What changes force a retest
Updated
A specification, a blend uniformity result and a potency assay all describe one configuration. The useful discipline is knowing what invalidates them.
The rule
If it changes what is in a serving, or how evenly it is distributed, or how long it survives, retest. Everything below is a consequence of that sentence.
Always
A new supplier for any component. Different particle size, moisture content and impurity profile all change blending behavior and potency, and a certificate from a new supplier is not a substitute for qualifying them.
A formula change, including a change in the amount of an existing component.
A process change: different mixer, different blend time, different order of addition.
A batch size change, because blend time and mixing energy do not scale linearly and a larger batch is not simply more of a smaller one.
A packaging change, because for a hygroscopic powder the moisture barrier is part of the stability case.
The ones people think are safe
A flavor change. Flavor systems are several percent of the blend and they alter particle size distribution and flow.
A sweetener change. Usually a very small component with very different properties from the bulk, which makes it a segregation risk out of proportion to its percentage.
A different grade of the same ingredient. Grade frequently means particle size, and particle size is what drives segregation.
A scoop change. A different scoop is a different serving unless recalibrated.
Building it in
Record the configuration each result belongs to: formula version, supplier lots, batch size, process parameters, packaging. Attach the test results to that record rather than filing them separately.
Then make a retest a required step in any change. The failure mode is never a decision to skip testing; it is six small changes over two years, each individually reasonable, after which nothing in the market matches anything that was tested.