What Are the Active Ingredients in Apple Extract Powder?

The Malus pumila Mill. plant of the Rosaceae family is native to Europe and Central Asia. It has a long history of cultivation and is grown in temperate regions around the world. In China, it is commonly cultivated in Liaoning, Hebei, Shanxi, Shandong, Shaanxi, Gansu, Sichuan, Yunnan and Tibet [1]. The Classic of the Famous Physicians records that the fruit of the apple is used in medicine [2]. The apple is therefore a fruit that can be used for both medicinal and food purposes, and has high nutritional and economic value. This paper summarizes the research progress of the chemical composition, pharmacological activity and dynamic changes of apples through literature searches at home and abroad, with a view to providing a reference for the rational development and utilization of apple resources. 

1 Active ingredients in apple extract

Apple extract powder contains a large amount of bioactive substances, and its chemical composition mainly includes flavonoids, phenolic acids and triterpenoids.

1.1 Flavonoids

Apples contain a large number of flavonoids, including flavones, flavonols, flavanols, anthocyanins and chalcones. So far, 35 flavonoid chemical components have been identified in apples: catechin[3]; epicatechin[3]; proanthocyanidin B1[3]; proanthocyanidin B2[3]; procyanidin B5[3] ; procyanidin C1[3] ; procyanidin tetramer[3] ; procyanidin pentamer[3] ; procyanidin hexamer[3] ; procyanidin heptamer[3] ; procyanidin octamer[3] ; root bark glycoside[3-6] ; root bark extract[3-6] ; quercetin[3-5] ; 3,5,7,3',4'-pentahydroxyflavanol-3-o-xyloside[3] ; 3,5,7,3',4'-pentahydroxyflavone-3-o-rhamnoside[3]; 3,5,7,3',4'-pentahydroxyflavone-3-o-galactoside[3]; 3,5,7,3',4'-pentahydroxyflavone-3-o-glucoside[3]; 3,5,7,3',4'-pentahydroxyflavone-3-o-arabinoside[3]; 3,5,7,3',4'-pentahydroxyflavone-3-o-rutinoside[3] ; anthocyanin[3] ; ideain ( cyanidin-3- galactoside) [3] ; 8-methoxyluteolin-7-o-rhamnoside[4] ; 6-methoxyquercetin [4-5]; quercetin-3-o-furanarabinoside [4];  genistein-6-o-β-D-glucopyranoside [4]; quercetin-3-o-D-xylose [4]; 6-methoxyluteolin-3-o-rhamnoside[5] ; 3-hydroxyquercetin[6] ; quercetin-2'-xylosylglucoside[6] ; 3-hydroxyquercetin-2'-xylosylglucoside[6] ; quercetin-2'-xyloside[6] ; kaempferol[6] ; quercitrin[6] ; quercetin-3-o-β-D-galactopyranoside[6].

1.2 Phenolic acids

Twenty-one phenolic acids have been identified in apple extract: p-coumaroylquinic acid[3], quinic acid[3], chlorogenic acid[3], caffeic acid[3,6], ferulic acid[3], malonic acid[3] ; maleic acid[3] ; p-coumaric acid[3,6] ; cinnamic acid[3,5] ; 2(R)-hydroxybutanedioic acid[3] ; 2(R)-hydroxybutanedioic acid 1-methyl ester[3] ; erlod- ictyol[4] ; methyl p-protocatechuate[4] ; 3,4-dihydroxybenzoic acid ethyl ester[4] ; methyl protocatechuate[5] ; 5-o-p-coumaroylquinic acid[6] ; 4-o- p-coumaroylquinic acid[6] ; 4-o-p-coumaroylquinic acid methyl ester[6] ; 1-o-p-coumaroylquinic acid methyl ester[6] ; 1-o-caffeoylquinic acid methyl ester[6] ; 1-o-caffeoylquinic acid ethyl ester[6] ; 3,5-di-o-caffeoylquinic acid[6].

