The production of biochemicals, materials and biofuels from renewables through biorefinery processes is important to reduce the anthropogenic impact on the environment. Chain elongation processes based on open microbiomes have been successfully developed to produce medium-chain fatty acids (versatile platform products) from organic waste streams. Yet, the sustainability of chain elongation can still be improved by reducing the use of electron donors and reducing chemicals use. This work aimed to in situ produce the electron donor lactate coupled to subsequent chain elongation for n-caproate production with decreased chemicals use for pH control. Food waste was used as substrate in repeated-batch fermentation experiments.