Introduction

Iodine is an essential mineral for normal growth, required for thyroid hormones syn- theses that have multiple functions as reg- ulators of cell activity (Nazeri et al., 2017). Iodine deficiency can exert a decisive influ- ence on the health status of a population, par- ticularly children at all stages of their devel- opment (Norouzian 2011).

In many countries, the provision of iodized salt was incredibly effective at eliminating the iodine deficiency, but there is currently a ma- jor global effort to reduce daily salt consump- tion to reduce the risk of health problems that resulted in an increased percentage of subjects with insufficient iodine status. For this reason, considering the other sources of dietary iodine is essential (Nazeri et al., 2017).

Due to the large proportion of iodine pres- ent in dairy products, they have been one of the most important sources  of  nutritional  iodine in several countries. For the UK adult popula- tion, dairy products typically contribute  35% to daily iodine intake (Bates et al., 2016). Also, milk and dairy products are a significant source (about 50%) of food-related iodine intake in Switzerland (Walther et al., 2018).

Analysis of iodine concentrations in dairy products in many countries has shown that the iodine content of dairy products clearly has implications for human iodine intake but there is no comprehensive analysis of the io- dine concentrations in Iranian milk and dairy product and the factors that affect it. There- fore, our research aims were to quantify the iodine concentrations in Iran retail milk, and estimate the contribution of milk to iodine intakes at the current Iran population milk intakes.

Materials and methods

In the first step, 10 commercial dairy farms

(with 100 or more dairy cows; five located in semi-arid area with an average annual rain- fall of 120 ± 42 mm and average annual tem- perature of 33 ± 6.5; tropical region, and five located in a semi-cold area with average an- nual rainfall of 315 ± 35 mm and an average annual temperature of 26 ± 5.5; cold region of the central area of Iran) were included in the study. During the period of study, the median herd size was 235 cows and the me- dian milk yield was 9200 kg/cow per year. Cows were milked three times a day. Nine raw milk samples were taken from each dairy farm bulk tank within a week. Samples were taken at all times of milking and 3 sub- samples were pooled according to milk pro- duction of the sampling time. A total of 90 samples were collected for analysis and after every collection, a 40 ml aliquot of each milk sample was taken and stored at –20 ºC in the same freezer until iodine content analysis.

In the second step, six different milk prod- uct types (different fat classes milk: pas- teurized whole milk and pasteurized semi- skimmed milk; different  processed milk: raw, sterilized and pasteurized milk) were purchased from five leading  supermarkets  in the Tehran province, giving a  total  of  389 samples. In the present study milk was pasteurized by typically high- temperature short time (HTST; 72 ºC for 15 s) method and sterilization was typically done at 140 ºC for 3–5 s. All milk samples were stored at

-20 ºC pending iodine analysis. The iodine concentration in milk samples was analyzed using the Sandell-Kolthoff (acid-digestion) reaction (Hedayati et al., 2007) and results were expressed as micrograms of iodine per liter of milk. Milk samples were carefully homogenized before the alkaline washing procedure.

Data were analyzed using the GLM proce- dure of SAS version 8 (SAS Institute, Cary, NC).  In the case of two groups,  Student’s   t test for independent samples was used.  The significance threshold was set at P-val- ue≤0.05.

Results and Discussion

The results of study 1 showed that the region of milk sampling had a significant influence (P≤0.05) on the iodine concentrations in raw milk (Table 1). The average iodine concentra- tion of milk samples from the cold region was significantly lower (50.7 ± 24.3 %) than the iodine concentration of milk samples from the tropical region. There is limited data on the ef- fect of region sampling on the iodine content of milk. National survey studies on the iodine concentration in milk showed considerable differences between countries, seasons, and production systems, such as conventional and organic farming (Payling et al., 2015; Crnkić

et al., 2015). Plant species and variety, soil of the region, and iodine content of intake water were reported to influence milk iodine content in these regions. In another study, Dahl et al. (2003) reported higher milk iodine content in the South and East regions of Norway. These authors demonstrated that this variation may have been due to the longer period of pas- ture feeding and differences in the access of dairy cows to iodine-fortified fodder. Anoth- er possibility for lower milk iodine content in cold-mountainous regions can be high precip- itation in these regions. All iodine compounds are readily soluble in water and high precipi- tation results in the release of much of their io- dine content into groundwater, which reduces plant access to this trace element. Therefore, plants and animal feed grown in this  area  will contain lower iodine concentration com- pared to another region with low participation (Watts and Mitchell 2009). No information on the iodine concentrations of the diets was sup-

Table 1. the effect of sampling region (cold versus tropical) on raw milk iodine content (µg/l)

plied in this study. Further investigations are required to determine the factors affecting the iodine concentration of milk.

