THE NUTRITIONAL VALUE OF SHEEP MILK

George F. W. Haenlein

Department of Animal & Food Sciences

University of Delaware, Newark, Delaware, 19717-1303 USA

Abstract

Reasons for sheep dairying in comparison to cow and goat dairying are presented in an extensive literature review. The cost of producing 1 kg sheep cheese, the main product in sheep dairying, is about $8.- compared to $5.- for goat cheese and $3.- for the same cheese made from cow milk. Net income from sheep milk production can be presently at least double per farm than that from wool and meat production. However, research emphasis worldwide has been on sheep wool and meat production and their characteristics, not on sheep milk. At least two dozen different dairy sheep breeds are recognized, mainly in the Mediterranean area, with different genetic milk yield merits, but all distinguished by higher milk fat and protein levels than in goat and cow milk. Some sheep milk protein polymorphisms and their relationships to different cheese making parameters have been identified. Sheep milk composition can also be influenced by different feeds, grazing systems and by subclinical mastitic conditions. The fatty acid composition in sheep milk is easily altered by different feed supplements. Average composition of milk from sheep, goats, cows and humans is comprehensively documented and compared relatively to the nutrient supply from human milk and to the recommended human daily dietary allowances. The unique richness in short chain and medium chain fatty acids in sheep milk, sheep cheeses, sheep butter (so far very neglected commercially), and their special values in human health and as treatment for many disease conditions is discussed extensively. This can lead to sheep dairying alongside the cow milk industry as a unique and justified niche industry in many countries with considerable growth potential, that has been estimated in England alone to be worth more than US $100 million.

Keywords:

1. Introduction

Why milk sheep or even goats, when cows give much more milk and require less maintenance and labour per animal unit? That is a serious question often heard among economists. It is true that a unit of milk is more costly produced from sheep, also from goats, than from cows. Therefore, dairy products like cheeses, the main form of sale of sheep milk in most countries, are more expensive for consumers than cow milk products, e.g. cow milk cheese (Table 1), provided the sheep milk producer wants to not go bankrupt and out of business after a few years of hard toil, but actually make a living for his family, and some profit. On the other hand, milk is presently a better avenue to make a living in many parts of the sheep world (Table 2), since wool has a poor market return, and sheep meat, mutton and lambs is in a tight profit position except for ethnic markets.

Thus, why milk sheep? Is it because it is a better alternative to wool and meat sheep farming, or is it an alternative to cow dairying, which has had times of uncontrolled surplus production, quota restrictions, and cow milk intolerance problems, which the medical research community has not adequately addressed?

But how to justify milk sheep economically? Just because sheep milk has a higher solids content than cow and goat milk, and therefore has a much better cheese yield? That is not necessarily a good enough reason for the general negative economics of scale. Actually, buffalo (16.9%), reindeer (36.7%) and yak (17.9%) milk have at least the same high solids contents (Path, 1995; Kon and Cowie, 1961) as sheep milk for superior cheese yield compared to cow milk (12.6%), and they would have the economics of scale advantage as cows do. However, the large population size of sheep breeds, their wide adaptability to climates and forages make sheep much more widely accepted and genetically of greater potential.

2. Research publications

What other reasons for milking sheep can be found, than the addiction of the sheep farmer to nothing but sheep? Matching sheep milk products to the consumer is probably the best reason for milking sheep. The Commonwealth Agricultural Bureaux (CAB) considered the British Sheep Dairy News worthy for regular inclusion in their world wide computer scientific journal abstract service.

Searching the CAB for the past 10 years, the numbers of publications world wide on sheep milk, sheep cheese and allergies related to sheep milk are minor compared to those on cow milk (Table 3). Certainly there are more publications in earlier years, but those would be based on less sophisticated analytical methods. Publications on the nutritional value of sheep milk often give only gross composition without detailed identification of more specific unique components of benefit in human nutrition, although trade magazines contain many anecdotal reports, that are waiting for scientific evaluation. However, among the about 1,000 research projects on sheep world wide (Table 3), less than 4% deal with topics of sheep milk and sheep dairying.

