Method for producing citric acid by degrading roughages with the rumen fungus-methanogen co-culture from qinghai yaks

A method for producing citric acid by degrading roughage with YakOH5, wherein YakOH5 is a natural co-culture isolated from the rumen of Qinghai yaks that comprises N. frontalis and M. gottschalkii, and the method comprises the following steps:

• S1) Preparation of the YakQH5 inoculum by:
• inoculating the YakQH5 into an anaerobic medium with wheat straw as substrate at an inoculation amount of 10% v/v, adding antibiotics and performing anaerobic culture to obtain high-activity inoculum;
• and
• S2) Fermentation and degradation of roughage to produce citric acid in a vessel by:
• collecting the inoculum prepared in step S1, inoculating it into an anaerobic medium with 1% w/v roughage as substrate according to an inoculation amount of 10% v/v, adding 1% v/v compound antibiotics at the same time, fermenting this inoculated anaerobic medium to degrade the roughage until citric acid is produced, and isolating the citric acid;
• in the step S1, the temperature of anaerobic culture is 39° C. for 72 hours;
• in step S2, the temperature of anaerobic culture is 39° C., for 5 days;
• the antibiotics are penicillin sodium and streptomycin sulfate, and the concentrations in the anaerobic medium are 1600 IU/mL and 2000 IU/mL respectively;
• the formula of the anaerobic medium is as follows:
• 1.0 g yeast extract,
• 1.0 g peptone,
• 7.0 g NaHCO3,
• 1 ml of 1.0 g/L reazurin,
• 1.7 g L-cysteine hydrochloride,
• 170 mL of rumen fluid supernatant obtained by centrifuging rumen fluid collected from the Qinghai yaks before morning feeding at 8000×g centrifugation at 4° C. for 20 min, then isolating the supernatant,
• 82.5 mL salt solution I,
• 16.5 mL salt solution II, and
• 1000 ml distilled water;
• wherein the formula of the salt solution I is as follows:
• 6 g NaCl;
• 3 g (NH4)2SO4,
• 3 g KH2PO4,
• 0.4 g CaCl2·2H2O,
• 0.6 g MgSO4·2H2O, and
• distilled water to make up to 1000 mL;
• and
• wherein the formula of the salt solution II is as follows:
• 4 g K2HPO4,
• And distilled water to make up to 1000 ml;
• S2 comprises deoxygenating the vessel after adding the substrates by filling it with carbon dioxide, and sterilizing it at high temperatures and high pressure;
• the substrates added in step S2 are selected from the group consisting of wheat straw, flax straw, potato straw, cotton straw, soybean straw, sainfoin straw, reed straw, sisal, bran, wheat husk, filter paper, tomato residue, and alfalfa.