Journal Pre-proof

Sheep milk: a pertinent functional food

Arpita Mohapatra, Ajay Kumar Shinde, Raghvendar Singh

PII: S0921-4488(19)30190-7 DOI: https://doi.org/10.1016/j.smallrumres.2019.10.002 Reference: RUMIN 6001

To appear in: Small Ruminant Research

Received Date: 14 June 2019 Revised Date: 25 September 2019 Accepted Date: 3 October 2019

Please cite this article as: Mohapatra A, Shinde AK, Singh R, Sheep milk: a pertinent functional food, Small Ruminant Research (2019), doi: https://doi.org/10.1016/j.smallrumres.2019.10.002

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© 2019 Published by Elsevier. Sheep milk: a pertinent functional food

Arpita Mohapatra1, Ajay Kumar Shinde2 and Raghvendar Singh3

1Scientist, Animal Physiology and Biochemistry Division, ICAR – Central Sheep and Wool

Research Institute, Avikanagar, Rajasthan 304501 India

2Principal Scientist, Livestock Production and Management, ICAR – Central Sheep and Wool

Research Institute, Avikanagar, Rajasthan 304501 India

3Principal Scientist, Animal Physiology and Biochemistry Division, ICAR – Central Sheep and

Wool Research Institute, Avikanagar, Rajasthan 304501 India

Corresponding Author: Arpita Mohapatra, Scientist, Animal Physiology and Biochemistry

Division, ICAR – Central Sheep and Wool Research Institute, Avikanagar, Rajasthan 304501

India. E mail: [email protected]. Mobile no. +91-8901501283

Highlights  Milk and its products play pivotal role in Diet- health research.  Health conscious people are now trying to stay away from red meat.  Dairy food from small ruminants, which generally rely on greens, act as a vehicle of bioactive components.  India stands third in sheep population but its sheep are mostly unexploited for milk yield potentialJournal. Pre-proof

Abstract

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Modern human diet and changes in lifestyle are emerging as a challenge in developing countries resulting to endless ailments. Thus, in modern spectra of human health, bioactive foods play a pivotal role. Under the umbrella of food and nutrition security, functional dairy foods have become the need of the hour. Sheep milk is one of the functionally active dairy foods and it is also considered as nutritional powerhouse. The beneficial role of sheep milk results from its fatty acid, immunoglobulin and non-immune protein contents. In human gut, milk proteins turn into excellent source of bioactive peptides with antioxidative, antimicrobial, antihypertensive, immunomodulatory and antithrombotic role. It is also used in anti-ageing formulations and cosmetic soap preparations to soothe psoriasis and skin eczema like chronic conditions. The unique physicochemical and biochemical properties of sheep milk also include prebiotics and probiotics which make it perfect functional food for human health promotion and disease risk reduction. The milk from Indian sheep is relished by the shepherds and their households. They claim that it has many health benefits, but it is an untapped area by the Indian researchers. The major challenge in Indian prospect is non availability of dairy sheep breed, but their milk functional potential cannot be ignored. This review is focused on worldwide work done on sheep milk for its unique functional characteristics.

Keywords: Sheep milk; Milk composition; Functional food; bioactive peptides

Introduction Sheep rearingJournal in developing countries is primarily Pre-proof aimed at livelihood security, not on commercial benefit. In these countries, sheep means only mutton and to some extent wool. In many developing countries, the demand as well as visible supply of sheep milk is negligible.

India accounts for 65 million sheep and ranks 3rd in world sheep population, but most sheep here

2 are raised for meat and wool purposes. Thus, contribution to world milk production is insignificant. The concept of milking sheep in developing countries is relatively new but in the new era of functional foods, sheep milk formulations will play pivotal roles in providing human health benefits. Changing diet and lifestyle of modern days increase the disease risks in human communities. People are now searching for natural foods with health benefits instead of medications which have numerous side effects. For many people, especially for the infants, small ruminant milk and their products are also medicinal necessities (Haenlein, 2001). Although the medical importance of sheep milk has been supported by certain anecdotal experiences

