Maize comes in a variety of colors, including white, yellow, red, blue, and purple, which is due to the presence of phytochemicals such as carotenoids, anthocyanins, flavonoids, phytosterols, and some hydroxycinnamic acid derivatives. In Mexico, maize is primarily grown for human consumption; however, maize residues comprise 51–58% of the total maize plant weight (stalks, leaves, ears, and husks) and are mainly used as livestock feed. These residues contain numerous bioactive compounds that interest the industry for their potential health benefits in preventing or treating degenerative diseases. This review explores the current knowledge and highlights key aspects related to the extraction methods and different techniques for identifying the bioactive compounds found in maize by-products.

Pistachio green hull (PGH) represents the non-edible fraction obtained after the seed is harvested and is an important source of phenolic compounds. Solid-state fermentation (SSF) is a viable biotechnological and economical technique for extracting phenolic compounds. This study aimed to evaluate the SSF with Aspergillus niger GH1 to recover total phenolic compounds (TPC) with antioxidant capacity (AC) from PGH. For this, the time of higher TPC and AC (DPPH [2,2-diphenyl-1-picrylhydrazyl], ABTS [2,2-azinobis-(3-ethylbenzothiazoline-6-sulfonate)], FRAP [ferric reducing antioxidant power]) was selected. Then, moisture, inoculum concentration, and aeration rate were evaluated. A. niger GH1 was able to grow and colonize the PGH, with the higher value of TPC (23.83 mg/g of dry mass (gdm)) obtained after 24 h of culture, which significantly correlated with AC (Pearson’s R = 0.69). Moisture and aeration rate were the main factors influencing TPC. The highest values for both TPC and AC were achieved in treatment 8 (60% moisture, 5 × 10⁶ spores/mL, and 1 L/Kgwm min), resulting in a 129% and 1039% increase, respectively. Gallic acid 4-O-glucoside and geranine were identified in the PGH extracts using high-performance liquid chromatography coupled with mass spectrometry. The SSF provides eco-friendly alternatives for releasing bioactive compounds from PGH, adding value to this waste.

Substantial agro-industrial waste is generated by the food industry, including pistachio green hulls (PGH), which can constitute 40% to 60% of the fresh fruit weight. This by-product contains bioactive functional components, especially phenolic compounds (PCs). An overview of research focused on PCs extracted from PGH is presented, highlighting their chemical composition, extraction methods, compound identification, and antioxidant and antibacterial activities. Extraction techniques such as ultrasound, microwave-assisted extraction, and solid-state fermentation are utilized, with mild organic solvents like water, ethanol, methanol, or their mixtures employed. The quantification of PCs is commonly performed using the Folin–Ciocalteu assay, HCl-Butanol technique, and aluminum chloride colorimetric assays. Furthermore, identification of compounds is generally accomplished through high-performance liquid chromatography (HPLC) or gas chromatography (GC), often coupled with mass spectrometry or photodiode-array detectors to enhance accuracy and reliability. Gallic acid, kaempferol, quercetin, cyanidin, and catechin are the main PCs identified, with their antioxidant activity validated by ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)), DPPH (2,2-diphenyl-1-picrylhydrazyl), and FRAP (ferric-reducing antioxidant power) assays. Antibacterial effectiveness has been demonstrated against bacteria using disk diffusion and minimum inhibitory concentration methods. These findings indicate potential uses of PGH by-products in the food, cosmetic, and pharmaceutical industries, contributing to a circular economy and enhancing agro-industrial waste management.

Corn, a widely cultivated cereal, is a rich source of several antioxidant compounds, including phenolic compounds. Some of these compounds are bound within the components of the corn cell wall, requiring specific treatments to release them. Solid-state fermentation (SSF) using Rhizopus oryzae enhanced the antioxidant capacity and phenolic content in legumes and grains. However, there is limited information regarding its application in blue corn. This study aimed to evaluate the effect of R. oryzae growth on the phenolic content and antioxidant capacity of blue corn kernels. The results demonstrated that R. oryzae used blue corn kernels for growth, which led to an increase in free phenolic compounds, reaching their highest concentration at 60 h of culture. Compared to the unfermented control (0 h), SSF increased by up to 161% in total phenolic content and a 94% increase in antioxidant capacity. The main phenolic compounds identified in the blue corn extracts at 60 h using RP-HPLC-ESI-MS included caffeic acid 4-O-glucoside, rosmanol, p-coumaroyl tartaric acid glucosidic ester, 5-heptadecylresorcinol, and caffeoyl tartaric acid. The SSF represents an eco-friendly method for enhancing the antioxidant capacity and phenolic compounds of blue corn grain.

Corn is one of the most widely cultivated cereal crops and is rich in antioxidant compounds, especially phenolics. However, many of these are bound to cell wall components, requiring pre-treatment for release. Solid-state fermentation (SSF) with Rhizopus oryzae has been used to enhance antioxidant capacity in grains and legumes, though its application in pigmented corn (PC) has not been reported. This study evaluated R. oryzae growth on PC via SSF and its effect on phenolic compound release and antioxidant capacity (AC). Variables such as temperature, pH, inoculum, and medium salts were tested for their influence on phenolic release and AC. Nutrient changes in PC due to SSF were also examined. HPLC-MS was used to analyze the phenolic compounds’ profile. R. oryzae grew effectively on PC, increasing total phenolic content (TPC) and AC by 131 and 50%, respectively. The pH was found to negatively impact phenolic release. The SSF also raised protein content by 10% and reduced carbohydrates and fiber by 3 and 8%. Thirteen phenolic compounds were identified, including Feruloyl tartaric acid ester and p-Coumaroyl tartaric acid glycosidic ester, with known anti-inflammatory properties. This process offers a sustainable method for enhancing the functional properties of pigmented corn.