The effect of heat processing (pasteuriza- tion and sterilization) on milk iodine concen- trations are shown in Table 2. Milk iodine concentration was significantly (P<0.05) influenced by heating processing. Sterilized milk (282.0 ± 111.0 µg/l) had higher and raw milk (224.3 ± 116.9 µg/l) had lower iodine concentration. Overall, the iodine concentra-

tion of raw milk was 20.5 ± 4.3 % and 11.8

± 1.7 % lower than sterilized and pasteurized milk, respectively. Also, the iodine  content of pasteurized milk was 9.9 ± 2.6 % lower than sterilized milk. Processing steps in the milk industry, such as heat treatment and skimming, are considered potential causes of iodine loss, but current data are limited and equivocal (Reijden et al., 2017). The iodine losses during pasteurization could also be one reason  for the differences  in the  iodine

concentrations of raw milk, in bulk milk or in milk samples from the food retail sector (Flachowsky et al., 2014). Norouzian et al. (2009, 2011) reported a 27-34% decrease in milk iodine concentration after HTST pro- cessing. In agreement with our results, Naze- ri et al. (2015) showed higher iodine concen- tration in sterilized milk as compared to raw and pasteurized milk. This increase of milk iodine after heating processing is probably due to condensation and reduction in the vol- ume of milk after the high temperature during sterilization (140 ºC) and pasteurization (72

ºC; Nazeri et al., 2015). Similarly, this effect was reported during the production of whey cheese whereby the whey is boiled in order to remove water, resulted in increases in the concentration of iodine in the remaining product and explains the higher iodine con- centration in whey cheese (Dahl et al., 2003). On the other hand, another possible reason for lower raw milk iodine concentration may be due to the variation of milk iodine con- centration in the different dairy farms. Thus, any high iodine concentrations in raw milk from one farm will be diluted by lower con-

Table 2. the effect of heat processing on milk iodine concentration (µg/l)

centrations in milk from other farms result- ing in lower iodine concentration of bulk raw milk (Soriguer et al., 2011; Bath et al., 2012). The effect of milk fat class (whole and semi-skimmed milk) on iodine concentra- tion is shown in Table 3. There was no effect of milk fat type on milk iodine concentration (P>0.05). Small (Soriguer et al.,  2011)  or no (Payling et al., 2015) variations in milk iodine concentration have been reported for different milk classes. In agreement with our findings, Arrizabalaga et al. (2015) reported that iodine concentration between 3 milk va- rieties: whole, semi-skimmed, and skimmed milk, had no significant differences. In the study of Soriguer et al. (2011) iodine con- centration of whole, semi-skimmed and skimmed milk was 251, 254 and 273 µg/l, respectively. No same effect was seen in the

present and another study (Payling et al., 2015). Milk iodine variation due to fat con- tent is very small compared with the large variation in iodine concentration seen in most studies (Payling et al., 2015).

The Iranian consumption of dairy products as milk equivalents is 112.5 kg per capita in 2016. In the present study the contribution  of raw, pasteurized, and sterilized milk  to the actual iodine supply of the Iranian pop- ulation was estimated (Figures 1-3). The contribution of the different groups of milk products was estimated based on of their per capita consumption in 2016.

The resulting estimation showed that daily intake of one and a half cup of raw, pasteur- ized and sterilized milk provides on average, 74.6, 84.6, 96.0 µg of iodine, approximately

29.8,  33.8,  37.6  % of the adult recommend-

ed dietary allowance (WHO 2007) for this nutrient, respectively. Based on calculations of Reijden et al. (2017), milk and dairy con- tribute 13-64% of the recommended daily iodine intake in industrialized countries.

In conclusion, the present study demon-

strated that this concentration of iodine in milk does not pose a public health threat con- cerning element; however, milk and dairy products possibly play an important role as iodine sources in the Iranian population.

Table 3. the effect of milk fat class on milk iodine concentration (µg/l)

Figure 1. Contribution of raw milk to the different groups recommended daily iodine intake (% of need)

Figure 2. Contribution of pasteurized milk to the different groups recommended daily iodine intake (% of need)

 Figure 3. Contribution of sterilized milk to the different groups recommended daily iodine intake (% of need)