Coalescence of oil droplets in plasticized starch matrix in simple shear flow

The formation of oil droplets in plasticized starch matrices is studied by using well-defined conditions similar to extrusion. A method based on double staining and confocal scanning microscopy was developed to investigate the role of coalescence during droplet formation. The results indicate that coalescence takes place at all process conditions studied. The apparent critical capillary number was used to quantify the coalescence intensity and to separate the influence of interdependent process parameters on the coalescence. The intensity of coalescence increased with shear rate and oil content whereas an increase in blend viscosity reduced the extent of coalescence. Besides, the results showed that the critical dispersed phase concentration (Φ_c), below which no coalescence takes place, depends on applied shear rate and decreases with increasing shear rate.     

Rheological properties of corn oil emulsions stabilized by commercial micellar casein and high pressure homogenization

The rheological behavior of corn oil emulsions prepared by high pressure homogenization (HPH) was investigated. Coarse emulsions of corn oil (10-30 g oil/100 g emulsion) in casein dispersions containing 0.5-3.5 g micellar casein/100 g casein dispersion in an oil-free basis were homogenized at 0-300 MPa. Flow behavior under continuous increasing (0-150 s⁻¹) or decreasing (150-0 s⁻¹) shear rate was tested. Emulsions that showed macroscopic change in consistency were tested for viscoelasticity (G′). Homogenization of emulsions with low oil concentration (10 g/100 g) resulted in Newtonian behavior for all treatment pressures. The rheological behavior of emulsions with higher oil concentration (30 g/100 g) was dependent on casein concentration in the aqueous phase and varied from Newtonian to shear thinning. Homogenization pressures between 20 and 100 MPa induced the formation of a gel-like structure possibly through pressure-induced interactions between caseins surrounding adjacent droplets.     

Characteristics of submicron emulsions prepared by ultra-high pressure homogenisation: Effect of chilled or frozen storage

Model O/W pre-emulsions at an initial temperature of 24 °C and pH 6.3, and containing (w/w) 4.3% whey proteins plus 15, 30 or 45% peanut oil were processed using a ∼15 L/h homogeniser with a high pressure (HP) valve immediately followed by cooling heat exchangers. The effect of ultra-high pressure homogenisation (UHPH) between 100 and 300 MPa (P₁) or of recycling (1–3 homogenisation passes) at 200 MPa was investigated on the droplet size distribution, size indices and viscosity. Fluid temperatures were measured at the inlet (T₁) and outlet (T₂) of the HP-valve, and after immediate cooling downstream of the HP-valve (T₃) as they varied throughout UHPH. Short-life heating phenomena and mechanical energy involved in droplet processing were clearly influenced by emulsion composition. Oil droplet diameters decreased when (P₁) increased from 100 to 300 MPa leading to submicron droplets at ≥200 MPa. Monomodal distributions with droplets well below 0.3 μm were obtained after recycling at 200 MPa for the three oil contents, with a peak at 138 nm (distribution in volume) or 60–70 nm (in number frequency). The emulsion behaviour varied from fluid (and quite Newtonian) to thick (and shear thinning) depending on the droplet size reduction and the oil volume fraction. Emulsions displayed an excellent stability vs. creaming and coalescence after 9 d storage at 5 °C. Freezing followed by 13 d storage at −24 °C then thawing, induced an increase in particle sizes depending both on the oil volume fraction and (P₁). After UHPH at 200–225 MPa (±recycling), the freezing/thawing process maintained most of oil droplet size below 1 μm at 15% (w/w) oil, and induced mainly oil droplet aggregation through SDS-labile interactions at higher oil contents.     

Microstructure & rheology of mixed colloidal dispersions: Influence of pH-induced droplet aggregation on starch granule–fat droplet mixtures

The creation of high quality reduced-fat food products is challenging because the removal of fat adversely affects quality attributes, such as appearance, texture, and flavor. This study investigated the impact of pH-induced droplet aggregation on the properties of model food systems consisting of fat droplets and starch granules. Oil-in-water emulsions (2 wt.% oil) containing whey-protein coated lipid droplets aggregated extensively when heated (90 °C, 5 min) at pH values around their isoelectric point (pH 5) but not at lower (pH 3.5) or higher (pH 7) values, which was attributed to changes in electrostatic repulsion. The physicochemical properties of mixed lipid droplet–starch dispersions (2 wt.% oil, 4 wt.% starch) prepared under similar conditions (pH 3.5, 5, and 7; 90 °C for 5 min) were also measured. At pH 5, extensive lipid droplet aggregation was observed in mixed systems, which led to a large increase in their yield stress and apparent viscosity when compared to mixed systems at pH 3.5 and 7. These results show that the rheological properties of mixed fat droplet–starch granule suspensions can be modulated by controlling the electrostatic interactions between the fat droplets so as to change their flocculation state. This study has important implications for fabricating high quality reduced-fat products with desirable sensory attributes.     

