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.     

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.     

The effect of partial glycerides on trilaurin crystallization

The effects of partial glycerides of fatty acid chainlength from C₁₀ to C₁₈ on trilaurin crystal growth rate were investigated. Glycerides of particular form were found to have similar effects. Free fatty acids or monoglycerides either had little effect or slightly increased growth rates, whereas diglycerides, especially 1,3-diglycerides reduced them. If a particular glyceride has shorter chainlengths than trilaurin, the inhibition of crystallization is extremely small. For slightly longer chainlengths, a significant reduction of crystallization rate is seen, although smaller than with the addition of partial laurates. Maximum inhibition occurs with chainlength matching, and the magnitude decreases with increasing difference between host and guest chainlength.     

Crystallization and pressure filtration of anhydrous milk fat: Mixing effects

Melt crystallization of anhydrous milk fat and subsequent filtration of the slurry is a common process for obtaining milk fat fractions with different physical and chemical properties. The crystallization mechanism is very complex and little is known about how the crystallizer conditions and the crystal size distribution (CSD) affect the filtration process. The objective of this study was to characterize the fractionation process and determine which geometric parameters of the crystallizer affect the filtration step. Two scales of fractionation were studied, 0.6 L and 3.6 L, with crystallization at 28°C. The slurry was pressure-filtered after 24 h at 500 kPa in a 1-L chamber. Impeller diameters and speeds were varied for both scales. Photomicroscopy and spectrophotometry were used to characterize the crystallization process, and filtration rates were measured by weighing the amount of filtrate passing through the filter. Filtration resistance values, calculated using the constant pressure filtration equation, as well as photomicroscopy results indicated that the agglomerates and crystals that formed had different morphological characteristics for the different mixing and flow regimes in the crystallizer. Crystallization conditions that provide an optimal filtration time, a solid fraction with minimal liquid entrainment, and a CSD with an intermediate range of sizes (80–500 μm) having good packing properties for filtration were found.     

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.     

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.     

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.     

Polymorphism of milk fat studied by differential scanning calorimetry and real-time X-ray powder diffraction

The crystallization behavior of milk fat was investigated by varying the cooling rate and by isothermal solidification at various temperatures while monitoring the formation of crystals by differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD). Three different polymorphic crystal forms were observed in milk fat: γ, α, and β′. The β-form, occasionally observed in previous studies, was not found. The kind of polymorph formed during crystallization of milk fat from its melted state was dependent on the cooling rate and the final temperature. Moreover, transitions between the different polymorphic forms were shown to occur upon storing or heating the milk fat. The characteristic DSC heating curve of milk fat is interpreted on the basis of the XRD measurements, and appears to be a combined effect of selective crystallization of triglycerides and polymorphism.     

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.     

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.     

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.     

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.     

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.     

A kinetic study on isothermal crystallization of palm oil by solid fat content measurements

Isothermal crystallization of plam oil was studied by means of differential scanning calorimetry (DSC) as well as by nuclear magnetic resonance spectrometry to monitor its solid fat content (SFC). The temperature of crystallization (Tc) varied from 0 to 30°C, depending on the method used. The plot of %SFC vs. time at 25°C was sigmoidal in shape. However, at lower temperatures, two consecutive curves were clearly visible. Results from DSC experiments showed the following interesting features. At each Tc, the crystals produced were of different compositions. From 0 to 8°C, the thermogram showed three peaks, with the first two peaks (I and II) sharp, and the third (III) rather broad. At elevated temperatures up to 20°C, peak II disappeared totally while peak III tended to shift toward peak I. Above 20°C, both peaks shifted downward to longer times. Peak I continued to be broadened, and then suddenly disappeared at Tc above 24°C. The melting thermograms of the crystals obtained above and below this cut-off point were distinctly different. Kinetic studies on isothermal crystallization based on the data of SFC measurements showed that the data fit well into the Avrami-Erofeev equation with n=3 over the first 70% of the crystallization.     

Lipase-catalyzed synthesis of waxes from milk fat and oleyl alcohol

A screening of five lipases was carried out for the synthesis of wax esters from stoichiometric amounts of oleyl alcohol and milk fat in which long-chain fatty acid content (myristic acid, palmitic acid, stearic acid, and oleic acid) represents 70% of the total fatty acid fraction. The lipases from Alcaligenes sp. and Chromobacterium viscosum both allowed for the best ester synthesis (around 60%) within 2 and 48 h, respectively. Enzeco® Lipase Concentrate gave 30% ester yield within only 2 h. During the time period of 166 h, less than 20% ester synthesis was obtained with Lipozyme? 10,000L whereas Enzeco® Lipase XX did not catalyze the reaction. Owing to commercial availability, the food-grade Enzeco® Lipase Concentrate preparation was selected for further experiments with a view to improve wax synthesis. Wax yields were compared for three substrate molar ratios, i.e., 0.5:1, 1:1, and 1.5:1 (alcohol/fatty acid). For 0.5:1 and 1.5:1 substrate molar ratios, the addition of water increased ester yields while the effect of silica gel addition was shown to be minor. The best improvement was obtained at a substrate molar ratio of 1.5:1 with addition of water, leading to 59% wax ester synthesis.     

Differently sized native milk fat globules separated by microfiltration: fatty acid composition of the milk fat globule membrane and triglyceride core

The aim of this study was to characterize the fatty acid composition of the core and membrane of differently sized milk fat globules separated by microfiltration, which can now be used to manufacture dairy products. Native milk fat globules of various mean diameters, ranging from d₄₃ = 2.3 µm to 8.0 µm, were obtained using microfiltration of raw whole milk. After milk fat globule washing, the milk fat globule membrane (MFGM) and the triglyceride core (TC) were separated by manual churning. After total lipid extraction from each fraction, their fatty acid composition was characterized using methyl ester analysis by gas chromatography. Regardless of season, no significant differences were observed in the fatty acid composition of the MFGM phospholipids. Conversely, significant differences were found in the fatty acid composition of TC; particularly, small fat globule TC contained more medium‐chain fatty acids and less stearic acid than large fat globule TC. These results show that the previously observed differences in total fatty acid composition among differently sized milk fat globules are due to their triglyceride composition and MFGM amount rather than to the composition of the MFGM.     

TAG composition of ewe's milk fat. Detection of foreign fats

The TAG composition of 45 samples of ewe's milk, collected throughout the year from five Spanish breeds, was analyzed according to their carbon number by using short capillary column GC. The TAG content had a bimodal distribution with maxima at C₃₈ (12.8%) and C₅₂ (8.4%). The TAG composition did not vary significantly with respect to the time of year of sampling but was affected by the breed. Multiple regression equations based on TAG content are proposed to detect foreign fats in ewe's milk fat. Analysis of known mixtures of lard, palm oil, and cow's milk fat with ewe's milk fat have experimentally confirmed the accuracy of the equations.     

Prooxidative and antioxidative effects of phospholipids on milk fat

The effects of dipalmitoylphosphatidylethanolamine (DPE) and dipalmitoylphosphatidylcholine (DPC) on milk fat oxidation was examined at 50°C and 95°C under various conditions by monitoring oxygen uptake and fatty acid composition. DPE strongly inhibited milk fat oxidation both at 50°C and 95°C in the absence of water. DPC was less effective than DPE. In aqueous systems, the reverse was observed. DPE accelerated milk fat oxidation at both 50°C and 95°C. DPC accelerated the oxidation at 50°C, but inhibited it at 95°C. The free amino group in DPE may be responsible for its inhibiting effect in the dry system. The accelerating activity of DPE in the aqueous system is probably due to the formation of a more dispersed structure with better oxygen accessibility.     

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.