(Razafindrakoto et al., 1993) but very little researches have been carried out in this course. Due to rich mineral, higher protein, beneficial fat contents and a better source of functional bioactive peptides the demand for sheep milk is increasing in the global market. It creates a scope to go for commercialization of sheep for its milk. Milk production is mainly dependent on quantity and quality of feed along with frequent milking stimulus (autocrine control). Nutritional manipulations can be tried with combinations of feed items, for promotion of sheep milk. Today the Indian farmers follow their age old practice; sheep are reared either by grazing in barren lands or by feeding crop residues and household wastes. Above said nutritional manipulations are under explored in developing countries where vast sheep resources are available. Therefore, there is a potential for increase in sheep milk production in developing countries. Again, because of their small sizes, they can be easily milked by household women. Finally, attempts can be taken to improve milk production of local breeds by crossing them with Awassi of Israel. The future of sheepJournal milk depends upon the ability to Pre-proofproduce more milk and quality milk products. This can be inferred from this review that nutrient enriched sheep milk can be preferably referred to as a functional food, but still more research in different sheep breeds are needed, to

3 validate the existing research as well to explore some unknown quality of sheep milk before formulation of health-enhancing nutraceuticals from these bio peptides.

Claeys et al. (2014) compared milk of horse, donkey and sheep with human milk and concluded that sheep milk can be a suitable alternative to breast milk and infant formula. Today, there is growing interest worldwide to undermine the hidden facts regarding sheep milk (Balthazar and

Cruz, 2018). Information on composition, physico-chemical characteristics and nutritional aspect of sheep milk are essential for creating demand among people, product making and marketing.

This paper was aimed at reviewing the worldwide work done on sheep milk composition as well as to discuss the medicinal and functional properties with emphasis on lipid and protein fractions including bioactive peptides. This review also emphasizes sheep milk quality as well as scope of sheep milk production in countries like India, where in spite of vast sheep resources, milk and milk products from sheep are least explored for human consumption.

Sheep milk production and use

Sheep and goat milk accounts for 36.5% and 63.5%, respectively of world total small ruminant milk production (FAO, 2017). Traditionally, any breed of sheep can be milked but some breeds are excellent milk producers (Table 1). Average lactation period of dairy sheep varies from 180-

240 days and a non-dairy breed has lactation period of 90-120 days. Sheep milk is more popular among consumers in form of cultured dairy products like cheese and yogurt. The world major producer of sheep milk is China. Top ranking 10 countries in sheep milk production are listed in Table 2. Journal Pre-proof The quality of sheep milk has paramount importance in controlling the quality of dairy products

(cheese, butter, ghee) made from it. In the world, most of sheep milk is transformed into cheese

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and yogurt. In Asian and African countries, sheep milk is transformed into butter and ghee. Fresh

sheep milk is rarely consumed because of its high fat and total solid contents. Total cheese

production from sheep milk in the world is 680.30 million kg and butter and ghee is 63.25

million kilogram (FAO, 2016).

Sheep milk composition

Milk compositions differ between species. Compositions of goat, sheep, camel, buffalo and cow

milk are different (Table 3). Again, they vary with diet, breed, individuals, parity, season,

feeding, management, environmental conditions, locality, stage of lactation, and health status of

the udder (Park, 2006). Besides, the visible anatomical differences of sheep teats (two teats) they

have small udder cistern volume than that of cattle and goats. Sheep and goats have apocrine

type of milk secretion whereas in cows it is merocrine. That’s why sheep and goat milk have

high somatic cell count (SCC) than that of cows (Caldwell, 2014). Haenlein and Wendorff

(2006) reported that sheep milk is quite different from cow milk, as it is predominantly produced

by seasonal breeding of ewes whereas cows have round the year breeding. Therefore, season of

year is an important factor that determines the sheep milk composition. Kanwal et al. (2004)

reported that cow milk contains approximately 12.5% total solids, 3.8% fat, 8.7% solid-not-fat

(SNF), 4.6% lactose, 0.8% ash and 3.1% protein. The corresponding values for sheep milk are

18.3%, 6.0%, 12.3%, 4.9%, 0.94% and 5.2%. Sheep colostrum is also higher in basic nutrients than cow colostrumJournal: fat 13.0 vs 5.1%, protein 11.8Pre-proof vs 7.1%, lactose 3.3vs 3.6%, minerals 0.9 vs 0.9%, total solids 28.9 vs 15.6%, respectively (Anifantakis, 1986). Zhang et al. (2006) reported

that sheep milk is rich in proteins, minerals and lipids; it contains higher amounts of calcium,

phosphate and magnesium. The lipid part also contains higher proportion of medium chain fatty

