Thermal analysis of isothermal crystallization kinetics in blends of cocoa butter with milk fat or milk fat fractions

The kinetics of isothermal crystallization of binary mixtures of cocoa butter with milk fat and milk fat fractions were evaluated by applying the Avrami equation. Application of the Avrami equation to isothermal crystallization of the fats and the binary fat blends revealed different nucleation and growth mechanisms for the fats, based on the Avrami exponent. The suggested mechanism for cocoa butter crystallization was heterogeneous nucleation and spherulitic growth from sporadic nuclei. For milk fat, the mechanism was instantaneous heterogeneous nucleation followed by spherulitic growth. For milk fat fractions, the mechanism was high nucleation rate at the beginning of crystallization, which decreased with time, and plate-like growth. Addition of milk fat fractions did not cause a significant change in the suggested nucleation and growth mechanism of cocoa butter.     

The effect of minor components on milk fat crystallization

Milk fat is composed of 97–98% triacylglycerols and 2–3% minor polar lipids. In this study triacylglycerols were chromatographically separated from minor components. Isolated diacylglycerols from the polar fraction were also added back to the milk fat triacylglycerols. The crystallization behaviors of native anhydrous milk fat (AMF), milk fat triacylglycerols (MF-TAG), and milk fat triacylglycerols with diacylglycerols added back (MF-DAG) were studied. Removal of minor components and addition of diacylglycerols had no effect on dropping points or equilibrium solid fat contents. Presence of the minor components, however, did delay the onset of crystallization at low degrees of supercooling. Crystallization kinetics were quantified using the Avrami model. Sharp changes in the values of the Avrami constant k and exponent n were observed for all three fats around 20.0°C. Increases in n around 20.0°C indicated a change from one-dimensional to multidimensional growth. Differences in k and n of MF-DAG from AMF and MF-TAG suggested that the presence of milk fat diacylglycerols changes the crystal growth mechanism. Apparent free energies of nucleation (ΔG_c,apparent) were determined using the Fisher-Turnbull model. (ΔG_c,apparent) for AMF was significantly greater than ΔG_c,apparent for MF-TAG, and ΔG_c,apparent for MF-DAG was significantly less than those for both AMF and MF-TAG. The microstructural networks of AMF, MF-TAG, and MF-DAG, however, were similar at both 5.0 and 25.0°C, and all three fats crystallized into the typical β′-2 polymorph. Differential scanning calorimetry in both the crystallization and melting modes revealed no differences between the heat flow properties of AMF, MF-TAG, and MF-DAG.     

Effect of milk fat fractions on fat bloom in dark chocolate

Anhydrous milk fat was dissolved in acetone (1∶4 wt/vol) and progressively fractionated at 5°C increments from 25 to 0°C. Six solid fractions and one 0°C liquid fraction were obtained. Melting point, melting profile, solid fat content (SFC), fatty acid and triglyceride profiles were measured for each milk fat fraction (MFF). In general, there was a trend of decreased melting point, melting profile, SFC, long-chain saturated fatty acids and large acyl carbonnumbered triglycerides with decreasing fractionation temperature. The MFFs were then added to dark chocolate at 2% (w/w) addition level. In addition, two control chocolates were made, one with 2% (w/w) full milk fat and the other with 2% (w/w) additional cocoa butter. The chocolate samples were evaluated for degree of temper, hardness and fat bloom. Fat bloom was induced with continuous temperature cycling between 26.7 and 15.7°C at 6-h intervals and monitored with a colorimeter. Chocolate hardness results showed softer chocolates with the 10°C solid fraction and low-melting fractions, and harder chocolates with high-melting fractions. Accelerated bloom tests indicated that the 10°C solid MFF and higher-melting fractions (25 to 15°C solid fractions) inhibited bloom, while the lowermelting MFFs (5 and 0°C solid fractions and 0°C liquid fraction) induced bloom compared to the control chocolates.     

