Rapid method for determination of milk casein content by infrared analysis.

Preliminary work was to develop a method for determination of milk casein content using commercially available infrared milk testing equipment. Fresh whole milk samples were split into two portions. One portion was tested for total protein with an infrared milk analyzer. The other portion was adjusted to pH 4.6 with a specific volume and concentration of phosphoric acid. Milk casein precipitated at pH 4.6 and was removed by filtration. Next, the noncasein protein filtrate was tested for protein content using an infrared milk analyzer. The difference between the percent protein of the original milk sample and the percent protein of the noncasein protein filtrate equals the percent milk casein. The infrared casein determination for 36 different individual herd milk samples was not significantly different from results obtained by the official International Dairy Federation casein testing method. The infrared method is faster than the chemical method and could be done with infrared milk testing equipment that is commonly available in many cheese manufacturing plants. Repeatability of the infrared method was comparable to the repeatability of the chemical method.

Variability in true protein, casein, nonprotein nitrogen, and proteolysis in high and low somatic cell milks.

Monthly variation in milk protein (total nitrogen X 6.38) and milk fat was determined for 115 farms over 2 yr. Yearly average milk protein and fat tests in each year were 3.16 and 3.62%, respectively. The mean regression coefficient for milk protein with respect to milk fat was .47 for the entire period. Twenty-four farms were selected and grouped high or low based on their previous 2-yr somatic cell history. Monthly milk samples for each farm were tested for direct microscopic somatic cell count, total nitrogen, noncasein nitrogen, and nonprotein nitrogen. No differences in monthly nonprotein nitrogen, true protein, and casein were found between groups. Casein as a percent of total nitrogen was significantly higher for the low somatic group for seven of the 12 mo studied but was significantly higher for 9 mo when expressed as a percent of true protein. The average increase in tyrosine value for incubated preserved milk was significantly higher for the high somatic cell milk, indicating higher proteolytic activity in high somatic cell milk. Electrophoretic analysis of high and low somatic cell milk indicated that there was substantial proteolytic breakdown of alpha S-casein and beta-casein by proteases associated with elevated somatic cell counts.

Iodine Content of Milk and Other Foods.

Excess dietary iodine intake has been identified as an issue of public health concern. The recommended dietary allowance for iodine is 100-150 (µg for adults and 70-120 (µg per day for children. A 1978 Food & Drug Administration survey found that milk and dairy products contributed more than 50% of the total food iodine intake for most age groups. A wide variety of dairy and food products were analyzed for iodine content. Red breakfast cereals and red candy (that contain FD&C Red No. 3), dairy products, eggs, milk, marine fish, and iodized salt contained the most significant quantities of iodine. Iodide content of individual raw milk samples from approximately 2500 farms in New York State was measured. Approximately 62% of all farms had milk iodide levels less than 200 µg/L, 28% between 200 and 499 µg/L, 7% between 500 and 1000 µg/L and 3% had greater than 1000 µg/L. The iodine content of all types of retail milk averaged 394.1 µg/L, cheese and cheese products averaged 15.2 µg/100 g. Most of the iodine partitions into the whey during cheese processing. For dairy powders (including whey), the average iodine content was 471.8 µg/100 g. Use of these powders as ingredients in other dairy and non-dairy products can contribute to high iodine content of other food products. In particular, the iodine content of ice cream was extremely variable, ranging from 18 to 359 µg/100 g. Generally, ice creams and ice milks that contained high proportions of whey and non-fat milk powders had higher levels of iodine in the finished product. Addition of FD&C Red No. 3 to foods substantially increases their total iodine content. However, the measured free iodide content of four brands of red breakfast cereal was higher than would be expected. Food and Drug Administration specifications for certified lots of FD&C Red No. 3 allows up to 0.4% sodium iodide as a contaminant from manufacture. The four brands of red breakfast cereal averaged 6% of their total iodine as free iodide (366 µg/30 g serving). This may indicate that free iodide may be released from FD&C Red No. 3 during processing.

Thermal Degradation of FD & C Red No. 3 and Release of Free Iodide.

Thermal degradation of certified FD & C Red No. 3 was evaluated at 20, 90, 150, 200, 220, 240, 260, 300, and 350°C using a differential scanning calorimeter. Release of free iodide was measured using an ion selective electrode. Under conditions used in this study, it was found that FD & C Red No. 3 begins to release significant amounts of iodide at temperatures between 200 and 210°C. In commercial food products such as breakfast cereals, where high processing temperatures are common, there may be significant thermal degradation of FD & C Red No. 3 that results in high levels of free iodide.