Solid-state fermentation (SSF) was evaluated to produce gluconic acid by metal resistant Aspergillus niger (ARNU-4) strain using tea waste as solid support and with molasses based fermentation medium. Various crucial parameters such as moisture content, temperature, aeration and inoculum size were derived; 70% moisture level, 30 degrees C temperature, 3% inoculum size and an aeration volume of 2.5 l min(-1) was suited for maximal (76.3 g l(-1)) gluconic acid production. Non-clarified molasses based fermentation media was utilized by strain ARNU-4 and maximum gluconic acid production was observed following 8-12 days of fermentation cycle. Different concentrations of additives viz. oil cake, soya oil, jaggary, yeast extract, cheese whey and mustard oil were supplemented for further enhancement of the production ability of microorganism. Addition of yeast extract (0.5%) was observed inducive for enhanced (82.2 g l(-1)) gluconic acid production. (c) 2007 Elsevier Ltd. All rights reserved.