Fat bloom formation and characterization in milk chocolate observed by atomic force microscopy

The surface microstructure and polymorphic behavior of milk chocolate subjected to multiple thermal cycles between 20 and 32, 33, or 34°C were examined using atomic force microscopy (AFM) and powder X-ray diffraction (XRD). The surface of unbloomed milk chocolate was smooth (surface roughness of 278 nm) and consisted of small, evenly distributed crystals. XRD results indicated the presence of mostly form V crystals and little or no form VI crystals. Cycling between 20 and 32°C resulted in little bloom formation and change in polymorphic behavior. Gradual bloom formation occurred as a result of cycling between 20 and 33°C, and was accompanied by the nascence of form VI crystals. Surface roughness increased gradually from 417 nm after one cycle to 476 and 521 nm after two and three cycles, respectively. Extensive bloom arose from cycling between 20 and 34°C. Surface roughness increased from 373 nm after one cycle to 603 and 736 nm after two and three cycles, respectively. This heavily bloomed chocolate consisted of jutting crystals and large raised, yet smooth areas that were haphazardly located within the chocolate matrix. In summary, a new perspective on the development of surface bloom due to thermal cycling is provided.     

Structure evolution and bloom formation in tempered cocoa butter during long‐term storage

Structural evolution in tempered cocoa butter (CB) and CB mixed with a cocoa butter equivalent (CBE) was examined during 26 wk of storage (at 25 °C) using atomic force microscopy, X‐ray powder diffraction, colorimetry and pulsed nuclear magnetic resonance. The form V‐to‐VI polymorphic transition in CB started after 1 week of storage. However, fat bloom was not detected until week 3 when large crystals started to appear on the CB surface. Changes in surface topography coincided with an increase in the surface whiteness index. Addition of CBE delayed bloom development by 1–2 wk. The solid fat content (SFC) of both CB and CB + CBE increased gradually during the early wk of storage before reaching a summit and then decreasing slowly with time (at 25 °C). Concurrently, the surface roughened and the whiteness index increased for both CB and CB + CBE. We postulate that, upon bloom formation, parallel phenomena took place: (i) There was exclusion of triglyceride molecules from the CB and CB + CBE fat crystal networks due to continued contraction, and (ii) less stable crystals melted due to the heat release from the (re)crystallization of liquid fat onto existing surface crystals and from the ongoing form V → VI polymorphic transition. These events resulted in the gradual decrease in SFC seen at longer storage times. In conclusion, this study demonstrated that kinetic and thermodynamic phenomena take place in CB long after it has been tempered.     

Thermal analysis of palm mid-fraction,cocoa butter and milk fat blends by differential scanning calorimetry

Commercial samples of anhydrous milk fat (AMF), Ivory Coast cocoa butter (CB) and palm mid-fraction (PMF) were blended in a ternary system. The melting characteristics of the blends were studied by differential scanning calorimetry (DSC). Results suggest that in the studies of interaction involving more than two fats, partial area (A_i) under the melting peak should be converted to partial enthalpy (ΔH_i) rather than to solid fat index. The ΔH values of the blends decreased as the amount of AMF was increased and increased as the amount of CB was increased. In general, the effect of PMF was less pronounced compared to the effect of the other two fats. Eutectic effects within the ternary system could be detected by measuring the deviation of melting enthalpy by DSC, and from the corresponding values that were calculated for the thermodynamically ideal blends. The deviation reached a maximum when the amount of AMF was about 33%. On the binary line of CB/PMF, the eutectic effect was maximum at about 50–75% PMF. The interaction effect in the system was more noticeable at 30 and 20°C than at lower temperatures. Evaluation at 30°C was preferred because both the effect of AMF in the ternary system and the effect of PMF on the binary line were more readily observed.     

Relation of fat bloom in chocolate to polymorphic transition of cocoa butter

A special chocolate with spray-dried sugar (50:50 w/w sucrose/20 Dextrose Equivalent corn syrup solids) was made to study the polymorphic changes in cocoa butter crystals using X-ray diffraction. Anhydrous milk fat (AMF) and high-, middle-, and low-melting milk fat fractions were used to replace 2% (w/w) of cocoa butter. Chocolates were tempered, and the consistency of temper among chocolate samples was verified by a temper meter. Chocolates were cycled between 19 and 29°C at 6-h intervals to induce fat bloom. The special chocolates were analyzed by X-ray spectroscopy and colormeter. X-ray analysis on the special chocolates showed polymorphic transition from the βV to the βVI form of cocoa butter. After a lag phase, the percentage of the βVI form rapidly increased. However, the sample made with the high-melting milk fat fraction transformed slowly to βVI. Visual bloom appeared rapidly on the special chocolates made with AMF, middle- and low-melting fractions, whereas visual bloom was very slow to appear on the special chocolates made with high-melting milk fat fraction and on the cocoa butter control. The commercial chocolate responded consistently; the control bloomed rapidly, the AMF exhibited some bloom resistance, and the high-melting fraction inhibited bloom. Despite the βV to βVI transition, the control chocolates with amorphous sugar did not bloom. Since the only difference in the chocolates was sugar microstructure, differences in bloom formation were caused by the microstructure, not the polymorphic transition.     