1.3 Triterpenoids

Research on the triterpenoids in apples has focused mainly on the apple peel, from which 24 triterpenoid chemical components have been found, mainly pentacyclic triterpenoids: ursolic acid[7]; 2α-hydroxyursolic acid[7]; 2α-hydroxy-3β-{[(2E)-3-phenyl-1-oxo-2-propenyl]oxy}urs-12-en-28-oic acid[7] ; 3β-trans-cinnamoyloxy-2α-hydroxyurs-12-en-28-oic acid[7] ; 3β-trans-p-coumaroyloxy- 2α-hydroxyurs-12-en-28-oic acid[7] ; 3β-cis-p-coumaroyloxy- 2α-hydroxyurs-12-en-28-oic acid[7] ; maslinic acid[7] ; 2α-hydroxy-3β-{[(2E) -3-phenyl-1-oxo-2-propenyl]oxy } -olean-12-en-28-oic acid[7] ; 2α- hydroxy-3β-{[(2Z ) -3-phenyl-1-oxo-2-propenyl]oxy }  olean-12-en-28-oic  acid[7] ; 3β-trans-cinnamoyloxy-2α- hydroxyolean-12-en-28-oic     acid[7] ;  3β-trans-p-cou- maroyloxy-2α-hydroxyolean-12-en-28-oic   acid[7] ;  3β- cis-p-coumaroyloxy-2α-hydroxyolean-12-en-28-oic    acid[7] ;  3β , 13β-dihydroxyurs-11-en-28-oic acid[7] ; 2α , 3β , 13β-trihydroxyurs-11-en-28-oic acid[7] ; 3β , 28-di- hydroxy-12-ursene[7] ;  olean-12-en-2α , 3β-diol[8] ; olean-12-en-3β-ol (β-amyrin)[8] ; olean-12-en-2α , 3β, 28-triol[8] ; urs-12-ene-2α , 3β-diol[8] ; urs-12-ene-3β-ol[8] ; urs-12-ene-2α, 3β, 28-triol[8] ; 3β-trans-p-cou- maroyloxy-2α, 3β,   13β-trihydroxy-urs-11-en-28-oic acid[8] ; 3β-cis-p-coumaroyloxy-2α, 3β, 13β-trihydroxy-urs-11-en-28-oic acid[8] ; urs-12-en-28-ol[8].

1.4 Other types of ingredients

In addition to flavonoids and phenolic acids, Li Zhi-hua [6] used an activity tracking method to isolate five flavonoid derivatives from apples: Cinchonain Ia; Rhinchoin Ia; Cinchonain Ib; Rhinchoin Id; and Cinchonain Id.

2 Pharmacological activity

Apples contain a variety of biologically active ingredients. These biologically active ingredients determine the various pharmacological activities of apples, which are mainly antioxidant, anti-aging, anticancer, antibacterial, hypoglycemic and liver protective.

2.1 Antioxidant and anti-aging effects

Apples contain a large amount of polyphenolic compounds. Polyphenolic compounds have the effect of scavenging free radicals, and the antioxidant and anti-aging effects of apples are related to polyphenolic compounds.

Kelly et al. [9] studied the total polyphenols and antioxidant activity of four apple varieties and found that the total polyphenol content in the apple peel was higher than that in the apple flesh, and it had better antioxidant activity.

Bai Xuelian [10] extracted apple polyphenols from apple pomace using 10 solvents and found that the extraction yield of polyphenols was high for highly polar solvents, and the scavenging effect on DPPH free radicals and hydroxyl free radicals was also better.

2.2 Antitumor activity

Flavonoids and isoflavonoids in apples have strong anti-cancer effects.

Daiki et al. [11] studied the effect of apple polyphenol extract on the proliferation and invasion of the rat ascites hepatoma cell line AH109A. The results showed that apple polyphenol extract inhibited the proliferation and invasion of liver cancer in a dose-dependent manner, significantly reduced the growth and metastasis of transplanted tumor rats, significantly reduced the serum lipid peroxide levels of transplanted rats, and reduced the serum very low-density lipoprotein (VLDL) -cholesterol levels. The results of in vitro and in vivo experiments all show that apple polyphenols have a good anti-liver cancer effect. Yang et al. [12] studied the anti-tumor effect of quercetin in apples and found that quercetin can enhance the anti-cancer effect of paclitaxel.

Reagan-Shaw et al. [13] studied the apple peel extract of Gala apples. The apple peel extract had an inhibitory effect on a variety of cancer cell lines, significantly reducing the growth and clonal survival of human prostate cancer cells CWR22Rv1 and DU 145 and breast cancer cells Mcf-7 and Mcf-7 / Her18.

2.3 Antibacterial effect

Apple polyphenols have good antibacterial activity against a variety of bacteria, including Staphylococcus aureus, Agrobacterium, Bacillus cereus, Bacillus subtilis SHL45, Escherichia coli, Pseudomonas fluorescens SHL5, Pseudomonas fluorescens SHL7, Bacillus subtilis, etc.

