3. Polyether defoamer
There are many kinds, including:

A. GP defoamer
It is prepared by addition polymerization of propylene oxide or mixture of ethylene oxide and propylene oxide with glycerol as initiator
GP defoamer has poor hydrophilicity and low solubility in foaming medium, so it should be used in thin fermentation broth. Its antifoaming ability is superior to that of defoaming, and it is suitable to be added in the basal medium to inhibit the foaming process of the whole fermentation process.

B. GPE defoamer, namely foam enemy
At the end of the PP chain link of GP defoamer, ethylene oxide is added to form a hydrophilic polyoxyethylene propylene glycerin, also known as polyoxyethylene propylene glycerin. According to the addition amount of ethylene oxide, 10%, 20% 50% is called gpe10, gpe20 GPE50.
GPE defoamer has good hydrophilicity, easy spreading in foaming medium, strong defoaming ability, but also large solubility, and short defoaming activity maintenance time, so it has better effect in viscous fermentation liquid.

C. GPEs defoamer: there is a new type of polyether defoamer. The end of the GPE defoamer chain is capped with hydrophobic stearate to form block copolymers with hydrophobic chains at both ends and hydrophilic chains at intervals. The molecules with this structure tend to accumulate on the gas-liquid interface in a supine state, so the surface activity is strong and the defoaming efficiency is high.

4. Silicon
The most commonly used is polydimethylsiloxane, also known as dimethylsilicone oil. It has low surface energy, low surface tension, low solubility and high activity in water and general oil. Its main chain is a silicon oxygen bond, which is a nonpolar molecule. It is not compatible with polar solvent water and general oil. It has low volatility, chemical inertness, stability and low toxicity. Pure polydimethylsiloxane is difficult to be used as defoamer without dispersing. It may be due to its high interfacial tension with water and low spreading coefficient, which is not easy to disperse on the foaming medium. Therefore, the complex formed by mixing silicone oil with SiO2 aerosol, i.e. the hydrophobic SiO2 aerosol mixed with dimethylsilicone oil, can be prepared after a certain temperature and a certain time treatment.