Mustafa Özilgen

Survival kinetics of lactic acid starter cultures during and after freeze drying

Survival kinetics of lactic acid starter cultures were modeled considering the microorganism and external medium interfacial area as the critical factors determining the resistance of the microorganisms to freeze-drying. Surviving fraction of the microorganisms increased with the increasing biomass concentration during freeze-drying, and this is attributed to the mutual shielding effect of the microorganisms against the severe conditions of the external medium. Survival of the microorganisms over the storage period after freeze drying was enhanced by the presence of dead microorganisms which reduce the interfacial area between the live cells and the external medium. Streptococcus thermophilus was found to be more resistant to freeze-drying conditions than Lactobacillus bulgaricus. Storage under vacuum or nitrogen was superior to storage under air. Poor survival rates under air was attributed to oxygen diffusion into the dry cells through the interfacial area.

Formation and stabilization of nanoemulsion-based vitamin E delivery systems using natural biopolymers: Whey protein isolate and gum arabic.

Natural biopolymers, whey protein isolate (WPI) and gum arabic (GA), were used to fabricate emulsion-based delivery systems for vitamin E-acetate. Stable delivery systems could be formed when vitamin E-acetate was mixed with sufficient orange oil prior to high pressure homogenization. WPI (d32=0.11 μm, 1% emulsifier) was better than GA (d32=0.38 μm, 10% emulsifier) at producing small droplets at low emulsifier concentrations. However, WPI-stabilized nanoemulsions were unstable to flocculation near the protein isoelectric point (pH 5.0), at high ionic strength (>100mM), and at elevated temperatures (>60 °C), whereas GA-stabilized emulsions were stable. This difference was attributed to differences in emulsifier stabilization mechanisms: WPI by electrostatic repulsion; GA by steric repulsion. These results provide useful information about the emulsifying and stabilizing capacities of natural biopolymers for forming food-grade vitamin-enriched delivery systems.

Nanoemulsion delivery systems for oil-soluble vitamins: Influence of carrier oil type on lipid digestion and vitamin D3 bioaccessibility.

The influence of carrier oil type on the bioaccessibility of vitamin D3 encapsulated within oil-in-water nanoemulsions prepared using a natural surfactant (quillaja saponin) was studied using a simulated gastrointestinal tract (GIT) model: mouth; stomach; small intestine. The rate of free fatty acid release during lipid digestion decreased in the following order: medium chain triglycerides (MCT) > corn oil ≈ fish oil > orange oil > mineral oil. Conversely, the measured bioaccessibility of vitamin D3 decreased in the following order: corn oil ≈ fish oil > orange oil > mineral oil > MCT. These results show that carrier oil type has a considerable impact on lipid digestion and vitamin bioaccessibility, which was attributed to differences in the release of bioactives from lipid droplets, and their solubilization in mixed micelles. Nanoemulsions prepared using long chain triglycerides (corn or fish oil) were most effective at increasing vitamin bioaccessibility.

The reaction engineering approach to modelling drying of thin layer of pulped Kiwifruit flesh under conditions of small Biot numbers

Abstract Drying is a subject of extensive research in the last three decades due to its importance in manufacturing many types of products. It has been modelled by adopting the drying characteristic curve method, the comprehensive transport phenomena approach or the entirely empirical correlation. Following the approach by Chen and Xie (Trans. IChemE Part C: Food Bioprod. Processing (1997) 213–230), this paper describes a mechanistic model of drying of thin layer material (Biot numbers for both heat and mass transfer are small) using a reaction engineering approach. This method considers drying as a competitive process between ‘evaporation reaction’ and ‘condensation reaction’. Here, the drying kinetics parameters have been established through the experiments performed on pulped Kiwifruit flesh spread onto a shallow metal tray (forming a layer) to simulate the process of making fruit leather. The small Biot number conditions have been employed. The activation energy has been obtained and correlated to the residue water content and the equilibrium sorption isotherm. This approach may offer an alternative for food drying modelling.

Mathematical analysis of freeze concentration of apple juice

Abstract Apple juice concentrate was produced by freeze concentration. A double-strength juice was produced in a single-stage process, whereas with a multi-stage process higher concentration levels were achieved. Lowering the refrigeration temperature shortened the time required to achieve a certain concentration level. To account for the distribution of the apple juice solids between the juice and ice, a distribution coefficient was found. A sigmoidal curve was fitted to describe the freeze concentration process and simulated the experimental data with correlation coefficients of approximately 0·99.

Astrocyte - neuron lactate shuttle may boost more ATP supply to the neuron under hypoxic conditions - in silico study supported by in vitro expression data

BackgroundNeuro-glial interactions are important for normal functioning of the brain as well as brain energy metabolism. There are two major working models - in the classical view, both neurons and astrocytes can utilize glucose as the energy source through oxidative metabolism, whereas in the astrocyte-neuron lactate shuttle hypothesis (ANLSH) it is the astrocyte which can consume glucose through anaerobic glycolysis to pyruvate and then to lactate, and this lactate is secreted to the extracellular space to be taken up by the neuron for further oxidative degradation.ResultsIn this computational study, we have included hypoxia-induced genetic regulation of these enzymes and transporters, and analyzed whether the ANLSH model can provide an advantage to either cell type in terms of supplying the energy demand. We have based this module on our own experimental analysis of hypoxia-dependent regulation of transcription of key metabolic enzymes. Using this experimentation-supported in silico modeling, we show that under both normoxic and hypoxic conditions in a given time period ANLSH model does indeed provide the neuron with more ATP than in the classical view.ConclusionsAlthough the ANLSH is energetically more favorable for the neuron, it is not the case for the astrocyte in the long term. Considering the fact that astrocytes are more resilient to hypoxia, we would propose that there is likely a switch between the two models, based on the energy demand of the neuron, so as to maintain the survival of the neuron under hypoxic or glucose-and-oxygen-deprived conditions.

