For thousand of years, sheep and goat milk has been the staple of life in many areas of the world. Sheep were certainly the first animals to be domesticated by humans in their effort at a sedentary and agricultural way of life. Eventually, the conservation of milk by its transformation into cheese was discovered and the best cheeses of the world were developed.

Although cows have replaced sheep as dairy animals because of their higher production potential, sheep dairying remains a strong and viable enterprise. Nowadays sheep dairy products (cheese, yogurt, ice cream) are particularly in demand because of their rich flavor and their exceptional nutritive value. Anyone who has tasted the famous Roquefort, the hard Pecorino Romano, the softer Pecorino Sardo, the melting Manchego, the tender Ossau-Iraty or the salty Feta, wants to have more than just a taste. In addition, for the master cheese maker, sheep's milk is a dream come true because of its composition and its cheese making properties.

Composition of sheep's milk

Sheep's milk contains almost twice as much solids than cow's milk as well as higher casein and fat content. Sheep's milk yields 18-25 % cheese; that is, it takes only 4 to 5 kg of sheep's milk to produce 1 kg of cheese (it takes 10 kg of cow's milk to produce the same amount). Moreover, the higher casein content makes the rennet coagulation time for sheep's milk shorter and the curd firmer (Jandal, 1996). The gross composition of sheep milk and milk of different other species are shown in the following table:

	Human	Cow	Sheep	Goat	Yak
Dry matter (%)	11.5-13.9	10.5-14.3	17.4-18.9	11.9-14.0	16.8-19.6
Fat (%)	3.7-4.6	2.8-4.8	6.0-7.5	4.1-4.5	6.5-7.8
Alb. Glob. (%)	0.8-1.7	0.3-0.8	0.9-1.1	0.4-1.0	0.6-1.9
Casein (%)	0.4	2.5-3.6	4.3-4.6	2.5-3.3	5.0-5.8
Lactose (%)	6.4-7.0	4.2-5.0	4.3-4.8	4.1-4.4	4.6-5.3
Ash (%)	0.2	0.7-0.9	0.9	0.8	0.9
Calcium (mg/l)		1360	2030
Sodium (mg/l)		460	360
Vit. A (mg/l)		0.3	0.5
Vit.E (mg/l)		7	15.8
Vit. C (mg/l)		22	40.0
Kcal/100g	73	73	113	77	114

Alfa-Laval (1981

Fat

Jandal (1996) shows that the fat of sheep's milk is in the form of globules with a size ranging from .5 to 25 microns with an average diameter of 3.3 microns (Assenat, 1985) which is smaller than fat globules of cow's milk (4.55 microns). The color of fat in sheep milk is very white because of a total absence of b -caroten (Assenat, 1985). Twenty percent of the fatty acids of sheep's milk and goat's milk are short-chain saturated fatty acids (C4:0 to C12:0) compared to 12% in cow's milk (table 2). Lipases attack the ester linkages of the short-chain fatty acids more rapidly, so these differences may contribute to more rapid digestion of goat and sheep's milk.

Moreover, Havel (1997) reports that short chain fatty acids have little effects on atherogenic lipoprotein concentration in blood plasma (cholesterol) of human. The amount of cholesterol in sheep's milk increases with the amount of fat and has been found to be between 150 and 300 mg/liter (Assenat, 1985). The higher proportion of short-chain fatty acids such as caproic, caprylic and capric in sheep's milk than in cow's milk (but less than in goat's milk) gives sheep's milk its special taste and aroma, although some other compounds such as phospholipids and phenols have an important role (Kim Ha and Lindsay, 1991).

Fatty acid	Goat (%)	Cow (%)	Sheep (%)
C4:0 (Butyric)	2.6	3.3	4.0
C6:0 (Caproic)	2.9	1.6	2.6
C8:0 (Caprylic)	2.7	1.3	2.5
C10:0 (Capric)	8.4	3.0	7.5
C12:0 (Lauric)	3.3	3.1	3.7
C14:0 (Myristic)	10.3	9.5	11.9
C16:0 (Palmitic)	24.6	26.5	25.2
C16:1 (Palmitoleic)	2.2	2.3	2.2
C18:0 (Stearic)	12.5	14.6	12.6
C18:1 (Oleic)	28.5	29.8	20.0
C18:2 (Linoleic)	2.2	2.5	2.1

Jandal (1996)

Properties	Cow's milk	Sheep's milk
Specific gravity	1.0231-1.0398	1.0347-1.0384
Viscosity, Cp	2.0	2.86-3.93
Surface tension (Dynes/cm)	42.3-52.10	44.94-48.70
Refractive index (nD 20 )	1.3344-1.3485	1.3492-1.3497
Conductivity (ohm -1 cm -1 )	.0040-.0055	.0038
Freezing point	-0.530 to -0.570 ° C	-0.570 ° C
PH	6.65-6.71	6.51-6.85
Acidity (Lactic acid %)	0.15-0.18	0.22-0.25

