication, UVC+mild heating, thermal pasteurization was extended up to 42 days while PEF prolonged up to 35 days at refrigerated storage. Multivariate data analysis revealed that HPP and PEF enhanced phytochemical characteristics of SJ compared to thermosonication and thermal pasteurization. Although UV-C irradiation did not alter physcicochemical properties, total phenolic content, and antioxidant activities of SJ, total anthocyanin content was significantly decreased by UV-C light compared to untreated juice.
  • Doctoral Thesis
    Investigation of the Effect of Different Processing Techniques on the Overall Quality and Shelf Life of Local Apricot Variety of Iğdır (prunus Armeniaca L., Cv. Şalak)
    (Izmir Institute of Technology, 2017) Hakgüder Taze, Bengi; Ünlütürk, Sevcan
    cv. Şalak apricot is a specific type of product with its unique size, shape and taste. However, its distribution and marketing is very limited due to its perishable nature and restricted shelf life. Since drying is not applicable for cv. Şalak apricot due to its low dry matter content, a novel method which would not alter the fruit quality and maintain the product safety was urged to be developed. The objective of this thesis was to explore the effect of combined processing (mild heat (MH) treatment+UV-C irradiation+CaCl2 dipping=MUC) and MAP on the shelf life of cv. Şalak apricots during cold storage. Characteristics of the samples and the effect of MUC on physicochemical properties, enzymatic activities and sensory were determined. At last, samples packed either with MAP or without MAP were stored at 1°C. Microbial quality and physicochemical properties were assessed throughout the storage. Optimum treatment conditions were 19 min at 3.23 mW/cm2 for UV-C irradiation; 6% of CaCl2 at 40°C for 21 min for MH + CaCl2 dipping. MUC treatment did not alter the physicochemical and sensorial properties, PPO and PME activities. After MUC, coliforms were completely inactivated whereas Total Aerobic Plate Count (TAPC) and Yeast and Mould Count (YMC) were reduced by 2.1-log and 2.67-log, respectively. Microbial shelf life of apricots could be extended from 3-5 days up to 14 days by MUC+MAP. Firmness was also maintained. But, MUC adversely affected the colour. On the other hand, MAP application alone was not sufficient to provide the microbial quality.
  • Doctoral Thesis
    Design of a Continuous Flow Uv Reactor for Opaque Liquid Foods by Using Computational Fluid Dynamics (cfd)
    (Izmir Institute of Technology, 2013) Atılgan, Mehmet Reşat; Ünlütürk, Sevcan
    UV-C radiation is usually applied as an alternative non-thermal process for clear liquid food products in order to achieve microbiologically safe and shelf stable products. The main objective of this Ph.D. study was to design a UV reactor inorder to provide efficient UV dose/intensity distribution to reach minimum 5 log microbial reduction in opaque liquid foods. For this purpose, a S-shaped, thin film, continuous flow UV reactor was designed and constructed. UV dose/intensity delivered in the UV system was measured and determined by means of biodosimetric, actinometric and computational fluid dynamics (CFD) methods. Inactivation performance of the UV reactor was assesed by using both clear and freshly squeezed opaque white grape juices. The number of spoilage yeast, lactic acid bacteria and E.coli K-12 (ATCC 25253) in opaque grape juice was reduced to 1.604, 4.133 and 5.431 log at UV dose levels of 247.302, 301.113 and 273.520 mJ/cm2 respectively. For clear grape juice samples inoculated with S.cerevisiae and E.coli K-12 (ATCC 25253), 6.498 and 5.986 log reductions were achieved at UV dose levels of 1001.618 and 577.245 mJ/cm2. The microbial shelf life of freshly squeezed grape juice was extended up to fourteen days which was two times longer compared to untreated samples at refrigerated conditions. The physicochemical properties of UV-C processed grape juice were also evaluated during storage. Although pH, total soluble content (Bix %) and total titratable acidity were not affected from UV treatment, significant changes observed in pH, turbidity, color properties, total soluble content (Brix %), total titratable acidity and ascorbic acid (Vitamin C) content during storage.