Sunflowers are grown primarily to produce vegetable oil. What remains after producing sunflower oil is a press cake which, despite its high nutritional value, is generally used only as animal feed. An innovative development process as part of the “SmartProSun” project has aimed to increase the value of such press cakes.

A seed full of valuable ingredients

Sunflower seeds contain extremely valuable substances: high-quality oil, polyphenols (secondary plant compounds) with a high antioxidant potential, dietary fibers in abundance and sulfur-rich amino acids as a valuable source of protein. However, their potential for human nutrition has not yet been fully exploited. The seeds are usually hulled for consumption or the sunflower oil extracted for use as cooking oil or the production of margarine.

There are basically three ways to extract sunflower oil: mechanical expeller pressing, solvent extraction after an initial pressing step, and direct extraction using solvents [1]. The hulls of the sunflower seeds are usually left in place for mechanical pressing because they increase the oil yield. The combination of mechanical pressing and solvent extraction is the most commonly used method to produce sunflower oil.

Unused potential of the proteins, dietary fibers and polyphenols

Oil production leaves various substances over, such as lecithin during degumming as part of the refining process. To obtain lecithin, the oil-insoluble sludge must be isolated from the oil by separators after hydration with water, and processed. The product can then be used as an emulsifier, slowing down the autoxidation and enzymatic hydrolysis of fats. The main residual product from oil extraction is the press cake of unhulled sunflower seeds. A number of factors influence the chemical composition of a press cake: oil extraction (mechanical processing and/or solvent extraction), differences in the seeds (e.g. origin) and the proportion of hulls removed. The crude protein and crude fiber concentrations are relatively high, ranging from 28 to 42% for protein and 18 to 28% for fiber [1, 2]. This makes the press cakes ideally suited for animal feed.

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Fig. 1 Structural formula of chlorogenic acid

Defatted sunflower meal consists of 4.2% phenolic components, of which 70% is chlorogenic acid [3-4] (Fig. 1). This is an ester of quinic and caffeic acids [5]. The 5-caffeoylquinic acid is the most abundant isomer [6].

Within the sunflower seed, the chlorogenic acid is unbound. It binds to the amino acids but only upon tempering (for example, in the course of oil pressing) or by increasing the pH, as is commonly done to extract proteins.

The aromatic hydroxyl groups (-OH) of phenolic substances help to protect the plant from oxidative damage when subjected to biotic or abiotic stress [7]. In humans, too, consuming chlorogenic acid can have health benefits. The intake of chlorogenic acid has been shown to correlate with anti-inflammatory, antioxidant, cancer preventive, analgesic, antihypersensitizing, skin protective, antibacterial, hypocholesterolemic, and muscle, nerve, and blood vessel calming effects [8]. The symptoms underlying all the named related ailments involve oxidative stress [9].

The oxidation of the chlorogenic acid’s OH groups converts the substance into a reactive quinone [10]. Chlorogenic acid quinones can react with amino acids to form green pigments [11] or, in the absence of amino acids, polymerize with each other to form brown pigments [12]. Reactions of such plant phenols like these hold immense potential for producing stable dyes of natural origin [13].

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Fig. 2 Protein obtained from the meal of sunflower seeds. The green coloration is due to oxidation of the contained chlorogenic acid when the pH is raised to over 8.

However, in the course of protein extraction, the chlorogenic acid binds covalently to amino acids, which means that it cannot be readily separated to be used selectively. Therefore both the chlorogenic acid and the sunflower proteins, are of limited use for food production. Under the influence of the pH and oxygen, the chlorogenic acid oxidizes, leading to darkening or a green coloration of the obtained protein preparation (Fig. 2). This limits its application potential to colored foods. At the same time, the digestibility and functionality of sunflower proteins, especially protein solubility and emulsifying capacity, are reduced. This severely limits their use in, for example, the production of vegan dairy product alternatives. Consequently, neither the protein nor the polyphenol chlorogenic acid are used as food ingredients.

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Fig. 3 An aqueous extraction process of defatted sunflower meal is used to develop three functional ingredients to enrich certain foods with either polyphenol, protein or fiber – for example dairy alternatives, baked goods, vegan meat alternatives, beverages and protein bars.

If it were possible to selectively fractionate the ingredients of sunflower seeds, all ingredients could be used specifically for the production of high-quality foods.

The aim of the “SmartProSun” research project, funded by the German Federal Ministry of Education and Research in its “BioEconomy International” funding program, is therefore to increase the value of dehulled and deoiled sunflower meal by fractionation of the ingredients. Thereby, enabling the production of three functional ingredients from sunflower seeds (Fig. 3) should be promoted:

• A biofunctional, polyphenol-rich fraction based on fractionated chlorogenic acid with antioxidant capacity
• A protein fraction with at least 75% protein content and with high protein solubility due to the removed chlorogenic acid
• A remaining fibre-rich protein concentrate, which can be used for the production of high-fiber food products

Exploiting the sunflower’s potential through kernel fractionation

Chlorogenic acid binding to protein is the main challenge to selectively fractionating the three target ingredients. To prevent this from happening, the focus has been on wet chemical depletion or fractionation of chlorogenic acid prior to protein extraction.

