Iodine content of semi-skimmed milk available in the Netherlands depending on farming (organic versus conventional) and heat treatment (pasteurized versus UHT) and implications for the consumer

Purpose: To study the iodine concentration in semi-skimmed cow's milk available on the Dutch market. Methods: The iodine content of 16 milk brands was determined at four times (June, September, December 2016 and March 2017) using inductively coupled plasma mass spectrometry. Brands included 12*pasteurized (n= 48 samples) and 4*UHT (n= 16 samples) milk, the brands subdivided into 8* organic and 8*conventional milk (each 32 samples). Results: The median iodine concentration was 15.9 mu g/100 g. The iodine concentration increased from June 2016 to March 2017 from 12.4 mu g/100 g to 19.9 mu g/100 g. The time trend with difference between summer and winter milk iodine content was stronger in organic than conventional producing farms indicating more uncertainties to supply iodized minerals by pasture feeding. Despite reflection of the time trend in the investigated kinds of milk, the distributions of iodine concentrations were similar between organic and conventional produced milk and between pasteurized and UHT milk. Conclusions: Milk was shown as a valuable iodine source for the Dutch consumer. The significant time trend in the iodine concentration in favor of the milk from a stable feeding period (Dec and March) compared with a pasture feeding period (June and September) is a challenge to keep all year round the cow's iodine supply level constant, mainly in organic producing farms.