Sheep’s Milk Yoghurt

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The technology of both traditional and industrial sheep’s yoghurt have been reported by Irvine (1989) and Anifantakis (1990). The main differences in the manufacturing stages are first, in the traditional process the milk is boiled, filled into containers at 95°C, allowed to cool to 45°C, inoculated with starter culture and fermented to the desired pH, and finally transferred to the cold store; such a method produces a set-type yoghurt with a crusty layer. Second, the industrial process may include standardisation of the fat content, homogenisation and heating the milk to 95°C only. The addition of aroma (e.g. fruit or flavouring substances) is optional because the majority of sheep’s yoghurt is sold unflavoured. The use of two-stage homogenisation at 13.8 MPa and 3.5 MPa, respectively, has been reported by Smith (1989), whilst Muir and Tamime (1993) have examined the effect of homogenisation of the milk on the extent of serum separation and firmness of set- and stirredtype sheep’s yoghurt (see Fig. 5.2). Furthermore, using milk from a commercial flock of milking sheep in Scotland, details of the effect of seasonal variation on the gross chemical composition, changes in indices of stability, microbiological quality and organoleptic properties of yoghurt have been given by Muir et al. (1993a–c) and Tamime et al. (1993) (see also Bonczar et al., 1998).
Inherently, sheep’s milk contains high levels of protein (c. 5.8 g 100 g-1 ), and does not require fortification of the milk SNF during the production of yoghurt (Muir et al., 1993a). As mentioned elsewhere, homogenisation of the milk can improve the firmness (see Fig. 5.2) and reduce syneresis of sheep’s yoghurt (Muir and Tamime, 1993), whilst Kisza et al. (1993) recommended heat treatment of the milk at 91°C for 30 s to reduce the fermentation time compared with cow’s milk. The same authors used a mixed starter culture consisting of S. thermophilus and L. Acidophilus which resulted in a superior product when compared with a yoghurt starter culture (see also Creed, 1996).
Since the lactation period of sheep is about 6 months, the availability of milk for processing in dairies all the year around is limited. Hence a problem is encountered in maintaining a steady output and availability of sheep’s yoghurt on the market.
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Some attempts have been made to preserve sheep’s milk by freezing (Young, 1986, 1987; Giangiacomo and Messina, 1991). The stability of the milk during storage is governed by the temperature of freezing and the size of the block being frozen.Anifantakis et al. (1980) recommended the addition of 2 g 100 g-1 Na-citrate and 0.1 g 100 g-1 ascorbic acid before freezing in order to improve the stability during storage (i.e. up to 11 months) and after thawing when it is heated for yoghurt making. Oxidation of the fat was more pronounced in a 7cm thick block of frozen milk stored
at -20°C, in the presence of ascorbic acid, and when compared with a 2cm block stored at -30°C; although the free fatty acid content increased during storage, the yoghurt made from the thawed milk was acceptable by the taste panel. However, in a recent study,Voutsinas et al. (1996a, b) concentrated sheep’s milk by RO (whole and skimmed – the latter was mixed with the cream after concentration) before freezing, and they reported: (a) no significant differences in lipolysis during storage at -20°C for up to 8 months, (b) although the initial total viable and coliforms counts were high, the number decreased during storage, and (c) the thawed and reconstituted concentrates were stable for the production of yoghurt especially for whole milk, but the product had a slight grainy texture and the extent of syneresis was higher when compared with yoghurt made from fresh sheep’s milk. These results may suggest, in part, some degree of storage stability of frozen sheep’s milk, but more research is required to overcome some of the faults observed during the manufacture of yoghurt.
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Isolates of S. thermophilus and L. delbrueckii subsp. bulgaricus from traditional Greek yoghurt have been characterised for flavour and proteolytic activity (Kalantzopoulos et al., 1990a, b; Georgala et al., 1995), and combinations of these organisms have been recommended for the industrial production of sheep’s milk yoghurt. In an earlier study Kehagias and Dalles (1984) noted that the b-galactosidase activity of starter cultures in sheep’s milk was double that observed in a similar product made from cow’s milk. However, the screening and selection of lactic acid bacteria from gioddu (a Sardinian fermented milk made with an “artisanal” starter culture plus enzymic extracts of aromatising yeasts) resulted in a sheep’s product with Good keeping quality, improved flavour and appearance, and a firmer product with low syneresis (Deiana et al., 1992).


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