Bioprocess Biosyst Eng (2010) 33:921–927 DOI 10.1007/s00449-010-0415-9 O R I G I N A L P A P E R Effects of shear stress on the microalgae Chaetoceros muelleri Michiel H. A. Michels • Atze J. van der Goot • Niels-Henrik Norsker • Rene´ H. Wijffels Received: 20 January 2010 / Accepted: 12 February 2010 / Published online: 27 February 2010 The Author(s) 2010. This article is published with open access at Springerlink.com Abstract The effect of shear stress on the viability of Chaetoceros muelleri was studied using a combination of a rheometer and dedicated shearing devices. Different levels of shear stress were applied by varying the shear rates and the medium viscosities. It was possible to quantify the effect of shear stress over a wide range, whilst preserving laminar ﬂow conditions through the use of a thickening agent. The threshold value at which the viability of algae was negatively inﬂuenced was between 1 and 1.3 Pa. Beyond the threshold value the viability decreased sud- denly to values between 52 and 66%. The effect of shear stress was almost time independent compared to normal microalgae cultivation times. The main shear stress effect was obtained within 1 min, with a secondary effect of up to 8 min. Keywords Microalgae Shear stress Viability Photobioreactor design Chaetoceros muelleri Aquaculture feed M. H. A. Michels (&) N.-H. Norsker R. H. Wijffels Bioprocess Engineering, Wageningen University, P.O. Box 8129, 6700 EV Wageningen, The Netherlands e-mail: michiel.michels@wur.nl M. H. A. Michels HZ University of Applied Sciences, P.O. Box 364, 4380 AJ Vlissingen, The Netherlands A. J. van der Goot Laboratory of Food Process Engineering, Wageningen University, Wageningen, The Netherlands Introduction Microalgae are cultivated to serve as feed for aquaculture (shellﬁsh, shrimps and ﬁsh), animal feed (pets and farm- ing), human nutrition, cosmetics and the production of high-value ingredients, such as polyunsaturated fatty acids and pigments [1]. The current market size of microalgal biomass is about 5,000 t dry matter/year of which at least 1,000 t dry matter/year is produced for aquaculture [2, 3]. We are interested in the production of microalgae as a feed for bivalves. In general, cultivation of microalgae for these applications takes place in transparent plastic bags. Because the capacity of these systems is relatively small, labor cost per amount of feed for bivalve production is high. This explains why alternative large scale production systems for microalgae are being investigated. Such sys- tems can consist of open ponds, bubble columns, ﬂat-plate photobioreactors (PBRs) and tubular PBRs [4–6]. PBRs seem to be most promising in cultivating these microalgae on a large scale, because of low contamination risk, low space requirement, almost no CO2 loss, high adaptability to many species and high biomass concentration [7, 8]. However, it is suspected that the large hydrodynamic for- ces present in the PBR could cause shear stress levels that might be too severe for the sensitive microalgae, which would lead to reduced growth or even cell death [9]. Nevertheless, a high mixing intensity is necessary to keep microalgae in suspension, to achieve a sufﬁcient light distribution and to enhance mass transfer. For the optimal design of PBRs, it is therefore important to know the maximum level of shear stress that can be tolerated by the microalgae. Damage because of shear stress has been demonstrated both in bubble columns due to gas sparging [10–14] and in tubular PBRs due to pumping action [12, 15, 16]. Although 123