5 acids. Higher fat and SNF content of sheep milk make it ideal for cheese and yogurt production

(Barłowska et al., 2011). Molik et al. (2008) reported that sheep milk has higher concentration of unsaturated fatty acids, calcium, phosphorus, iron and magnesium than cow milk. Lactose in sheep milk as in other ruminants is lower at the beginning of lactation in colostrum and towards the end of lactation, contrary to the behavior of fat and protein contents in milk (Pulina and

Bencini,2004 ; Haenlein and Wendorff, 2006). Casoli et al. (1989) reported concentration of fat varies among sheep breeds from 4.6% in the Iraqi Kurdi sheep to 12.6% in Dorset sheep milked in America, whereas protein concentration was less variable, from 4.8% in the Grade Precoce to

7.2% in the Armenian Corriedale.

Physico-chemical property of sheep milk

There are distinct differences in physico-chemical characteristics of goat, sheep and cow milk

(Table 4). Sheep milk has higher specific gravity, viscosity, titratable acidity, refractive index and lower freezing point than cow milk (Haenlein and Wendorff, 2006). The higher viscosity of sheep milk is due to the differences in total solid content of the milk, which caused a significant effect on the firmness of yogurt curd (Jumah et al., 2001). The higher viscosity may also be attributable to an increased water-binding capacity in the milk proteins (Labropoulos et al.,

1984).

NutritionalJournal and functional properties of sheep Pre-proof milk Energy content

Sheep milk is characterized by high energy value of 5932 kJ/kg (Park et al., 2007) whereas that of cow, buffalo, camel, goat and human are 3169 to 3730 kJ/kg (Barłowska, 2007), 3450 kJ/kg

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(Kanwal et al., 2004), 3283 kJ/kg (Shamsia, 2009), 3018 kJ/kg (Park et al., 2007) and 2407 kJ/kg

(Shamsia, 2009) respectively. The donkey milk is characterized by lowest energy value of -1842 to 2051 kJ/kg (Guo et al., 2007).

Milk proteins

In many countries, people demand for small ruminant milk based on its total protein content

(Pirisi et al., 2007). Milk proteins are divided into casein complexes and whey protein fractions.

Casein is the most abundant protein (80%), while the proportion of whey proteins is relatively low (20%) in milk of ruminants. Whey protein is composed of β-lactoglobulin, α-lactalbumin, immunoglobulins, glycomacropeptides, bovine serum albumin and minor proteins such as lactoperoxidase (LP), lysozyme and lactoferin(LF). Sheep milk is the richest source in whey proteins (1.02 g/100 g) and it also has highest concentration of casein- 4.18 g/100 g (Dario et al.,

2008).The β-lactglobulin is the major whey protein in sheep milk (Gaye et al., 1986) and α- lactalbumin content in sheep milk is also higher than that of bovine milk (Zhang and Brew,

2003).

There are four main casein fractions distinguished: αs1, αs2, β, and κ. their proportion is diverse and polymorphism of these proteins was demonstrated in most of the animal species (Barłowska et al., 2007). The human casein does not contain the αs1-fraction, which is the predominant factor causing milk protein allergy. However, it is rich in the β-fraction (Sood et al., 1997).

Conversely, αs1-fraction of casein in cow and buffalo milk is very abundant; 38.4% and 30.2% of total casein, respectively (Zicarelli, 2004). Studies related to availability of sheep milk casein fractions areJournal scanty, contradictory and inconclusive. Pre-proof Bramanti et al. (2003) reported that goat milk is rich in β-fraction and sheep milk is rich in αs1-fraction. Moatsou et al. (2004) reported that sheep milk casein has 9.1–10.2% κ-casein, 12–16.4% αs2-casein, 33.9–39.9% αs1 -casein

7 and 37–42.3% β-casein (while the same caseins in cow milk are approximately 12, 10, 37 and

35%, respectively. A recent study by Ruprichová et al.(2015) states that ewes’ milk had 5.3%

αS1-casein, 25.0% αS2 casein, 59.5% β-casein and 10.2% κ-casein. Table 5 shows concentrations of different protein fractions in sheep milk.

After analyzing the amino acid composition of different milk Barłowska et al. (2011) reported that the best composition of exogenous amino acids is found in the milk of goats and sheep.

