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.     

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.     

Effects of minor lipids on crystallization of milk fat-cocoa butter blends and bloom formation in chocolate

Minor lipids, such as diacylglycerols, monoacylglycerols, cholesterol, and phospholipids play a key role in crystallization of fats. In this study, the effects of minor lipid components on crystallization of blends of cocoa butter (CB) with 10% milk fat or milk-fat fractions, and on bloom formation of chocolate were investigated. Both removing the minor lipids from milk fat and doubling the level of minor lipids from milk fat resulted in longer nucleation onset time, slower crystallization rate, and rapid bloom development in chocolate. Removal of minor lipids resulted in the formation of irregular primary and secondary crystals with inclusions of liquid fat, whereas the crystals were spherical and uniform in shape in the presence of minor lipids. Minor lipids from milk fat, even at the low concentrations typically found in nature, affected the crystallization of milk fat-CB blends, impacted the chocolate microstructure, and affected bloom development in chocolate.     

Temperature and concentration dependent effect of partial glycerides on milk fat crystallization

Two diglycerides (distearin and diolein) and two monoglycerides (monostearin and monoolein) were added to milk fat in a concentration of 0.5% and 1%. The isothermal crystallization behavior was evaluated at 22 °C, 23.5 °C, 25 °C and 26.5 °C by DSC and pNMR. The crystallization kinetic was quantified by means of two models. It was noticed that the effect of the minor components on the crystallization behavior depends on temperature and concentration. The type of esterified fatty acids and the polar head of the amphiphilic molecule determine to what extent partial glycerides influence the nucleation and crystal growth of triglycerides. Moreover the degree of insolubility of partial glycerides in the melt determines which effect (on growth or on nucleation) predominates. Stearic acid based partial glycerides enhance nucleation at low temperatures, while at higher temperatures an interaction with the crystal growth predominates. Oleic acid based partial glycerides have an effect on the nucleation process while no interaction with the crystal growth was observed.     

On the use and misuse of the avrami equation in characterization of the kinetics of fat crystallization

The Avrami model is widely used in the analysis of crystallization kinetic data. Unfortunately, the use of the original model has been abandoned in favor of modified versions. The modifications are largely arbitrary and create a dependence between the Avrami constant and the Avrami exponent. From a curve-fitting point of view, no advantages exist in using the modified over the original form of the Avrami model. The order of a polynomial fit to crystallization data is not equivalent to the Avrami exponent. The use of turbidity measurements for the quantitative characterization of crystallization kinetics is not valid.     

Isothermal crystallization of hydrogenated sunflower oil: II. growth and solid fat content

Isothermal crystallization of sunflower seed oil hydrogenated under two different conditions was studied by means of pulse nuclear magnetic resonance (pNMR) and optical microscopy. Solid fat content (SFC) curves showed two different shapes depending on supercooling. When supercooling was high, hyperbolic curves were found, whereas with low supercooling sigmoidal curves were obtained. Curves were interpreted with the modified Avrami equation. Photographs of the crystals were taken from the beginning of crystallization, every 15 s until 15 min and every 5 min until 60 min. Samples which exhibited hyperbolic curves showed a slight increase in crystal number, and crystals were needle-shaped in all cases. Samples which had sigmoidal crystallization curves showed a marked increase in crystal number with time, and crystals were spherical in shape. Crystallization behavior was also in agreement with the chemical composition of the samples. Samples which had the highest content of high-melting triacylglycerols (especially trielaidin) showed only hyperbolic curves. Supercooling is a very important parameter that defines the way nucleation occurs. Depending on the initial number of nuclei, two different growth mechanisms were found: a uniform linear growth of the nuclei for a small initial number (sigmoidal curves) and an aggregate of the nuclei for a high initial number (hyperbolic curves).     

