4.2. Analysis of minerals

Minerals are usually extracted from food by complete digestion or decomposition of the material. A representative, homogenized food sample is weighed and mineralized by either dry-ashing or wet-digestion procedures (Mir-Marques et al 2016). Dry-ashing is performed at high temperatures (400-550 °C) followed by dissolving the ash in nitric acid or hydrochloric acid. However, losses of some elements like Se and Cl may occur due to volatilisation at this high temperature. Wet-digestion can be performed at lower temperatures (80-120 °C), in the presence of destructive chemicals like nitric acid plus hydrogen peroxide; a microwave oven is frequently used to facilitate the digestion process. The exact digestion protocol for complete decomposition varies between food materials, depending on their composition (e.g. liquid or solid).

After digestion, the levels of minerals are mostly performed by using by atomic spectrometry and mass spectrometry methods, including inductively coupled plasma-mass spectrometry (ICP-MS), inductively coupled plasma optical emission spectroscopy (ICP-OES), graphite furnace atomic absorption spectroscopy (GF-AAS) or flame atomic absorption spectroscopy (F-AAS). Nutrients essential for optimal growth of plants, like K, Mg, Ca, S, Fe and Zn are usually present at relatively high concentrations, while toxic elements as Pb, Cd, Hg and As should be absent, or only present at ultra-trace levels. For detection of both plant macro- and micronutrients, as well as possible toxic elements, either ICP-MS or GF-AAS are mostly used as they allow ultra-sensitive detection of elements at parts-per-billion (ppb) levels. ICP-OES and FAAS only work for the major/minor plant nutrients as they detect at parts-per-million (ppm) levels, thus at 1000-fold higher. ICP-MS is nowadays the most popular technique, as it is both ultra-sensitive and suitable for the simultaneous analysis of series of different elements in a single extract (Hansen et al. 2013 e.g. ).

For total N determination, including protein and non-protein N, the classical but rather insensitive Kjeldahl method is still frequently used. In this method the plant material is digested in sulphuric acid and the liberated N converted into and distilled as ammonia, which is finally quantified by titration.