Their milk, fully cover the requirement for all essential amino acids. Park et al. (2007) highlighted that taurine which play important role in bile acid conjugation, osmoregulation, calcium signaling and function as antioxidant in animal tissue can be obtained from goat and sheep milk. It is a valuable nutrient for the human neonate.

Milk lipids

Information on fatty acid (FA) profile is critical for judging sheep milk for health benefits and promotion of derivative dairy products. Among the bioactive constituents of sheep milk, lipids are important due to their high nutritional value and effect on physicochemical, sensory and manufacturing properties of dairy products (Park et al. 2007).

Owing to the smaller size of fat globules, sheep milk aid in higher digestibility and efficient metabolism compared with cow milk fat (Park, 1994). Some studies found that the average fat globule size is smallest in sheep milk followed by goat milk (Mens. 1985). Mehaia

(1995) reported average fat globule sizes in this decreasing order: cow > sheep > goat.

The fatty acid profile of milk fat is affected by a number of genetic (breed, genotype), physiologicalJournal (age, stage of lactation, season) andPre-proof environmental (feeding, grazing) factors as well as their interactions (De La Fuente et al. 2009). Sinanoglou et al. (2015) extensively analyzed fatty acid profile and lipid quality indices in raw milk and cream fat of some

8 indigenous sheep breeds of Greece. They found that sheep raw milk and cream fat have high proportions of short- and medium-chain FA, which are of value for a healthy diet. Raw milk

FA profile was influenced to a higher extent by the lactation stage than by the breed factor.

The relative proportions of most short- and medium-chain FA decreased as lactation progressed, whereas unsaturated FA proportion increased. Palmitic and oleic acids showed their highest proportion during late lactation. Atherogenic and thrombogenic indices decreased; on the other hand, the hypocholesterolaemic/hypercholesterolaemic (h/H) fatty acid ratio increased at late lactation. The lower the atherogenicity (AI) and thrombogenicity (TI) index values, the healthier the food. These indexes show the relationship between fatty acids in food and their contribution to the prevention of coronary diseases (Turan et al., 2007).

Levels of the metabolically valuable short and medium chain FA, caproic (C6:0) (2.9, 2.4,

1.6%), caprylic (C8:0) (2.6, 2.7, 1.3%), capric (C10:0) (7.8, 10.0, 3.0%), and lauric (C12:0) (4.4,

5.0, 3.1%) are significantly higher in sheep and goat than in cow milk, respectively (Alonso et al., 1999 and Goudjil et al., 2004). These FA are associated with the characteristic flavors of cheeses and can also be used to detect admixtures of milk from different species.

Characteristic of sheep milk is a higher concentration of butyric acid (C4:0), conjugated linoleic acid (CLA) and omega 3 fatty acid content than other ruminant milk (Collomb et al., 2008). The presence of the essential ω-3 and ω-6 FA in milk fat as well as other less common FA, like linoleic acid isomers, has gained an increasing interest due to the consumer demand for a healthy diet. The greatest seasonal differences were measured in sheep milk CLA, 1.28% in summer andJournal 0.54% at the end of the winter period Pre-proof (Jahreis et al., 1999). Milk minerals and vitamins

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Milk is an important source of mineral substances, especially calcium, phosphorus, sodium, potassium, chloride, iodine, magnesium, and small amounts of iron. In a modern European diet, milk is the main source of calcium. Gueguen and Pointillart (2000) reported that calcium bound to casein (both in organic and mineral form) exhibits significant availability during the milk digestion process. Thus, the bioavailability of this element is closely correlated with a higher concentration of casein (Gaucheron, 2005). The highest concentration of this, and other mineral elements, is specific to sheep milk. The concentrations of Ca, P, Fe and Mg in sheep milk are

193, 158, 0.08 and 18 mg ⁄100 g milk, respectively (Park et al. 2007).

A review by Paccard and Lagriffoul (2006a) and (2006b) for sheep milk, demonstrated the high content in B vitamins especially niacin. Milk is a valuable source of vitamins, both water- soluble and fat-soluble ones. Goat and sheep milk are characterized by higher vitamin A concentration in comparison with cow milk (Park, 2007). All of the β-carotene in milk from goats and sheep is converted into retinol, resulting in the white color of that milk. Goat milk is a good source of vitamin A, niacin, thiamin, riboflavin, and pantothenic acid.

