Thermal and Structural Behavior of Anhydrous Milk Fat. 3. Influence of Cooling Rate

Abstract

The crystallization behavior of anhydrous milk fat has been examined with a new instrument coupling time-resolved synchrotron x-ray diffraction as a function of temperature (XRDT) at both small and wide angles and high-sensitivity differential scanning calorimetry. Crystallizations were monitored at cooling rates of 3 and 1 degrees C/ min from 60 to -10 degrees C to determine the triacylglycerol organizations formed. Simultaneous thermal analysis permitted the correlation of the formation/melting of the different crystalline species monitored by XRDT to the thermal events recorded by differential scanning calorimetry. At intermediate cooling rates, milk fat triacylglycerols sequentially crystallize in 3 different lamellar structures with double-chain length of 46 and 38.5 A and a triple-chain length of 72 A stackings of alpha type, which are correlated to 2 exothermic peaks at 17.2 and 13.7 degrees C, respectively. A time-dependent slow sub-alpha <--> alpha reversible transition is observed at -10 degrees C. Subsequent heating at 2 degrees C/min has shown numerous structural rearrangements of the alpha varieties into a single beta' form before final melting. This polymorphic evolution on heating, as well as the final melting point observed (approximately 39 degrees C), confirmed that cooling at 3 degrees C/min leads to the formation of crystalline varieties that are not at equilibrium. An overall comparison of the thermal and structural properties of the crystalline species formed as a function of the cooling rate and stabilization time is presented. The influence on crystal size of the cooling rates applied in situ using temperature-controlled polarized microscopy is also determined for comparison.

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.…”

“…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 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 .…”

“…• Dehydration using extraction of fat with the use of solvents (hexane) according to the methodology [15].…”

“…In numerous publications concerning butter analyzed using DSC [11,12], various procedures of its dehydration were used, employing both physical and chemical methods. For example, dehydration by evaporation [4], by centrifugation [2,3,13], centrifugation and evaporation [14], and extraction [15] were applied. However, there are no studies comparing the melting and crystallization processes for milk fat in case of different butter dehydration methods.…”