Foam control in biopesticide production from sewage sludge AS Vidyarthi 1 , M Desrosiers 2 , RD Tyagi 1 and JR Vale Âro 2 1 Institut National de la Recherche Scientifique, Universite  du Que Âbec, 2700 rue Einstein, Ste-Foy, Que Âbec, Canada G1V 4C7; 2 Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre, 1055 du PEPS, P.O. Box 3800, Ste-Foy, Que Âbec, Canada G1V 4C7 Several antifoam agents were evaluated for the ability to control foam in the production of Bacillus thuringiensis - based biopesticides using sewage sludge as a raw material. Experiments were conducted in shake flasks as well as in 15 l fermentors with controlled parameters. Polypropylene glycol ( PPG ) , the most commonly used antifoam agent in B. thuringiensis fermentation, inhibited cell growth, sporulation and decreased the entomotoxicity yield even at a concentration of 0.1% ( v / v ) in sewage sludge medium. About 40% reduction in entomotoxicity was observed when PPG was used at 0.3% ( v / v ) . The impact of PPG on sporulation and toxin synthesis in tryptic soy yeast broth ( TSYB ) medium was also studied. The inhibitory effects were less severe in TSYB than in sludge medium. Another silicone - based antifoam agent, ``Antifoam A'', showed less severe effect on growth and stendotoxin production. The problem of the inhibitory effect of chemical antifoam agents on growth and endotoxin production was minimised substantially with the use of vegetable oils such as canola, olive, and peanut oils. Canola and peanut oil stimulated both sporulation and  -endotoxin synthesis. The stimulus effect varies with the monounsaturated fat contents of oils. Journal of Industrial Microbiology & Biotechnology (2000) 25, 86±92. Keywords: foam control; biopesticide production; Bacillus thuringiensis; entomotoxicity;  -endotoxin; spores; Choristoneura fumiferana; spruce budworm; natural oils; sewage sludge Introduction In addition to aeration, agitation and other hydrodynamic conditions, the protein content of the microbial growth medium is the most vulnerable factor for foam formation in fermenta- tion systems. Intensive foam formation frequently engenders productivity losses such as reduction in working volume of a bioreactor [8], product and biomass losses, cell lysis, enhanced gas hold up, decreased power dissipation and liquid circulation rate, lower mass and heat transfer and increased chances of contamination during fermentation [18]. The concentration of components initially present in the medium and products of biochemical reactions as well as operating conditions signifi- cantly influenced the stability and pattern of foam formation [20]. Foam control needs more attention when sewage sludge is used as a sole medium due to the fact that sludge is composed of approximately 95% bacterial cell mass (dry sludge basis) and is rich in proteins. Foam formation in biopesticide fermentation is controlled mainly by the addition of polypropylene glycol (PPG) and silicone-based antifoam agents [5,10]. These antifoam agents affect the respiratory activity by affecting transport of nutrients through the cell walls, including oxygen transport. They influence the physiology of cells and cells grown in the presence of these agents resemble physiologically those grown under oxygen limitation [20]. High aeration rates are necessary to maximize cell growth, sporulation and  - endotoxin production by Bacillus thuringiensis (Bt) [3,5,13], and causes severe foaming in fermentation processes. The dissolved oxygen concentration in the fermentation liquid was also correlated with cell respiration rates [9] and  -endotoxin synthesis by Bt [1]. Many problems associated with chemical antifoam agents can be eliminated with the use of natural oils which, apart from controlling foam [17,18], work as an oxygen vector [12]. The selection of an antifoam agent depends on its ability to suppress foam, volume required and unit cost of antifoam. This report deals with the selection of the best antifoam agent based on overall efficiency of foam suppression in Bt fermentation using sewage sludge as a raw material. Materials and methods Bacterial strain B. thuringiensis var. kurstaki HD-1 (ATCC 33679) was used. It was subcultured and streaked on tryptic soy agar plates [TSA: 3.0% Tryptic Soy Broth (Difco, Becton Dickinson & Company, Sparks, MD, USA)+1.5% Bacto-Agar (Difco, Becton Dickinson & Company, Sparks, MD, USA) ], incubated for 24 h at 3018C and preserved at 48C for future use. Antifoam agents The antifoam agents used in this study include ``Antifoam A'' (Sigma Chemicals, St. Louis, MO, USA), PPG (Aldrich Chemical Company, Milwaukee, WI, USA) and most common natural oils available in the market. The natural oils used were canola oil (Crisco brand, Proctor and Gamble Inc., Toronto, Ontario, Canada), corn oil (President Choice, Sun Fresh Ltd., Toronto, Ontario, Canada), olive oil, peanut oil, soybean oil (all of Generation, Minix Inc., Montreal, Que Âbec, Canada), sesame (China Sun, National Importers Ltd., New Westminster, BC, Canada) and sunflower oil (Safflo, Unico Inc.). All Correspondence: RD Tyagi, Institut National de la Recherche Scientifique, Universite  du Que Âbec, 2700 rue Einstein, Ste-Foy, Que Âbec, Canada G1V 4C7 Received 9 February 2000; accepted 6 June 2000 Journal of Industrial Microbiology & Biotechnology (2000) 25, 86±92 D 2000 Society for Industrial Microbiology 1367-5435/00 $15.00 www.nature.com/jim