The invention pertains to a process for manufacturing a glycerol-tricarboxylic acid polyester foam which comprises the steps of combining glycerol and a tri-carboxylic acid to provide a liquid reaction mixture and contacting the reaction mixture with a substrate under polymerization conditions, wherein the substrate has a top layer comprising one or more of metal, metal oxide, and metal halide. The invention also pertains to a glycerol-tricarboxylic polyester foam, in particular a glycerol-citric acid polyester foam, which has a closed cell foam structure wherein at least 90 vol.% of the foam, preferably at least 95% of the foam, is built up from cells having a diameter below 2 mm. The foamed polyester of the present invention is "green", biodegradable, and non-toxic, and can be cleanly combusted. It finds application in, int. al., packaging materials, insulation materials, and materials with a short life cycle.