Anifantakis, 1985

Proteins

Quantitatively and qualitatively, proteins constitute the most important fraction of milk. Two types of proteins can be found in milk: the casein and the serum or whey proteins. In sheep's milk, casein enters for 80% of the total proteins. Because of a higher casein content, sheep's milk has better coagulation properties and better cheese making potential than cow's milk. Caseins are made of 5 major components: a s 1 , a s 2 , b , k and g , and the percentage of the 5 components varies from one specie to the other. The percentage of a s 1 and a s 2 is higher in sheep's milk than in goat's milk but significantly lower than in cow's milk. Casein b , however, represent 50% of the total casein in sheep's milk for 2/3 in goat's milk and 1/3 in cow's milk. Those variations in percentages of casein explain the difference in micelle structure and the absence of bitterness taste in sheep's milk cheeses.

	Cow		Goat		Sheep
Casein fractions	Caseins in % total casein		Caseins in % total casein		Casein in % total casein
a s 1	36	45.5	12.6		15.5	30.2
a s 2	9.5		14.7
b 1	33		35.9	75.3	18.9	47.1
b 2			39.4	28.2
k	9.4		8.1		7.3
g	6.8		3.9		15.4

Assenat, 1985

Most of the sheep's milk produced in the world is transformed into cheese, and is rarely consumed directly (with the exception of the United Kingdom). For this reason, Bencini and Pulina (1997) refer to the quality of sheep's milk as its capacity to be transformed into high quality products and to produce high yield of these products which is referred to as the processing performance of the milk. The processing performance of the milk (yield, composition and taste) is mainly dependent on the clotting properties of the milk, a combination of renneting time, rate of curd formation and consistency of the curd. The clotting properties of the milk are widely affected by its composition.

Factors affecting the composition of sheep's milk

Bencini and Pulina (1997) cite several factors that would affect the composition of the milk:

Somatic cell count

In sheep's milk, only 10% of the somatic cells are mammary glands cells (eosinophils, epithelial cells), normally secreted together with the milk as a result of cellular turnover in the mammary gland. The remaining 90% of the somatic cells are blood cells (macrophages, leucocytes, lymphocytes). These normally contribute to the immune defense of the mammary gland, but their number increases considerably in the case of inflammatory or pathological process within the mammary gland. Therefore, a high somatic cell count is a sign of general infection of the animal. A high somatic cell count results in changes in the composition of milk with a higher pH, a reduction in fat, casein, total solids, soluble calcium, and an increase in total nitrogen, non-protein nitrogen and whey proteins (Pirisi et al., 2000). These changes in composition lead to a considerable slow-down of coagulation and serious difficulties in the structuring of the curd and consequently a lengthening of the cheese making process as well as a decrease in cheese yield (Pirisi, 2000). High somatic cell count in cow's milk has been associated with problems in the quality of cheese. However, Pirisi et al. (1996, 2000) report that, in a study conducted in sheep's milk, although cheese yield was reduced, ripened cheeses did not show significant differences for chemical parameters and sensorial characteristics. Wendorff (2000), however, found a greater tendency for cheese to develop a rancid flavor.

Some cheese makers might set an artificial limit of somatic cells over which milk would be discounted. This is a very serious incentive for producers to make all possible efforts to decrease their overall (bulk tank) somatic cell count.

		SCC<500	500<SCC>1000	1000<SCC>2000
		x 1000/ml
SCC	x 1000/ml	229 ± 55	653 ± 250	1200 ± 214
Ph		6.52	6.62	6.68
Dry Matter	g/100g	17.03	17.15	16.89
Lactose	g/100g	4.74	4.54	4.38
Fat	g/100g	6.61	6.34	6.36
True Protein	g/100g	5.25	5.45	5.51
Casein	g/100g	4.18	4.26	4.20
Soluble Casein	%	6.51	6.98	7.77
Whey Protein	g/100g	1.07	1.19	1.30
Non-Protein N	gN/100g	.06	.05	.05
Casein:Protein	%	79.71	78.16	76.27
Urea	mg/100ml	54.21	54.18	52.86
Total Ca	g/l	2.21	2.14	2.26
Soluble Ca	g/l	.46	.38	.36

Pirisi et al., 2000

Microbial count

Many microorganisms present in the milk are advantageous for its transformation into cheese ( Lactobacillus spp., Lactococcus ssp., Streptococcus spp.). However, others can cause serious human diseases ( Salmonella, Listeria, Brucella ), and others can create problems in the maturation of cheese ( Enteriobactericeae, coliforms, psychrotrophs ). Some psychrotropic bacteria thrive at temperature below 7 ° C and produce enzymes that destabilize the casein and modify the clotting properties of the milk.

