Valorization of purple corn pericarp using two-pot emerging thermal and non-thermal extraction technologies and evaluation of product quality, structure, and bioactivity
Authors:   , Boateng Isaac Duah 
Published: October 2023

The pericarp of purple corn has traditionally been thought of as being a byproduct. However, it is an excellent source of high-value polyphenols (such as anthocyanins, proanthocyanins, etc.) that could be extracted and used as nutraceuticals, cosmeceuticals, and valuable starting material for synthesizing food colorants and food additives. Hence, the first investigation was to turn the purple corn pericarp (waste) into polyphenol-rich extract using a two-pot extraction technique and evaluate the phenolic profiles, antioxidant properties, product quality, and structural characteristics of the recovered extracts. According to Plackett-Burman design (PBD) screening, the critical extraction variables identified were acetic acid concentration, extraction time, and ethanol concentration. This was followed by optimization using Box-Behnken design (BBD), facial central composite design (FCCD), and full-factorial design (FFD), and the best design was identified after comparative analysis. The models were acceptable, as evidenced by the desirability values of 0.836 - 0.933. A comparison of BBD, FCCD, and FFD was performed, and it was found that the PBD-BBD produced a lower residual standard error (RSE) than the PBD-FCCD for most of the responses and yielded the lowest extraction time (~8.6 h), proving that the BBD could forecast the data more accurately. To evaluate the extracts' bioactive extraction efficiency, antioxidant properties, product quality, and structural parameters, samples from a single factor (SF) experiment and BBD were compared. The BBD extract had better color properties (lower ?E, higher a*, C, and H0) and a greater extraction yield (17.5 percent) than SF and was confirmed by the Principal component and Hierarchical cluster analysis. Although the scanning electron microscopy and the optical profilometer for BBD revealed irregular, loosely arranged granules, it had a less adverse effect on overall product quality and aligned with the Fourier transform infrared spectroscopy results. The BBD samples had lower levels of phenolic acids but higher levels of total phenolic content (118.86 [plus or minus] 3.60 g GAE/kg), condensed tannins (231.74 [plus or minus] 8.45 g EE/kg), antioxidant activity, cyanidin-3-glucoside (90.46 [plus or minus] 4.47 mg/g), naringin (7.68 [plus or minus] 0.58 mg/g), and cyanidin-chloride (49.81 [plus or minus] 1.78 mg/g). This showed that BBD can effectively be used to optimize the extraction of polyphenols from PCP using optimal extraction conditions, resulting in a higher polyphenol yield and the extract with superior antioxidant capacity. The second experiment involved the use of process intensification to improve the extraction of the bioactive compounds while maintaining the desired structural properties and reducing the extraction time. For the first time, the polyphenols in PCP were extracted using a two-pot microwave extraction (MAE) technique, an emerging thermal method. The MAE conditions were optimized, and response surface methodology was used to understand the association between independent variables and their responses and used further to decipher the underlying mechanisms through visualization. PBD was used to screen significant extraction parameters, and further optimization was done using BBD. Under the optimum conditions (ethanol (42.4 percent v/v), temperature (75 degreesC), and time (29 min)), total anthocyanin content (TAC), total phenolic content (TPC), and condensed tannins (CT) to the tune of 38.73 g/kg, 138.62 g/kg and 279.48 g/kg pericarp, respectively were obtained with a desirability function value of 0.838. Monomeric anthocyanins degraded and polymerized to 3-deoxyanthocyanin, whereas phenolic acids such as chlorogenic, caffeic, ferulic acid, and hesperidin increased as the microwave temperature and time increased. The MAE's extraction yield was 38.11 percent higher than the conventional extraction (CE) observed in research 1. The CE process took ~8.6 h, whereas MAE took ~0.5 h to extract the phenolics. The MAE samples had a higher TAC, TPC, CT, phenolic acids (chlorogenic acid, caffeic acid, ferulic acid, and hesperidin), total flavonoid content (TFC), and antioxidant activities than CE samples. Nevertheless, individual anthocyanins concentrations and color qualities were lower in MAE than in CE, despite a substantial increase in extraction yield and phenolic acids. Thus, requiring the need to explore an efficient non-thermal extraction technology. Hence, in the final experiment, ultrasound, a non-thermal extraction technique, was employed to extract phenolic compounds from PCP for the first time using the PBD coupled with the BBD to elucidate how the ultrasonic parameters affect the phenolic extraction. According to PBD's screening, the significant parameters were ethanol concentration, extraction time, temperature, and ultrasonic amplitude that affected TAC, TPC, and CT. These parameters were further optimized using the BBD method for the RSM. The RSM analysis showed a linear curvature for TAC and a quadratic curvature for TPC and CT with a lack of fit > 0.05, indicating that the models were trustworthy. Under the optimum conditions (ethanol (50 percent v/v), time (21 min), temperature (28 degreesC), and ultrasonic amplitude (50 percent)), a maximum TAC, TPC, and CT of 34.99 g C3G/kg, 121.26 g GAE/kg, and 260.59 of EE/kg pericarp, respectively were obtained with a desirability 0.952, which was regarded as acceptable and excellent, according to the desirability scale. The UAE sample was compared to MAE sample for product quality, bioactives, antioxidant potential, and structural properties (from research 2). It was found that although UAE sample had a lower extraction yield, TAC, TPC, and CT, the UAE gave a higher individual anthocyanin, flavonoid, phenolic acid profile, and antioxidant activity. The UAE took 21 min, whereas MAE took 30 min for maximum extraction. Regarding product qualities, the UAE extract was superior, with a lower total color change (?E) and a higher chromaticity (C), L*, a*, and H0 and WSI. Structural characterization using SEM showed that MAE extract had severe creases and ruptures, whereas UAE extract had less noticeable alterations which was attested by an optical profilometer. It was observed that ultrasound, an emerging non-thermal technology, might be used to effectively extract phenolics from PCP as it requires lesser time and improves phenolics yield, antioxidant potential, structure, and product qualities.