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U radu je ispitana mogućnost proizvodnje fermentiranih mliječnih napitaka uz primjenu koncentriranog kombucha inokuluma i s dodatkom transglutaminaze (TG). Glavni ciljevi rada bili su ispitati utjecaj koncentriranog inokuluma kom¬buche i dodatka transglutaminaze na teksturalna, reološka i senzorska svojstva čvrstih i tekućih fermentiranih mliječnih napitaka. Rezultati pokazuju da dodatak transglutaminaze u mlijeko značajno utječe na teksturalna i reološka svojstva kombucha fermentiranih mliječnih napitaka. Čvrstoća i konzistencija kombucha fermentiranih mliječnih napitaka značaj¬no je povećana primenom transglutaminaze u odnosu na kontrolni uzorak. Čvrsti uzorci s transglutaminazom imali su 90 % veću površinu histerezisne petlje u usporedbi s tekućim uzorcima, dok je površina histerezisne petlje tekućih uzoraka bez transglutaminaze bila oko 60 % manja u usporedbi s čvrstim uzorcima kombucha fermentiranih mliječnih proizvoda. Enzimski tretman mlijeka s transglutaminazom poboljšao je teksturalna i reološka svojstva, i senzorske karakteristike čvrstih i tekućih fermentiranih mliječnih napitaka dobivenih primjenom koncentriranog kombucha inokuluma., The possibilities of manufacturing fermented milk beverages by applying kombucha inoculum concentrated by evaporation and with addition of transglutaminase have been investigated. The main aims of this study were to investigate influence of concentrated kombucha inoculum and the addition of transglutaminase (TG) on textural, rheological and sensory properties of set and stirred fermented milk beverages. The results showed that the addition of transglutaminase had a great influence on textural characteristics and rheological properties of kombucha fermented milk beverages. Firmness and consistency of kombucha fermented milk products were significantly increased with the incorporation of TG compared to the control sample. Set samples with TG addition had approximately 90% higher hysteresis loop area compared to stirred samples, while hysteresis loop area of stirred samples without TG was 60% lower compared to set samples. The enzymatic treatment of milk with TG improved texture, rheology and sensorial characteristics of set and stirred fermented milk beverages obtained by concentrated kombucha inoculum.

U ovom radu ispitani su proteinski profil, in vitro gastrointestinalna digestija i antioksidativni potencijal dva uzorka svježeg sira dobivenog korištenjem kombucha inokuluma i tradicionalne starter kulture. Utvrđena je značajna razlika u proteinskom profilu i biološkom potencijalu između uzoraka svježeg sira. Kombucha svježi sir sadržava veći udio proteina, pretežno frakcija kazeina (α-kazein i β-kazein) u odnosu na svježi sir koji je dobiven tradicionalnom starter kulturom. Tijekom gastrointestinalne digestije, frakcije kazeina brže se razgrađuju pepsinom u uzorku svježeg sira s tradicionalnim starter kulturom nego u kombucha svježem siru. Nakon gastrointestinalne digestije (s pepsinom i pankreatinom), u uzorku kombucha svježeg sira utvrđena je znatno veća količina topljivih proteinskih frakcija i viši stupanj hidrolize u odnosu na uzorak dobiven tradicionalnom starter kulturom. Hidrolizati sira dobiveni inokulumom kombuche sadržavali su veće koncentracije fenola i imali veću sposobnost uklanjanja antiradikala od hidrolizata svježeg sira proizvedenog s tradicionalnom starter kulturom. Rezultati pokazuju da primjena inokuluma kombuche doprinosi poboljšanju biološkog potencijala svježeg sira., Protein profile, in vitro gastrointestinal digestion and antioxidant potential of two fresh cheese samples obtained by kombucha inoculum and traditional starter culture have been investigated in this study. There is a significant difference in protein profile and bioactive potential between fresh cheese samples. Kombucha fresh cheese contains higher proportion of protein, predominantly the casein fractions (α-casein and β-casein) compared to fresh cheese obtained by traditional culture. After gastrointestinal digestion (with pepsin and pancreatin), significantly higher amount of soluble protein and higher degree of hydrolyses were measured in kombucha fresh cheese sample than sample obtained by traditional culture. During gastrointestinal digestion, for fresh cheese with traditional starter both fractions of casein are degraded more rapidly by pepsin than in kombucha fresh cheese. The hydrolysates of cheese obtained by kombucha inoculum showed higher phenolic content and higher antiradical scavenging abilities than hydrolysates of fresh cheese produced with traditional starter. These results suggest that kombucha inoculum contributes to improvement of product’s bioactive potential.

