Methods for assessing process performance should be identical between scales. These include sample-dilution schemes and measurement
times for calculating culture optical densities, wet and dry cell-weights, media metabolite levels, and protein expression.
For example, if spectrophotometers are used to make culture optical-density (OD) measurements, use the same model in the scaled-down
process. If the same model is not available, use a spectrophotometer with the same path length and instrument precision (relative
to one used in manufacturing). To verify, use several solution standards of known optical density.
At the small scale, minimize reactor sampling volumes and times as much as possible. Since each sample that is taken in the
scale-down process represents a larger percentage of the total process volume, the percentage of loss at the small-scale must
be minimized to prevent depletion of culture broth beyond acceptable levels. If the sample size cannot be reduced, then adjust
the frequency of sampling. For example, optical-density or wet cell-weight measurements performed every two hr at the manufacturing
scale could be done every ten hr in the scale-down process, as long as the sampling does not occur at a crucial time such
as at the start or finish time of feed delivery.
Maintenance of equivalent oxygen transfer (k
α) and control of dissolved oxygen across process scales is the most important requirement for most fermentation scale-down
strategies (see Table 1 for equations). However, the efficiency of oxygen transfer and control in a production-scale fermentor
(10,000 to 100,000 L) is often significantly lower than in laboratory-scale (1 to 100 L). The overall oxygen-control strategy
— including sparger design, calibration, and placement within the reactor — should be identical to the large-scale process.
If the sparger design is different between scales, then agitation, aeration, and oxygen enrichment may need to be adjusted
to provide equivalent oxygen transfer in the scaled-down process.
Table 1. Scale-down Equations for a Typical Fermentation Process
For inoculum development and expansion, conserve the vessel geometries, incubation conditions, and working volumes at each
step whenever possible. Therefore, if 2-L baffled shake-flasks are used for seed expansion in the full-scale process, use
the same volume and flask type in the scale-down process. All process control setpoints and ranges (temperature, shaker speed,
stroke length, pH, and dissolved oxygen) should be the same. If the same equipment is not available, then vessels with different
volumes but similar geometries may be used. However, the operational control parameters may need to be adjusted to account
for different vessel geometries.
OPERATIONAL SCALE-DOWN GUIDELINES
Set up and sterilize all process vessels and tanks according to the current manufacturing procedure for the process. Sterilization
temperatures, procedures for probe and flowmeter calibration, and post-use cleaning protocols should be the same as the large-scale
process. If extended (or multiple) sterilization cycles are necessary for any piece of equipment, assess the impact on process
performance and individual medium component stability.