The influence of the supplementation with SPI on the performance

performance of yogurt starter during storage at 4

C

6.3.1.1Lactose metabolism

Figure 6.1 presents the lactose concentration in both SY and USY during the storage period. As shown in the figure, the initial lactose contents in SY and USY were 43.95 and 47.03 mg/g, respectively, although the lactose contents in the mix prepared for SY and USY were at 65.42 mg/g (data not shown). Hence, during the fermentation of the

yogurt mixes, significantly (P<0.05) higher amount of lactose was utilised by S.

thermophilus 1342 and Lb 11842 in SY than that in USY by 4.7%. This result suggests that the supplementation with SPI to SY significantly promoted the lactose metabolism by the yogurt starter during the fermentation process. This could be due to the enrichment of nitrogen source through SPI for the yogurt starter. Since SPI contains 18 amino acids including 11.0 mg/g of tryptophan and 65.9 mg/g of arginine which are complementary to the inadequate nitrogen source such as tryptophan and arginine in SMP (Nutrition Data, 2007; Poch & Bezkorovainy, 1988)). To synthesise enzymes involved in lactose utilisation, several amino acids are needed. Consequently, the rich source of amino acids released from SPI during the hydrolysis by probiotic organisms in SY could help the yogurt starter to utilise lactose more efficiently (Vedamuthu, 2006). It also appears that, during the entire storage period, the lactose content in SY was always significantly (P < 0.05) lower than that in USY. For instance, at 21 d of the storage period, the lactose content in SY was 41.72 mg/g yogurt compared to 45.01 mg/

biotransformation of isoflavones during storage period

g in USY. However, the amounts of lactose utilised by the yogurt starter during the entire 28 d of the storage period in SY and USY were similar (2.21 and 2.08 mg/g yogurt, respectively). The reason is possibly due to the inactive status of enzymes such as lactase that are involved in lactose utilisation in low pH and temperature condition of storage period in both SY and USY.

6.3.1.2Organic acids production

Figure 6.2 illustrates the organic acids concentration including lactic and acetic acids, and the pH values of SY and USY during the storage period. Although other organic acids such as orotic, citric, pyruvic, uric and formic acids are present in yogurt, however, they are found in very low concentration (Fernandez-Garcia & McGregor, 1994). Lactic and acetic acids are the two dominant organic acids in yogurt. Especially, lactic acid is used as an indicator to evaluate the fermentation of the yogurt starter (Vedamuthu, 2006). As shown in Figure 6.2, the acetic acid concentration in SY was insignificantly (P>0.05) higher than that in USY. In contrast, the lactic acid concentration in SY is insignificantly (P>0.05) lower compared to that in USY. As a result, the ratio of lactic acid to acetic acid in SY was lowering than that in USY. At 28 d of storage period, the ratios were 8.61 and 10.33, respectively. Hence, our study suggests that the presence of SPI slightly altered the production of lactose metabolism of the yogurt starter. This was in agreement with the study of Gomes et al. (1998), who also reported that the production of lactic acid decreased and acetic acid increased in milk supplemented with a rich protein source, milk hydrolysates, for fermentation by lactic acid bacteria.

As shown in Figure 6.2., the pH values in SY were always lower than those in USY. Our study suggests the supplementation with SPI could reduce the fermentation time of yogurt. In fact, after the same fermentation time of 8 h, the pH values of SY and USY were 4.55 and 4.60, respectively. The reason might be that the reconstituted SMP exhibited a stronger buffering capacity than reconstituted SPI. The maximum buffering capacity of milk is around 5.1, considerably close to the pH zone of yogurt at 4.6 (Figure 6.2) (Chandan, 2006). Consequently, the pH of USY was in range of 4.35 to 4.60 compared to the range of 4.15 to 4.55 in SY, during 28 d of the storage period.

biotransformation of isoflavones during storage period

Thuy Thi PHAM- PhD Thesis, Victoria University, 2009

6.3.1.3Viability of yogurt starter

Figure 6.3 presents the viability of the yogurt starter including Lb 11842 and S.

thermophilus 1342 in SY and USY during the storage period at 4 oC. For the first 7 d of the storage period, the survival of the yogurt starter in SY was significantly higher (P<0.05) than that in USY. The reason could be the yogurt starter was provided more nutritious by SY than USY. However, from 14 d of the storage period, the viability of

both S. thermophilus 1342 and Lb 11842 in SY were significantly lower (P < 0.05) than

those in the control USY. In addition, during the storage period, the viability of S.

thermophilus 1342 and Lb 11842 in SY decreased by 0.94 and 0.61 log CFU/g, respectively, compared to 0.36 and 0.27 log CFU/g in USY. Thus, the pH values may play a key role in lowering the survival of the yogurt starter in SY since from 14 d of the storage period, pH of SY was 0.20 – 0.27 lower than that in USY (Figure 6.2).

However, the viable counts of both S. thermophilus 1342 and Lb 11842 in SY were still

in the range of 8.84-9.78 and 8.11-8.72 log CFU/g, respectively. Those were higher than the minimum concentration required at 7.0 log CFU/g to have health benefits (Frye,

2006). Although S. thermophilus 1342 and Lb 11842 are not classified as probiotic

organisms, these bacteria can improve lactose digestion and may help promote a healthy immune system. Hence, it is desirable that they remain alive at a high concentration during storage in order to have beneficial effects (Zonis, 2007).