In this paper are presented results of mutual comparison of the switching waveforms and energy losses, recorded during the turn-on and turn-off switching transients in GaN HEMTs. Switching losses were examined with the double-pulse test, using a SPICE computer simulation program. Detailed simulation models of one each cascode and enhancement-mode GaN HEMTs were used. Generated simulation results were compared with the experimental results procured on a bridgeless totem-pole power factor correction circuit, utilising the tested cascode GaN HEMT. While cascode HEMTs, in general, demonstrated higher switching losses, e-mode transistors were much more prone to the subcritically dumped oscillatory response during the hard switching. Influences of the implemented packaging and technology on the recorded response of cascode and e-mode HEMTs were analysed.

High electron mobility transistors (HEMT) have a growing application in the modern power converters. Gallium nitride (GaN) field effect transistors (FET) are particularly promising solutions for operation at very high switching frequencies. The aim of this paper is to examine the harmonic spectra of the high-frequency AC/DC power converter based on GaN HEMTs, tested both during its intermittent and continuous operation. Particular interest is dedicated to the effects of high order harmonics (above the 50th), not considered by international standards. Experiments were performed on the 4-kW, single-phase AC/DC converter, a power converter based on the bridgeless totem-pole power-factor correction (PFC) circuit. During the operation with minor load current (200 mA, being 2 % of the nominal output current), power converter operated in the intermittent mode, with bursts of control pulses succeeding the idle mode, when there were not pulses on the power switches. There was observed the great difference in the current total harmonic distortion (THDi) of the intermittent operation, comprising of these two modes. In the idle mode were dominant very-high order harmonics (one after the other, the 693rd, the 1384th and the 230th), with THDi being approximately 6.5 %. In a burst mode, the 3rd harmonic was dominant, while current distortion was very high, reaching THDi = 23 %. On the other hand, during the continuous operation with 70 % of the nominal load, only low-order harmonics became dominant, with THDi being less than 3.6 %.

Ageing of storage batteries leads to a gradual degradation of their characteristics. Particularly sensitive are valve-regulated lead-acid batteries, when the long-term operation at high temperatures may lead to a significant loss of available capacity, or even to a complete failure of the storage battery. Therefore, it is important to define the procedures for determination of the battery state-of-health (SOH) during its operation or planned maintenance. The paper describes a simple procedure for assessment of the VRLA battery state-of-health. The procedure is based on the monitoring of a cell voltage relative standard deviation in a storage battery chain. In two uninterruptible power supply systems, the state of the batteries was periodically monitored over a period of 12 years, i.e., the nominal exploitation period for these stationary storage batteries. The influence of the exploitation conditions and regular maintenance on battery's state-of-health and service life has been analysed. It has been found that the annual battery cycling, performing the discharge of 80% of the nominal battery capacity, has an indispensable importance for prevention of the premature loss of the VRLA battery capacity. The relative standard deviation of battery cell voltages increases rapidly during the final period of exploitation, simultaneously with spotting of characteristic degradation of the ever increasing number of battery cells. Described phenomenon can be a trustworthy sign for replacement of a worn-out absorbent glass mat VRLA battery.

