1. Raw materials

Currently, the raw materials actually used for citric acid fermentation production can be divided into two main categories: sugar raw materials and petroleum alkane raw materials. Sugar raw materials include sweet potatoes, dried sweet potatoes, cassava, potato chips, potato residue, etc.; grains such as corn, wheat, flour, and rice; various starches, starch hydrolysates (monosaccharides, disaccharides, dextrin, maltose, glucose mother liquor), molasses, and various sugar-containing fruits and by-products of sugar food processing, etc. Petroleum alkane raw materials include n-alkanes, acetic acid, ethanol, etc. In industrial production, the selection of raw materials needs to consider many comprehensive factors. In addition to low price, high acid production, stable production, and technical feasibility, it is also necessary to consider factors such as wide availability, easy purchase, convenient storage and transportation, high comprehensive utilization value, production safety, and low waste.

2. Pretreatment of raw materials

Raw materials containing starch or sugar often need to be pretreated before fermentation, such as removing impurities, removing mold, crushing, liquefying, and removing metal ions. (1) Liquefaction of starch raw materials. Although the citric acid producing bacteria Aspergillus niger can produce and secrete amylase and saccharifying enzyme, in order to shorten the fermentation time, various starches, as well as dried sweet potatoes, cassava, corn flour, etc., need to be liquefied in advance. At present, citric acid fermentation production all adopts enzymatic liquefaction. α-amylase is directly added to the feed solution and acted on for a certain period of time at a certain temperature (80~105℃, depending on the heat resistance of the enzyme). Usually, the endpoint of liquefaction is controlled when the feed solution no longer turns blue when reacting with iodine. For citric acid producing bacteria that use starch as raw material, the DE value of the liquefied liquid should be around 20%. (2) Pretreatment of molasses raw material. Citric acid production abroad mainly uses molasses as raw material. In addition to containing abundant sugar, molasses also contains a large number of metal ions and growth factors such as thiamine and riboflavin. Among them, metal ions will significantly affect the ability of Aspergillus niger to produce citric acid, so pretreatment is required. There are many pretreatment methods, the most common being the addition of potassium ferrocyanide (K4(CN)6Fe) or treatment with sulfuric acid or phosphoric acid. Activated carbon, EDTA, tannins, lime, and ion exchange resins can also be added, and several methods can be used alternately. Potassium ferrocyanide can be added once or multiple times, the amount depending on the metal ions and total nitrogen in the molasses, generally between 0.06% and 0.3%. A small amount of potassium ferrocyanide is added before inoculation after sterilization, followed by additional additions in stages based on cell growth and acid production.

3. Nutritional Requirements for Citric Acid Fermentation

There are two nutritional requirements during fermentation: firstly, to provide the nutrients needed for the growth and reproduction of Aspergillus niger and the energy to maintain its life activities in the initial stage of fermentation; and secondly, to serve as a substrate for the biosynthesis of citric acid. The former requires not only carbon (energy) sources but also a certain amount of nitrogen and other growth factors. Since excessive nitrogen is detrimental to citric acid production, controlling the nitrogen content in the citric acid fermentation medium is crucial. Currently, the main raw materials for citric acid fermentation media used in China are various starch-containing agricultural by-products. These products are rich in carbohydrates (the main component) and also contain certain nitrogen sources such as proteins and trace elements. However, since the protein content of various agricultural by-products varies, it is necessary to appropriately adjust the ammonia content. Table 13-1 lists the nitrogen adjustment measures for domestic citric acid fermentation raw materials. Besides the organic ammonia contained in the raw materials, physiologically acidic substances that lower the pH of the fermentation medium as nitrogen is consumed should be used as nitrogen sources. Ammonium salts (ammonium sulfate, ammonium chloride, urea, etc.) are the best, while nitrates are not suitable.