Phenolic compounds are secondary metabolites with an important antioxidant activity that can be classified as extractable polyphenols (EP) and non-extractable polyphenols (NEP). EP is easily extracted with solvents, whereas NEP need acidic or basis hydrolysis to be broken, so they can be extracted and quantified in vitro.
1.1. Persimmon Fruit
Martínez-Las Heras et al.
[1] analyzed the content of EP and the antioxidant capacity of the fruit, leaves, and fiber extracted from
Rojo Brillante persimmon during its in vitro digestion. The results showed that the oral phase and the presence of α-amylase were the factors that most affected the reduction in the recovery index of the EP, especially in the leaves of persimmon. An increase in the recovery index of EP was observed in persimmon fruit and fibers during the small intestine phase. This could be because of the extended period of this phase (>2 h) and the effect of intestinal enzymes and bile salts, which could facilitate the release of polyphenols from the persimmon matrix. However, antioxidant capacity during digestion resulted in total losses at the end of digestion in leaves, persimmon fruit, and fibers. The bioaccessibility of the EP in the persimmon fiber was higher than in the fruit and persimmon leaves. Moreover, the bioaccessibility of the total antioxidant capacity was lower than those of EP and never exceeded 40%. They concluded that the EP and total antioxidant capacity of the aqueous extract of persimmon leaf were more sensitive to the gastrointestinal environment than those derived from persimmon fruits or fibers. Although the bioaccessibility of the total antioxidant compounds in the persimmon fruit and the fiber was greater than in the aqueous extract of persimmon leaves, an infusion with persimmon leaf (1.5 g in 110 mL of water) and persimmon fruit (200 g) would provide similar bioaccessible antioxidants at the end of digestion.
Zhu et al.
[2] analyzed the inhibitory effect of tannins extracted from persimmon
Niuxin on pancreatic lipase. This critical enzyme is associated with hyperlipidemia and obesity. The results showed the tannins extracted from persimmon had a high affinity for pancreatic lipase and inhibited the activity of this enzyme; the interaction was spontaneous through non-covalent bonds. Therefore, the binding and inhibition capacity of persimmon tannins on lipid digestive enzymes may have effectiveness for the treatment and prevention of obesity.
Li et al.
[3] showed the effect of the tannins extracted from the fresh
GongChengYueShi persimmon on the digestibility of corn starch and on the activity of α-amylase and α-glucosidase; two of the main digestive enzymes involved in the hydrolysis of the starch. The digestibility of starch decreased with the addition of tannins from persimmon; the higher the concentration of added persimmon tannins, the greater the inhibition of starch digestibility. Moreover, the results showed persimmon tannins interacted with the starch, interacting with amylose more than amylopectin. However, tannins exerted a great inhibitory effect on α-amylase and α-glucosidase. Generally, data suggest that tannins in persimmon may help reduce postprandial hyperglycemia, regulating glucose levels in the human body.
Lee et al.
[4] evaluated the polymers and oligomers of proanthocyanidins from persimmon peel against diabetes. The oligomeric proanthocyanidins exerted a higher interaction with α-glucosidase and the polymeric proanthocyanidins had greater interaction with α-amylase. This suggests that the inhibition of both enzymes depends on the degree of polymerization of the phenols. Therefore, persimmon peel could have an antidiabetic action.
In a study conducted by Zhou et al.
[5], the total content of EP and NEP present in
Mopan persimmon was evaluated. The action against α-glucosidase of EP and NEP and their antioxidant capacity were also compared. In addition, the release of EP from NEP was studied throughout in vitro digestion, considering the oral, gastric, and small intestinal phases. The results showed the NEP content in fresh persimmon was higher than EP. The EP had a greater inhibition capacity of α-glucosidase than the NEP. This could be because EP comprised low molecular weight molecules, which can be more easily attached and have a greater ability to interact with the enzyme. Both had a higher α-glucosidase inhibition capacity than acarbose—an oligosaccharide used as a drug reducing the speed of carbohydrate digestion. After in vitro digestion of NEP, the polyphenols content and the antioxidant capacity were lower in the oral phase and significantly increased in the gastric and intestinal phases. The acidic conditions of the stomach environment can enhance the release of NEP. Therefore, NEP was released after simulated gastrointestinal digestion, and the gastric phase played a key role in their release. Thus, NEP was the most effective antioxidant in persimmon fruit after digestion. Nevertheless, Zhou et al.
