Effect of pH and Ionic Strength on the Physicochemical Properties of Coconut Milk Emulsions

ABSTRACT:  Coconut milk (16% to 17% fat, 1.8% to 2% protein) was extracted from coconut ( Cocos nucifera L.) endosperm and diluted in buffer to produce natural oil-in-water emulsions (10 wt% oil). The effect of pH (3 to 7) and NaCl (0 to 200 mM) on the properties and stability, namely, mean particle size, ζ-potential, viscosity, microstructure, and creaming stability, of the natural coconut milk emulsions was investigated. At pH values close to the isoelectric point (IEP) of the coconut proteins (pH 3.5 to 4) and in the absence of NaCl, coconut milk flocculated, but did not coalesce. Flocculation corresponded to low surface charges and was accompanied by an increase in emulsion viscosity. Adding up to 200 mM NaCl to those flocculated emulsions did not change the apparent degree of flocculation. Coconut milk emulsion at pH 6 was negatively charged and not flocculated. Upon addition of salt, the ζ-potential decreased from –16 to –6 mV (at 200 mM NaCl) but this was not sufficient to induce flocculation in coconut milk emulsions. At low pH (< IEP), the positively charged droplets of coconut milk emulsions only flocculated when the NaCl concentration exceeded 50 mM, as the ζ-potential approached zero.     

慢消化椰油乳液系列体系的构建及其微流变特性和体外消化特性对比

本文以椰子油为芯材,乳清分离蛋白(Whey protein isolate,WPI)为壁材制备单层椰油乳液,再以单层椰油乳液为芯材,分别以羧甲基纤维素钠(Carboxmethylcellulo sesodium,CMC)、纤维素纳米晶体(Cellulose nanocrystals,CNC)、壳聚糖(Chitosan,CNI)、微晶纤维素(Microcrystalline cellulose,MCC)为壁材制备四种双层椰油乳液,进而探究各乳液体系的微流变特性和体外消化特性。结果显示,WPI-CNC稳定的椰油乳液体系粘弹性最高(P<0.05),乳液中的粒子不能自由运动,乳液的固液平衡值最低(P<0.05),乳液中粒子运动的速率低;WPI-CNC稳定的椰油乳液有最低的肠释放率,且释放速率最为缓慢;除WPI-CNC稳定的椰油乳液外,各乳液体系经胃相消化后均出现明显聚集,小肠消化后聚集程度增加;WPI、WPI-CNC、WPI-CMC稳定的椰油乳液经过口腔、胃、肠消化后平均粒径依次增加,粒径分布出现多峰现象;肠消化后,各乳液表面负电位增大。综上,椰油乳液的流变学特性显著影响其体外消化率,WPI-CNC稳定的椰油乳液体外消化率最低且消化最慢。     

The use of sedimentation field-flow fractionation in the size characterization of bovine milk fat globules as affected by heat treatment

Size distribution of fat globules affects the appearance, taste and stability of milk and milk-based products. Full-fat, semi-fat and chocolate bovine milk were subjected to heat treatment within a temperature range of 50–125 °C for 1 h. Sedimentation field-flow fractionation was employed to determine the changes in mean particle diameter of milk fat globules as affected by heat treatment. The mean particle diameter of fat droplets increased with increasing heating temperature for most samples. The particle size of fat globules increased on average 40 nm (4.65%) for full-fat and 72 nm (8.52%) for semi-fat milk following the heat treatment (50–125 °C). Chocolate milk exhibited considerable increase in particle size (104 nm, 12.53%) within a certain temperature range (50–110 °C), followed by a decrease in particle size when heated at 125 °C for 1 h. Heat-induced flocculation due to attractive interactions between hydrophobic sites on denatured protein molecules on different droplets was assumed to be mainly responsible for the increases in particle size observed in this study. Extensive heat-induced denaturation of milk proteins was also indicated by Native PAGE. Sedimentation field-flow fractionation proved to be a useful technique for adequately monitoring heat-induced changes in particle size distributions in milk.     

