*2.2.1.1 Factors and responses*

The FFR, spray gas flow rate, and outlet temperature were defined, in this study, as the potentially critical process parameters, also known as factors in this study. The factors and investigated level ranges are listed in **Table 1.** The levels selected represent the experimental space of the DoE. The selection was based on previous in-house trials and technical restrictions.

The influence of the factors and their interactions resulted in varying powder qualities and characteristics. These effects were measured via the selected output responses: powder output, reconstitution performance, *h*EGF/powder, and total protein/powder to evaluate in this study; see **Table 2**.


**Table 1.**

*Input factors selected in the DoE custom design used for this case study.*

*Excipient-Free Spray Drying of Bioactive Recombinant Proteins Produced in Plants DOI: http://dx.doi.org/10.5772/intechopen.112944*


#### **Table 2.**

*Output responses selected in the DoE custom design used for this case study.*
