Dry milling characteristics and polyphenolic contents of adapted colored corn varieties and polyphenol extraction by combining deep eutectic solvents and ultrasound
Authors:   , Kumar Ravinder 
Published: October 2023

Although conventional yellow dent corn (Zea mays L.) has been extensively grown and utilized in the US for food, feed, and numerous industrial applications, the degree of associated value-addition is rather modest. Colored corn varieties have recently drawn increased attention due to their relatively higher concentrations of diverse phytochemicals, such as anthocyanins, proanthocyanins, and flavonoids, which may be utilized for a variety of agro-industrial applications. Corn pericarp is a low value cellulosic coproduct of the processing industry. However, in case of colored corn varieties, most of the polyphenolic compounds such as anthocyanins, flavonoids, flavanols, tannins and phenolic acids are concentrated in corn pericarp which can be separated and selectively processed for their economic recovery. The corn endosperm, containing most of the kernel starch and proteins and of prime interest to the processing industry remains unaffected and can be utilized further. Due to the diverse applications of recovered value-added phytochemicals, colored corn varieties can supplement the lower economic value of yellow dent corn. However, polyphenol-rich colored corn varieties need to be adapted to the midwestern US climatic conditions for their economic exploitation. In this study, dry milling characteristics and polyphenolic contents (anthocyanins, total phenols, and condensed tannins) of fourteen conventionally bred and midwestern US grown colored varieties developed by the USDA-ARS colored corn breeding program at the University of Missouri and their milling coproducts were ascertained and compared with a commercial purple corn. With mean large grits yield of 22.98 percent (db), colored varieties had a dominant softer endosperm composition and were found to be more suitable for wet milling and dry grind industry, processing most of the corn grown in the US. The mean starch content of colored varieties was lower than conventional yellow dent corn (~9.91 percent, db), and the mean protein content was higher (~1.70 percent, db). Although commercial purple corn contained the highest amounts, four experimental varieties were identified with high concentrations of polyphenols. Through HPLC analyses, a diverse mix of various anthocyanin forms, phenolic acids, and flavonoids were identified. The antioxidant potential of polyphenolic extracts from five selected varieties were evaluated using four different assays, and significant antioxidant activities were observed. Histological analysis was conducted on polyphenol-rich varieties, and most of the pigments were found to be concentrated in pericarp. The pericarp thickness was greatest for the variety containing the maximum polyphenolic compounds. Adapted colored corn varieties grown in the midwestern US can be valorized to recover value-added polyphenolic compounds in line with the circular bioeconomic paradigm. Polyphenolic compounds can be extracted from plant matrices using a wide array of solvents. However, conventional extraction methods involve the use of organic solvents, which are of concern due to their adverse environmental impacts and generation of waste streams. Therefore, there is a growing demand for green solvents that are renewable, non-toxic, biodegradable, and economically viable. In second study, a combination of deep eutectic solvents (DESs) and ultrasound-assisted extraction were investigated for extracting anthocyanins, condensed tannins, and flavonoids from purple corn pericarp. Initially, nineteen DESs with different constitutes were prepared and characterized. It was observed that the DESs formed strong intermolecular bonds, their viscosities ranging from 0.0209 to 0.7463 Pa.s at a constant temperature of 50 degreesC and shear rate of 10 s-1. During NIR analysis, dominant stretching of O-H and C-H bonds was observed. The highest and lowest polarities ranged between 50.63 (DES 4) and 46.80 kcal.mol-1 (DES 5), respectively, and all the solvent mixes were acidic in nature. In a comparatively shorter extraction time (10 min), significant amounts of anthocyanins (27.41 mg cyanidin-3-glucoside C3G equivalent/g pericarp), condensed tannins (249.33 mg epicatechin EE equivalent/g pericarp), and total flavonoids (36.87 mg catechin CE equivalent/g pericarp) were recovered. Ultrasound-assisted extraction in DESs was found to enhance the extraction efficiency compared to the conventional methods utilizing ethanol and water. This improved extraction efficiency could be attributed to a greater solvent penetration into the plant cell matrix. The highest CUPRAC antioxidant capacity of 392.27 mg Trolox equivalent (TE)/g of pericarp, DPPH free radical scavenging activity of 82.70 mg TE/g of pericarp, and ABTS activity of 34.87 mg TE/g of pericarp, respectively, were observed. C3G, delphinidin, cyanidin chloride, and peonidin as high as 20.56, 9.62, 11.91, and 2.90 mg/g of pericarp, respectively, were detected during anthocyanin profiling of the extracts. Phenolic acids profiling identified caffeic acid, ferulic acid, gallic acid, hesperidin, and chlorogenic acid, the highest values being 3.29, 1.72, 0.63, 15.18, and 5.23 mg/g pericarp, respectively. Finally, epicatechin, naringin and kaempferol concentrations of 102.73, 19.93, and 1.55 mg/g pericarp, respectively, were eluted during flavonoids profiling by HPLC. This work demonstrated a highly efficient and green extraction of bioactive compounds from colored corn pericarp by combing DESs and ultrasound.