Tissue-engineering approaches for cartilage repair hold promise for the treatment of cartilage defects. Various methods to prevent or reduce dedifferentiation during chondrocyte expansion are currently under investigation. In the present study we evaluated the effect of oxygen on chondrocyte proliferation, as oxygen has received increased attention as a possible regulator of chondrocyte differentiation and its effect during expansion is uncertain. Therefore, the effect of three oxygen tensions (4, 10.5, and 21%) was investigated in a bioreactor microcarrier culture, which allows precise control of the oxygen tension in the liquid phase. During culture cells acquired a round shape on microcarriers. No differences in proliferation rate of chondrocytes were observed within the range of oxygen tensions evaluated. Cells exhibited predominantly anaerobic metabolism and, per mole of glucose, approximately 2 mol of lactate was produced independent of oxygen tension. Cellular oxygen consumption was comparable for all bioreactor cultures. Nevertheless, specific consumption rates were relatively high (2-4 x 10(-17) mol. cell(-1). s(-1)), in comparison with chondrocytes in cartilage (0.8-2.2 x 10(-18) mol. cell(-1)). Subsequent cartilaginous tissue formation in pellets was not affected as qualitatively assessed by safranin-O staining. At the oxygen concentrations evaluated, no effect of oxygen tension was observed on proliferation, oxygen consumption, and yield of lactate on glucose administration. For future investigations of chondrocytes and oxygen, the bioreactor system, which allows precise control and monitoring of oxygen tension, holds promise.

Lipid-accumulating fungi may be able to produce biodiesel precursors from agricultural wastes. As a first step in understanding and evaluating their potential, a mathematical model was developed to describe growth, lipid accumulation and substrate consumption of the oleaginous fungus Umbelopsis isabellina (also known as Mortierella isabellina) in submerged chemostat cultures. Key points of the model are: (1) if the C-source supply rate is limited, maintenance has a higher priority than growth, which has a higher priority than lipid production; (2) the maximum specific lipid production rate of the fungus is independent of the actual specific growth rate. Model parameters were obtained from chemostat cultures of U. isabellina grown on mineral media with glucose and NH4+. The model describes the results of chemostat cultures well for D > 0.04 h−1, but it has not been validated for lower dilution rates because of practical problems with the filamentous fungus. Further validation using literature data for oleaginous yeasts is described in part II of this paper. Our model shows that not only the C/N-ratio of the feed, but also the dilution rate highly influences the lipid yield in chemostat cultures.

The production of lipids by oleaginous yeast and fungi becomes more important because these lipids can be used for biodiesel production. To understand the process of lipid production better, we developed a model for growth, lipid production and lipid turnover in submerged batch fermentation. This model describes three subsequent phases: exponential growth when both a C-source and an N-source are available, carbohydrate and lipid production when the N-source is exhausted and turnover of accumulated lipids when the C-source is exhausted. The model was validated with submerged batch cultures of the fungus Umbelopsis isabellina (formerly known as Mortierella isabellina) with two different initial C/N-ratios. Comparison with chemostat cultures with the same strain showed a significant difference in lipid production: in batch cultures, the initial specific lipid production rate was almost four times higher than in chemostat cultures but it decreased exponentially in time, while the maximum specific lipid production rate in chemostat cultures was independent of residence time. This indicates that different mechanisms for lipid production are active in batch and chemostat cultures. The model could also describe data for submerged batch cultures from literature well.

The kinetic resolution of a range of methyl-substituted 1-oxaspiro[2.5]octanes by yeast epoxide hydrolase (YEH) from Rhodotorula glutinis has been investigated. The structural determinants of substrate specificity and stereoselectivity of YEH toward these substrates appeared to be the configuration of the epoxide ring and the substitution pattern of the cyclohexane ring. For all compounds tested, O-axial epoxides were hydrolyzed faster than the corresponding O-equatorial compounds. In concern of the ring substituents, YEH preferred methyl groups on the Re side of the ring. Placement of substituents close to the spiroepoxide carbon decreased the reaction rate but increased enantioselectivity. YEH-catalyzed kinetic resolutions of 4-methyl 1-oxaspiro[2.5]octane epimers were most enantioselective (E > 100).

In the search for new transport fuels from renewable resources, biodiesel from microbial lipids comes into view. We have evaluated the lipid yield and energy use of a process for production of biodiesel from agricultural waste using lipid‐accumulating yeast and fungi. We included different bioreactors for submerged and solid‐state fermentation in our evaluation. Using existing kinetic models, we predict lipid yields on substrate between 5% and 19% (w/w), depending on the culture system. According to the same models, improvement of the yield to 25–30% (w/w) is possible, for example by genetic modification of the micro‐organisms. The net energy ratio of the non‐optimized systems varies between 0.8 and 2.5 MJ produced per MJ used; energy use for pre‐treatment and for oxygen transfer are most important. For the optimized systems, the net energy ratio increases to 2.9–5.5 MJ produced per MJ used, which can compete very well with other biofuels such as bioethanol or algal biodiesel. This shows that, although quite some work still has to be done, microbial lipids have the potential to be tomorrow's source of biodiesel. © 2013 Society of Chemical Industry and John Wiley & Sons, Ltd