Droplet breakup and coalescence in a twin-screw extrusion processing of starch based matrix

Formation of oil droplets during twin screw extrusion processing of maize starch was investigated by analyzing the droplet breakup and coalescence mechanisms separately. For this purpose, the flow was characterized by computational fluid dynamics (CFD) using material data derived from online rheological measurements. The simulated results on local flow conditions were coupled to experimental data on the dispersed phase morphology, which was analyzed by confocal laser scanning microscopy (CLSM). This was used to elucidate the influence of process characteristics relevant for droplet breakup and coalescence. The results showed that increasing screw speed does not necessarily result in smaller droplet sizes. This could be related to the contradictory effects: increasing screw speed improves droplet breakup but also increases the rate of coalescence. Smaller droplet sizes were obtained at higher blend viscosities, which could be achieved either by increasing the feed rate or by using screw configuration that applies less mechanical stress. The results suggest that an increase in blend viscosity reduced the rate of coalescence. Moreover, increasing oil content led to an increased rate of coalescence, and therefore to remarkably bigger droplets. Selection of process parameters (e.g. screw configuration, feed rate, screw speed) based on the findings of this study allowed enhancing the dispersive mixing efficiency of triglyceride droplets during extrusion processing of maize starch.     

Steady shear flow behavior of gum extracted from Ocimum basilicum L. seed: Effect of concentration and temperature

Steady shear flow behavior of basil seed gum (BSG) was investigated between 0.5% and 2% (wt/wt) concentration and temperatures of 5-85 °C. BSG showed shear thinning behavior at all concentrations and temperatures. The Herschel-Bulkley model was employed to characterize flow behavior of BSG solutions at 0.1-1000 s⁻¹ shear rate. The pseudoplasticity of BSG increased markedly with concentration. Flow behavior of 1% BSG indicated a higher viscosity of this gum at low shear rates compared to xanthan, konjac and guar gum at similar concentration. The activation energy of BSG quantified using an Arrhenius equation increased from 4.9 × 10³ to 8.0 × 10³ J mol⁻¹ as concentration changed from 0.5% to 2% wt/wt. This indicated a heat-resistant nature of BSG. Increasing the apparent viscosity of BSG as temperature increase from 60 °C showed a sol-gel behavior of BSG based on dynamic oscillatory measurements. The static yield stress was obvious between shear rates 0.001-0.1 s⁻¹ (9.98 Pa for 1% BSG at 20 °C). The existence of the yield stress, high viscosity at low shear rates and pseudoplastic behavior of BSG make it a good stabilizer in some food formulations such as mayonnaise and salad dressing.     

Influence of interfacial composition on oxidative stability of oil-in-water emulsions stabilized by biopolymer emulsifiers

The objective of this research was to evaluate the influence of storage pH (3 and 7) and biopolymer emulsifier type (Whey protein isolate (WPI), Modified starch (MS) and Gum arabic (GA)) on the physical and oxidative stability of rice bran oil-in-water emulsions. All three emulsifiers formed small emulsion droplets (d₃₂ < 0.5 μm) when used at sufficiently high levels: 0.45%, 1% and 10% for WPI, MS and GA, respectively. The droplets were relatively stable to droplet growth throughout storage (d₃₂ < 0.6 μm after 20 days), although there was some evidence of droplet aggregation particularly in the MS-stabilized emulsions. The electrical charge on the biopolymer-coated lipid droplets depended on pH and biopolymer type: −13 and −27 mV at pH 3 and 7 for GA; −2 and −3 mV at pH 3 and 7 for MS; +37 and −38 mV at pH 3 and 7 for WPI. The oxidative stability of the emulsions was monitored by measuring peroxide (primary products) and hexanal (secondary products) formation during storage at 37 °C, for up to 20 days, in the presence of a pro-oxidant (iron/EDTA). Rice bran oil emulsions containing MS- and WPI-coated lipid droplets were relatively stable to lipid oxidation, but those containing GA-coated droplets were highly unstable to oxidation at both pH 3 and 7. The results are interpreted in terms of the impact of the electrical characteristics of the biopolymers on the ability of cationic iron ions to interact with emulsified lipids. These results have important implications for utilizing rice bran oil, and other oxidatively unstable oils, in commercial food and beverage products.     