Interactions between food components in ice cream. Part 1: unfrozen emulsions

Ice cream was manufactured on a pilot plant and the structure of the emulsion was estimated in terms of droplet size distribution and protein composition of the aqueous phase after homogenization (two stages: 19 MPa + 3 MPa, 70°C) and after ageing ( 18 h, 4°C). Four different factors were studied: the nature of the milk protein [skim milk powder (SMP), skim milk replacer (SMR) or whey protein concentrate ( WPC) ], the nature of the emulsifier (saturated monoglycerides or Sugin Fl50, which is apolysorbate 80-based emulsifier) and its concentration (0.17–0.67% w/w for Sugin F150; 0.20–0.54% wlw for saturated monoglycerides), and the amount of butter oil (8–12% wlw). Freshly homogenized mixes containing either SMP or an SMR were stable during the ageing stage, irrespective of the nature and the concentration of the emulsifier. WPC-based mixes, however, were destabilized after homogenization: this destabilization was found to be flocculation only, which shows that whey proteins are efficient against coalescence. The quantity of adsorbed protein per surface unit was systematically higher for SMP mix than for both SMR and WPC. After the ageing stage, the structure of the mixes containing monoglycerides or WPC + polysorbate 80 remained unchanged. However, polysorbate 80 used in combination with both SMP and SMR led to a destabilization of the mix during the ageing stage: this destabilization was found to depend upon the mass/surface ratio of polysorbate 80 to butteroil.     

Effect of different treatments for the destabilization of coconut milk emulsion

Coconut milk is an emulsion which is stabilized by naturally occurring proteins. The main objective of the present work is to explore different methods employing thermal, pH, chilling, enzyme treatments and combination of enzyme treatments followed by chilling and thawing for effective destabilization of the coconut milk emulsion. Stability of emulsion is evaluated by measuring the creaming index and observed for the changes in structure of oil droplets, using phase contrast microscope. Combination of treatments (enzyme treatment at 37 °C followed by chilling and thawing) of coconut milk emulsion has resulted in highest yield of 94.5%. Physico-chemical properties and fatty acid compositions are evaluated for coconut oil obtained by combination of treatments and compared with that of commercial coconut oil. It is found that the oil obtained by combination of treatments is low with respect to free fatty acids and peroxide value and high in lauric acid content.     

Formation, Stability, and Properties of an Algae Oil Emulsion for Application in UHT Milk

This study aimed to develop a technology for producing ultrahigh temperature ultrahigh temperature-treated (UHT) milk enriched with docosahexaenoic acid. Starch hydrophobically modified with octenyl succinic anhydride (OSA starch) was used as an emulsifier to make algae oil emulsion in UHT milk. In this study, the stability of oil-in-water emulsions was examined. The emulsification of 10 % algae oil model emulsion with 10 % OSA starch and 40 % corn syrup had small droplets and was completely stable. Milk enriched with unsaturated fatty acids was heated using an indirect UHT treatment, and the milk was then stored at different temperatures. The oxidative stability of fish oil-enriched milk was investigated by measuring peroxide value, measuring volatile secondary oxidation products, and carrying out sensory analysis. All of the milk samples were stable. In summary, fish oil-enriched milk is resistant to oxidation. Algae oil-enriched drinking milk is a stable product during 11 weeks of storage. Application of high storage temperature (40 °C) does not significantly increase the oxidation process. The present study suggested that stable algae oil emulsion can be formed by OSA starches with corn syrup, and a food formulation test confirmed the successful application of algae oil emulsion to extend the shelf life of milk.     

Physicochemical Properties of Whey Protein-Stabilized Emulsions as affected by Heating and Ionic Strength

Corn oil-in-water emulsions (19.6 wt%; d₃₂～ 0.6 μm) stabilized by 2 wt% whey protein isolate (WPI) were prepared with a range of pH (3–7) and salt concentrations (0–100 mM NaCl). These emulsions were heated between 30 and 90°C and their particle size distribution, rheological properties and susceptibility to creaming measured. Emulsions had a paste-like texture around the isoelectric point of WPI (～φ 5) at all temperatures, but tended to remain fluid-like at pH >6 or <4. Heating caused flocculation in pH 7 emulsions between 70 and 80°C (especially at high salt concentrations), but had little effect on pH 3 emulsions. Flocculation increased emulsion viscosity and creaming. Results were interpreted in terms of colloidal interactions between droplets.     

The influence of enzymatic cross‐linking of proteins on the Maillard reaction in milk

The influence of transglutaminase (TGase) on the Maillard reaction was investigated in skimmed milk samples during heat treatment. TGase‐treated and control samples were heated at 80, 120 and 140 °C for 1, 5, 15, 30, 40 and 60 min. Compared with the TGase‐treated samples heated at 80 and 120 °C, the sample heated at 140 °C showed a larger decrease in furosine concentration. It was also found that TGase did not affect the formation of hydroxymethylfurfural and lactulose at 120 °C, whereas their concentrations increased in the presence of TGase at 140 °C. It was concluded that blockage of lysine residues via enzymatic cross‐linking of milk proteins had a limited effect on the Maillard reaction.     

