Non-processed plant food frequently contains relative high levels of phenolic compounds and oxidizing enzymes, including polyphenol oxidases and peroxidases, that upon tissue disruption (cutting, blending, extraction) can initiate diverse chemical and enzymatic reactions. If not prevented sufficiently, these processes may result in rapid and severe losses of phytonutrients including vitamins. For instance, enzyme activities during blending of vegetables may result in vitamin C losses of 60% in tomato fruit up to 95% in broccoli (Lopez-Sanchez et al. 2015). Moreover, many phytochemicals are labile and sensitive to heat, pH or light. The food matrix has strong influence on this potential loss. For instance, vitamin C can be easily lost upon extraction from leafy vegetables (having relative high pH of around 6) while in fruits like tomato and citrus (having pH of around 4) vitamin C is more stable. Likewise, upon thawing of frozen food samples, either on purpose or accidentally, phytonutrients can be rapidly degraded by endogenous plant enzymes due to loss of subcellular compartmentalization. So, both good sample handling practice (see §4.1) as well as applying suitable analysis methods are key in nutrient analyses.
Lopez-Sanchez P, De Vos RCH, Jonker HH, et al. 2015. Comprehensive metabolomics to evaluate the impact of industrial processing on the phytochemical composition of vegetable purees. Food Chemistry 168: 348–355. DOI: 10.1016/j.foodchem.2014.07.076.