Metabolic inhibitors as stimulating factors for citric acid production

摘要

The effect of some metabolic inhibitors on citric acid production by Aspergillus niger in cane molasses medium was investigated. Addition of 0.01-0.1 iodoacetic acid and sodium arsenate, 0.05 -1.0 sodium malonate, 0.01 sodium azide , 0.01-0.05 sodium fluoride , 0.1-1.0 EDTA stimulated citric acid production (5-49%), but not necessary total titratable acids. Higher concentrations 10mM of iodoacetic acid , sodium malonate and 0.5 mM sodium azide caused a complete inhibition of fungal growth. 0.2 mM of iodoacetic acid, sodium arsenate and sodium fluoride caused a remarkable inhibition of citric acid production. The implications of those preliminary functions was discussed. Introduction Many strains of Aspergillus niger are well known for their capacity to produce citric acid under suitable conditions. By carefully selection strains and improving conditions, 80 to 85 % of the weight of the initial sugar substrate can be converted into citric acid (Kiel et al., 1981). Citric acid is a commodity chemical, so it is necessary to use inexpensive and readily available raw materials in industrial process molasses is a desirable raw material for citric acid fermentation because its availability and relatively low price (Pazouki et al., 2000; Haq et al., 2002)Many investigators have tried to improve the production of CA by various additives. Moyer (1953) found that methanol, ethanol and isopropanol decreased with parallel increase in CA production from cane and beet molasses media. Since that time , a lot of work had beeen done on the efectg of alcohol on citric acid fermentation (Hamissa, 1966; Szczodark and Ilczuk, 1975; Maddox et al., 1986; El-Batal et al., 1995 and Roukas, 1999)Millis et al. (1963) increased CA yield by about 20-50% by the addition of some natural oils with a high content of unsaturated fatty acids, also supplementation of a surface culture with some oil makes a direct increase in CA yield possible in using molasses medium (Adham, 2002).Specific inhibitors such as fluoroacetate and iodoacetattors, are particularly useful (Peter, 1957; Packer and Krimsky, 1952).Addition of some metabolic inhibitor to synthetic medium stimulated citric acid production (Agrwal et al., 1983). Ali and Haq, 2005 discussed the role of different additives and metabolic micro minerals on the enhanced citric acid production by Aspergillus niger using different carbohydrate materials. They found that ethanol and cocnut oil at 3.0 5(v/w) increased citric acid productivity fluoroacetate at a concentration of 1.0 mg/ml bagass enhanced theyield of citric acid significantly. Also, addition of copper sulphate and molybdenum sulphate remarkably enhanced the production of citric using molasses medium.The present study was undertaken mainly to determine the effect of some metabolic inhibitors on citric acid production by Aspergillus niger in cane molasses medium as cheap resources. Materials and Methods Microorganisms and culture conditionsA strains of Aspergillus niger Van Tieghem 595, 599 were kindly provided by the culture collection of Northern Regional Research Laboratory (NRRL), USA. Aspergillus niger A10 , A20 were kindely provided by the center of culture collection National Research Center (NRC), Egypt. Aspergillus niger EMCC III, EMCC103, EMCC 147 were obtained from the Cairo Mercen (CAIM), Egypt.The slants (PDA) of A.niger were incubated at 30 C for 7 days. Inoculum prepared form spore suspension (105-106 spore/ml) in 0.01% v/v tween 80.Cane molassesThe cane molasses samples used in the present study were kindly supplied by the Cane Sugar Factory, Egypt.Citric acid fermentation.Fermination medium were prepared by diluting cane molasses (CM) with tap water to approximately 15 % sugar concentration, preparation of molasses have been reported previously (Mohamed and Adham , 2003). Stationary cultures were grown on cane molasses media containing different concentration of the inhibitors and incubated in slating position (surface fermentation) at 30