Effect of SPME extraction conditions and humidity on the release of volatile lipid oxidation products from spray-dried emulsions.

The aim of the study was to investigate the effects of headspace solid-phase microextraction (HS-SPME) conditions and relative humidity (RH) on the release of volatile lipid oxidation products from spray-dried emulsions. Two model spray-dried sunflower oil emulsions with a Na-caseinate-maltodextrin matrix were oxidised, stabilised at five RHs and analysed by HS-SPME-GC-MS for volatiles. Increased extraction temperature raised not only the overall release of volatile compounds but also altered the volatile profile. The obtained volatile profiles were dependent on the RH. This was mainly due to the humidity response of the matrix affecting e.g. its binding ability and hydrophilicity. Cross-linking of the emulsifying protein had a minor influence on the release. Both matrix-related factors and extraction conditions should thus be taken into account in interpretation of the HS-SPME results. On the other hand, being sensitive to changes in matrix composition and structure, the HS-SPME allows studying of matrix-related changes in foods.

Study of oxygen transfer across milk proteins at an air-water interface with scanning electrochemical microscopy.

Scanning electrochemical microscopy (SECM) combined with a Langmuir trough was used for studying oxygen transfer across protein films at an air-water interface. The method allows the comparison of the oxygen permeability of different emulsifiers without any concerns of interference of atmospheric oxygen. Two milk proteins, ß-lactoglobulin and ß-casein, were compared, and the permeabilities obtained were for ß-casein PD ≈ 2.2 × 10(-7) cm(2)/s and for ß-lactoglobulin PD ≈ 0.6 × 10(-7) cm(2)/s, which correspond to the lowest limit of the diffusion coefficients and are 2 orders of magnitude lower than the diffusion coefficient of oxygen in water, yet several orders of magnitude higher than previously reported for milk protein films. The method allows characterization of the oxygen barrier properties of liquid interfacial films, which is of crucial importance for understanding the role of the interface in the inhibition of oxygen transport and developing modified interfaces with higher oxygen blocking efficacy.

Interfacial protein engineering for spray-dried emulsions - part I: effects on protein distribution and physical properties.

Distribution of protein and oil in aqueous and spray-dried emulsions and the effect of protein cross-linking on emulsion properties and matrix-water interactions were investigated. Sodium caseinate and sunflower oil were used to make emulsions which were spray dried using maltodextrin as a wall material. 3% Na-caseinate concentration showed optimum emulsion and process stability as observed in CLSM images, droplet size data and in the amount of heptane-extractable oil from spray-dried emulsions. Transglutaminase cross-linking prior to emulsification slightly increased the amount of protein both on the oil droplet interface and on the particle surface as confirmed by analysis of continuous phase protein in the feed emulsion and by XPS measurements from the powder surface. DSC and water sorption measurements were used to study the physical state of the matrix. Glass transition occurred between RH 54% and 75% at room temperature and it was not affected by cross-linking.

Interfacial protein engineering for spray-dried emulsions - part II: oxidative stability.

The aim of this work was to investigate how the oxidative stability of encapsulated oil is affected by the humidity response of a Na-caseinate-maltodextrin matrix. Furthermore, the effect of modification of the interfacial Na-caseinate layer through cross-linking was studied. For this purpose, two model spray-dried emulsions containing sunflower oil, maltodextrin, and either non-cross-linked or cross-linked Na-caseinate were stored at different relative humidities (RHs; ∼0%, 11%, 33%, 54%, and 75%). Increasing RH improved the oxidative stability of the spray-dried emulsions. This behaviour was mainly linked to the loss of individual powder particles upon caking and collapsing of the matrix at RH 75%. Oxidation of non-encapsulated surface lipids with a proportion of ca. 5% of total lipids was only twofold compared to total lipids. Excess protein on particle surfaces may have delayed oxidation, e.g., by its radical scavenging activity. Under several storage conditions, cross-linking of the protein slightly improved the oxidative stability.