Study on crystallization behavior of Tibet butter

Chuanguo Ma,Songo WangTianlei Sia,,Jieyu LiXiaowei Chen

a Grain&Corn Engineering Technology Research Center,State Administration of Grain,Zhengzhou 450001,China

b College of Food Science and Engineering,Henan University of Technology,Zhengzhou 450001,China

ABSTRACT Tibetan butter(TB),generally called butter,is a solid oil product extracted from yak milk in the Qinghai-Tibet plateau area.However,due to the limitations of raw material sources and production technology,there is a shortage of TB,so it is important to find substitutes of TB.This paper studied the crystallization behavior of six kinds of commercial TB products in Tibet to provide the theoretical basis for the development of TB substitutes for the production of TB lamps and flowers.This study assessed the crystallization behaviors of the TB samples,including isothermal crystallization process and non-isothermal crystallization process.The microstructure,isothermal crystallization of TB were evaluated by polarized light microscopy and low-pulse NMR spectrometry,respectively.The non-isothermal crystallization process of TB under temperature scanning were investigated using Rheometer,and the crystallization behavior under different cooling rates were determined by differential scanning calorimetry(DSC)procedures.The results showed that the TB was crystallized at a higher supercooling(-10,0 and 10°C).Dimensional growth is dominant;at 20°C,spiral growth dominates.The TB has complex crystallization nucleation behavior.The crystal types of TB are mainly β'-crystal form and β'-like crystal form.The lower the cooling rate,the shorter the crystallization induction time,and the more the number of crystal nuclei.Therefore,during producing TB substitutes in the factory,the cooling rate can be controlled at 10°C/min to reduce energy consumption and production costs.

Keywords:Tibetan butter Crystallization kinetics Isothermal crystallization Non-isothermal crystallization Nucleation

1.Introduction

Tibetan butter(TB),traditionally made from yak milk,is the main food on the Qinghai-Tibet Plateau.It has high energy,rich nutrition,plasticity and unique nutritional and medicinal functions.It has a long history and a good reputation[1,2].TB is rich in nutrients,including protein,fat and carbohydrates[2-4],and is an essential part of the Tibetan diet.In addition,TB is also the main raw material for Tibetan people to make butter lamps and butter flowers.

However,the distribution of TB is limited as a result of its geographical origin,season of production,and storage and transport conditions.Additionally,the natural source characteristics of TB are not sufficient to satisfy the high demand,and the role of saturated fats in health failure has been highlighted with considerable evidence from the World Health Organization(2018),so it is necessary to study the characteristics of TB to provide data support for the development of alternatives[5-7].However,simply decreasing trans fats could negatively impact the functionality unless care and attention are taken to crystal form details[8-10].

It is well known that the physical and mechanical properties of crystalline polymers depend on the morphology,the crystalline structure and the degree of crystallization[11].The behavior of polymers during non-isothermal crystallization from the melt is of increasing technological importance,because these conditions are closest to real industrial processing conditions.Thus,an understanding of physical properties such as crystallization behavior,polymorphism and crystal structure in fats is necessary to control production processes so that they can be‘tailor-made’to suit particular applications.Plastic fats,such as margarine,shortening and butter,show a certain spread ability and plasticity in the application,which is because different crystal structures will be formed with the change of temperature during the processing of fats.The root cause that affects the crystal structure of plastic fat is the crystallization behavior of fats,which is also important for the processing and storage of plastic fat.Fredrick et al.[12]studied the crystallization kinetics of milk fat and found that there is a relationship between the crystallization rate constant and the change in microstructure,and there is also a certain relationship between microstructure and hardness.Changes in the kinetics of crystallization have been shown to have profound effects on both microscopic and macroscopic properties of various fat systems.Therefore,the crystalline nature has important significance for the quality of plastic fats.In this article,we would like to focus on the crystallization behaviors of TB.

Over the past several years,various studies have been focused on the characteristics of TB,mainly including fatty acid composition and physico-chemical indicators[13,14].There are a few papers dealing with the crystallization behavior of TB.In this paper,the crystallization behavior of TB was studied,including isothermal crystallization process and non-isothermal crystallization process.The results will provide theoretical support for the development of TB substitutes.In our future research,we will prepare TB substitutes on this basis.

2.Methods

2.1.Microscopic observation

Images were obtained using a polarized light microscope(Carl Zeiss AG.,Ltd.,Germany).A small amount of the molten sample was placed on a glass slide,covered with a cover glass,and left at room temperature for 24 h.The microscopic crystal morphology of the sample was observed using a polarizing microscope.

