Certain thermal processing methods for foods can reduce their phosphorus levels significantly without affecting protein content, which could help dialysis patients control their phosphorus levels while maintaining proper nutrition, according to a recent study.

These methods include boiling in water and stewing in oil containing some water, researchers in Croatia reported online ahead of print in the Journal of Renal Nutrition. Soaking meat in cold water for 1 hour prior to thermal processing can decrease phosphorus content even more.

Thermal processing may allow the design of menus “that take into account the changes in nutritional content that occur during thermal processing of foodstuffs,” the investigators led by Ines Panjkota Krbavčíc, PhD, of the Faculty of Food Technology and Biotechnology at the University of Zagreb, wrote. “This may allow hospitals to simultaneously reduce the risk of hyperphosphatemia and malnutrition in dialysis patients.”

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Dr. Krbavčíc’s group used Standard Official Methods of Analysis methods to determine protein, calcium, and phosphorus levels in potatoes, fresh and frozen carrots, frozen green beans, chicken, beef, and pork, frozen hake, past, and rice. They measured the levels before and after boiling in water, steaming, stewing in oil or water, or roasting.

Boiling in water and stewing in oil containing some water significantly decreased phosphorus levels in fresh and frozen vegetables by 27% to 43%, in meat by 10% to 49%, in pasta by 7%, and in rice, by 22.8% on a dry weight basis.

Frying potatoes in oil lowered phosphorus content by 37%, whereas steaming them reduced phosphorus content by 27%. Roasting frozen hake in oil led to a 49% decrease in phosphorus content. Soaking pork in water and then roasting it in oil led to a nearly 12% decrease in phosphorus content. Eliminating the soaking step eliminated this decrease, the investigators noted.

Boiling increased the calcium content in all foods because of calcium absorption from the hard water, but stewing in oil containing a small amount of water decreased calcium content of vegetables by 8% to 35% and of chicken by 12% to 40% on a dry weight basis, according to the researchers.

With a few exceptions, protein content did not decrease significantly for the foodstuffs and thermal processing methods tested, the researchers stated. One exception was stewing fresh diced carrots in oil containing water; in this case, protein content on a dry weight basis decreased by 39.5%. Another exception was stewing frozen green beans in the oil-water mixture, which led to a 46% decrease in protein content.

“On the basis of these results, we modified the thermal processing methods used to prepare a standard hospital menu for dialysis patients,” the authors wrote. “Foodstuffs prepared according to the optimized menu were similar in protein content, higher in calcium, and significantly lower in phosphorus than foodstuffs prepared according to the standard menu.”