Investigations on how an alkali-acid treatment alter the structures of plant-derived components (2023.10-2024.12)

Introduction

Plant-derived components often form various food structures, leading to distinct properties and functions. For instance, crystalline curcumin exhibits strong intra- and intermolecular interactions, resulting in a high melting temperature and very low water solubility. However, its solubility can be significantly changed when they are complexed with other plant-derived components, like the proteins and polysaccharides. Additionally, different processing methods could also alter the native structures of plant-derived components, like the use of different pH conditions and temperatures. It is therefore crucial to understand their impacts on structures of plant-derived components. During my first postdoctorial research career, I investigated how an alkali-acid treatment can alter the food structures, like the crystalline curcumin, turmeric, and other plants.

1. pH-induced structural changes of crystalline curcumin enhance its encapsulation in emulsions

Abstract: The simple and green pH-based method shows promise for encapsulating hydrophobic molecules in delivery systems to enhance their bioavailability. However, there is still a limited understanding of the pH-induced structural changes that are involved. In this study, we combine experimental techniques with molecular dynamics simulations to investigate pH-induced structural changes in curcumin crystals. An alkali-acid pretreatment was introduced to encapsulate curcumin, where curcumin is first dissolved in an alkaline solution and then rapidly acidified to form aggregates. Remarkably, these curcumin aggregates can be spontaneously encapsulated into emulsions, even at high concentrations (1 mg/mL). Microscopy images suggested that this pretreatment disrupts the crystalline structure of curcumin. Molecular dynamics simulations further demonstrated that the hydroxyl groups of curcumin form hydrogen bonds with water molecules, while the hydrophobic interactions dominate within pH-treated curcumin aggregates. The structural changes increase the solvent-accessible surface area and promote the rapid solubilization of curcumin into emulsions or milks.

Figure 1. The pH-based versus direct approaches for incorporating hydrophobic curcumin crystals into emulsion systems (such as nanoemulsions).

Reference

Gong, X.; Suryamiharja, A.; Zhou, H.* pH-induced structural changes of crystalline curcumin enhance its encapsulation in emulsions. ACS Food Science & Technology 2024. DOI: https://doi.org/10.1021/acsfoodscitech.4c00595. Link

2. Harnessing pH for sustainable and effective synthesis of phenolic compound-loaded nanoparticles directly from raw plants

Abstract: While considerable efforts have been made to develop phenolic compound-loaded nanoparticles for applications in foods, pharmaceuticals, and agriculture, current production methods fall short in sustainability, efficiency, and cost-effectiveness. This study introduces a pH-based “raw-to-nano” strategy to produce phenolic compound-loaded nanoparticles directly from raw plants. Curcumin-loaded nanoparticles were first formulated from raw turmeric, with an average size of 141.3 ± 2.8 nm and a surface charge of −23.3 ± 0.7 mV. Nanoparticles are stabilized by electrostatic interactions at pH 7, but stability decreases under acidic conditions (pH < 5), which could limit certain applications in acidic beverages. Based on pH effects and microstructures, a core-shell model is proposed, where acidic polysaccharides coat the surface, and insoluble branched starches form the inner phase, trapping hydrophobic curcumin molecules. This strategy successfully applies to other plants like ginger, paprika, and thyme, enabling versatile nanoparticle synthesis for practical applications in various aspects.

Figure 2. Formation of curcumin-loaded nanoparticles from raw turmeric. a) Production process of the Turmeric-pH 7 nanocomplex, where both the filtered Turmeric-pH 13 liquid and sediment were neutralized by citric acid. b) Particle size distributions of Turmeric-pH 13 and Turmeric-pH 7 nanocomplexes (peaks labeled). The Z-average of Turmeric-pH 13 is 152.2 ± 3.9 nm with a PDI of 0.27 ± 0.01, which reduces to 141.3 ± 2.8 nm with a PDI of 0.20 ± 0.00 upon neutralization to Turmeric-pH 7. Insert images show the appearance of each nanocomplex solution. c) Zeta potential distribution of the Turmeric-pH 7 nanocomplex (peaks labeled). The averaged zeta potentials for Turmeric-pH 13 and Turmeric-pH 7 are −23.3 ± 0.4 mV and − 23.3 ± 0.7 mV, respectively.

Reference

Gong, X.; Wang, M.; Zhou, H.* Harnessing pH for sustainable and effective synthesis of phenolic compound-loaded nanoparticles directly from raw plants. Food Chemistry 2025, 467, 142327. DOI: https://doi.org/10.1016/j.foodchem.2024.142327. Link

3. An improved pH-driven method for upcycling polyphenols from plants or byproducts into foods

Abstract: The incorporation of polyphenols into food systems provides various health benefits, yet their stability and bioactivity are often compromised by processing conditions. In this study, we advanced the pH-driven method for processing highly pH-sensitive polyphenols, such as quercetin, by optimizing operating conditions, including minimizing oxygen exposure and reducing operating times. As a result, an improved post-pH-driven (PPD) method was developed to encapsulate pH-sensitive quercetin into nanoemulsions with an encapsulation efficiency exceeding 95%, indicating that this method could be broadly applicable for incorporating various polyphenols. For example, it has been successfully applied to upcycle plant polyphenols from peanut skin into nanoemulsions, serving as a representative food model. The PPD method demonstrated superior performance compared to a conventional water-based method, achieving 1.8 times higher remaining percentage of total polyphenolic content. Additionally, the PPD-based nanoemulsions exhibited significantly enhanced antioxidant properties, with DPPH and ABTS radical scavenging activities increasing by 3.7 and 2.8 times, respectively, compared to the water-based method. These findings underscore the potential of the PPD method as a versatile and efficient approach for developing polyphenol-powered foods by upcycling plant byproducts and improving processing efficiency.

Figure 3. Upcycling polyphenols from peanut skin into foods via an improved post pH-driven (PPD) method. The left panel illustrates the PPD-based extraction process, and the right panel shows the remaining total polyphenol content (TPC), DPPH, and ABTS antioxidant activities, with the PPD-based method demonstrating significantly higher efficiency compared to the water-based method.

Reference

Gong, X.; Wang, M.; Lu, P.; Zhou, H.* An improved pH-driven method for upcycling polyphenols from plants or byproducts into foods. Foods 2024, 13 (23), 3945. DOI: https://doi.org/10.3390/foods13233945. Link