Comparison of the effects of high-pressure microfluidization and conventional homogenization of milk on particle size,water retention and texture of non-fat and low-fat yoghurts

The effect of high-pressure homogenization using a Microfluidizer^® on texture, water-holding capacity, and extent of syneresis on stirred yoghurts was compared with that of conventional homogenization. The effect of homogenization condition on particle size was also assessed in milk and in yoghurt. Stirred yoghurts were prepared from recombined milk samples (0 and 1.5% fat) heat-treated (95 °C, 2 min) and then treated by conventional valve homogenization at 25 MPa or microfluidization at 150 MPa. Homogenization conditions influenced the particle size in milk, gel particle size, and textural quality of stirred yoghurts in a manner dependent upon fat content. Milk microfluidized at 150 MPa had smaller particle size than homogenized milk, but resulted in larger particles in yoghurt. Microfluidization of low-fat milk modified the microstructure of yoghurt, giving more interconnectivity in the protein networks with embedded fat globules, but with similar texture profiles and water retention compared with yoghurt made from conventionally homogenized milk.     

The effects of microfluidization on the physical,microbial, chemical,and coagulation properties of milk

This work examines the use of mild heat treatments in conjunction with 2-pass microfluidization to generate cheese milk for potential use in soft cheeses, such as Queso Fresco. Raw, thermized, and high temperature, short time pasteurized milk samples, standardized to the 3% (wt/wt) fat content used in cheesemaking, were processed at 4 inlet temperature and pressure conditions: 42°C/75 MPa, 42°C/125 MPa, 54°C/125 MPa, and 54°C/170 MPa. Processing-induced changes in the physical, chemical, and microbial properties resulting from the intense pressure, shear, and cavitation that milk experiences as it is microfluidized were compared with nonmicrofluidized controls. A pressure-dependent increase in exit temperature was observed for all microfluidized samples, with inactivation of alkaline phosphatase in raw and thermized samples at 125 and 170 MPa. Microfluidization of all samples under the 4 inlet temperature and processing pressure conditions resulted in a stable emulsion of fat droplets ranging from 0.390 to 0.501 μm, compared with 7.921 (control) and 4.127 (homogenized control) μm. Confocal imaging showed coalescence of scattered fat agglomerates 1 to 3 μm in size during the first 24 h. We found no changes in fat, lactose, ash content or pH, indicating the major components of milk remained unaffected by microfluidization. However, the apparent protein content was reduced from 3.1 to 2.2%, likely a result of near infrared spectroscopy improperly identifying the micellar fragments embedded into the fat droplets. Microbiology results indicated a decrease in mesophilic aerobic and psychrophilic milk microflora with increasing temperature and pressure, suggesting that microfluidization may eliminate bacteria. The viscosities of milk samples were similar but tended to be higher after treatment at 54°C and 125 or 170 MPa. These samples exhibited the longest coagulation times and the weakest gel firmness, indicating that formation of the casein matrix, a critical step in the production of cheese, was affected. Low temperature and pressure (42°C/75 MPa) exhibited similar coagulation properties to controls. The results suggest that microfluidization at lower pressures may be used to manufacture high-moisture cheese with altered texture whereas higher pressures may result in novel dairy ingredients.     

Coconut Milk and Coconut Oil: Their Manufacture Associated with Protein Functionality

Coconut palm (Cocos nucifera L.) is an economic plant cultivated in tropical countries, mainly in the Asian region. Coconut fruit generally consists of 51.7% kernel, 9.8% water, and 38.5% shell. Coconut milk is commonly manufactured from grated coconut meat (kernel). Basically, coconut milk is an oil‐in‐water emulsion, stabilized by some proteins existing in the aqueous phase. Maximization of protein functionality as an emulsifier can enhance the coconut milk stability. In addition, some stabilizers have been added to ensure the coconut milk stability. However, destabilization of emulsion in coconut milk brings about the collapse of the emulsion, from which virgin coconut oil (VCO) can be obtained. Yield, characteristics, and properties of VCO are governed by the processes used for destabilizing coconut milk. VCO is considered to be a functional oil and is rich in medium chain fatty acids with health advantages.     