Kinetics of amino acid production by over-producer mutant microorganisms

Abstract Experimental data were modeled based on the literature for the production of nine different amino acids by ten different mutant over-producer microorganisms. Mathematical modeling helped to better understand common aspects of these fermentations, which were not stressed previously by plotting the data only. A Luedeking-Piret type of a model was found to be valid for representing all the amino acid production data. This result indicated the similarity among the amino acid production patterns of all the mutant microorganisms concerned. The results also indicated that amino acid production by the mutant microorganisms was mostly non-growth associated, and that the substrate was allocated by all the microorganisms, mainly for biomass and amino acid production. The amino acid production pattern of the microorganisms did not change when the microorganisms reached the stationary phase, but the depletion of the substrate stopped production immediately.

A Review on Grain and Nut Deterioration and Design of the Dryers for Safe Storage with Special Reference to Turkish Hazelnuts

Referee: Dr. X. Dong Chen, Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Auckland New Zealand Turkey produces about 80% of the total hazelnut crop of the world. About 75% of the production are exported. In Turkey hazelnuts are traditionally sun dried, and may be subject to mold growth and subsequent mycotoxin formation due to prolonged drying time under humid and rainy weather conditions. Drying hazelnuts in a reasonable time after harvest is necessary for mycotoxin-free, high-quality products. In general, nuts and cereals contaminated by the toxins pose a potential hazard not only to the people of the producer countries, but also to people of the importing countries, if they should be regarded as safe by inefficient sampling plans, therefore preventing toxin formation actually benefits very large populations. Deterioration and health hazards associated with toxin contaminated hazelnuts and other nuts and cereals have similar causes and consequences; therefore, deterioration of the nuts and cereals in storage has been reviewed by considering as many grains and nuts as possible, then special reference was made to hazelnuts. Proper preharvest practices followed by proper drying and safe storage reduces the hazards associated with contamination by the toxins. This article reviews the pre- and post-harvest practices, and the grain- and nut-drying systems required for toxin-free products. Because drying is the major unit operation involving this process, the drying systems and the mathematical models required for their design is also discussed.Turkey produces about 80% of the total hazelnut crop of the world. About 75% of the production are exported. In Turkey hazelnuts are traditionally sun dried, and may be subject to mold growth and subsequent mycotoxin formation due to prolonged drying time under humid and rainy weather conditions. Drying hazelnuts in a reasonable time after harvest is necessary for mycotoxin-free, high-quality products. In general, nuts and cereals contaminated by the toxins pose a potential hazard not only to the people of the producer countries, but also to people of the importing countries, if they should be regarded as safe by inefficient sampling plans, therefore preventing toxin formation actually benefits very large populations. Deterioration and health hazards associated with toxin contaminated hazelnuts and other nuts and cereals have similar causes and consequences; therefore, deterioration of the nuts and cereals in storage has been reviewed by considering as many grains and nuts as possible, then special reference was made to hazelnuts. Proper preharvest practices followed by proper drying and safe storage reduces the hazards associated with contamination by the toxins. This article reviews the pre- and post-harvest practices, and the grain- and nut-drying systems required for toxin-free products. Because drying is the major unit operation involving this process, the drying systems and the mathematical models required for their design is also discussed.

Effects of velocity and temperature of air on the drying rate constants of apple cubes

Abstract Drying rate constants of apple cubes were calculated after performing experiments with a computer-controlled laboratory scale tunnel drier in 1·7–3·0 m/s air velocity and 78–94 °C air temperature ranges. There was only one falling rate period in the experiments with linearly decreasing air temperatures, but two consecutive falling rate periods were observed when air temperature was constant. Variation of the drying rate constants with temperature were described by an Arrhenius-type expression. The Arrhenius expression and drying rate constants of the falling rate period increased with air velocity. The second falling rate period was a diffusion-controlled process. It was confirmed by the experimental results that the velocity of air flow does not affect the drying rate constants in the second falling rate period. Within the range of the experiments, drying rate constants decreased with the initial particle size of the cubes, implying that the thickness effects suppressed the surface area effects.

Mixed culture growth kinetics of Streptococcus thermophilus and Lactobacillus bulgaricus

A simple microbiological technique was used to differentially enumerate growth of Streptococcus thermophilus and Lactobacillus bulgaricus in a mixed culture. The growth of the microorganisms in the mixed culture was satisfactorily simulated with a set of modified logistic equations. This simple model was valid for various initial biomass concentrations and their ratios. It did not need substrate or product data for simulation of biomass growth, which may simplify the calculations in fermenter design. It was shown that our model may also be regarded as a special case of a common mixed culture model: Volterras competition analysis.