The influence of α-amylase-hydrolysed barley starch fractions on the viscosity of low and high purity barley β-glucan concentrates

Gelatinized barley starch was hydrolysed using porcine pancreatic α-amylase for various time intervals and the hydrolysate fractionated according to molecular weight distribution (low, medium and high) by gel permeation chromatography. The effects of hydrolysed starch fractions (2.5%, w/w) on the solution viscosity of low- (∼50%, w/w) and high- (∼88%, w/w) purity barley β-glucan (0.75%, w/w,) at different temperatures (20 and 37 °C) were determined and compared to that of a control. The results indicated that none of the hydrolysed starch fractions significantly influenced the solution viscosity of high purity β-glucan. However, addition of the medium molecular weight fraction to low purity β-glucan significantly increased its viscosity when determined at low shear rates (1.29–12.9 s⁻¹). Marginal changes in viscosity were observed at shear rates exceeding 12.9 s⁻¹. This study suggested that a non-β-glucan component in the low purity β-glucan concentrate probably influences the solution viscosity of “β-glucan–hydrolysed starch” blends.     

Phase structures impact the rheological properties of rennet-casein-based imitation cheese containing starch

The dynamic rheological and microstructural properties of rennet-casein-based imitation cheeses containing various concentrations of potato starch were investigated using a stress-controlled rheometer and confocal laser scanning microscopy. The influence of added starch on the size of the oil droplets in the imitation cheeses was also examined. Imitation cheeses with 0–15% protein replaced by starch were processed in a Rapid Visco Analyser (RVA) at 90 °C for 10 min at a shear rate of 800 rev/min and were then evaluated using oscillatory shear measurement and a temperature sweep (20–90 °C). The storage modulus (G′) of the rennet casein imitation cheeses increased abruptly at added starch concentrations >4%. In the temperature range 20–90 °C, tan δ of the imitation cheeses decreased with increasing starch concentration and was <1 at added starch concentrations >4%. A binary continuous phase consisting of a protein phase and a starch phase was observed in systems containing >4% starch, whereas the starch was dispersed in the protein matrix as small particles of irregular shapes at added starch concentrations ≤4%. As the dispersed phase, the size of the oil droplets increased with starch addition in the imitation cheeses. The marked increase in G′ and the reduction in tan δ may be attributed to the formation of a binary continuous separated phase structure in imitation cheeses containing added starch that is driven by thermodynamic incompatibility between rennet casein and starch.     

Rheological behaviour of tamarind seed gum in aqueous solutions

Tamarind seed gum as seed polysaccharide from Tamarindus indica L. has been characterized for physicochemical and rheological properties in the present work. The structural analysis determined the presence of glucose:xylose:galactose in a molar ratio of 2.61:1.43:1.The Huggins and Kraemer plots obtained by capillary viscometry gave an intrinsic viscosity of 4.7 dl g⁻¹ and the viscosity average molecular mass was calculated to be 9.18 × 10⁵ g mol⁻¹ using the Mark-Houwink relationship. The steady shear and dynamic viscoelasticity properties of tamarind seed gum in aqueous solutions at different concentrations were investigated at 20 °C using a Haake Rheometer RS75. The tamarind seed gum solutions clearly exhibited shear-thinning flow behaviour at high shear rate and Newtonian region occurred at low shear rate range, however, at higher concentrations, pronounced shear thinning was observed. The value of zero shear viscosity (η₀) was estimated by fitting Cross and Carreau models. The specific viscosity at zero shear rate (η_sp0) was plotted against the coil overlap parameter (C[η]) and the slopes of the lines in the dilute and semi-dilute regions were found to be ∼2.2 and 4.3, respectively. The value of the critical concentration (C^∗) was about 4.23/[η]. While, the mechanical spectra in the linear viscoelastic region of tamarind seed gum solutions showed the typical shape for macromolecular solutions. Plots of η versus γ and η^∗ versus ω were superimposable and hence obey the Cox-Merz rule.     

A rheological characterization of semi-solid dairy systems

Dispersions of cross-linked starch in full fat milk, taken as models of custard model systems, have been characterized by different rheological means: viscoelastic measurements, classical flow measurements and ‘vane’ rheometry. From viscosity measurements, the flow behaviour was described within the shear rate range 0.01–100 s⁻¹. The flow curves were fitted using the Herschell–Bulkley equation over the shear rate range 0.1–100 s⁻¹ while a deviation was found towards the low shear rate range, making the determination of the yield stress non realistic. Instead, measurements with the ‘vane’ device in low shear conditions provided a way to estimate the yield stress, at rest and after shearing, but the entire flow curve was not described. From the viscoelastic measurements at low strain amplitude, the mechanical spectra were obtained. Linearity tests beyond the linearity limits provided the critical stress corresponding to the G′–G″ cross-over. The parameters obtained from these different rheological methods are discussed.     