Effect of DAG on milk fat TAG crystallization

The effect of milk fat and standard DAG on the crystallization behavior of milk fat TAG (MF-TAG) was investigated. When milk fat DAG were added to MF-TAG at the 0.1 wt% level, crystallization was delayed. Racemic purity was shown to be an important factor in the ability of DAG to influence TAG crystallization. Only sn-1,2 isomers of blends of MF-TAG with 0.1 wt% of the racemic mixtures of dipalmitin and diolein increased the activation free energy barrier to MF-TAG nucleation (ΔG _c ) and delayed the subsequent crystallization process by increasing the crystallization induction time (τ_SFC) determined from solid fat content-time measurements. Although crystallization kinetics were affected, the properties of the resulting network structures remained unchanged upon addition of milk fat minor components at the 0.1 wt% level     

Composition and crystallization of milk fat fractions

Milk fat was fractionated by solvent (acetone) fractionation and dry fractionation. Based on their fatty acid and acyl-carbon profiles, the fractions could be divided into three main groups: high-melting triglycerides (HMT), middle-melting triglycerides (MMT), and low-melting triglycerides (LMT). HMT fractions were enriched in long-chain fatty acids, and reduced in short-chain fatty acids and unsaturated fatty acids. The MMT fractions were enriched in long-chain fatty acids, and reduced in unsaturated fatty acids. The LMT fractions were reduced in long-chain fatty acids, and enriched in short-chain fatty acids and unsaturated fatty acids. Crystallization of these fractions was studied by differential scanning calorimetry and X-ray diffraction techniques. In this study, the stable crystal form appeared to be the β′-form for all fractions. At sufficiently low temperature (different for each fraction), the β′-form is preceded by crystallization in the metastable α-form. An important difference between the fractions is the rate of crystallization in the β′-form, which proceeds at a much lower rate for the lower-melting fat fractions than for the higher-melting fat fractions. This may be due to the much lower affinity for crystallization of the lower-melting fractions, due to the less favorable molecular geometry for packing in the β′-crystal lattice.     

Influence of minor components on fat crystallization

Crystallization in fats is of fundamental importance in the production and consumption of fats per se and of food and home and personal care (HPC) products in which fats form a major part. While crystallization of fats as such has been extensively reviewed over the past decade there has been less emphasis on the role of minor components. A review by Smith et al. [1] redressed this; this article is based on that review.     

The effect of phospholipids and water on the isothermal crystallisation of milk fat

Different amounts of phospholipids (0.00‐0.07%) and water (0.00‐0.70%) were added to milk fat. The mixtures were crystallised under isothermal conditions and the crystallisation was monitored by differential scanning calorimetry and pulsed nuclear magnetic resonance. The crystallisation behaviour was described with the Avrami and Gompertz model which was fitted by non‐linear regression. Variance analysis revealed significant effects, whereas especially the induction time was influenced: higher concentrations of water seemed to decrease the induction time, while higher amount of phospholipids delayed the onset of crystallisation. No interaction effects between phospholipids and water were observed. An attempt to explain the effect of phospholipids on the induction time, based on the molecular interactions between phospholipids and triglycerides is proposed. This principle can be applied for sn‐1, 2 diglycerides as well.     

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.     

Monitoring milk fat fractionation: Filtration properties and crystallization kinetics

The effect of fractionation temperature, residence time, and agitation rate on the chemical composition of the stearin and olein milk fat fractions was studied. During fractionation, filtration properties of the crystal suspension were monitored; crystallization kinetics was determined by ¹H NMR. Higher fractionation temperatures result in a lower stearin yield, more oil entrapment, and a lower final solid fat content of the crystal suspension. On the other hand, the chemical composition of the resulting fractions is not influenced. Longer residence times lead to longer filtration times and lower oil entrapment, whereas the yield is not affected. Longer residence times induced lower growth rates, but chemical composition is not influenced. Agitation rates varying from 10 to 15 rpm have no influence on the chemical composition of stearin and olein milk fat fractions. Higher agitation rates decrease the filtration quality and increase stearin yield, causing a softer stearin. In designing and monitoring milk fat fractionation, filtration experiments and the assessment of crystallization kinetics are valuable techniques, but compositional chemical analysis is not favorable.     