Fattouch et al. [14] reported that apple phenolic compounds can effectively inhibit the growth of Staphylococcus aureus, Agrobacterium, and Bacillus cereus.

Sun Aidong et al. [15] used the Oxford cup method to study the antibacterial effect of apple polyphenol extract. The results showed that apple polyphenol extract had a strong inhibitory effect on bacteria, and its minimum inhibitory concentration (MIC) for test bacteria such as Bacillus, Escherichia coli, and Pseudomonas was 0.1%. The antibacterial activity of apple polyphenol extract has good thermal stability, and its antibacterial effect is best under environmental conditions of pH = 5–6 and less than 0.3 mol/L inorganic salt.

Sun Aidong et al. [16] also used the double-layer plate perforation method to study the inhibitory effect of apple polyphenols at different mass concentrations (2 g/L, 6 g/L, 10 g/L) on 6 bacteria, 3 yeasts and 3 molds. For the selected test strains, 0.3 g/L, 0.4 g/L, 0.5 g/L, 0.6 g/L, 0.7 g/L, 0.8 g/L, 1.0 g/L, and 2.0 g/L. The lowest inhibitory concentration within the tested concentration range was tested. The results showed that the minimum inhibitory concentrations of apple polyphenols on the test strains of Bacillus subtilis SHL45, Escherichia coli, Pseudomonas fluorescens SHL5, Pseudomonas fluorescens SHL7, Bacillus subtilis and Staphylococcus aureus were 0.6 g/L, 0.7 g/L, 0.6 g/L, 0.6 g/L, 0.8 g/L and 0.5 g/L, respectively, and had a significant inhibitory effect on the six test strains.

Hao Shaoli et al. [17] used the filter paper disc method to study the antibacterial activity of apple polyphenols. The results showed that apple pomace polyphenols had a strong inhibitory effect on bacteria, but had no significant inhibitory effect on the test molds and yeasts. This result is consistent with the results of Sun Aidong et al.

Liao Chunli et al. [18] used the filter paper disc method to study the antibacterial activity of apple polyphenols. The results showed that apple polyphenols had a significant inhibitory effect on three test strains of Escherichia coli, Staphylococcus aureus and Bacillus subtilis, and the lowest inhibitory concentrations in the test concentration range were 0.6 g/L, 0.7 g/L and 0.8 g/L, respectively. This result further demonstrates that apple polyphenols have good antibacterial activity.

2.4 Hypoglycemic effect

Various active substances in apple extracts, such as pectin, have good hypoglycemic activity.

Adyanthaya et al. [19] reported that apple polyphenols can regulate postprandial blood glucose by inhibiting the activity of glucosidase. Pectin, which is mainly found in apple branches, leaves and peel, has a good hypoglycemic effect. Pectinin binds to the co-transport of glucose and Na+ and inhibits the expression of the glucose transporter gene Glut2, thereby reducing blood glucose levels [20].

Li Zhihua [6] used an activity tracking method to determine the active site of apple polyphenol extract by testing a diabetic mouse model, from which 25 compounds were isolated. The isolated compounds were tested for islet cell proliferation, and it was found that the compounds had an effect on islet cell proliferation, with coumaric acid, 1-o-caffeoylquinic acid methyl ester, 3-hydroxyquercetin, 3-hydroxyquercetin-2-xylosylglucoside, quercetin-2-xylosylgalactoside, root bark has a proliferative effect on islet cells, and 3-hydroxyroot bark-2-xylosyl glucoside and root bark-2-xylosyl galactoside have a stronger proliferative effect on islet cells.

2.5 Hepatoprotective effect

Li Yan [21] studied the hepatoprotective effect of apple polyphenols using various chemical liver injury models. The results showed that at doses of 200 mg/kg, 400 mg/kg and 800 mg/kg, apple polyphenols significantly improved the pathological changes in liver tissue caused by CCl4, D-galactosamine, ethanol and cisplatin. Apple polyphenols at a dose of 800 mg/kg significantly increased the relative liver weight and spleen index of mice undergoing partial hepatectomy, while a dose of 400 mg/kg significantly increased the relative liver weight. Both 400 mg/kg and 800 mg/kg significantly increased the bile secretion of normal mice. This indicates that apple polyphenols have a significant protective effect on various liver damage models and have a choleretic effect. The mechanism of their hepatoprotective effect may be related to scavenging free radicals, inhibiting lipid peroxidation, improving the body's antioxidant capacity, promoting the repair and regeneration of liver cells, and protecting the function of liver cell membranes and mitochondria.

