Main Factors Affecting Spray Drying

1. Inlet and Outlet Temperature

The temperature in a spray drying chamber typically refers to the temperature of the hot air entering the tower. The drying temperature is one of the most crucial factors influencing the physical and chemical properties of the spray-dried powder. Higher drying temperatures provide more heat to the chamber, increasing the drying rate and reducing moisture content in the final product.

Kha et al. noted that increasing the spray drying temperature from 120°C to 200°C reduces the moisture content in the dried powder from 5.29% to 3.88%. The particle size of spray-dried products is also affected by the inlet temperature. As the drying temperature rises, the evaporation of moisture speeds up, causing microspheres to form more quickly without sufficient time to shrink, resulting in larger particle sizes.

Tonon et al. found that as the inlet drying temperature increased from 138°C to 202°C, the particle size of açaí powder increased from 13.38 µm to 20.11 µm. Similarly, the particle size of guava juice powder significantly (p < 1%) increased with higher inlet temperatures. The bulk density of spray-dried powders decreases with increasing temperature. Larger particles may be hollow or have a porous or fragmented structure due to higher evaporation rates. Typically, porous or fragmented particles exhibit lower bulk density.

Chegini et al. demonstrated that because water has a higher density than most dried food solids, powders produced at higher temperatures have lower bulk density compared to those produced at lower temperatures. They also observed that smaller powder particles have higher bulk density.

The flowability of spray-dried powders is somewhat influenced by the drying temperature; as the temperature rises, flowability tends to decrease. Solubility is also an important quality characteristic of powdered products, directly affecting the reconstitution behavior of spray-dried foods. As the spray drying temperature increases from 120°C to 160°C, the solubility of the powder increases.

2. Wall Material

Sugar-rich substances, such as fruit and vegetable juices, are difficult to spray dry directly without a coating agent. The wall material is a polymer used to encapsulate active ingredients during the spray drying process, making it one of the most important factors in spray drying.

Wall materials can enhance the glass transition temperature and yield in spray drying, while reducing the stickiness and moisture absorption of the powdered product. Common wall materials include gum Arabic, maltodextrin, gelatin, starch, pectin, methylcellulose, alginates, and calcium phosphate, among others.

The choice of wall material primarily depends on the purpose of spray drying and the physicochemical properties of the processed materials. Wall materials should be highly soluble in the process solvent and possess sufficient film-forming ability to produce low-viscosity solutions even at high concentrations. For spray drying, they must have high molecular weights and high glass transition temperatures to improve the final product's anti-sticking properties. They should also protect sensitive compounds from heat, oxygen, and light.

Commonly used wall materials for spray drying include carbohydrates, mainly:

1. Starch and its derivatives (starch, maltodextrin, dextrin, and cyclodextrin);
2. Gums (gum Arabic or mixtures of gum Arabic and acacia gum);
3. Cellulose and its derivatives (cellulose, carboxymethylcellulose, and hydroxypropyl methylcellulose).

Starch and its derivatives exhibit good spray drying performance, such as high molecular weight and high glass transition temperature, are highly soluble in low-viscosity cold water, and can produce relatively dense powders. However, their film-forming ability is relatively poor, which is detrimental to drying efficiency, especially for sensitive compounds.

Compared to starch, gums have better film-forming ability but a relatively lower glass transition temperature. Cellulose and its derivatives have good film-forming characteristics and surface activity but are not easily digestible. Combining starch or starch derivatives with gums can improve spray drying performance, although the gum content should be lower than that of starch or its derivatives.

Reports suggest that proteins, particularly whey protein, have excellent film-forming ability and nutrient retention capability and are often used alongside starch or its derivatives.

3. Feed Rate

In the spray drying process, the feed rate is one of the significant influencing factors. The feed rate determines the residence time of the material in the drying chamber, separator, and conveyor, and also affects the atomization of the material and the size of the droplets. The feed rate primarily depends on the speed of the atomizer; the higher the pump speed, the faster the feed rate. However, a higher feed rate can slow down heat transfer, making it difficult for droplets to dry fully and potentially leading to wall sticking.

Additionally, Tonon et al. observed that droplets can directly fall into the drying chamber due to incomplete atomization at high feed liquid flow rates, resulting in reduced yield. A higher feed rate leads to insufficient interaction time between the droplets and hot air, increasing the moisture content in the spray-dried powder. Moreover, shorter contact times can reduce heat and mass transfer efficiency, resulting in higher moisture levels in the final product.