Secondary adsorption of milk proteins from the continuous phase to the oil–water interface in dairy emulsions

Low protein surface concentration emulsions are susceptible to secondary protein adsorption where protein moves from the continuous phase to the existing, oil–water interface. The resulting increase in protein surface concentration can greatly alter emulsion properties. Butteroil was emulsified with whey proteins and the emulsion was combined with a solution of dissolved skim milk powder (SMP), producing mixes with fat and protein levels representative of ice cream. The primary adsorbed layer was modified by heating the whey protein solution prior to emulsion formation (70°C, 80°C, 90°C), by heating the emulsion (70°C, 80°C, 90°C) or by pH adjustment of the emulsion (6–8). Modifications of the SMP solution included heat treatment (80°C, 95°C) or sugar addition with or without κ-carrageenan. The effect of addition of SMP solution on the protein surface concentration and shear stability of the diluted emulsions was determined. Addition of untreated solution to the control, heated or pH adjusted emulsions greatly reduced shear destabilization and increased the protein surface concentration. Addition of heat treated or sugar containing SMP solution to the control emulsion produced the same result. However, sugar and carrageenan in the mix maintained the susceptibility to partial coalescence and reduced the secondary adsorption of caseins and whey proteins.     

The effect of homogenization of whole milk, skim milk and milk fat on nisin activity against Listeria innocua

Whole milk, skim milk and an emulsion of milk fat in water, inoculated with approx. 10(5) cfu/ml of Listeria innocua, were treated at 30 degrees C with 100 IU/ml of nisin, homogenization at 200 bar or both procedures. Nisin activity and survival of L. innocua after treatments were determined. Recovery of nisin activity from non-homogenized whole milk treated with 100 IU/ml of nisin was complete, whereas a loss of 18 to 28% of activity was detected in non-homogenized fat-in-water emulsion. Loss in nisin activity due to homogenization represented up to 64% in whole milk and 62% in fat-in-water emulsion. Nisin addition by itself achieved a reduction in L. innocua counts of 3.7-3.8 log units in whole milk and 3.6 log units in fat-in-water emulsion compared to numbers in untreated samples. When nisin-containing whole milk and fat-in-water emulsion were homogenized, L. innocua counts were only reduced by 2.6-2.9 log units and 2.5 log units, respectively, compared to numbers in untreated samples. Homogenization of nisin-containing skim milk resulted in a loss of nisin activity of 20% but achieved a reduction of 3.0 log units in L. innocua counts.     

Characterization and acid-induced gelation of butter oil emulsions produced from heated whey protein dispersions

Whey protein isolate was dispersed at 4% or 8% (w/v) and heated at neutral pH to produce protein polymers. Butter oil, up to 20%, was homogenized in heated whey protein dispersions at pressure ranging from 10 to 120 MPa. Emulsion gelation was induced by acidification with glucono-δ-lactone. Whey protein polymers produced finely dispersed emulsions with fat droplet diameter ranging from 340 to 900 nm. Homogenization pressure was the main factor influencing droplet size. At low fat volume fraction, the emulsions exhibited Newtonian behaviour. As fat content increased, shear thinning behaviour developed as a result of depletion flocculation. Emulsion consistency index increased with protein and fat concentrations. Increasing homogenization pressure had no effect on Newtonian emulsions but promoted flocculation and significantly increased the consistency of high fat emulsions. Protein concentration was the main factor explaining emulsion gel hardness and syneresis. Syneresis decreased with increasing fat content in the gel.     

Droplet Size and Coalescence Stability of Whey Protein Stabilized Milkfat Peanut Oil Emulsions

Droplet diameter and the polydispersity of droplet size tended to decrease with increased proportion of peanut oil. Macromolecular additives affected droplet size. Xanthan gum or sodium carboxymethyl-cellulose (Na-CMC) produced smaller droplets than the controls, but the average diameter was independent of the composition of the dispersed phase. At 50°C the coalescence stability of these emulsions qualitatively correlated with the initial droplet diameter. Xanthan gum and Na-CMC, despite increasing continuous phase viscosity, gave lower stability than controls, primarily the result of flocculation of droplets due to depletion of the highly hydrophilic macromolecule from the intervening region between approaching droplets. At 25°C, stability increased as solid butter oil content of the dispersed phase increased.     

Emulsifying Properties of Bovine Milk Fat Globule Membrane in Milk Fat Emulsion: Conditions for the Reconstitution of Milk Fat Globules

A procedure for the reconstitution of milk fat globules (MFG) stabilized with milk fat globule membrane (MFGM) was developed. MFG was reconstituted by homogenizing a mixture of 1% MFGM and 25% milk fat at 45°C and at pH 7.0 for 1 min. The emulsifying properties of MFGM were evaluated by emulsifying activity (EA), emulsion stability (ES), whippability and foam stability. Of the variables affecting the reconstitution of MFG, prolonged homogenization decreased EA and ES. About 25% milk fat gave maximal EA and ES, increasing the MFGM concentration increased both EA and ES, which were also influenced by the pH level. Foam disappeared at >30°C.     

