Elements in fruits and vegetables from areas of commercial production in the conterminous United States

The mean concentrations of 27 chemical elements in eight kinds of fruits and nine kinds of vegetables were estimated from field col­ lections within 11 areas of commercial production. Water-content and ash-yield measurements permitted the element concentrations to be expressed on fresh-, dry-, and ash-weight bases. A three-level sampling design was used; and estimates were made of the chemical variation in the produce from among the areas, among fields within each area, and between sites within fields. Most significant varia­ tion was found to be among areas: concentrations of some elements in some kinds of produce were found to vary tenfold. Soils in which the produce grew were sampled also, and analysis revealed strong differences among areas that are attributed, from place to place, to cultivation practices, contamination by pesticides, or pollution, as well as to climate and underlying geology. In general, little relation­ ship was found between total element concentration in the soil and in the produce that could be assigned to natural causes, but where the soils are highly contaminated the levels of the contaminating element were found to be high in the produce. The concentrations of elements in fruits and vegetables generally differ least in the macronutrients potassium, phosphorus, magnesium, and sulfur that are essential for plant growth. Trends in concentrations of the micronutrients boron, copper, iron, and zinc are similar but not as pronounced as those of the macronutrients. The concentrations of the nonnutritive nontoxic elements barium, cobalt, lithium, titanium, and zirconium tend to have greater ranges than do those of the nutritive elements. Concentrations of elements generally considered toxic to organisms exhibit erratic distribu­ tions among areas and kinds of produce, and a wide range of values is indicated. INTRODUCTION The chemical elements in food plants are of interest primarily because these plants constitute the major source of essential elements (excluding oxygen and hydrogen) in human nutrition (Underwood, 1971); lesser amounts of these elements are derived from water, soil and rocks, and air. The elements contained in the plants may be consumed directly in vegetables and fruits, or indirectly in meat and milk from animals that have eaten the plants. Numerous studies have been made of the elements in man's total diet, and analyses of many food plants are given in the reports of the studies. A lack of uniformity among these studies exists, however, in sampling techniques, methods of analyses, kinds of plants sampled, and bases used for reporting element concentrations. In­ adequate, or no, descriptions of the origins of the samples further reduce the usefulness of many reports. A National Research Council report (Morrison and others, 1974, p. 92) stated, "no systematic study has been made of sampling of fruits and vegetables for trace elements. More attention has been paid to food processing and its effect on changes in trace element composition of fruits and vegetables. " The effects of processing on the element content of food plants cannot be determined unless reliable bases are available for estimating the typical concentrations of the elements that are in the edible portion of the plants as they grow in the field. Regulations governing allowable increases or decreases of element content caused by processing food should take into account not only the concentrations of elements of interest char­ acteristic of the different food plants, but also the variation in chemical composition of the plants among the areas of commercial production throughout the country. Some fruits and vegetables are consumed that have had little or no processing; therefore, the introduction of extraneous elements is minimal, and only the elements acquired by the plants while growing in the field are generally present. Yet the sources and concen­ trations of elements in these, as well as in other, food plants may be greatly different among areas where the plants are grown, as influenced by factors of soil chemistry, agricultural practices, climate, and the ex­ tent to which environmental pollution affects the pro­ duce. Plant species react differently to these influences on element content because of their genetic control of growth processes and characteristics. If the edible parts of the plant are leaves and stems, atmospheric pollution may considerably influence the kinds and concentrations of elements in these parts. If roots or tubers are used for food, the elements in the soil may exert the greatest effect, whereas if fruits are the edible portion, only the elements that can be readily ELEMENTS IN FRUITS AND VEGETABLES, CONTERMINOUS UNITED STATES transported from the roots to the fruit are likely to be greatly influenced by differences in geochemical environments. Quarter-man (1973, p. 171) stated, "The amount of a particular trace element in a plant food can depend on the species of plant, the breed or strain, and which part of the plant is eaten. It can also depend on the season of the year and the climate, on the soil type and pH, the proximity of other plants, manuring and various forms of contamination. " These factors may differ greatly among regions of foodplant production; there­ fore their influence creates an interest in determining the presence and extent of regional variation in the elemental composition of fruits and vegetables. The chemical composition of fruits and vegetables is of interest to the growers of produce because it can be used as an index of the nutritional status of the plants. Some large-scale commercial operations make frequent analyses of the plants during the growing season for as many as eight elements; deficiencies in elements that may affect yield or market quality are determined and are promptly corrected by soil or foliar applications of the deficient elements. Analyses are commonly made of leaf or stem tissue, but may also use fruit tissue. In addition, visible symptoms of specific element defi­ ciencies may be used to initiate corrective measures. The data on elemental composition of food plants given in reports of these practices are of limited usefulness in establishing baseline values to be applied to problems of human nutrition, because the emphasis in these reports is on plant nutrition or pathology. These reports include, however, maximum concentra­ tions of certain elements in the plants, concentrations that are potentially toxic to humans. Examples of com­ prehensive studies of this type include those of Goodall and Gregory (1947), Chapman (1966), and Kenworthy (1967). The emphasis in fruit and vegetable growing is on an adequate yield of produce that is of acceptable market quality to return a profit to the grower. Quality in this context is measured by the ability of the produce to withstand harvesting, processing, and marketing operations, and to be adequate in such factors as color, flavor, and texture. The nutritional value of the pro­ duce generally is given only minor, or no, considera­ tion, although Beeson (1957, p. 258) pointed out, "The term 'crop quality' means both marketable quality and nutritional quality of a crop* * *. Nature has not always combined two aspects of crop quality in one package, and man has seldom improved matters in his efforts to breed plants and manage soil so as to pro­ duce crops that are both attractive and high yielding. " A task force of the Food and Drug Administration (Miller, 1974) proposed monitoring the content of two elements, magnesium and calcium (along with protein; vitamins A, B6, and C; thiamin; riboflavin; and niacin) in nine food crops. It would seem to be of equal or greater importance to monitor some other elements that, for man, are obtained principally from food plants—copper, for example. In a paper giving quantitative data on the occur­ rence in plant tissue of 71 of the 94 naturally occurring elements, 46 elements were reported in measurable concentrations in the edible portion of food plants (Shacklette and others, 1978). The mean concentra­ tions, deviations, and observed ranges of 30 elements in many vegetables were reported by Connor and Shacklette (1975), who gave element values for the material actually analyzed, whether it consisted of plant ash or dry plant material. Extensive tables giving element concentrations in many food plants were published by Beeson (1941), Monier-Williams (1949), and Diem (1962); and although the data in these reports generally based the concentrations on dry weight of the plant material, values based on fresh weight occur at places. Only the report by Diem gave the water content of the material that was analyzed. The absence of clearly stated bases for calculating element concentrations in samples of foods and other biological materials is a deficiency in many published reports, and for these one can only assume which bases were used (that is, fresh weight, dry weight, or ash weight) by judging the kind of material analyzed and the magnitude of the concentrations that were reported. The use of various bases for expressing element concentrations in organic materials was discussed by Goodall and Gregory (1947) and is also discussed later in this report. The elemental composition of a variety of foods from tropical plants was given by Duke (1970) in an ethnobotanical report on some Central American In­ dians. Many reports of the elements in foods have been published by U.S. Government agencies, including the Department of Agriculture handbooks (for example, see Watt and Merrill, 1950, which gives both water and ash contents of the food plants that were analyzed). Most of these reports include only the major and minor nutritive elements. The concern with environmental contamination has resulted in many publications which include food-plant analyses for toxic elements. In a report on toxicants occurring naturally in foods, Underwood (1973) pro­ vided some background ran