BACKGROUND Currently, microbial products are gaining worldwide importance. Arthrospira (Spirulina) maxima is one of the most widely studied microorganisms whose industrial importance is due to its ability to accumulate high amounts of good‐quality proteins; as a consequence, research efforts of many health food and aquaculture industries are focused on the development of tools to optimize the commercial exploitation of this microalgae (cyanobacteria). Kinetic models can appropriately describe the patterns of growth and product formation, which are necessary for any biotechnological process using microorganisms. Some primary, secondary or even more complex kinetic models have been used to depict the growth of microalgae, but to the best of our knowledge, little has been done to choose the most appropriate one. RESULTS The present contribution addresses the selection of a kinetic model to describe the growth curve of Arthrospira maxima by considering Akaike's weights as a measure that simultaneously considers the goodness‐of‐fit and the complexity of the model (i.e. number of parameters). Specifically, Haldane‐based, Monod‐based, and logistic kinetic models are considered as candidates to describe the growth of A. maxima under autotrophic conditions. CONCLUSION The logistic model, which is the most parsimonious, should be selected to describe the growth of A. maxima within a photobioreactor either in continuous light operation or in dark/light cycles at several light intensities. The selection of this model will allow implementation of simpler monitoring, control, and optimization schemes. © 2016 Society of Chemical Industry
An efficient recovery of the active spore-crystal complex of Bacillus thuringiensis was achieved by using either a disk-stack centrifuge or a rotary vacuum filter. In both operations the spore recovery efficiency was higher than 99%. Nevertheless, the concentration of dry solids produced by each method strongly differed, being 31.5% for filtration and 7.5% for centrifugation, while the organic solids concentration increased 9.2 and 2.5 times, respectively. Adjustment of the broth to pH 4 improved the solid-liquid separation efficiency. IntroductionBioinsecticides derived from Bacillus thuringiensis contain inclusion bodies, spores, cell debris, and other residual solids, which are all recovered from the broth at the end of the fermentation and then formulated, packed, or dried (Rowe and Margaritis, 1987). In 1963, Bonnefoi patented a method for producing biological pestdestroying reagents, where spores and inclusion bodies were collected through centrifugation or filtration. In the early 1970s, the production of wettable powders was achieved via the acetone precipitation process, a method that has largely been replaced by centrifugation and spray-drying.In this work, the influence of different factors on the separation efficiency and performance of continuous centrifugal separation equipment and a continuous vacuum filter was studied on the laboratory and pilotplant scales. Then, performance of both unit operations for B. thuringiensis-product separation was compared. Materials and MethodsMicroorganism, Culture Medium, and Cultivation Conditions. The strain B. thuringiensis var. kurstaki HD-73 was used. Culture medium and fermentation conditions on the bench scale were described previously (Rodríguez and de la Torre, 1996). In the pilot-plant runs, a 1100-L stirred-tank fermentor (made in Mexico) and the same culture medium were utilized. Temperature and pH were controled as in the batch runs, and dissolved oxygen was kept above 20%.Fermented Broth Conditioning. The pH of 10 mL of fermented broth (5.0-5.2% total solids and 2-2.3% suspended solids) was adjusted to either 4 or 7. Then the chosen flocculant was added and the sample was incubated at constant temperature (30 or 60°C) for 15 min. Later the sample was centrifuged at 535g for 10 min, and the transmittance of the supernatant liquid was measured at 700 nm. The flocculants tested were poly-(acrylamide) WT-2640 (Calgon Co.) and aluminum sulfate, both at 200 ppm. The effect of each parameter on the sedimentation velocity was evaluated on the laboratory scale by using a bench tube centrifuge (Sorvall SS-3).Centrifugation. Tests were run in a disk bowl centrifuge (Westphalia Separator SA-1) of equivalent area 1042 m 2 (9470 rpm, 34 disks, outside radius of disks ) 0.0512 m 2 , inside radius of disks ) 0.0258 m 2 , R ) 38.73°), which allows the continuous removal of supernatant liquid and intermittent solids discharge. The experimentation was done as suggested by Kroog (1988). The pH of the fermented broth was adjusted to pH 4.0.Filtration. On the bench scale, a...
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