FAT RICH DAIRY PRODUCTS

Module1. Introduction to fat rich dairy products

Lesson 1
STATUS OF LIPIDS IN MILK

1.1 Introduction

The total milk production of India during the year 2010-11 is 116 million tonnnes out of which 33 percent was converted into ghee. As per FAO statistics, about 3.08 million tonnes of butter and ghee is being marketed in India during the year 2007. The amount of table butter produced in organized sector is 0.055 million tons during the year 2005 both in cooperative and private dairies and the value is 7,700 million rupees, which will be increased to 0.1 million tons and 22,500 million rupees respectively during the year 2011.

1.2 Status of Milk Lipids

Milk contains approximately 3.4% fat. Of all edible fats, milk fat has the most complex fatty acid composition. Over 400 individual fatty acids have been identified in milk fat. However, approximately 15 to 20 fatty acids make up 90% of the milk fat. The major fatty acids in milk fat are straight chain fatty acids that are saturated and have 4 to 18 carbons (C4:0, 6:0, 8:0, 10:0, 12:0, 14:0, 16:0, 18:0), monounsaturated fatty acids (C16:1, 18:1), and polyunsaturated fatty acids (C18:2, 18:3). Some of the fatty acids are found in very small amounts but contribute to the unique and desirable flavor of milk fat. For example, the C14:0 and C16:0 ß-hydroxy fatty acids spontaneously form lactones upon heating which enhance the flavor of butter.

The fatty acid composition of milk fat is not constant throughout the cow's lactation cycle. The fatty acids that are 4 to 14 carbons in length are synthesized in the udder. Some of the C16 fatty acids are made by the animal and some come from the animal's diet. All of the C18 fatty acids come from the animal's diet. There are systematic changes in milk fat composition due to the stage of lactation and the energy needs of the animal. In early lactation, the animal's energy comes largely from body stores and there are limited fatty acids available for fat synthesis, so the fatty acids used for milk fat production are obtained from the diet and tend to be the long chain C16:0, 18:0, 16:1 and 18:2 fatty acids. In late lactation, more of the fatty acids in milk are formed in the mammary gland so that the concentration of the short chain fatty acids such as C4:0 and 6:0 are higher than in early lactation. These changes in fatty acid composition do not have a great impact on nutritional properties of milk, but may have some effect on processing characteristics for products such as butter.

The fatty acids are arranged on the triglyceride molecule in a specific manner. Most of the short chain fatty acids are at the bottom carbon position of the triglyceride molecule, and the longer fatty acids tend to be in the middle and top positions. The distribution of the fatty acids on the triglyceride backbone affects the flavor, physical, and nutritional properties of milk fat.

Milk fat melts over a wide temperature range, from approximately - 40°C to 40°C. This is best illustrated by the firmness of butter at refrigerator temperature versus room temperature. At refrigerator temperature butter is approximately 50% solid, but is only about 20% solid at room temperature, which is why it spreads more easily as the temperature increases. The melting properties of milk fat are a result of the melting points of the individual fatty acids that make up milk fat and their arrangement on the triglyceride molecule.

The triglycerides of milk fat are in the form of globules. The globules are surrounded by a protein and phospholipid membrane that stabilizes the globules in the serum (water) phase of milk. The native globules range in size from less than 1 µm to over 10 µm. The uneven size distribution allows the larger globules to float in a process called creaming, thus resulting in a “cream line” at the top of the container. Milk is homogenized to reduce the size of the large globules to less than 1 µm, create a uniform distribution of globules throughout the serum phase, and minimize creaming.

Milk fat can be degraded by enzyme action, exposure to light, and oxidation. Each of these processes proceeds through different mechanisms. Enzymes that degrade fat are called lipases, and the process is called lipolysis. Lipases split fatty acids from the glycerol backbone of the triglyceride. Usually the action of lipase causes undesirable rancid flavors in milk. Pasteurization inactivates native lipase and increases the shelf life of milk. However, in some cheeses, such as Blue cheese and Provolone, a small amount of lipolysis is needed to achieve the characteristic flavor. Light induced degradation can happen fairly rapidly in milk and produces a characteristic off-flavor. The majority of this off-flavor is caused by protein degradation. Storing milk in opaque containers minimizes this process. Milk fat can also be degraded by a classical chemical oxidation mechanism i.e., the attack on double bonds in the fatty acids by oxygen. Oxidation of the unsaturated phospholipids in milk produces off-flavors that are described as painty, fishy, or metallic. Milk fat though quite bland in taste, imparts richness and smoothness to fat containing dairy products.

