1H NMR to assess bioaccessibility and stability of designed astaxanthin nanoemulsions

• Effect of HLB interface on the stability and bioaccessibility of nanoemulsions was evaluated. • Higher HLB values improved nanoemulsion stability and increased astaxanthin bioaccessibility. • Astaxanthin bioaccessibility increased with the oil-water interface tension gradient. • 1 H NMR confirmed astaxanthin's structure remained stable in nanoemulsions after-digestion. High-concentration astaxanthin nanoemulsions (NEAX) were formulated using a mixture of soy lecithin and non-ionic surfactants with different hydrophilic-lipophilic balance (HLB) values. Nanoemulsions were prepared using high-power ultrasound. The surface properties of the emulsifiers, including their HLB value and surface tension, were related to the bioaccessibility and stability of the nanoemulsions. Additionally, proton nuclear magnetic resonance ( 1 H NMR) was used to assess the stability of astaxanthin after in vitro digestion. Particle sizes of approximately 200 nm and a high astaxanthin concentration of up to 2.8 ± 0.02 mg/mL were achieved. A higher HLB significantly enhanced astaxanthin's stability and bioaccessibility during digestion. The high-HLB value nanoemulsion released 45 % of astaxanthin during intestinal digestion, compared to 4 % bioaccessibility in oil. Furthermore, NEAX achieves maximum bioaccessibility within the initial minutes of intestinal digestion and provides a sustained release throughout its passage through the intestine. The 1 H NMR analysis showed that astaxanthin in the nanoemulsions did not undergo significant structural changes after digestion. These results showed how 1 H NMR can be applied to assess astaxanthin delivery, when HLB-optimized interfaces are used to improve bioaccessibility (45 % vs. <6 % in oil) and stability. Hence, high-HLB nanoemulsions are a promising strategy to improve astaxanthin delivery, offering enhanced bioavailability for functional foods and nutraceuticals.