# Powdered Foods: Structure, Processing, and Challenges: A Review
## 3 Powdered Food Production Processes  
Production of powdered foods is usually divided into two main processes: transformation from liquid to powder and solid to powder. For solid materials, the powder production process involves reducing the size of the material (crushing, grinding, or pulverization), followed by granulation and mixing. This size reduction occurs via the application of energy to the solid material, and this mechanical activity can generate stress on the food structure, leading to rupture and size reduction. Furthermore, moisture content is inversely proportional to grinding process efficiency, which requires a higher energy consumption due to the water content, making the material breaking process more difficult [4; 14; 15].  
To achieve smaller-sized particles, the superfine grinding method is an interesting emerging technique, capable of obtaining sizes on the nanometer scale. This technique can improve several food characteristics, including flow properties, morphological structure, bioactivity, and bioavailability [16]. [PERSON] et al. [17], for instance, evaluated the morphological and antioxidant properties of ginger, obtaining particles ranging in size from 1 to 100 nm from high-energy ball milling equipment. Their findings suggested that the superfine grinding process could extract more phenolic content, resulting in higher antioxidant activity due to enhanced interaction between the solvent and the food matrix [17]. Similarly, [PERSON] et al. [18] observed a significant increase in polyphenol and anthocyanin content in rose myrtle powder, with particle sizes ranging between 0.79 and 41.57 \(\upmu\)m.  
On the other hand, when the material is liquid, the process can occur via spray-, freeze-, drum-, or belt-drying; crystallization; or other methods [4]. In this review, the spray- and freeze-drying processes will be covered, as they are the most widely used technologies, as well as their respective processes to obtain powdered foods (Figure 2).  
Figure 1: Keyword map representing publications on powdered foods. The circle size indicates the number of publications and the circle color represents the average year of publication. Two circles are close to each other if the keywords co-appeared more frequently. The data were obtained from the Scopus database and analyzed using VOSviewer software version 1.6.20.  
To produce powdered foods on an industrial scale, [PERSON] [19] highlights some aspects associated with production difficulty. Among them, the author included the flow properties of powdered foods, agglomeration in tanks, contamination, and structural problems in silos [19]. Regarding flow, at the beginning of the process, material blockage can occur due to the accumulation of material at the exit of the silo, which requires methods to force product flow, such as air blowers or sledgehammers. The ratholing process can also occur, characterized by material friction on the silo walls, leading to flow only in the central internal part [19].  
In addition to powder properties, factors such as temperature, relative humidity, and storage time also impact food flow. For instance, the presence of lactose at low relative humidity can result in the caking effect (agglomeration of particles and sticky material), leading to a decrease in the flow rate [20].  
Flow limitations can also occur, caused by the exit of air between the powdered material, resulting in the formation of a vacuum, requiring the application of a countercurrent flow of air or gas. Therefore, these flow challenges, among other factors, contribute to the complexity of establishing an industry for powdered products, which are needed to evaluate the physical and chemical characteristics, silo properties, and solids flow [19].