The colour of fat depends upon its carotene content and varies with the species, breed and feed of the animal. The yellow colour of cow milk is because of carotene. Buffalo milk does not contain carotene. Milk fat also contains cholesterol (0.23 to 0.1 %) and phospholipids (lecithin, phosphatidyl serine, sphingomyelin, inositol and cerebrosides) some of which serve as antioxidants in prolonging the shelf life of ghee.

Milk fat is the highly expensive item among all the constituents of milk. The shelf life of fat rich products is longer than that of milk. Fat rich products are important milk products from which the dairy gets more profits. At last, the fat rich products are utilized since years as wound healing agent, lighting the wicks of cotton and used in performing homa’s, further even now the house wives are using milk fat (ghee) as cooking medium.

1.3 General Composition of Milk Fat

Milk fat, though quite bland in taste, imparts richness/smoothness to fat-containing dairy products. In freshly secreted milk, it occurs as a microscopic globular emulsion of liquid fat in an aqueous phase of milk plasma. Fat is the most variable component of milk. The average size of fat globules in buffalo milk is larger (4.15 to 4.60 µm) than in cow milk (3.36 to 4.15 µm). Milk fat globules fall in to three overlapping size distributions (Table 1.1).

Table 1.1 Size distribution of milk fat globules

Class	Diameter (µm)	Proportion of the total globule population (%)	Fraction of total milk lipid (%)
Small	Below 2	70-90	<5
Intermediate	3-5	10-30	90
Large	8-10	0.01	1-4

The fat globules are stabilized by a very thin membrane of 5-10 µm thickness, closely resembling plasma membrane. The membrane consists of proteins, lipids, lipoproteins, phospholipids, cerebrosides, nucleic acids, enzymes, trace elements and bound water (Table 1.2). The membrane is important in keeping fat from separating as free oil when it is subjected to physical abrasion during handling/ processing of milk. It also protects milk lipids against the action of enzymes, notably lipase, in development of rancidity. Certain enzymes such as alkaline phosphatase and xanthine oxidase, as well as certain important minerals like iron and copper, are preferentially attached to the fat globule membrane.

Table 1.2 Average compositions of milk fat globules membranes

Component	Buffalo	Western cow
Lipid content	38.5	35.7
Neutral lipids	74.4	65.7
Phospholipids	19.7	18.6
Triglycerides	52.7	45.5
Diglycerides	7.4	5.5
Sphingomyelin	4.3	3.2
Phosphatadyl serine	3.1	2.4
C 18:0 Stearic acid	34.4	24.1
C16:0	22.4	23.8
C18:1	14.5	14.5
C14:0	12.6	14.4
Protein (g/100 g fat globule)	1.36	1.8

1.3.1 Fatty acid profile of milk fat

Both buffalo and cow-milk fats consist chiefly of the triglycerides of fatty acids, which make up 95-99 per cent of milk fat. The remaining portion of milk fat is composed of diglycerides (about 4.1% in buffalo milk and 1.26-1.59% in cow milk), monoglycerides (about 0.7% in buffalo milk and 0.016-0.038% in cow milk). High, medium and low molecular weight triglycerides in buffalo milk occur in the proportion of 42.5, 17.1 and 40.5 per cent, respectively and corresponding values for cow milk fat are 52.9, 18.9 and 28.2 per cent, respectively. Free fatty acid content of buffalo milk fat is lower (0.22%) as compared to that in cow milk fat (0.33%).

The functional properties of milk fat are attributed to its fatty acid make-up. Milk fat contains approximately 65% saturated, 30% monounsaturated, and 5% polyunsaturated fatty acids. From a nutritional perspective, not all fatty acids are considered equal. Saturated fatty acids are associated with high blood cholesterol and heart disease. However, short chain fatty acids (C4 to 8) are metabolized differently than long chain fatty acids (C16 to 18) and are not considered to be a factor in heart disease. Conjugated linoleic acid is a trans fatty acid in milk fat that is beneficial to humans in many ways. Milk also contains 7 per cent short-chain fatty acids (C4-C8), 15-20 per cent medium-chain fatty acids (C10-C14) and 73-78 per cent long-chain fatty acids (C16 or higher). In all, milk fat contains 19 or more fatty acids (Table 1.3).

