Spray-dried whey protein/lactose/soybean oil emulsions. 2. Redispersability, wettability and particle structure

In the present paper redispersion and wettability experiments of spray-dried whey protein-stabilized emulsions are presented. Emulsion droplet size after redispersion gives information about eventual coalescence between emulsion droplets in the powder matrix during drying or storage, resulting in an increase in emulsion droplet size after redispersion. Results from redispersion experiments are combined with previously presented knowledge about powder surface composition and particle structure to elucidate internal processes in the powder matrix and external processes on the powder surface during drying and storage of whey protein powder. The results show that with addition of lactose to whey protein-stabilized emulsions, emulsion droplet structure remains intact in the powder matrix during drying since the emulsion droplet size in the redispersed spray dried emulsion is unchanged. In the absence of lactose there is a growth in emulsion droplet size after redispersion of the spray-dried whey protein-stabilized emulsion, showing that a coalescense of emulsion droplets occurs during the drying or redispersion process. Storage of the whey protein-stabilized powders in a humid atmosphere (relative humidity 75%, 4 days) induces changes in some powders. When the powder contains a critical amount of lactose there is a remarkable increase in emulsion droplet size after redispersion of humid stored powders compared with the emulsion before drying and with the redispersed dry stored powder. In addition, there is a release of encapsulated fat after humid storage of lactose-containing powders detected by electron spectroscopy for chemical analysis. For powders which do not contain any lactose there is no increase in emulsion droplet size after storage in a humid atmosphere compared with the redispersed dry stored emulsion. Addition of only a small amount of lactose prevents coalescence of emulsion droplets and the subsequent increase in droplet size during drying. If the lactose content is kept rather low neither an effect on the droplet size after storage under humid conditions nor a release of fat onto powder surfaces is detected. Furthermore, wettability of the spray-dried whey protein-stabilized emulsions by water is presented. It is concluded that it is beneficial to wettability in water to have as high a coverage of lactose on the powder surface as possible. In addition, a review of particle structure for powders of various composition is presented.     

Invited review: spray-dried dairy and dairy-like emulsions--compositional considerations

Milk constituents [caseins, whey proteins (WP), lactose, and anhydrous milk fat] are used widely in the manufacture of dehydrated dairy and dairy-like emulsions. When sodium caseinate- (NaCas) and WP-stabilized emulsions with an oil-to-protein ratio ranging from 0.25 to 5 are dehydrated, NaCas is a more effective encapsulant than WP because of its superior emulsifying properties and resistance to heat denaturation. Denaturation degree of WP during drying has been associated with increased powder surface fat and larger droplet size after reconstitution. Encapsulation of NaCas-stabilized emulsions improves in the presence of lactose; powder surface fat was reduced from 30 to <5% when lactose was added at a 1:1 ratio to NaCas in an emulsion containing 30% (wt/wt) oil. This has been related to the ability of lactose to form solid-like (or glassy) capsules during sudden dehydration. Encapsulation of WP-stabilized emulsions is not improved by addition of lactose, although there are conflicting reports in the literature. Storage stability of dehydrated dairy-like emulsions is strongly linked to lactose crystallization as release of encapsulated material occurs during storage at high relative humidities (e.g., 75%). The use of alternative carbohydrates as “matrix-forming” materials (such as maltodextrins or gum arabic) improves storage stability but compromises the emulsion droplet size after reconstitution. The composition of the powder surface has been recognized as a key parameter in dehydrated emulsion quality. It is the chemical composition of the powder surface that dictates the behavior of the bulk in terms of wettability, flowability, and stability. Analyses, using electron spectroscopy for chemical analysis of the surface of industrial milk powders and dehydrated emulsions that mimicked the composition of milk, showed that powder surface is covered mainly by fat, even when the fat content is very low (18 and 99% surface fat coverage for skim milk and whole milk powders, respectively). The functional properties of milk constituents during emulsion dehydration are far from being thoroughly understood; future research needs include a) the encapsulation properties of pure micellar casein; b) a deeper understanding of colloidal phenomena (such as changes in the oil-water and air-oil interfaces) that occur before, during, and after dehydration, which ultimately define emulsion stability after drying; and c) reconciliation of the current different views on powder surface composition.     