[5] suggested further investigations to explain the composition and structure of EP and NEP to clarify their differences in biological activities.
1.2. Persimmon Derived Products
Several authors have studied the in vitro digestibility of polyphenols in products such as persimmon flour, spaghetti, and pork liver pâté enriched with persimmon flour, persimmon peels, and dehydrated persimmon. Lucas-González et al.
[6] evaluated the recovery index, bioaccessibility of EP, and total soluble flavonoids during the in vitro digestion of persimmon flour derived from the
Rojo Brillante and
Triumph varieties. The recovery index of the EP in both varieties was similar, except in the oral phase, where
Triumph showed lower values, and the recovery index of total soluble flavonoids was higher in the
Rojo Brillante variety. These could be related to the different compositions of the persimmon varieties (total dietary fiber is higher in the
Triumph variety) and the interaction of phenolic compounds with α-amylase. The highest recovery index was obtained after the gastric phase and the lowest after the small intestine phase in both varieties. This could be explained by the different pH in the digestion phases. At the acidic pH present in the stomach, phenols are often found in a very stable chemical form—the Flavylium cation. Therefore, the bioaccessibility in the stomach could be higher. However, this bioaccessibility drastically decreases in the intestine because of interactions with dietary compounds such as fiber, chemical reactions of oxidation, and polymerization, or molecular changes because of enzyme action. Regarding the bioaccessibility of EP, the results indicated both flours presented similar values.
Triumph presented greater bioaccessibility of soluble flavonoids than the
Rojo Brillante variety, probably because of the different flavonoid profiles of the persimmon flours. Therefore, the different fiber content, the interaction of α-amylase with polyphenols, the pH during the gastric and intestinal phase, and the total content of polyphenols and flavonoids in the persimmon samples are the main factors affecting the behavior of these bioactive compounds during digestion
[6].
In another study, Lucas-González et al.
[7] formulated spaghetti with 3% and 6% of the persimmon flours obtained from the
Rojo Brillante and
Triumph varieties. The profile of EP and NEP and their bioaccessibility and antioxidant capacity after simulated in vitro digestion were determined. Spaghetti enriched with persimmon flours modified the polyphenolic profile with the appearance of two new compounds, gallic acid and p-coumaric-o-hexoside, increasing the antioxidant capacity. After in vitro digestion, numerous polyphenols remained bound to the cell wall or to indigestible polysaccharides. The EP bioaccessibility determined in the small intestine phase was poor and did not improve with the addition of persimmon flours; many EP could become part of the NEP. The authors concluded that although most NEP did not release from the food matrix during gastrointestinal digestion, they may still have a health-promoting effect as they could be available in the colon
[7].
Furthermore, Lucas-González et al.
[8] enriched pork liver pâté with 3% and 6% of
Rojo Brillante persimmon flour. In both samples of enriched pâté, 2 EP and 21 NEP were detected, provided by the persimmon flour. After in vitro digestion of the pâté samples, EP and NEP were evaluated. The in vitro digestion consisted of the oral, gastric, and two small intestine phases; one with pancreatin high lipase activity (C1) and the other with pancreatine low lipase activity (C2). More NEP than EP was detected in all digestion stages. In addition, it was observed that the intestinal phase C1 was more suitable to recover NEP after digestion than the intestinal phase C2. This could be associated with a greater release of fatty acids in the digestive environment, which could have a protective effect on polyphenols by interacting with them. However, the observed polyphenols were NEP, which were not released. Therefore, they probably reached the colon intact, and some could be metabolized by the intestinal microbiome. Lucas-González et al.
[8] concluded that high-fat foods such as pâté are excellent vehicles for preserving NEP, which could reach the colon intact and be metabolized by the intestinal microbiome. However, more studies are needed on lipid digestibility, colonic fermentation, and polyphenol transformations to achieve the complete health implications of fortifying meat products with persimmon flours.
Liu et al.