Some studies have shown [22] that apple total polysaccharides and total flavonoids also have a protective effect on liver damage in mice.

2.6 Cardiovascular protective effect

Studies have shown that the combination of flavonoids and nitrates can enhance the production of nitric oxide in the stomach, but their combined effect in the circulatory system is not yet clear. Therefore, Bondonno et al. [23] studied the independent and additive effects of flavonoid-rich apples and nitrate-rich spinach on nitric oxide levels, endothelial function and blood pressure. The results showed that the apple-spinach combination had no additive or synergistic effect on nitric oxide levels, endothelial function and blood pressure. Flavonoid-rich apples and nitrate-rich spinach can independently increase nitric oxide levels, enhance endothelial cell function, and significantly lower blood pressure. This indicates that apples have a certain cardiovascular protective effect.

2.7 Other effects

Apple extract also has anti-influenza virus and lead-promoting effects [24-25].

Li Runfeng et al. [24] used a model of cell damage (CPE) caused by influenza A virus infection of dog kidney cells (MDCK) to evaluate the inhibitory effect of apple polyphenols on influenza virus replication in vitro and in vivo. The results showed that apple polyphenols have a significant anti-influenza effect in vitro and in vivo.

Ai Zhi-lu et al. [25] used a lead-contaminated mouse model to study the lead-promoting effect of apple polyphenols. The results showed that after lead-contaminated mice were treated with apple polyphenols, the blood lead and lead content in the femur and liver were significantly lower than those in the positive control group, while the urinary lead was significantly higher than that in the positive control group. This indicates that apple polyphenols have a good lead-promoting effect.

3 Results of dynamic change studies

The content of bioactive substances varies according to the variety of apple, the different parts of the same variety, and the growth process of the apple. Some studies have shown that the phenolic substances in the peel are higher than those in the flesh, and 46% of the apple polyphenols are found in the peel. The main polyphenols in ripe apples are catechins, proanthocyanidins and chlorogenic acids, while immature apples contain more dihydrochalcones and flavonol compounds. The synthesis of proanthocyanidins in apples continues to increase throughout the growth and development period. The synthesis of many polyphenols begins 40 days after flowering, and the content of flavonoids shows a significant downward trend within a range of about 35 to 100 days after flowering [2].

Zhao Yanmin et al. [26] studied the changes in the content of rutin and quercetin in apple branches and leaves. It was found that both apple branches and leaves contain rutin, but the characteristics of the changes are different: apple leaves contain quercetin, while apple branches were not detected.

4 Conclusion

At present, research on the active ingredients in apple extract powder mainly focuses on the peel, leaves, roots, branches and flesh, with relatively little research on the chemical composition of apple flowers. The chemical composition of apple flowers is dominated by flavonoids. Our research group previously studied the chemical composition of apple flowers and isolated and identified nine compounds: pyracanthoside, kaempferol, kaempferol-7-o-β-D-glucopyranoside, kaempferol-3-o-β-D-glucopyranoside, kaempferol-3-o-α-L-arabinopyranosiduronic acid. The contents of root bark glycoside, astragaloside and afzelin in apple flowers were simultaneously determined using ionic liquid microextraction-high performance liquid chromatography. After the addition of ionic liquid, the total extraction rate of root bark glycoside, astragaloside and afzelin in apple flowers can reach 25.4%. The effect of apple flower extract on tyrosinase activity was studied, and it was found that methanol extract of apple flower had a certain activating effect on tyrosinase, while methanol ionic liquid extract had a certain inhibitory effect on tyrosinase [27].

Yin Yizhen et al. [28] used high-performance liquid chromatography to study the fingerprints of apple flowers from different origins and identified 12 common peaks. By comparing with the reference substance, three peaks were identified: quercetin, kaempferol and kaempferol.

Dynamic studies on apple flowers have mainly focused on the dynamic changes in amino acids and proteins, and there have been no reports on the research of secondary metabolites in apple flowers. Apple flowers have similar chemical compositions as the fruit, and can also be used as a flower tea, so research on apple flowers can be increased to enrich the study of apples, with a view to making fuller use of apple resources.

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