## Lesson 31. FRACTIONATION OF MILK FAT

Module 10. Special butter and related products

Lesson 31
FRACTIONATION OF MILK FAT

31.1 Introduction

The physical properties such as melting point and consistency of butter are depending on chemical composition of the milk fat. The different triglycerides have different melting points and can therefore easily divided into different fractions consists of different fatty acids. When molten milk fat is slowly cooled, a crystalline solid and an uncrystallized liquid phase are formed. Separation of these phases yields fractions with high and low softening points respectively. Due to the fact that fat is composed of triglycerides of various molecular weights with different physical properties, fractionation of milk fat into fractions markedly different from one another in composition and physical properties is the most logical basis of modification. Economic fractionation of milk fat into oil and hard fat fractions will facilitate an increased utilization of milk fat in many food applications, such as chocolate, confectionary and bakery products and in developing new convenient (e.g. freeze spreadable) and dietetic (e.g. cholesterol reduced and short and medium chain enriched triglycerides) butter types. Differences in molecular weight, melting temperatures (molecular weight and entropy of fusion), volatility and intermolecular interaction energy of constitutive triglycerides, can provide the physical basis for fractionation of milk fat triglycerides. Fractional crystallization is most promising process to separate milk fat, a laboratory method of fractional crystallization is explained in detail in following paragraph.

31.2 Melting Characteristics of Milk Fat

More than 400 fatty acids have been identified in milk fat and their melting point varies from - 40°C to 40°C. This is due to variation in the composition of milk fat with season, region, breed of cows and type of feed. Following tables gives the melting point of major fatty acids of milk fat.

Table 31.1 Classification of milk fat fraction based on melting temperature

 Milk Fat Fraction Melting Temperature Range (°C) Very High Melting Fraction >50°C High Melting Fraction 32 - 50°C Middle Melting Fraction 25 - 32°C Low Melting Fraction 10 - 25°C Very Low Melting Fraction <10°C

Table 31.2 Melting point characteristics of common fatty acids of milk fat

 Carbon Number Common Name Melting point (Deg C) Type Typical Composition (%w/w) 4:0 Butyric -8 Short chain, Saturated 3.9 6:0 Caproic -4 Short chain, Saturated 2.5 8:0 Caprylic 17 Short chain, Saturated 1.5 10:0 Capric 32 Medium chain, saturated 3.2 12:0 Lauric 44 Medium chain, saturated 3.6 14:0 Myristic 54 long chain, saturated 11.1 16:0 Palmitic 63 long chain, saturated 27.9 18:0 Stearic 70 long chain, saturated 12.2 18:1 Oleic 16 long chain, unsaturated 21.1 18:2 Linoleic -5 long chain, saturated 1.4 18:3 Linolenic -10 long chain, saturated 1.0 Others long chain, saturated 10.6

31.3 Fractionation Technologies

Following are the various methods for fractionation of milk fat

1. Crystallization from Melted Milk Fat(Dry method)

2. Crystallization using Solvents(Wet method)

3. Supercritical Fluid Extraction

31.3.1 Crystallization from melted milk fat

It is basically temperature controlled process. Dry fractionation consists of two steps, a partial crystallization of triacylglycerols from a melt and a subsequent separation. More than 400 fatty acids have been identified in milk fat and their melting point varies from - 40°C to 40°C. This is due to variation in the composition of milk fat with season, region, breed of cows and type of feed. The driving force for melt crystallization is the difference between the melting point of a substance and the crystallization temperature. The efficiency of separation of the liquid (olein) from the crystalline phase (stearin) influences the quality of the solid fraction to a great extent. The crystallization step has a greater influence on the chemical composition of the pure fractions. Crystallization in general can be divided into two characteristic process steps: nucleation and growth. To crystallize a fat compound, super saturation or super cooling is necessary. This is the driving force for both crystallization steps. For fat systems, crystallization is complex because natural fats are a mixture of various triacylglycerols. Consequently, the concentration of each triacylglycerol is low and, for example, increased super cooling is needed to achieve nucleation of this low concentrated species. Furthermore, triacylglycerols are characterized by a complex melting behavior. They can solidify in three different crystal structures (α, β′, β) (polymorphism). Different crystal grid structures result depending on the magnitude of the driving force of crystallization. Less stable modifications require a lower driving force. The different polymorphic modifications have different thermodynamic stability, and the metastable polymorphic modifications (α, β′) are transformed with time to the stable β form.