[9] selected optimal deastringency methods and evaluated the bioaccessibility of polyphenols in
Yongding persimmon peels treated with 12 combinations of CO
2 and ethanol. EP and NEP content after the deastringency treatments as well as the antioxidant capacity and bioaccessibility of the EP after in vitro digestion, were determined. The results indicated that the EP content decreased, and the NEP content increased with increasing ethanol and CO
2 concentration in the non-digested samples. After in vitro digestion, there was also a significant decrease in the EP and antioxidant capacity of persimmon peels. This could be related to a higher NEP formation after the digestion process. The ethanol (30%) and CO
2 vapor (70%) methods were the most effective, with the highest bioaccessibility of EP. Therefore, it could be considered the best deastringency method.
Recent studies investigated the recovery index of EP, soluble flavonoids, and antioxidant capacity of persimmon affected using drying methods—ultrasound-assisted vacuum drying (USV), freeze-drying (FD), infrared drying (ID), and hot air drying (HAD)
[10]. The results showed that the USV, ID, and HAD led to a significant increase in the bioaccessibility of EP, soluble flavonoids, and antioxidant capacity compared to fresh persimmon. The samples obtained by FD did not show significant differences in the recovery index with respect to fresh persimmon. The recovery index increment was produced because of the heat treatment during the drying process. The heat facilitated the release of bioactive compounds from the food matrix. This would also explain the higher bioaccessibility obtained for ID and HAD because they are the drying processes with the highest thermal load. Therefore, the best dehydration processes to obtain higher bioaccessibility of EP, soluble flavonoids, and antioxidant capacity are HAD and ID.
Bas-Bellver et al.
[11] analyzed the effect of the gastric, small intestine phase, and colonic fermentation on the phenolic compounds and the antioxidant capacity of
Rojo Brillante persimmon powders obtained using the HAD and FD methods. In other studies, EP and antioxidant capacity increased after the gastric phase and decreased after the intestinal phase. As they stated, most of the solubilized polyphenolic compounds remained in the precipitate. The bioaccessibility of the EP of both samples obtained by both drying treatments showed no differences between them, whereas the bioaccessibility of the antioxidant capacity increased in the samples treated by freeze-drying. After the colonic fermentation of the predigested samples, a growth of beneficial bacteria such as
Bifidobacterium and
Faecalibacterium prausnitzii was observed. Likewise, positive correlations were detected between polyphenols and
Actinobacteria, Akkermansia, and
Ruminococcaceae; bacteria genera related to beneficial effects on the immune system and health status. Lactic acid bacteria,
Streptococcus, and
Lactobacillus also showed higher abundance after fermenting the digested persimmon samples. In addition, butyrate-producing bacteria such as
Faecalibacterium and
Ruminococcaeae also showed higher abundance after fermentations. Butyrate is a short-chain fatty acid produced by intestinal bacteria because of the fermentation of indigestible polysaccharides. This metabolite is a critical mediator of the colonic inflammatory response and a contributor to the immune system. Thus, persimmon powders could be used in food formulation to improve the content of bioactive compounds and could influence human health.
Matsumura et al.
[12] investigated the in vitro antioxidant potential of NEP from
Hohrenbo dehydrated persimmon using HAD. They performed the in vitro digestion divided into four phases: oral, gastric, small intestine, and large intestine and determined the antioxidant capacity in each phase. The antioxidant capacity in the oral phase was low but increased in the gastric and small intestine phases. However, the highest values of antioxidant capacity were obtained in the large intestine phase. In the large intestine phase, the intestinal microflora produced the fermentative decomposition of the non-extractable fraction of the dried persimmon, enhancing its antioxidant capacity. Moreover, the authors concluded that more studies are required to confirm the health benefits of NEP and to distinguish between the dietary functions of EP and NEP.
Hamauzu and Suwannachot
[13] analyzed the EP and NEP fractions in
Ichida-gaki persimmon samples dehydrated using natural drying. After simulating the gastric and small intestine phase in vitro, the NEP fraction presented a strong bile acid-binding capacity. Therefore, dehydrated persimmon with a large amount of NEP could act as a cholesterol-lowering agent.