D-value (Decimal reduction time)
Japanese: D値（十進法減少時間）
The heating time required to reduce viable microorganisms by 90% at a specific temperature. Used in designing sterilization conditions.
D-value is the time required to reduce viable microorganism count by one log (90%) at a given temperature. If a microbe's D₁₂₁ is 2 minutes, heating at 121°C for 2 minutes reduces the count to 1/10.
D-value changes with temperature—higher temperatures yield smaller D-values (faster kill) and lower temperatures yield larger ones. Sterilization conditions for canned and retort foods are designed based on target microorganisms' D-values.
In fermented foods, D-value concepts inform pasteurization temperature and time settings. Not all microbes need to be killed—food hygiene sets 'safe sterilization conditions' based on D-values of spoilage and pathogenic organisms. It is essential for food safety design. Where fermentation meets science, new discoveries await.