1,721,118 research outputs found
Resuspended freeze-dried Nannochloropsis as a model laboratory system for concentrated fresh Nannochloropsis in ultrasound cell disruption experiments
No full textMicroalgae have rigid, complex cell walls hindering direct lipid extraction. Cell disruption techniques are used to rupture these cellular structures to increase lipid extraction. Researchers investigating the downstream processing of microalgae do not always have access to microalgal cultivation systems to generate large amounts of fresh microalgal biomass. Using resuspended freeze-dried microalgal biomass as a model laboratory system for concentrated fresh biomass during cell disruption experiments offers greater flexibility in experimental planning and omits investment costs of microalgal cultivation equipment. So far, it however remains unclear whether freeze-dried resuspended biomass can be used as a model laboratory system to represent concentrated fresh biomass during cell disruption and lipid extraction experiments. This paper thus evaluated the suitability of resuspended freeze-dried Nannochloropsis as a model laboratory system for concentrated fresh Nannochloropsis during cell disruption. Ultrasound assisted cell disruption was used as example cell disruption technique and lipid extraction efficiency and free fatty acid content were investigated. Tap water and 3% sodium chloride are both suitable resuspension media for the resuspension of freeze-dried Nannochloropsis. Resuspension duration should be limited (< 120 min) to prevent the formation of free fatty acids. The condition of the biomass (concentrated fresh, or resuspended freeze-dried) prior to ultrasound assisted cell disruption did not influence the resulting lipid extraction efficiency. Resuspended freeze-dried Nannochloropsis biomass in tap water or 3% sodium chloride can thus be used as a model laboratory system for fresh microalgal biomass during research on ultrasound assisted lipid extraction. The generalization of the results to other cultivation conditions, cell disruption techniques, components of interest or microalgal species should be carefully assessed.The author(s) declare that financial support was received for the research, authorship, and/or publication of this article. This work was supported by Flanders’ Food and funded by Flanders Innovation and Entrepreneurship (VLAIO) through the cSBR
project EffSep (Grant number HBC.2019.0012)
Ultrasound assisted extraction of Nannochloropsis: Effects on lipid extraction efficiency and lipid stability
No full textUltrasound assisted extraction of microalgal lipids from Nannochloropsis in presence of hexane/isopropanol extraction solvent was studied. The results were compared with adequate control extractions, using the same solvent system, solvent contact time and extraction temperature but in the absence of ultrasound. Ultrasound assisted extraction at 0.45 W/mL and control extractions at elevated temperatures (comparable to the temperature induced by the ultrasound) resulted in similar lipid, free fatty acid and pigment extraction efficiencies and the free fatty acid and pigment content of the lipid extracts was comparable. The control and ultrasound assisted extractions did not influence the lipolytic and oxidative stability of the lipids during extraction. These results indicated that the positive impact of ultrasound assisted extraction can almost completely be attributed to the ultrasound induced increased temperature. Simpler extraction techniques applying moderate heating (+/- 40 degrees C) in presence of lipid extraction solvents are most probably more straightforward to implement industrially compared to ultrasound assisted extraction.The authors want to thank VLAIO and Flanders' FOOD for their funding and support during the EffSep Project [c-SBO project HBC.2019.0012]. The authors wish to thank Dr. Jonas Blockx for proofreading the paper. The authors also wish to thank C´eline Dejonghe for integrating part of the FAME chromatograms
Enzyme-assisted disruption of oleaginous microalgae to increase the extraction of lipids: Nannochloropsis as a case study
No full textsponsorship: Acknowledgements This work was supported by the EffSep project. The EffSep project is financed by the Vlaams Agentschap Innoveren en Ondernemen (VLAIO) of Belgium (grand ID: c-SBO project HBC.2019.0012) . The authors also wish to thank Flanders' Food for their support. The authors sincerely thank Dr. Elaine Kozma for revising the language of the paper. (Vlaams Agentschap Innoveren en Ondernemen (VLAIO) of Belgium|HBC.2019.0012)status: Publishe
Flocculation based harvesting processes for microalgae biomass production
No full textThe global demand for biomass for food, feed, biofuels, and chemical production is expected to increase in the coming decades. Microalgae are a promising new source of biomass that may complement agricultural crops. Production of microalgae has so far however been limited to high-value applications. In order to realize large-scale production of microalgae biomass for low-value applications, new low-cost technologies are needed to produce and process microalgae. A major challenge lies in the harvesting of the microalgae, which requires the separation of a low amount of biomass consisting of small individual cells from a large volume of culture medium. Flocculation is seen as a promising low-cost harvesting method for primary concentration. In this study, we overview the challenges and possible solutions for flocculating microalgae focusing on three flocculation modes: flocculation using cationic starch, electro-coagulation and flocculation induced by high pH. Secondly, those modes were compared to two reference modes (flocculation using aluminum sulphate and chitosan) in function of their interaction with algal organic matter and floc properties. We evaluated the potential of cationic starch as a flocculant for harvesting microalgae using jar test experiments. Cationic starch was an efficient flocculant for freshwater (Parachlorella, Scenedesmus) but not for marine microalgae (Phaeodactylum, Nannochloropsis). At high cationic starch doses, dispersion restabilization