The present invention relates to the fields of industrial biotechnology, renewable raw materials and production of organic acids. Specifically, the invention relates to a method of producing an organic acid or a salt thereof. Still, the present invention relates to a mixture comprising legume molasses and calcium hydroxide or a mixture obtained by combining a carbon substrate composition comprising legume molasses and an aqueous suspension comprising calcium hydroxide, for producing an organic acid or a salt thereof by a microorganism, and use of said mixture for producing an organic acid or a salt thereof. Still further, the present invention relates to a method of producing one or more products selected from the group consisting of polymers, polyesters and polylactic acids.