Sensitivity of SDI for experimental errors

Silt density index (SDI) testing is a widely-accepted method for estimating the rate at which colloidal and particle fouling will occur in water purification systems when using reverse osmosis (RO) or nanofiltration (NF) membranes. However, the SDI has several deficiencies. For example, the SDI has no linear relationship with the particle concentration, is not based on any fouling mechanism, and is not corrected for temperature, pressure and membrane resistance. The accuracy and reproducibility of the SDI is often questioned. In this study, mathematical models were developed to investigate the sensitivity of SDI for the following types of errors: errors due to inaccurate lab or field equipment, systematic errors, and errors resulting from artifacts and personal observations and experience. The mathematical results were verified experimentally. Both the mathematical models and experimental results show that the membrane resistance RM has the highest impact on the SDI results. The allowable ASTM variation in RM is responsible for a deviation in SDI between 2.29 and 3.98 at a level of SDI = 3. Besides that, a 1 s error in measuring the time to collect the second sample t2 results in ±0.07 at SDIO = 3. The artifacts and personal experience also influence the SDI results. The total error in measuring SDI was estimated to be equal to ±2.11 in the field and only ±0.4 in the lab in level of SDIO = 3. Furthermore, several recommendations are mentioned based on these theoretical results and our personal experience. This study demonstrates the sensitivity of the SDI for errors in RM and the accuracy of the equipments, and explains the difficulties in reproducing SDI results for the same water

Functional analysis of circular and linear bacteriocins of Gram-positive bacteria

SUMMARY & GENERAL DISCUSSION Preservation methods such as drying and fermenting to store food products for longer periods of time have been used for centuries. Nowadays, people desire fresh and minimally processed food, to be prepared with minimal cooking. This sets new targets for the food industry, as such products can only contain a minimum of additives and at the same time need to have a shelf life that provides optimal consumer safety. One approach to accomplish this could be the addition of biological preservatives such as bacteriocins. Bacteriocins are antimicrobial peptides produced by bacteria that can kill or inhibit other bacteria. A few bacteriocins have already reached the stage of industrial application e.g. nisin and pediocin. New bacteriocins are identified on a regular basis and are studied to determine if they have novel properties that would make them desirable as preservative. Properties which are used to judge the effectiveness of bacteriocins as preservative are activity level, activity spectrum, biostability, and bioavailability. Chapter one presents an overview of the known anti-microbial peptides produced by bacteria. Genome organization, action mechanism, bacteriocin immunity by the producer and bacteriocin resistance are discussed. Chapter two describes the effects of amino acid changes on the activity of pediocin PA-1, produced by Pediococcus acidilactici. An inducible gene expression system for the production of pediocin PA-1 mutants with altered bactericidal activities was devolped in Lactococcus lactis. In a twin-plasmid system the expression of the pediocin structural gene was effectively uncoupled from the genes that encode the pediocin secretion machinery. The secretion machinery (encoded by the pedC and pedD genes) is expressed under the control of a NaCl-inducible promoter. The effects on bacteriocin activity of amino acid changes at certain positions in the pediocin PA-1 molecule are discussed. Replacing the alanine residue at position 34 in pediocin PA-1 by a positively or negatively charged residue strongly reduces bacteriocin activity. Changing the aspartic acid residue at position 17 alters target cell specificity. Chapter three describes the purification and (partial) amino acid sequencing of two novel antibacterial peptides which are active against the cheese spoilage bacterium Clostridium tyrobutyricum: closticin 574 and circularin A produced by C. tyrobutyricum ADRIAT 932 and C. beijerinckii ATCC 25752, respectively. Based on the obtained amino acid sequences the structural genes encoding closticin 574 and circularin A were identified. Closticin 574 is synthesized as a pre-proprotein of 309 amino acids that is possibly secreted via the general secretion pathway. After secretion it is likely to be hydrolyzed at an Asp-Pro site yielding a mature antimicrobial peptide of 82 amino acid residues. Circularin A is produced as a prepeptide of 72 amino acids. Cleavage between the third and fourth amino acid followed by a head-to-tail ligation between the resulting N- terminus and C-terminus creates a circular antimicrobial peptide (Fig. 1). The occurrence of circular proteins is rare but not unprecedented: Enterocin AS-48, a homologue of circularin A produced by Enterococcus faecalis S-48 is also cyclic. Other examples of circular proteins are cyclotides (plant defense peptides) (1), the pilus proteins TrbC (Escherichia coli) and VirB2 (Agrobacterium tumefaciens) (2). The identification of genes involved in the production of circularin A is described in Chapter four. Flanking the circularin A structural gene, a region of 12 kb containing 12 putative genes was identified and sequenced. Genes in this region are organized in overlapping open reading frames, which is indicative of translational coupling. Upon heterologous expression of circularin A in E. faecalis, five genes, i.e. cirABCDE, were identified to be minimally required for bacteriocin production and secretion. CirA is the structural gene and cirE confers immunity towards circularin A. CirBD is the putative transporter, which also confers partial resistance, and CirC putatively harbors the circularization activity, either alone or in concert with CirBD. Deletion of either of the three genes cirBCD prevents bacteriocin production. Upstream of cirA, four genes are present: cfgRK encode a two-component system, cfg01 encodes a protein with homology to AgrB, a membrane protein involved in post-translational modification of an auto-inducing quorum-sensing peptide (6) and cfg02 encodes a protein lacking any homology with proteins in the available databases. Downstream of the minimal region three genes (cirGHI) were identified, of which the function has not yet been clarified. Homologues (BacGHI) of the encoded proteins CirGHI are involved in increasing the expression of enterocin AS-48 (4), while other homologues (LolCDE) are involved in removing lipid-modified proteins from the membrane of E. coli (5). Heterologous expression of circularin A was initially attempted in L. lactis but this proved to be unsuccessful due to a deleterious effect of cirB. When cirB is omitted from the minimally required region the remainder can be introduced in L. lactis, but as expected, does not lead to circularin A production in this host. Other novel putative circular bacteriocins are described in Chapter 5. This was achieved by surveying genome databases for translation products with homology to CirC. CirC was used instead of CirA, as small peptides are less likely to yield homologies and have a tendency to be under-estimated in automatically annotated genome sequences, depending on the ORF-size cutoff used in the annotation procedure. Three novel bacteriocin coding regions were identified on the basis of CirC homologs and subsequent studies of the chromosomal region: one in each of four Staphylococcus aureus strains (Mu50, N315, MW2 and 467), one in the chromosome of Geobacillus stearothermophilus DSM13240 and one in the chromosome of Oenococcus oeni PSU-1. Functions of genes in these regions are assigned on the basis of homologies of the encoded proteins to those in the CirA encoding region. The probable head-to-tail ligation sites of the novel putative bacteriocins are predicted on the basis of homology to known circular bacteriocins. An interesting challenge for future work is to confirm or disprove aspects of the working model presented here and elucidate in particular the mode of action of CirA, the mechanism behind immunity, the regulation of CirA production and secretion and, most definitely, the bacteriocin circularization mechanism.

