Protein films can be applied to improve food quality and to reduce packaging waste. To overcome their poor water barrier properties, lipids are often incorporated. The function of incorporated lipid depends on the interface between filler and matrix. This study aimed to tailor the properties of a protein-lipid film by designing the oil/water interface to see if the concept of inactive/active filler is valid. Therefore, we varied the emulsifier stabilizing solid lipid nanoparticles (SLN) to promote (via β-lactoglobulin) or to minimize (via Tween 20) interactions between particle surface and protein. SLN were incorporated into protein films and film properties were determined. Addition of SLN led to significantly decreased water vapor permeability (WVP) of protein films. However, WVP was mainly affected by the emulsifiers and not by the lipid. Protein-stabilized SLN (BS) replaced a lacking protein in the protein network and therefore did not influence the mechanical properties of the films at ambient temperature. BS-composite films were temperature sensitive, as lipid and sucrose palmitate melted at temperatures above 40°C. Tween 20-stabilized SLN (TS) led to reduced tensile strengths, probably due to perturbative effects of TS and plasticizing effects of Tween 20. Dynamic mechanical analysis showed that TS and Tween 20 increased film mobility. Melting of lipid and emulsifiers, and temperature-dependent behavior of Tween 20 led to a strong temperature dependence of the film stiffness. By designing the interface, particles can be used to tailor mechanical properties of protein films. Tuned edible films could be used to control mass transfers between foods.
In order to apply emulsion-based delivery systems to food, they have to be stable in a protein rich environment. This study investigated the stability of solid lipid nanoparticles (SLN) during heat treatment in the presence or absence of β-lactoglobulin (BLG). SLN were stabilized either by Tween 20 (TS) or by the protein itself (BS) and were enriched to a total BLG content of 56 mg/mL. The sizes of both types of SLN were initially in the range of 170 nm. The amount of free protein was determined before and after enrichment with BLG. As revealed by particle size and zeta potential measurements, a protein layer of BLG (hard corona) adsorbed on BS but not on TS. By contrast, a soft corona was formed around both BS and TS. SLN were heat treated in the presence and absence of protein and were characterized regarding size and zeta potential. According to transmission electron microscopy imaging, heating did not affect the shape of TS and BS: TS were platelets, whereas BS exhibited a spherical or platelet like shape. Upon heat treatment, the particle size of TS increased to about 3.5 fold of the initial size (to appr. 600 nm) in the presence and in the absence of excess protein. The cloudy protein layer (soft corona) around TS could thus not prevent coalescence of TS. By contrast, BS did not experience a change in particle size. Hence, by the choice of emulsifier, an encapsulation system that is stable against heat treatment can be obtained.
The influence of sucrose palmitate, Tween 20, and lecithin on the properties of heat-induced aggregates and cold-set gels of β-lactoglobulin was studied based on an experimental mixture design with a fixed total emulsifier concentration. Emulsifiers were added to the protein solution before heating. Aggregate size and absolute values of ζ potential increased with the addition of emulsifiers, among which lecithin had the most pronounced effect. The water retention of the aggregates correlated positively with the aggregate size. Gels had reduced fracture stress and strains with increasing sucrose palmitate and decreasing Tween 20 contents. The fracture properties correlated with the ζ potentials of the aggregates, and larger aggregates led to gels with higher water-holding capacities. The emulsifiers hence influenced the gel properties indirectly via the aggregate properties. The impact of emulsifiers on food structures should therefore be considered when a food product is designed.
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