Individual chemical compounds with activity against food-spoiling moulds and yeasts. They range from weak organic acids, which work in their undissociated form at low pH, to potent polyene antibiotics such as natamycin that bind fungal membrane sterols. Researchers screen plant extracts, fermentation metabolites and synthetic molecules for new candidates, driven by consumer preference for natural preservation and by resistance to established agents. Every candidate faces the same gauntlet: proven efficacy in real foods, full toxicological assessment, regulatory approval as a preservative, and acceptable flavour impact at effective doses.