Effect of Oil-Droplet Diameter on Lipid Oxidation in O/W Emulsions.

The effect of oil-droplet diameter on lipid oxidation in O/W emulsions is unclear, and conflicting results have been reported. These conflictions may be due to different experimental conditions being used, such as the type of oil, the type of emulsifier, temperature, and the range of oil-droplet diameters tested. The method used to evaluate the oxidation could also have varied among studies. In O/W emulsions, oxygen dissolved in the aqueous phase is transferred to the oil phase through the oil-water interface and is consumed in the oil phase by oxidation. Therefore, the effect of the oil-droplet diameter on the lipid oxidation rate was evaluated by simultaneously solving the mass balance equations of oxygen and oil in the oil phase. The simulation showed that the oil-droplet diameter does not affect the lipid oxidation rate in O/W emulsions with oil-droplet diameters on the order of micrometers or less because the oxidation reaction itself is rate-limiting.

Kinetic analysis of thermal degradation of betanin at various pH values using deconvolution method.

Although betanin is commonly used as a water-soluble red colorant in foods, it is unstable when heated. In this study, the discoloration of betanin in a buffer solution was kinetically studied in the pH range of 2.6 to 7.6 at 40 °C to 80 °C. The absorption spectrum of betanin was deconvoluted into the spectra of betanin and its degradation product, betalamic acid. Subsequently, a reversible consecutive reaction, in which betanin was reversibly degraded to betalamic acid, and betalamic acid was further degraded, was assumed to calculate the first-order rate constant of each step. The apparent reaction mechanism varied depending on the pH values. Based on these rate constants, the activation energy of each process, as well as the equilibrium constant and enthalpy change for the reversible reaction between betanin and betalamic acid were evaluated.

Interdependent Oxidation of Eicosapentaenoic Acid and Docosahexaenoic Acid in Isada Krill Oil Encapsulated with Maltodextrin by Spray-drying.

The major polyunsaturated fatty acids in krill oil extracted from Euphausia pacifica, known as Isada on the Sanriku coast, are eicosapentaenoic acid (EPA) and docosahexaenoic (DHA) acid. A kinetic model was proposed to explain the relationship between the fractions of unoxidized EPA (Y E) and unoxidized DHA (Y D) in the oil spray-dried with maltodextrin and stored at 25, 50, and 70℃. The relationship between Y E and Y D during storage was independent of the temperature and could be expressed using the proposed model. This indicated that the oxidation of EPA and DHA in krill oil was interdependent.

Solubility and mass transfer coefficient of oxygen through gas- and water-lipid interfaces.

The solubility of oxygen and its transfer rate to the lipid phase play important roles in lipid oxidation, which affects the taste and safety of lipid-containing foods. In this study, we measured the Henry's constants (solubility) of oxygen for fatty acids, fatty acid esters, and triacylglycerols (TAGs; vegetable oils), as well as the mass transfer coefficients of oxygen at the gas- and water-lipid interfaces. The constants and coefficients were estimated by analyzing the change over time in the oxygen partial pressure or concentration in the closed container based on the mass balance equations of oxygen in the gas and liquid phases. The constant for water obtained by the method used in this study was in agreement with the previously reported value to confirm the validity of the method. The constants for lipids depended on the lipid type, and were higher in the order of fatty acid ester, fatty acid, and TAG. That is, the solubility of oxygen decreased in this order. For all lipids, the constant increased as the number of carbon atoms in the fatty acid chain increased. The constants for fatty acids and their esters were linearly correlated with the enthalpies of evaporation of the lipids. The mass transfer coefficients of oxygen at the gas-liquid interface were on the order of 10-5 m/s for water and methyl dodecanoate and of 10-6 m/s for TAG (rapeseed oil). The coefficient at the water-lipid interface was on the order of 10-6 m/s. PRACTICAL APPLICATION: The Henry's constants (solubility) and transfer rate of oxygen to the lipid phase, fatty acids, fatty acid esters, and triacylglycerols (TAG) were measured. The lipids solubilized three to five times more oxygen than water, and mass transfer rate of oxygen at gas- and water-lipid interfaces were almost same. The constants for fatty acids and fatty acid esters were linearly correlated to their enthalpies of evaporation, and this correlation is expected to be useful for estimating the Henry's constants for other fatty acids and their esters.

