Riitta Partanen

Effect of glycerol on behaviour of amylose and amylopectin films

Abstract The effect of water and glycerol on sorption and calorimetric Tgs of amylose and amylopectin films were examined. The mechanical properties of the films were also analysed under varying glycerol content at constant RH and temperature. Based on changes observed in sorption and tensile failure behaviour glycerol was strongly interacted with both starch polymers. Even though water was observed to be more efficient plasticiser than glycerol, glycerol also affected the Tg. But in spite of the observed decrease in Tg under low glycerol contents brittleness of the films increased based on changes in elongation. The increase in brittleness of both polymers was also in agreement with their actual behaviour. At around 20% glycerol great change in the rheological properties occurred. Above 20% glycerol amylose film showed much larger elongation than the low glycerol content films and was still strong but the amylopectin produced a very week and non-flexible film.

Crosslinking Food Proteins for Improved Functionality

Different possibilities for protein crosslinking are examined in this review, with special emphasis on enzymatic crosslinking and its impact on food structure. Among potential enzymes for protein crosslinking are transglutaminase (TG) and various oxidative enzymes. Crosslinking enzymes can be applied in cereal, dairy, meat, and fish processing to improve the texture of the product. Most of the current commercial applications are based on TG. The reaction mechanisms of the crosslinking enzymes differ, which in turn results in different technological properties.

Effect of Relative Humidity on Oxidation of Flaxseed Oil in Spray Dried Whey Protein Emulsions

Flaxseed oil was emulsified in whey protein isolate (WPI) and spray-dried. Powder characteristics and oxidative stability of oil at relative humidities (RH) from RH approximately 0% to RH 91% at 37 degrees C were analyzed. Oil droplets retained their forms in drying and reconstitution, but the original droplet size of the emulsion was not restored when the powder was dispersed in water. The particles seemed to be covered by a protein-rich surface layer as analyzed by electron spectroscopy for chemical analysis (ESCA). Oxidation of flaxseed oil dispersed in the WPI matrix was retarded from that of bulk oil but followed the same pattern as bulk oil with respect to humidity. A high rate of oxidation was found for both low and high humidity conditions. The lowest rate of oxidation as followed by peroxide values was found at RH 75%, a condition that is likely to diverge significantly from the monolayer moisture value. A weak baseline transition observed for the WPI matrix in a differential scanning calorimetry (DSC) thermogram suggested a glassy state of the matrix at all storage conditions. This was not consistent with the observed caking of the powder at RH 91%. Scanning electron microscopy (SEM) images revealed a considerable structural change in the WPI matrix in these conditions, which was suggested to be linked with a higher rate of oxygen transport. Possible mechanisms for oxygen transport in the whey protein matrix under variable RHs are discussed.

Improving laccase catalyzed cross-linking of whey protein isolate and their application as emulsifiers.

Whey protein isolate (WPI) was chemically modified by vanillic acid in order to enhance its cross-linkability by laccase enzyme. Incorporation of methoxyphenol groups created reactive sites for laccase on the surface of the protein and improved the efficiency of cross-linking. The vanillic acid modified WPI (Van-WPI) was characterized using MALDI-TOF mass spectrometry, and the laccase-catalyzed cross-linking of Van-WPI was studied. Furthermore, the vanillic acid modification was compared with the conventional approach to improve laccase-catalyzed cross-linking by adding free phenolic compounds. A small extent of the vanillic acid modification significantly improved the cross-linkability of the protein and made it possible to avoid color formation in a system that is free of small phenolic compounds. Moreover, the potential application of Van-WPI as emulsifier and the effect of cross-linking on the stability of Van-WPI emulsion were investigated. The post-emulsification cross-linking by laccase was proven to enhance the storage stability of Van-WPI emulsion.

Transglutaminase catalyzed cross-linking of sodium caseinate improves oxidative stability of flaxseed oil emulsion.

