Phd Degree / Doktora

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End date: 2019Department: search.filters.department.Thesis (Doctoral)--İzmir Institute of Technology, Food Engineering

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Now showing 1 - 10 of 28
  • Doctoral Thesis
    Exploring Lysozyme-Nisin Antimicrobial Synergy at Different Conditions for Novel Food Applications
    (Izmir Institute of Technology, 2019) Sözbilen, Gözde Seval; Yemenicioğlu, Ahmet
    The main objective of this thesis is to explore lysozyme-nisin antimicrobial synergy in combination with refrigeration, mild heating or edible packaging for novel food applications. The thesis is based on the following four chapters: (1) Characterization of biochemical properties of lysozyme at different conditions; (2) Lysozyme-nisin synergy in combination with mild heating against Listeria innocua: test of findings in inoculated raw-milk (3) Lysozyme-nisin synergy in combination with chitosan coating against Listeria innocua: test of findings on coated inoculated sprouting seeds; (4) Lysozyme-nisin synergy in combination with refrigeration against lactic acid bacteria (LAB): test of findings in boza, a traditional fermented beverage. The results of this thesis clearly showed that the use of lysozyme-nisin synergy in combination with mild heating at 50 ºC or in combination with edible chitosan coating could be employed to reduce risk of listeriosis from raw-milk and sprouted seeds, respectively. It is also clearly demonstrated that the lysozyme-nisin synergy in combination with refrigeration could be used to delay acidic spoilage of boza without reducing LAB below 106 CFU/mL. This thesis clearly showed the high potential of using lysozyme-nisin synergy against pathogenic or spoilage bacteria as an effective hurdle.
  • Doctoral Thesis
    Chemical and Enzymatic Interesterification of Tallow With Different Oils
    (Izmir Institute of Technology, 2019) Aktaş, Ayşe Burcu; Özen, Fatma Banu; Yemişçioğlu, Fahri
    The purpose of this study is to manufacture structured lipids by enzymatic and chemical interesterification of tallow with corn, canola and safflower oils individually and to investigate the effects of several process parameters on various chemical and physical properties of structured lipids. Moreover, collection of Fourier-transform mid infrared (FT-MIR) and near infrared (FT-NIR) spectra during interesterification process is also aimed in order to monitor the processes and to construct chemometric models for the prediction of chemical and physical properties of the interesterified products. Both enzymatic and chemical interesterification provided modification of the properties of tallow. The blend ratio is the most significant factor among the parameters investigated for both types of interesterification. Longer reaction time for enzymatic interesterification caused undesirable changes in physical properties of fats. Interesterified lipids have generally low trans fatty acids and they tend to have lower consistencies and solid fat contents compared to their physical blends and the tallow; as a result, they also acquired better spreadable and plastic behaviors. The structured lipids produced with chemical interesterification of tallow with corn oil have better physical properties, higher oxidative stability and lower free fatty acid content compared to structured lipids produced with other vegetable oils. Chemical and physical properties of interesterified fats could be predicted accurately with chemometric analysis of FT-NIR spectra.
  • 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
    Exploring the Factors Modulating the Solubilization of Ss-Carotene in Dietary Mixed Micelles Through Computer Simulations
    (Izmir Institute of Technology, 2018) Tunçer, Esra; Bayramoğlu, Beste
    β-carotene is one of the major carotenoids beneficial to human health in the prevention of a variety of diseases. However, due to its hydrophobic nature, the bioavailability of β-carotene is very low. The solubilization of β-carotene in the duodenal mixed micelles is considered as a key step in their bioavailability. Therefore, understanding the factors and the molecular mechanisms behind their solubilization within the dietary mixed micelles are of great importance. Based on this motivation, the aim of this dissertation was to systematically investigate this phenomenon via extensive coarse-grained molecular dynamics simulations in three steps: (i) the characterization of the self-assembly and structure of the mixed micelles of bile salts and phospholipids at fasted and fed state conditions, (ii) determination of the effects of addition of the fatty acids with different chain lengths and degrees of unsaturation ,(iii) solubilization of different amounts of β-carotene in the selected mixed micelles. As it would be computationally too expensive to equilibrate the systems with full atomistic resolution, the MARTINI force field, which is a well-established model for lipid systems, was used. All the simulations were carried out using the GROMACS simulation package. Validations of results were made by comparison of the micellar properties with experimental data in the literature when available. The knowledge gained by this study provides important information to be utilized in the design of effective nutraceutical delivery systems that optimize the bioaccessibility of β-carotene.
