How standardized is lab-grade skim milk?

Question

I was surprised to learn in the answers to this question that in many cases the preferred cryoprotectant for many organisms is skim milk, rather than something more simple and well-defined like glycerol. And indeed, Sigma will happily sell you lab-grade dried non-fat cow's milk for lab-grade prices.
It seems to me that this could potentially cause significant variability and reproducibility issues. Milk is pretty sensitive to the condition of the producing animal, and when dealing with a cow we're talking about a very large and complex source that's not hyper-standardized like lab-grade rodents.
I don't seem to be able to find anything much about specification on Sigma's site, however (the spec sheet for milk has only the obvious like "opaque white fluid").
Does anybody know to what degree lab-grade milk is actually standardized, and if this should be a concern for protocols like cryoprotection?

Catherine · Answer

There is a degree of variability in milk, and in some cases this cause problems in protein work. Despite being a standard for ELISAs, it's not known to be the most reliable blocking buffer. As far as cryoprotection goes, variability doesn't seem to be a major concern.
Standardization varies depending on manufacturer. The Sigma link you included reports a quality level of MQ 200. The MQ level is something Sigma created in an attempt to increase clarity about quality controls. Products rated MQ 200 are for non-regulated applications. For this level they will provide a site-quality self-assessment, a certificate of quality/analysis, and specifications. They also have a notification system that customers can opt-in to for updates on quality control features. There's information on what each quality level meansand a breakdown of what they will notify you about on the Sigma website.

How standardized is lab-grade skim milk?

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