Exploring the potential of native and modified starch and starch nanocrystals in Pickering emulsions: current advances, future perspectives, and challenges

Nanocrystals derived from organic particles, such as starch, can be produced through acid or enzymatic hydrolysis reactions to serve as stabilizers in Pickering emulsions. Starch nanocrystals (SNCs) offer the advantage of providing enhanced storage stability and controlled in vitro and in vivo release compared to conventional surfactant-stabilized emulsions. This article highlights recent advances in Pickering emulsions stabilized with both native and modified starch, including SNCs, and it demonstrates that there are some limitations in the modification methods, with the most common being the addition of octenylsuccinic anhydride (OSA). Additionally, the article discusses the primary methods of SNC production, frequently used starch sources, employed operational conditions, and resulting characteristics such as crystallinity, size, shape, and applied pretreatments. Promising trends in optimizing the SNC production process through combined starch modification pretreatments are also addressed. The main challenge lies in developing a safe starch modification method that facilitates acid or enzyme diffusion to optimize the process and reduce environmental impacts. In addition to showcasing the main applications of Pickering emulsions in the food industry, drug delivery, and their impact on human health. Finally, the study emphasizes the importance of exploring new pathways and methodologies for the production of nanomaterials intended for food applications using more sustainable processes.

Highlights

• The limitation in stabilizing Pickering emulsions with native starch was discussed
• Methods of modifying native starch were demonstrated aiming at stabilizing emulsions
• Use of sulfuric acid to produce starch nanocrystals is harmful to the environment
• The production route of starch nanocrystals through enzymes must be explored
• Physical and enzymatic methods are indicated to modify starch and produce nanocrystals

Introduction

Emulsions are inherently thermodynamically unstable systems, making them susceptible to destabilization phenomena like flocculation and coalescence due to both external environmental conditions and their own properties (Wang et al., 2023c). The use of starch granules as stabilizers in Pickering emulsions for pharmaceutical and cosmetic products offers the advantage of causing fewer adverse effects on the skin compared to surfactants (Marto et al., 2016; Almeida et al., 2024). Due to their dual wettability and increased stability, especially at high internal phase ratios, this characteristic facilitates the formation of large and stable droplets, reaching sizes of up to millimeters (Chevalier & Bolzinger, 2013a; Marto et al., 2015). In the case of food emulsions, it is essential to maintain stability during storage, requiring the use of stabilizers, where the use of starch and starch nanocrystals (SNC) as stabilizers offers a sustainable and environmentally beneficial alternative for the production of stable emulsions (Campelo et al., 2020).

It has been reported in the literature that studies using native starches for Pickering emulsion stabilization are quite limited due to their low affinity with both oil and aqueous phases, as exemplified by quinoa and amaranth starch (Kierulf et al., 2020), rice starch (Dun et al., 2020), and corn starch (Yang et al., 2020). In contrast, the use of modified starches is more extensive, especially through chemical modifications such as OSA (Domian et al., 2015), esterification with lauric acid and ethanol (Wang et al., 2023a), esterification with citric acid (Lee & Chang, 2019), and esterification with lauroyl chloride (Leal-Castañeda et al., 2018).

Hydrolytic processes are commonly employed to obtain SNC, involving the use of strong acids or specific enzymes (amylases) to target the amorphous regions within the starch structure. This leads to the release of highly crystalline nanoscale crystals (George et al., 2021). SNC typically exhibits a “platelet” shape, offering excellent mechanical strength and modulus, as the amorphous portions of the layered “onion” starch granules are removed during hydrolysis (Zhang et al., 2020). The yield and properties of SNC are influenced by various factors, including the starch source, type, and concentration of acid, as well as the temperature and duration of the process, as highlighted by Lin et al. (2011).

In recent years, there has been growing interest in utilizing starch and SNC for the preparation of Pickering emulsions (Azfaralariff et al., 2020). Pickering emulsion gels, stabilized by solid particles or surfactants, exhibit improved storage stability and controlled release in vitro compared to conventional emulsions (Zhou et al., 2018). The use of particles derived from natural biopolymers, such as starch, as stabilizers has attracted attention from industries (Li et al., 2019b). Pickering emulsions stabilized with starch nanocrystals are considered eco-friendly or “green” since starch is edible, non-allergenic, and Generally Recognized as Safe (GRAS) (Zhu, 2019).

Currently, the production of SNC primarily involves the acidic hydrolysis route, using sulfuric acid as the main agent to break the amorphous chains of starch granules. However, this reaction is time-consuming, taking about 5 to 7 days when applied to native starch and potentially up to 9 days of environmentally unfavorable processing (López-Hernández et al., 2022; Li et al., 2023). The difficulties in stabilizing Pickering emulsions with starch and SNC can be related to the size and distribution of the crystals, compatibility with other components of the emulsion such as bioactive and fragrances, processing conditions such as starch concentration, water/oil ratio, shear, pH, and temperature, as well as precise control of rheology (Tan & McClements, 2021).

Therefore, this review aims to highlight the methodologies and operational conditions commonly used for the production of SNC, as well as the starch sources that have been explored. Additionally, recent articles that have utilized both native and modified starch, as well as SNC, to stabilize Pickering emulsions were emphasized. New approaches for native starch modification through non-thermal, thermal, and enzymatic treatments were demonstrated, as well as combined methods that have not been applied to emulsions and could represent a new route for optimizing the Pickering emulsion stabilization process. The discussion also encompassed how Pickering emulsions stabilized with starch and SNC can be utilized in the food industry and food packaging. Finally, we sought to raise awareness regarding the use of sulfuric acid for SNC production and OSA for starch modification, presenting the enzymatic route as a solution.

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Raphael Lucas Jacinto Almeida, Newton Carlos Santos, Shênia Santos Monteiro, Nathália Saraiva Rios, Everaldo Silvino dos Santos, Exploring the potential of native and modified starch and starch nanocrystals in Pickering  emulsions: current advances, future perspectives, and challenges, Food Bioscience, 2024, 104675, ISSN 2212-4292, https://doi.org/10.1016/j.fbio.2024.104675.