water content. This can be done accurately for simple mixtures, since the
water content can be unequivocally determined. For mixtures of solutes, the
problem remains. As a rule of thumb, the value ofc^0 Wis generally not far
removed from 0.2.
Conclusion. Most foods that are aqueous solutions and/or
dispersions can be freeze-concentrated without crystallization of solutes
and so become mixtures of ice crystals and a glassy material. The glass is of
mixed composition, and roughly 20%of its mass is unfrozen water. The
frozen food is stable to all physical changes and to many chemical reactions,
provided that the temperature is kept belowTg^0.
Question
Suppose that a company wants to produce ‘‘aromatic sugar,’’ i.e., sucrose containing
aroma (volatile flavor) compounds. The product should be in a fine granular or
powdered form, readily dissolvable in cold water, and be stable, in particular to loss
of aroma. What manufacturing methods would be suitable? Give particulars,
especially on the temperature–water content sequence.
Answer
Grinding crystalline sucrose and adding the aroma compounds will not work: most
of these compounds are somewhat hydrophobic and will not at all or only slightly
adsorb onto sucrose. Even if sufficient ‘‘aroma’’ can be adsorbed, it soon will be lost
by evaporation.
The aroma compounds should therefore be incorporated into a sucrose glass.
(The chance that compound crystals of sucrose and aroma components can be
formed is very small.) The sugar can be melted, mixed with the aroma compounds,
and then cooled to belowTg. However, owing to its high melting temperature
(192 8 C: Table 16.1), the sucrose will show caramelization and browning, which is
presumably undesirable; moreover, aroma compounds may be lost or broken down.
Manufacture must then start by making an aqueous sucrose solution and
adding the aroma. The sucrose concentration can be high, saycs¼ 0 :5, but it must
be checked that sufficient water is present to dissolve the aroma compounds. A
convenient process then would be spray-drying, which makes a powdered, glassy
material. A considerable proportion of the aroma compounds can often be retained
in spray-drying, since the outside of a drying drop very soon becomes glassy, greatly
reducing diffusion of the compounds, whereas water diffusivity is still considerable.
However, as seen in Figure 16.6, the drying temperature should be at most 40 8 C, to
leave a mere 2%water and still be beyondTg. It will be quite difficult to dry fast
enough to such a low water content at such a low temperature.