Rheological and microstructural characterization of WPI-stabilized O/W emulsions exhibiting time-dependent flow behavior

The rheological behavior of oil-in-water (O/W) emulsions stabilized by whey protein isolate (WPI) and its relationship with the microstructural changes caused by shearing was studied. O/W emulsions (50, 55, and 60 g oil/100 g) were made using ultrasound and their rheological properties were determined by: flow curve test, constant shear rate test, and hysteresis loop test. Microstructural changes were evaluated in terms of droplet size and droplet size distribution. Emulsions containing 50 and 55 g oil/100 g showed a Newtonian behavior, whereas those with 60 g oil/100 g exhibited shear-thinning behavior. Under constant deformation, the apparent viscosity of the emulsions decreased with time. The hysteresis loop test revealed that increasing oil content increased the degree of thixotropy of the emulsions. Moreover, before and after the constant deformation test droplet size distributions did not show differences, indicating that the decrease in the apparent viscosity may be promoted by breakdown and further deformation and/or reorganization of oil droplets flocs. In turn, experimental data obtained from the constant shear rate test was fitted to a structural kinetic model. The rate constant values showed no particular trend with oil content and shear rate, implying that probably wall slip occurred at high shear rates and high oil contents.     

Off-line capillary rheometry of corn starch: Effects of temperature, moisture content and shear rate

A capillary rheometer was used to determine the rheological behaviour of corn starch with moisture contents ranging from 27 to 37 g/100 g (wet basis), at temperatures of 85, 100 and 120 °C, and true wall shear rates ranging from 100 to 2000 s⁻¹. It was found that the rheology of the system followed a pseudoplastic law and the interactions between the processing variables such as temperature, moisture content and shear rate were well correlated with viscosity for the experimental range of conditions considered in this work. The apparent viscosity decreased as moisture content, temperature and shear rate increased in agreement with previous work.     

Physicochemical,thermal and pasting properties of starch separated from γ-irradiated and stored potatoes

The morphological, thermal and pasting properties of starch separated from potatoes of three varieties (Kufri Chandramukhi, Kufri Jyoti and Kufri Chipsona-2), treated either with CIPC (isopropyl N-(3 chlorophenyl) carbamate) or γ-irradiation (Co⁶⁰, 0.1 and 0.5 kGy) and subsequently stored at 8, 12 and 16 °C for 90 days, were studied. Scanning electron microscopy (SEM) showed the presence of oval and irregular shaped starch granules with a diameter range of 15–16 μm. Mean granule size of starch separated from potatoes stored at 12 °C ranged from 18–25 μm and irradiation treatment resulted in an increase in the proportion of small size granules. The irradiation of potatoes with 0.5 kGy resulted in starch with significantly lower peak-, trough- and breakdown-viscosity as compared to starch from potatoes treated with either CIPC or 0.1 kGy irradiation. The irradiation of potatoes with 0.5 kGy caused a significant increase in setback and pasting temperature. Pasting temperature of starch was observed to vary with the storage temperature. Starch separated from potatoes stored at higher temperature showed lower pasting temperature and vice versa. The starch from potatoes stored at 8 °C showed higher peak-, trough- and breakdown-viscosity and lower setback. Peak viscosity increased and swelling volume decreased with increase in storage temperature. FTIR spectra showed that the starch from irradiated potatoes displayed a significant decrease in the intensity of the C–H stretch region between 2800 and 3000 cm⁻¹, which was observed to be irradiation dose-dependent, and higher with 0.5 than 0.1 kGy. However, a slight broadening of O–H stretch (3000–3600 cm⁻¹) in starches from irradiated potatoes was observed. The spectral changes caused by γ-irradiation were apparent in the O–H stretch (3000–3600 cm⁻¹), C–H stretch (2800–3000 cm⁻¹) and bending mode of water (1600–1800 cm⁻¹).     