Sodium caseinate was modified by transglutaminase catalyzed cross-linking reaction prior to the emulsification process in order to study the effect of cross-linking on the oxidative stability of protein stabilized emulsions. The extent of the cross-linking catalyzed by different dosages of transglutaminase was investigated by following the ammonia production during the reaction and using SDS-PAGE gel. O/W emulsions prepared with the cross-linked and non-cross-linked sodium caseinates were stored for 30 days under the same conditions. Peroxide value measurement, oxygen consumption measurement, and headspace gas chromatography analysis were used to study the oxidative stability of the emulsions. The emulsion made of the cross-linked sodium caseinate showed an improved oxidative stability with reduced formation of fatty acid hydroperoxides and volatiles and a longer period of low rate oxygen consumption. The improving effect of transglutaminase catalyzed cross-linking could be most likely attributed to the enhanced physical stability of the interfacial protein layer against competitive adsorption by oil oxidation products.

Sodium caseinates with an altered isoelectric point as emulsifiers in oil/water systems.

Sodium caseinate was chemically modified in order to alter its isoelectric point (pI). Negatively charged carboxylic groups were introduced to lower the pI, and positively charged amino groups to achieve the opposite. Different chemical amino acid modification approaches were studied and the modified proteins were characterized using free amino group assays, SDS-PAGE, MALDI-TOF mass spectrometry, and zeta potential measurements. Oil-in-water emulsions were prepared using these modified caseinates. The pH stability behavior of the emulsions was monitored, and interestingly, the stability of the emulsion could be modulated through steering the pI of caseinate. Using different modified caseinates, it was possible to create emulsions that were stable in the acid, neutral, and alkaline regions of the pH spectrum. The stability behavior of the emulsions correlated well with the theoretical and experimentally determined pI values of the caseinates. Storage stability of emulsions was also studied at pH values around 7, and emulsions made of modified caseinates showed storage stability similar to that of unmodified caseinate emulsions.

Loosening of globular structure under alkaline pH affects accessibility of β-lactoglobulin to tyrosinase-induced oxidation and subsequent cross-linking

Globular proteins such as β-lactoglobulin (BLG) are poorly accessible to enzymes. We have studied susceptibility of BLG to oxidation by Trichoderma reesei (TrTyr) and Agaricus bisporus (AbTyr) tyrosinases and subsequent intermolecular cross-linking with respect to pH-induced structural changes. We evaluated pH-induced structural changes in BLG using circular dichroism, tryptophan fluorescence and small angle X-ray scattering (SAXS) measurements, where after these results were correlated with the analysis of cross-linking by sodium dodecyl sulphate polyacrylamide gel electrophoresis. Oxygen consumption measurement and changes in radii of gyration determined by SAXS during the enzyme-induced oxidation at the respective reaction conditions were also followed. Intermolecular cross-linking of BLG by TrTyr was found at pH 9 but not at pH 7.5. AbTyr was unable to catalyze cross-linking at pH 7.5 or pH 9. Increased accessibility and cross-linking by TrTyr was addressed to loosening of the three dimensional structure of the protein, increased flexibility of the backbone as well as partial hydrolysis. In addition to basic research of the effect of protein folding on enzymatic cross-linking the research results have significance on the exploitation of TrTyr at alkaline conditions.

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.

Charge Modifications to Improve the Emulsifying Properties of Whey Protein Isolate

Whey protein isolate was modified by ethylene diamine in order to shift its isoelectric point to an alkaline pH. The extent of the modification was studied using SDS-PAGE and MALDI-TOF mass spectrometry. The modified whey proteins were used as an emulsifier to stabilize oil-in-water emulsions at acidic and neutral pH ranges, and their emulsifying properties were compared with that of the unmodified whey proteins and with the previously studied ethylene diamine modified sodium caseinate. The emulsifying activity of the modified whey proteins was similar to that of the unmodified ones, but the stability of an emulsion at pH 5 was significantly improved after the modification. Charge and coverage of droplet surface and the displacement of the interfacial proteins by surfactant Tween 20 were further studied as a function of pH. As compared with the unmodified whey proteins, the modified ones were proven to cover the interface more efficiently with extensive surface charge at pH 5, although the interfacial layer was less resistant to the surfactant displacement.

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, β-lactoglobulin and β-casein, were compared, and the permeabilities obtained were for β-casein PD ≈ 2.2 × 10(-7) cm(2)/s and for β-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.