A batch culture operation is characterized by no addition to and withdrawal from the culture of biomass, fresh nutrient medium and culture broth (with the exception of gas phase). The batch operation is initiated by addition of a small amount (with respect to sterile nutrient medium) of a cell culture (the so-called "inoculum") to a sterile nutrient medium. The inoculum is derived from serial batch cultures, the so-called starter cultures or subcultures. Some of the culture conditions (such as pH and dissolved oxygen level) during each subculture are usually left uncontrolled. A typical batch culture operation is strictly not a batch operation since it may involve addition of an acid/base for pH control and antifoam to suppress foaming in the culture and withdrawal of small portions of culture for assessing the status of the culture. Any net volume changes due to these additions and withdrawals axe usually minimized by using concentrated acid/base and antifoam solutions and by keeping the number and volume of samples withdrawn within limits. As concerns cell mass accumulation resulting from uptake and utilization of nutrients, the batch culture is characterized by a lag phase (during which period cells in the inoculum adjust to the shock in their environment and accelerate synthesis of enzymes needed to utilize nutrients in the liquid medium), which is followed by an active growth phase (both the cell number and cell mass usually increase exponentially with time in this phase). Cell growth continues in the next phase, albeit at a slower rate since substantial consumption of nutrients has already occurred and such growth is usually referred to as non-exponential growth. Production of some of the metabolites (including a variety of antibiotics and enzymes of commercial importance), whose synthesis is not necessarily directly proportional to cell growth, is accelerated (and in some cases initiated) in this second growth phase. These metabolites are usually referred to as secondary metabolites. The growth phase is followed by a stationary phase upon near complete exhaustion of one or more nutrients essential for cell growth. Synthesis of secondary metabolites is usually promoted in the stationary phase. The stationary phase is followed by the death phase, which is characterized by a significant decline in the cell number density (or the viable cell concentration). A batch operation is usually terminated near the end of the growth phase or during the stationary phase. In industrial bioprocesses, serial batch culture operations are very common. In a typical operation, a portion of the culture from the previous batch is used as inoculum for the next batch. Since there is in a sense recycle of culture from batch to batch, these operations are referred to as repeated batch operations with recycle. If there is no transfer of culture from a batch to the next, the serial operations are referred to as repeated batch operations without recycle and are indistinguishable from a single (once-through) batch culture or parallel batch cultures (conducted simultaneously).
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