Table 1.3 Fatty acid profile of buffalo and cow milk fat

Fatty acids	Common name	Composition (wt/wt %)
		Buffalo milk Cow milk
C4:0	Butryic	4.36	3.20
C6:0	Caproic	1.51	2.11
C8:0	Caprylic	0.78	1.16
Cl0:0	Capric	1.28	2.57
Cl0:1	-	-	0.31
C12:0	Lauric	1.78	-
C14:0	Myristic	10.81	11.93
C14:1	Myristoleic	1.27	2.12
C15:0	-	1.29	1.23
C 16:0 (branched)	-	0.18	0.30
C16:0	Palmitic	33.08	29.95
C16:1	Palmitoleic	1.99	2.16
C17:0	-	0.58	0.34
C18:0 (branched)	-	0.24	0.35
C18:0	Stearic	11.97	10.07
C18:1	Oleic	27.15	27.42
C18:2	Linoleic	1.51	1.49
C18:3	Linolenic	0.47	0.59

Buffalo milk fat contains appreciably higher butyric acid in its triglycerides in comparison to cow milk fat. However, other short-chain fatty acids (caproic, caprylic, capric and myristic) are lower in buffalo milk fat. The content of long-chain fatty acids (palmitic and stearic) is relatively higher in buffalo milk. The unsaturated fatty acid level of buffalo and cow milk is comparable. Milk fat also consists of varying quantities of other lipids such as phospholipids, sterols, carotenoids, vitamins A, D, E and K and some traces of free fatty acids.

There are characteristic physico-chemical differences between fats of buffalo and cow milk. (Table 1.4) summarizes some of these differences.

1.3.2 Cholesterol

The cholesterol content of milk is significantly affected by the species, breed, feed, stage of lactation and season of the year. Generally, the cholesterol content is higher in the western breeds of cattle (317-413 mg/100 g fat), followed by zebu (desi) cow (303-385 mg/100 g fat). It is lowest for buffalo (235-248 mg/100 g fat). Generally lowest at the beginning of the lactation period, the cholesterol content progressively rises to reach its highest level towards the end. It is quite high in colostrum, being 570 to 1950 mg per 100 g fat in the first milking after parturition, progressively declining to normal levels during subsequent milkings.

Table1.4 Some physico-chemical properties of buffalo and cow milk fat

Physico-chemical Properties	Buffalo milk fat	Cow milk fat
Softening point range	34.3 - 36.3°C	33.5 - 35.9°C
Melting point range	33.4 - 46.4°C	31.5 - 35.2°C
Acid value	0.17 - 0.352	0.26
Refractive index	1.4515 -1.4533	1.4498 -1 .4530
BR reading	41.00 - 43.50	41.05 - 42.40
Saponification number	218.23 - 236.10	221.0 - 238.0
Iodine value	27.00 - 33.90	27.70 - 37.32
Reichert-Meissel value	27.83 - 35.50	24.60 - 29.70
Polenske value	0.70 - 1.60	1.30 -1.80
Density	0.905 - 0.917 g/ml	0.888 - 0.911 g/ml
Grain size	0.20 - 0.41 mm	0.098 - 0.190 mm
Fat globule size	4.15 - 4.60 µm	3.36 - 4.15 µm
Unsaponifiable matter	392 - 398 mg/100ml	414 - 450 mg/100ml

1.3.3 Phospholipids

The total phospholipid content of buffalo milk fat averages 21.04 mg/l00 ml of milk, whereas for cow milk the corresponding value is 33.71mg/100 ml. (Table 1.5) shows the concentration of various fractions of phospholipids present in buffalo and cow milk.

Table 1.5 Concentration of phospholipid fractions in buffalo and cow milk

Phospholipid fraction	Concentration(mg/100ml)
	Buffalo milk	Zebu cow milk
Lecithin	7.29	10.01
Cephalin	9.12	15.57
Sphingomyelin	4.63	8.13