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 at the same facility. Chemical analyses showed the presence of theobromine, which is found in almost no other plant besides cocoa, No extraneous chemical contaminants were found. Also, the color (a specific shade of brown) 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 containers before getting their sub-sample).

It this happens in regular paperboard gable top containers, it would be much less noticeable than in clear plastic jugs.

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

Small portion in test tube spun down in a centrifuge

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Here are 2 references discussing the physical stability problem of cocoa particles during chocolate milk production. The larger particles don’t completely dissolve, but disperse and form a suspension that settle over time.

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Photos of another incident from the same plant, showing how the material was isolated. More material was present this time.

Letting the material settle in a corner before collecting with 11 mL wide-mouth microbiology pipets

After spinning down the two 15 mL glass centrifuge tubes

Drawing off most of the supernatant milk with a long-tipped Pasteur pipet, without disturbing the brown plug

In this case the remaining fluid could be drained off while the plug stays in place