TRACE METALS IN FOODS 5 



Nutrients in Dry Matter 



Although based on a limited number of samples, the analytical data seem to 

 warrant the followmg deductions. The major fruits are notable primarily for 

 their high carboh^'drate content. The berries average considerably lower in 

 carbohydrates and contain more protein, fat (wax), and fiber. The Rubus 

 varieties are relatively high in crude fiber. The garden fruits, as a whole, contain 

 more protein and ash than the fruits previously considered, with the exception 

 of the pumpkin, winter squash, and muskmelon. 



The beans and peas, in common with most leguminous crops, contain con- 

 siderable protein, and some varieties of soybeans are high in fat. 



The leaf and stem vegetables constitute one of the most diversified groups. 

 Wide variations in composition occur in the same variety grown under different 

 environmental conditions. Onions, cabbage, cauliflower, celery, and lettuce 

 frequently contain 50 percent or more of nitrogen-free extract, and the quality, 

 in some instances at least, appears to increase with the percentage. Onions are 

 low in protein, while leaf greens, celery, and lettuce are fairly high in protein 

 and ash, and asparagus in protein. Most root vegetables are high in carbohy- 

 drates, with mangels and radish containing more than average amounts of protein 

 and ash. 



The cereals of low fiber content are similar to the root vegetables in total 

 extract, although this is more largely starch. The other cereals reflect a higher 

 fiber content. The nuts, including the peanut, are unlike any other group and 

 contain a very high percentage of fat. 



Processed human foods resemble the cereals from which many are derived. 

 The cattle feeds are industrial by-products and vary with the original substances 

 and subsequent treatment. 



Forage plants, fed green or dry to farm animals, are naturally high in fiber and 

 ash and when cut early contain considerable protein, especially in the case of 

 leguminous forage. 



Trace Metals in Dry Matter 



Innumerable determinations of the more prominent inorganic elements in 

 plant products have been reported in the literature during the past few decades. 

 These results were obtained largely by the gravimetric methods then in vogue. 

 More recently, with the introduction of the so-called microchemical or colorimetric 

 methods, the determination of the rarer mineral elements has received con- 

 siderable impetus. The results herein reported were determined exclusively by 

 colorimetric methods based on visual readings with a Duboscq Comparator. 

 In the preparation of solutions, wet combustion was found more serviceable, 

 more rapid, less subject to contamination, and of wider application than dry 

 combustion when many samples are invohed. Dry combustion was used for 

 the determination of zinc, but later investigation has shown that with a pre- 

 liminary treatment the regular sulfuric acid solution can be employed. Iron 

 was determined by the thiocyanate reaction; copper, by sodium diethyl dithio- 

 carbamate reagent and extraction with carbon tetrachloride; zinc, by diphenylthio- 

 carbazone reagent and extraction with carbon tetrachloride; manganese, by 

 periodate oxidation; and phosphorus, as molybdenum blue. The methods are 

 presented in some detail on pages 24-31. 



Many practitioners and dietitians assume that foodstufTs containing appre- 

 ciable amounts of these elements may serve as curative or at least preventive 

 agents in cases of malnutrition and functional derangements. Furthermore some 



