Researchers in New Zealand have discovered that iron levels and bioavailability in meat are dependent on the degree to which meat is cooked and the type of cut.
In recent years, industry-sponsored advertising campaigns have got across the message that red meat is a more readily available source of iron than chicken or spinach on a weight basis but until now little was known about how iron levels were affected by the way meat is cooked or cut.
Iron in meat is more readily taken up into the body than iron from most other sources because it is mainly present as the more available haem iron, and because meat contains an as-yet-unidentified "meat-factor" that enhances the uptake of not only meat iron, but also other iron in the same meal, research claims.
Research recently published by a joint Massey University , AgResearch group working under contract to Meat New Zealand has found iron bioavailability does vary between cuts and that long slow cooking also reduces the amount of iron available.
Dr Roger Purchas from the Institute of Food, Nutrition and Human Health, says the group's research found that, as expected, iron levels tended to be higher in beef than in comparable cuts of lamb, and also for levels to be higher in darker red cuts such as the bolar from the shoulder than in paler cuts such as the eye of the round in the hindquarter that is often part of a corned silverside.
"Iron needs to be in a soluble form in order to be absorbed into the blood, so the percentage of iron in the soluble-haem form was of particular interest, and this proved to be higher in beef than lamb by about 25 per cent. Much of the extra iron of beef relative to lamb appeared as the desirable soluble-haem iron. In a similar way those cuts with more iron tended to have a higher percentage as soluble haem iron."
These results for uncooked meat are useful, but almost all meat is consumed cooked and is digested before it reaches the site where it is absorbed into the blood.
Dr. Purchas said: "Comparisons of meat cooked to various final temperatures and for different times showed that much of the soluble-haem iron was quickly converted to an insoluble form with cooking, and with more severe cooking some of the haem iron was converted to the less desirable non-haem form. The extents to which these changes are reflected in a lower bioavailability remain to be determined. Interestingly the amount of iron lost in juices released during cooking was highest at a lower cooking temperature (60°C as opposed to 80 or 98°C) apparently because the slower cooking meant the haem iron remained soluble for longer. This loss in cooking juices was up to 16 per cent of the total iron in some cases, which suggests that devising ways of retaining the cooking juices from meat may be beneficial with respect to iron intakes."
Dr Purchas also added that, according to the research, a significant amount of iron was also found to be lost in the juices released from meat during storage, with up to 13 per cent of the total iron being lost over eight days for meat that had been frozen and thawed. This was mainly in the valuable soluble-haem form, so devising culinary strategies to capture this source of iron will also be worth while. These losses were reduced by about half for meat that had not been frozen and thawed.
The group's research, recently published in the International Journal of Food Science and Technology , is part of a larger quest by Meat New Zealand to identify ways in which the bioavailability of iron and other nutrients in meat vary.
"Considering the proportions of various forms of iron is a start on this road, but eventually the results need to be backed up by demonstrating that products predicted to have strengths in this area, actually demonstrate such strengths when consumed by people," Dr. Purchas said.