  • Doctoral Thesis
    Preparation, Production and Industrial Application of Cheese Protective Cultures
    (Izmir Institute of Technology, 2018) Şatana, Elçin; Harsa, Hayriye Şebnem
    Cheese industry is always under the risk of microbiological contaminations. These contaminants can be either bacteria or fungal species; both affect the final product quality and acceptance. Globally, 20% of the cheese produced industrially is lost due to microbial contaminations. Fungal contaminations can be overcome by the usage of chemicals; sorbate, benzoate, natamycine, propionate, etc. or by natural biopreservatives such as protective cultures or some metabolites of antifungal cultures. Chemical preservatives have a legal limit for their usage and these limits are generally not effective to prevent the product from fungal growth. Also consumers demand goes toward the clean-labeled products and force the manufacturers to use bioprotectants if necessary. There are many types of bioprotectants commercially available with defined lactic acid bacteria combinations which are not so effective for some fungal species due to the species that they include. In this study, several lactic acid bacteria (LAB) strain has been selected to prepare efficient protective culture combinations e.g. Lactobacillus rhamnosus, Lactobacillus plantarum and Lactobacillus paracasei spp. paracasei. The maximum antifungal effect was determined by using mixed culture combination of lactic acid bacteria strains with equal volume. In cheese applications, 3x108 cfu/ml antifungal bacteria mixture including; Lactobacillus rhamnosus, Lactobacillus plantarum and Lactobacillus paracasei spp. paracasei, was inoculated into the curd before fermentation and found more effective than surface applications to the final products. In yoghurt production, same antifungal lactic acid bacteria mixture was found to be effective when applied before fermentation with 106 cfu/ml initial cell counts.
  • 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
    Development of Novel Antimicrobial, Antioxidant and Bioactive Edible Gels Intended for Food Preservation and Promotion of Human Health
    (Izmir Institute of Technology, 2017) Boyacı, Derya; Yemenicioğlu, Ahmet
    The aim of this thesis was the development of edible gelatin (GEL) based composite gels with antimicrobial, antioxidant, and bioactive properties. For this purpose, composite gels of gelatin with rice starch (RS), candelilla wax (CW) and soy protein isolate (SPI) were incorporated with lysozyme (LYS) and green tea extract (GTE). LYS and GTE release profiles of GEL and its composites were tested in aqueous media and cold-smoked salmon selected as a model food. GEL, GEL/RS and GEL/CW gels did not show considerably different LYS and GTE release profiles in aqueous media, but their release profiles in the model food system were different. GEL/SPI gels bound LYS and prevented its release from the gels, but they released GTE. GEL showed the most rapid LYS delivery onto smoked salmon, while GEL/RS and GEL/CW caused sustained LYS delivery profiles. The total LYS delivered from GEL and GEL/RS gels onto smoked salmon were similar while GEL/CW gels caused almost 30% lower LYS delivery onto food samples. On the other hand, GEL/CW gels caused the highest amounts of GTE delivery onto model food. In broth media at 4°C, all LYS containing gels (with or without GTE) showed antimicrobial activity with a minimum 1.5 decimal reduction in L. innocua that was used as a target bacterium. In smoked salmon samples, the LYS containing gels inhibited the growth of L. innocua, but they did not cause a significant reduction in its counts. Antioxidant capacity, antihypertensive and antidiabetic activity, and cytotoxicity tests against human colon carcinoma cells (Caco-2) conducted with released GTE proved the bioactive potential of developed gels. The gels were also tested for their water binding capacity, and physical (textural properties) and morphological (SEM images) properties. This work clearly showed the possibility of using active gels as multifunctional delivery tools (as coating material, pad or pre-cast film) to prevent microbial and oxidative changes in food and to enhance their bioactive properties.