Functional Property

Casein micelle characteristics of sheep milk

Caseins are phosphoproteins and are organized within micelles, where the calcium-sensitive αs1,

αs2-, and β-caseins are stabilized within the micelle by κ-CN (Rijnkels, 2002). The micelles are interlinked by calcium phosphate and small amounts of magnesium, sodium, potassium, and citrate, diffusing the light and lending the milk its opaque white appearance (Park et al., 2007). Caseins playJournal a nutritive function as a source of aminoPre-proof acids, calcium, and phosphorus for the neonates (Holt and Sawyer, 1988).Micelle structures of sheep milk differ from cow milk in average diameter, hydration and mineralization. Sheep milk is similar to goat milk in micelle

10 characteristics. Average mineralization levels of micelles in goat and sheep milk are higher than in cow milk. That means these small ruminant’s casein micelles contain more calcium and inorganic phosphorus. Here, casein micelles are less solvated, less heat stable, and lose β-casein more readily than bovine casein micelles (Jenness, 1980 and Remeuf and Lenoir, 1986). There is an inverse relationship between the mineralization of the micelle and its hydration and direct relationship exist between mineralization and bioavailability.

Micelles are characterized by different sizes in milk from different animal species (Bornaz et al.,

2009). Camel milk contains micelles of largest diameter (380 nm), whereas goat milk having 260 nm, sheep milk having 180 nm and cow milk having the smallest of 150 nm. Attia et al. (2000) and Bornaz et al. (2009) showed a negative correlation between the casein concentration and micelle size. The rennet clotting time of milk depends upon micelle size; small and medium sized micelles having the shortest clotting time. Therefore, casein curd obtained from goat milk is weaker and more susceptible to tearing than curd from cow and sheep milk. Brule et al. (2000) reported that big micelles contain higher concentrations of calcium phosphate, whereas smaller micelles contain more κ-casein. Similar results were published by Bornaz et al. (2009), stating that κ-casein share decreases with an increase of casein micelle diameter. Moatsou et al. (2004) reported that a high concentration of protein, fat, and calcium per casein unit in sheep milk predestinates it as an excellent material for cheese making. This is the main reason for higher cheese yield in case of sheep milk from same volume of cow milk. Whey proteins may impair cheese making (cheese yield and whey draining, especially for heat treated milks) but their amino acidJournal profiles are of interest with a high levelPre-proof in essential amino acids (e.g. tryptophane and lysine) (Raynal-Ljutovac et al., 2008). The β-caseins are described as ‘calcium-sensitive’ because they precipitate in the presence of low concentrations of this cation. They are important

11 in determining the surface properties of casein micelles and essential for rennet curd formation.

Among the most studied casein is casein kappa (κ-CN), probably because of its importance in the stability of the micelle and its role in dairy processing.

Bioactive peptides

Dietary proteins possess some potent biologically active peptides in them which remain latent and inactive until hydrolysis. Enzymatic hydrolysis (animal, plant or microbial source) of inactive milk proteins can release fragments (biologically active form) able to exert specific biological activities, such as antihypertensive, antimicrobial, opioid, antioxidant, immunomodulant, or mineral binding. Such protein fragments, known as bioactive peptides, are formed from the precursor inactive protein during gastrointestinal digestion and/or during food processing (Korhonen and Pihlanto-Lepp ala, 2003). In last few decades dairy bioactive peptides

(particularly from cow milk) had gain the attention of researchers for their physiological and physico-chemical versatility and used for health promoting foods or pharmaceutical applications.

Angiotensin Converting Enzyme (ACE) inhibitory peptides are important bioactive peptide gaining special attention due to their potentially beneficial effects in the treatment of hypertension. ACE is a multifunctional enzyme, located in different tissues, and able to regulate several systems that affect blood pressure. It is responsible for generating vasopressor angiotensin II and the inactivation of the vasodepressor bradykinin. Milk proteins are the main source of ACE inhibitory peptides. Most publications on ACE inhibitory and/or antihypertensive peptides have used peptides from cow milk (Korhonen and Pihlanto-Lepp¨al¨a, 2003; Gobbetti et al., 2004; SilvaJournal and Malcata, 2005; Meisel, 2005). Pre-proof However, in recent years, the sheep and goat milk proteins havebecome an important source of ACE inhibitory peptides. Hern´andez-Ledesma et al. (2002) and Chobert et al. (2005) studied the ACE inhibitory activity of hydrolysates of β-