Surface Properties of Heat‐Induced Soluble Soy Protein Aggregates of Different Molecular Masses

Suspensions (2% and 5%, w/v) of soy protein isolate (SPI) were heated at 80, 90, or 100 °C for different time periods to produce soluble aggregates of different molecular sizes to investigate the relationship between particle size and surface properties (emulsions and foams). Soluble aggregates generated in these model systems were characterized by gel permeation chromatography and sodium dodecyl sulfate‐polyacrylamide gel electrophoresis. Heat treatment increased surface hydrophobicity, induced SPI aggregation via hydrophobic interaction and disulfide bonds, and formed soluble aggregates of different sizes. Heating of 5% SPI always promoted large‐size aggregate (LA; >1000 kDa) formation irrespective of temperature, whereas the aggregate size distribution in 2% SPI was temperature dependent: the LA fraction progressively rose with temperature (80→90→100 °C), corresponding to the attenuation of medium‐size aggregates (MA; 670 to 1000 kDa) initially abundant at 80 °C. Heated SPI with abundant LA (>50%) promoted foam stability. LA also exhibited excellent emulsifying activity and stabilized emulsions by promoting the formation of small oil droplets covered with a thick interfacial protein layer. However, despite a similar influence on emulsion stability, MA enhanced foaming capacity but were less capable of stabilizing emulsions than LA. The functionality variation between heated SPI samples is clearly related to the distribution of aggregates that differ in molecular size and surface activity. The findings may encourage further research to develop functional SPI aggregates for various commercial applications.     

The effect of heat treatment of caprine milk on the composition of cheese whey

In three independent trials, caprine milk from the same batch was divided into three lots, which were heated at 65 °C for 30 min, 80 °C for 5 min or 90 °C for 5 min. Representative whey samples collected during the whole cheese making process were analysed for fat, protein and dry matter contents, which decreased as the heating temperature of milk increased. Percentages of serum albumin and β-lactoglobulin in the total proteins of whey decreased as the heating temperature of milk increased, while α-lactalbumin and glycomacropeptide increased, particularly in the 90 °C whey. Lactoferrin and the immunoglobulin-heavy chain were only detected in the 65 °C whey.     

Disulfide-mediated Polymerization Reactions and Physical Properties of Heated WPI-stabilized Emulsions

Heating a 19 wt% corn oil-in-water emulsion stabilized by 1 wt% whey protein isolate from 30 to 70°C and then cooling to 25°C for at least 15 hr, brought about minimal changes in droplet aggregation, apparent viscosity and susceptibility to creaming. At 75°C, droplet aggregation occurred but this decreased on heating to 90°C. The apparent viscosity and susceptibility of droplets to creaming increased as the degree of droplet aggregation increased. Inclusion of the sulfhydryl blocking agent N-ethylmaleimide to inhibit thiol/disulfide interchange reactions did not affect droplet aggregation but resulted in higher apparent viscosity values and susceptibility to creaming at 85 and 90°C and not at lower temperatures. The results suggest that droplet aggregation results from noncovalent interactions between unfolded protein molecules adsorbed on different droplets and that the interactions are strengthened by disulfide bonds.     

Interfacial and emulsifying properties of sucrose ester in coconut milk emulsions in comparison with Tween

In this study, sucrose esters were presented as a promising alternative to petrochemically synthesized Tweens for application in coconut milk emulsions. The interfacial and emulsifier properties of sucrose ester (SE), mainly sucrose monostearate, had been investigated in comparison with Tween 60 (TW), an ethoxylate surfactant. The interfacial tension measurement showed that SE had a slightly better ability to lower the interfacial tension at coconut oil–water interface. These surfactants (0.25 wt%) were applied in coconut milk emulsions with 5 wt% fat content. The effects of changes in pH, salt concentration, and temperature on emulsion stability were analyzed from visual appearance, optical micrograph, droplet charges, particle size distributions, and creaming index. Oil droplets in both SE and TW coconut milk emulsions extensively flocculated at pH 4, or around the pI of the coconut proteins. Salt addition induced flocculation in both emulsions. The pH and salt dependence indicated polyelectrolyte nature of proteins, suggesting that the proteins on the surface of oil droplets were not completely displaced by either added nonionic SE or TW. TW coconut milk emulsions appeared to be thermally unstable with some coalesced oil drops after heating and some oil layers separated on top after freeze thawing. The change in temperature had much lesser influence on stability of SE coconut milk emulsions and, especially, it was found that SE emulsions were remarkably stable after the freeze thawing.