INTRODUCTION

Adequate dietary iodine intake is essential for the production of thyroid hormones. Milk has become a major source of dietary iodine for humans. Iodine content of milk has been investigated extensively (Alderman and Suanks, 1967; Berg et al., 1988; Bruhn and Franke, 1985; Conrad and Hemken, 1978; Hemken et al., 1972) but relatively little definitive information on the effect of heat proceessing on iodine content in milk is available (Magee and Glennie, 1928). Therefore, this study was conducted for determination of the effect of heating during pasteurization process of milk on iodine concentration.

MATERIALS AND METHODS

This research is a part of study that was conducted in Ferowsi University of Mashhad, Iran for survay of relationship between of iodine content in diet and milk. In this study 16 Holstein cows, with an average daily milk yield of 32.9±2.4 kg and 189±27 days in milk, were assigned to 4 treatments in a completely randomized design. The experimental diets were the control, the control diet plus 2.5, 5 and 7.5 mg Potassium Iodide kg^-1 diet DM. The experiment lasted after 8 weeks. Diets were formulated accordance to NRC 2001 recommendations.

Table 1: Treatments and iodine content of experimental diets *Iodine Supplement was Potassium Iodide; **The iodine content of each diet was including iodine Diet, Water and Iodine supplementation

The content of iodine in basal diet was 0.534 mg kg^-1 DM and other treatments included basal diet plus different levels of iodine supplementation (Table 1). The iodine content of raw and pasteurized milk was determined. In the end of any week, 10 mL of samples saved in 4°C until send to laboratory. Milk pasteurized by HTST (High Temperature, Short Time) method. Iodine content determined after acid digestion with use of Sandell-Kolthoff reactions (Endocrine Research Center, Shaheed Beheshti University of Medical Sciences, Tehran, Iran). Data were statistically analyzed by repeated measurement procedure with mixed model of SAS software.

RESULTS AND DISCUSSION

Milk iodine affect by treatment significantly (p<0.01) and increased (Table 2). Milk is an important way for excretion of additive iodine in body and increase of iodine intake cause to increase in excretion via milk.

Table 2: Least square mean of iodine concentration raw and pasteurized milk

Results of 17 studies with radioisotope iodine were shown that the excretion ratio of iodine via milk is 8-12% of iodine intake (Swanson et al., 1990). Decrease of iodine content in all treatments after pasteurization were significant (p<0.01). Average decrease of iodine after pasteurization was 27.15±7.13% (Table 2 and Fig. 1).

Magee and Glennie (1928) found 20 and 26% decrease of iodine content of milk in pasteurization and boil point, respectively.

The reason of decrease of iodine concentration after heat processing is sublimation characteristic of iodine element. Iodine in milk includes tow forms, Protein Binding Iodine (PBI) and free. Magee and Glennie (1928) reported that free form of iodine in milk is 83%. High amount of iodine in free form in milk cause to decrease of iodine content after heating.