Recovery of polyphenols from the by-products of plant food processing and application as valuable food ingredients
Introduction
In contrast to tropical and subtropical fruits, those from the temperate zone are characterized by a large edible portion, while the proportion of waste accruing from fruit processing is relatively low. Nevertheless, total waste arising from apple juice processing and winemaking accumulates to huge amounts, thus being a valuable source of waste valorization. Solely in Germany, 200,000–250,000 tons of wet apple pomace is available per year. According to literature reports, total mass of grape pomace resulting from European wine production was estimated to vary between 5 and 14.5 million tons (Meyer et al., 1998, Schieber, Stintzing and Carle, 2001b, Torres and Bobet, 2001, Torres et al., 2002). Similarly, sunflower expeller resulting from oil extraction is abundant, since sunflower is one of the four most important annual crops grown for edible oil production. Its annual world production amounted to 37.4 million tons in 2012 (The Statistics Division of the Food and Agriculture Organization of the United Nations; FAOSTAT). Sunflower meal obtained from the press cake is so far mostly used as animal feed. It contains up to 4% of polyphenols being considered as anti-nutritive in animal nutrition. In addition, due to rapid polyphenol oxidation and their concomitant interactions with proteins under alkaline condition, the conventional extraction of sunflower proteins from the press cake is still challenging (Weisz et al., 2013, Weisz et al., 2010).
Polyphenols are unevenly distributed in plant tissues, and processing may result in the separation of those parts of the fruit and grain being particularly rich in polyphenols. This holds true for apple, grape, cereals and oilseeds, where UV absorbing flavonoids and phenolic acids, attractant anthocyanins and deterrent tannins are mainly deposited in the outer layers of the skin, aleurone cells, seed coats and hulls, respectively. On the other hand, the apple core and the seeds of apple and grape are rich in astringent polyphenols. Consequently, apple and grape pomace as well as sunflower hulls containing the core, skin and pips, and pigmented seed hulls represent rich sources of polyphenols worth being exploited for the recovery of such components as natural food ingredients.
Polyphenols are characterized by high structural diversity, and may be subdivided into two major groups, the flavonoids and non-flavonoid compounds. Flavonoids share a common C6–C3–C6 carbon skeleton comprising flavanones, flavan-3-ols, flavan-3,4-diols, flavones, flavonols, dihydroflavonols as well as chalcones, dihydrochalcones, and aurones. The non-flavonoids are represented by phenolic acids, such as gallic acid, protocatechuic acid, and cinnamic acid derivatives, stilbenes and lignans. Polyphenols are present in plant tissues either in non-glycosylated form or as glycosides and/or associated with various organic acids and/or complex polymerized molecules with high molecular weight such as tannins.
Beginning with the first reports of the so-called “French paradox” (Renaud & De Lorgeril, 1992), interest in polyphenols has distinctly been boosted because of their putative health benefits (Schieber et al., 2001b). During the last two decades, polyphenols have been associated with a multitude of health beneficial effects possibly preventing damages and diseases caused by oxidative stress (Havsteen, 2002, Kammerer et al., 2007).
Due to their widespread occurrence in fruits, vegetables, nuts, cereals, and oilseeds, phenolic compounds are an integral part of human nutrition. However, their dietary intake considerably varies depending on age, gender and nutritional habits (German National Nutrition Survey II, 2008). According to Scalbert and Williamson (2000) the daily per capita polyphenol intake amounts to an estimate of 1 g.
Since the consumers' interest in healthy food has significantly increased within the last few years, there is a steady trend toward the production of functional foods and functional food ingredients. Among other secondary plant metabolites, polyphenols are believed to contribute to the health protective effect of many food commodities. Therefore, they have even been named “vitamins of the 21st century” (Stich, 2000). Consequently, supplementation of foods with polyphenols recovered from waste materials accruing from fruit, vegetable, cereals and oilseed processing may be a valuable strategy to increase the dietary ingestion of such compounds.
For an economic recovery of polyphenols sufficient availability of raw material is mandatory. As earlier stated, in the case of pomace from apple juice extraction and winemaking such by-products are abundant. The same is true for sunflower press cake arising from oil extraction. Therefore, these by-products have been the focus of our research aiming at sustainable food production. In the following, the recent developments in the field of recovery and application of polyphenols from wastes accruing from food processing are exemplified by our attempts of apple pomace, grape pomace and de-oiled sunflower meal valorization.
Section snippets
Exploitation of apple pomace as a valuable source of polyphenols and pectin
The global apple (Malus domesticus Borkh.) production is steadily growing with an annual world production amounting to 76.1 million tons in 2011. Within the last 50 years (1961–2011), apple production area has almost been tripled reaching its maximum extension (6.3 million ha) in 1995; however, subsequently being reduced to 4.7 million ha corresponding to the latest records (FAOSTAT). According to recent estimates, 70%–75% of the apple is freshly consumed, while only about 25%–30% of the annual
Exploitation of grape pomace as a valuable source of phenolic colorants and antioxidants
Apart from apples, grapes belong to the most important fruit crops with an annual world production quantity of around 69 million tons in 2011, being nearly equivalent to that of oranges (FAOSTAT). Grapes may be processed into a variety of common food products, such as grape juice, jams, raisins and wine, with the latter predominating by far. Around 80% of the world grape production is estimated to be processed into wines resulting in the accumulation of great amounts of grape pomace (Kammerer
Exploitation of residues from sunflower oil extraction as a valuable source of polyphenols and protein
Sunflower (Helianthus annuus L.) oil is one of the four most important oilseed crops. Its worldwide production steadily grew within the last three decades to reach a global production of 15.2 million tons in 2012 (FAOSTAT), and FAO forecasted world production of sunflower oil to amount to around 22.4 million tons toward 2050 (ASAGIR, 2012). About 90% of sunflower seed oil consists of unsaturated fatty acids. Since edible oils being rich in mono-unsaturated fatty acids are considered to be most
Conclusions
Directive 2008/98/EC (the so-called ´Waste Framework Directive´) implemented basic concepts for waste management based on a hierarchical system, where the most favored option is the avoidance of waste accruing from processing, followed by waste minimization, reuse and recycling of waste streams. The least favored options are energy recovery from waste materials, and finally, waste disposal. Furthermore, the a.m. EU directive sets rules applying the “polluter pays principle,” and defined two
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Present address: WALA Heilmittel GmbH, Department of Analytical Development & Research, Section Phytochemical Research, Dorfstraße 1, 73087 Bad Boll/Eckwälden, Germany.
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Present address: lege artis Pharma GmbH + Co. KG, Breitwasenring 1, 72135 Dettenhausen, Germany.
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Further affiliation: Biological Science Department, King Abdulaziz University, P.O. Box 80257, Jeddah 21589, Saudi Arabia.