12 lactalbumin from sheep and goat milk with enzymes of digestive and microbial origin. Minervini et al. (2003) isolated and characterized bioactive peptides from 6 different species (cow, sheep, goat, buffalo, pig & human) by using Lactobacillus derived proteases and obtained 5 different peptide fractions from αs1 (f1-6, f1-4, and f4-8) and αs2 (f182-185 and f186-188) casein of sheep milk showing ACE inhibitory activities. The κ-casein hydrolysis also results formation of

ACE inhibitory biopeptides (Manso and L´opez-Fandi˜no, 2003). Correa et al. (2011) used

Bacillus derived protease to get bioactive peptides and estimated them for ACE inhibitory activities, antioxidant and antimicrobial activities. They concluded that synergistic action of various peptide fragments obtained is responsible for bioactivities. These biopeptides are useful in food industries for increasing the shelf-life of food and in therapeutic preparation for their nutritional values. Apart from sheep milk, its products like cheese and yogurts are also been studied for bioactivities.

Bioactive proteins and peptides derived from milk have been reported to provide a non-immune disease defense and control of microbial infections (McCann et al., 2005). It has been proved, that milk proteins can also act as antimicrobial peptide precursors, and in this way might enhance the organism’s natural defenses against invading pathogens. Consequently, food proteins can be considered as components of nutritional immunity (Pellegrini, 2003). Both whey protein and casein derived bio-peptides have antimicrobial action. Peptides derived from lactoferrin (LF) are antibacterial peptides. Tomita et al. (1991) first reported the enzymatic release of antibacterial peptides with more potent activity than the precursor lactoferrin. Similar to whey protein, casein also serve Journalas antimicrobial bio-peptide precursor. Pre-proof Lopez-Exposito et al. (2006) identified four antibacterial peptides from a pepsin hydrolysate of ovine αs2-casein. The peptide fragments are

αs2-casein fragments f(165-170), f(165-181), f(184-208) and f(203-208), and these peptides

13 showed a strong activity against Gram negative bacteria. Of them, the fragment f(165-181) was the most active against all tested bacteria. Recio et al. (2005) concluded that the ovine αs2-casein f(203-208) is a good example for a multifunctional peptide, because it exhibits not only antimicrobial activity, but also potent antihypertensive and antioxidant activity.

Medicinal and therapeutic properties of sheep milk with respect to specialized biomolecules

Milk and milk products from sheep are very important for proper human nutrition at places where cow milk is not readily available or affordable (Haenlein, 2001). In countries like

Somalia, Greece, Bangladesh, Iraq, Iran etc. more than one half or at least one third of all milk is supplied by sheep and goats, which is sufficient to meet the protein and calcium requirement of people living at that place. In India, a lot of people are vegetarian; they rely on plant sources for protein and calcium requirement. Animal protein is considered to be more complete, because it contains all of the essential amino acids that the human body needs to function effectively. Plant protein cannot serve as an alternative to animal protein. The recommended daily intake for an adult person per day is at least 60 g of animal protein and 800-1000 mg of calcium (National

Institute of Nutrition, 2009) which is apparently not achieved in many countries like India. Sheep milk is an excellent source of protein and calcium. It is a better alternative for these countries.

The major mineral compounds of milk are calcium and phosphorus, which are essential for bone growth and the proper development of newborns (Al-Wabel, 2008). A recent study showed that sheep milk consumption resulted to higher trabecular bone surface density and trabecular bone surface to volumeJournal ratio in growing rats compared Pre-proof to cow milk having equal total solid (Burrow et al., 2018). It is clear from the above study that sheep milk has positive effect on bone structural integrity and bone health.

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Sheep milk is the richest source of conjugated linoleic acid and α-linolenic acid (Markiewicz-

Kęszycka et al., 2013). It is claimed to inhibit the occurrence and development of cancer

(Parodi, 1999), in the prevention of coronary heart disease and atherosclerosis (Gavino et al.

