Apple pomace was used as a substrate for microbial production of citric acid using Aspergillus niger. A packed-bed bioreactor consists of jacket and four stages with dimensions of 100 x 16 cm(2) was constructed and operated for solid state fermentation. The operation of this bioreacter was optimised using Taguchi statistical method of analysis. The optimum parameters was obtained as: aeration rate; 0.8 l min(-1); bed height, 10 cm; particle size, 0.6-2.33 mm moisture content, 78% (w/w). Under these optimised conditions, 124 g citric acid was produced from I kg dry apple pomace with yield of 80% based on total sugar. (C) 2002 Elsevier Science Ltd. All rights reserved.