Petr Dejmek

In situ visualization of the effect of a pulsed electric field on plant tissue

A new method was developed for in situ visualization of changes related to electropermeabilization of plant tissue. Onion epidermis stained with neutral red was subjected to a pulsed electric field (PEF), and serial images were captured by a camera connected to a stercomicroscope. Sample resistance was recorded simultaneously. Above a threshold level of a field strength of 0.35 kV/cm, it was possible to distinguish the individual permeabilized cells by their colour. Over 90-95% of the PEF-induced colour changes occurred during the first 2-3 min after the electric pulse and the rest after some 20 min. The size and rate of the observed changes were correlated with the severity of PEF, and were further influenced by the pH and conductivity of the solution used in mounting the epidermis, the sampling location on the onion epidermis, and cell size and number. The final conductivity increase was directly proportional to the number of permeabilized cells. Permeabilization was not randomly distributed but occurred along preferential paths connecting the electrodes

Quinoa starch granules: a candidate for stabilising food-grade Pickering emulsions.

BACKGROUND Particle-stabilised emulsions, so-called Pickering emulsions, are known to possess many beneficial properties, including being extremely stable. Starch granules isolated from quinoa have been used as emulsion stabilising particles. The granules were intact, 1-3 µm in diameter and modified with octenyl succinic anhydride to increase their hydrophobicity. Starch granules, as opposed to most other particles used to generate Pickering emulsions, are edible, abundant and derived from natural sources. RESULTS Emulsions produced by high shear homogenisation had droplet sizes of 9-70 µm depending on the starch-to-oil ratio. Droplet size decreased with increasing starch-to-oil ratio, but was unaffected by the oil phase volume over a range of 5-33% oil (v/v). Although the drops were large and subject to creaming, their size remained unchanged over a period of 7 days. By adjusting the starch-to-oil ratio drops could be made to be buoyancy neutral to prevent creaming. Rheological characterisation indicated a gel structure with an elastic modulus in the range 200-2000 Pa depending on droplet size. CONCLUSION This work has demonstrated the successful use of starch granules to stabilise emulsions which may find applications beyond that of food, for example in cosmetics and pharmaceutical formulations.

Emulsion stabilizing capacity of intact starch granules modified by heat treatment or octenyl succinic anhydride

Starch granules are an interesting stabilizer candidate for food-grade Pickering emulsions. The stabilizing capacity of seven different intact starch granules for making oil-in-water emulsions has been the topic of this screening study. The starches were from quinoa; rice; maize; waxy varieties of rice, maize, and barley; and high-amylose maize. The starches were studied in their native state, heat treated, and modified by octenyl succinic anhydride (OSA). The effect of varying the continuous phase, both with and without salt in a phosphate buffer, was also studied. Quinoa, which had the smallest granule size, had the best capacity to stabilize oil drops, especially when the granules had been hydrophobically modified by heat treatment or by OSA. The average drop diameter (d32) in these emulsions varied from 270 to 50 μm, where decreasing drop size and less aggregation was promoted by high starch concentration and absence of salt in the system. Of all the starch varieties studied, quinoa had the best overall emulsifying capacity, and OSA modified quinoa starch in particular. Although the size of the drops was relatively large, the drops themselves were in many instances extremely stable. In the cases where the system could stabilize droplets, even when they were so large that they were visible to the naked eye, they remained stable and the measured droplet sizes after 2 years of storage were essentially unchanged from the initial droplet size. This somewhat surprising result has been attributed to the thickness of the adsorbed starch layer providing steric stabilization. The starch particle-stabilized Pickering emulsion systems studied in this work has potential practical application such as being suitable for encapsulation of ingredients in food and pharmaceutical products.

Quinoa starch granules as stabilizing particles for production of Pickering emulsions

Intact starch granules isolated from quinoa (Chenopodium quinoa Willd.) were used to stabilize emulsion drops in so-called Pickering emulsions. Miglyol 812 was used as dispersed phase and a phosphate buffer (pH7) with different salt (NaCl) concentrations was used as the continuous phase. The starch granules were hydrophobically modified to different degrees by octenyl succinic anhydride (OSA) or by dry heat treatment at 120 degrees C in order to study the effect on the resulting emulsion drop size. The degree of OSA-modification had a low to moderate impact on drop size. The highest level of modification (4.66%) showed the largest mean drop size, and lowest amount of free starch, which could be an effect of a higher degree of aggregation of the starch granules and, thereby, also the emulsion drops stabilized by them. The heat treated starch granules had a poor stabilizing ability and only the starch heated for the longest time (150 min at 120 degrees C) had a better emulsifying capacity than the un-modified native starch granules. The effect of salt concentration was rather limited. However, an increased concentration of salt slightly increased the mean drop size and the elastic modulus.