Effects of feed composition,protein denaturation and storage of milk serum protein/lactose powders on lactosylation

During whey powder production, the feed is subjected to several heat treatments which can cause lactosylation of proteins. In this study, lactosylation of whey proteins was evaluated in spray-dried powders before and after storage by varying the native protein fraction as well as the serum protein/lactose ratio in the powders. The lactosylation of native α-lactalbumin and β-lactoglobulin in the powders before storage was not affected to a large extent by the protein denaturation or if the feed had been heat treated in a high or low lactose environment. After storage (relative humidity of 23.5%, 30 °C, 25 days), the kinetic of lactosylation tended to increase with increasing native protein fraction and bulk protein content in the powders. An explanation could be that proteins dissolved in the lactose glassy structure might have a lower reactivity, while proteins present in the protein glassy structure with dissolved lactose may display higher lactosylation reactivity.     

Microencapsulating Properties of Whey Protein Concentrate 75

ABSTRACT Emulsions containing various levels of soya oil dispersed in solutions of whey protein concentrate (WPC) 75 (5% w/v) were spray-dried to yield powders with oil contents ranging from 20% to 75% (w/w). The effect of homogenizing pressure and oil/protein ratio on oil globule size distributions and protein load of the emulsions and the microencapsulation efficiency (ME) and redispersion behavior of the powders were examined. Emulsion oil droplet size decreased with increasing homogenization pressure but was not affected by oil/protein ratio. Emulsion protein load and ME of the powders were negatively correlated with increasing oil/protein ratio. Powders with an oil/protein ratio < 0.75 were least susceptible to destabilization during spray-drying.     

热处理对大豆分离蛋白稳定乳液包埋特性的影响

本实验通过喷雾干燥前对SPI溶液95℃1 5 min热处理及形成乳液后加入乳糖溶液制备粉末样品,并将部分干粉储存于RH 75%环境中记录其7 d内等温吸湿线,待吸湿稳定后得到湿粉样品,测定原始乳液及干、湿粉末复溶乳液的粒径大小分布,干、湿粉末的水分含量、包埋效率(ME)、溶解速率并用扫描电镜(SEM)观察其微观结构。结果表明热处理和加糖处理能显著提高喷雾干燥SPI稳定乳液的包埋效率,高达98.68%,相对于未经处理的SPI乳液包埋效率高出1倍以上,此外含糖粉末表现出良好的溶解性,但潮湿环境对其溶解性、包埋效率及微观结构有较大影响。     

Stability to Oxidation of Spray-Dried Fish Oil Powder Microencapsulated Using Milk Ingredients

ABSTRACT: Microencapsulation of fish oil was achieved by spray-drying homogenized emulsions of fish oil using 3 different types of casein as emulsifier and lactose as filler. As the degree of aggregation of the casein emulsifier increased, the vacuole volume of the microencapsulated powders decreased. The shelf life of the powders increased as the degree of aggregation of the casein emulsifier increased at the high homogenization conditions. When micellar casein was used as emulsifier, the shelf life also increased as homogenization conditions increased. Free fat but not surface fat was inversely related to shelf life. Since the type of casein used was confounded with the powder vacuole volume, the increased shelf life may have been due to either factor.     

Microencapsulation properties of soy protein isolate: Influence of preheating and/or blending with lactose

Properties and storage stability of spray-dried emulsions stabilized by unheated and preheated (95 °C, 15 min) soy protein isolates, alone or in combination with lactose, were investigated. In general, the heat pretreatment greatly improved retention efficiency (RE), redispersion behavior, glass transition temperature (T_g) and thermal stability of the emulsion powders, but accelerated instability of the reconstituted emulsions. Additional blending with lactose further considerably improved the RE and dissolution behavior, but significantly decreased the stability of reconstituted emulsions and T_g. Storage at 75% relative humidity resulted in considerably increased droplet size of reconstituted emulsions, as well as decreased RE, wettability and T_g, especially in the powders containing lactose. Microscopic observations confirmed that the changes in properties and stability of the powders upon storage were closely related to rupture of particle structure, and/or particle agglomeration. These findings provide fundamental understanding for the development of microencapsulated products using soy proteins as the wall materials.     

