Trace Element Speciation in Food: A Combined Enzymolysis—SEC-ICP-MS Approach

Trace elements in food can be either desirable, tolerable or undesirable. They occur in the diet as natural constituents of agricultural produce, as deliberate additives and as contaminants from environmental and processing sources. Historically, their significance to man has been assessed on the basis of total concentration of the element in the food item-or the total amount in the diet as a whole (with the exception of the measurement of mercury and to a lesser extent arsenic where specific forms are sometimes measured). In recent years the inadequacy of simply measuring the total concentration has been increasingly appreciated and efforts to provide information which is targeted more accurately to the needs of the nutritionist and the toxicologist are now being made. Over the last few years, the MAFF Food Science Laboratory has had a small programme investigating ways of obtaining more meaningful information about the chemical forms of trace elements in food. As a first stage of a general approach to the problem, attention has been restricted to species which are soluble at around neutral pH after treatment with normal monogastric mammalian digestive enzymes. A procedure has been developed in which food is treated sequentially with pepsin at around pH 2 and with pancreatic enzymes at around pH 7 at 37 'C. A limitation of this procedure is that it is a "static" system, whereas absorption is dynamic; if there is equilibrium between soluble and insoluble species, the system will underestimate the amount which is potentially available biologically. In some very simple experiments, food composites representing the cereal, meat, fish and green vegetable components of the UK diet were examined with and without the addition of small amounts of copper, zinc, cadmium and lead as inorganic salts. The properties of the endogenous element which was soluble after enzyme treatment has been compared with that of the added element. For all the foods studied, endogenous and added copper behaved similarly, but there were marked differences for some of the other elements. For instance, zinc from fish is largely insoluble, and added zinc also becomes insoluble; other foods release much zinc in soluble form. On the other hand, iron from meat is largely soluble but added iron becomes insoluble [1]. This investigation has been developed further in studies of the effect of one food upon the release of trace elements from another. The amount of acidsoluble cadmium available from crab meat is reduced by 75% by digesting it along with wholemeal bread; white bread has very little effect. Similar experiments show that addition of 10% soya to ground beef gives a much lower release of zinc from the beef-presumably a soya phytate effect. Processing of beef can affect the solubility of iron; inorganic iron as taken in dietary supplements is much more soluble if digested along with cereals and vegetables than with meat and fish [2]. The broad conclusions of these experiments were that: a) acidity enhances solubility, b) enzyme action changes solubility, c) processing can affect solubility, and d) presence of other foods can affect solubility. Even where an element was soluble, there were indications of changes in chemical form as a result of enzyme action, processing or the presence of other foods. In devising an investigation of these changes, the criteria were that it should be, i) with foods containing levels of toxic elements typical of those in the normal food supply; ii) with systems which,