It is a weak organic acid naturally found in citrus fruits, providing the acidic taste and makes citrus fruits taste sour. Also, it plays an important part in the metabolism of most living things as an intermediate in the tricarboxylic acid cycle or Krebs cycle.

Two types in market
There are two forms of citric acid sold in the market, anhydrous and monohydrate. Monohydrate can turn to anhydrous in the condition of heated above 37 °C.

Natural sources
Citrus fruits have a high level of citric acid. Fruit containing citric acid includes:

Lemon
Lime
Pineapple
Oranges
Tomatoes
Strawberries
Peaches
Apples
Grapefruits
Blueberries
Bananas
Concentration
Lemon juice and lime juice are rich in citric acid. The concentration is around 41 mg/kg in ready-to-consume lemon juice and around 31 mg/kg in concentrates. The numbers are 39 mg/kg and 30 mg/kg in lime juice. In commercial lemon juice-based drinks, such as lemonade, the content ranges from 0.62 to 0.96 mg/kg. (1)

How is it made?
Citric acid is commercially produced by microbial fermentation of a glucose or sucrose carbohydrate substrate derived from corn.

Generally made from feeding substrate to black mold. Aspergillus niger is the major organism used in the microbial production of citric acid.

There are three manufacturing processes approved by the FDA as follows (2):

From plant sources, e.g. lemon or pineapple juice.
By mycological fermentation using Candida spp.
From Aspergillus niger fermentation liquor.
A study published in Toxicology Reports in 2018 said that approximately 99% of the world’s output of citric acid is using the fungus Aspergillus niger since 1919. So we can conclude that the third process method above is the most used one. (3)

Specification
Other Names   
2-hydroxy-1,2,3-propanetricarboxylic acid
β-Hydroxytricarballylic acid
CAS Number   
77-92-9 (anhydrous)
5949-29-1 (monohydrate)
Chemical formula   
C6H8O7 (anhydrous)
C6H8O7·H2O (monohydrate)
Molecular Weight   
192.13 (anhydrous)
210.14 (monohydrate)
Melting Point    156 °C
Boiling point    310 °C
Structure
citric acid chemical structure

Properties
PKa
PH
Solubility
Appearance
A white crystalline powder or granular with a strongly acidic taste.

Pka
PKa is used to describe the acid dissociation. Citric acid is a weak organic acid containing three carboxylic acid functional groups and as a result it has three PKa values, PKa1 = 3.14, PKa2 = 4.77 and PKa3 = 6.39.

PH
PH measures the concentration of H+ ions or H3O+ ions of the citric acid solution. The PH value depends on the citric acid concentration and its dissociation. It is indicated that the PH value of citric acid is 2.62 in the concentration of 10 mM (0.01mol/L) (4)