Chemistry Reference
In-Depth Information
remain soluble in 100 g DL after the crystallization of the excess lactose.
From the value of the equilibrium ratio, it can be calculated that of the 5.4 g
oflactoseinsolution,2.02gwouldbe
-lactose and 3.38 g
-lactose. Summar-
izing: 3.70-2.02
¼
1.68 g of
-lactose would crystallize and 6.2-3.38
¼
2.82 g of
-lactose would first mutarotate to
-lactose and then crystallize. In this
context, the rate of crystallization of lactose in DL depends not only on the
rate of crystal formation itself but also on the mutarotation rate, and the
slowest reaction will limit the crystallization kinetics.
The presence of salts and sugars in the system impacts on the kinetics of
mutarotation in opposite directions. Citrate and phosphates, at the concen-
trations found in milk, accelerate mutarotation to a rate twice as high as in
pure water, whereas in contrast, concentrated sucrose slows down the muta-
rotation reaction. This latter effect is only slight up to 40% (w/v) sucrose, but
above this level, the mutarotation rate decreases and the catalytic impact of
milk salts is counteracted (Holsinger, 1988). Temperature and pH also influ-
ence the mutarotation rate. This effect of pH is minimum in a range of 2.5-7.5
but increases dramatically both at lower and higher pH values. The effect of
temperature is shown in Table II.2.
Taking into account the environmental conditions in DL and the data
above, rate of lactose mutarotation in DL should be low, as sucrose is highly
concentrated, the pH is between 5.57 and 5.97 and the storage temperature is
always below 208C (Ferramondo et al., 1984; C odigo Alimentario Argentino,
2007).
On the other hand, the formation of lactose crystals in the intermediate
zone (Figure II.2) may be very slow. In this zone, seeding with lactose crystals
can induce crystallization but otherwise supersaturated solutions will be
stable (Holsinger, 1997).
Either way, the slow or rate-limiting step may be mutarotation or
crystallization, depending on the characteristics of the system containing
the lactose. Haase and Nickerson (1966a,b) found that mutarotation is very
fast and consequently crystal formation is the rate-limiting step under the
environmental conditions that exist in most dairy products. Under other
conditions, for example, when an extensive nucleation area is available for
lactose crystallization, it has been observed that neither of the two steps can
be clearly identified as the rate-controlling step (Tweig and Nickerson, 1968).
Table II.2.
Effect of temperature on the rate of lactose mutarotation
Temperature, 8C
75
25
15
0
Reaction completion (% in 1 h)
100 (instantaneous)
51.7
17.5
3.4
Adapted from Holsinger (1988).