Evaluation of tripalmitin crystallization in sesame oil through a modified avrami equation

Tripalmitin (TP) crystallization in sesame oil solutions (0.98, 1.80, and 2.62%, wt/vol) was investigated by utilizing a modification of the Avrami equation. The modified equation retains the original correspondence to the nucleation process (i.e., n) and crystal growth and simply corrects the value of the crystallization rate constant (z) by eliminating the influence of n. The energy of activation (E _a ) values for TP crystallization in sesame oil solution, calculated with the modified z, were quite similar to those calculated with the reciprocal of time required to achieve 50% of TP crystallization (t _F =0.50⁻¹). However, E _a values calculated with z from Avrami’s original equation were quite different from those obtained with t _F =0.50⁻¹. Thus, z and E _a values calculated through the Avrami equation yield erroneous results, especially when comparing crystallization processes having different magnitudes of n, as in this study. Additional analysis that considered the viscosity of the TP oil solutions concluded that, at equal supercooling conditions (e.g., 22.0–22.5), the magnitude of z and E _a became more dependent upon the crystal growth process as oil viscosity decreased. In contrast, as viscosity of the oil phase increased, the main crystallization process, evaluated through z and E _a′ was nucleation. Furthermore, within the supercooling interval achieved at the temperatures utilized, the increase in supercooling at constant viscosity conditions (e.g., 5.25–5.5 dynes/cm²) would produce a higher degree of nucleation without an appreciable effect on TP crystal size. The results obtained indicate that investigating the effects of supercooling, molecular diffusion (i.e., viscosity) and TP concentration on the magnitude of z and E _a during TP crystallization in sesame oil requires a multiple variable statistical approach.     

Milk fat fractionation by solid-layer melt crystallization

The layer crystallization process has the potential to produce the same milk fat fractions as can be obtained by the suspension crystallization process. That is, milk fat fractions with solid fat content melting profiles similar to those obtained by suspension fractionation can be produced with this technique. The fatty acid profiles as well as the melting enthalpies of the different fractions confirm the separation of milk fat by the layer technique. Furthermore, there is potential to improve the results of separation presented in the first part of this paper. The two sources of improvement, temperature control of the process and controlled nucleation, lead to (i) a smooth crystalline layer with a low amount of entrapped mother liquor, contrary to the layers composed of agglomerated needles, and (ii) a good quality of attachment of the crystalline layer to the cooled surface. Moreover, the product quality can be increased using sweating as a postcrystallization step. “Sweating by warm gas” seems to have a better outlook concerning handling and controlling the process than “sweating by warm tube” because sloughing of the crystal layers can be avoided. Further investigations of the mass ratio of sweating fraction and amount of product as well as the aspect of energy consumption will determine the technical feasibility of solid-layer crystallization for fractionation of milk fat.     

Isothermal crystallization of tripalmitin in sesame oil

Crystallization of tripalmitin (TP) in sesame oil was investigated under isothermal conditions at a cooling rate similar to the one achieved in industrial crystallizers (1 K/min). The results obtained indicated that, at TP concentrations <0.98%, triacylglycerides of sesame oil developed mixed crystals with TP. However, at concentrations within the interval of 0.98 to 3.44%, tripalmitin crystallized independently from sesame oil. Within this concentration interval, discontinuities were observed in the behavior of the induction time of TP crystallization (T _i) in sesame oil as evidenced by differential scanning calorimetry, polarized microscopy studies, and determination of the Avrami index (n). In general, the discontinuities in T _i were associated with different polymorph states developed by TP in sesame oil as a function of its concentration and crystallization temperature. Thus, TP crystals obtained at temperatures above 296 K with 1.80 and 2.62% TP solutions had n values close to 3 and developed lamellar-shaped crystals that are characteristic of β tripalmitin. In contrast, the crystals obtained at temperatures of 296 K and below with 1.80% and 2.62% TP solutions provided n values close to 3. Axialite-shaped β′ TP crystals were obtained under these conditions. For the 0.98% TP solution, simultaneous production of α and β′ crystals occurred below 291 K. However, at temperatures above 291 K, a crystallization process with n=3 was obtained, and it developed a different polymorph state, i.e., β, with lamellar-shaped TP crystals.     

