Leaf proteins for foods.

Green leaves may contain 5-10% protein, and direct extract ion of certain leaves can yield up to 2,000 kg of crude protein per hectare per year. Actual yields of leaf protein vary significantly with species, variety, age, composition, and physiological state of the plant; length of postharvest period; drying t reatment ; degree of macerat ion of leaves; solvation rat io; pressure applied; solvent(s) used; additives (ammonia, bisulfite); duration, pH and temperature of extract ion; separation techniques; number of extractions; method of precipi tat ion (acid, salt, heat); method of recovering and drying the protein. Protein recoveries range from 40-80%, with the bet ter methods yielding 60% of the original leaf protein. Good extracts contain 65-70% protein, i.e., leaf protein concentrates (LPC); up to 10% lipids and around 6.0% ash. Fur ther refining may yield protein isolates with over 90% protein. The term LPC has been applied to leaf proteins prepared from numerous different plants by a variety of methods, hence claims about propert ies and effects of leaf protein (LP) may not be generally valid because of variations in composit ion. Extensive research has shown that LP has potential as a source of food protein. However, LP need not and should not be considered as a sole source of dietary protein, rather it initially might be introduced as a supplementary source of protein to complement other conventional food sources. In considering the use of LP, knowledge of the eating habits and preferences, technological and educational status, educational level and motivat ion of the target consumer is important . Before their general introduct ion, leaf preparations must meet a number of critical criteria, e.g., organoleptic, functional, nutr i t ional and safety properties. Assessment of nutr i t ional value has been a major preoccupat ion of many researchers s tudying LP. LP from many plant sources possess a good amino acid composi t ion, being apparent ly deficient in only methionine. Methionine and lysine, being quite reactive, are easily destroyed during extraction, drying, and storage. Heating and storage in the presence of oxidizing lipids reduce the concentrat ion of these amino acids and result in LP o f inferior nutrit ive value. Discrepancies between in vitro digestibility studies (pepsin/ trypsin or pancreatin) are largely due to different methods of extract ion, refining, precipi tat ion, washing, drying, and storing. These affect the composi t ion (e.g., protein content , ant i t rypt ic factors) of the LP preparation. The more refined (low yield) LP are very digestible and appear to have a well balanced amino acid composi t ion. Numerous feeding trials have reported PER values ranging from 1.5 to 2.5. These vary with protein preparation. Supplementat ion of LP with methionine (.02%) and lysine increases PER values to 3.0. Trials w i t h human subjects have demonstra ted the value of LP as a supplementary source of dietary protein, and the amino acids present indicate its value for complement ing cereal-based diets. In developing LP for foods (or feeds for monogastric animals) appropria te processes for eliminating the antinutrients ( trypsin inhibitors), toxins (cyanogenic glycosides in cassava, thioglycosides in brassica, goitrogenic compounds, alkaloids), and physiologically active compounds (oestrogenic materials, photosensit ive molecules) that may be present in the original leaves must be incorporated. Much of the research on LP has not been conducted with a specific objective in terms of ut i l izat ion as a food ingredient; rather, localized use as a condiment or supplement has been envisaged. However, for ul t imate widespread acceptance and use by food manufacturers and consumers, leaf proteins like conventional and food proteins must have appropriate functional properties. Funct ional propert ies is a collective term for those physicochemical propert ies of proteins which determine the overall behavior or performance of proteins in foods during manufacturing, processing, storage, preparat ion, and consumption. They reflect those propert ies of the protein per se and its interact ions with other food components as affected by the immediate environment. The part icular functionali ty required varies with each food application, and of course no single protein can perform all of these functions. Ideally, leaf proteins should have good nutr i t ional value, satisfactory color, odor, flavor, and texture, but they also should possess addit ional functional properties, e.g., solubility, surface activity, coagulabili ty, thermal stabil i ty, adsorption properties, etc. for successful performance in a variety of applications in foods. These criteria are frequently overlooked when considering new proteins where yield and biological value are emphasized. The successful supplementat ion of t radi t ional foods, the replacement of protein in or the simulation of t radit ional proteinaceous foods, and the fabrication or engineering of new foods will depend on the availability of new proteins possessing the requisite functional properties. Thus, leaf protein to be adopted by the food industry and to gain general consumer acceptance must possess appropriate functional properties. Addit ional research on leaf protein is needed to develop improved methods for protein extract ion; to develop refining procedures for removing undesirable colors and flavors that are compat ible with re tent ion of reasonable functional propert ies; to establish nutritive value and safety and to develop and demonstra te acceptable applications. When such informat ion is available, consumer and political acceptabil i ty may be more accurately assessed, and more definitive economics can be considered.