was observed. The required cationic starch dose to induce flocculation increased linearly with the initial algal biomass concentration. Of the two commercial cationic starch flocculants tested, Greenfloc 120 (used in wastewater treatment) was more efficient than Cargill C*Bond HR 35.849 (used in paper manufacturing). For flocculation of Parachlorella using Greenfloc 120, the cationic starch to algal biomass ratio required to flocculate 80% of algal biomass was 0.1. For Scenedesmus, a lower dose was required (ratio 0.03). Flocculation of Parachlorella using Greenfloc 120 was independent of pH in the pH range of 5 to 10. Measurements of the maximum quantum yield of PSII suggest that Greenfloc 120 cationic starch was not toxic to Parachlorella. It could thus be concluded that cationic starch may be used as an efficient, nontoxic, cost-effective, and widely available flocculant for harvesting microalgal biomass. Secondly, the use of electro-coagulation flocculation (ECF) as a method for harvesting a freshwater (Chlorella vulgaris) and a marine (Phaeodactylum tricornutum) microalgal species is evaluated. ECF was shown to be more efficient using an aluminum anode than using an iron anode. Furthermore, it could be concluded that the efficiency of the ECF process can be substantially improved by reducing the initial pH and by increasing the turbulence in the microalgal suspension. Although higher current densities resulted in a more rapid flocculation of the microalgal suspension, power consumption, expressed per kg of microalgae harvested, and release of aluminum were lower when a lower current density was used. The aluminum content of the harvested microalgal biomass was less than 1% while the aluminum concentration in the process water was below 2 mg L−1. Under optimal conditions, power consumption of the ECF process was around 2 kWh kg−1 of microalgal biomass harvested for Chlorella vulgaris and 0.3 kWh kg−1 for Phaeodactylum tricornutum. Compared to centrifugation, ECF is thus more energy efficient. Because of the lower power consumption of ECF in seawater, it is a particularly attractive method for harvesting marine microalgae. We explored the potential of flocculation induced by high pH for harvesting Chlorella vulgaris. Our results demonstrated that flocculation can be induced by increasing medium pH to 11. Although both calcium and magnesium precipitated when pH was increased, only magnesium (0.15 mM) by the formation of magnesium hydroxide proved to be essential to induce flocculation. The costs of four different bases (sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide) were calculated and evaluated and the use of calcium hydroxide appeared to be the most cost-efficient. Flocculation induced by high pH was thus shown to be a potentially useful method to preconcentrate freshwater microalgal biomass during harvesting. Microalgae excrete relatively large amounts of algal organic matter (AOM) that may interfere with flocculation. The influence of AOM on flocculation of Chlorella vulgaris was studied using five different flocculation methods: aluminum sulfate and chitosan as reference modes in comparison with cationic starch, electro-coagulationflocculation (ECF) and pH-induced flocculation. The presence of AOM was found to inhibit flocculation for all flocculation methods resulting in an increase of dosage demand. For pH-induced flocculation, the dosage required to achieve 85% flocculation increased only 2-fold when AOM was present, while for chitosan, this dosage increased 9-fold. For alum, ECF and cationic starch flocculation, the dosage increased 5 to 6-fold. Interference by AOM is an important parameter to consider in the assessment of flocculation based harvesting of microalgae. Flocculation of microalgae should not only be effective in terms of flocculation efficiency, but also in terms of settling rate and concentration of the biomass. Floc characteristics such as settling velocity, concentration factor and floc size were therefore studied for the five previously mentioned flocculation modes for Chlorella vulgaris. These floc characteristics were influenced by the flocculation mode, which depends on the coagulation mechanism: adsorption charge neutralization, sweeping or bridging. Secondly, the influence of the presence of AOM was evaluated. This resulted in a decrease of the concentration factor. The floc characteristics upon flocculation using cationic starch were least affected by the presence of AOM, while flocculation using chitosan was most affected. The impact on floc characteristics is an important parameter to consider next to flocculation efficiency in the assessment of flocculation-based harvesting of microalgae.status: Publishe
Ontwikkeling en validatie van een ultrasone reflectie-techniek gebruikmakend van transversale golven om de kristallisatie van cacaoboter op te volgen
No full textFat crystallization plays a critical role in determining the macroscopic properties of fat containing food products, such as chocolate, butter and margarine. However, most of the currently used techniques to monitor fat crystallization are off-line techniques, while on-line techniques could have considerable financial benefits by avoiding expensive rework or disposal of out-of-specification products. The aim of this PhD research was therefore to study the potential of an advanced ultrasonic shear wave reflection (USWR) technique based on low intensity ultrasound which has the potential to measure the crystallization behavior on-line. This reflection set-up was chosen to avoid problems with the high attenuation of fats and shear waves were used to obtain information about the microstructure of fats. The proposed technique was developed under both static (chapter 3) and dynamic (stirring) conditions (chapter 6) using cocoa butter as a substrate. The validation of both techniques (chapter 5 and 8) was conducted by varying process conditions and adding minor components to cocoa butter. Limonene and lecithin were chosen as minor components and their effects on the crystallization behavior of cocoa butter were also studied in depth by conventional techniques (chapter 4 and 7).