Influence of Surface Structure on Cholesterol Oxidation in Model Food Powders

Three milk-resembling powders having different oil phases were prepared and stored for six months at room temperature. Samples were taken monthly for estimation of the surface composition by ‘electron spectroscopy for chemical analysis’ (ESCA) and analyses of the level of cholesterol oxides by gas chromatography. The highest fat coverage was obtained with technical tristearin as the oil phase. This preparation had around 75% of the powder surface covered by fat; during storage this coverage decreased. This powder also had the largest increase in cholesterol oxides during storage, the final level being 159 μg g⁻¹ cholesterol. The powder containing high-melting pure tristearin had a low surface coverage of fat, originally about 25%, which decreased during the storage period. Little increase in cholesterol oxides was observed, the final level being 52 μg g⁻¹ cholesterol. The third powder containing liquid triolein as oil phase, had a surface coverage of about 50% throughout the storage period. The cholesterol oxidation rate was in between that of the two tristearin powders, the final level of oxides being 75 μg g⁻¹ cholesterol. The results for the investigated powders indicate that the surface composition is of major importance for the oxidation of cholesterol. No correlation between cholesterol oxidation and solvent extractable fat (free fat) was found.     

Enhancement of emulsifying properties of whey proteins by controlling spray-drying parameters

Whey protein concentrate is the main source of globular proteins in food products which are principally used as emulsifying, foaming and gelling ingredients. These whey proteins are commonly used in powder form obtained by a spray-drying process. It is well known that β-lactoglobulin, the major protein component in whey, is greatly affected by heat treatments, with consequences on its adsorption properties at fluid–fluid interfaces. This study concerned four whey protein powders obtained using spray-drying at four different air inlet temperatures (from 170 to 260 °C), leading to different levels of protein solubility, denaturation and end-use properties. After evaluation of the protein denaturation by HPLC, the emulsifying properties were studied through particle size parameters and rheological properties in relation with spray-drying parameters. Our results indicated that oil-in-water emulsions, stabilized by 5% (w/w) protein samples, exhibited a shear-thinning flow behaviour, and the harsher the spray-drying conditions (the higher the protein denaturation), the less viscous were the emulsions. The apparent viscosity of emulsions measured at 20 °C and 50 s⁻¹ shear rate was around 0.08 Pa s when containing whey proteins before drying, and around 0.05–0.018 Pa s after drying at air inlet temperatures from 170 to 260 °C. These differences in emulsion rheological properties were related to particle size effects, in regards to analysis of particle size distributions which showed a finer emulsion according to spray-drying intensity. Our results will be presented and discussed in terms of optimization of spray-drying process relative to globular protein surface activity.     

Microencapsulation properties of protein isolates from three selected Phaseolus legumes in comparison with soy protein isolate

The spray-drying microencapsulation properties of protein isolates from three selected Phaseolus legumes (kidney, red and mung beans; KPI, RPI and MPI), at a specific concentration (6 g/100 mL) and oil/protein ratio (1:1, w/w) were compared with soy protein isolate (SPI). The oil retention efficiency (RE), redispersion and dissolution behavior, as well as microstructure of the spray-dried powders were characterized. The influence of storage at 75% relative humidity for 7 days on these characteristics was also evaluated. The results indicated that the microencapsulation properties (except RE in the KPI case) of the three protein isolates were considerably poorer than SPI, though their emulsifying ability was even superior. The microencapsulating properties of these protein isolates were largely associated with their interfacial properties, especially the interfacial protein concentration. Among the three protein isolates, the spray-dried powders with KPI exhibited highest RE but least redispersion and/or dissolution behavior. The storage resulted in a severe loss of RE and ability to be redispersed and/or dissoluted, with much higher extent observed for the KPI powder. These results suggest that appropriate modifications, especially in interfacial properties, should be conducted on these proteins to warrant their application as wall materials in spray-drying microencapsulation.     