Methods and means for producing improved dairy products

Bacteriocins produced by "food-grade" lactic acid bacteria (LAB) are successfully used as food preservatives. However, they are often characterised by a narrow inhibitory spectrum, acting mainly on closely related bacteria. In order to specifically prevent the growth of spoilage Clostridia, we set out to isolate bacteriocins from closely related Clostridium spp. The cognate genes are cloned and introduced in a starter or non-starter (adjunct) LAB, to be used in cheese manufacturing.</p

The use of flow-field flow fractionation as a process control tool for membrane filtration in water treatment

In this paper, we discuss two methods to predict filtration characteristics of a given feed water with a given membrane in the application of dead-end ultrafiltration. The first method aims to determine the critical flux for irreversibility: The flux below which fouling built up during filtration is completely removed by backwashing. The critical flux and filtration behavior in general, were strongly influenced by the presence of an electrolyte in the solution. The second method is flow field-flow fractionation coupled to multi angle light scattering (flow FFF). We show that flow FFF can be used to determine particle-membrane interactions and particle size and molar mass distributions of a complex, model solution. The results of flow FFF were used to explain the filtration results obtained with the critical flux method for the sodium alginate in the presence and absence of salt

Unraveling ultrafiltration of polysaccharides with flow field flow fractionation

We used flow field flow fractionation (flow-FFF) coupled with multi-angle-light scattering (MALS) to study the conformation of alginate molecules in ultrapure water and in a 10 mM salt solution. In particular, we investigated the behavior of alginates under filtration conditions. The flow-FFF results were compared to ultrafiltration experiments with hollow fiber membranes. Membrane material and hydrodynamic conditions were comparable. We demonstrated that, depending on the ionic environment, the alginates formed aggregates in the presence of calcium, a stretched conformation in ultrapure water, and a collapsed state in the presence of KCl. Flow-FFF showed that ionic environment also affected alginate adsorption and membrane fouling

Towards controlled fouling and rejection in dead-end microfiltration of nanoparticles – Role of electrostatic interactions

Membrane technology proves to be effective in the removal of nano-sized contaminants from water. However, not much is known on the filtration and fouling behavior of manufactured nanoparticles. The high surface-area-to-volume ratio of nanoparticles, significantly increases the effect of surface interactions on the stability of nanoparticle suspensions. Also, the stability of nanoparticle suspensions and their tendency to aggregate strongly affects the fouling mechanism during membrane filtration of nanoparticles. In this experimental study, fouling development and rejection mechanisms of mono-disperse silica model nanoparticles were investigated in great detail. A microfiltration hollow fiber membrane was employed in dead-end filtration mode for the filtration of commercially available silica nanoparticles under constant pressure. By applying a low concentration of nanoparticles and a large difference between the membrane pore size (∼200 nm) and the nominal size of the nanoparticles (22 nm), a detailed investigation of the fouling mechanisms was allowed. Five subsequent fouling stages were postulated: adsorption, unrestricted transport through pores, pore blocking, cake filtration and cake maturation. Higher concentrations of nanoparticles did not change the behavior of these fouling stages, but were found to lead to their acceleration. Fouling severity and occurrence of dynamic transitions between these fouling stages were quantitatively evaluated. The presence of salts, pH and valency of the cation strongly influenced nanoparticle properties and interactions and thus occurrence and character of the fouling stages. Lower repulsive interactions between the nanoparticles accelerate fouling by faster pore blockage and aggregation on the membrane surface. Porosity and permeability of the formed filtration cake layer are strongly dependent on the repulsive interactions between nanoparticles, with a lower repulsion leading to denser cake layers. This paper clearly shows that fouling development and rejection of nanoparticles by microfiltration membranes easily can be adjusted by tuning the electrostatic interactions between the suspended nanoparticle