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

Abstract

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.Milk fat is one of the main constituents of milk and determines the specific properties of butter and cream. It is also an important ingredient in many bakery and confectionery industry applications. The various applications require different properties of milk fat, which in turn requires improved functionality control. The functional properties of milk fat are strongly related to the amount and type of milk-fat crystals at the temperature of application. The crystalline part of the fat determines to a large extent the firmness of products in which fat is present as the continuous phase, such as butter and butter oil, and the stability of products containing an emulsion of milk fat, such as cream. Milk fat has a broad melting range due to a large number of triglycerides with a wide range of chain lengths and degrees of saturation. Moreover, the phase behavior is complicated because of the polymorphism of the solid phase.Polymorphism of the crystallized phase is a general feature of triglycerides. The different polymorphic forms can be identified by X-ray diffraction (XRD). The polymorphic forms are characterized by the d-spacings (short-spacings) of the crystal lattices (typically between 3 and 6 Å) as observed in XRD patterns, which correspond to the distances associated with the lateral packing of the fatty acid hydrocarbon chains. Polymorphs with similar packing of the fatty acid hydrocarbon chains were found for pure tristearin (1), and for natural oils and fats (2), including milk fat (3). The d-spacings are characteristic for the type of polymorph, and this has led to the nomenclature given by Larsson (4) that is now widely accepted. Table 1 lists the d-spacings of the polymorphs of triglycerides (2). In general, the stable polymorph of triglycerides is either a β′-or a β-crystal form. The density of the β-crystal form is higher than that of the β′-crystal form, and this leads to more severe packing constraints for the first form as compared with the latter (5). As a result, asymmetrical triglycerides, i.e., triglycerides of the SSU or UUS type, in which the single unsaturated (U) or saturated (S) fatty acid resides in either the sn-1 or ...

Melting and crystallization DSC profiles of milk fat depending on selected factors

“…The temperature, at which milk fat begins to crystallize (T c on), decreases with an increase in cooling rate from 21.35°C for the rate of 1°C min -1 to 11.1°C for the rate of 20°C min -1 . Similar dependencies in milk fat were also observed by others authors [15,16]. It is a different situation than in case of pure substances.…”

“…In contrast to the crystallization process, in the melting process no statistically significant differences were found between values of temperatures (T m 1, T m on, T m 2, T m end) in the range of heating rates from 2 to 20°C min -1 . A lack of significant differences between values of final melting temperature (T m end) were also found in other studies [15,16,24]. Values of enthalpy did not differ for heating rates from 2 to 10°C min -1 , while significantly different values were recorded for 1 and 20°C min -1 .…”

“…It was observed that for the rate of 1°C min -1 values of crystallization points are shifted more markedly than in case of other rates, which may results from a different mechanisms of crystallization at very low rates and the formation of other polymorphic forms with higher crystallization points. This was confirmed by XRD analyses conducted by ten Grotenhuis et al [16], who stated for cooling rate C2.5°C min -1 the formation of a forms together with the c form, while for the rate of B1°C min -1 found mainly the formation of a more stable form b 0 , with a higher crystallization point. The recorded values of temperatures and enthalpy at different cooling rates are very similar to those presented by Lopez et al [15] and ten Grotenhuis et al [16].…”

“…This was confirmed by XRD analyses conducted by ten Grotenhuis et al [16], who stated for cooling rate C2.5°C min -1 the formation of a forms together with the c form, while for the rate of B1°C min -1 found mainly the formation of a more stable form b 0 , with a higher crystallization point. The recorded values of temperatures and enthalpy at different cooling rates are very similar to those presented by Lopez et al [15] and ten Grotenhuis et al [16]. A reduction of crystallization point for triacylglyceroles with an increase in cooling rate was also found in pure triacylglycerols [17] and different vegetable oils [18][19][20].…”

“…In case of the rate of 1°C min -1 significantly higher temperatures (T m 1, T m on, T m 2, T m end) were also found, which may confirm the formation of more stable crystalline forms. Ten Grotenhuis [16] using XRD observed that at a rate of 1°C min -1 during crystallization only the stable b 0 form is formed. This phenomenon may be confirmed by the disappearance of the first peak T1 at a rate of 1°C min -1 .…”