Process Biochemistry 35 (1999) 127 – 133 Effect of substrate composition on the mycelial growth of Pleurotus ostreatus. An analysis by mixture and response surface methodologies Oscar Soto-Cruz a , Gerardo Saucedo-Castan ˜ eda a, *, Jose ´ Luis Pablos-Hach b , Mariano Gutie ´rrez-Rojas a , Ernesto Favela-Torres a a Departamento de Biotecnologı ´a, Uniersidad Auto ´noma Metropolitana, Iztapalapa, A.P. 55 -535, C.P. 09340, Mexico D.F., Mexico b Facultad de Medicina Veterinaria y Zootecnia, UNAM, Ciudad Uniersitaria, Mexico D.F., Mexico Received 23 September 1998; received in revised form 26 February 1999; accepted 19 March 1999 Abstract The effect of the composition of a mixture containing, oat straw (OS), oat bran (OB) and copra cake (CC), on the mycelial growth of Pleurotus ostreatus was studied using mixture and response surface methodologies. The applied constraints to the mixtures were: moisture content higher than 70%, C/N ratio less than 30 and total mixture cost less than 2% of the retail cost of fruiting bodies of P. ostreatus. The maximum observed value of apical growth rate (0.50 0.02 cm day -1 ) was obtained using 0.633, 0.284 and 0.083 (g g -1 mixture, dry basis) for OS, CC and OB mass fractions, respectively. Under these conditions the C/N ratio was 22.4–23.2. Loss of dry matter decreased from 16.9 to 8.5% as the OS fraction (lignin and cellulose source) was increased from 0.55 to 0.80 (g g -1 mixture, dry basis). The utilisation of mixture and response surface methodologies was an useful approach to evaluate the relationship between substrate composition and mycelial development of P. ostreatus. © 1999 Elsevier Science Ltd. All rights reserved. Keywords: Pleurotus ostreatus ; Solid substrate cultivation; Mycelial growth; Mixture methodology; Response surface methodology www.elsevier.com/locate/procbio 1. Introduction Pleurotus ostreatus is an edible fungi with a nutri- tional value similar to milk and meat [1]. Large scale production of this fungi has been attempted in various developing countries including India, Brazil and Mex- ico [2]. Pleurotus world production has increased from 7.7 in 1986 to 24.2% in 1990 [3]. In the case of Mexico, production was estimated at 1825 ton in 1997 [4]. Mushroom cultivation is a simple, low cost and environmentally friendly technology for the utilisation of rural and agro-industrial residues [5]. The cultivation of edible fungi on agricultural by-products like sugar cane bagasse, coffee pulp and straws could be consid- ered as a valuable approach for the production of protein enriched food [6]. Additionally, fermented residues could be used as animal feed after mushroom cultivation [7,8]. The final product, fruiting bodies of P. ostreatus, is obtained after solid substrate cultivation (SSC) in two stages. In the first stage mycelia develop- ment requires 2 weeks incubated at 28°C in the dark- ness. Once the substrate has been totally invaded, the second stage involves the production of fruiting bodies taking a further 2–3 weeks. The second stage requires humid saturated air and alternated periods (12 h) of light and darkness [7]. The biological efficiency (fresh weight of fruiting bodies per dry weight of the substrate) of the final product depends on the development of mycelia in the first culture stage. Contaminations can decrease biolog- ical efficiency from 40 to 100% [9]. The risk of contam- ination is maximal at the beginning of the process and it could be inversely related to Pleurotus mycelia devel- opment [10]. In this sense, as long as the duration of mycelia development is reduced, the risk of substrate contamination could also be reduced. * Corresponding author. Tel.: +52-5-7244999; fax: +52-5- 7244712. E-mail address: saucedo@xanum.uam.mx (G. Saucedo-Castan ˜ eda) 0032-9592/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved. PII:S0032-9592(99)00043-6