The statement ‘Let thy food be thy medicine and medicine be thy food’, often ascribed to Hippocrates (400 BC), shows the early recognition of health benefits of healthy diets. The concept of functional food originated in Japan in the early 1980s to refer to processed foods that possess disease-preventing and/or therapeutic properties beyond their basic nutritional roles [11,13,14]. This concept often overlaps with nutraceuticals, which was coined by Stephen DeFelice to emphasize the complementarity between nutrition and pharmacology [13,15]. Sumac fruit has been recognized as a functional food due to the vast proportion of key nutritionally and pharmacologically important constituents such as proteins, essential oils, fatty acids, fiber, minerals, tannins, phenolic acids and anthocyanins [16]. Sumac plant (Rhus coriaria) is considered a rich and valuable dietary source of important nutrients which include unsaturated fatty acids, vitamins and minerals. Studies have shown estimates of the overall composition of sumac fruit as illustrated in Table 1 and Table 2.

In a study, Kizil and Turk [12] detected oleic acid (37.7%), linoleic (34.8), palmitic (27.4%), stearic acids (17.3%) and other fatty acids from the petroleum ether extract of the fruit of sumac through GC-MS analysis, indicating the content of unsaturated fatty acids. Analyses of some sumac samples from Iran detected a total of 21 oil compounds representing 86.6% of the fatty acids present in the fruit samples [18]. Proximate analysis of different types of Sumac fruits in Turkey revealed 12.5% oil and 3.5% protein. In this study, Dogan and Akgul [18] further revealed the contents of important fatty acids which include: myristic acid (0.25%), palmitic acid (23.1%), stearic acid (3.1%), and oleic acid (37.5%). Interestingly, the study reported linoleic (omega 6) (34.84%) and α-linolenic acid (omega 3) (1.88%). Alsamri, Athamneh [7] reviewed that, sumac fruit contains important minerals including potassium, calcium, magnesium, phosphorus, iron, sodium, manganese, and copper. The mineral contents were found to be influenced by the prevailing environmental factors and geographic locations [12]. Although, studies on the vitamin composition of Rhus coriaria are still fragmentary. A study revealed that, sumac fruit is rich in pyridoxine, ascorbic acid, thiamine and riboflavin [19]. Other vitamins reported include cyanocobalamin, nicotinamide, and biotin [7].

Most studies on sumac have revealed similar phytochemical composition, with polyphenols such as tannins, flavonoids and conjugated phenolics as major constituents.
The term phenolics refer to phytochemicals comprising aromatic ring with at least one hydroxyl group. Such compounds with one or more aromatic rings possessing more than one hydroxyl groups are referred to as polyphenols (or polyphenolic compounds). These compounds are basically categorized into flavonoids and non-flavonoid compounds. Flavonoids, the dominant class of plant-derived polyphenols comprise two phenolic rings connected by a three-carbon bridge with a common C6-C3-C6 structural skeleton  [20]. Non-flavonoids include phenolic acid derivatives, stilbenes, tannins and lignins [21]. In recent decades, tannin-rich plants extracts are becoming popular as additives in ruminant diets to improve the quality of animal products. Plant-derived tannins can be classified into hydrolyzable tannins and condensed tannins. Hydrolyzable tannins contain polyphenol nuclei having molecular weights from 500 to 3000 Daltons (Da), while condensed tannins are oligomeric or polymeric flavonoids comprising flavane-3-ols, including catechin, epicatechin, gallocatechin, and epigallocatechin with molecular weights ranging from 1000 to 20,000 Da [22]. The condensed tannins are not easily degraded by anaerobic enzymes and can depolymerize only with strong oxidation. Several reports which have documented the bioactive components of sumac have revealed hydrolysable tannins as the most prominent group of compounds in the Sumac fruits, followed by flavonoids and phenolic acids [23]. In their study, Abu-Reidah, Ali-Shtayeh [24] detected 211 compounds from methanol extract of sumac fruit samples, with majority belonging to phenolics compounds through an extensive and sophisticated chromatographic analysis. A similar study that employed methanol extract of sumac fruit samples, detected 191 compounds comprising 78 hydrolysed tannins, 59 flavonoids, 9 anthocyanins, and 40 other compounds such as butein [25]. In the case of sumac leaf, analysis of ethereal leaf extract using coloring method identified myricetin alongside gallic acid as the coloring agent of sumac leaves [7,26]. The flavonols quercetin, myricetin, and kaempferol alongside gallic acid, methyl gallate, m-digallic acid, and ellagic acid were identified from ethyl acetate and methanol extracts of sumac leaf samples [7]. Other compounds such as organic acids which
include malic, citric, fumaric, and tartaric were also detected from the aqueous extract. Most of the compounds have been categorized into hydrolysed tannins, phenolic acids, conjugated phenolic acids, organic acids, anthocyanins, flavonoids, coumarins, xanthones, terpenoids, steroids, and essential oils.

Conclusion

Sumac is a versatile food spice with a wide spectrum of possible uses in human nutrition, pharmaceutical industries and food industries. Various studies have shown the functionality and therapeutic effects of extracts and fractions from this plant which could be attributed to the constituent bioactive compounds. Such plant extracts
could be exploited in the search for bioactive natural products that may be useful in the developing new drugs. Polyphenol-rich sumac extracts and its constituent polyphenolic bioactive ingredients could be further exploited towards developing new food products which do not only address consumers’ interests regarding organoleptic, nutritional value and techno-functionality of food, but also meet the growing need for ‘clean label’ as well as value addition with respect to antioxidant potentials, disease prevention, and health promotion in humans. Utilization of sumac in the food industry is increasingly gaining prominence not only as flavor enhancers but also as natural preservatives to extend the shelf-life of the products. More possibilities of sumac extracts could still be explored and exploited on as a preservative and tenderizer for several products like meat and fish. Moreover, polyphenol-rich sumac extracts hold a great potential as natural antioxidant in the production of high-fat foods which are prone to oxidative deterioration during processing. Therefore, polyphenol-rich sumac extracts and the bioactive ingredients could be exploited towards developing new food products which do not only address the growing consumers’ interests regarding organoleptic, nutritional value and techno-functionality of food, but also meet the growing need for ‘clean label’ as well as value-addition with respect to antioxidant potentials, disease prevention, and health promotion in humans.