U. Kulozik

Technische Universität München, München, Bavaria, Germany

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Publications (42)42.72 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: We investigated the fractionation of casein micelles and the whey protein β-lactoglobulin (β-LG) of skim milk by crossflow microfiltration (0.1 μm) for the first time by a novel approach as a function of membrane length and membrane resistance. A special module was constructed with 4 sections and used to assess the effects of membrane length by measuring flux and β-LG permeation (or transmission) as a function of transmembrane pressure and membrane length. Depending on the position, the membranes were partly controlled by a deposit layer. A maximum for β-LG mass flow through the various membrane sections was found, depending on the position along the membrane. To study the effect of convective flow toward the membrane, membranes with 4 different intrinsic permeation resistances were assessed in terms of the permeation and fouling effects along the flow channel. From these findings, we derived a ratio between transmembrane pressure and membrane resistance, which was useful in reducing the effect of deposit formation and, thus, to optimize the protein permeation. In addition, the fouling effect was investigated in terms of reversible and irreversible fouling and, in addition, by differentiation between pressure-induced fouling and adsorption-induced (pressure-independent) fouling, again as a function of membrane length.
    Journal of Dairy Science 04/2012; 95(4):1590-602. · 2.57 Impact Factor
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    ABSTRACT: There is still lack of the insight into the storage stability of dry probiotics produced by vacuum drying. Therefore, in this study we assessed the stability of a vacuum-dried Lactobacillus paracasei F19 under varying storage conditions. L. paracasei F19 was vacuum-dried with and without sorbitol and trehalose. The dried cells were stored at 4, 20 and 37°C, and at aw=0.07, 0.22 and 0.33. The survival was determined by viable counts on MRS agar plates. The inactivation rate constants were determined for each storage condition. The survival after drying of cells dried without and with trehalose and sorbitol was 29, 70 and 54%, respectively. All vacuum-dried cells were very stable at 4°C. However, high stability at non-refrigerated temperatures was obtained only in the presence of sorbitol. In contrast to sorbitol, the supplementation of trehalose did not stabilize cells during storage. This is supposedly due to the rapid crystallization of trehalose during storage. While glass transition temperatures of dry cell-sorbitol increased from −32°C to 12°C during storage at 37°C and aw=0.07, Tg of dry cell-trehalose (−15°C after drying) could not be determined after storage for only 24h. In conclusion, we showed that high stability of probiotic cells at non-refrigerated temperatures could be obtained by vacuum drying process with appropriate protectant.
    Food and Bioproducts Processing - FOOD BIOPROD PROCESS. 04/2012;
  • [show abstract] [hide abstract]
    ABSTRACT: Casein micelles undergo shape changes when subjected to frontal filtration forces. Grazing incidence small angle X-ray scattering (GISAXS) and atomic force microscopy (AFM) allow a quantification of such structural changes on filtration cakes deposited on smooth silicon micro-sieves. A trans-membrane pressure of deltap = 400 mbar across the micro-sieve leads to an immediate film formation after deposition of casein solution. We observe significant changes in the GISAXS pattern depending on how many layers are stacked on top of each other. Compared to a deposit formed by one layer, GISAXS on a deposit formed by three layers of casein micelles leads to less scattering in the vertical and more scattering in the horizontal direction. Simulations show that the experimental results can be interpreted by a structural transformation from an originally spherical micelle shape to an ellipsoidal-deformed shape. The results are supported by AFM measurements showing a reduced lateral size of casein micelles deposited on top of a membrane pore. The observed shape changes could be due to filtration forces acting on densely packed deposits confining the micelles into ellipsoidal shapes.