The approaches described below were designed to prevent irreversible binding of chlorogenic acid to proteins and thus to achieve fractionation and production of light-colored food ingredients:

• Using antioxidant substances. There are two possible reaction mechanisms: reaction with the o-quinones of chlorogenic acid to form colorless reaction products, and inhibition of polyphenol oxidase. Sodium bisulfite is special in that it acts via both mechanisms [14]. However, since sulfites are part of the big eight allergens, there is a health risk for allergy suffering individuals [15], which might limit their use in the production of food ingredients.
• Using synthetic preservatives that are non-toxic and highly effective even at low concentrations (0.01–0.02%). These can scavenge emerging chlorogenic acid radicals and form relatively stable products, which can then be removed. Two important antioxidants are 2,6-Di-tert-butyl-p-cresol (BHT) and tert-butyl-4-hydroxyanisole (BHA), the latter being a mixture of two isomers [16].
• Using acidifying agents. Here, the additive sodium hydrogen sulfate known as E 514 [17] and the chelating agent citric acid were used – according to Bau et al. (1983), citric acid also has an antioxidant effect and highlights thus a double potential to yield only lightly colored products by also inhibiting chlorogenic acid oxidation [18]. Furthermore, processing was performed in the absence of oxygen by gassing with nitrogen during extraction, to prevent the oxidation of polyphenols [19].

The resulting three sunflower fractions

After extraction of chlorogenic acid as described above, the proteins were alkaline extracted and either isoelectrically precipitated or concentrated by ultrafiltration. Drying of the remaining protein concentrate yielded the high-fiber food ingredient. For each experiment, all three fractions were characterized.

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Fig. 4 Chromatographic separation by HPLC of chlorogenic acid and further phenolic compounds of the polyphenol-rich fraction from sunflower seeds

The protein-rich fractions had protein contents of at least 84% after isoelectric precipitation and 87% after ultrafiltration. The studies on acidic pre-extraction of chlorogenic acid showed a complete functionalization of the protein fraction obtained by ultrafiltration, especially when sodium bisulfite was used for pre-extraction. Protein solubility could be increased by only 8% overall compared to the deoiled sunflower meal and was similar among all protein-rich fractions. This low solubility increase is attributed to the depletion of soluble albumins during acidic extraction. Besides, emulsifying capacity was raised from 350 mL oil/g protein in the deoiled meal to 390 mL/g after the production of a protein-rich fraction and reached values of 590 mL/g when sodium bisulfite was used for pre-extraction. This reflects the successful depletion of the chlorogenic acid and functionalization of the protein-rich sunflower fraction. This protein fraction was also particularly light-colored with an L* = 81. Upon a concentration of 12% w/v, all the protein-rich fractions proved suitable for the production of a cuttable gel.

While only traces of chlorogenic acid could be determined by liquid chromatography using HPLC in both the protein fraction and the dietary fiber rich fraction, the polyphenol-rich extracts had chlorogenic acid contents of up to 75%. The protein content here reached up to 24%. In addition to quantification of the polyphenols by HPLC (Fig. 4), the antioxidant potential of the samples was determined photometrically by the Folin-Ciocalteu method. The antioxidant potential was proportional to the chlorogenic acid concentration and thus high for the polyphenol-rich fractions.

The fractions rich in dietary fiber showed particularly high water but also oil binding capacities and were equally light-colored, with values of up to L* = 81.

Concluding remarks and outlook

It was found that adding sodium bisulfite during acidic pre-extraction of the chlorogenic acid in combination with citric acid to reduce the pH, nitrogen gassing during extraction, water degassing and sunflower meal vacuuming resulted in an exceptionally light-colored product with lower total polyphenol content. Moreover, this was the only sample obtained via acidic pre-extraction with preservatives that did not lead to green coloration during gel formation. Thus, the objective of fractionating the sunflower meal into a polyphenol-enriched fraction, a polyphenol-depleted, protein-rich fraction and a fiber-rich fraction was successfully achieved.

The protein fractions displayed particularly stable pudding-like gel properties, which can be attributed to the high protein content. Their cut resistance makes them suitable for use in vegan meat substitute products. The high water binding properties of the fiber fractions, which are rich in dietary fiber, make them particularly suitable to produce baked goods. The high antioxidant potential of the polyphenol-rich fractions allows them to be used as a biofunctional ingredient in functional food products to protect these products against so-called “free radicals”, to maintain their color or to protecting them from vitamin degradation. Moreover, preventing the interaction of chlorogenic acid with proteins allowed their beneficial properties to be used individually and specifically to produce high quality food products in the future.

Acknowledgment

The work presented in this article was carried out within the framework of the project “SmartProSun - Extraction and evaluation of functional proteins, dietary fibers and polyphenols from sunflower flour” funded by the German Federal Ministry of Education and Research in its funding program “BioEconomy International”, with Brazil as a research partner, funding code 031B0941, duration from 01.04.2020 to 31.03.2023.

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Infobox

Research project “SmartProSun”
Sustainable utilization of sunflowers

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The “SmartProSun” project is funded by the German Federal Ministry of Education and Research (BMBF), in its funding program “BioEconomy International”, with Brazil as a research partner.

The demand for plant proteins and vegetable oil for both human consumption and bioenergy production is increasing globally. In the “SmartProSun” project, which is coordinated by the Fraunhofer Institute for Process Engineering and Packaging, the German Federal Ministry of Education and Research (BMBF), with Brazil as a research partner, is funding the development of sustainable processes that enable the complete use of sunflower seeds to produce food and bioenergy, as part of the “BioEconomy International” project funding. By closing the water cycle, recycling and filtering the wastewater, the process developed by the Fraunhofer IVV minimizes the consumption of process water and the use of additives.

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Kategorie: Food Processing Technology | Alternative Protein Sources

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