Analysis of Nonisothermal Crystallization of Rapeseed Oil by Deconvolution of Differential Scanning Calorimetry Curve.

Crystallization of vegetable oil affects the efficiency of the oil-refining process and the quality of foods containing the oil. However, the crystallization of the oil is a complicated phenomenon. The crystallization behavior of rapeseed oil was investigated using differential scanning calorimetry (DSC) at a cooling rate of 0.5-10°C/min. It was found that multiple crystal structures with a metastable crystal formed at high cooling rates. A deconvolution technique was applied to analyze kinetically the DSC curves with some peaks. Each DSC curve was separated into 3-5 peaks fitted with an asymmetric double sigmoid function and a Lorentz one. Each peak was normalized by its integrated value, and the crystallinity was calculated. Using the crystallinity values, Avrami, Ozawa, and Friedman plots were obtained. The cooling rate was found to affect the crystallization mechanism and triacylglycerol (TAG) composition of the crystal in the separated peak. Even if the peak numbers are the same, the TAG composition of the crystals might be different depending on the cooling rate. Judging from the peak top temperature and area of the separated peaks, trioleoyl glycerol and 1,2-dilinolenyl-3-linoleyl glycerol would be included in crystals in the peaks with the largest and lowest peak top temperatures, respectively. The relationship between the peak top temperature and cooling rate was unique. This suggests that the crystallization behavior during isothermal storage was different from that in a nonisothermal process.

Effects of Sodium Chloride and Acetic Acid Concentrations on Dispersion Stability of Mayonnaise.

Mayonnaise is an oil-in-water (O/W) emulsion containing oil at the weight fraction of about 70%, which is stable for over a year when stored at room temperature. The stability examination of more stable mayonnaise can be time-consuming. Mayonnaise rapidly separates into oil and aqueous phases when water in the mayonnaise evaporates, which increases the ionic strength of the aqueous phase and reduces electrostatic repulsion among oil droplets. Simulating this phenomenon under reduced pressure, the stability of mayonnaises with different sodium chloride concentrations [0, 1, 3, 5 or 8% (kg/kg-aqueous phase)] and acetic acid concentrations [0, 1, 5, 10, 15 or 20% (kg/kg-acetic acid solution)] was evaluated by comparing the duration of time before each mayonnaise sample separated into oil and aqueous phases. The durations (destabilization times) correlated with the sodium chloride concentrations for mayonnaises with 1% (kg/kg-acetic acid solution) acetic acid solution. Destabilization times were independent of the sodium chloride concentration, however, for mayonnaises with greater than 10% (kg/kg-acetic acid solution) acetic acid solution. The differences in destabilization times were ascribed to denaturation of egg yolk granules. The destabilization time of commercially available mayonnaise can be similarly explained. The results of this study, which showed that the increase in the ionic strength of the aqueous phase by evaporation assessed the stability of mayonnaise in expedition way, were consistent with previously reported findings.

Induction Periods for Lipid Crystallization of Various Vegetable Oils.

The induction period for crystallization, defined as the time required to crystallize, was discussed during isothermal storage at -5 to -45°C for various vegetable oils; olive, safflower, rapeseed, corn, rice bran, soybean, and linseed oils. The induction periods, largely dependent on the oil type and storage temperature, were classified into two groups. The induction periods of corn, rice bran, soybean, and linseed oils were monotonically shortened as the storage temperature decreased. On the other hand, the induction periods for rapeseed, olive, safflower oils and the mixtures of rapeseed and soybean oils, and of olive and safflower oils did not simply change or elongate with a decrease in storage temperature. The induction periods could be formulated using two parameters. One was an expected value of the melting point of the oil, which was calculated from its fatty acid composition. The other was a molar fraction of triacylglycerol composed of the same fatty acids in the oil. The crystallization and melting processes of the oils under nonisothermal conditions were also analyzed by differential scanning calorimetry (DSC). It was suggested that the induction period was also predictable from the peak shape, peak number, onset temperature, and peak area in the DSC curve of the oil during the crystallization process.

