Overview of citric acid production from Aspergillus niger

Figures & data

Figure 1. Structural formula of citric acid, characterized by three –COOH groups.

Figure 2. Schematic representation of the main metabolic reactions involved in the production of citric acid by Aspergillus niger (Soccol et al. 2006).

Figure 3. Schematic representation of the metabolic reactions involved in citric acid production, the enzymes (italics), the known feedback loops (dashed lines) and their locations within the cellular structure of Aspergillus niger (Papagianni 2007).

Table 1. Raw materials used for citric acid fermentation.

Type of fermentation	Raw materials
Surface	Brewery waste, carob sugar, cotton waste, tubers of Asphodelus aestivus, turnip whey (enriched with molasses)
Submerged	Bagasse hydrolysates, beet molasses, black strap molasses, brewery wastes, cane molasses, carob pod extract, coconut oil, corn starch, date syrup, glycerol, hydrolysate starch, n-paraffin, palm oil, olive oil, soya bean oil, rapeseed oil, spent grain liquor, wood hemicellulose, whey permeate, xylan hydrolysate, yam bean starch
Solid state	Apple pomace, carob pod, carrot waste, cassava bagasse, cellulose hydrolysate and sugar cane, coffee husk, corncob, deoiled rice bran, grape pomace, kiwi fruit peel, kumara, molasses, mussel processing wastes, okara, orange waste, pineapple waste, rice bran, sucrose, sugarcane-pressmud, wheat bran

Table 2. Factors affecting citric acid production.

Factors	Effects
Carbon source	Positive: sucrose, glucose, fructose, galactose
	Negative: starch, xylose, arabinose, sorbitol, pyruvic acid
Phosphorus source	Positive: potassium dihydrogen phosphate (0.5–5.0 g/l)
Nitrogen source	Positive: ammonium nitrate, ammonium sulphate, peptone, malt extract, urea (0.1–0.4 gN/l)
	Negative: high concentrations lead to biomass production
pH	Positive: < 3 (depends on species)
Trace elements	Positive: zinc, copper, magnesium sulphate (low levels)
Lower alcohols	Positive: methanol, ethanol, n-propanol, iso-propanol, methylacetate (1–4% v/w)

Table 3. Applications of citric acid.

Applications	Industry	Uses
Food	Animal feed	Complementary feed
	Candies	Acts as acidulant. Prevention of sucrose crystallization provides dark colour in candies, sucrose inversion. Provides tartness
	Dairy products	Emulsifier (ice creams and processed cheese), antioxidant and acidifying agent (cheese products)
	Fats and oils	Stabilizing action. Synergist for other antioxidants as a sequestrant
	Frozen fruits	Inactivates trace metals to protect ascorbic acid. Lowers pH to inactivate oxidative enzymes. Neutralizes the residual lye
	Jellies and jams	Adjusts pH (to the range where pectin acts as a gelling agent). Acts as an acidulant. Provides tartness, tang and flavour. Increases the effectiveness of antimicrobial preservatives
Beverages	Fruit and vegetable juices	Stabilizer in commercially prepared juices
	Soft drinks and syrups	Simulates fruit flavour and tartness. As an acidulant in carbonated and sucrose-based beverages
	Wines and ciders	Browning prevention in some white wines. Turbidity prevention. Oxidation inhibition. pH adjustment
Pharmaceutics	Pharmaceuticals	Provides rapid dissolution of active ingredients. Acidulant in mild astringent formulations, anticoagulant. Effervescent in powders and tablets in combination with bicarbonates, solubilization action for cathartics, antioxidant in vitamin preparations
	Cosmetics and toiletries	Metallic-ion chelator and buffering agent. Adjusts pH. Antioxidant
Other	Other	Buffering agent, sequesters metal ions, neutralizes bases
		Metal oxide removal from ferrous and non-ferrous metal surfaces, operational cleaning of iron and copper oxides
		In electroplating, copper plating, metal cleaning, leather tanning, printing inks, bottle washing compounds, floor cement, textiles, photographic reagents, concrete, plaster, adhesives, paper, polymers, tobacco, waste treatment, chemical conditioner on teeth surface, ion complexation in ceramic manufacture, etc.

Soccol CR, Vandenberghe LPS, Rodrigues C, Pandey A. 2006. New perspectives for citric acid production and application. Food Technology & Biotechnology. 44:141–149.

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Papagianni M. 2007. Advances in citric acid fermentation by Aspergillus niger: biochemical aspects, membrane transport and modeling. Biotechnol Adv. 25:244–263. doi: https://doi.org/10.1016/j.biotechadv.2007.01.002

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