Goat’s Milk Yoghurt

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The gross chemical composition of goat’s milk can vary considerably and the total solids (TS) may range between 11.3 and 15.9g 100 g-1 (Robinson and Vlahopoulou, 1988); the main causes of this variation are breed, stage of lactation, geographical location and diet. Such a view was confirmed by Kehagias et al. (1989) who reported that the best quality set-type goat’s yoghurt was made from milk of indigenous breeds because it contained the highest TS. Whilst in India, Singh et al. (1991, 1996) reported that the growth of starter cultures in pasteurised goat’s milk was faster than in boiled milk, that significant variation (P < 0.01) was observed in the growth of three mesophilic and four thermophilic starter cultures in milks obtained from four breeds of goat and that the lowest sensory scores were awarded to yoghurts made with Lactobacillus acidophilus and L. delbrueckii subsp. bulgaricus, and the highest to products made with single strains of Lactococcus species. The use of mixed strain starters improved the firmness of dahi (an Indian fermented milk) made from cow’s, buffalo’s or goat’s milk (Katara and Lavania, 1991). However, the rate of acid development of S. thermophilus and L. delbrueckii subsp. bulgaricus in sterilised milk was in the following order: goat > goat + cow > cow (Bozanic and Tratnik, 1997; Bozanic et al., 1998).
Thus, milk high in TS (c. protein 3.8 g 100 g-1 ) should be used for yoghurt making and, as with cow’s milk, different methods of fortification and processing of the milk can be used (Table 5.1, see also Park, 1994). However, the selection of starter cultures can greatly influence the organoleptic characteristics of goat’s yoghurt (Castagnetti and Turtura, 1994). Whilst Ibrahim et al. (1990) observed enhanced growth, acid development and peptidase activity of L. delbrueckii subsp. Bulgaricus in goat’s milk, the observed inhibition of the yoghurt starter cultures in goat’s milk could be associated with either milk containing strong “goaty” flavours or a higher concentration of free fatty acids than in cow’s milk (Abrahamsen and Rysstad, 1991). Litopoulou-Tzanetaki et al. (1993) achieved a higher than usual concentration of acetaldehyde, diacetyl and acetoin in fermented goat’s milk by using a mixture of a commercial yoghurt starter culture plus Lactococcus lactis biovar diacetylactis. In general, the citrate content in goat’s milk is rather low when compared with cow’s milk and, as a consequence, such milk may not be suitable for diacetyl production by mesophilic lactococci alone (Abrahamsen and Rysstad, 1991). However, low levels of acetaldehyde in goat’s yoghurts have been attributed to the relatively high concentration of glycine in the milk; glycine can inhibit the enzyme involved in the conversion of threonine to acetaldehyde and glycine (Abrahamsen and Rysstad, 1991). The addition of threonine to goat’s milk stimulated acetaldehyde production (Marshall and El-Bagoury, 1986; Rystaad et al., 1990) and some relevant information regarding the behaviour and proteolytic activities of the yoghurt starter cultures in goat’s milk have been reported by Telles (1988) and Abd-Rabo et al. (1992).
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Inoculation rates (£1.5%) of the yoghurt starter culture have been recommended by Vlahopoulou et al. (1994) to produce firmer gels, but other researchers have used ≥2% (Marshall and El-Bagoury, 1986; El-Samragy, 1988; Araujo et al., 1988; Alexiou et al., 1990; Baltadzhieva et al., 1991). However, the viscoelastic properties of goat’s yoghurt when using exopolysaccharide (EPS) cultures were lower (storage modulus G¢ and loss modulus G≤ module) than those made from non-ropy starter cultures (Vlahopoulou and Bell, 1993) and similar observations were also reported for cow’s milk yoghurt
Nevertheless, EPS starter cultures produce thicker yoghurt and the product can be diluted with water (ratio 1 : 0.3 or 1 : 0.4) and 7 g sugar 100 g-1 added for the production of drinking yoghurt (van Dender et al., 1991), whilst Hashimoto and Antunes (1997) recommended the heat treatment of goat’s milk at 90°C for >5 min during the production of yoghurt using EPS cultures. Alternatively, UF goat’s milk retentate has been used to improve the characteristics and composition of a cultured-type beverage (Miocinovic et al., 1990), whilst in Poland consumer acceptability of goat’s fermented milk products were in the following order: drinking yoghurt > cultured acidophilus milk > kefir (Pieczonka and Pasionek, 1995).

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