Proteini imaju osnovnu ulogu u životnimprocesima, usled čega je isptivanje raznovrsnostinjihove strukture jedna od ključnih potrebamodernih bioloških istraživanja. Cilj ovedoktorske disertacije je predviđanjetrodimenzionalne strukture beta-galaktozidazaodabranih bakterijskih sojeva (Streptococcusthermophillus, Lactobacillus acidophilus, iBifidobacterium animalis ssp. lactis) kao ikarakterizacija njihovih aktivnih mesta iodređivanje aktivnih rezdua. Takođe, ciljdisertacije obuhvata i ispitivanje beta-galaktozidaze Bacteroides thetaiotaomicron-a,kao i ispitivanje aktivnosti beta-galaktozidazeKluyveromyces lactis-a. Na osnovu sličnostianalizirane sekvence se mogu podeliti u dvaklastera. Članovi klastera 1 pripadaju GH-2subfamiliji, dok članovi klastera 2 pripadaju GH-42 subfamiliji. Na osnovu klaster analize i poravnanja sekvenci predložene su katalitičkiaktivne rezidue beta-galaktozidaza ispitivanihsojeva: GLU458/GLU546 kod S. thermophillus-a,GLU148/GLU307 kod L. Acidophilus-a iGLU164/GLU324 kod B. animalis ssp. lactis-a.Konstruisani su homologi modeli beta-galaktozidaza S. thermophilus, L. acidophilus, B. animalis ssp. lactis. Iz rezultata simulacijemolekularnog dokinga svih beta-galaktozidazauočljivo je da se supstrat veže za aktivno mestokoje se nalazi u regionu visokeelektronegativnosti. Simulacije dokinga supotvrdile aktivna mesta predložena poravnanjemsekvenci i otkrile rezidue koje učestvuju uvezivanju laktoze i galaktoze kao supstrata.Obzirom da je objavljena samo jedna kristalnastruktura beta-galaktozidaze sa laktozom uaktivnom mestu, analiza molekularnog dokingalaktoze kao supstrata je sprovedena i sa beta-alaktozidazama koje imaju određenutrodimenzionalnu strukturu. Na osnovu slučnostisekvenci može se zaključiti da je nukleofilnarezidua beta-galaktozidaze B. thetaiotaomicron-aGLU259. Analizom konzervativnih regiona, kao idoking simulacije, može se zaključiti da uloguproton donora ima rezidua GLU182. Hemijskisastav mleka za ispitivanje aktivnosti beta-galaktozidaze Kluyveromyces lactis-a je u skladu sa pravilnikom o kvalitetu proizvoda od mleka istarter kultura koji izdaje Ministarstvopoljoprivrede i zaštite životne sredine RepublikeSrbije. Celokupna laktoza se razgradila upermeatu na temperaturi od 40°C i prikoncentraciji enzima od 0,1% nakon 60 minuta odtrenutka dodatka enzima. Maksimalna brzinapostiže se toku prvih 10 minuta. Iz dobijenihrezultata može se zaključiti da beta-galaktozidazaK. lactis-a ima visoku efikasnost prilikomrazgradnje laktoze što je čini pogodnom zakomercijalnu upotrebu u industriji mleka. U ovojdisertaciji predstavljen je protokol zamodelovanje komercijalno važnih beta-galaktozidaza i analiziranje njihovih interakcija sa laktozom kao supstratom. Dobijeni rezultati trebada pruže okvir za molekularna istraživanjavezivanja i razgradnje laktoze, kao i za budućedizajniranje komercijalno interesantnih beta-galaktzidaza.&nbsp, Proteins have an essential role in all lifeprocesses. Therefore, investigation of theirstructure diversity is one of the key focuses ofmodern biological researches. The aim of thisPhD thesis is prediction of three-dimensionalstructure of beta-galactosidases of selectedbacterial strains (Streptococcus thermophillus,Lactobacillus acidophilus and Bifidobacteriumanimalis ssp. lactis), as well as characterizationof their active sites and active residues.Furthermore, this thesis include investigationBacteroides thetaiotaomicron andKluyveromyces lactis beta-galactosidases. Basedon sequence similarity analyzed sequences canbe clustered in two clusters. Members of cluster1 belong to GH-2 subfamily, while members ofcluster 2 belong to GH-42 subfamily. Clusteranalysis and sequence alignment revealedpotential active residues: GLU458/GLU546 - S.thermophillus, GLU148/GLU307 - L.Acidophilus and GLU164/GLU324 - B.animalis ssp. lactis. Homologous models ofbeta-galactosidases from S. thermophilus, L.acidophilus, B. animalis ssp. lactis areconstructed as a result of this PhD thesis.Molecular docking simulations of all beta-galactosidases revealed that substrate binds inregion with high electronegativity. Furthermore,docking simulations confirmed active sitessuggested by sequence alignment. As only onecrystal structure with lactose in its active sitehas been published, docking simulations withlactose as a ligand were also performed withexperimentally determined beta-galactosidasesstructures. Also, lactose and alactose bindingresidues are predicted. Based on sequencesimilarity GLU259 can be recognized asnucleophile of B. thetaiotaomicron beta-galactosidase. By analysis of conserved regionsand docking simulation, GLU182 is predicted asa proton donor residue. Chemical compositionof milk used for Kluyveromyces lactis beta-galactosidase research had appropriate qualityaccording to directions of Ministry ofagriculture and environmental protection ofRepublic of Serbia. Total lactose content wasdegraded in permeate after 60 min usingenzyme concentration of 0,1% at 40°C.Maximal degradation was reached in the first 10min. Based on the results, it can be concludedthat Kluyveromyces lactis beta-galactosidase hashigh efficiency for lactose degradation, whichmakes it commercially interesting for milkindustry. Protocol for modeling of commerciallyimportant beta-galactosidases and theirinteractions with lactose is developed as a resultof this PhD thesis. Obtained results shouldprovide the frame for molecular investigation oflactose binding and degradation, as well as fordesigning of commercially suitable beta-galactozidases.&nbsp

