Citric acid and gluconic acid are produced by Aspergillus niger. Citric acid is a preservative and flavouring agent which can also be used in medicines engraving, dyeing and inks. Gluconic acid is employed in pharmaceuticals. Citric acid is also used by Mucor species and yeast (when medium is deficient in Fe and Mn). Lactic acid is obtained by the activity of number of bacteria like Streptococcus lactis, Lactobacillus
delbrenci, L. bulgaricus and fungus Rhizopus over a variety of nutrient media. It is used in clearing, flavouring and preserving confectionery, dyeing, etc. Acetic acid is both microbial and synthetic in origin. It is produced by Acetobacter aceti and used in plastics, pharmaceuticals, colouring agents, etc.