Investigation of breakage behavior and its effects on spray-dried agglomerated whey protein-lactose powders: Effect of protein and lactose contents

Particle breakage of dairy powders occurs easily during many processes, reducing the powder functionality. The characteristics of particles and the applied stress from processing conditions on the particles are 2 main factors that can be manipulated to reduce breakage. In this study, we explored the effect of whey protein and lactose contents on dynamic breakage in agglomerated whey protein-lactose powders to provide useful information, in terms of particle characteristics, for controlling unwanted dairy powder breakage. A series of model agglomerates with different whey protein:lactose ratios were produced under the same spray-drying conditions, through a pilot plant trial. We evaluated physical characteristics, composition, and structure of samples; analyzed dynamic breakage under different mechanical stresses; and investigated the rehydration and water adsorption properties of model powders before and after breakage. The particle size and irregularity of agglomerates with more lactose was significantly higher than of samples that contained more protein. This resulted in higher particle breakage during dynamic breakage for samples with more lactose. The breakage of agglomerates was affected by the moisture content of powders and fatigue, where particle breakage happens when mechanical loads, lower than the strength of particles, occur multiple times. Breakage changed the morphology and surface composition of particles and decreased particle size. It also decreased the dispersibility of powders and increased the wetting time of wettable samples but decreased the wetting time of powders with poor wettability. Breakage accelerated time-dependent crystallization and decreased the crystallization temperature but did not affect the glass transition temperature of samples. Thus, under the same drying conditions, composition of powders significantly affected breakage, mainly by altering the physical properties of their particles, which resulted in deteriorated functionality.     

Viscosity of Molten Milk Chocolate with Lactose from Spray-Dried Whole-Milk Powders

Dry whole-milk powders containing 0, 30, 50 and 70% nominal lactose prepared by spray-drying alone, or followed by post-drying crystallization, were incorporated into milk chocolate to give 0–50% substitution of lactose for sucrose. Increasing the concentration of amorphous lactose from spray-dried powders in the chocolates decreased viscosity, increased particle size of refined chocolate mass, and lowered the concentration of surface-active agents at which a minimum in Casson yield value was observed. Increasing the concentration of crystalline lactose from milk powders in the chocolates increased viscosity, decreased particle size, and increased the concentration of surface-active agents at which a minimum in Casson yield value was observed. Conditions which affect lactose crystallinity in milk powders, such as improper storage and handling prior to use in chocolate production, could be responsible for variations in chocolate viscosity noted sometimes by processors.     

In-situ lecithination of dairy powders in spray-drying for confectionery applications

Powders are essential ingredients of chocolate. In particular for milk chocolate milk and whey powders are important, together with sucrose, lactose and cocoa solids. During processing to maintain a good flow of the molten chocolate mass, particles with hydrophilic surfaces, such as dairy powders and sugars, are coated with a surface-active compound. Only lecithin and polyglycerol polyricinoleate (PGPR) (at a limited level) are allowed in chocolate, and as these are expensive as little as possible is added, whilst maintaining rheological properties. Conventionally, lecithin is added during conching, and through the intense kneading of the chocolate mass it is distributed throughout the mass. Usually about 0.5% is added, although the level depends upon the composition of the chocolate. Here we present a new approach to lecithination of spray-dried milk and lactose powders, which we call in-situ lecithination. It has been found that the surface of a spray-dried powder is dominated by any surface-active species, and in a competitive situation, the most rapidly adsorbing species dominates. This behaviour is utilised when lecithin is added to the spray-dryer feed, and through the competitive adsorption of surface-active agents during the drying process, it dominates the powder surface composition as measured by X-ray photoelectron spectroscopy (XPS). This is also seen in differences in sedimentation rate when the powders are mixed with cocoa butter to assess the rheological properties of the powder dispersions. The effect was large for lactose powders, but smaller for skim milk powder and whey powder.     

The Influence of Drying Process Conditions on the Physical Properties,Bioactive Compounds and Stability of Encapsulated Pumpkin Seed Oil

In this study, the influence of encapsulation process conditions on the physical properties and chemical composition of encapsulated pumpkin seed oil was investigated. Four variants of encapsulated oil were prepared: spray-dried non-homogenized emulsions at the inlet temperatures of 180 and 130 °C, spray-dried homogenized emulsion at the inlet temperature of 130 °C, and freeze-dried homogenized emulsion. The emulsion was prepared by mixing 10.6% oil with 19.8% wall materials (15.9% maltodextrin + 0.5% guar gum + 3.9% whey protein concentrate) and 69.6% distilled water. The quality of encapsulated pumpkin seed oil was evaluated by encapsulation efficiency, surface oil, total oil and moisture contents, flowing properties, color, and size. Additionally, fatty acid composition, pigment characteristics, and the content of bioactive compounds (tocopherols, squalene, and sterols) were determined. Changes of these components after the encapsulation process in comparison to the control pumpkin seed oil were considered as stability parameters. The highest encapsulation efficiency was obtained by spray-drying at the inlet temperature of 130 °C. Generally, the spray-drying process had a positive effect upon the physical parameters of encapsulated pumpkin seed oil but results were dependent on process conditions. The higher inlet temperature generated more surface oil, but capsules obtained at the lower temperature were greater in size and more deformed. Although freeze-drying proceeded at a very low temperature, the powder obtained with this technique was characterized by the highest bioactive compound losses (with the exception of sterols) and the lowest stability. The homogenization process applied before spray-drying affected greater polyunsaturated fatty acid, squalene, and pigment degradation. In conclusion, results of the study showed that the spray-drying non-homogenized emulsion was a more recommendable technique for the encapsulation of pumpkin seed oil because of smaller changes of native compounds and better oxidative stability.     

