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?