Mulberry is rich in nutrients, approximately 0.5–1.4% protein and about 7.8–9% carbohydrates [17]. Mulberry contains neutral sugars such as arabinose, galactose, glucose, rhamnose, xylose, mannose and also contains a large amount of uronic acid, namely in the form of galacturonic acid and glucuronic acid [18,19,20,21]. The most abundant amino acid in mulberry is glutamate, which accounts for approximately 20%, followed by glycine and aspartate [22]. It also contains lysine, leucine, isoleucine, histidine, threonine, tryptophan and glycine, among others. Among these amino acids, leucine, threonine, isoleucine, glycine, threonine, valine, tryptophan, arginine, aspartic acid and serine are found in a higher content in white mulberry when compared with black mulberry. In contrast, the content of lysine, histidine, and proline are higher in black mulberry and lower in white mulberry [23]. The fat content of mulberry is extremely low, and linoleic acid, oleic acid, palmitic acid and stearic acid make up 69.66–78.02% of the total fatty acids [24]. The vitamins in mulberry are mainly vitamin C, vitamin A and some B groups [16]. The organic acids of mulberry are succinic acid, acetic acid, malic acid, citric acid and tartaric acid [3]. The content of titrable acid is 0.20–2.65%, and the content in black mulberry is higher than that in white mulberry [4,23,25,26,27]. The minerals in mulberry are potassium, calcium, phosphorus, sodium, zinc, copper and selenium. Studies have shown that the content of potassium in black mulberry is much higher than that in other fruits [13]. Soluble solids mainly include sugar, acid, vitamins and minerals, and their content can directly affect the taste of fruits and vegetables. The total soluble solid content of mulberry is 6.2–25.8% [4,23,25,26].

Mulberry contains many phenolic compounds, and Table 1 contains the results of the determination of total phenol and total flavonoids from recent years. In addition to the above ingredients, mulberry also contains alkaloid compounds (quinine, 1-deoxynojirimycin) and α-glucosidase inhibitors [28,29,30]. Different varieties of mulberry possess different chemical compositions and nutritional statuses, which are related to the climate, topography and soil conditions [7].

Polyphenols play an important role in promoting human health and are the most relevant family of phytochemicals [53].

Among them, flavonoids constitute a very wide range of groups and are distributed in a variety of vegetables and fruits. They have a common basic structure: C6-C3-C6, which usually forms an oxygen-containing heterocycle. Flavonoids are usually associated with sugars (glycosides), and therefore, they tend to be water-soluble.

Mulberries are especially rich in flavonoids, specifically anthocyanin [4,31,32,54,55]. Anthocyanin is the main active ingredient and chromogenic substance of mulberry, which is why mulberry is considered to be an important source of anthocyanin in the diet [33]. Its anthocyanin content is high, pigment is stable, can dissolve completely in water and has increased bioactive activity; therefore, it becomes a fruit pigment that cannot be replaced. Mulberry is an optimal source for extracting anthocyanin from pine bark, pine needles and grape seeds. Therefore, together with sea buckthorn, it is listed as a functional health food with the international development of the third generation of “fruit resources”. The most abundant anthocyanin in mulberry is cyanidin-3-glucoside (C3G), representing 53.94–78.23% of the total anthocyanins; cyanidin-3-rutinoside (C3R) accounts for 19–43.83%, and pelargonidin-3-glucoside (P3G) is measured in a proportion close to 5% [34,56,57,58,59,60]. The content of mulberry anthocyanins is shown in Table 2; white varieties do not contain any anthocyanins [24,25]. The content of anthocyanin in mulberry is related to mulberry variety, maturity, climate, soil, pruning of mulberry trees, pest control and other factors [3,7,61].

Mulberries contain phenolic acids (chlorogenic acid, gallic acid, protocatechuic acid, p-coumaric acid, O-coumaric acid, ferulic acid, caffeic acid, and vanillic acid), rutin, quercetin and resveratrol [3,15,56,66,67]. The main phenolic acids in mulberry are hydroxycinnamic acid derivatives [25]. Gallic acid and protocatechuic acid are the main derivatives of hydroxybenzoic acid in black mulberry [35,66]. The content of phenolic acids in mulberry is 0.02952–0.17564 mg/g fw [68]. Chlorogenic acid is the main phenolic component in black mulberry, while ruin is the dominant phenolic in white and red mulberry [3]. Chlorogenic acid is the main acid in the sugarless extract of black and white mulberry [36]. Gundogdu et al. described the concentration range of chloric acid in mulberry as 0.119–3.106 mg/g fw [3]. Butkhup et al. found that its concentration in white mulberry was 0.01 to 0.06 mg/g dw [35], and the concentration range of chlorogenic acid in white mulberry and black mulberry was reported by Sanchez-Salcedo et al. as 0.15–0.97 mg/g dw and 0.35–3.18 mg/g dw [13], respectively. The amount of chlorogenic acid in black mulberry was higher than that in white mulberry. As mulberry matures, the amount of chlorogenic acid and its isomers gradually decreases [69]. Song et al. detected resveratrol in 38 mulberry varieties from China at 0.0021–0.0053 mg/g [52]. Chon et al. found a significant difference in the total phenol content measured by different solvents [37]. The content of total phenolic compounds in mulberry is shown in Table 1, with black mulberry containing a higher amount than white mulberry [12,38].