Most textbooks on "Milk" deal only with cow milk, and human milk to some extend (Kon and Cowie, 1961; Falconer, 1971; Renner, 1982; Gravert, 1983). The only major sources of research information on sheep milk appear to be the bulletins and proceedings of the International Dairy Federation Seminars held in Greece in 1985 and 1995 (IDF, 1983; 1986; 1996). Not even the 1st World Congress of Dairy Products in Human Health and Nutrition (Serrano Rios, 1994) recognised the existence or contribution of sheep milk products to human nutrition, nor does the handbook of the US Sheep Industry Development Program (Scott, 1986). In France, the scientific journal Le Lait devoted an entire issue to goat milk, including its role as an alternative to cow milk (Le Lait, 1993). A bibliography on goat milk (Freund, 2000), and the proceedings of a national colloquium on the nutritional and dietetic qualities of goat milk (Freund, 1996) have been published, but similar efforts on sheep milk are still missing. Even a book on buffalo milk is available (Deepak Sahai, 1996).

3. Effects of breeds, season, feeds, mastitis

Among the more than 25 recognised dairy sheep breeds, which are mostly in the Mediterranean region, great genetic variation exists in milk composition, lactation length, lactation yield and seasonality of milk production. Using artificially controlled photoperiods of daylight can change milk production by 25-38% with concomitant changes in fat and total solids contents (Bocquier et al., 1997). Milk yields can differ more than ten times between breeds and within breeds, and lactation lengths can vary by 100%, which is a considerable potential for genetic selection and improvement. Even normal sheep milk composition may differ between 6 and 9% for fat, 4 and 7% for protein, 17 and 21% for total solids, 4 and 6% for lactose (Table 4) (Dario et al., 1995; Margetin, 1996; Simos et al., 1996), making sheep milk the composition leader next to reindeer among the 8 farm species, that produce milk for man (Path, 1995; Renner, 1982).

Genetic differences in milk protein polymorphisms are of great interest to the cheese maker, because of correlations to curd firmness, coagulation time, casein content and cheese yield. Many studies on cow and goat milk proteins have shown that genetic selection for polymorphisms can have advantages for cheese processing, but much less is known for sheep milk. New analytical techniques during the last 5 years have made identifications of sheep milk protein polymorphisms possible (Pirisi et al., 1999b). Sheep milk protein comprises caseins, and the whey proteins beta-lactoglobulin, alpha-lactalbumin, serum albumin, and immunoglobulin mainly. The caseins comprise 4 main fractions, alpha s-1, alpha s-2, beta and kappa, and each has a number of genetic polymorphs, which are due to different phosphorylation levels, amino acid substitutions, glycosylation differences, alterations in electrical charge, molecular weight, and hydrophobicity of the proteins. The hetrogeneity of alpha s-1 casein polymorphisms in sheep milk consists of either A, B, C, D, E, or F genes (Pirisi et al., 1999b), contributed by each parent. The D gene has a frequency of <0.01 in Lacaune, but 0.03 in Sarda sheep, and may be associated with low levels of fat, total protein, casein and poor curd forming characteristics. CC sheep milk had higher casein content by 3.5% than CD, and by 8.6% than DD milk (Table 5). CC milk also had higher protein : fat ratio, smaller casein micelle diameter, and better renneting properties. Three genetic polymorphisms of beta-lactoglobulin, A, B, and C, in sheep milk have been reported, and homozygous AA had higher cheese yield and fat contents than AB or BB (Pirisi et al., 1999a). Kappa casein has A and B variants, alpha s-2 casein 3 variants A, B, and C, and beta casein has 3 variants, beta-1, 2, and 3 (Martin and Addeo, 1996).

Milk composition during the lactation follows typical curves. Therefore tables of average milk composition of any species do not tell the whole story, since fat, protein and ash contents increase considerably towards the end of lactation, while lactose contents decrease (Casoli et al., 1989; Fenyvessy et al., 1991; Dario et al., 1995; IDF, 1996; Ploumi et al., 1996). This influences the taste of milk, as it may be more salty at the end of lactation, and also affects cheese making characteristics (Piredda et al., 1996; Perea et al., 2000).

