An antibacterial agent suppresses the growth of gram-positive contaminating bacteria. The antibacterial agent is selected from: a lactylate in accordance with the general formula (R—(O—CH(CH3)—CO)aO)bM; a glycerol ester in accordance with the general formula CH2OR1—CHOR2—CH2OR3; and, mixtures thereof. In the general formulae: R represents a C4-C18 acyl group, R1, R2 and R3 are each independently selected from H or a C4-C18 acyl group, the acyl group having an alkyl or alkenyl chain which may be branched or unbranched, with the proviso that at least one of R1, R2 or R3 is H and at least one of R1, R2, or R3 is an acyl group; M represents a proton (H+) or a counter-cation chosen from the group Li, Na, K, Ca, Mg, Zn, Fe(II), Cu, Mn, Ag, ammonium or substituted ammonium having one or more (C1-4)alkyl optionally substituted with one or more hydroxyl.