2000; Tricon et al. 2004), inhibits the development of osteoporosis (Watkins and Seifert 2000) and stimulates the immune system (O’Shea et al. 2004). The CLA has multiple functional properties. Sheep milk and their products are a medical necessity for many people as an alternative to cow milk. It is presently used in anti-ageing formulations and cosmetic soap preparations which have soothing action on psoriasis and skin eczema like chronic diseases due to high fat and mineral content (Belanger, 2018)

Conclusion

Sheep rearing is an integral part of human culture in India. It was domesticated by early humans and now also small and marginal farmers earn their livelihood from sheep rearing. Research on sheep milk will be beneficial to this underprivileged section of human society. Sheep milk has positive effect on bone growth, skin moisturizing, blood vascular system and cardiac health. The significant impact of sheep milk on human health importance is the neglected area of research.

This review enlightens the researchers to consider sheep milk not only a source for delicious cheese and creamy yogurt, but to appraise it as a vehicle for good health.

Conflict ofJournal interest Pre-proof We hereby disclose that there is no actual or potential conflict of interest including any financial, personal or other relationships with other people or organizations within three years of

15 beginning the submitted work that could inappropriately influence, or be perceived to influence, their work.

Journal Pre-proof

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Journal Pre-proof

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Table 1. Dairy sheep breeds in world and their production

World Sheep population 1202 million

Dairy Sheep 246 million (21%)

Sheep milk production in world 10.42 million tonnes

Well known dairy sheep breeds Awassi, Assaf, East Friesian, Lacaune

(>200 lit/lactation) Sarda, Chios, Manchega

Source: FAO, 2017, Shinde and Naqvi, 2015

Table 2. Leading sheep milk producing countries

Rank Country Sheep Milk Production Contribution in total domestic milk

(Metric Tonnes) production (%)

1 China 1,540,000 3.34

2 Turkey 1,101,013 4.49

3 Greece 705,000 39.04

4 Syria 684,578 29.10

5 Romania 632,582 12.19

6 Spain 600,568 6.40

7 Sudan 540,000 9.06

8 Somalia 505,000 17.77 9 JournalIran 470,000 Pre-proof5.88 10 Italy 383,837 3.41

Total 10, 422, 522

Source: FAO, 2017

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Table 3. Milk composition of different dairy animals

Parameters Sheep Goat Buffalo Cow Camel

Protein (%) 4.50-6.60 2.80-3.70 4.38 3.20-4.00 3.26

Fat (%) (5.30-9.30) 6.99 (3.40-4.50) 4.07 7.73 (3.40-4.50) 4.09 3.80

Lactose (%) 3.90-4.90 3.90-4.80 4.79 4.60-4.90 4.30

Solid not fats 12.00 8.90 9.50 9.00 10.36

(SNF) (%)

Water (%) 82 83.20 83.18 87.80 86.50

Total solid (%) 18.50 12.50 18.00 13.80 14.00

Source: Kapadiya et. al., 2016; Giambra et. al., 2014

Table 4. Physical properties of sheep, goat and cow milk

Properties Sheep milka Goat milkb Cow milkc

Specific gravity (density) 1.0347–1.0384 1.029–1.039 1.0231–1.0398

Viscosity (Cp) 2.86–3.93 2.12 2.0

Surface tension (Dynes/cm) 44.94–48.70 52.0 42.3–52.1 ConductivityJournal (Ω−1 cm−1) 0.0038 0.0043 Pre-proof–0.0139 0.0040–0.0055 Refractive index 1.3492–1.3497 1.450 ± 0.39 1.451 ± 0.35

Freezing point (−°C) 0.570 0.540–0.573 0.530–0.570

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Acidity (lactic acid %) 0.22–0.25 0.14–0.23 0.15–0.18 pH 6.51–6.85 6.50–6.80 6.65–6.71

Source- a Haenlein and Wendorff, 2006 ; b Parkash and Jenness, 1968 ; c Park et al., 2007

Table 5. Concentration of protein fractions of sheep milk

Protein Unit

Total protein (g kg-1) 45-66

Total casein (g kg-1) 42-52

αs1- casein (% of total casein) 6.66

αs2- casein (% of total casein) 22.84

β- casein (% of total casein) 61.60

κ- casein (% of total casein) 8.90

Whey proteins (g kg-1) 10-13

α- Lactalbumin (% of total whey protein) 13.50

β- Lactalbumin (% of total whey protein) 46.70

Minor whey proteins (% of total whey protein) 39.80 Source- HaenleinJournal and Wendorff, 2006 ;Degan, 2007 ;Pre-proof Pandya and Ghodke, 2007 ; Park et al., 2007 ; Barłowska et. al., 2011 ; Selvaggi et. al., 2014

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