Proposing Novel Encapsulating Matrices for Spray-Dried Ginger Essential Oil from the Whey Protein Isolate-Inulin/Maltodextrin Blends

The aim of this study was to evaluate the effects of the blending of whey protein isolate (WPI) with maltodextrin (MD) and inulin (IN) biopolymers as encapsulating matrices for spray-dried ginger essential oil. Encapsulation was performed by ultrasound-assisted emulsification and using spray drying, and the stability parameters of the emulsion (with or without ultrasound-assisted) were evaluated. The influence of these different wall material systems was investigated based on various functional properties of microparticles such as stability of the emulsion, encapsulation efficiency, reconstitution properties, chemical profile, microparticle stability, morphology, particle size distribution, and crystallinity. Higher viscosity values were obtained for the emulsions prepared with WPI and IN which had the apparent viscosity increased by the ultrasound-assisted emulsification process. Creaming index values indicated that ultrasound-assisted emulsions had higher stability. The composition of the wall materials did not affect the solubility and the moisture content of the particles. The wettability property of the powders was improved by the addition of IN. The lowest level of water adsorption under conditions of high relative humidity was also observed in microparticles containing IN. The partial replacement of WPI by MD significantly affected the efficiency of encapsulation. Moreover, MD led to high thermal microparticle stability. Larger particles were observed in the powders prepared with WPI. The powders obtained from WPI, WPI:IN, and WPI:MD treatments exhibited amorphous structures and did not have any cracks on the surface. The findings of this study indicate that IN and MD together with WPI proved to be good alternative secondary wall materials for spray-dried ginger oil.     

Influence of particle size on the physicochemical properties and stickiness of dairy powders

The compositional and physicochemical properties of different whey permeate (WPP), demineralised whey (DWP) and skim milk powder (SMP) size fractions were investigated. Bulk composition of WPP and DWP was significantly (P < 0.05) influenced by powder particle size; smaller particles had higher protein and lower lactose contents. Microscopic observations showed that WPP and DWP contained both larger lactose crystals and smaller amorphous particles. Bulk composition of SMP did not vary with particle size. Surface composition of the smallest SMP fraction (<75 μm) showed significantly lower protein (−9%) and higher fat (+5%) coverage compared with non-fractionated powders. For all powders, smaller particles were more susceptible to sticking. Hygroscopicity of SMP was not affected by particle size; hygroscopicity of semi-crystalline powders was inversely related to particle size. This study provides insights into differences between size fractions of dairy powders, which can potentially impact the sticking/caking behaviour of fine particles during processing.     

Development and application of confocal scanning laser microscopy methods for studying the distribution of fat and protein in selected dairy products.

Confocal scanning laser microscopy (CSLM) methods were developed to identify fat and protein in cheeses milk chocolate and milk powders. Various fluorescent probes were assessed for their ability to label fat or protein in selected food products in situ. Dual labelling of fat and protein was made possible by using mixtures of probes. Selected probes and probe mixtures were then used to study (a) structure development of Mozzarella cheese during manufacture and ripening, and (b)) the distribution of fat and protein in milk chocolate made with milk powders containing varying levels of free fat. Microstructural changes in the protein and fat phases of Mozzarella cheese were observed at each major step in processing. Aggregation of renneted micelles occurred during curd formation; this was followed by amalgamation of the para-casein into linear fibres during plasticization. Following storage, the protein phase of the Mozzarella became more continuous; entrapping and isolating fat globules. Chocolate made with a high free-fat spray-dried powder blend showed a homogeneous fat distribution, similar to that of chocolate made with roller-dried milk. Chocolate made with whole milk powder containing 10 g free fat/100 fat showed a non-homogeneous fat distribution with some fat occluded within milk protein particles. These differences in fat distribution were related to Casson yield value and Casson viscosity of the chocolates.     

