Biochemical Engineering Journal 79 (2013) 7–14 Contents lists available at ScienceDirect Biochemical Engineering Journal jou rnal h om epage: www.elsevier.com/locate/bej Regular article Experimental study of microorganism disruption using shear stress Talal Yusaf ∗ National Centre for Engineering in Agriculture (NCEA), Faculty of Engineering & Surveying, University of Southern Queensland, Toowoomba 4350, QLD, Australia a r t i c l e i n f o Article history: Received 13 February 2013 Received in revised form 27 June 2013 Accepted 3 July 2013 Available online 12 July 2013 Keywords: Yeast Shear treatment Cell disruption Energy Water a b s t r a c t There has been a broad spectrum of theoretical and experimental works on microorganism disruption methods undertaken in the past. However, there is a lack of understanding regarding the actual reasons for microorganism disruption using ultrasound and whether it is caused by shock or shear. In the case of shear stress, which is the focus of this paper, analysis of the intense turbulent flow region of an in- house built shear apparatus combined with the experimental results demonstrated that when the energy dissipation rate in the turbulence region is high, and the size of the eddy is smaller than the size of the cell, the likelihood of yeast disruption is high. The mechanical properties of yeast cells combined with the calculated energy dissipation rate were used to evaluate the yeast disruption efficiency (log reduction). The results show that the shear apparatus can efficiently and effectively disrupt S. cerevisiae at different treatment times, suspension temperatures and rotor speeds. The experimental work suggests that maximum yeast log reduction was achieved when the maximum power dissipation of 2.095 kW was recorded at 10,000 RPM, while suspension temperature was controlled below 35 ◦ C. The corresponding shear stress at 10,000 RPM was 2586.2 Pa. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Recent work on biological cells revealed that mechanical meth- ods such as microfluid and shear stress devices can create many opportunities for investigating the dynamic mechanical behavior of a single biological cell [1]. Microstreaming causes large localized forces to shear the cell wall surfaces resulting in physical dam- age to the cells [2]. Doulah et al. [3] and then Doulah [2] describe the theory of shear stress as a primary cause of a microorganism rupturing. Doulah [2] has also reported that when small gas bub- bles oscillate during the compression and rarefaction phases of the sound wave, strong eddies are developed in the area surrounding the bubbles which ultimately spread into the liquid. According to Refs. [2,4,5], this effect, known as microstreaming, leads to a sig- nificant localized shear force that rubs the cell wall surfaces of surrounding organisms and causes the cell wall to rupture. Wil- son and Kohles [6] reported in their recent simulation research work that his team have identified a range of potential mechan- ical strains that can be produced in multiaxial fluid-induced stress. The mathematical model can provide a glimpse into the potential for rapid microfluidic flow rates and the response of weaker cells to increased fluid-induced stresses. ∗ Tel.: +61 7 4631 2691/1373. E-mail addresses: yusaft@usq.edu.au, talaloo@hotmail.com Sowana et al. [7] suggest that the cell disruption correlates well with the local energy dissipation in the fluid mechanics using Kol- mogorov’s theory of isotropic turbulence. According to Walstra [8], Doulah [2] and Doulah et al. [3], the use of the Kolmogorov theory for universal balance is essential for understanding the rupturing of the cell wall where the flow is turbulent. Others believe that both laminar and turbulent flow can produce the same amount of cell disruption, which indicates that eddies are not an essential feature for disruption [2]. Although the Zhang et al. [9] model was used for animal cells, the approach suggested by Zhang et al. [9] may still provide an estimate of the energy required for rupturing the yeast cell wall. It is noticed that while Zhang et al. [9] applied the approach to animal cells, earlier workers such as Doulah et al. [3] and Doulah [2] essentially used the same approaches to evaluate yeast cell dis- ruption. Doulah et al. [3] used the principle of liquid drop breakage and hydrodynamic flow to estimate the yeast disruption and the energy required to rupture a yeast cell wall using kinetic energy and strain (elastic) energy. Doulah et al. [3] reported that a reason- able agreement was found between the experimental results (refer to yeast disruption) and the principle of the liquid drop breakage theory expression in the homogenizer. Several mechanical devices that use shear stress for microorganism disruption have been inves- tigated by other researchers including homogenization in the dairy product industry [10], bead mills and microfludization [11]. The apparent cause of the cell disruption in these devices appears to be the shearing effect. A homogenizer is a device that is commonly used in the dairy industry to break-up fat globules into smaller 1369-703X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.bej.2013.07.001