1.1.Reducing production capacity
In the fermentation tank, the loading capacity is reduced in order to contain the foam and prevent overflow. Coefficient of loading: ratio of loading capacity to volume. The coefficient of loading for fermentation tank is generally 0.7.
1.2.Leading to waste of raw materials
If the volume of the equipment can not leave room for the foam, the bubbles will cause the loss of raw materials, so it causes the waste.
1.3.Affecting the respiration of bacteria
If the bubble is stable and not broken, then as the microorganisms breathe, the bubble is filled with carbon dioxide, and it cannot exchange with the oxygen in the air, which affects the respiration of the bacteria.
1.4.Leading infection of bacteria
As the foam increases and it causes the liquid to overflows, then the cultivation medium is stuck in the exhaust pipe, which lead the bacteria grows. As time goes on, the infectious microbe will enter into the fermentation tank and cause infection of bacteria. A large amount of foam further permeates into the shaft seal from the top of the tank, and the lubricating oil at the shaft seal can play a little role of defoaming, and the foam falling from the shaft seal often causes contamination of infectious microbe.
1.5.Affecting fermentation products
Some fermentation products, especially proteases, the fermentation foams may inactivate these proteases.
(1) The intensity degree of ventilation stirring: the stronger the ventilation stirring in the fermentation process, the easier it is to produce foam.
(2) Composition of cultivation medium ratio and raw material : When the cultivation medium ratio and raw material composition are richer, the content of organic nitrogen source is higher , and the stability of generated foam is higher.
(3) Growth state of bacterial: When the bacterial is growing vigorously, the protein concentration of the fermentation liquid will increase, and the concentration will be enhanced, which will ultimately leading to the generation of bubbles.
(4) Cracking of bacterial in fermentation tank : When bacterial experiences the cracking in fermentation tank , bacterial protein is massively released, it will lead to rapid foaming process for a short time in the cultivation medium.
(5) Environmental factors: The fermentation process is affected by the environment, so that the temperature rises rapidly during fermentation.Moreover,the continuous agitation is conducted during the fermentation process, and it leads to the gradual increase of bubbles.
(6) Composition of fermentation liquid : When the viscosity and concentration of fermentation liquid is rather large, more foam will be produced. In order to control the production of fermentation foam, various methods can be adopted, such as adjusting the intensity of ventilation stirring, optimizing the composition of medium and raw materials, controlling the growth of bacteria, avoiding the cracking of bacteria in the fermentation tank, controlling environmental factors and adjusting the composition of fermentation liquid.
3.1.Mechanical elimination
The use of strong mechanical vibration or through pressure changes to promote foam breaking is a way to use physical action to eliminate foam. In the mechanical defoaming method, the simplest way is to install the defoaming paddle on the upper layer of the stirring shaft, and use external force to break the foam to achieve the purpose of defoaming. The advantage is that there is no need to add other substances to the fermentation liquid, which saves raw materials and reduces the risk of contamination of miscellaneous bacteria. However, the defoaming effect is relatively poor, and the factors causing foam stability can not be fundamentally eliminated, so it can only be used as an auxiliary method for defoaming.
3.2.Elimination of defoamer
The defoamer can cause the foam to burst. Because there are many factors which lead to the formation of foam, there are two kinds defoaming mechanism of defoamer as follows:
(1) When there is the polarity surface active substance on the surface of the foam to form the double electric layer, another surfactant with opposite charge can be added to reduce its mechanical strength, or some substances with strong polarity can be added to compete with the foaming agent for the space of the liquid film, and reduce the mechanical strength of the liquid film, and then promote the foam to burst.
(2) When the liquid film of the foam has the large surface viscosity, some substances with small molecular cohesion can be added to reduce the surface viscosity of the liquid film, thereby it will promote the liquid loss of the liquid film and cause the foam to burst.
The good defoamer can have the above two properties at the same time, that is, it can simultaneously reduce the mechanical strength of the liquid film and reduce the surface viscosity of the liquid film. In addition, in order to make the defoamer easily dispersed on the foam surface, the defoamer should have characteristics such as small surface tension and water solubility, but when the defoamer is used improperly, it will interfere with the metabolism of microorganisms and affect biosynthesis.
4.1.Types of defoamer
(1) Natural oils and fats: such as corn oil, soybean oil, rice bran oil, cottonseed oil, rapeseed oil and lard;
(2) Polyethers: such as polypropylene glycerin, which is referred to as GP type, polyoxyethylene oxypropylene glycerin, which is referred to as CPE, also known as bubble enemy;
(3) Higher alcohols (octadecanol and polyglycol), fatty acids and esters;
(4) Silicone: such as polydimethylsiloxane and its derivatives, it is most commonly used in microbial drug fermentation.
In the past, vegetable oil was used for defoaming, but at present, silicone is chosen for defoaming, and there are also silicone and polyethers mixed with each other, the effect is good. One or several defoamers can be selected and used together, and the toxicity test is carried out. For example, Bacillus subtilis is commonly used to emulsify silicone oil for defoaming; The fermentation of saccharomyces cerevisiae can be combined with bubble enemy alone, or soybean oil and bubble enemy can be mixed in a certain proportion for defoaming. The effect of different kinds of defoamers on fermentation of different strains is different, so it is recommended to do more experiments.
4.2.Basis for selecting defoamer
(1) Fermentation type: Different fermentation types have different requirements for defoamer. For example, beer fermentation needs to choose the fast foaming defoamer to quickly eliminate the foam produced in the fermentation process; In order to avoid foam interference in antibiotic production, antibiotic fermentation requires the selection of low foam and high efficiency defoamer.
(2) Properties of fermentation liquid: Properties of fermentation liquid include liquid depth, viscosity, pH, etc. These properties affect the effectiveness of the defoamer. For example, the acidic fermentation solution needs to choose the defoamer that is resistant to acid and alkali to ensure that the defoamer can still play the good role of defoamer in the acidic environment. In addition, for the fermentation liquid with high viscosity, the defoamer with strong adsorption should be selected to better adhere on the surface of the fermentation liquid and reduce the surface tension of the liquid.
(3) Temperature and pressure: Temperature and pressure can also affect the effectiveness of the defoamer. In high temperature and high pressure environments, it is necessary to choose the defoamer which can resist the high temperature and high pressure to ensure that the defoamer can still maintain good defoamer performance under high temperature and high pressure.
(4) Performance of defoamer: The selection of the appropriate defoamer needs to take the foamability, adsorbability, durability and other factors of the defoamer into account. Foamability indicates the ability of the defoamer to produce foam in the solution, adsorbability indicates the ability of the defoamer to adhere to the surface of the solution, and durability indicates the time that the defoamer continues to play an effect in the solution. According to the actual needs, the defoamer with appropriate foaming and adhere properties is selected to ensure that the defoamer can play the role of defoaming in the appropriate time and it will play the role for time of duration.
(5) Economical efficiency: Economical efficiency is also one of the factors to be considered when selecting defoamer. In the selection of defoamer, it is necessary to choose the defoamer with appropriate price according to their own economic situation. Under the premise of ensuring the defoaming effect, the cost-effective defoaming agent is selected to reduce the production cost of industrial fermentation.
In short, the selection of defoamer in industrial fermentation needs to consider fermentation type, properties of fermentation liquid , temperature and pressure, performance of defoamer, economical efficiency,etc . The stability, efficiency and product quality of fermentation process can be effectively improved by the correct selection of appropriate defoamer. In the actual operation process, the above factors can be balanced and selected according based on the actual situation to obtain the best defoaming effect.