A process for manufacturing a thermoset polyester foam includes the following successive steps: (a) providing an expandable and thermosetting composition containing a polyol component including at least one element selected from glycerol, diglycerol and glycerol oligomers; a polyacid component including citric acid; a surfactant selected from alkyl polyglycosides and mixtures of an anionic surfactant and a cationic surfactant, and an esterification catalyst; (b) introducing the expandable and thermosetting composition into a mold or applying the expandable composition to a support; and (c) heating the expandable and thermosetting composition at a temperature at least equal to 135° C. so as to react the polyol component with the polyacid component and form a block of thermoset polyester foam.