The possibility of kombucha application cultivated on two different tea types in combination with probiotics for milk fermentation at different temperatures, as well as the physicochemical and textural properties of manufactured fermented milk products were investigated. Combination of probiotic starter culture and kombucha inoculums, cultivated on Camellia sinensis (black tea) and Thymus serphyllum (thyme tea) were used for milk fermentation. Obtained results showed that kombucha inoculums cultivated on different tea types could be used for fermented milk products in combination with probiotic starter culture. Different temperatures of milk fermentation had no significant effect on chemical characteristics of kombucha fermented dairy products. As the results of factorial ANOVA test revealed the significant influence of the interaction of factors Temperature*Tea type on L–lactic acid content. Tea type had singificant influence on textural characteristics of fermented dairy products, especially after storage. After 10 day of storage, all textural characteristics except firmness had highly significant diferences depending on the tea type used for kombucha inoculum cultivation. Significant effect on textural characteristics showed interaction of factors Tea Type*Day of storage. Key words: Fermented milk product, kombucha, textural properties, physicochemical properties.

Radiation response of the serial isolated collector vertical pnp transistor in the voltage regulator L4940V5 is a complex function of the radiation-induced trapped charge concentration, load current, bias voltage and negative feedback reaction. Input bias voltage and oxide trapped charge above the base area improved radiation hardness of the serial transistor, whilst the load current and interface traps above the base decreased radiation tolerance of the examined devices. Additive radiation effects of interface traps and oxide trapped charge above the emitter area had a negative influence on radiation hardness of the serial pnp transistor. Negative feedback reaction heavily affected operation of voltage regulator L4940V5, causing a significant shift of the serial transistor's operating point on the characteristic beta(V-BE). An analogy was perceived between characteristics of the serial transistor's base current in a moderately loaded voltage regulator L4940V5 and McLean's curves on variation of the interface traps concentration in a biased oxide.

Examinations of the minimum dropout voltage with a high load current demonstrated proper operation of voltage regulators L4940V5 for gamma radiation doses of up to 500 Gy(SiO2). An increase in the minimum dropout voltage of approximately 0.5 V was measured. Due to the relatively small dissipation, the thermal protection circuit did not significantly affect the operation of the voltage regulator L4940V5. According to the line regulation characteristics, obtained after absorption of a total ionising dose of 300 Gy(SiO2), the voltage reference was negligibly affected by the irradiation. Variations in the line regulation characteristics after irradiation were less than 1% in comparison with initial values. Despite the operation with a load current of 400 mA, the serial PNP transistor emitter injection efficiency remained high after irradiation, with an estimated value greater than 0.93. The interdigitated structure of the serial transistor's base-emitter junction led to an abrupt increase of the surface recombination in a gamma radiation environment, followed by a great rise in the excess base current. This effect was enabled by the feedback circuit reaction and its influence on a driver transistor, affecting the sharp increase of the serial transistor's base current. On the other hand, the antisaturation circuit prevented a rise in the total voltage regulator's quiescent current above the limit of approximately 40 mA.