  • Doctoral Thesis
    Feasibility of Eugenol Encapsulated Poly (lactic Acid) (pla) Films Via Electrospinning as a Novel Delivery System for Volatile Compounds in Food Packaging Systems
    (Izmir Institute of Technology, 2017) Arserim Uçar, Dilhun Keriman; Korel, Figen; Korel, Figen; Yam, Kit L.
    Food safety and quality are important issues in food industry. The aim of this research was to evaluate the feasibility of delivering eugenol via poly(lactic) acid (PLA) emulsion fibers-grafted PLA films with bacterial cellulose into the package headspace. For this purpose, first, bacterial cellulose crystals as a natural carrier for eugenol were produced. The influence of hydrolysis temperature, time, and acid to cellulose ratio, acid concentration and type with the addition of the neutralization step on the structure, and the properties of bacterial cellulose crystals were studied. Nanocrystals, which had high thermal stability and high crystallinity bacterial cellulose, were produced. Bacterial cellulose stabilized oil-in-water Pickering emulsions were produced as carriers for eugenol. The emulsion formulations consisting of cellulose fibers and crystals, eugenol, and surfactants were characterized for food packaging applications. PLA films were produced with obtained eugenol emulsions and poly(lactic) acid which were obtained via the electrospinning method. The produced films revealed a significant antibacterial effect on L. innocua, and E. coli inoculated tomato stem scars as real food model. The fabricated films also had significant antifungal activity on B.cinerea inoculated table grapes. Developed novel biodegradable-PLA cellulose composite films had a great potential for delivering bioactive volatile compounds for intelligent food packaging applications. The findings of this research supports the technical feasibility of delivering eugenol for antimicrobial active packaging applications via electrospun fibers.
  • Doctoral Thesis
    Prebiotic Oligosaccharide Production From Hazelnut Wastes
    (Izmir Institute of Technology, 2017) Sürek, Ece; Büyükkileci, Ali Oğuz
    Turkey is the world leader in hazelnut production and a large amount of wastes is produced during its harvesting and processing. Obtaining valuable products from hazelnut by-products can add value to those. In this study, xylooligosaccharide (XOS), which has prebiotic potential and not manufactured in Turkey, was selected as the target product. Hazelnut wastes (shell, green leafy cover and pruning wastes) are suitable sources for production of XOS. Although there are some studies on valorization of hazelnut wastes, there is no study about XOS production. In this study, xylan in the shell was hydrolyzed to XOS by autohydrolysis and enzymatic hydrolysis at different conditions. The highest XOS yield was obtained as 62.93% after autohydrolysis at 190 oC for 5 min. The content of XOS with low degree of polymerization (DP<6) increased from 10.13 to 32.74% after hydrolysis with xylanase. XOS from enzymatic hydrolysis was purified by ultrafiltration through 10 and 2 kDa size membranes. The percent of low DP-XOS in liquor was increased after ultrafiltration; however monomers could not be separated. Prebiotic activity and stability of XOS were determined under different conditions. The growth of probiotic bacteria was observed after in vitro fermentation of XOS. XOS showed a good stability to food production and gastrointestinal digestion conditions. The objective of this study was to produce of a high value product such as XOS with prebiotic potential from hazelnut wastes by autohydrolysis and enzymatic hydrolysis. This study was highly original since XOS has been produced from hazelnut wastes for the first time.
  • 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.