Effect of processing conditions on crystallization kinetics of a milk fat model system

Crystallization behavior of three blends of 30, 40, and 50% of high-melting fraction (MDP=47.5°C) in low-melting fraction (MDP=16.5°C) of milk fat was studied under dynamic conditions in laboratory scale. The effect of cooling and agitation rates, crystallization temperature, and chemical composition of the blends on the morphology, crystal size distribution, crystal thermal behavior, polymorphism, and crystalline chemical composition was investigated by light microscopy, differential scanning calorimetry (DSC), X-ray diffraction (XRD) and gas chromatography (GC). Different nucleation and growth behavior were found for different cooling rates. At slow cooling rate, larger crystals were formed, whereas at fast cooling rate, smaller crystals appeared together. Slowly crystallized samples had a broader distribution of crystal size. Crystallization temperatures had a similar effect as cooling rate. At higher crystallization temperatures, larger crystals and a broader crystal size distribution were found. Agitation rate had a marked effect on crystal size. Higher agitation rates lead to smaller crystal size. Cooling rate was the most influential parameter in crystal thermal behavior and composition. Slowly crystallized samples showed a broader melting diagram and an enrichment of long-chain triacylglycerols. Crystallization behavior was more related to processing conditions than to chemical composition of blends.     

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.     

The role of amino acids in the autoxidation of milk fat

The effects of amino acids and their analogs on milk fat oxidation were examined under various conditions by measuring oxygen consumption and total unsaturated fatty acids. All the amino acids tested acted as antioxidants, characteristically extending the induction period (IP). Not only primary amino groups are responsible for the antioxidative activities of amino acids, but also the side-chain groups contribute, at least partially, to the protective effects of L-cysteine, L-tryptophan and L-tyrosine. In aqueous and HCL solutions, the antioxidative effects of L-alanine were significantly reduced. The freeze-dried L-lysine-HCL and L-alanine-HCL accelerated, while the corresponding control amino acids inhibited, milk fat oxidation.     

Thermal behavior of fat droplets as related to adsorbed milk proteins in complex food emulsions. A DSC study

The thermal behavior of hydrogenated palm kernel oil-in-water emulsions, which differed in their milk-protein composition, was studied in parallel with other characteristic parameters such as the aggregation/coalescence of fat droplets, and the proportion of adsorbed proteins at the oil/water interface. DSC was applied to monitor the crystallization and melting behavior of nonemulsified and emulsified fat samples. Comparison between nonemulsified and emulsified fat samples showed that in emulsified samples the initial temperature of fat crystallization and the temperature of the completion of melting were invariably lower and slightly higher, respectively. Furthermore, in complex food emulsions the supercooling temperature needed to initiate fat crystallization and the variation in its growth rate in the cooling experiment were dependent on the amount and nature of the adsorbed proteins. Our results indicate that the total replacement of milk proteins by whey proteins affected the fat crystallization behavior of emulsified fat droplets, in parallel with changes in their protein surface coverage and in their physical stability against fat droplet agglomeration.     

Effect of phospholipids on isothermal crystallisation and fractionation of milk fat

Milk fats with different concentrations of water and phospholipids (PL) were crystallised isothermally under static conditions and their crystallisation behaviour was monitored by Differential Scanning calorimetry (DSC) and pulsed nuclear magnetic resonance (pNMR). The Avrami and the Gompertz models, which were fitted by non‐linear regression, described the crystallisation process. A significant effect of phospholipid concentration was observed using both techniques (DSC and pNMR). Especially the induction time and the Avrami growth rate constant were altered: higher amounts of PL delayed the onset of static crystallisation. A similar effect of PL on the crystallisation kinetics was observed in a small‐scale fractionation. Moreover, the filtration time of the crystal suspension and melting properties of the stearin were strongly affected by the presence of higher concentrations of PL. These observations emphasise the importance of the adequate removal of PL during anhydrous milk fat production.