The development in PL and PLG derived activities in Milk72 are
shown in Fig. 4. As can be seen, PLG derived activity decreased
rapidly, while PL activity increased. This indicates activation of PLG
into PL during storage, and no PLG derived activity was detectable
after 6 weeks storage. A similar development has been reported in
direct heated UHT milk (Manji & Kakuda, 1986). The increase in PL
activity could be fitted well to a first order reaction kinetic
(k¼0.1828, R2
¼0.995),while PLGderived activity on the otherhand
decreased from 0 to 6 week in a linear fashion (y ¼104.7x þ 603;
R2
¼ 0.997). The inhibitors of the PL system are heat labile: under
pasteurisation conditions (74.5 C for 15 s) PAI is completely inac-
tivated, while about 36% of PI is inactivated (Prado et al., 2006). PA
have been shown to have a similar or even higher heat stability than
PL and PLG (Lu & Nielsen,1993; Saint-Denis et al., 2001), suggesting
that a significant proportion of PA remained inMilk72 after the heat
treatment. This, together with the inactivation of PAI, explains the
fast rate of PLG activation in Milk72. After 4 weeks storage, the PL
activity inMilk72was comparable to the PL activity found in the low
pasteurised milk (632 28 UmL1
) prior to heat treatment. The
gelation of Milk72 had no apparent effect on PL activity.
The development in PL and PLG derived activities in Milk72 areshown in Fig. 4. As can be seen, PLG derived activity decreasedrapidly, while PL activity increased. This indicates activation of PLGinto PL during storage, and no PLG derived activity was detectableafter 6 weeks storage. A similar development has been reported indirect heated UHT milk (Manji & Kakuda, 1986). The increase in PLactivity could be fitted well to a first order reaction kinetic(k¼0.1828, R2¼0.995),while PLGderived activity on the otherhanddecreased from 0 to 6 week in a linear fashion (y ¼104.7x þ 603;R2¼ 0.997). The inhibitors of the PL system are heat labile: underpasteurisation conditions (74.5 C for 15 s) PAI is completely inac-tivated, while about 36% of PI is inactivated (Prado et al., 2006). PAhave been shown to have a similar or even higher heat stability thanPL and PLG (Lu & Nielsen,1993; Saint-Denis et al., 2001), suggestingthat a significant proportion of PA remained inMilk72 after the heattreatment. This, together with the inactivation of PAI, explains thefast rate of PLG activation in Milk72. After 4 weeks storage, the PLactivity inMilk72was comparable to the PL activity found in the lowpasteurised milk (632 28 UmL1) prior to heat treatment. Thegelation of Milk72 had no apparent effect on PL activity.
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