Brown sediment in milk

I encountered two instances over the years where plastic bottles/jugs of milk had a brown sediment settled at the bottom. It turned out to be cocoa particles, presumably from carryover of chocolate milk production. Chemical analyses showed the presence of theobromine, with no extraneous chemical contaminants. Also, the color was a dead giveaway. Isolation of the unknown substance was hampered because the material got resuspended during shipment to the lab, and by Microbiology testing prior to Chemistry testing (their protocol requires thoroughly shaking the milk before sampling).

Photos of bottom of the milk jug sticking out from a shelf

After spinning down with a benchtop centrifuge in 15 mL glass test tube

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Here are 2 references discussing the physical stability problem of cocoa particles during chocolate milk production

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