In-line measurement of tempered cocoa butter and chocolate by means of near-infrared spectroscopy

In the present work cocoa butter and chocolate were precrystallized by means of a newly developed shear crystallizer. The shear crystallizer was integrated into a circular loop. The handling of precrystallized cocoa butter showed a high dependency on the timing of applied analysis. Differential scanning calorimetry, calorimetry, rheometry, and in-line near-infrared (NIR) were all directly influenced by the fat crystal structure. Nevertheless, for cocoa butter it was shown that mechanical energy input (rpm) had a significant influence on viscosity, melting enthalpy, and slope at the second point of inflection of a temper curve. Experiments with cocoa butter at constant exit temperature showed a linear increase of viscosity between 0.1 and 0.8 Pa·s in the range of 300 to 1300 rpm. Melting enthalpy increased in the same rpm interval from 0.02 to 2.5 J/g. Solidification time (from 4.5 to 0.5 min) and slope (from 0.82 to 0.15, second point of inflection of temper curve) consequently decreased (both with exponential approximation). For cocoa butter, slope and solidification time correlated linearly whereas solidification time and viscosity followed a power law fit. This proved that defined relationships exist between rheological data and data from temper curve measurements. Viscosity was linearly dependent on crystal content. By means of NIR spectroscopy good correlation models for cocoa butter viscosity, enthalpy (crystal content), and slope values were found. For precrystallized chocolate, analytical values such as viscosity and slope values were detected off-line and used for calibration of NIR spectroscopy.     

Rheometry and polymorphism of cocoa butter during crystallization under static and stirring conditions

In this work the association between polymorphism and the crystal network structure developed by the TAG of cocoa butter (CB) was investigated under static and stirring crystallization conditions using a dynamic mechanical spectrometer. The results obtained showed that parameters obtained through oscillatory rheometry (i.e., phase shift angle, δ) followed the polymorphism of CB during static crystallization. Although standard DSC was not capable of differentiating the α to β′ phase transformation from the direct β′ crystallization from CB melt, δ rheograms measured these two processes separately. Additionally, through oscillatory rheometry, we followed the dimensionality of the crystal network during CB crystallization. Within this context, the pre-exponential term (In γ) from the weak-link regime equation for colloidal dispersions was much more sensitive than the fractal dimension (D) to differences in crystal size, spatial distribution of the crystal network, and melting temperature of the β′ phase of CB. On the other hand, torque measurements obtained during CB crystallization under stirring conditions showed a shear rate effect that favored TAG development in the β phase at temperatures of 19, 22, and 26.5°C, particularly at shear rates of 120 and 400 rpm. In contrast, under static conditions CB did not develop in the β phase at any of the crystallization temperatures investigated (i.e., 18 to 26.5°C).     

Effects of ultrasonic irradiation on crystallization behavior of tripalmitoylglycerol and cocoa butter

Effects of application of ultrasonic power (20 kHz, 100 W) on the crystallization behavior of tripalmitoylglycerol (PPP) and cocoa butter have been examined in terms of rate of nucleation and polymorphic control. High-purity PPP (>99%) and low-purity PPP (>80%) samples were employed to mimic real fat systems, which usually have higher concentrations of minor components in addition to the main component. For both the high-purity and low-purity PPP, the application of ultrasonic power accelerated the rate of nucleation as measured by induction time for the occurrence of crystals and by the number of crystals nucleated. As for the polymorphic influences, the nucleation of both the β′ and β forms was accelerated by the ultrasound, yet the β′ form nucleation was more accelerated when the low-purity PPP samples were employed. As for cocoa butter, sonication for a short period accelerated the crystallization of Form V. The present results indicate that ultrasound irradiation is an efficient tool for controlling polymorphic crystallization of fats.     

