Utilization of plant by-products for the recovery of proteins, dietary fibers, antioxidants, and colorants

Abstract

Processing of fruits, vegetables, and oilseeds results in high amounts of waste materials such as peels, seeds, stones, and oilseed meals. Disposal of these materials usually represents a problem that is further aggravated by legal restrictions. Plant waste is prone to microbial spoilage; therefore, drying is necessary before further exploitation. The cost of drying, storage, and transport poses additional economical limitations to waste utilization. Therefore, agroindustrial waste often is utilized as feed or fertilizer. However, demand for feed or fertilizer varies and depends on agricultural production. Moreover, valuable nutrients contained in agroindustrial wastes are lost. Thus new aspects concerning the use of these wastes as by-products for further exploitation on the production of food additives or supplements with high nutritional value have gained increasing interest because these are high-value products and their recovery may be economically attractive. It is well known that agroindustrial by-products are rich in dietary fibers, some of which contain appreciable amounts of colorants, antioxidant compounds or other substances with positive health effects, while some of them, like the oilseed meals, are rich in proteins. Dietary fibers consist of polysaccharides and lignin that are neither digested nor absorbed in the human small intestine. The beneficial role of dietary fibers in reducing risk of coronary heart disease and certain types of cancer has been pointed out by several epidemiological studies. The recommended daily intake is 30-45 g per day and person, while the estimated consumption in the West is about 20 g per day and person (Thebaudin et al., 1997). Cereals make up to 35- 60% of the actual consumption while fruits and vegetables contribute only 7-15% (Grigelmo-Miguel and Martin-Belloso, 1999b). Cereals are rich in insoluble dietary fibers, which are mainly related to intestinal regulation, while fruits and vegetables contain appreciable amounts of soluble dietary fibers, which help in lowering blood cholesterol and glucose intestinal absorption. Soluble dietary fibers should account for 30-50% of the total dietary fibers intake to improve dietary benefits. Apart from health effects dietary fibers show some functional properties, such as water-holding capacity, swelling capacity, increasing viscosity or gel formation which are essential in formulating certain food products. Formulated food products with high dietary fibers content are now commercially available. Dietary fibers incorporated in these products are obtained mainly by cereals. Citrus and apple pomace are also raw materials for the production of dietary fiber powders. However, the use of byproducts from other fruits and vegetables seems promising, since all these materials are rich in soluble dietary fibers. Several other organic micronutrients, such as carotenoids and polyphenols, impart health benefits. Carotenoids are mostly used as natural food colorants, but also some of them (β-carotene, apocarotenal) have a vitamin A activity as well as antioxidant activity in vitro and in vivo (Kiokias and Gordon 2004). Polyphenols also act as antioxidants; they scavenge free radicals which are responsible for serious diseases and for the oxidation of lipids, proteins, and DNA. Several studies have revealed their antimicrobial, antithrombotic, antimutagenic, and anticarcinogenic activities (Kandaswami and Middleton, 1997; Sahu and Green, 1997). In addition to health benefits, the supplementation of food products with antioxidants delays the formation of off-flavors and rancidity and extends the shelf life of the product. Carotenoids and polyphenols are extensively distributed in several plant by-products. Thus most fruit and vegetable by-products could serve as raw materials for their recovery. Moreover, these antioxidants or colorants would have a natural origin, which is in accordance with the demands of consumers for "all natural" because of the possible toxicity of synthetic additives. The role for food proteins in human nutrition is substantial, while according to modern nutrition recommendations the supply of proteins in human diet should rely mostly on vegetable proteins, i.e., cereals, legumes, oilseeds, etc., than on those from animal sources, i.e., meat. Because the amounts of proteins required to cover the nutritional needs of the world population are continuously increasing, the investigation and finding of new protein sources are of interest. In addition to their nutritional value, proteins offer great potential as functional food ingredients providing useful properties when incorporated into foods. In order to utilize a byproduct as a protein source it should both present high protein content and protein value (quality) based on well-balanced essential amino acids. An additional requirement to utilize a material for food purposes is the absence of allergic or toxic substances or the application of a proper pretreatment for their efficient removal. Several protein products (flours, concentrates, or isolates), depending on their protein content, can be introduced to food products in order to improve their nutritional value as well as their sensory and functional properties. Such protein products already have been used as meat extenders in processed meat products or as protein-enriching agents in nutritional beverages. Particularly, soy protein products are used for the production of several foods such as: imitation cheese, whipped toppings, soy milk, and baked products. Today consumers are highly aware of the close relationship between nutrition and health and they want to include health-promoting ingredients in their diets. Foods fortified with nutraceuticals, or functional foods in other terms, are expected to be the new food category with an expanding demand by future generations. Therefore, food scientists focus their effort on the development of new products with improved nutritional profiles. Natural ingredients recovered from agroindustrial by-products have specific dietary and functional properties and can be utilized effectively to develop this new food category.