 In this module, we will talk about the role of oxygen delivery system in aseptic operation of the fermentation process, how we can maintain the aseptic operation. If we assure that the air entered into the fermentation vessel that is properly sterilized when that exit from the fermentation vessel that is properly free from those organisms which we cultured inside the fermentation vessel. So, during the sterilization, the concept of maintaining the positive pressure. So, if all the time during the fermentation process, we keep the pressure positive inside the fermentation vessel. Then the maintain the positive pressure means that during the sterilization, cooling and the filling and if appropriate the fermentation proper air must be pumped into the reactor. So, in the time of the sterilization, in the time of the addition of the inoculum or when we run the fed batch fermentation, then we have to add something inside the fermenter. So, in all these cases, when there is a positive pressure inside, we cannot just off the system of the oxygen delivery system. We have to keep the on and in a continuous supply of the air just to make the positive pressure. Why we will talk about in later slide, but in this way the reactor is always pressurized and thus the aerial contaminant will not be sucked into the reactor. Because when there is a negative pressure then there is a chance of the contaminant to be sucked inside the fermenter. So, it is very important that positive pressure is maintained when the bio reactor is cooled following sterilization. So, when there is a strain then there is expansion and when there is a cooling then there is a contraction. So, while expanding and construction then there is a suction of the material. So, that is why there is a chance of contamination. So, due to the vacuum, small kind of the vacuum that produce sometimes there is a different gadgets which are having attachment to the surface plate and then they can have some leakage then there is a chance of that. So, that is why if there is a positive pressure then there is a no risk of such inward entry. In this slide you can see that in case of the negative pressure inside when there is no aeration. So, you can see here there is no aeration, no oxygen supply or any other gas supply. So, what happened then there is a negative pressure inside when there is a negative pressure. Contaminants can sucked into that. So, in other case when the PI inside pressure is more than the pressure outside then what happened then that keep away those contaminants from the fermentation vessel. So, that is why by this diagram you can easily understand that what is the role of positive pressure inside the fermentation vessel. So, we should always keep the positive pressure, but very careful by having a positive pressure up to a certain limit that can withstand the pressure that can withstand in the fermentation vessel and mostly critical when the lab scale fermenters when there is a glass vessel. So, by maintaining the positive pressure at all the stages of the fermentation setup and operation is most important aspect just to reduce the risk in the fermentation vessel of a contaminants. So, that is the major role of the positive pressure and how we can control the aseptic operation by oxygen delivery system.