Melting and solidification characteristics of confectionery fats: Anhydrous milk fat,cocoa butter and palm kernel stearin blends

Differential scanning calorimetry measurements of crystallization and melting characteristics of commercial samples of anhydrous milk fat (AMF), cocoa butter (CB) and hydrogenated palm kernel stearin (PKS) in ternary blends were studied. Results showed that stabilization at 26°C (either for 40 h or 7 d) did not greatly affect the melting thermogram trace of PKS. However, the effect of stabilization became prominent as CB was added into the system. Deviation of measured enthalpy from the corresponding values, calculated for thermodynamically ideal blends, showed clear interaction between all three fats. At 20°C, the strongest deviation occurred at about the AMF/CB/PKS (1∶1∶1) blend, whereas at 30°C the deviation moved toward the CB/MF (1∶1) blend. The presence of 25% AMF in PKS had little effect on its solidification capability, but solidification was adversely affected with inclusion of CB.     

Dynamic crystallization of cocoa butter. I. characterization of simple lipids in rapid- and slow-nucleating cocoa butters and their seed crystals

Six cocoa butters with different crystallization induction times and their seed crystals were analyzed for simple lipid composition. The rapid-nucleating cocoa butter samples had higher concentrations of 1-palmitoyl-2-oleoyl-3-stearoylglycerol and 1,3-stearoyl-2-oleoylglycerol (SOS), and lower concentrations of the diunsaturated triacylglycerols, 1-palmitoyl-2,3-oleoylglycerol and 1-stearoyl-2,3-oleoylglycerol, as well as higher stearic acid concentrations within their diacylglycerol fractions when compared to the slow-nucleating samples. At the early stages of crystallization, under agitation conditions at 26.5°C, cocoa butters solidified into two fractions, high-melting and low-melting. The low-melting fractions were composed of polymorphs IV and V of cocoa butter, as indicated by the onset melting temperatures of the endotherms from differential scanning calorimetry. The high-melting fractions, which had wide melting ranges, had peak maxima of 38.5–52.2°C. Seed crystals isolated at the early stage of crystallization were characterized by high concentrations of complex lipids, saturated triacylglycerols, saturated fatty acid-rich diacylglycerols, and monoacylglycerols. The rapid-nucleating seed crystals had higher concentrations of SOS when compared to their respective cocoa butters. The slow-nucleating seed crystals did not exhibit this characteristic.     

Prediction of migration fat bloom on chocolate

The objective of this study was to investigate whether it is possible to predict migration fat bloom based on measurements shortly after production. At different storage times shortly after production (0, 1, 4 h), the chocolate batches, varying in tempering method, tempering degree and amount of added butter oil, were evaluated by DSC, pNMR and texture analysis. Discriminant analysis and principal component analysis were combined to investigate the potential towards prediction. The batches were classified into groups depending on the time when white spots appeared (<8 wk, 8–13 wk, >13 wk). A good separation (100% correct classifications, 100% using cross‐validation) was obtained using the afore‐mentioned analyses and storage times. It was also shown that it is possible to exclude DSC analyses or analyses at 0 h storage time without compromising the classification performances too drastically. The study further elucidated that the tempering method has no significant effect on visual fat bloom development. Furthermore, undertempered chocolates bloomed quicker than well‐tempered ones, while fat bloom was delayed on overtempered chocolates. Addition of 6% butter oil promoted fat bloom development, while no significant difference was detected between chocolate with no added butter oil and chocolate with 3% butter oil added.     

The effect of supercooling on crystallization of cocoa butter-vegetable oil blends

