Effects of Grilling on Total Polyphenol Content and Antioxidant Capacity of Eggplant (Solanum melongena L.)

Abstract

Cooking can change the polyphenol contents of eggplant. This study elucidated the effects of grilling on total polyphenol content (TPC), antioxidant capacity, and the inner structures of eggplant. After identical hollowing, cylindrical eggplant samples were prepared and were then grilled until their center temperatures (CT) respectively reached 50, 65, 75, 85, and 95 °C. Chemical assays and observations of the inner structures clarified that TPC and 1, 1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity decreased as CT increased when CT was below 65 °C. Results also showed that TPC and DPPH radical scavenging activity increased as CT increased when CT was between 65 °C and 95 °C. For CT 65 °C, the samples retained polyphenol oxidase (PPO) activity up to 40% of the raw state activity. The 3 grilled eggplant models, chlorogenic acid, chlorogenic acid-sugar and chlorogenic acid-amino acid model, yielded results showing that phenol functional groups on chlorogenic acid were thermally stable and that phenol functional groups on chlorogenic acid reacted neither with sugar nor with amino acids. Results show that PPO activity is a primary reason for the decrease of the 2 indices. Optical microscopic and scanning electron microscopic observations revealed collapsed cells and inter-tissue cracks around the surface area for CT 85 and 95 °C. Scanning electron microscopic observations clarified that intercellular bonds for CT 85 and 95 °C became thinner than those for CT 75 °C around the middle area. The phenomena explained above are reasons for the increase of TPC and DPPH radical scavenging activity.

A heating method for producing frozen pizza ingredients with increased total polyphenol content and 2,2‐diphenyl‐1‐picrylhydrazyl radical scavenging activity

“…As shown in Figure a, onion TPC increased during the first 1.5 min of hot plate heating before decreasing slightly, with AHT onion having significantly higher TPC than raw onion and both samples having significantly higher TPC than conventional onion (7.6 ± 0.1, 7.0 ± 0.4, and 6.3 ± 0.3 μmol Teq/g dw, respectively). The decrease in TPC after steaming may be in part due to oxidation by polyphenol oxidase, the activity of which may persist during the initial heating of vegetables (Uchida et al., ). The TPC of raw onion measured falls within the range (2.5–24.49 mg GAE/g dw) of those reported in other studies (Gorinstein et al., ; Kähkönen et al., ).…”

“…TPC of both pepper types was found to first decrease before increasing as heating progressed (i.e., after hot plate heating of 0.5 and 1.5 min for Japanese green and red peppers, respectively). Decrease in TPC at the start of heating may be attributed to the enzymatic degradation of polyphenols by polyphenol oxidase, and the delayed increase of red pepper TPC relative to Japanese green pepper TPC could be explained by the higher thermal stability of red pepper polyphenol oxidase (Castro et al., ; Uchida et al., ). The TPC of peppers in this study was lower than that of red pepper (52.27 ± 2.03 mg GAE/g dw), as reported by Chen and Kang ().…”

“…With the exception of corn, the TPC of all vegetable toppings had significant (onion and Japanese green pepper) or near significant (red pepper) positive correlation with internal temperature. As discussed above, this phenomenon may be attributed to the heat‐induced dissociation of previously bound polyphenols, which may lead to a change in the quantity and quality of antioxidative compounds, coupled with heat inactivation of polyphenol‐degrading enzymes (Dewanto et al., ; Uchida et al., ).…”

“…Although conventional thermal blanching processes such as steaming and boiling are effective at preserving fresh attributes (e.g., bright colors and structural integrity) and decreasing microbial load, blanching‐induced tissue softening has been reported to result in losses of both polyphenols and antioxidant capacity (Castro et al., ; Chuah et al., ; Myojin et al., ; Puupponen‐Pimiä et al., ). In contrast, heating various vegetables by methods not typically used for blanching such as oven‐baking, stir‐frying, or microwaving may yield products with polyphenol contents or antioxidant capacity that remain comparable or even exceed their raw counterparts (Chuah et al., ; Uchida, Tomita, Takemori, & Takamura, ; Yamaguchi et al., ). It is plausible, then, that the aforementioned cooking processes could be used as methods for increasing bioavailable polyphenol content and potential antioxidant capacity in processed vegetables.…”

Effect of Eggplant Flour (Solanum melongenaL.) associated with hypoenergetic diet on antioxidant status in overweight women ‐ a randomised clinical trial

“…Other study showed that on total polyphenol content and antioxidant capacity in eggplant decreased during grilling when centre temperatures was below 65 °C, but they increased as centre temperatures increased to 95 °C. (Uchida et al ., )…”