    Faraday Discussions 01/2012; 158:77-88; discussion 105-24. · 3.82 Impact Factor
  • Foerst P, Kulozik U
    [show abstract] [hide abstract]
    ABSTRACT: In order to better understand inactivation of cells during a drying process, the inactivation kinetics of concentrated Lactobacillus paracasei ssp. paracasei (F19) was measured under stationary conditions for different combinations of water activities and temperatures in a water activity range of a w = 0.23–a w = 0.75 and temperatures between 4°C and 50°C. It was shown that the inactivation kinetics of the probiotic bacterium L. paracasei at moderate temperatures could, for all conditions, be formally described by a first-order reaction with activation energies that are much lower than for thermal inactivation (E a = 61 kJ/mol). With regard to the water activity, the reaction rate constants exhibit a maximum inactivation rate at intermediate water activity a w = 0.52. As this behavior has direct implications for the stability of cells in a drying process, the stationary data were used to model the inactivation during test vacuum drying processes, where both temperature and water activity dynamically change. It is shown that—depending on the drying rate—dynamic effects have to be taken into account when modeling the survival during drying. Nevertheless, the model based on stationary inactivation data is capable to predict the characteristics of inactivation during a drying process. Therefore, it can serve as basis to optimize the drying process with regard to maximum survival of cells. However, a further refinement of the model with regard to the drying rate is necessary.
    Food and Bioprocess Technology 01/2012; · 4.12 Impact Factor
  • R Gebhardt, C Vendrely, U Kulozik
    [show abstract] [hide abstract]
    ABSTRACT: The objective of this study was to determine the effect of size-fractionation by centrifugation on the film structure of casein micelles. Fractionated casein micelles in solution were asymmetrically distributed with a small distribution width as measured by dynamic light scattering. Films prepared from the size-fractionated samples showed a smooth surface in optical microscopy images and a homogeneous microstructure in atomic force micrographs. The nano- and microstructure of casein films was probed by micro-beam grazing incidence small angle x-ray scattering (μGISAXS). Compared to the solution measurements, the sizes determined in the film were larger and broadly distributed. The measured GISAXS patterns clearly deviate from those simulated for a sphere and suggest a deformation of the casein micelles in the film.
    Journal of Physics Condensed Matter 11/2011; 23(44):444201. · 2.36 Impact Factor
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    ABSTRACT: The influence of low temperature vacuum drying process parameters on the survival, metabolic activity and residual water content of three different bacterial strains (Lactobacillus paracasei ssp. paracasei, Lactobacillus delbrueckii ssp. bulgaricus and Bifidobacterium lactis) was investigated. Shelf temperature and chamber pressure were varied and optimized by response surface methodology with regard to survival and residual water content. It is shown that the survival rate after low temperature vacuum drying is comparable to that of freeze drying. Based on the optimization experiments the combined influence of fermentation pH and drying process parameters was studied for the most detrimental and the best process condition, respectively. The results show that interactions between process and fermentation conditions have to be taken in account and that these influences are highly strain specific.
    Journal of Biotechnology 06/2011; 159(4):351-7. · 3.18 Impact Factor
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    ABSTRACT: Assay systems were developed to determine bioactive sialylated structures in complex and crude mixtures as they occur in milk fractionation processes. The sialic acid-binding proteins Maackia amur- ensis lectin and Sambucus nigra agglutinin as well as Siglec-2 and Siglec-4 were used in inhibition assays. The complementary linkage speci � cities of these lectins allow the determination of a2,3- and a2,6-linked sialic acids even in the presence of high lactose concentrations that are common in milk products. The functionality and linkage speci � city of the assays have been evaluated using oligosaccharides from human and bovine milk. Furthermore, the presence of bioactive sialoglycoconjugates has been deter- mined in fractions from a bovine milk fractionation process like sweet buttermilk, lactose-reduced serum, skim milk proteins, caseins and glycomacropeptides.