Effect of the matrix system in the delivery and in vitro bioactivity of microencapsulated Oregano essential oil

The effect of encapsulating matrix on retention, protection and delivery of Oregano essential oil (EO) was studied. EO was encapsulated in rice starch porous spheres, inulin and gelatine/sucrose capsules by spray drying. Gelatine/sucrose matrix was also dried by freeze drying. Experimental designs were applied to test the effect of bonding agents and solids content for rice starch and drying temperature and solids content for inulin and gelatine/sucrose systems. The ratio of gelatine/sucrose was also tested. EO was identified (confocal laser scanning microscopy and FT-IR) in all tested matrices and the release profiles, antioxidant activity and antimicrobial activity of encapsulates evaluated. Results showed that the three tested materials are able to encapsulate Oregano EO. Higher diffusion coefficients were obtained for starch microcapsules (about 10⁻¹³ m²/s) followed by spray-dried gelatine/sucrose systems (about 10⁻¹⁵ m²/s) and inulin microcapsules (about 10⁻¹⁶ m²/s). Gelatine/sucrose microparticles exhibit high antioxidant and antimicrobial activity while inulin and rice starch microencapsulates ensure higher stability.     

Large microchannel emulsification device for producing monodisperse fine droplets

In this paper, we present a novel microchannel emulsification (MCE) system for mass-producing uniform fine droplets. A 60×60-mm MCE chip made of single-crystal silicon has 14 microchannel (MC) arrays and 1.2×10⁴ MCs, and each MC array consists of many parallel MCs and a terrace. A holder with two inlet through-holes and one outlet through-hole was also developed for simply infusing each liquid and collecting emulsion products. The MCE chip was sealed well by physically attaching it to a flat glass plate in the holder during emulsification. Uniform fine droplets of soybean oil with an average diameter of 10 μm were reliably generated from all the MC arrays. The size of the resultant fine droplets was almost independent of the dispersed-phase flow rate below a critical value. The continuous-phase flow rate was unimportant for both the droplet generation and the droplet size. The MCE chip enabled mass-producing uniform fine droplets at 1.5 mL h–1 and 1.9×10⁹ h^–1, which could be further increased using a dispersed phase of low viscosity.     

Droplet characterization and stability of soybean oil/palm kernel olein O/W emulsions with the presence of selected polysaccharides

Droplet characteristics, flow properties and stability of egg yolk-stabilized oil-in-water (O/W) emulsions as affected by the presence of xanthan gum (XG), carboxymethyl cellulose (CMC), guar gum (GG), locust bean gum (LBG) and gum Arabic (AG) were studied. The dispersed phase (40%) of the emulsions was based on soybean oil/palm kernel olein blend (70:30) that partially crystallized during extended storage at 5 °C. In freshly prepared emulsions, the presence of XG, CMC, GG and LBG had significantly decreased the droplet mean diameters. XG, LBG, GG and CMC emulsions exhibited a shear-thinning behavior but AG emulsion exhibited a Bingham plastic behavior and control (without gum) emulsion almost exhibited a Newtonian behavior. Both control and AG emulsions exhibited a severe phase separation after storage (30 days, 5 °C). The microstructure of stored XG emulsion showed the presence of partially coalesced droplets, explaining a large increase in its droplet mean diameters. Increases in droplet mean diameters and decreases in flow properties found for stored GG and LBG emulsions were attributed to droplet coalescence. Nevertheless, the occurrence of droplet coalescence in these emulsions was considered to be small as no free oil could be separated under centrifugation force. Increases in flow properties and excellent stability towards phase separation found for stored CMC emulsion suggested that CMC could retard partial coalescence. Thus, the results support the ability of CMC, GG and LBG in reducing partial coalescence either by providing a sufficiently thick continuous phase or by acting as a protective coating for oil droplets.     

Impact of iron encapsulation within the interior aqueous phase of water-in-oil-in-water emulsions on lipid oxidation

Iron (Fe³⁺) was encapsulated within the internal aqueous phase of water-in-oil-in-water (W/O/W) emulsions, and then the impact of this iron on the oxidative stability of fish oil droplets was examined. There was no significant change in lipid droplet diameter in the W/O/W emulsions during 7 days storage, suggesting that the emulsions were stable to lipid droplet flocculation and coalescence, and internal water diffusion/expulsion. The initial iron encapsulation (4 mg/100 g emulsion) within the internal aqueous phase of the water-in-oil (W/O) emulsions was high (>99.75%), although, a small amount leaked out over 7 days storage (≈10 μg/100 g emulsion). When W/O/W emulsions were mixed with fish oil droplets the thiobarbituric acid-reactive substances (TBARS) formed decreased (compared to fish oil droplets alone) by an amount that depended on iron concentration and location, i.e., no added iron < iron in external aqueous phase < iron in internal aqueous phase. These differences were attributed to the impact of W/O droplets on the concentration and location of iron and lipid oxidation reaction products within the system.