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Significance of Thermal Isomerisation on the Quantitation of Total Vitamin D3 in Foods

Gill, Brendon D., Gilliland, Donald L., Indyk, Harvey E., Wood, Jackie E., Woollard, David C.
Food analytical methods 2019 v.12 no.4 pp. 998-1006
ambient temperature, analytical methods, cholecalciferol, ergocalciferol, food matrix, foods, isomerization, isotope labeling, saponification, storage temperature
Sample preparation techniques for the analysis of vitamin D₃ in food matrices typically utilise a saponification step, either at room temperature or at elevated temperatures. A calciferol (vitamin D₂ or isotope labelled vitamin D₃) is generally chosen as the internal standard to compensate for changes of previtamin D₃–vitamin D₃ isomerisation during analysis, as well as to correct for analyte loss through complex sample preparation steps. Manufacturing practices and processing parameters contribute to previtamin D formation in food products. A significant proportion (5.6–8.3%) of the total vitamin D₃ in premixes was found as previtamin D₃, indicating that it is likely, depending upon storage temperature and the time since manufacture, that a vitamin D₃-fortified food product will contain a similar proportion of previtamin D₃ prior to analysis. Conversely, freshly prepared internal standard solutions have low previtamin D levels (< 1%). In lieu of direct measurement, this discrepancy in previtamin D content between the internal standard and analyte forms of vitamin D will lead to analytical bias. To mitigate this as a source of potential error, it is recommended that sample pretreatment steps are appropriately set and controlled. Based on this work, saponification times greater than 300, 120, or 60 min for temperatures of 60, 70, or 80 °C respectively should be employed and that saponification at room temperature be avoided.