2.2.Isothermal crystallization process

The isothermal crystallization process of TB was studied by measuring the change of the Solid Fat Content(SFC)at the same temperature.Six kinds of commercial TBs products(Farmers'market,Tibet,China)were transferred into NMR tubes for a low-pulse nuclear magnetic resonance instrument(Oxford Co.,Ltd.,British)and melted at 80 °C for 30 min to erase crystal memory.Then the NMR tube was quickly moved to a water bath circulator(PolyScience Co.,Ltd.,USA)at different temperatures(-5,0,10,20°C),and the SFC value was measured every 30 s within the first 2 min.Record the SFC value every 2 min until the measured SFC value does not change significantly.

2.3.Crystallization kinetics model

The Avrami equation has been used widely to describe isothermal crystallization kinetics of polymers[15]:

where X(t)is the relative crystallinity degree,n is the Avrami exponent,representing the nucleation mechanism and growth dimension,t is time and k is the growth rate constant.

where Stis the solid fat content at time t,Smaxis the maximum solid fat content at a certain temperature.Eq.(2)can be changed to:

Using Eq.(1)connects Eqs.(2)and(3),plot ln[-ln(1-X)]against lnt,then k and n can be determined and the plots are in reasonable linearity.

2.4.Non-isothermal crystallization process

The TB samples were placed between rheometer(Thermo Fisher Scientific,USA)sample tables with the size of 35 mm and the spacing of 1 mm.The temperature sweep tests were carried out under the strain control mode with constant frequency of 0.02%.The change of storage modulus and loss modulus with temperature was measured,and the temperature range was 0-40°C.

2.5.Differential scanning calorimetry(DSC)procedures

To measure the exothermic process at different cooling rates by differential scanning calorimetry(DSC)(TA Instruments,USA),nine milligrams of the sample was pipetted into a DSC aluminum pan and hermetically sealed.It was heated from 25 °C to 80 °C at a rate of 20 °C/min under nitrogen atmosphere(flow rate is 40 mL/min).The sample was kept for 10 min at this temperature to erase any crystal history before cooling at a specified cooling rate.Constant cooling rate 1,5,10,15 and 20 °C/min were applied.The thermograms corresponding to the heating and cooling cycles were recorded and analyzed to estimate the non-isothermal crystallization kinetics[16].The data was processed by software TA.

3.Results and discussion

3.1.Microstructure

Butter,shortening,margarine and other products have certain smear,plasticity,etc.,which are related to the mechanical properties of the internal network structure,and the mechanical properties of the network structure are mainly related to the crystal form formed during the processing of the oil[17].As a kind of solid-like fat,TB also forms crystals during processing.The internal microstructure of TB is characterized by polarized light microscopy and laser scanning confocal microscopy.The polarizing microscope image of TB was shown in Fig.1.There are large and uniform spherical crystals in TB1,small and uniform crystal forms are formed in TB2and TB3,and a small amount of needle crystals can be seen.There is a relatively large spherical or dish-like crystal in TB4.Both TB5and TB6are very fine and uniform crystals.The crystal forms of these TB are different and may be related to the manufacturing process.Most TB products form relatively uniform spherical crystals.It is generally acknowledged that the larger the volume of spherulites,the less force between them,and the lower the mechanical strength of crystalline network structures[18].The needle crystals were observed,which was the main body of spherical crystal formation.The formation of needle crystals was caused by the growth of different crystal nuclei[19].Due to the uniform crystal,the TB texture is soft,ductile and plastic.

3.2.Isothermal crystallization kinetics analyses

Many methods have been suggested to study the isothermal crystallization of oils[20].In this experiment,the isothermal crystallization process of TB was studied by nuclear magnetic resonance method,and the change of the solid fat content of the sample at different temperatures was measured.The isothermal crystallization curve was shown in Fig.2.The kinetic parameters of the isothermal crystallization curve were analyzed using the Avrami equation.The kinetic parameters of the isothermal crystallization were shown in Table 1.

Fig.1.Polarized light micrographs of Tibetan butter melted and kept at room temperature for 24 h.

Fig.2.Solid fat content versus time profiles during isothermal crystallization of Tibetan butter.