Breed of ewes

The breed of sheep can affect the composition of the milk mostly because there is a negative correlation between milk yield and concentration of milk components. Therefore, breeds highly selected for dairy production tend to have a lower concentration of fat, protein and total solids. As a consequence, with high milk production, the total amount of cheese produced from the milk will be higher but the relative yield of cheese from each liter of milk will be lower.

Age and parity

Although reports are somewhat contradicting, it seems that the milk of young ewes contains a lower concentration of fat, proteins and total solids. The concentration of total solids increases with the parity number.

Stage of lactation

The amount of fat, protein, total solids, and somatic cells is high at the beginning and at the end of lactation and low at the peak of lactation. The processing performance of the milk tends to decrease as the lactation proceeds, with an increase in renneting time and rate of curd formation and a decrease in the consistency of the curd (Ubertalle, 1989, 1990 cited by Bencini and Pulina, 1997).

Season of milking

Many researchers cited by Bencini and Pulina (1997) have shown that sheep milk produced in summer has poor cheese making performance due to long renneting time, poor consistency of the curd, and high proteolytic and lipolytic activities. Mendia et al., 2000) also found that Idiazabal cheeses made in February earned higher sensory analysis scores for characteristic odor and taste and higher sensory scores than cheese made in June. It seems that hot temperatures do not affect the composition of the milk as much as the length of days. Long days result in a lower protein concentration and reduced rate secretion of fat and protein.

Nutrition

Nutrition affects the total milk production as well as the quality of the milk. The concentration of fat in the milk is correlated with the concentration of fiber in the diet.

Conservation of milk

Fresh milk

In order to avoid the multiplication of bacteria, the milk must be cooled to 1 ° C-4 ° C and maintained at this temperature from milking to delivery to the processing plant. The low temperature will prevent bacterial multiplication for 24-48 hours. However, the presence of psychrotrophic bacteria, i.e., those able to multiply, albeit slowly, at 5 ° C, can result in marked increases in bacterial count during longer storage periods. Cousins and McKinnon (1977) have shown a significant increase in bacteria numbers after 3 days at 5 ° C and that the effects of relatively small increases in storage temperature were dramatic.

Ineffectively cleaned and disinfected milking equipment, particularly the bulk tank, is the major source of psychrotrophic bacteria in milk.

210 gallon bulk tank

Frozen milk

In North America sheep dairy producers are few, separated by great distances and producing modest amount of milk daily. Therefore, the collect of fresh milk on a daily basis by a processing plant is economically difficult for the time being. American producers generally freeze the milk until a sufficient quantity is stored for delivery to a cheese maker. Moreover, the production of sheep milk is very seasonal and fresh milk becomes unavailable from October to February. The freezing of milk helps remedy the shortcoming of production. Bastian (1994) at the University of Minnesota, studied the effect of freezing on the quality of the milk for cheese making. He found that the freezing and thawing of sheep milk did not change rennet coagulation properties, compared to fresh, unfrozen sheep milk. Wendorff, (2000) at the University of Wisconsin-Madison tried to determine the storage stability of milk frozen at 2 different temperatures. Samples of raw sheep milk were frozen at -12 ° C and at -27 ° C. Samples were thawed at 1, 2, 3, 6, 9, and 12 months and analyzed for total bacteria, coliform bacteria, acid degree value (ADV), and intact protein. Intact protein was defined as the total protein content of milk minus the protein present in sediment at the bottom of the recipient in which the milk was frozen. Results indicate that milk frozen in a standard home freezer at -12 ° C was not as stable as milk frozen in a commercial hardening room at -27 ° C. After 6 months of storage at -12 ° C, about one third of the casein was destabilized and precipitated out upon thawing. The raw milk stored at the lower temperature was stable up to 12 months. No evident protein precipitation was noticed throughout the study.

Time of storage (Mo)	Coliforms (CFU/ml)	SPC (CFU/ml)	ADV	TCA-ppt. Protein
Stored at –12 ° C
0	44	8200	.220	5.09
1	26	4100	.250	5.06
2	21	2500	.420	5.01
3	12	3400	.350	5.06
6	<1	2200	.410	5.02
9	<1	340	.420	3.37
12	<1	610	.490	3.90
Stored at –27 ° C
0	44	8200	.220	5.09
1	10	4100	.260	5.05
2	9	3200	.320	4.99
3	12	3700	.290	4.96
6	8	2800	.310	4.89
9	8	2700	.280	4.92
12	5	1800	.350	5.07

Wendorff., 2000

The best freezing is obtained when the milk is placed in food approved plastic bags with central cap (75 cm x 40 cm) holding approximately 18-20 liters of milk. The bags are placed flat on shelves inside the freezer so that the thickness of the bags is no more than 6-7 cm allowing for a quick and uniform freezing. As soon as the milk is solid, the bags can be stacked easily in the freezer.