    International Dairy Journal 01/2011; 21(6):413-420. · 2.33 Impact Factor
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    ABSTRACT: Dispersed air provides an additional phase within gel-type foods may accommodate new textural and functional demands. This paper addresses the effect of using whey protein β-lactoglobulin (β-lg), with different degrees of denaturation, as stabilizing agent in the formation of aerated gelatin gels using ultrasound as a novel method to incorporate bubbles in model foods. The heat denaturation, aggregate formation and surface properties of β-lg dispersions were studied at three pHs (6.0, 6.4 and 6.8) and at a heating temperature of 80 °C. β-Lg dispersions with four degrees of denaturation (0%, 20%, 40% and 60%) were used to stabilize bubbles generated by high intensity ultrasound in aerated gelatin gels. Experimental methods to determine gas hold-up, bubble size distributions and fracture properties of aerated gelatin gels stabilized by β-lg (AG), as well as control gels (CG), aerated gelatin gels without β-lg, are presented. Gas hold-up of AG peaked at a degree of denaturation of 40% when AG were fabricated using β-lg heated at pH 6.4 and 6.8, whereas using β-lg heated at pH 6.0 gas hold-up decreased constantly with increasing degree of denaturation. The use of β-lg as surfactant at pH 6.8 and 6.4 reduced the bubble sizes of AG compared with CG, but no effect was observed at pH 6.0. AG showed values of stress and strain at fracture lower than CG (5.86 kPa and 0.62), probably because of the lower gas hold-up of CG. However, both type of aerated gels were weaker and less ductile than non-aerated gels, with a decrease in stress and strain at fracture for AG between 56–71% and 33–43%, respectively. This study shows that the presence of bubbles in gel-based food products results in unique rheological properties conferred by the additional gaseous phase.Graphical abstractThe use of β-lactoglobulin as surfactant to stabilize bubbles incorporated by ultrasound increased the gas hold-up of aerated gelatin gels in comparison to control gels (gelatin gels without β-lactoglobulin).
    Food Hydrocolloids 01/2011; 25(5):958-967. · 3.49 Impact Factor
  • C. Guyot, U. Kulozik
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    ABSTRACT: A novel method of transglutaminase (TGase) treatment for skim milk yoghurt production was investigated. In contrast to previous studies, TGase pre-treated skim milk powder (SMP) was used as protein fortification for yoghurt making, instead of treating the entire yoghurt milk. When the TGase concentration for powder production was increased from 0 to 10 U g−1 protein, the viscosity of stirred skim milk yoghurt produced with addition of TGase-treated SMP increased from 247 to 453 mPas and the serum loss assessed using a centrifugation method decreased from 57.1% to 52.6%. Furthermore, by using enzyme-modified SMP, only half of the protein addition was required to obtain an equivalent viscosity compared to the control. The study showed that crosslinking the caseins by TGase only in the added SMP yields the desired positive effects while allowing for a complete elimination of the residual enzyme activity.
    International Dairy Journal - INT DAIRY J. 01/2011; 21(9):628-635.
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    ABSTRACT: The kinetics of heat denaturation and aggregation for beta-lactoglobulin dispersions (5% w/v) were studied at 3 pHs (6, 6.4, and 6.8) and at a heating temperature of 80 degrees C. Protein aggregates were characterized for hydrodynamic diameter, microstructure, and molecular weight by means of dynamic light scattering, transmission electron microscopy, and polyacrylamide gel electrophoresis, respectively. Concentration of native beta-lactoglobulin decreased with holding time and with a decrease in the pH. Apparent rate constants were calculated for beta-lactoglobulin denaturation applying the general kinetic equation solved for a reaction order of 1.5. Values of the apparent reaction rate constant k = 7.5, 6.3 and 5.6 x 10(-3) s(-1) were found for pH 6, 6.4, and 6.8, respectively. Decreasing the pH of the dispersions produced higher aggregate sizes. After a holding time of 900 s, average hydrodynamic diameters for beta-lactoglobulin aggregates at pH 6, 6.4, and 6.8 were 96, 49, and 42 nm, respectively. These results were confirmed by transmission electron microscopy images, where a shift in the size and morphology of aggregates was found, from large and spherical at pH 6 to smaller and linear aggregates at pH 6.8. beta-Lactoglobulin formed disulfide-linked intermediates (dimers, trimers, tetramers) and so on) which then formed high molecular weight aggregates. From the results obtained by DLS, TEM, and SDS-PAGE a mechanism for beta-lactoglobulin aggregation was proposed. This study shows that heat treatment can be used to produce protein aggregates with different sizes and morphologies to be utilized as ingredients in foods.