Crystallization can generally be classified into two steps:nucleation and growth.Nucleation involves the formation of molecular aggregates which exceeded a critical size and therefore are stable.Once nuclei have formed,they grow and develop into crystals.The nucleation process depends on supersaturation or supercooling[21].When the crystal begins to grow,the driving force required for crystal growth is small[22].The growth rate of the crystal depends on the different molecular composition and the type of crystal at the same position.In addition,the growth rate of the crystal is also related to the degree of supercooling of the oil and the degree of supersaturation of the solution[23].It can be seen from Fig.2 that the trend of the isothermal crystallization curves of the SFCs of the six types of TBs over time is similar,and they gradually decrease with increasing temperature.It can be confirmed that temperature is the main factor affecting the TBs crystallization.The crystallization rate of TB changes the fastest at-5°C then 0°C,followed by the crystallization rate at 10 °C,and the slowest at 20 °C.Crystallization usually occurred at the beginning,especially in the first five minutes,and the crystallization process was almost completed.However,the long time required for crystallization to complete at 20°C indicated that the higher the temperature,the slower the crystallization process.The crystallization curve can be divided into two types:first,the fast crystallization rate in the early stage,the slower in the middle,and the stable hyperbola in the later stage;second,the long induction time in the early stage,the fast speed in the middle,and the stable S-curve in the later.The crystallization curves of the six kinds of TB are hyperbolic at-10,0 and 10 °C,and S-shaped at 20 °C.The difference in crystallization curves indicated that the crystallization mechanism was different,and different crystallization mechanisms usually also lead to different transformation modes of the crystalline forms[24].During processing,the influence of temperature on plastic fat is not only reflected in the change of solid fat content,but also has an important effect on its crystallization behavior,thereby affecting the texture properties of the product.

Crystallization kinetics could be quantified using the Avrami model.By fitting data to the Avrami model,parameters that describe the kinetics of crystallization and the nucleation and growth modes can be obtained,and information on the mechanism of nucleation and crystal growth via the exponent(n)[25].The Avrami exponent of TB is not a positive integer.In theory,the value of n should be an integer,but in practice,it is usually a non-integer.The value of n may be a non-integer because of two or more crystallization methods during the crystallization process[22].Spherical crystals may also be formed during the rodshaped(or needle-shaped)or dish-shaped(or plate-shaped)crystals formed in the initial stage,or they may have formed needle-shaped or rod-shaped crystals during the initial stage of the spherical crystal process.In these cases,the value of n is constantly changing.According to the microstructure of TB,it can be seen that clustered spherical crystals are formed inside TB.It is speculated that during the crystal growth process of TB,needle-like crystals may be formed at the initial stage,and then continue to grow into rod-and dish-shaped crystals,then the crystals polymerized,and finally formed larger spherical crystals.The n value of TB increases with the crystallization temperature,which is similar to the result of the isothermal crystallization kinetics of palm stearin[26].Generally,high values of k and low values of n are associated with an increased rate of crystallization and a more instantaneous nucleation process with a shorter induction time.Putte's research[27]showed that the growth mode of the crystal conformed to the screw dislocation mechanism.When the supercooling degree is high,the growth mode of the crystal is mainly two-dimensional growth.In the case of lower supercooling,the energy required for two-dimensional growth is very high,so the crystal growth screw mechanism is dominant.It can be seen from Table 1 that the crystallization rate constant k and the Avrami index n change in opposite directions.The k value decreases with increasing temperature,indicating that the higher the temperature,the slower the crystallization rate,especially at 20°C,the k value is the smallest,which is consistent with the isothermal crystallization results.

Table 1 Isothermal crystallization parameters of Tibetan butter.

3.3.Non-isothermal crystallization kinetics analyses

Isothermal crystallization of fats is the process of crystal growth of fats at a specific temperature,but in the actual production process,most of the crystallization is carried out under variable temperature conditions,so it is of great significance to study the non-isothermal crystallization of fats[28].Some studies use the rheological behavior of fats to characterize its crystal formation process.For instance,the temperature of crystallization can be determined by the intersection of the elastic modulus G′and the viscous modulus G″in the temperature scan.In the rheological curve,the temperature at which the viscosity changes suddenly is the crystallization temperature[29,30].In this study,the crystal nucleation process of the samples was observed by temperature scanning curve.Because TB has a certain lubricity,in order to prevent the sample from sliding on the test platform,a constant strain of 0.02%was selected during the temperature scan.The temperature scanning curves of the TB samples were identified as shown in Fig.3.

The elastic modulus G′and viscous modulus G″of all TB samples increased with decreasing temperature,indicating that TB gradually changed from molten to solid or semi-solid with decreasing temperature,and has certain elasticity and viscosity.The crystal melted at a higher temperature,and the internal two-dimensional or threedimensional network structure was broken,so the elastic modulus(G′)decreased with increasing temperature.For the six types of TB samples,in the range of 20-25°C,the elastic modulus G′and the viscous modulus G″had a sudden change process and the intersection point appeared,indicating that at this temperature the TBs began to change from a molten state to a gel state crystals,thus exhibiting a solid or semi-solid state.As the temperature continued to decrease,especially below 20 °C,it could be observed that there were multiple intersections of the elastic modulus G′and the viscous modulus G″,indicating that the crystal nuclei grew rapidly,and a large number of crystals had been formed.TB samples nucleated twice or more within a certain temperature range.In the process of crystallization and nucleation,a variety of different crystallization and nucleation methods appeared simultaneously.

