led to equilibrium. In a spray-dried product, the particles are for the most part about
0.05 mm in diameter or larger. Given the small diffusion coefficient at the prevailing
water contents, it will take at least some weeks before a homogeneous distribution of
water throughout the powder particles is obtained. This implies that the outside of
the powder particles would contain more water than the inside. Keeping the powder
will thus lead to a lower water content in the outer layer. Since the water activity as
observed will roughly correspond with that at the outside, its value will decrease
upon storage.
Note The results would be rather different if a considerable part of the
material could crystallize, say in step (c).Question 2
Suppose that you make a dough from 1 kg wheat flour, 600 g water and 15 g salt. The
flour contains 14%water and 65%starch. Assume that the dough can be considered
as a two-phase system: (a) containing starch and part of the water and (b) the
remaining components, in which gluten will take up most of the dry matter. How can
you establish the water content of both phases?
Answer
First, determine the water activity of the system; suppose that the answer is 0.975.
Then find out what the water content is of starch at thatawvalue; Figure 8.4a shows
that it will be about 0.4 g water per g dry starch. This then would mean that the
starch phase contains 650 6 0.4¼280 g water, or 30%. This leaves for the rest 600þ
0.14 61000 280 ¼460 g water, leading to a water content of 67%. It is thus clear
that the ‘‘gluten phase’’ contains far more water than the ‘‘starch phase.’’
Notes- Actually, Figure 8.4a relates to potato starch, not wheat starch, but the
difference will be small. - To check the calculation, the water activity of a gluten system of
comparable composition should be found or estimated. - In doing this exercise, you will see that the sorption isotherms should
be exactly known and that even at the high vales ofawinvolved, hysteresis
of the isotherms cannot be fully neglected.