The triglycerides are separated according to their melting points by filtration or centrifugation. After the process of crystal formation and growth, a filtration step is used to separate the solid product from the remaining melt. Filtration efficiency is determined by the size, shape, and mechanical stability of the milk fat crystals and depends on the amount of the mother liquor adhering to the crystals.

Another method which is widely used for crystallization is Solid layer melt crystallization. This process has only recently been applied to fractionation of milk fat (4–6) although it is widely used in the chemical processing industry. In layer crystallization processes, crystals generally grow on the cooled surface of a specially designed multi-tube or plate heat exchanger. The crystalline product is removed by re-melting crystals after draining the residual melt.

Fig. 31.1 Flow diagram of crystallization from melted milk fat

On laboratory method is explained below, wherein milk fat was melted completely and washed several times with warm water, dried under vacuum and filtered at 50-60°C.

Fig. 31.2 Fractionation by crystallization of molten milk fat in laboratory

500g sample was melted in a beaker which is kept in thermostat controlled water bath at 600C. Fat was stirred at about 100rpm. Then crystallization temperature was adjusted from 25°C to 32°C at an interval of 1°C for six hours. After crystallization at one of these temperatures fat was filtered on a Buchner funnel resulting in a liquid and a solid fraction at each of the selected temperatures. Fatty acid composition was determined using gas-liquid chromatographic determination.

The solid fraction (stearin) and the liquid fraction (olein) displayed a different triacylglycerol (TG) composition. Stearin fraction was enriched in long-chain fatty acids, whereas olein fraction was enriched in short-chain and unsaturated fatty acids. Determinations of fatty acid composition by GLC showed that unsaturated and short chain fatty acids were present in increased concentration in the liquid fraction (average 37·8 and 12·4% as compared with 32·1 and 10·8% in the original milk fat) and long chain saturated acids in the solid fraction (average 57·8 as compared with 53·8%). There was some concentration of carotene and vitamin A, and to a lesser extent of cholesterol, in the liquid fraction

31.3.2 Crystallization using solvents

This process involves dissolving melted milk fat in a solvent prior to crystallization. Solvents employed are generally acetone, ethanol, pentane or hexane. Melting temperature used is similar to dry fractionation. Crystals separation is done by filtration. Fractions are heated to remove the solvent. However, the costs incurred by solvent recovery, the hazardous nature of the operation, and the process losses make this process less frequently used than crystallization and filtration.

Isopropanol is used as solvent for fraction of milk fat. It should be added (4 mL/g butter oil), to butter oil. Mixture should keep at different temperature viz., 15, 20, 25 and 30°C and stirred by a low-speed mechanical stirrer for 1 h. Then, solid part and liquid part were separated by filtration under vacuum. The fractions were desolventized at 80°C for 1 h at 10 mm Hg pressure, weighed, and stored in a refrigerator.

Both the stearin fractions and the olein fractions show differences in the fatty acid compositions as reported by Bhattacharyya et al., 2000 The stearin shows higher content of short-chain acids and other saturated acids than the oleins.

The melting point of the stearin fraction at 15°C has increased from original butter oil by 4.1°C due to the increase in both palmitic acid (from 31.7 to 39.1%) and stearic acid (from 14.0 to 21.3%). On the other hand, in the olein fraction, palmitic acid and stearic acid have both decreased from 31.7 to 24.7% and 14.0 to 12.6%, respectively, while oleic acid has increased. The fatty acid compositions of the stearin and olein fractions obtained at 30°C and also at 25°C from isopropanol closely matches with the pattern composition of fatty acids of the corresponding fractions isolated by dry fractionation technique.