    Journal of Food Science 06/2010; 75(5):E261-8. · 1.78 Impact Factor
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    ABSTRACT: The effect of colloidal interactions between casein micelles on the flux of a tubular ceramic membrane at cross-flow microfiltration (MF) of skim milk was studied. Filtration experiments were performed at a pH range of 6.8–5.9. Compared to filtration of milk at its native pH (6.8), the flux was reduced and membrane fouling proceeded faster when acidified milk was filtered. To explain the observed flux behavior, a new interaction model for casein micelles was developed, which incorporates on the basis of the extended DLVO (xDLVO) theory hydrophilic and hydrophobic interactions in the form of Lewis acid–base forces, which were derived from protein surface energies. It could be shown that deposit layer build-up is strongly influenced by the charge-dependent protein surface hydrophilicity, whereas electrostatic interactions between proteins can be neglected in high ionic strength fluids like milk.
    Journal of Membrane Science. 01/2010;
  • Journal of Biotechnology - J BIOTECHNOL. 01/2010; 150:61-62.
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    ABSTRACT: To examine changes in membrane fatty acid profile attributed to the physiological adaptation of Lactobacillus helveticus during vacuum drying. The viability and membrane integrity of the cells after vacuum drying were measured by plate counts and DNA fluorescence dyes. The physiological adaptation of cells dried in the presence of sorbitol was observed by determining changes in membrane fatty acid composition using gas chromatography. Results showed that viability and membrane integrity of Lact. helveticus cells increased when drying in the presence of sorbitol. The occurrence of the very low melting point polyunsaturated fatty acids linoleic and arachidonic acid was observed in cells dried in the presence of sorbitol. The physiological adaptation of cells occurred with cell membrane of Lact. helveticus during vacuum drying of cells in the presence of sorbitol. The study showed that physiological adaptation with membrane of the cells occurred during the drying process. The insight implies that instead of viability improvement of dried cells by the conventional stress induction during cultivation, the induction may be exercised thereafter without compromising growth of the cells.
    Letters in Applied Microbiology 08/2009; 49(4):516-21. · 1.63 Impact Factor
  • P Foerst, J Reitmaier, U Kulozik
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    ABSTRACT: The aim of this work was to study the effect of sorbitol as protective agent on the survival of the probiotic strain Lactobacillus paracasei ssp. paracasei (F19) after vacuum drying. The survival was studied after different drying times and for various concentrations of sorbitol by plate count method. Furthermore, time domain 1H NMR studies on dehydrated suspensions of Lact. paracasei ssp. paracasei were performed to study the proton mobility in the dried samples. From the obtained signal, T2 relaxation times of single components and fractions with different proton mobility were determined. It was found out that the survival is increased by the presence of a minimum amount of sorbitol that is dependent on drying time. Furthermore, it is shown that the protective effect can only be observed below a critical water content of c. 20%. Nuclear magnetic resonance (NMR) results indicate a transition of sorbitol from liquid to solid like behaviour during drying. The onset of the transition coincides with the critical water content found for a protective effect. The data suggest that sorbitol protons are incorporated into the dried cells of Lact. paracasei ssp. paracasei (F19) below the critical water content and therefore leading to an enhanced survival. The results help to better understand the underlying mechanism of protection of Lact. paracasei ssp. paracasei using sorbitol and to establish vacuum drying as potential alternative drying technique to standard freeze drying.
