Chitosan-alginate polyelectrolyte complexes for encapsulation of low molecular weight fish bioactive peptides

Encapsulation of low molecular weight (M w ) bioactive peptides (BAPs) in electrostatically mediated polyelectrolyte complexes (PECs) of sodium alginate (ALG) and chitosan (CS) was studied, formed via a simple one-step mixing process. PECs were characterized via dynamic light scattering (DLS), mixed-mode phase analysis light scattering (M3-PALS), static light scattering (SLS), small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The encapsulation efficiency (EE) and in vitro release of low M w antihypertensive LKPNM and LKP BAPs (derived from fish hydrolysates) were measured, under both gastric and intestinal pH conditions, via high performance liquid chromatography (HPLC). Two different ALG were tested (M w ≈ 8 and 21 kDa, with mannuronic: guluronic ratios 5.1 and 1.4, respectively), whilst the M w and degree of acetylation (DA) of the CS were ≈111 kDa and ≈10 %, respectively. At the pH (5.5) of PEC formation the BAPs were positively charged. As the molar charge ratio of alginate to chitosan (n − /n + ) was increased from 0.1 to 0.6 the PEC size reduced from ca. 800 to 300 ± 50 nm, indicating more compact structures, but increased again significantly to >10 μm around charge neutralisation (n − /n + = 1) and net PEC ζ-potential swapping from +ve to –ve. The size then decreased again to between 1 and 10 μm as n − /n + → 10, as expected if the more prevalent polysaccharide (ALG) coated the surface of the PECs. However, higher M w ALG gave significantly smaller (more compact) and more highly negatively charged PECs for n − /n + > 1. The PECs showed high (≈80%) EE but low (10–20%) release in gastrointestinal conditions, highlighting their potential as effective carriers of such BAPs. • Bioactive peptides are efficiently encapsulated in alginate + chitosan complexes. • Alginate: chitosan molar charge ratio has a large influence complex size. • Higher M w alginate gives significantly smaller, more compact complexes. • The peptides apparently aid cross-linking of the alginate and chitosan. • Peptide release from complexes at gastrointestinal pH and salt levels was low.