The SFC of the stearin fractions indicated significant values, viz. 62–67 at 10°C, 39–51 at 20°C, and 21–34 at 30°C. The stearin fractions obtained at 30, 25, and 20°C are all fairly similar in properties. This suggests that the temperature of fractionation, between 30, 25, and 20°C, does not lead to significant differences in physical characteristics.

31.3.3 Supercritical fluid extraction (SFE)

Supercritical CO2 extraction may be used in batch or continuous systems to fractionate anhydrous milk fat into fractions with specific properties to enhance its use. A gas above its critical pressure and temperature exhibits unique solvent properties. SFE of milk fat is generally performed with carbon dioxide. Milk fat fractions are selectively dissolved in SFE CO2 and separated when pressure and temperature return to atmospheric conditions. Supercritical carbon dioxide (SC-CO2) fractionation holds promise as a means to turn milk fat into a value-added ingredient.

Fig. 31.3 Supercritical state of fluids

The removal of cholesterol and fractionation of butteroil with supercritical fluids have been reported. The use of co-solvents to improve the removal efficiency in the extraction of cholesterol with supercritical CO2 have been evaluated. It has been reported that only 5% of the cholesterol in the initial butter using supercritical extraction followed by adsorption on silica gel.

In one study conducted by Torres et al.,( 2009), wherein butteroil is fractionated based on the individual fatty acid types via countercurrent CO2 extraction at pressures ranging from 8.9 to 18.6 MPa and at 2 different temperatures (48 and 60°C). Using this methodology, fractions as high as 70% of SCFA and MCFA ethyl esters were obtained. Figure 13.3 shows a flow diagram of the countercurrent supercritical fluid extraction system employed in this study.

Fig 31.4 Countercurrent supercritical fluid extraction equipment

The countercurrent extraction column (316 stainless steel) is 100 cm × 12 mm i.d. and is packed with Fenske rings (3 × 0.5 mm). The countercurrent supercritical fluid extraction device also includes 2 separator cells (S1 and S2) of 270 mL capacity each (where a cascade decompression takes place) and a cryogenic trap at atmospheric pressure. Both CO2 and liquid feed sample were preheated at the exit of their respective pumps before introduction into the extraction column. All units were equipped with electrical thermostats. The device has computerized programmable logic controller-based instrumentation and a control system with several safety devices including valves and alarms. During the extraction, a continuous flow of CO2 was introduced into the column through the bottom. When the operating pressure and temperature were reached, the liquid sample was pumped (100 mL/h) from the top during the entire extraction time (60 min). The first separator was maintained at 6 MPa and 20°C and the second separator cell was maintained at low pressure and temperature (2 MPa and 10°C). The raffinate and liquid fractions collected in the separators were weighted and analyzed. The material balance closed in all experiments with an inaccuracy <7.4%.

 Crystallization form melted fat Crystallization from solvents Supercritical CO2 extraction Advantages •No additives •Simple process •Successfully commercialized •More discrete fractions produced •Can use low temperatures •Reduced time for crystal formation •No additives •CO2 is nontoxic •More discrete fractions produced Disadvantages •Less pure fractions •Limited temperature range •Long residence time for crystal formation •Potential toxicity of solvent •Flavour changes in milk fat •High cost of operation and solvent recovery •High capital investment

Uses of Milk Fat Fractions

1. Low Melting Fraction <15°C

i. Has strong butter flavour

ii. Can be incorporated into milk powder to improve functionality

iii. Has applications in confectionery products

iv. Can be used to make normal butter spreadable at refrigerator temperatures

2. Medium -Melting Fraction 15 - 30°C

i. Can be used as shortening to provide crusty, flaky texture to croissants and pastries

ii. Can be used in making cakes and biscuits such as shortbread

3. High-Melting Fraction > 30°C

i. Hard fraction can be used in chocolate manufacturing instead of cocoa butter

ii. Has been reported to act as bloom inhibitor in dark chocolate

iii. Can be used as a flavour and texture agent in milk chocolate

iv. Hard fraction can improve the whipping properties of cream which is desirable in ice cream manufacturing