In chapter 3, an USWR technique to monitor the isothermal crystallization process of cocoa butter under static conditions was developed. The custom-built experimental set-up basically consists of a cylindrical sample holder, which is open at both sides, with the bottom side connected to a thick plexiglass plate. A shear wave transducer is positioned at the bottom of the plexiglass plate. The remarkable oscillatory damped response in the shear wave reflection coefficient (swRC) as function of the crystallization time could be explained by constructive and destructive interference of a first reflection at the boundary between the plexiglass delay line and the crystallized cocoa butter and a second reflection occurring at the interface between crystallized and liquid substance. This hypothesis was supported by the excitation frequency dependence of the oscillations. An inverse model was developed based on the oscillatory damped response including four parameters: tind (induction time), K (crystallization rate constant), vs2 (shear ultrasonic velocity) and as2 (shear ultrasonic attenuation coefficient).
Before this technique was validated, the effect of limonene on the crystallization behavior of cocoa butter was first studied with conventional techniques (chapter 4). Differential scanning calorimetry (DSC) and X-ray diffraction measurements revealed that the effect of limonene on the crystallization kinetics was temperature dependent. This could be explained by a two-sided effect: the addition of limonene resulted in a reduction of the amount of unstable crystals and an acceleration of polymorphic transitions. At 17°C, the crystallization process was accelerated due to the acceleration of the formation of more stable polymorphic forms, while there were insufficient a crystals for an a mediated b’ nucleation at 20°C, resulting in a slower crystallization process.
In chapter 5, both the temperature effect and the effect of limonene in different concentrations on the isothermal crystallization of cocoa butter were studied by way of the USWR technique and the associated inverse model. Subsequently, the USWR parameters were compared to results of conventional techniques to monitor fat crystallization (DSC, microscopy). The study showed that tind and K provide information on the kinetics of the microstructure development, as the results of the USWR technique were much more comparable to microscopy measurements providing information about the developed microstructure than to DSC experiments which are known to measure the primary crystallization. The parameter vs2 is related with the equilibrium SFC (solid fat content), while as2 is both influenced by the SFC and the organization of the crystals in the network, yielding information about the microstructure of the crystallized samples.
In chapter 6, the monitoring technique was adapted for dynamic conditions by placing an overhead stirrer above the sample holder. Under dynamic conditions, the typical pattern of the swRC exhibited a monotonic decline instead of the oscillating pattern and also the frequency dependence disappeared. This drastic change could be explained by the fact that in this case a homogeneous sample was measured, without the formation of additional layering and surfaces, and hence without interferences. Therefore, the inverse model has also been adjusted to be able to fit this monotonic decline, resulting in two parameters: t1 and Kini, indicating the start and initial rate constant of the microstructural development respectively. Only the first 30 minutes were modeled as a much faster stick release between the plexiglass and the sample was observed under dynamic conditions, making a further measurement impossible.
In chapter 7, the effect of different lecithins on cocoa butter crystallization was studied with conventional techniques (DSC, rheology and a simple dynamic experiment). It was however very difficult to infer general conclusions as the effect of lecithin depended on the concentration and the matrix to which they were added. Furthermore, the observed effects depended on the measuring technique used to study the crystallization, whereby only the dynamic experiment could detect a difference between the different lecithins.
Therefore in chapter 8, the effect of different lecithins was investigated with the more advanced dynamic USWR technique. To investigate the sensitivity of this technique, the stirring rate was first varied, and it was observed that a higher stirring rate resulted in a lower t1 and higher Kini, which is consistent with a faster microstructural development. Furthermore, one sunflower lecithin demonstrated a lower t1 and a much faster release between the plexiglass and the sample, suggesting that certain differences in microstructural development could be detected by the dynamic USWR technique, while no differences were observed by static DSC experiments measuring the primary crystallization.
In conclusion, the proposed USWR technique shows definite promise to monitor the fat crystallization process. Its major advantages are the non-destructive character, the information about the microstructural development and the ability to measure under dynamic conditions.status: Publishe
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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