Citric Acid Production

INDUSTRIAL MICROBIOLOGY

1. CITRIC ACID PRODUCTION

INTRODUCTION

It is Tri carboxylic acid and was isolated from lemon juice and crystallised in 1784 by Scheele.It is found as a natural constituent of variety of foods. Over the year ,a large number of organisms including fungi, yeast and bacteria have been screened for citric acid.Aspergillus niger still remain the organism of choice for industrial preparation of citric acid.

PRODUCTION FROM FUNGI:

Fungal strain of various fungi which has been found to accumulate citric acid in their culture media included strains of Aspergillus niger , Aspergillus penicillium, Aspergillus flavus and Mucor.Strains of Aspergillus niger have dominated on other in large production as well as industrial scale production of citric acid.

ADVANTAGES OF ASPERGILLUS FOR PRODUCTION :

• It is easy to handle culture medium

• Cheap raw material that it can utilise

• High end constitute consistent yields ,clearly making the process economical

TYPES OF FERMENTATION USED IN PRODUCTION:

• Surface fermentation

• Solid state fermentation

• Submerged state fermentation

1 SURFACE FERMENTATION

In this method ,sterile nutrients medium containing Agar is allowed to flow into stainless steel trays which are arranged in sterile fermentation chambers.

This is then inoculated with spores of Aspergillus niger . The Organism grows and spreads over the surface, fermentation is completed within eight to twelve days . The fermentation liquor is then drained off and processed further for recovery of acid

2 SOLID STATE FERMENTATION

It is first described by Cohan in 1935 .In this process the fermentation medium is impregnated in porous solid material such as sugar cane , potato pulp, fine apple pulp etc.,sterilise the medium and then inoculated with a suspension of fungal spores .The mash ,then incubated in trays for 6 to 7 days ,after fermentation the mash is extracted with water ,concentrated it and processed it for citric acid precipitation

3 SUBMERGED STATE FERMENTATION

This technique now have been adopted by most newly built plants. In this ,the nutrients medium after inoculation are subjected to vigorous controlled aeration and agitation in large fermenters . The time interval involved is much shorter i.e 3 to 5 days in the process.

• Culture condition or media composition

Media composition for citric acid production by fungi is vary strain to strain and also depend upon type of process. It includes following sources:

• Carbon sources -a variety of carbon sources such as sucrose,starch,cane and beet molasses have been used for citric acid production yet sucrose and molasses have remained the substrate of choice

• Nitrogen source -the nitrogen requirement for citric acid production is generally meet by the addition of inorganic nitrogen sources such as ammonium sulfate ,nitrate ,soil nitrate urea etc in addition to carbon and nitrogen.

• Phosphate constituent is the third essential element required.Some trace elements like irons ion,Copper ion, zinc ion ,magnesium ions and manganese ions etc. a variety of divalent trace elements also required for growth of Aspergillus niger and citric acid production.

Pre treatment of raw materials:

Since the concentration of trace elements affect the production of citric acid .So high concentration of these matters allow the vegetative growth of organism ,at the cost of acid excretion, various techniques have been developed to minimised the concentration of these metals in fermentation medium .For these two approaches have been made

1 pre treatment of raw materials with chemicals ,ion exchange, resins etc. to reduce trace elements concentrations.

2 development of strains of fungi which have ability to produce citric acid in the presence of high concentration of trace elements

• pH

Maintenance of a favourable pH is very essential . The initial pH required depends on carbon source used . Example :when sucrose or glucose are used ,a low pH that i.e 3 is desirable . Low initial pH has some advantages like it prevent contamination and suppressed oxalic acid formation

• Inoculum

Inoculum for fermentation can be the spores or pre grown mycelium. In case of spores ,suspension is freshly prepared in sterile water or Sterile water containing tween- 80 chemical is used to inoculate fermentation medium.

• Aeration

The citric acid fermentation is essentially an aerobic fermentation and organism needs an abundant supply of . There are three ways to ensure aeration are:

• surface cultivation

In this, oxygen diffuses through the surface of medium which is maintained in a static condition

• submerged aeration

In this, the sterile air or oxygen is found to form beneath the surface of culture

• shaked culture technique

This is the process of both surface as well as submerged aeration.

• Temperature

Aspergillus niger and other fungi used for citric acid production have an optimum temperature of 25 to 30 degrees Celsius and an increase in temperature beyond 30 degrees Celsius have been found to decrease the yield

• Time of incubation

it depends on type of fermentation

• Additives and stimulants:

A variety of stimulants have been tested for improving the yield by Aspergillus Niger. The most important stimulant is methanol ,it is added to allow the tolerance of fungi to trace elements and it is an oxidising agent Methylene blue and hydrogen peroxide is also added to stimulates the citric acid production and enhances the flavor.

2. CITRIC ACID PRODUCTION BY YEAST

It is now well known that many kinds of yeast also accumulated citric acid in their growth media along with relatively large amount of iso- citric acid . Important species of yeast are Candida ,Hensenula, Rhodotorula. Out of these generous species of Candida are the one That are widely used for the production of citric acid.

• Substrates for citric acid production

Yeast utilise mainly glucose, acetate and molasses

• Other conditions

Fermentation is always carried out under aerobic conditions and temperature between 20 to 30 degrees Celsius is optimum .One advantage of using yeast for citric acid production as compared to Aspergillus is the tremendous potential for developing a continuous process.

3. CITRIC ACID PRODUCTION BY BACTERIA

Some bacteria such as Bacillus subtilis, Brewbacterium flavum have been found to produce citric acid from either glucose isocitric acid or from hydrocarbons. Besides glucose the medium supplemented with nitrogen source such as urea, ammonium sulfate or glutamate and the carbon source used is n-paraffin.