Milk composition of major and minor components is affected by feeding regimes, ration components and forage : grain ratios (Perea et al., 2000). Supplementation of a mixed ration of rye grass pasture, vetch-oats and alfalfa hay with 20% calcium soaps of palm oil fatty acids increased milk fat content by 23%, yield by 16%, in milk of Manchega ewes, but decreased milk protein content by 9%; while 30 or 45% rumen undegradable protein had little effect on milk composition (Table 6) (Casals et al., 1999).

Milk fatty acid composition is of particular interest to consumers with concern of health and heart disease. Studies aiming to modify the fatty acid profile of milk lipids have shown that type of diet will be effective. Adding supplementary rumen protected fat can increase the proportion of unsaturated fatty acids in sheep milk. Adding supplementary rumen protected methionine and lysine increased the levels of Cl6:0-Cl8:3 by 5%, the unsaturated : saturated fatty acid ratio by 4%, but reduced C4:0-C12:0 by 12%, and left C12:0-C16:0 unchanged in milk of Comisana ewes (Table 7) (Sevi et al., 1998). Feeding ammonia treated olive tree leaves to Karagouniko ewes also decreased C6:0-C6:1, while increasing C18:0-C18:l fatty acids (Fegeros et al., 1995).

Mastitic conditions affect milk composition in sheep as it does in cow or goat milk, except that the monitoring indirect parameter of somatic cell counts (SCC) in milk is much less related to pathogenic conditions in sheep as it is in goat udders compared to cow udders, and that high SSC (>1 million/ml) do occur in normal sheep and goat milk, especially towards the end of lactation (Bufano et al., 1996). Nevertheless, with increasing SCC it has been reported that milk pH, whey protein, fat contents, rennet clotting time, and rate of clot firming time rise, while lactose, casein contents, and clot firmness decreased (Table 8) (Diaz et al., 1996). It has not been determined, however, whether the deterioration of renneting characteristics is due primarily to the normal physiological SCC increase at the end of lactation and the concurrent change in milk composition, or to subclinical and clinical mastitic conditions in the udder, which also produce high SCC and changed milk composition.

4. Standard nutritional reference tables

Average nutritional values of sheep milk in comparison to its 3 main competitors, cow, human and goat milk have been published years ago most comprehensively, but have not been updated apparently (Tables 9, 10, 11) adapted from (Kon and Cowie, 1961; Posati and Orr, 1976; Renner, 1982). They do show, however, the superiority of sheep milk composition, in relative terms compared to the composition of human, cow and goat milk (Tables 12, 13, 14); and in such critical nutrients as protein, calcium, iron, magnesium, zinc, thiamin, riboflavin, vitamin B6, vitamin B12, vitamin D, medium chain fatty acids, monounsaturated fatty acids, linolenic acid, and all 10 essential amino acids. This is shown also in (Table 15) in actual values compared to the recommended daily minimum allowances for human nutrition, when 2 cups (490 g) of milk of one of the 4 species are consumed. Most importantly, daily calcium requirements and those of riboflavin and 5 of the essential 10 amino acids are completely covered by just drinking 2 cups of sheep milk without eating anything else. As drinking sheep milk is not widespread, more likely 2 cups of sheep milk yoghurt will meet those daily requirements, or the milk equivalent in 93 g of sheep cheese.

The nutritional values of sheep cheeses have not been published much nor in detail compared to those of cow cheeses. (Table 16) attempts to show how sheep cheeses compared to cow cheeses can make a valuable contribution for consumers. More significantly this is demonstrated in (Table 17) for MCT, the medium chain fatty acids C6:0-C14:0, especially on a total solids or dry matter basis.

