Protein contents of plants

The protein content of plant foods is most of the time so different from that of animal-derived foods that large amounts of plants will have to be consumed with relatively little effect on consumer 5 N values. For an archaeologically meaningful interpretation of isotopic measurements this weighting problem needs to be considered. 

Because Mo promotes the utilization of absorbed nitrates in plants and promotes N fixation in leguminous plants, it is likely to influence the protein content of plants. Applications of Mo have been shown to increase the contents of soluble protein in maize (Figure 4.3) (Agarwala et al., 1978) and beans (Domska, Benedycka, and Krauze, 1989). 

The analyst who desires a method for the determination of the protein content of plant or animal material is confronted with a voluminous and confusing literature. No better indication could be found of the unsatisfactory state of analytical development in this field. The reasons are apparent. Proteins form a very diverse group of similar compounds of extraordinary complexity, with widely different compositions and properties, yet difficult to separate completely, to purify and to dry. Their amphoteric nature, high adsorptive capacity, hydration properties and sensitivity to electrolytes cause them to vary widely in behavior depending on the composition, pH and temperature of the solvent medium (1). Moreover, they usually occur in mixture with each other in variable concentration ratios and in various solid and dissolved states. The analyst may be concerned with the content of a particular protein in such a mixture, or commonly, he wishes to know the total pro in content when that content is itself made up of a mixture of variable and uncertain composition. 

Protein Content. The protein content of milk can be determined using a variety of methods including gasometric, Kjeldahl, titration, colorimetric, and optical procedures (see Proteins). Because most of the techniques are too cumbersome for routine use in a dairy plant, payment for milk has seldom been made on the basis of its protein content. Dye-binding tests have been appHed to milk for determination of its protein content these are relatively simple to perform and can be carried out in dairy plant laboratories. More emphasis will be given to assessing the nutritional value of milk, and the dependence on fat content as a basis for payment will most likely change. 

Table V. Weights and Protein Contents of Leaves from Four Tropical Plants [from Nagy et al. (4 )]...

For measuring water absorption by the excess water method, the techniques developed by Janicki and Walczak (described by Hamm, 21) for meats and by Sosulski (22) for wheat flour are modified. Lin et al. (17) modified the Sosulski technique for use with sunflower and soy meal products. This modified procedure has been employed for much of the research on water absorption of plant protein additives. Water absorption capacities of a soy flour, two soy concentrates, and two soy isolates were compared by Lin et al. (17) to those of a sunflower flour, three sunflower concentrates, and one sunflower isolate. The percent water absorption of the soy products increased as the total protein content of the samples increased from flour to isolate. The soy flour absorbed 130% water, the soy concentrates absorbed an average of 212% water, and the soy isolates absorbed an average of 432% water. No calculations were made, however, that related the percent water absorbed to protein content of the samples. The sunflower products, though similar in protein content, did not respond in the same magnitude or direction as the soy products. 

In terms of percentage of protein content of basic sources, the animal sources far excel the plant sources. For example, the protein content of some typical unfortified foods is as follows 20-30% for cooked poultry and meats 19-30% for cooked or canned fish 25% for cheese 13-17% 17% for cottage cheese 16% for nuts 13% for whole eggs 7-14% for dry cereals 8.5-9% for white bread 7-8% for cooked legumes and about 2% for cooked cereals. 

The compositional variation of samples of wet DG received from a number of different corn dry mill ethanol plants is given in Table 1. Two of the entries show the variation between samples of wet DG removed from the production line centrifuge of a dry-grind ethanol plant several months apart. The crude protein content was found to vary among the different plants from 31.2 to 35.2%. These results can be compared with those of a study of six Minnesota dry-grind ethanol plants conducted by the Department of Animal Sciences at the University of Minnesota that found crude protein content of DDG and DDGS from these plants ranging from 28.7 to 31.6 % (4). This variation is of some concern to animal feedlot operators. 

Proteins occur in animal as well as vegetable products in important quantities. In the developed countries, people obtain much of their protein from animal products. In other parts of the world, the major portion of dietary protein is derived from plant products. Many plant proteins are deficient in one or more of the essential amino acids. The protein content of some selected foods is listed in Table 3-1. 

Safflower is a minor oilseed crop limited in production by environmental constraints and by the plant s spiny nature. Unless the seed is well dehulled, the oilcake resulting from oil extraction will have a high fiber content. Undecorticated oil cake has a protein content of 20-22% and an end use as manure. In contrast, removal of the hull improves the protein content to 40%, making it acceptable as cattle feed despite low lysine levels. Leftover hulls and husks are added to cattle feed or are used to manufacture cellulose, insulation, and abrasives (5, 49). 

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