Results were presented on examinations of two valve-regulated lead-acid (VRLA) batteries AGM SVT 300 after six-year exploitation in two industrial systems of uninterruptible power supply. Batteries of the same type, with the nominal service life of twelve years, operated with similar load and the number of cells, but in various temperature conditions. Battery B1, operating without a proper airconditioning for a long time, expressed failure of three battery cells (out of total 106). These failures were followed by significant characteristic degradation of another four cells after completion of the charging process of the previously deeply discharged battery. In the case of the battery B2, exploited with good airconditioning , characteristics of the 100 battery cells (out of total 102) were spotless during the discharge of the 80% of nominal capacity, as well as during the following charging. From two cells from the battery circuit, not being from the same batch as the remaining hundred cells, was discharged only 30% of the nominal capacity. Voltage variations of the weak battery cells were analyzed, as well as their failure mechanisms. Implemented procedure for equalization of the VRLA battery's cells after accomplishing charging process was described. A practice of batteries operation with rectifiers that would enable their operation during the entire declared service life was proposed.

In the paper the upgrade procedure for high-power thyristor rectifiers in the thermal power plant "Nikola Tesla A" is described. Originally, thyristor rectifiers were not designed for operation without batteries. Thus it may happen that, in a critical situation, battery failure would induce the thyristor rectifier switch off, leaving all the consumers without a direct voltage. In such a situation, it is very important for rectifier to stay in operation, continuing power supply of the critical consumers with a stable direct current, even if the extremely heavy electrical disturbance occurs in a local direct current grid. Improvements of the thyristor rectifiers based on the digital controller of type "DRI 07" were described in detail. Responses of the rectifier digital regulators were presented in the cases of the battery disconnection and connection, in circumstances of permanent operation in the voltage regulation mode, as well as during the alternation of the voltage and current regulators in a trickle charge mode. The effect of the adaptive PI regulator on dynamic characteristics of rectifier was analyzed during the abrupt load current variations. Applied procedure for the battery availability detection was described, as well as its influence on the characteristics of the storage battery.

In this paper аre presented line-frequency phase-controlled rectifiers with digital control units developed in Institute “Nikola Tesla”. Digital control units of the type “DRI 07” are based on microcontroller „Intel” 80C196KB16. Implemented technical solutions are described, with emphasis on capabilities to establish digital communication with a central process computer implementing standard industrial communication protocols. Electrically isolated „half-duplex” serial communication of RS-485 standard enables easy connection of numerous, mutually remote rectifiers with one programmable logic controller adjusted for the conversion of communication protocols, also as further proceeding of data on the state of all rectifiers in a facility to the process control system. Results of the examinations of power factor, total current harmonic distortion and overall efficiency are presented. Comparison of procured results with corresponding data related to the rectifiers made by the leading west-european manufacturers. On phase-controlled rectifier rated for 90 kVA, loaded with 50% of its nominal current, overall efficiency of at least 96.6% was measured.

Thyristor rectifiers from the series DRI-PT with analogue control electronics printed circuit board "DIGISP 09" and serial diodes towards the load represent an additional improvement of the existing rectifiers based on the programmable logic controller (PLC). Along with the concomitant direct current distribution board it represents the redundunt system for the direct current supply of consumers in a hydro power plant. New implemented functions are the output voltage compensations depending on the battery temperature and the battery cables voltage drop, active sharing of the load current during the parallel operation of two rectifiers, manual battery presence check, automatic and manual battery state check (periodical automatic discharging of the predefined battery capacity in the adjusted moment) and limited operation time in the equalize charging mode. Maximum battery current ripple was reduced to 5% of the nominal value utilizing the double LC filter in the power circuit. Digital controllers were implemented in the form of adjustable PID regulators with the linear adaptive gain scheduling. Beside the basic digital regulators, control electronics also possess an analogue PI regulator, enabling the reliable rectifier operation in the float charging mode even in the case of PLC fault. In the paper were presented results of the examinations of rectifier's power factor, total harmonic current distortion and the overall efficiency. High values of overall efficiency for nominally loaded thyristor rectifiers of approximately 95.5% were measured.

In this paper was presented new model of rectifier based on programmable logic controller and touch panel. Significant difference in regard with previous series of rectifiers represents implementation of redundant parallel DC/DC converters toward the load, with constant voltage in all modes of battery charging. Another innovations were separate measurement and regulation of battery current and total rectifier current, as well as remote signalization of rectifier's currents and voltages by electrically isolated current loops 4-20 mA. Accomplished modifications of "soft start" function, battery presence check, regulators, protective functions and signalizations were also described.