Effect of osmotic pretreatment and pulsed electric field on the viscoelastic properties of potato tissue

Abstract Compression stress relaxation of potato tissue exposed to pulsed electric field (PEF) treatment, with or without osmotic pretreatment, was measured and modelled with five parameter generalised Maxwell model. The changes in viscoelastic model coefficients were quantified as a response to applied field strength, pulse length, and pulse number and were correlated with conductivity changes. Using the same approach, additional effects of different osmotic treatments along with constant PEF treatment were also studied. As measured by the post-PEF conductivity, the residual elasticity was the parameter most affected at lower levels of PEF treatment. At high PEF levels, the longer of the relaxation times dropped from 10 to 2–3 s range. Maximal PEF treatment had a similar effect to 0.7 M hyper-osmotic treatment, implying that the effect of PEF on relaxation behaviour was dominated by loss of turgor. Neither hypo-osmotic nor hyper-osmotic pretreatment appeared to interact with the PEF treatment.

Heat induced aggregation of b-lactoglobulin studied by dynamic light scattering

The in situ heat-induced aggregation of commercial β-lactoglobulin as such, or after further purification, was followed to a z-average hydrodynamic diameter of 15–20 nm at 59–63 °C by dynamic light scattering. In this temperature range, measurable increase of hydrodynamic diameter occurred after an apparent lag period, which was strongly dependent on heating temperature, pH and initial protein concentration. The changes in time scale of the aggregation process agreed with changes in amount of unfolded β-lactoglobulin, assuming a two-state model of the denaturation. The pH dependence reflected the midpoint unfolding temperature and not the sulphydryl group reactivity, suggesting that this reactivity was not rate limiting in the aggregation. The aggregation process was modelled numerically with FuchsSmoluchowski kinetics.

Interactions between EPS-producing Streptococcus thermophilus strains in mixed yoghurt cultures

Mixed cultures of different EPS-producing Streptococcus thermophilus strains in combination with a Lactobacillus delbrueckii subsp. bulgaricus strain with negligible EPS-production were used for yoghurt production. The yoghurt texture was characterised with respect to sensory, rheological and microstructural properties and the EPS-concentrations were determined. The cultures resulted in yoghurts with highly different texture properties, and positive interactions between certain Streptococcus thermophilus strains were observed. The underlying properties of yoghurt texture are multidimensional, but a number of microstructural characteristics were apparent in the yoghurts with the highest mouth thickness, creaminess and viscosity. A strong protein network, not too dense and with medium size pores containing EPS, seems associated with these properties. The presence of capsular polysaccharides (CPS) also appeared to be beneficial as did a combination of EPS types, which were distributed differently in the protein network (in serum pores, respectively in association with protein). Obviously, a certain concentration of EPS must be present to provide for these effects on yoghurt texture, but other factors than concentration per se seem more important.

Characterization of a cold-gelling whey protein concentrate

Abstract Cold-gelling whey protein concentrates, produced by heat pre-treatment during manufacture, gelled without heating when dissolved at sufficient protein concentration and under suitable conditions of pH and ionic strength. A cold-gelling whey protein concentrate was dissolved at low concentrations in water or NaCl solutions of varying ionic strength, pH and with the addition of a reducing agent, dithiothreitol (DTT) for studies by rheology, light microscopy and atomic force microscopy. The cold-gelling whey protein powder was found to consist of large, micron-sized, drying-induced, weak aggregates consisting of primary disulfidebridged aggregates of 20–30 nm in diameter. In dilute solutions under conditions of large repulsive forces (low ionic strength and pH far from the isoelectric point) the large aggregates dissolved slowly over many hours. In less good solvents, larger aggregates remained or were formed.

Time-Resolved Shear Viscosity of Wheat Flour Doughs—Effect of Mixing, Shear Rate, and Resting on the Viscosity of Doughs of Different Flours

ABSTRACT The shear viscosity of three doughs of different wheat cultivars mixed to a farinograph level of 500 BU was measured at low shear rates as a function of the shear deformation using a cone-and-plate viscometer. Cyanoacrylate adhesive was used to attach the dough samples to the instrument surfaces to eliminate wall slip. Flours used were Dragon, Kosack, and a fodder wheat. A distinct difference was observed between the viscosities of the different flour cultivars. The strongest dough (Dragon), with the highest protein content and a good resistance in the farinograph, had the highest maximum viscosity. The doughs showed distinct strain hardening, more pronounced for the strong doughs. Maximum viscosity was obtained at a strain of ≈4, almost independent of the shear rate, but at higher values for stronger doughs (5 for Dragon, 4 for Kosack, and 3.5 for fodder wheat). The maximum was most pronounced for well-mixed doughs after resting. The viscosity and its variation with strain may be used as a measu...

Minimizing whey protein retention in cross-flow microfiltration of skim milk

Abstract The feasibility of fractionating casein and whey protein in skim milk was studied by measuring the permeation of whey protein in a laboratory microfiltration set-up in relation to temperature, circulation velocity, transmembrane pressure (TMP) and permeate flux. A tubular, ceramic membrane from Tech-Sep with a nominal pore size of 0.14 μm was used. The protein content in the permeate was found to be independent of TMP up to 190 kPa for the highest circulation velocity, 8 m s −1 , at 55 °C. For lower circulation velocities and at 15 °C, the protein content decreased when the TMP was increased. For every circulation velocity, a lowest value of the TMP corresponding to a limiting flux was found. The value of the limiting flux increased with increasing temperature and circulation velocity. The highest value for the limiting flux was 145 L h −1 m −2 for the circulation velocity 8 m s −1 at 55 °C. All results could be roughly correlated as limiting flux = 0.0025 Re.