Red ginseng extract capsule fabricated using W/O/W emulsion techniques

The objective of the present study was to investigate the physicochemical characteristics and storage stability of water-in-oil-in-water (W/O/W) emulsions and spray-dried red ginseng extract capsules (RGEC) prepared using different secondary coating materials such as maltodextrin (MD), whey protein concentrate (WPC), or arabic gum (AG). The particle sizes of W/O/W emulsions and spray-dried RGEC coated with MD were considerably lower than those of W/O/W emulsions and spray-dried RGEC coated with WPC or AG. The zeta-potential value (?36 mV) of spray-dried RGEC coated with MD was significantly higher than that of spray-dried RGEC coated with WPC or AG (p < 0.05), indicating that spray-dried RGEC coated with MD were much more stable than spray-dried RGEC coated with WPC and AG. In the storage stability study, it was found that the refrigerator temperature (4°C) was preferred for storing spray-dried samples for a long time. Based on these results, it was confirmed that MD could be the most efficient coating material for W/O/W emulsions and nano-sized spray-dried RGEC.     

Crystallization and X-ray Diffraction of Crystals Formed in Water-Plasticized Amorphous Spray-dried and Freeze-dried Lactose/Protein Mixtures

ABSTRACT: Effects of proteins (whey protein isolate [WPI], Na-caseinate, and gelatin), drying method, storage relative vapor pressure (RVP), and time on lactose crystallization and crystals formed were investigated using x-ray diffraction (XRD). Crystallization was observed from increasing peak intensities of XRD patterns. Lactose in lactose/protein (5:1,3:1) mixtures crystallized in samples stored at RVP of 44.1% and above in both spray-dried and freeze-dried materials, except in freeze-dried lactose/Na-caseinate and lactose/gelatin mixtures, which showed lactose crystallization at 54.5% RVP and above. The rate of crystallization increased with increasing RVP and storage time. The rate of crystallization in spray-dried materials was higher than in freeze-dried materials, and the crystallization rate decreased with increasing protein content. Lactose crystallized mainly as α-lactose monohydrate in spray-dried lactose/WPI and lactose/gelatin mixtures. Crystals formed in freeze-dried lactose/WPI and lactose/gelatin mixtures were anhydrous β-lactose and α-lactose monohydrate crystals. Lactose crystallized as a-lactose monohydrate in both spray-dried and freeze-dried lactose/Na-caseinate mixtures. Trace amounts of anhydrous β-lactose were present in spray-dried lactose/WPI (5:1) and lactose/gelatin (5:1) mixtures. Peak intensities of XRD patterns for anhydrous β-lactose decreased with increasing protein content and storage time. The crystallization data were successfully modeled using Avrami equation at an RVP of 65.6% and above. These data are important in understanding and predicting storage stability of lactose- and protein-containing food and pharmaceutical materials.     

Properties of spray-dried food flavours microencapsulated with two-layered membranes: Roles of interfacial interactions and water

Engineering the interface of oil-in-water emulsion droplets with biopolymers that modify its permeability could provide a novel technique to improve flavour retention in dry powders. The objective of this study was to determine if volatile compounds were more retained in dry emulsions stabilized by pea protein isolate (PPI)/pectin complex than that stabilized by PPI alone. The retention of ethyl esters during spray-drying increased with decreasing volatility of the encapsulated compound and ranged from 28% to 40%. The addition of pectin to feed emulsions was quite effective in markedly improving the retention of the three studied flavour compounds. In our previous work (Gharsallaoui et al., 2010), we showed that pectin was able to improve physical integrity of emulsion oil droplets during spray-drying. However, the pectin positive effect on both the droplet stability and the flavour retention at the time of spray-drying can also be explained by a protein molecular structure protective effect. Indeed, the obtained FTIR results showed that pectin was able to preserve the β-sheet secondary structure of pea protein when pea globulins/pectin complexes are heated. The study of the release characteristics of a flavour compound from dried powders showed that pectin addition did not affect the release profile mainly accomplished by the diffusion mechanism.