Effect of Corn steep liquor supplementation and scale up on Lactococcus starter production M. Hamdi, S. Hamza, N. Mtimet, N. Hmida C. Cornelius, S. Zgouli, A. Mahjoub, Ph. Thonart Summary Three Lactococcus strains (Lactococcus ssp. lactis var. diacetylactis, Lactococcus ssp. lactis cremoris and Lactococcus ssp. lactis var. lactis) isolated from the Tuni- sian lben were grown at constant pH on CSL medium in stirred fermentors for lactic starters production. The agi- tation required to homogenate alkali used to pH control should be low because it affects the Lactococcus growth. Scale up from 20-liter fermentor to 400-liter fermentor was carried out at constant impeller tip speed below 150 cm s )1 . The CSL supplementation and fed-batch with glucose increased the yield in the upper 10 10 cfu/ml. The con- sumed glucose during fermentation was converted into lactic acid and cell. Before fed-batch, the maximum spe- cific growth rate of Lactococcus ssp. lactis var. diacetylactis was around 1 h )1 and the number of cells increased 20 to 40 times according to inoculum size. After fed-batch, the glucose consumption rate remains constant but specific growth rate decreased and number of cell trebled only. 1 Introduction The use of appropriate starter cultures in dairy fermented milk allows us to obtain a uniform product of high quality. The function of lactic acid bacteria (LAB) is twofold: the acidification of milk by lactic acid production and the production of flavor ingredients that gives the final product taste. Moreover LAB produce bacteriocins which kill several pathogenic bacteria [1]. Fermented milks (yo- gurt, kefir, koumiss, lben) are made in most parts of the world, and the lactic bacteria species involved in these dairy products are different [2]. Lben is one of the oldest fermented milk known espe- cially in the Arab countries, and the Lactococcus species are dominants [3, 4, 5, 6]. For lactic starter production, three Lactococcus strains were selected from Tunisian lben for their high rate of development of acid, the degree of acidity obtained, and the odor and flavor evaluated in the lben produced [7]. Corn steep liquor (CSL) and yeast extract (YE) have been used as source of nitrogen for LAB production [8]. Only a few reports concerning lactic starter production in stirred fermentors and data remain part of industrial secrets. Problems of scale up in a fermentor are associated with the behavior of liquid in the fermentor and the metabolic reactions of the organisms [9, 10]. The mixing is required for LAB in fermentor for transfer of mass, and especially to homogenate rapidly alkali used to control pH during fer- mentation and then to reduce the inhibitory effect of this alkali. The pH control at 6 increased LAB density up to 10 10 cfu/ml and enhanced survival cell after freeze-drying [11]. However, the shear force effect caused by agitation can affect the cultivated microrganisms especially micro- bial flocs or chain-forming bacteria [12, 13]. In the present studies, the influence of initial Corn Steep Liquor concentration, the initial inoculum size, and scale up on the Lactococcus strains production in stirred fer- mentor were investigated. 2 Material and methods 2.1 Strains and preculture conditions Lactococcus strains used in this work were isolated from the Tunisian lben [7]. These strains were indentified as Lactococcus ssp. lactis var. diacetylactis (SLT 6), Lac- tococcus ssp. lactis cremoris (SLT 7) and Lactococcus ssp. lactis var. lactis (SLT 10). These strains are preserved in glycerol at )180 °C. The M17 agar was prepared for the growth, viable count (colony-forming units: cfu) and maintenance of Lactococcus strains. Colony from M17 agar was inoculated into a preculture medium in a Erlenmeyer flask full at 80%. The preculture medium was the M17 broth where lactose was changed by 20 g/l of glucose. Incubations were performed statically at 30 °C for 16 hours. 2.2 Fermentors operations Batch cultures and fed-batch cultures were performed in 20-liter and 400-liter stirred fermentors (Biolafitte, France) at 30 °C. The agitation system was a Rushton-like turbine. The impeller’s diameters of 20-liter and 400-liter stirred fermentors were 10 cm and 26 cm respectively. Bioprocess Engineering 22 (2000) 23–27 Ó Springer-Verlag 2000 23 Received: 25 January 1999 M. Hamdi (&), S. Hamza, N. Mtimet, N. Hmida, A. Mahjoub Ecole Supe´rieure des Industries Alimentaires de Tunis (ESIAT), 58 Avenue Alain Savary 1003 Tunis – Tunisie C. Cornelius, S. Zgouli, Ph. Thonart Centre Wallon de Biologie Industrielle (CWBI) 49 Bd du Rectorat Sart Tilman Universite´ de Lie`ge 4000 Lie`ge – Belgique We wish to thank the Belgium government (AGCD) and the Tunisian government (IRESA) for supporting this research.