Lactose hydrolysed yoghurt (LHY)

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During the manufacture of yoghurt, only part of the available lactose is utilised by the starter culture bacteria as an energy source with the production of lactic acid.
The excess lactose could be utilised to sweeten the yoghurt without increasing its calorific value. This effect could be achieved by hydrolysing the lactose using bgalactosidase (in powder or liquid form), which splits the lactose into glucose and galactose; the relative sweetness of lactose and these monosaccharides is, compared to a degree of sweetness for sucrose equal to 1, as follows: lactose 0.4, galactose 0.6 and glucose 0.7. Commercial preparations of b-galactosidase are mainly produced from yeasts, fungi and, to a lesser degree, bacteria (Broome et al., 1983, Gunther, 1984). However, Engel (1973) observed that only 50% hydrolysis of the lactose was necessary to produce an acceptable yoghurt in terms of sweetness. Up until the late 1970s, relevant data on the manufacture of LHY were reported by Tamime (1977a, b, 1978a) and reviewed by Driessen and Loones (1992), IDF (1993) and Khedkar et al. (1994). The process of lactose hydrolysis in milk could be carried out using one of the following methods:
• Process A – low temperature hydrolysis at <10°C during overnight storage;
• Process B – high temperature hydrolysis at 30–35°C for 1 –2 hour;
• Process C – high temperature hydrolysis at 30°C where the enzyme is added to the processed milk base along with the starter culture.
In processes A and B it is essential to agitate the milk and to adjust the pH to about 6.6; proceed to manufacture the yoghurt as illustrated in Fig. 5.1. Inactivation of the b-galactosidase is achieved by the heat treatment. In process C, the slow rate of acid development by the starter culture gradually reduces the b-galactosidase activity and total activation may occur bel Hydrolysis is only desirable, of course, during the manufacture of fruit/flavoured yoghurt, since plain/natural yoghurts are not sweetened at all. Nevertheless, although a reduction in the level of lactose in natural yoghurt does improve its therapeutic value (Gallagher et al., 1974), current clinical studies confirm that b-galactosidase originating from the starter culture is sufficient for lactose maldigestors, and there is no need to hydrolyse the lactose in the milk base (Rosado et al., 1992; Rosado, 1998; see also Chapter 9). However, work in this field has associated the enhanced activity of S. thermophilus and L. delbrueckii subsp. Bulgaricus in lactose hydrolysed milk with the availability of glucose and/or galactose in the milk. Hemme et al. (1978, 1979) and Marschke and Dulley (1978) have detected some proteolytic activity in commercial samples of b-galactosidase (possibly due to contamination during its preparation) and the improved activity of the yoghurt starter culture may be associated with the liberation of essential amino acids (Lee et al., 1990a) rather than with the presence of glucose and/or galactose. Nevertheless, despite these contradictory views, many researchers have reported shorter coagulation times for the lactose hydrolysed milks (Ismail and El-Nimer, 1980; Dariani et al., 1982; Effat et al., 1983; Shchelokova et al., 1985; Kreuder, 1988; Arsov, 1990). However, Arsov and Godic (1993) and Arsov and Torkar (1995) concluded that the causes of increased activity of starter cultures in lactose hydrolysed milk could be determined more clearly only using a pure culture of S. thermophilus. In a separate study, Arsov (1990) observed no enhanced activity by one of two commercial yoghurt starter cultures, while Sharma and Dutta (1986) suggested that stimulation of acid production by either of the yoghurt organisms in hydrolysed milk was strain dependent.
The quality of LHY may be influenced by a multitude of factors. First, lactosehydrolysed milk can have an inhibitory effect on the growth of some strains of S. thermophilus and L. delbrueckii subsp. bulgaricus in mixed culture (Abd El-Hady et al., 1985). Second, some b-galactosidase preparations may cause off-flavours in the product when hydrolysis levels exceed 60%, while others are suitable at 80% hydrolysis (Dariani et al., 1982; Broome et al., 1983;Toba et al., 1986a). Consequently, yoghurt treated with b-galactosidase during fermentation received slightly lower scores for flavour, texture and consistency than the control (Ismail et al., 1983).
On some occasions it may be desirable to use the b-galactosidase of Aspergillus oryzae to obtain a higher oligosaccharide content (4–19 times) then that obtained with the control yoghurt (Toba et al., 1986b). However, the use of hydrolysed whey concentrate or hydrolysed reconstituted SMP and dried whey may cause gelation of the milk base or affect curd stability of the yoghurt, and the recommended processing methods for LHY have been reported by Shah and Jelen (1987), Shah et al. (1993) and Atamer et al. (1995). Also, if the use of lactulose in the manufacture of LHY is desired, it should be added after the lactose hydrolysis in order to reduce the loss of lactulose due to b-galactosidase activity (Olano et al., 1986). Furthermore, an alcoholic LHY beverage can be made from either whey or skimmed milk using b-galactosidase from A. oryzae and fermentation with Zymomonas mobilis and L.
delbrueckii subsp. bulgaricus for ethanol and lactic acid production, respectively (Miyamoto et al., 1987).
Further processing methods to produce low calorie and low lactose yoghurt may include: (a) combined UF and b-galactosidase hydrolysis of milk which produces yoghurt with a lactose level <0.1 g 100 g-1 (Streiff et al., 1990; Khorshid et al., 1993; Abbas et al., 1996a–c; see also Shady and Abdel-Razik, 1997), (b) production of aow pH 5.0 (see also Lelieveld, 1984).
good quality yoghurt with L. delbrueckii subsp. bulgaricus alone in hydrolysed milk fortified with glucose oxidase and hydrogen peroxide (Tahajod and Rand, 1993), and (c) the use of a b-galactosidase preparation from lactic acid bacteria rather than yeasts (Sinha and Dutta, 1985; Kobayashi et al., 1989; Toba et al., 1990; Yang et al., 1993; Somkuti and Steinberg, 1995).
It is clear, therefore, that yoghurt can be produced from lactose hydrolysed milk, but the incentive for commercial production is limited because the process is still not economic in comparison with the addition of normal sweetening agents.
However, Smith and Bradley (1984) have reported a net saving of U.S.$0.0061 per 227 g cup of sundae-style LHY and a similar view was confirmed by Botha et al.
(1987). Alternatively, the production cost of LHY could be reduced by replacing SMP with a whey/caseinate blend in the milk base before hydrolysis (Whalen et al., 1988). It could be argued of course, that the use of immobilised enzymes might offer an attractive solution, but the economics of the process will be the decisive factor.

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