The solid fat content (SFC), Avrami index (n), crystallization rate (z), fractal dimension (D), and the pre-exponential term [log(γ)] were determined in blends of cocoa butter (CB) with canola oil or soybean oil crystallized at temperatures (T _Cr) between 9.5 and 13.5°C. The relationship of these parameters with the elasticity (G′) and yield stress (σ^*) values of the crystallized blends was investigated, considering the equilibrium melting temperature (T _M ^o) and the supercooling (i.e., T _Cr ^o−T _M ^o) present in the blends. In general, supercooling was higher in the CB/soybean oil blend [T _M ^o=65.8°C (±3.0°C)] than in the CB/canola oil blend [T _M ^o=33.7°C (±4.9°C)]. Therefore, under similar T _Cr values, higher SFC and z values (P<0.05) were obtained with the CB/soybean oil blend. However, independent of T _Cr TAG followed a spherulitic crystal growth mechanism in both blends. Supercooling calculated with melting temperatures from DSC thermograms explained the SFC and z behavior just within each blend. However, supercooling calculated with T _M ^o explained both the SFC and z behavior within each blend and between the blends. Thus, independent of the blend used, SFC described the behavior of G′_eq and σ^* and pointed out the presence of two supercooling regions. In the lower supercooling region, G′_eq and σ^* decreased as SFC increased between 20 and 23%. In this region, the crystal network structures were formed by a mixture of small β′ crystals and large β crystals. In contrast, in the higher supercooling region (24 to 27% SFC), G′_eq and σ^* had a direct relationship with SFC, and the crystal network structure was formed mainly by small β′ crystals. However, we could not find a particular relationship that described the overall behavior of G′_eq and σ^* as a function of D and independent of the system investigated.     

Surface roughness during storage of chocolate: Fractal analysis and possible mechanisms

Laser scanning microscopy and fractal analysis were used to determine roughness in the surface of chocolate samples stored under cycling temperature conditions (16 to 26°C) for 24 d. The four samples varied in the source of fat: 100% cocoa butter (CB), lauric and nonlauric fat replacers, and CB with 2% of nonlauric fat replacer. The response variable was the area-scale fractal complexity (Asfc), equivalent to a fractal dimension. Asfc increased with time to an asymptotic value (AV) in much the same way as whiteness index, both being accepted proxies of surface bloom. Images produced from topographical data revealed clearly the increase in roughness. Chocolate samples prepared with CB replacers exhibited an induction period and a slower rate of change in surface roughness than chocolate containing only CB. A linear relationship between a normalized roughness and the square root of time was followed by CB chocolate samples for the period before reaching AV. This result suggests that either diffusion or capillary flow may be the mechanism involved in fat migration to the surface.     

Phase transitions and polymorphism of cocoa butter

The polymorphism and phase transitions of cocoa butter (CB) have been reexamined separately by differential scanning calorimetry (DSC) and X-ray diffraction as a function of temperature (XRDT) at scanning rates between 0.1 to 5°C/min and 0.1 to 2°C/min, respectively. A new instrument, which allowed simultaneous DSC and XRDT recordings from the same sample by taking advantage of the high-energy flux of a synchrotron, was employed for characterization of the intermediate phase transitions. These techniques allowed us to confirm the existence of the six polymorphic forms of CB (called I to VI) by in situ characterization of their formation in the DSC + XRDT sample holder. A detailed study of Form I structure led us to propose a liquid-crystal organization in which some of the chains displayed sharp long-spacing lines (d₀₀₁=52.6±0.5 Å) and a β′ organization (4.19 and 3.77 Å), while the others remained unordered with broad scattering (maxima at about 112 and 36.5 Å). The organization of this liquid crystalline phase is compared to that of fat and oil liquids. This liquid crystalline phase progressively transformed on heating into a more stable phase (Form II, α type, d₀₀₁=48.5±0.5 Å and short-spacing at 4.22 Å). Form III was only observed in a sharp temperature domain through its specific short-spacings. The existence of the six species has been essentially related to the crystallization of monounsaturated triacylglycerols (TAG), while trisaturated species were found partly solid-soluble in these six polymorphic forms. An insoluble fraction crystallized independently of the polymorphism of the monounsaturated TAG in a separate phase with long-spacings that were either of the α (49.6±0.5 Å) or β (44.2±0.5 Å) form. In mixture with Form V, this fraction melts and solubilizes in the liquid phase at 37.5°C. Isolation of these high-melting crystals shows a melting point of about 50°C. High-performance liquid chromatography analysis of this fraction confirmed an increase from 3.0 to 11.3% of saturated TAG and their association with part of the 1,3-stearoyl-2-oleoylglycerol preferentially to 1-palmitoyl-2-oleoyl-3-stearoylglycerol and (1,3-palmitoyl-2-oleoylglycerol).     

