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A spray-drying process was optimised by response surface methodology for the development of a powder formulation of Sinorhizobium meliloti (produced in starch industry wastewater). The process parameters comprised drying temperature, air drying rate, spray pressure and feed sample rate. Cheese whey powder, sorbitol and sucrose were added at 10, 1 and 1% w/v, respectively before drying. Responses studied comprised cell viability, moisture and outlet temperature. Under the optimal conditions (maximum desirability = 0.81), the formulation showed cell viability of 6.8 x 10(9) CPU g(-1), moisture of 11.4% and outlet temperature of 42 degrees C. During 4 months of refrigerated storage, viability was maintained at >10(9) CPU g(-1). Analysis of inoculated alfalfa plants showed that powder formulation induced similar shoot yield on dry basis (144.6 mg) (p = 0.23) and nodulation index (15.2) (p = 0.18) compared to fresh cells cultured in standard media (shoot yield and nodulation index were 131 and 14 mg, respectively). Micrographs showed that cells were embedded in the mixture of additives (whey-sucrose-sorbitol) which acted as a matrix. (C) 2013 IAgrE. Published by Elsevier Ltd. All rights reserved.