Retention of Aroma in Drying Process

The retention of aroma in drying process

The retention of aroma in liquid dehydration process has been studied in the past twenty years. Great progress has been made on the understandings of aroma retention and principles of optimization of aroma retention. Compared to concentration process, the retention of aroma in drying process has drawn more attention and effort. Most work has been done by Holland researchers Thijssen and colleagues, Karel, Flink and their colleagues at MIT; King and colleagues at University of California Berkeley; Loncin and most recently, Toei et al. in Japan. Spray drying and frozen drying are two common models for the study. The study with other models results in similar conclusions. Here discussed are only the first two drying methods.

1. The mechanism of aroma retention in drying process

There are two widely accepted theories on the retention mechanism of aroma.

1. The selective diffusion theory proposed by Thijssen
2. The micro entrapment proposed by Flink and Karel

According to the selective diffusion concept, in the drying process of liquid foods, transport of water and aroma components are controlled by molecular diffusion if there is no inner turbuluration. The diffusion coefficients for water and aroma component, D_w and D_a, decrease quickly as the water content decreases. And D_a decreases faster than D_w. When the surface water content drops as the drying process starts, a water concentration gradient is actually formed. As the surface water content drops to a certain level so that the D_a is far smaller than D_w, the aroma can not escape any more from the surface. The food surface now actually act as a semi-permeable membrane. This theory has been demonstrated in the spray drying and extractive drying models.

The micro-entrapment theory explains that in a drying process, the aroma retention is realized by the encapsulation of aroma by food matrix. In non-crystalline form of food solids, this encapsulation can be done by quick cooling or drying (spray drying, frozen sublimation, fluidized bed cooling or drying). The structure of food solids is metastable and not thermodynamically stable. If the molecules in the metastable structure have enough motility, the structure can turned into stable form. This transformation is called collapse in the study of aroma retention. When the temperature exceeds the collapse temperature (T_c), the collapse can be seen. T_c depends on a variety of factors, the major one moisture content. The aroma entrapment depends largely on the transformation. It is generally agreed that the entrapment of the aroma components with higher evaporation pressure compared to water is due to the selective diffusion of water. There is a linear relationship between the degree of collapse and the retention of aroma. More collapse results in less entrapment of the aroma. Because the moisture content determines the collapse, the loss of aroma is related to the moisture content. In the early study, Karel et al attributed the loss of aroma to the heterogeneity of the food matrix, while Ometette and King have another explanation. They think that at low moisture content, the diffusion coefficient is very low, Dt/L2 (t is time and l is thickness) is not big enough for aroma to escape. At high moisture content, the diffusion coefficient is large and the aroma easily escapes. At middle levels of moisture content, collapse may not happen. If collapse does not happen, a model with constant time and thickness can be used to explain the loss of aroma. If it does happen, the loss of aroma can be explained using a model with the thickness increasing as time. Their explanation has been supported by their experimental evidence.

The encapsulation properties of the carbohydrates make the food matrix with carbohydrate suitable for the entrapment of aroma during the drying of the carbohydrate based foods. As to the spray drying and frozen drying, the encapsulation mechanisms are summarized in Table 2 and 3.

From Table 2 and 3, the explanation offered by both theories are consistent. The diffusion theory explains from a micro viewpoint and the encapsulation from a macro viewpoint.

2. The methods for aroma retention in drying processes

Based on the above discussion, the practical methods are listed in Table 4 and Table 5 for retaining aroma in spray drying and frozen drying processes. They can be referred to when choosing the drying conditions.

Table 2 The mechanism of aroma retention involved in the spray drying once drops form, water on the surface evaporates quickly and form a membrane with selective permeability to water and aroma components. Therefore, the retention of aroma can be improved by following methods:
--- shorten the lag time before the drop formed;
--- once the drop form, let it evaporate quickly;
--- form a membrane with high selectivity;
--- avoid formation of the inner circulation;
— avoid breakage of the surface of the drop.

Table 3
The entrapment mechanisms involved in the frozen drying process
Once the ice crystal forms, the unfrozen part of the solution is viscous and it prevents the diffusion of aroma components. When the center is cooled, the surface of the solution becomes heterogenic solids among them the selective diffusion can function. Then the key factors are:
--- due to the distances crystals formed;
--- due to the selectivity of heterogenic solids;
--- avoid collapse;
--- avoid breakage;

Table 4
The controllable parameters for retaining aroma in spray drying process

1. The feasible maximal viscosity of the liquid which allows quick drying of the drops;
2. High nozzle pressure;
3. Add surface active agents;
4. Add non-volatile oils;
5. Incorporate N2 to avoid the air inner circulation;
6. Use high gas temperature and low air moisture;
7. Add polymers to avoid breakage or shrinkage of drops;
8. High solid concentration;
   Avoid formation of gas bubbles in the feed.

Table 5
The key factors influencing the aroma retention in frozen drying process

1. High solid concentration;
2. Add polymer to increase the viscosity of the feed;
3. Keep low pressure in the drying chamber;
4. Slow freezing;
5. Avoid collapse and foaming;
6. Avoid structure breakage;
7. Avoid plastic stream;
8. Control the shape of the solid.

This is part of an old article published in another language nearly forty years ago.