    Journal of Applied Microbiology 08/2009; 108(3):841-50. · 2.20 Impact Factor
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    ABSTRACT: The water content of casein micelle films in water vapor atmosphere is investigated using time-resolved grazing incidence small-angle neutron scattering (GISANS). Initial dry casein films are prepared with a spin-coating method. At 30 degrees C, the formation of a water-equilibrated casein protein film is reached after 11 min with a total content of 0.36 g of water/g of protein. With increasing water vapor temperature up to 70 degrees C, an increase in the water content is found. With GISANS, lateral structures on the nanometer scale are resolved during the swelling experiment at different temperatures and modeled using two types of spheres: micelles and mini-micelles. Upon water uptake, molecular assemblies in the size range of 15 nm (mini-micelles) are attributed to the formation of a high-contrast D2O outer shell on the small objects that already exist in the protein film. For large objects (>100 nm), the mean size increases at high D2O vapor temperature because of possible aggregation between hydrated micelles. These results are discussed and compared with various proposed models for casein micelle structures.
    Langmuir 05/2009; 25(7):4124-31. · 4.19 Impact Factor
  • Chemie Ingenieur Technik - CHEM-ING-TECH. 01/2009; 81(8):1149-1149.
  • J. Behr, U. Kulozik, P. Först
    Chemie Ingenieur Technik - CHEM-ING-TECH. 01/2009; 81(8):1264-1265.
  • [show abstract] [hide abstract]
    ABSTRACT: The preservation of lactic acid starter cultures by drying are of increased interest. A further improvement of cell viability is, however, still needed, and the insight into inactivation mechanisms of the cells is a prerequisite. In this present work, we review the inactivation mechanisms of lactic acid starter cultures during drying which are not yet completely understood. Inactivation is not only induced by dehydration inactivation but also by thermal- and cryo-injuries depending on the drying processes employed. The cell membrane has been reported as a major site of damage during drying or rehydration where transitions of membrane phases occur. Some drying processes, such as freeze drying or spray drying, involve subzero or very high temperatures. These physical conditions pose additional stresses to cells during the drying processes. Injuries of cells subjected to freezing temperatures may be due to the high electrolyte concentration (solution effect) or intracellular ice formation, depending on the cooling rate. High temperatures affect most essential cellular components. It is difficult to identify a critical component, although ribosomal functionality is speculated as the primary reason. The activation during storage is mainly due to membrane lipid oxidation, while the storage conditions such as temperature moisture content of the dried starter cultures are important factors.
    Journal of Applied Microbiology 08/2008; 105(1):1-13. · 2.20 Impact Factor
  • A. Tolkach, U. Kulozik
    Chemie Ingenieur Technik 07/2008; 80(8):1165 - 1173. · 0.70 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: Crossflow microfiltration of skimmed milk to fractionate casein micelles and whey protein was investigated regarding length dependency of flux and whey protein permeation using a 1.2m long, 0.1μm tubular ceramic membrane. A special module consisting of four sections was constructed allowing to assess the effects of membrane length online by measuring flux and permeation of the whey protein β-lactoglobulin as a function of local processing conditions. It was found that under the applied filtration parameters (mean transmembrane pressure ΔpTM,m=0.5bar; temperature ϑ=55°C; wall shear stress τw=115Pa) main parts of the membrane were controlled by a deposit layer. In consequence, the transmission of the whey protein β-lactoglobulin increases from 38% to 87% from membrane inlet to membrane outlet. Results show that a local optimum for protein fractionation exists regarding membrane resistance and process conditions.
    Journal of Membrane Science - J MEMBRANE SCI. 01/2008; 325(2):887-894.

Publication Stats

207 Citations
42.72 Total Impact Points


  • 2002–2012
    • Technische Universität München
      • Chair of Food Process Engineering and Dairy Science
      München, Bavaria, Germany
  • 2010
    • Technische Universität Kaiserslautern
      Kaiserlautern, Rheinland-Pfalz, Germany
  • 2002–2003
    • Hohenheim University
      Stuttgart, Baden-Württemberg, Germany