Tannins of Aronia

by Lilian

The genus Aronia is represented by two species Aronia melanocarpa (Michx.) Elliot (black chokeberry) and Aronia arbutifolia (L.) Pers. known as red chokeberry. Aronia melanocarpa is the predominant commercial chokeberry cultivar and has gained increased popularity due to its assumed health-promoting effects, which have been reviewed by Sidor et al. (2019) and Kokotkiewicz et al. (2010) together with the berries pharmacologically relevant constituents. Chokeberries are very valuable as a food ingredient and are used in the food industry mainly for the production of juice, jam and wine, and as a natural colorant.

The fruits have an astringent taste due to a high tannin content (Wu et al., 2004). They mainly contain polymeric proanthocyanidins (DP > 10) which account for about 66% to 82% of total polyphenolic compounds (Oszmiański and Wojdylo, 2005, Denev at al., 2018; Wu et al., 2004). Even higher amounts are present in smaller fruits (Wangensteen et al., 2014) and chokeberry leaves, for which 22% higher levels of proanthocyanidins were reported when compared to ripe fruits (Teleszko and Wojdylo, 2015). According to literature, their levels in fruits vary from 522 mg/100 g FW to 3671mg/100 g FW (Denev et al., 2018; Wu et al., 2004; Taheri et al., 2013), however, no significant differences were reported between black, red and purple (Aronia prunifolia) coloured chokeberry fruits (Taheri et al., 2013). Proanthocyanidins predominated in the berry flesh (70%) followed by the skin (25%) and kernels (5%) (Mayer-Miebach et al., 2012).

An increasing trend in their content was observed with prolonged harvest time (Poyraz Engin and Mert, 2020). Chokeberry proanthocyanidins have been identified exclusively as procyanidin B-type, containing epicatechin as the main monomer unit (Oszmiański and Wojdylo, 2005). Cultivated and wild A. melanocarpa fruits have a similar oligomeric proanthocyanidin composition (Sueiro et al., 2006). Different chokeberry varieties contain 80-95% extractable proanthocyanidins (Hellström et al., 2009; Taheri et al., 2013), while ellagitannins have not been detected (Kähkönen et al., 2001). In contrast, the chokeberry leaves,
which are not used as functional food, are characterized by high proportions of flavanols (Teleszko and Wojdylo, 2015). The total proanthocyanidin content ranges from approximately 1408-1579 mg/100 g DW for chokeberry juice to 8192-9586 mg/100 g in pomace (Oszmiański and Wojdylo, 2005; Rodríguez-Werner et al., 2019) and depends on genetic attributes, harvest date, cultivation location and practice, processing and storage. Their levels remain stable upon blanching, and then increase by 11% after enzyme treatment, probably due to the disruption of cell wall polysaccharides and proteins to which polymeric procyanidins are bound. The higher losses of about 40% of proanthocyanidins occurred during the pressing operation after which the majority of them remains in the pomace (Mayer-Miebach et al., 2012). Oszmiański and Lachowicz (2016) reported that the content of procyanidin polymers in juices prepared from crushed fruits before processing was higher by over 62% higher than in juices prepared from non-crushed berries. Juices stored for 6 months at 25 °C, retained more than 90% of the total proanthocyanidins (Wilkes et al., 2014). They are quite stable as no degradation was noticed after heating purees up to 100°C for 20 min (Mayer-Miebach et al., 2012

Berry fruits are important sources of tannins. Their content and chemical composition depend on the species, variety, cultivation practice, and treatment before and after harvest. Ellagitannins are found in strawberries and raspberries, but are less common in other berry fruits. The major class of tannins in blueberries and chokeberries are proanthocyanidins, while strawberries are characterized by both ellagitannins and proanthocyanidins. Chokeberries are characterized with the highest content of condensed tannins among 100 plant foods investigated. All these berries can be consumed fresh or processed into purees, juices, syrups, and jams, due to their short shelf life. They also can be preserved by deep freezing or by different drying techniques. Currently, cold storage or freeze-drying is the most effective strategy to preserve the colour and polyphenol content in berries and their products. Tannins are lost during processing to varying degrees, depending on the production technology. In general, processes comprised of more steps (e.g. juice production) result in the greatest losses. As large amounts of bioactive compounds are annually discharged in food by-products, challenges exist to improve the most critical steps and to retain these compounds in berry products. During processing and storage of berry products, the tannin composition is altered.
What exactly happens to the various compounds belonging to the class of tannins during these processes is poorly
understood and requires further consideration.

SOURCE: Agricultura 17: No 1-2: 27-36 (2020)

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