Crystallization and polymorphic behaviors of cocoa butter alternatives: A review

Although cocoa butter (CB) has remarkable physical properties, its high price, growing difficulties, and increased consumption have been the main incentive to explore alternatives to replace or improve it. The potential of fats systems obtained from tropical butters, mixtures of them with vegetable oils, or marine fats as cocoa butter equivalent (CBE), extender (CBEx), substitute (CBS), replacer (CBR), or improver (CBI) have been deeply investigated and their physical chemical properties have been compared to those of CB. The TAGs composition of fats systems is a key factor that determines fats crystallization and polymorphic behaviors, and the suitability for an application. Fats with high concentrations of the TAGs StOSt, StOA, or StOB and low concentrations of POP compared to CB show some incompatibility with CB as analyzed by iso-solid diagrams and a more complex polymorphic behavior. The presence of low melting TAGs such as POO, StOO, and AOO also leads to significant differences in physical behavior compared to CB. In those fats systems, co-crystallization and polymorphic transitions of co-existing solid solutions were reported. A few of the studied fat systems may behave as CBE. However, most of them have potential as CBS, CBR, CBEx or CBI in confectionery products. Studies reported the relevance of fractionation and interesterification processes to modify TAGs composition and the need of finding the right processing conditions and additives to extend fats applications.     

The use of atomic force microscopy to measure the formation and development of chocolate bloom in pralines

Atomic force microscopy (AFM) has been used to study the surface of chocolate as well as the progress of chocolate bloom over time. Fresh chocolate was found to be relatively smooth but with deep holes. These could be pipes leading deep down into the body of the chocolate, perhaps reaching the filling. After storage for a few weeks, we observed the growth of small drops around these holes. With increasing time, these drops became larger and more structured. After further storage, a crystalline structure and bloom were revealed. These results suggest that bloom growth in pralines is a two-phase process, with drops initially forming on the surface and then bloom crystals nucleating and growing from them. Further, we deduced pipes leading down into the center of the chocolate through which the migration of filling fats can preferentially occur.     

Effect of cooling rate on crystallization behavior of milk fat fraction/sunflower oil blends

The effect of cooling rate (slow: 0.1°C/min; fast: 5.5°C/min) on the crystallization kinetics of blends of a highmelting milk fat fraction and sunflower oil (SFO) was investigated by pulsed NMR and DSC. For slow cooling rate, the majority of crystallization had already occurred by the time the set crystallization temperature had been reached. For fast cooling rate, crystallization started after the samples reached the selected crystallization temperature, and the solid fat content curves were hyperbolic. DSC scans showed that at slow cooling rates, molecular organization took place as the sample was being cooled to crystallization temperature and there was fractionation of solid solutions. For fast cooling rates, more compound crystal formation occurred and no fractionation was observed in many cases. The Avrami kinetic model was used to obtain the parameters k _n and n for the samples that were rapidly cooled. The parameter k _n decreased as supercooling decreased (higher crystallization temperature) and decreased with increasing SFO content. The Avrami exponent n was less than 1 for high supercoolings and close to 2 for low supercoolings, but was not affected by SFO content.     

Heat-resistant cocoa butter extenders from mahua (Madhuca latifolia) and kokum (Garcinia indica) fats

Cocoa butter extenders with heat-resistant properties were prepared using mahua and kokum fats. The stearin fraction [Fraction (Fr.) 1, 77–80% yield] obtained by solvent fractionation of 50:50 blends of these fats showed a steep melting profile with a higher solid fat content (SFC) at 32.5°C than cocoa butter, even after mixing with it at 25 or 50% levels. The solidification characteristics showed that the Fr. 1 had a supercooling property similar to cocoa butter, but showed higher temperature rise with less crystallization time on the cooling curve, which is advantageous for chocolate molding. Fr. 1 was compatible with cocoa butter at all proportions, as revealed by cooling curves and isothermal solid diagrams. The stearin fraction obtained by dry fractionation of mahua/kokum blend (Fr. 2, 77% yield), though, had similar solidification characteristics and showed lower SFC compared to that of Fr. 1. Fr. 1 and Fr. 2 have high levels of 2-oleo-distearin triacylglycerols (46–51%), which are responsible for better stand-up property at high temperatures compared to cocoa butter. The suitability of the blends of mahua/kokum fats and mahua stearin/kokum fats as cocoa butter extenders was also evaluated. The isothermal solid diagrams showed complete miscibility of the two fats fractions. The results showed that a series of cocoa butter extenders with varying melting characteristics could be prepared by fractionating and by physical blending of mahua and kokum fats in selected proportions.