Zinc deficiency is a global public health issue with about 17 % of the world's population estimated to be at risk of zinc deficiency mainly due to zinc deficient foods. Improving zinc intake through diet is a possible solution, and zinc-enriched yeast (ZnYeast) as a biological source, with zinc integrated into its own structures, is a promising option that can be developed as a supplement or functional food, much like the already established selenium yeast.
However, the safety of ZnYeast for nutritional purposes has not yet been confirmed by the EFSA. For this pending safety assessment, more comprehensive data regarding the zinc uptake and accumulation in yeasts, and, most importantly, the chemical speciation and bioavailability of the zinc compounds present in these zinc-enriched yeasts is essential.
Two industrial yeast strains of the Saccharomyces pastorianus and S. cerevisiae species were selected as the most promising strains for zinc enrichment [1]. Those yeasts were supplemented with 10 mM of ZnSO4 during micro-scale fermentation. The zinc content was quantified by ICP-MS which showed a significant increase in zinc accumulation compared to control yeasts.
The localisation of the zinc enrichment was imaged by Nanoscale secondary ion mass spectrometry (NanoSIMS). This technique allowed the visualisation of the subcellular distribution of zinc and other essential and trace elements in the yeast cells with a spatial resolution ≥ 100 nm, and zinc was found to be concentrated in spots, mainly attached to the cell walls. Afterwards, X-ray Absorption Near Edge spectroscopy (XANES) analyses were carried out to evaluate zinc speciation in fresh and freeze-dried ZnYeast samples, which proved the transformation of the ZnSO4 into both inorganic and organic forms of zinc during fermentation. These findings are fundamental to understand zinc metabolization in yeast and the gastrointestinal bioaccessibility and bioavailability of the resulting zinc species, in order to assess the suitability of ZnYeast to be implemented as a commercial dietary zinc supplement.