Dispersion Stability of O/W Emulsions with Different Oil Contents Under Various Freezing and Thawing Conditions.

Freezing and thawing of oil-in-water (O/W) emulsion-type foods bring about oil-water separation and deterioration; hence, the effects of freezing and thawing conditions on the destabilization of O/W emulsions were examined. The freezing rate and thawing temperature hardly affected the stability of the O/W emulsion. O/W emulsions having different oil fractions were stored at temperatures ranging from -30 to -20 °C and then thawed. The stability after thawing depended on the storage temperature, irrespective of the oil fraction of the emulsion. A good correlation was found between the time at which the stability began to decrease and the time taken for the oil to crystalize. These results indicated that the dominant cause for the destabilization of the O/W emulsion during freezing and thawing is the crystallization of the oil phase and that the effects of the freezing and thawing rates on the stability are insignificant.

Dispersion and oxidative stability of O/W emulsions and oxidation of microencapsulated oil.

Oil-in-water (O/W) emulsions are among the dispersion systems commonly used in food, and these emulsions are in thermodynamically unstable or metastable states. In this paper, various methods for preparing O/W emulsions are outlined. Since the commodity value of food is impaired by the destabilization of O/W emulsions, experimental and theoretical approaches to assess the stability of O/W emulsions are overviewed, and factors affecting the dispersion stability of emulsions are discussed based on the DLVO theory and the concept of the stability factor. The oxidation of lipids in O/W emulsions is unhealthy and gives rise to unpleasant odors. Factors affecting the autoxidation of lipids are discussed, and theoretical models are used to demonstrate that a reduction of the oil droplet size suppresses or retards autoxidation. Microencapsulated lipids or oils exhibit distinct features in the oxidation process. Models that explain these features are described. It is demonstrated that a reduction in the oil droplet size is also effective for suppressing or retarding the oxidation of microencapsulated oils.

Destabilization of mayonnaise induced by lipid crystallization upon freezing.

The thermal and rheological history of mayonnaise during freezing and its dispersion stability after the freeze-thaw process were investigated. Mayonnaise was cooled to freeze and stored at -20 to -40 °C while monitoring the temperature; penetration tests were conducted on the mayonnaise, which was sampled at selected times during isothermal storage at -20 °C. Significant increases in the temperature and stress values due to water-phase crystallization and subsequent oil-phase crystallization were observed. The water phase crystallized during the cooling step in all the tested mayonnaise samples. The oil phases of the prepared mayonnaise (with rapeseed oil) and commercial mayonnaise crystallized during isothermal storage after 6 and 4 h, respectively, at -20 °C. The dispersion stability was evaluated from the separation ratio, which was defined as the weight ratio of separated oil after centrifuging to the total amount of oil in the commercial mayonnaise. The separation ratio rapidly increased after 4 h of freezing. This result suggests that crystallization of the oil phase is strongly related to the dispersion stability of mayonnaise.

Effects of Vegetable Oil Type and Lipophilic Emulsifiers on the Induction Period of Fat Crystallization.

The induction period of crystallization, which is defined as the time required for oil to start to crystallize, is useful indicator of the freeze-thaw stability of food emulsions such as mayonnaise. We investigated the induction period of vegetable oils with low melting points, such as rapeseed and soybean oils, which are commonly employed for mayonnaise production. The induction period was measured by monitoring the temperature of a specimen during storage at low temperature. The induction period depended on the type of oil and lipophilic emulsifier, emulsifier concentration, and storage temperature. The effect of the oil type on the induction period depended on the composition of the oil. Differential scanning calorimetry (DSC) analyses of the lipophilic emulsifiers suggested that the melting trend of the emulsifier is strongly related to the induction period.

Effect of oil droplet size on activation energy for coalescence of oil droplets in an O/W emulsion.

The activation energy of a reasonable order of magnitude was estimated for the coalescence of oil droplets in an O/W emulsion by formulating the balance of forces acting on a droplet that crosses over the potential barrier to coalesce with another droplet by the DLVO theory and Stokes' law. An emulsion with smaller oil droplets was shown to be more stable.