Summary Losses of vitamin B12 during the processing and/or storage of foods and supplements have been associated with both reducing and oxidising agents. We evaluated the effects of six redox active (Cu2+, Fe2+, Fe3+, Mn2+, Ce3+, and Cr6+) and one redox inactive (Zn2+) metal ions on cyanocobalamin stability in heat‐treated, milk protein‐based matrices. The metal ion could either decrease (by ≤76%) or increase (by ≤56%) cyanocobalamin loss, depending on metal ion concentration and redox potential, as well as on matrix properties (metal ion binding, redox activity, and reactive oxygen species generation) and on protein properties (cysteine content, conformation, and charge density). The largest losses of cyanocobalamin appeared to be caused by redox cycling mechanisms (for Cu2+ and Fe2+); however, indications were that such losses could be mitigated by metal ion modification of the matrix. Only the redox inactive metal ion Zn2+ inhibited cyanocobalamin loss in all experimentally evaluated matrices.

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