5. MCT and other uniqueness

Medium chain length fatty acids or medium chain triglycerides (MCT) have been recognised as unique lipids with unique applications (Babayan, 1981; Babayan and Rosenau, 1991; Haenlein, 1992). Their medical and nutritional values have been the subject of at least 102 publications during the past 10 years demonstrating real benefits in malabsorption syndroms, chyluria, steatorrhea, hyperlipoproteinemia, and in cases of intestinal resection, coronary by-pass, premature-infant feeding, childhood epilepsy, cystic fibrosis and gallstones, because of their unique metabolic ability to provide energy instead of contributing to adipose tissues as other lipids do. MCT also inhibit or limit cholesterol deposition, dissolve cholesterol gallstones, and contribute to normal growth of infants. MCT are unique, because they do not follow the general lipid transport pathway through the lymphatic system and into the prostaglandin metabolism (Babayan and Rosenau, 1991). Instead they go through the portal system directly into the liver, where they are oxidised into quickly available energy for body utilisation. Thus, patients have been prescribed daily intake of MCT supplements for at least 40 years.

The unique content of about 25% MCT in total sheep milk fat and its possible quantitative modification through feeding has not been exploited commercially nor explored in research. Daily recommended patient intakes of 15 g MCT (Dulloo et al., 1996) could be provided by about 60 g sheep butter, but sheep butter making may have to be reinvented. A novel approach has been proposed by Babayan and Rosenau (1991) to make cheese, either cheddar or fontinella, enriched with MCT oil as a palatable and more attractive option for patients who suffer from various malabsorption syndroms and must consume MCT.

Another interesting research programme is pursued at the University Wisconsin (Wendorff 1998) in utilising the unique properties of sheep milk, especially its flavourful fatty acids, to enrich the flavour of low-fat cow milk cheeses such as Muenster or Manchego by adding 20% sheep milk. The blended 9-months aged Manchego-type cheese has become popular at the dairy store of the University of Wisconsin in Madison. Odorous substances are actually at a higher level in sheep milk than in goat or cow milk, except for buffalo milk (Table 18) (Moio et al., 1993). Another potentially important study by Ramet (1996) has demonstrated that the poor cheese making properties of camel milk can be significantly improved by adding from 10 to 50% sheep milk.

Of particular interest to dairy sheep farmers is the possibility of using sheep milk as an alternative to cow milk in cases of cow milk allergy. Medical research, however, has not adequately pursued the many practical experiences and anecdotal reports of benefits to patients suffering from cow milk allergy. (Table 3) shows that there has been a fair number of publications on cow milk allergy, but few on sheep milk as an alternative. Actually there are at least 3 papers during the last 10 years indicating allergy also to sheep milk, because of cross reactivity between the milk proteins of bovine, ovine or caprine origin (Wuthrich and Johansson, 1995; Fenyvessy and Csanadi, 1999; Umpierrez et al., 1999).

6. Niche marketing

Various nutritional preparations are on the market for use by people in need or interested in boosting their energy. Sheep milk has been proposed as a more natural and better tasting alternative with great nutritional and clinical potential (Hardy, 2000), because of its many valuable constituents, discussed here, and in a market estimated e.g. in the U.K. as worth $100 million. Of course, the sheep cheese market is well established and growing, but there is much room for innovations as the new dairy sheep industry in USA has demonstrated in artisanal and direct marketing of sheep cheeses and yogurt with the support of the American Cheese Society during the last 10 years.

Today's consumer is much more interested in low-fat than whole milk. Therefore to market any substantial amounts of whole sheep milk with its very much higher fat content than cow milk may not be very successful. Low-fat sheep milk apparently has not been tried in marketing, but because of its high protein content and full taste, it should have potential. Also the resulting cream can be a good market in itself or as sheep butter or for sheep ice cream production, always utilising the name "sheep" as a special label for sales with possibly a higher price. Promotion of sheep milk and its products could be much better organised and a greater market share realised, not just a special niche for connoisseurs, but also for medical needs, if producers, at least in developed countries, would make a regular percentage check-off money of their product sales available for a central agency for advertisements and research. In many developing countries it is also of vital economic importance to assure a fair price for sheep milk products to maintain the farmer's future on the farm, besides providing critical protein, calcium and energy food for subsistence and to fight malnutrition. In this effort, much is needed to improve the genetic ability of many dairy sheep populations to have longer lactations and to focus on much better udder morphology.

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