Free radicals may mediate the conversion of Amadori compounds to AGEs . It has been reported that catechins display potent scavenging activity towards free radicals when compared to the positive control aminoguanidine, removing free radicals mainly through the phenolic hydroxyl group of the B-ring . These findings suggest that catechins are involved in the inhibition of AGE formationat each stage of the route, ranging from the Amadori compounds to the formation of AGEs. Furthermore, the antioxidant activity of procyanidin varies depending on the degree of polymerization, the binding site, and the tertiary structure of the polymer, and the antioxidant capacity increases as the degree of polymerization rises . This blocks chain oxidative reactions by acting as a free radical scavenger that quenches free radicals .
Procyanidin is thought to be involved in the inhibition of AGE formation at each step because the AGE formation route involves stepwise oxidative reactions. Here, the structure-activity relationship of the isolated compounds reveals that compound 7, a dimer of compound 6 , shows higher activity than compound 6. There is a difference between compounds 8 and 9 in that the former has protocatechuic acid attached to procyanidin B, and the latter has gallic acid attached to procyanidin B. However, compound 9, which has more hydroxyl groups, showed higher activity. In addition, compound 10 was a trimer with the highest degree of polymerization and the highest activity among these compounds. In the comparison of extracts, when the extracts of immature persimmons were compared with those of ripe persimmons, the activity in Tone Wase persimmons that had not undergone astringency removal was higher than that in immature persimmons. However, this activity was greatly reduced by astringency removal. Regarding Fuyu persimmons, activity in ripe persimmons was lower than that in immature persimmons.
As a result, activity in immature persimmons can be considered to be higher than that of the persimmons that we usually eat. This is thought to be due to the effect of tannins. First, when the Fuyu ripens, acetaldehyde generated inside the fruit forms polymers by cross-linking the soluble tannins, making the tannins insoluble. The Tone Wase has a high soluble tannin content of 1–2% when ripe. The anaerobic treatment of persimmon with carbon dioxide or ethanol increases the acetaldehyde content, rendering the tannins insoluble. Based on the above and the other results of this study, high activity was observed in immature persimmons containing soluble tannins and in ripe Tone Wase persimmons that had not undergone astringency removal, but activity was not observed in ripe Tone Wase persimmons that had undergone astringency removal and contained insoluble tannins or in ripe Fuyu persimmons. Therefore, it is presumed that soluble tannins have an inhibitory effect on AGE formation, while insoluble tannins may not have such activity. In the beverage and flavoring experiments, the extracts of immature persimmons and gallic acid had no inhibitory effect on deterioration. For TPA, deterioration was inhibited at lower sample concentrations except for dodecane, and for GA, the concentration that showed the greatest inhibition was 5 mg/L.
It is believed that low concentrations did not provide sufficient antioxidative potency to protect the citral and failed to inhibit its deterioration. The following two points can be considered as possible reasons for the failure to sufficiently inhibit deterioration when concentrations became too high. First, it is thought to be due to the acidic nature of the sample, which led to its deterioration. Other causes include the fact that gallic acid is also an acid, and the extracts of immature persimmons that contain it are also thought to be acidic. The deterioration may have been accelerated due to the more acidic nature of the sample added as its concentration increased. Second, pro-oxidants may be produced due to the generation of other radicals associated with the elimination of oxidizing radicals. In a previous study, the inhibition of the deterioration of citral was investigated using ubiquinol as an antioxidant.
The results showed that neither too low nor too high a concentration sufficiently inhibited deterioration, indicating the existence of an optimal concentration . Although it is difficult to elucidate the detailed mechanism, similar phenomena have been observed for other antioxidants, such as carotenoids . In addition, there were no deteriorated substances such as p-methyl acetophenone , which exhibited a petroleum scent, and the deteriorated substances formedwere myrcenol and linalool oxide . As neither of the two deteriorated substances was cyclized, it is thought that the deterioration of p-menthadien-8-ol was inhibited at the preliminary stage of its formation. In the presence of the emulsifier SY-Glyster ML-750, citral was converted into alcohols by the Cannizzaro reaction and proceeded to become linalool, which was ultimately converted into linalooloxide and myrcenol.