DSC can record the melting and crystallization process of the sample with the change of temperature,so as to analyze the thermodynamic properties of the sample.This method has important guiding significance for the theoretical research of fat crystallization and the actual operation process of industrial production of shortening,cream and other products[31].The crystallization curve of the samples was studied by DSC at different cooling rates,and the crystallization behavior of TB during the rapid cooling process was further discussed.The crystallization curves of the six TB samples at 1,5,10,15,and 20°C/min cooling rate were shown in Fig.4.

Fig.3.The temperature scanning curves of Tibetan butter at a constant frequency of 0.02%(temperature range 0 to 40°C).

Fig.4.Crystallization curves of Tibetan butter at different cooling rates.

Table 2 Parameters of non-isothermal crystallization of Tibetan butter.

There are two crystalline peaks at different cooling rates for all samples.However,the diffraction intensity of the crystallization peak at 0-10°C is stronger,indicating that a large number of crystals are mainly formed at low temperatures(i.e.higher supercooling),which is consistent with the previous isothermal crystallization results.In addition,as the cooling rate decreases,the crystallization peak of TB tends to be flat(diffraction intensity weakens),indicating that as the cooling rate increased,the crystals were easier to form and had more quantity.At the second peak(crystallization peak in the range of 0-20°C),the crystallization temperature range became wider because the fat rapidly crystallized under the promotion of higher supercooling,forming mixed crystals.As a result,the crystallization temperature of the sample also became lower[32],which leads to the results in Fig.4.Studies have shown that a higher degree of supercooling will cause the viscosity of the oil phase in the system to increase,inhibit the rearrangement of triglyceride molecules,and thus promote the formation of more stable crystals in the system.The non-isothermal crystallization results of TB were shown in Table 2.The initial temperature,peak temperature and enthalpy change of the crystallization peak decrease with the increase of the cooling rate,indicating that the higher the cooling rate,the greater the internal energy of the crystal bonding and the faster the crystal formation.Che Man et al.[33]studied the crystallization curve of oils and found that the low temperature peaks correspond to α and β′crystals,and the high temperature peaks correspond to β and β′crystals,so it can be speculated that TB mainly contains β′crystal form.It can be seen from the crystallization curves of the six samples that the greater the cooling rate,the shorter the crystallization induction time and the greater the number of crystal nuclei produced.Therefore,in the factory production process,the cooling rate can be controlled to 10°C/min to reduce energy consumption and production costs.

4.Conclusions

In the study of the isothermal crystallization behavior of TB,temperature is the main factor affecting the crystallization.The complex non-isothermal crystallization process showed that within a certain temperature range,TB had secondary nucleation,even multiple nucleations.Therefore,more than two different types of crystallization and nucleation methods were observed during the nucleation of TB.In addition,as the cooling rate increases,the shorter the induction time of crystallization,the easier the formation of crystals and the greater the number of crystal nuclei.In general,the size of the crystals is influenced by the crystallization conditions applied,and the quantitative data on crystallization kinetics are a useful tool to optimize and control such a process.So it is important to know the extent to which a higher degree of supercooling will cause the viscosity of the oil phase in the system to increase,inhibit the rearrangement of triglyceride molecules,and thus promote the formation of more stable crystals in the system,or the degree of crystallization of the fat mixture at both the practical temperature and various other tempreatures.Therefore,the research results of this article provide us with information on the crystallization of TB,which will help us to further explore the development of TB substitutes on this basis.Progress in the understanding of the function of the crystallization in food now means that the fat processor and food manufacturer can work together to improve the food products available to the consumer.

Author Contributions

Chuanguo Ma:Writing-review&editing,methodology,investigation,project administration;Songbo Wang:Data curation,writing-original draft,validation;Tianlei Si:Resources,conceptualization,supervision,project administration;Jieyu Li:Data curation,resources;Xiaowei Chen:Conceptualization,supervision.

Conflicts of Interest

The authors declare that there are no conflicts of interest.

Acknowledgements

The authors wish to thank the Director-General,specifically,and management of the Malaysian Palm Oil Board(MPOB).The work was carried out with funding from the MPOB(PORTSIM 045/2016),and the National Key R&D Program of China(2016YFD041404).