The main highlight of the work presented in this paper is that is one of the few studies using cereals as substrates for fermentation of probiotic microorganisms. ► It is also one of the few attempts trying to study the effect of mixed cultures in these beverages. ► The use of mixed cultures is common practice in the production of dairy based probiotic formulations commercially available. ► Mechanisms for growth and substrate production of single culture fermentations have been widely studied, but the potential effects of the used of combined cultures is not well understood. ► This works proves that even though the effect of the mixture of microorganisms does not seen to greatly affect cell populations, it does affect the production of flavour attributes, which could have a considerable effect on consumers' acceptance of these products and future developments of novel probiotic beverages.