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By Kantha Shelke, Ph.D. and David Feder, R.D. | 02/24/2006
The USDA estimates animal products provide half of the zinc and one-fifth of the iron in U.S. diets. While plant-based foods might serve as good sources for these minerals, not much is readily absorbed for functional use by the body.
Cereals, nuts and beans, although rich in the mineral by percentage are poor sources of iron because of a high content of phytic acid (phytates). Polyphenols, such as tannic acids in tea, coffee, eggs, red wine and soy protein (independent of the phytic acid in soy) also are powerful inhibitors of plant based-iron absorption.
Since these trace minerals are critical to healthy growth and development, children are at special risk for deficiency. The recent modification of the U.S. National School Lunch Program, allowing for complete replacement of meat, poultry, and fish products with alternate plant protein products, requires careful iron and zinc fortification of such products.
The FDA list of iron compounds generally recognized as safe (GRAS) includes elemental iron, ferrous ascorbate, ferrous carbonate, ferrous citrate, ferrous fumarate, ferrous gluconate, ferrous lactate, ferrous sulfate, ferric ammonium citrate, ferric chloride, ferric citrate, ferric pyrophosphate and ferric sulfate. This list, long as it may be, is actually restrictive because there is no single compound that can be added to foods without some kind of complication or caveat for functionality.
|Fortitech's Ram Chaudhari advises using caution when fortifying cereals with iron, as the wrong compound can result in undesirable flavor and color changes.|
Other forms, such as ferrous fumarate, ferrous succinate and ferrous saccarate, although less reactive and highly bioavailable, are off-limits to formulators because they tend to become reactive during storage and exposure to high humidity — a prevailing condition in most manufacturing plants. Low water-solubility forms (ferric orthophosphate, ferric ammonic orthophosphate, ferric pyrophosphate and elemental iron powder) may seem like a logical solution for fortification. However, these forms suffer from poor bioavailability.
“To establish a food fortification program, several important factors should be considered, including the choice of food vehicle and iron compound, the fortification level and ways to enhance iron bioavailability from the fortified food,” says Ram Chaudhari, Ph.D., senior executive vice president and chief scientific officer at Fortitech Inc. (www.fortitech.com) Schenectady, N.Y. “Cereal products are difficult to fortify with such compounds due to unacceptable organoleptic changes — rancidity-related flavor and color changes — that occur during storage and/or preparation.”
Zinc is part of every tissue in the body and is an important component in literally hundreds of body functions. It plays an integral role in muscle growth, injury healing and immunity building. The FDA requires iron addition to refined flour by “enrichment” during manufacturing. Zinc, also one of the lost nutrients, however, is not regulated for replacement.
As with iron, zinc from meat is better absorbed than zinc from plant sources. Absorption is further confounded by foods high in fiber and phytate. Zinc also can compete with iron for absorption when the two are eaten in tandem.
According to Arthur Grider, Ph.D., zinc researcher at the University of Georgia, Athens, there is more than one way to ensure maximum absorption and bioavailability or iron and zinc: “Loading a food with the fortifier compound doesn’t mean greater availability. Instead, reduce the anti-nutritional factors in the food. The addition of ascorbic acid in quantities that exceed the RDA can enhance iron absorption in foods with high levels of phytate. Zinc can be enhanced with the addition of the enzyme phytase to inactivate the phytates.”
According to Grider, the growing number of methods available to degrade or remove phytic acid and other anti-nutritional factors has yet to be applied to large-scale commercial production.
Mineral oxides, although most poorly absorbed, are used for the convenience of their small particle size and inertness with little effect on food shelf life, texture and taste. Mineral sulfates are more easily absorbed than oxides, but they also tend to provoke lipid oxidation and rancidity by reacting with other food components. The FDA lists five zinc compounds as GRAS for foods: zinc chloride, zinc gluconate, zinc oxide, zinc stearate and zinc sulfate.
Gluconates, acidic forms of glucose, are better absorbed than sulfates and oxides. Amino acid chelates — a stable chemical complex of the mineral with amino acids — are produced by different processes and therefore have different degrees of absorption properties. Independent research indicates amino acid chelates are better absorbed than their inorganic counterparts; iron bis-glycinate in bread, for example, is absorbed at twice the rate of iron sulfate. Similarly superior absorption properties are reported for zinc methionine chelate in contrast with zinc sulfate and zinc oxide. Zinc picolinate chelate reportedly is absorbed better than zinc gluconate and zinc citrate.
Chelate absorption rates can’t, however, be compared gram-for-gram because ingredient suppliers use proprietary processes to produce chelates, and different processes create a wide range of results. Also, the many methods to evaluate bioavailability can produce conflicting results, rendering a one-to-one comparison meaningless.
Zinc and iron with lactate and citrate ligands are gaining popularity for their enhanced functional bioavailability. Chicago-based Tropicana uses calcium citrate malate marketed as FruitCal by Procter & Gamble. It’s a proprietary ligand product in its fruit juices to provide dietary calcium without influencing zinc or iron nutriture, a common concern with other calcium forms that have antagonistic interactions with both zinc and iron.
Kraft Foods, Northfield, Ill., recently applied for the GRAS status of sodium iron EDTA for use in iron fortification of powdered soft drinks (at a level of 2.5 mg iron/200 ml of reconstituted beverage).
Microencapsulation is one innovative way to supplement iron. Thanks to microencapsulated forms of iron, polyglycerol monostearate (PGMS) can now be added to flours. Previously, iron PGMS was off limits because this compound provokes rancidity during extended storage and exposure to humid conditions. Microencapsulation also is particularly helpful because iron absorption is higher when the particle size is smaller.
Absorption antagonism — in which one mineral inhibits the absorption of another mineral — is a major concern for formulators. Processors need to determine if their product should provide both minerals in balance. If so, they should leverage the antagonism and provide each mineral adequately.
Crafty formulators are exploring enhancement of mineral functionality with compounds such as superoxide dismutases and metallothionein for zinc, and the use of flavonoids and other phytochemicals, such as lycopene, to help increase concentration of minerals in certain tissues.
Small amounts can make a big difference. Iron and zinc can be toxic. Ensure fortification is accurate and appropriate and, if possible, get the mineral compounds in a premix for easier measuring. Mineral compounds should be used well within specified shelf life. Out-of-date minerals can create drastic changes in the texture, taste and appearance of finished products.
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