The type and size of freezing containers, however, will depend on the wish of the cheese makers and on his thawing possibilities. Before deciding on a freezing method the producer or group of producers should discuss with the cheese maker the best suitable method.

Sheep's milk stored in a commercial freezer at -25 degrees C.

Sheep's milk in FDA approved bags and placed on shelves for quick freezing

The use of sheep's milk

Fluid products

Sheep's milk has some unique nutritional qualities that could be used in specific markets. It is richer in vitamins A, B and E, calcium, phosphorous, potassium and magnesium than cow's milk. Sheep's milk contains 1.08-1.44% whey proteins while cow's milk contains only 0.54-0.88%. It is also richer in C4-C12 fatty acids. Sheep's milk provides some relief for allergy sufferers who cannot tolerate cow's milk proteins. In spite of the added nutritional qualities, only small quantities of sheep's milk are consumed as fluid milk. In Spain, 7.2% of the sheep's milk is consumed as fluid milk. This is certainly due to the fact that most of the sheep's milk is produced in countries where consumption of fluid milk has always been traditionally low. Moreover, with the high solids content of sheep's milk, it is more readily accepted for manufacturing of semi-solid or hard dairy products, e.g. yogurt or cheese. However, in Great Britain, a large portion of the sheep's milk production is consumed as fluid milk. The milk is pasteurized and stored in ½ liter carton containers (pint). The milk is then generally frozen and sold to health food stores.

Dried products

In spite of the high solids content of sheep's milk, there is little activity reported in the area of dried sheep's milk or non-fat milk products. There appears to be a significant demand, at the current time, for dried sheep's milk for blending with cow's milk for specialty cheese production. Some of the concerns about these products would be the stability of the milk fat and shelf life of the dried milk products.

Yogurts

The solids content of sheep's milk make it a natural for production of premium yogurt products similar to the Greek-style yogurt. With solids content of 16-18% in the milk, yogurts can be produced without the need of added solids or stabilizers. With the higher fat in the sheep yogurt, the potential harshness of the lactic acid may be lessened. Sheep yogurt also shows a greater cold storage stability. Sheep yogurt needs to be marketed as premium specialty product to avoid competing with commodity yogurts produced from cow milk. Low fat yogurt can also be made after separation of the cream. The cream can be used for the manufacturing of butter.

	Separated serum (ml)
Type	1 day	10 days
Cow	41	40
Sheep	6	7
Goat	23	15

Kehagias et al, 1986

Characterisics	Initial milk	Stored at -12 ° C	Stored at -27 ° C
Titratable acididty, %	1.25	0.90	1.18
Syneresis, %	75.1	79.7	77.5
Water holding capacity, %	28.5	25.7	30.4
Firmness, g	125	72	109

Wendorff, 2000

A small yogurt plant in Wisconsin

Freshly made yogurts

Cheese

Traditionally, production of cheese is the greatest market for sheep's milk throughout the world. With the high solids and smaller fat globules sheep's milk is an outstanding substrate for manufacturing cheese of high quality. Normally, 15% solids in milk are about the most efficient for obtaining maximum output per vat per day and while allowing for sufficient syneresis of the curd for proper moisture control in the final cheese.Typical cheese yields for cow and goat milk are 9-10% while sheep milk yield approximately 18-25% according to the type of cheese. Cheese yield is much more dependent on protein content than on fat content.

A method of milk payment that would involve milk composition should therefore favor the protein content of the milk more than the fat content. However, fat content is important for the texture and savor of the cheese. It is generally admitted that the ratio Fat/Protein should not be below 1.10.

The most famous sheep milk cheeses produced in the world are listed below:

W hite fresh cheeses

Burgos (Spain)

Villalon (Spain)

Cachat (France)

Perail (France)

A nice sheep milk cheese display in Scotland

Brined cheeses

Feta (Greece, Italy, France)

Teleme (Romania)

Sirene (Bulgaria)

Halloumi (Cyprus)

Hard and semi-hard cheeses

Pecorino Romano, Sardo, Siciliano, Toscano (Italy)

Kefalotyri (Greece)

Idiezabal (Spain)

Manchego (Spain)

Roncal (Spain)

Ossau-Iraty (France)