Validation of D value by different SCD culture medium manufacturer and/or different SCD culture medium constituent.

Variation in decimal reduction (D) time/value for attaining sterility assurance was reported in terms of difference between lot to lot and/or manufacturer to manufacturer of soybean casein digest (SCD) culture medium. The D value variation phenomenon itself due to SCD culture medium was reported by Graham et al; however, it was not sufficiently clarified what constituent(s) of SCD culture medium would mainly contribute to the variation of D value. If it is not well clarified, the reproducible sterility assurance cannot be successfully attained. The authors studied individually the constituent of SCD broth (SCDB), SCD agar (SCDA) and SCD broth plus 1.5% agar (SCDB/A). As a result, the culture medium constituent causing D value variation among SCD culture medium was determined as the difference of calcium (Ca) amount as well as the presence and the absence of K2HPO4 in SCD culture medium. Additionally, it was clarified D value differed significantly between SCDB and SCDA. D value using the former was around 1 min lower than that using the latter. This phenomenon can be explained from the formation of insoluble CaHPO4 in SCDB culture medium, resulting in insufficiency of usable Ca to B. stearothermophilus in SCDB culture medium.