What goes wrong

Off flavors

Updated October 2026

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Bitter Yeasty or fizzy Cardboard or metallic Cooked or scorched Taste against a reference, not against memory Smell the milk before you blame the process References

Dairy picks up flavors from everything around it, and four named off notes cover most complaints, each with a different cause.

Bitter

Usually protein breakdown, from a culture that ran too long or from organisms that were not meant to be there. In an aged product it can be part of the character; in a fresh one it is a signal that the fermentation went past its endpoint or that something else grew.

Yeasty or fizzy

Yeast, which belongs in a kefir and does not belong in a yogurt. In a product that should not have it, this says contamination, usually from equipment or from an environment where another fermentation is running nearby. It is one of the strongest arguments for separating kefir production from everything else.

Cardboard or metallic

Oxidation, and in dairy it is frequently light induced. A product in a clear container under shop lighting develops it within days, which is the reason a dairy container is so often opaque and the reason 64 of the containers on this shelf are light protective.

Cooked or scorched

From the heat step, either too hot or held too long, or from product sitting against a hot surface. It is more noticeable in a plain product than a flavored one, which means a flavored line can hide a process drift that a plain line would have shown you.

Taste against a reference, not against memory

Keep a unit from each batch and taste the new one alongside the last good one. Slow drift is invisible batch to batch and obvious against something three months old.

Smell the milk before you blame the process

An off note that was in the raw material survives everything you do to it, and in a plain product it is the whole flavor. Checking incoming milk by smell takes seconds and is the only point at which rejecting it is still an option.

References

21 CFR 131.200, Yogurt: not less than 3.25 percent milkfat and not less than 8.25 percent milk solids not fat, with “a pH of 4.6 or lower measured on the finished product within 24 hours after filling.” law.cornell.edu

21 CFR 1240.61, Mandatory pasteurization for all milk and milk products in final package form intended for direct human consumption, which prohibits delivering such a product into interstate commerce unless it has been pasteurized or is made from pasteurized dairy ingredients. law.cornell.edu

Written from federal regulation and FDA guidance, not from a laboratory. Pasteurization of milk and milk products in final package form for interstate commerce is a federal requirement, and what your state permits inside its own borders is a separate question. Anything here is a starting point for a conversation with your state department of agriculture, not a determination that you comply.